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Extra long period (20–40 day) oscillations in the mesospheric and lower thermospheric winds: observations in Canada, Europe and Japan, and considerations of possible solar influences
Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 835–852
www.elsevier.nl/locate/jastp
Extra long period (20 – 40 day) oscillations in the mesospheric and lower thermospheric winds: observations in Canada, Europe and Japan, and considerations of possible solar in/uences Y. Luoa;∗ , A.H. Mansona , C.E. Meeka , K. Igarashib , Ch. Jacobic a Institute
of Space and Atmospheric Studies, University of Saskatchewan, Saskatoon, Canada S7N5E2 b Communications Research Laboratory, Tokyo, Japan c Institute for Meteorology, University of Leipzig, Leipzig, Germany
Received 10 January 2000; received in revised form 16 August 2000; accepted 1 September 2000
Abstract An extra long period (20 – 40 day) oscillation has been identi9ed in the mesospheric and lower thermospheric (60 –100 km) winds observed simultaneously by radars (MF, LF) at four sites from 70◦ N to 30◦ N in the northern hemisphere during the winter of 1995=1996. A long-term (1980 –1999) investigation of this oscillation at Saskatoon and Collm is also carried out to obtain climatological and statistical characteristics. Spectral analysis has shown that this oscillation is a common feature of the winter (November–March) atmosphere, having strong amplitudes throughout the mesosphere (∼10 m=s) and lower thermosphere (∼5 m=s), and being much stronger at mid-low latitudes. Although the oscillation has a climatology similar to the long period normal mode planetary waves (10 –16 day), the phases at the various sites are very similar, and not consistent with a freely propagating wave. Comparisons with geomagnetic=solar wind parameters and solar UV radiation suggest that the oscillation could be related to the short-term solar rotation period (ca. 27 days) in some way. However the range of observed wind periods is very broad and this raises questions about this interpretation. Nevertheless the inter-annual variations of this 20 – 40 day oscillation indicate a weak 11-year solar cycle correlation in the mesosphere (positive) and the lower thermosphere (negative). Also, the cross-correlation between the winds and solar radiation shows signi9cant quasi 27-day correlation and the wind lags behind the solar radiation a few days in the mesosphere. In general it is implied that the atmosphere could react c 2001 Elsevier Science Ltd. All rights reserved. to the solar activity in an indirect way due to certain dynamical mechanisms. Keywords: Mesospheric and lower thermospheric dynamics; 27-day oscillation
1. Introduction When analyzing the long-period oscillations of the wind in the mesosphere and lower thermosphere (MLT), apart from periods related to the well-known normal mode 2–16 day planetary waves, there also exist much longer oscillations with periods ranging from 20 to 40 days (Luo et al., 2000a). In fact there have been a few other reports of such ∗ Corresponding author. Tel.: 1-306-9662337; fax: 1-3069666400. E-mail address: [email protected] (Y. Luo).
long oscillations in the mesosphere. Ebel et al. (1986) investigated the response of the middle atmosphere to solar activity oscillations of quasi-27 day. They used coherence estimates relating solar radiation /ux and atmospheric oscillations from the lower thermosphere down to the middle stratosphere (30–120 km), which were inherent in winds from Saskatoon MF radar and from radiosondes, and concluded that main source of the 27-day oscillations induced by solar ultraviolet (UV) radiation may be near the stratopause. A 30-day wind oscillation of as large as 10 m=s in winter in the meteor region (80 –110 km) was also displayed when studying the strato-meso-thermosphere coupling (Cevolani,
c 2001 Elsevier Science Ltd. All rights reserved. 1364-6826/01/$ - see front matter PII: S 1 3 6 4 - 6 8 2 6 ( 0 0 ) 0 0 2 0 6 - 6
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1991). Pancheva et al. (1991) showed some /uctuations of 24 –30 day in the ionospheric absorption of radio waves at 80 –95 km altitudes and related them to variations in the neutral atmosphere at those periods. They conjectured that they might be of solar origin in some way. Unlike the normal mode (free travelling) planetary waves which have been comprehensively reported and studied, both observationally and theoretically by a great number of authors, the 20 – 40 day oscillation has not yet been clearly identi9ed because of its complexity. The oscillations have, on the one hand, periods comparable to the solar short-term variation, i.e. the solar 27-day rotation eEect, naturally implying a solar-induced oscillation. On the other hand, it will be shown that their latitudinal structure and seasonal variation often resemble that of some stationary or transient planetary waves, which are strongest at mid-high latitude during fall-spring epoch. This contradictorily suggests an excitement of, or linkage with, the planetary waves (Ebel et al., 1981; Chandra, 1985). Anyway it has been rather suggested that solar activity can modulate existing planetary waves or even generate wave-like circulation perturbations, probably via changing UV and corpuscular radiation /uxes in the middle atmosphere. Speci9cally, one likely cause of such a solar-terrestrial link, or source of forcing, is via the absorption of the solar UV radiation by ozone around the stratopause (40 –50 km). This solar forcing or generation of perturbations by solar activity seems capable of playing a signi9cant role in the entire middle atmosphere (10–100 km). For example, a number of modelling and observational studies have shown that a 27-day oscillation near the stratopause can propagate downward or aEect the lower atmospheric structure, possibly through dynamic processes such as planetary waves (Ebel et al., 1986; Dameris et al., 1986), and even penetrate into the middle atmosphere. Otherwise there have been few studies concerning its in/uence on the upper atmosphere, e.g. the mesosphere and lower thermosphere. Preliminary assessments of planetary wave responses to solar forcing at periods near 30 day, within the global scale wave model (GSWM), have produced very small amplitudes (C. Meyer, 1999, private communication). Another practicable energy and momentum transport from the sun to the earth atmosphere is through the interaction of the solar wind with the earth’s magnetosphere, i.e. the reaction of the neutral wind to particle precipitation and ionospheric current variations probably recurring with a quasi-period of 27 day at high latitudes (Hocke and Schlegel, 1996). In this paper we will make a detailed investigation of the “extra long period” wind oscillations (by that term we will be referring to the period ranging from 20 to 40 days) and of comparisons with the solar parameters. Firstly, the mesospheric winds observed by ground-based radars simultaneously at four northern hemispheric sites (from 70◦ N to 30◦ N) during the winter of 1995=1996 are displayed. After that the continuous daily winds from 1980 to 1999 at Saskatoon and Collm are used to obtain statistical charac-
teristics, climatological patterns, and long-term variations of this ‘extra long period oscillation’. Finally comparisons with solar-related parameters are made. 2. Data sets and processing methods The winds are measured by the “spaced-antenna” technique in the medium-frequency (MF), and in the low-frequency (LF) ranges. Three MF radars used here are located at TromsH (70◦ N, 19◦ E), Saskatoon (52◦ N, 107◦ W), and Yamagawa (30◦ N, 130◦ E) with operating frequencies of 2.8, 2.2, and 2.0 MHz, respectively. They use the partially re/ected signals from the D- and lower E-region of the ionosphere to derive the wind pro9les. The Saskatoon MF radar and the spaced antenna “full correlation analysis” method have been described in detail elsewhere (Manson et al., 1981). The TromsH system is virtually identical to that at Saskatoon. For these two radars the winds are sampled every 5 min (post-integration time) with height samples at 3 km intervals from about 60 up to 110 km (virtual heights, which assume the radio wave propagates as it does in a vacuum). It is generally realized that the virtual height is equal to the real height up to ca. 100=95 km for the winter=summer season. Above these altitudes a certain amount of group retardation should be considered. Uniquely, the Saskatoon MF radar has been operated continuously since 1979, and the data can therefore be used for long-term (e.g. solar cycle) analysis. At Yamagawa the winds are detected with samples at intervals of 2 minutes and 2 km (Igarashi et al., 1996). In this paper the LF measurements carried out at Collm (52◦ N, 15◦ E) are also used. The Collm radar uses the re/ected ‘sky wave’ of commercial radio transmissions at 177, 225 and 270 kHz from the lower boundary of the E-region of the ionosphere, and supplies the night-time and twilight wind around 95(±5) km (Schminder and KMurschner, 1994). We are mainly concerned with two aspects of solar activity in this paper. First is the solar ultraviolet radiation represented by the Mg II index. In this paper the Mg II index (data version V19r2) is computed from the daily mid-resolution spectrum in the vicinity of 280 nm measured by the solar ultraviolet spectral irradiance monitor (SUSIM) on the upper atmospheric research satellite (UARS) (Floyd et al., 1998). As for the solar cycle (∼11 yr) variation, the solar 10.7 cm /ux can be used as the proxy indicator of the slowly varying component of the solar UV /ux in the 160 – 400 nm region. Another useful solar parameter is the solar wind. In this paper the hourly data from the solar wind experiment (SWE) on the WIND spacecraft are used. Also, there is interest in the eEect of magnetospheric particles upon the earth upper atmosphere and dynamo region, so for the purpose of assessing the short- (∼27 days) and long-term (solar cycle) variations, the Kp-index is a good representative. All wind analyses in this paper are based on daily mean data. For the MF radar the hourly mean wind is 9rstly
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obtained from the 5 (or 2)-min wind and is only considered valid when there are at least 2 (or 4) values within an hour. Secondly, the daily mean wind is derived from these hourly data and requires at least 3 values per day. Actually the real situations are far better than the above criteria, usually with the number of hourly values greater than 12 (and approaching 24 by 80 km), which can for the most part reduce the tidal=semi-tidal contamination. For the LF radar only night and twilight measurements can be derived due to the strong absorption of the sky wave during the day. Therefore a multiple regression analysis is used to handle these inhomogenously distributed data and to determine estimates of the daily prevailing winds (Jacobi et al., 1998). The data gaps in the daily-mean data set are 9lled with linear interpolation when the length of gap is less than 10 days (about 1=3 of the period of interest). Otherwise, when gaps are longer than 10 days but less than 1=3 of the length of the time window for spectral analysis (64 day long in this paper except in Fig. 2), they are 9lled with random values having the same means and standard deviations as the rest of the available data set. Lastly if gaps are longer than the above-mentioned 9t-length, analysis does not proceed. All data are detrended with polynomial 9tting of order 2 before any spectral analysis is applied. For detrending we have also tried polynomials of order 1 (linear) and 3. The order 1 process still leaves some variations of long-term comparable to the window width (∼50 days). On the other hand, the order 3 process suppresses a great deal of the components longer than about the 2=3 of the length of the window (∼40 days) and alters the position of spectral peaks within the range of 20 – 40 days. For the purpose of this paper, order 2 will be the compromise. The data are also modulated by a Hanning window before spectral analysis to reduce the eEect of spectral leakage. In the time domain, to get a 9lter of a band-pass nature, the simple and convenient method of the inverse-transform of appropriate components from the Fourier analysis is used for a short-term sequences. For long-term sequences an FIR (9nite impulse response) 9lter is operated. For spectral purposes a couple of techniques were employed. The 9rst is harmonic 9tting with the period sweeping through the range we are interested in. Also the Lomb–Scargle (LS) method (Scargle, 1982) and Fourier transform are used to get estimates of the spectral amplitudes and con9dence levels as well as the phase information. Although the LS method is suitable for unevenly time spaced data or data with gaps (in fact only for short gaps, to be exact, as we state next), we have tried to 9ll the gaps as possible as already described. Scargle’s de9nition of the time-translation invariance of the periodogram makes it exactly equivalent to a Least Square Fitting of a sine wave to data (Scargle, 1982). The nature of any harmonic 9tting is such that whether it is intentionally or not, one is making assumptions for the missing interval. By completely ignoring the missing interval, we are actually making the assumption that whatever happens in this interval is acceptable. When the missing interval becomes
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longer the spectral analysis may produce results appropriate to an enormous false burst within the gap interval. Therefore adding any a priori knowledge to the missing data is adding constraints and thus will potentially enable us to obtain more realistic results. Zhou et al. (1997) provided a detailed test for this problem. This was also tested by ourselves by using real data, and arti9cially creating gaps. Finally a cross-spectral method based on the Fourier transform, and a cross-correlation analysis, are applied to check the spectral relationship and phase diEerence between the winds and solar parameters. 3. Results and analysis 3.1. Winter of 1995=1996 The spectra of the detrended daily mean wind, both zonal and meridional components, observed at TromsH, Saskatoon, Yamagawa and Collm during the year of 1995=1996 (October–April) are shown in Fig. 1a,b,c and d, respectively. We have an extended set of data for Saskatoon and Collm, but data only starting from November 1995 for TromsH and Yamagawa. For the MF radar sites, data from four selected altitudes (76, 82, 88 and 94 km) are displayed, but for the LF radar there is only one layer (∼95 km) available. The spectral amplitudes in the contour plots are calculated by harmonic 9ttings within a set of time-windows of 64 days length, whose center is chosen as the corresponding time for the plots. This window is shifted by 5 days each time to get a “continuous” distribution of the spectra with time. With this window the frequency resolution is 0.016 cycle-per-day; converting to periods, they are approximately 4.8–5.2 day at 5 day, 9.3–10.8 d at 10 d, 13.4 –17.0 d at 15 d, and 24.3– 39.2 d at 30 d. The well-known planetary wave periods can be clearly seen as bands at most altitudes and sites; such as the 6-, 12-, 17-day oscillation at TromsH, the 5-, 8-, 10- and 16-day at Saskatoon and the 6-, 10-, 16-day at Yamagawa. Besides the planetary waves, longer period oscillations with stronger amplitudes also appear at all altitudes and sites. They fall into the period range of 20 – 40 day, and the spectral peaks generally change in period a little with time, e.g., conspicuously, the zonal winds at TromsH, Saskatoon and Yamagawa at 76 km change periods from 30 to 25 day from February through March. The amplitude is as strong as 10 m=s or even more at lower altitudes (76 –82 km), but usually decreases to ∼5 m=s at greater heights (∼94 km). In general, oscillations in this band are richer and stronger in the zonal wind component than in the meridional one. Considering variations with time, at lower altitudes the oscillation displays a clear seasonal feature with stronger contours in winter-centered months, and with a tendency toward peaks during the late fall to early spring epoch. This character is very similar to that of the large-scale westward traveling planetary waves with phase speeds smaller than the background /ow, such as the 16-day wave of zonal wave number one which can only propagate in eastward /ow, i.e. in the
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Fig. 1. (a) The 1995=1996 amplitude spectral contours of the zonal and meridional winds at four altitudes (76, 82, 88, and 94 km) at TromsH (19◦ E, 70◦ N). The individual spectra are calculated by “swept harmonic 9tting” within a 64-day window shifted by 5 days; (b) The same as (a) but for Saskatoon (107◦ W, 52◦ N); (c) The same as (a) but for Yamagawa (130◦ E, 31◦ N); (d) The same as (a) but for one altitude at Collm (15◦ E, 52◦ N).
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Fig. 1. (continued).
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Fig. 1. (continued).
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Fig. 1. (continued).
“winter” season (Luo et al., 2000a). Also from Fig. 1 we note that strong 20 – 40 d oscillations frequently occur when the 16-day wave activity is strong. Considering the latitudinal diEerences, as for the four sites involved in this paper (31–70◦ N), it seems that at middle to low latitudes the amplitude becomes the largest. An extension of the 16-day wave study to include radars from 2 to 70◦ N is processing (Luo et al., 2000b), and those preliminary results show no evidence that the long-period (20 – 40 day) oscillation is larger at tropical sites than at Yamagawa. This is consistent with normal mode structure, and why the ‘extra long period oscillation’ initially looks like a normal mode wave. As mentioned before, the period of this oscillation varies with time. Besides the behavior at 76 km, it can also be seen that at 82 km, at all three sites, a strong zonal long-period oscillation begins in January 1996 with a period at about 35 – 40 days, then decreases with time: TromsH to 25 days in March, Saskatoon to as low as 20 days in March=April, and Yamagawa to 28 day in March=April. This may be one of the global characteristics of this kind of oscillation. We now present a detailed analysis for the interval from January to March of 1996 when the ‘extra long period oscillation’ is most conspicuous at all sites. Except for Collm, heights within the 76 –85 km layer are used, where the amplitudes are largest at all sites. The oscillations of winds as well as solar parameters are shown in Fig. 2. In Fig. 2a and b the daily zonal and meridional winds at four sites are displayed as time series on the left panels. The dotted lines are the FFT-based band-pass (∼23–32 day) 9ltered results consistent with the spectral peaks. For winds the 9ltered amplitudes are displayed twice as large as their actual values for easier inspection. Corresponding to these time series, on the right panels are their LS spectra with 95% con9dence level marked. Please note that the amplitude scales for each plot are diEerent. The data have only a few gaps so that should not aEect the determination
of the spectral features at longer periods. Meanwhile, the daily solar wind, Kp-index, solar 10.7 cm /ux and UV radiation are also shown in the same way in Fig. 2c. This is to explore any possible relationships between solar activity and the ‘extra long period oscillations’ in the winds. As expected, the UV and F10.7 sequences and spectra are quite similar, and in this case have almost a pure 25 day oscillation throughout the sequence. The oscillations in the solar-wind and Kp-index sequences, however, seem more complicated but have dominant periods at 14 days and subordinate ones at about 30 days. The wind ‘extra long period oscillations’ are prominent (well above the con9dence levels) and quite similar at diEerent sites for this time interval. In the zonal component three cycles of oscillation are clearly displayed and are almost in phase between sites. Among them the spectra for Saskatoon and Yamagawa have peak amplitudes at 30 days, and for TromsH at 25 days. The amplitude at Collm is smaller, probably because the wind is observed at higher altitude (∼95 km), but it also has a peak at 27 days. In the meridional component there is a manifested 30-day peak only at Saskatoon; at other sites the 25 –30 day oscillations, if there are any, are merged with unknown longer oscillations. But the band-pass outputs still indicate a tendency for in-phase variations between sites. Furthermore, the in-phase feature also applies to the comparison between the meridonal and zonal components. When comparing oscillations of the wind and solar parameters one should ask if the wind oscillations are correlated with the solar short-term variations. From Fig. 2 the phases of band-pass 9ltered outputs can be seen over almost 3 cycles and comparisons made. The wind oscillations are apparently nearly out of phase with the UV variations. The more quantitative correlations are shown in Fig. 3, where cross-spectral analysis has been used. The solid curves are the amplitude product and the cross symbols denote the phase diEerences. The largest response peaks between 25
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and 29 day period can be seen at all sites. Caution is needed in interpretation of the amplitude of cross-spectra in that the dominant spectral peak (here 25 days in the UV compared to 25 –30 days for the winds) can lead to a slight shift of the dominant cross-spectral peaks in the plots. The phase diEerences are fairly stable and /at around these peaks and this also indicate good correlation at these periods. Furthermore, the phase diEerences are all nearly 270◦ which means the wind oscillations at diEerent locations are in phase. In other words the oscillation is more like a symmetrically forced or standing oscillation instead of a free travelling one. When considering the correlation between winds and Kp-index (not shown) the cross-spectra do not show a convincing phase relationship.
So far we have considered the 76 –85 km altitude range for each site, but it is interesting to see the situations at other altitudes within the MF radar’s detecting range. The height pro9les, both of phase and amplitude, of the ‘extra long period oscillation’ are shown in Fig. 4. It presents the monthly wave from January to April of 1996 observed at three MF radar sites. For each month a 64-day long data set centered at the middle of that month was used in the spectral analysis, and the quasi 27-day (23–32 day) component is extracted. This band was chosen because it includes the range of spectral peaks shown in Fig. 2. The left-half of each diagram denotes the phase (all calibrated to January 1st) at each altitude calculated from the Fourier transform.
Fig. 2. (a) On the left are the time series of the daily mean zonal winds (m/s) at four sites from day 1 to 80 of 1996. The dotted lines are the band-pass (22–33 day) 9ltered outputs (values are doubled in the plots). On the right are their corresponding amplitude spectra by the Lomb–Scargle method with 95% con9dence level marked. (b) The same as (a) but for meridional winds. (c) The same as (a) but for the daily solar wind velocity, Kp-index, 10.7 cm /ux and UV radiation (MgII index).
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Fig. 2. (continued).
The right-half illustrates the wave amplitude (curve with diamonds), its percentage con9dence levels (dotted line) and the mean zonal wind (thick curve). Generally speaking, the large amplitudes almost always occur at lower latitude locations and lower altitudes, and with higher signi9cance levels. For example at Saskatoon and Yamagawa the peaks are as large as 15 m=s always below 85 km. But at TromsH the amplitudes are smaller and have no prominent peak values. When comparing the extra long period amplitude with the background (mean) wind, it seems that when there is a strong eastward /ow will there be a greater likelihood of a stronger ‘extra long period oscillation’. The phase pro9les, though usually varying through the height range, are very consistent and continuous with height. The exception is during April at TromsH, which looks sparse and random, consistent with the small amplitudes there. If we only consider the phase at
lower altitudes (¡ 85 km), the oscillations at the three sites are almost in phase in the 9rst two months of 1996, and then change to diEerent values of phase in the next two months probably due to the period shifting to diEerent values at different sites. It is therefore suggested that they have a common beginning but individual evolution, and this can also be veri9ed by checking the variation of period in Fig. 1. It is additionally noticeable that at Saskatoon the phase pro9le has a rapid change or even discontinuity at about 80 –85 km in most of months, and that meanwhile a subpeak in amplitude appears at a higher altitude near 95 km. Con9dence levels are high in these two regions. This probably implies diEerences of the wave characteristics, their sources or related dynamical processes, aEecting the propagation conditions for the higher (thermosphere) and lower (mesosphere) atmosphere.
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Fig. 2. (continued).
3.2. The Climatology and Inter-annual Variation (1980 –1999) One of the signi9cant conclusions reached through analyzing the 1995=1996 data, is that the wind’s ‘extra long period oscillation’ is more likely to be a winter phenomenon and is stronger at lower altitudes. Now the question is whether this is a common feature for all other years. Fortunately we have Saskatoon data continuously since 1979, so it is appropriate and ideal for investigating the so-called ‘climatology’ of this oscillation. A simple and convincing way is a statistical average because we have such a long history of data. At each altitude the same spectral method used for Fig. 1 was used again for each individual year to obtain totally 20 annual spectral contours (1980 –1999) of the zonal wind. They were then arithmetically averaged to acquire a
mean annual spectra. Fig. 5 shows these mean spectra at eight selected altitudes (58–100 km at 6 km increment) for the zonal wind oscillations. Obviously a strong oscillation of 20 – 40 day occurs in ‘winter’ (November to March), especially for the lower altitude ranges (58–76 km), and the amplitude decreases monotonically with height from ∼10 to ∼5 m=s. The amplitude-peak’s period of oscillation also changes gently with increasing altitude from longer to shorter. For example, in the November–December epoch the period of the main peak at 58 km is around 35 days, but changes to 30 days at 76 km, and further to 25 days at 88 km. Above 82 km the winter=summer distinction is not so prominent with oscillations appearing in most months. After the averaged situation is viewed another interesting question is whether the ‘extra long period oscillation’ in the MLT region has an inter-annual variation, or a long-term
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Fig. 3. Cross-spectra between the zonal winds and the solar UV sequences which are exactly displayed in Figs. 2a and b. The solid line is for the amplitude product, cross symbol for the phase diEerence.
(such as the 11-year solar cycle) variation. We have applied the FIR 9lter with order length (window) of 64 days and band width of 20 – 40 day to the Saskatoon daily mean winds from 1980 through 1999. Three altitude ranges, 61– 67, 73–79 and 85 –91 km, in which the winds are altitudinally averaged before input to the 9lter, are displayed in Fig. 6. In order to make a comparison with the solar parameters easier, the same 9lter outputs but for the solar 10.7 cm /ux and the geomagnetic Kp-index are also given in Fig. 6, respectively. In Fig. 6 each tick on the x-axis denotes the beginning of that year. So at the lower altitudes the wind oscillation is a maximum in every mid-winter. This
annual variation, as already shown in Fig. 5, is predominant throughout the 20 years. But on the other hand, the peak-to-peak variation, or the envelope of the wave, gives an indication of a solar cycle variation; most of the larger wind oscillation bursts are seen during the solar maximum years. As a reference the solar cycle can be clearly seen in the 27-day oscillations of solar 10.7 cm /ux but not in the Kp-index. This is consistent with the notion that the extra long period wind oscillations might be associated with the solar UV radiation, at least at lower altitudes. The 9ltered wind amplitudes at higher altitudes, however, are smaller and irregular, and it is hard to see such long-term variations.
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Fig. 4. The monthly pro9les of phase and amplitude of the ‘extra long period oscillations’ at three sites. In each panel the left-half denotes the phase (all calibrated to January 1st). The right-half illustrates the wave amplitude (curve with diamonds), its signi9cance (dotted line) and the zonal mean wind (thick curve).
If anything, the largest extra long period wind oscillations above 70 km occurred during a solar cycle minima (1984 –1988). In order to have a more accurate or quantitative check, a linear regression analysis has been made for the 20 year variation of wind oscillations depending on the solar cycle. From the spectra of each individual year, which have been produced for the calculation of Fig. 5, the yearly ‘extra long period oscillations’ are extracted based on the average of each winter (November–March). Then the amplitudes of each year versus the F10.7 cm /ux are plotted in Fig. 7. The 12 altitudes from 61 to 94 km are shown in three divided diagrams; meanwhile the regression lines for each altitude are also plotted and the correlation coeQcients are denoted. According to the t-distribution test, for our cases the con9dence levels of 99, 95, 90 and 80% correspond to the coeQcients of 0.559, 0.442, 0.377 and 0.299, respectively. Therefore at lower altitudes (61– 67 km) they show weak positive correlation with con9dence levels between
90 and 95%. Above this altitude range until the mesopause there is not any hint of a signi9cant correlation. But going up again into the lower thermosphere just above the mesopause (∼82 km) negative correlation is displayed at 88–91 km with con9dence level greater than 95%. In addition, the same wind oscillations (20 – 40 days) at Collm from 1980 to 1999 versus F10.7 cm are also shown in Fig. 7, but they do not indicate a signi9cant correlation. Given this possible correlation between wind and solar radiation at the solar rotation period, it is natural to check their phase relationship in this period band. We have already done this for a particular event in Fig. 3, but here all available winters will be inspected with a statistical description. A cross-correlation technique has been applied to the wind and F10.7 cm /ux of all winters (November–March, 1979 – 1999), and the results are shown in Fig. 8. Cross-correlation analysis provides a set of time-lag-dependent correlation coeQcients of two signals. If the two time series have the same
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Fig. 5. The Saskatoon (1980 –1999) averaged annual spectra of the zonal daily mean wind from 58 to 100 km. The same spectral technique as in Fig. 1 is employed.
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Fig. 6. From the upper to the bottom are the FIR band-pass (20 – 40 day) 9lter outputs of the zonal daily mean winds (m/s) at Saskatoon at 85 –91 km, 73–79 km and 61– 67 km layers, and the daily solar F10.7 cm /uxes (10−22 w=m2 =Hz) and the Kp indexes.
periodical variations but diEerent phases, their correlation coeQcients will also have that periodical variation against the time lags, and the diEering lags at a particular phase of the oscillation represent their phase diEerences. Fig. 8 shows in most of years there are quasi-27-day (25 –30 day) variations with coeQcient peaks around ±0:2 (con9dence level ¿ 95%), but in a few years, such as 85=86; 86=87; 89=90 and 93=94, the correlation coeQcients are irregular and without a long-period oscillation presented. The latter situations seem to occur most often during the solar minimum years, but an exception is for the year 89=90. If we look back at Fig. 6 for this winter the amplitude of the wind’s ‘extra long period oscillation’ is abnormally small compared with those of other maximum years. Focusing on the lower altitude range (say, ¡ 82 km), if the lags (phases) of the quasi periodic cross-correlation are compared to zero lad, there is in general a negative lag of 0 to a few days. This means that the F10.7 cm variation leads the wind oscillation by a few days.
For example, in 79=80 winter the lag at the crest is almost 0 at 61 km and changes slightly to about −8 days at 79 km, meaning the oscillations are almost in phase in this range. In the next year the troughs are near −6 days up through to 82 km, which close to an out of phase situation. Again in the third year it (the crest) is at −8 days up to 73 km. It might be noted that in the above the crest and trough of cross-correlations appear alternatively close to zero lag. If one looks at other years (except those without quasi period oscillation) this point of view can often be seen, such as from 87=88–92=93 and 94=95–98=99, and therefore presents a quasibiennial oscillation (QBC). On the other hand, when the higher altitude range is involved, such variations cannot be seen. Instead the cross-correlation sequences are usually out of phase with those at lower altitudes when the latter has negative correlation (a trough near zero lag); for instance, in 83=84; 90=91; 92=93 and 95=96. In other words for these cases the correlations at zero lag at higher altitudes
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while in others the clear oscillation is restrained to the lower heights, e.g. 1987=1988. 4. Concluding remarks
Fig. 7. Scatter plots of the wind ‘extra long period oscillation’ (20 – 40 days) amplitudes averaged over each winter (November– March) from 1980 to 1999 at Saskatoon and Collm, versus the solar 10.7 cm /uxes (units of 10−22 w=m2 =Hz), and their linear regressive lines. In each diagram for the Saskatoon winds four altitude situations are displayed, from high to low, of amplitudes by circles, diamonds, triangles and squares symbols; regressive lines by solid, dashed, long dashes and dash dot lines, respectively. The correlation coeQcients are denoted on the right of each diagram.
are always positive. This kind of discrepancy between altitude ranges is also seen in Fig. 4 where the phase pro9le for 1995=1996 has a jump or discontinuity around 80 –85 km. It is interesting that in some years the cross-correlations are regularly oscillatory from 61 to 94 km, e.g. 1998=1999,
We have shown the 9rst detailed assessment of the MLT ‘extra long period oscillations’ and also compared these with solar parameters. The four stations (31◦ N–70◦ N) show generally similar behaviors and climatologies: there are winter-centered months of strongest amplitudes with responses being strongest at lower heights. In contrast to the spectral bands for the normal mode waves (5 –16 days), the periods of the extra long oscillation are in a quite broad band and often vary systematically with time (and heights). The coherence of the extra long period wind oscillations with the solar 10.7 cm /ux and the geomagnetic Kp-index quasi 27-day variations favors some physical relationship, and that the UV as likely to be dominant in the lower mesosphere. The behaviors of wind oscillations at four sites indicate a zonally stationary wave structure with strongest amplitude at mid-low latitudes. It is known that the UV forcing (mainly absorbed by ozone in the stratosphere) cannot directly aEect the wind 9eld in the mesosphere. The fact that the ‘extra long period oscillations’ at mesospheric levels are the strongest during winter (i.e. depends upon the background wind) suggests a plausible connection with some longer period planetary waves, which are only large in the winter, and which are also aEected by forcing from the troposphere=stratosphere. On the other hand, for long period planetary wave runs in a numerical planetary wave model (i.e. GSWM), there was no indication of a substantial peak near 27 days for a westward propagating zonal wave number one oscillation with the ‘winter’ (January and April) climatological wind 9elds (C. Meyer, 1999, private communication; for a description of the model see Hagan et al., 1999). Studies of the stratospheric response to solar radiation, however, give strong evidence that atmospheric dynamics play an important role in the response of the middle atmosphere to short-term (13–27 days) solar variability. This suggests that the response of the atmosphere is controlled and presumably modi9ed by stationary and transient planetary waves (Chandra, 1985; Ebel et al., 1988). Furthermore, the model calculation by Ebel et al., (1988) indicated that the 16-day wave proved to be a suitable one, of those planetary waves studied, for the generation of a seasonal variation in the magnitude of the quasi-27 day perturbations induced by weak external forcing (i.e. solar UV). We argue there may be a similar mechanism for the response of the mesospheric winds following the propagation of the wave into that region. This hypothesis is certainly consistent with the earlier 9nding (Section 3) that the seasonal variation and latitudinal structure of the extra long period “wave” is very similar to that of the 16-day wave, and that even the appearance of bursts of the oscillation is accompanied by a strong 16-day occurrence. Another possible
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Fig. 8. Cross-correlations of the Saskatoon zonal winds at 61–94 km and the solar 10.7 cm /uxes in 20 winters (November–March of 1979 –1999).
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dynamic eEect is the modulation of the 27-day periodicity by the background wind, which has annual and semiannual variations (Ebel et al., 1981). This can cause the oscillation period to depart from the 27-day to shorter or longer periods. A three-dimensional model used to explore the dynamical response of the atmosphere to possible forcing by solar activity eEects indicated that nonlinear dynamical responses are so important that the oscillation periods are diEerent from that of the most prominent periodicities of the solar activity signal (i.e. 27 day) (Dameris et al., 1986). For example, a nonlinear interaction between the 27-day and semiannual (∼180 day)=annual (∼360 day) variations would produce a 23–32 and 25 –29 day oscillations. This may explain the broad band features (25 –29 day) we have seen in the observations (e.g. Fig. 2), especially in the lower altitude layer where the annual variation of the background /ow is strong. This rather broad band reaction was also observed in the stratospheric temperature=ozone mixing ratio which is in a continuous state of oscillation with period varying between 3 and 7 weeks (Chandra, 1986). The tendency for small negative time-lags (phases) between the wind and F10.7 cm oscillations on many occasions indicate a certain delay of the atmospheric reaction on the solar sources, but otherwise an in-phase tendency. A similar feature was also reported for the stratospheric temperature and ozone mixing ratio responses to the solar F10.7 cm where a 5 –10 day delay was shown (Chandra, 1985). However, the range of phase diEerences for the quasi 27-day oscillation between the wind and the UV are not easily explained; it can either show the preferential “in-phase” or an obvious “out-of-phase” (after ignoring the delay of a few days) correlation, suggesting wave re/ections and standing waves. The positive and negative correlations in the correlograms (in phase, out of phase) appear in alternative winters and give a hint of a QBO variation. A very recent study using global stratospheric data by Labitzke and van Loon (2000) showed evidences that the correlations between the winter stratospheric geopotential heights, especially at high latitudes, and the solar 11-year cycle are aEected or connected with the QBO in the winds of the lower equatorial stratosphere. They found a positive correlation in the QBO westward-wind years and negative in the eastward-wind, and related this to the fact that the stratospheric mid-winter warmings in the westward phase of QBO primarily happen at peaks in the solar cycle. As for our own case, at present there is no generally accepted explanation of this behavior. In the lower thermosphere (above ∼85 km), however, a diEerent dynamical process is implied as mentioned earlier. Possibly magnetospheric activity could aEect the wind 9eld. Hocke and Schlegel (1996) suggested that particle precipitation and electric current variations in the ionosphere at high latitudes would generate gravity waves which propagate to mid-latitudes, namely providing a source of energy=momentum transfer from high to mid-latitudes. This kind of transfer is supported by the observational results
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that the tidal and gravity wave amplitudes in the lower thermosphere (¿ 90 km) increase during the geomagnetic disturbances (Manson and Meek, 1986, 1991). Because coronal holes are most prominent during the declining phase of the solar cycle, the tendency for the amplitude of ‘extra long period oscillations’ (20 – 40 days, but often with periods near the solar rotation period ∼27 day, e.g. Fig. 8), to be in anti-correlation with solar cycles above 80 km (cf. Fig. 7), also indicate a possible transfer of energy from the sun to the wind 9eld. Earlier work for the tidal variations based on the Saskatoon MF radar data suggested a negative correlation of the semidiurnal tidal amplitudes at 79–97 km layer with solar activity. (Namboothiri et al., 1993). As for the negative correlations, these are not unique, as the radio absorption /uctuations (at 80–95 km) of 30 days period which were claimed to correspond to the variations in the mesopause winds, are largest near solar activity minimum (Pancheva et al., 1991). Using multi-station data from radars (D1 and D2 wind measurements) the zonal prevailing wind as well as the semi-diurnal tidal amplitudes were shown to be weakly negatively correlated with solar activity (Bremer et al., 1997). In this paper we have explored the ‘extra long period oscillations’ of 20 – 40 day in the MLT region using both case and statistical studies. Their behavior has demonstrated a possible link to a solar source through dynamical mechanisms which apparently involves the stratosphere, mesosphere and thermosphere coupling. Thus MLT data alone are not suQcient to obtain a complete morphology and satisfying explanation. On the theoretical side there is a need to more carefully assess the possibility of normal-mode coupling with other planetary waves, and the possibility of quasi 27-day wave oscillations in some sophisticated models, e.g. the GSWM and TIME-GCM (thermosphere ionosphere mesosphere electrodynamics general circulation model). Advantageously, the second model involves the processes of dynamics, chemistry and energetics, with inputs including the solar /ux, auroral particles, high-latitude electric 9elds and upward propagating tides and gravity wave /uxes. We expect that models such as these can shed light on the generation and evolution of the ‘extra long period oscillations’ in middle and upper atmosphere. Returning to observational aspects, the analyses used here can also be repeated and improved using new data from the PSMOS (planetary scale mesopause observing system) campaigns (1998–2002). We will leave these to future studies. Acknowledgements We are grateful to the SUSIM experiment team at Naval Research Laboratory for their free supply of the solar UV data, and the NGDC for the well-formatted Kp-index in their database. We acknowledge Dr. K.W. Ogilvie (NASA GSFC) and CDAWeb for the solar wind data from the SWE on WIND available. The authors also thank Dierk
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KMurschner, Collm, for his work in maintaining the Collm measurements. The Saskatoon MF radar is supported by the Natural Sciences and Engineering Research Council of Canada, and the Institute of Space and Atmospheric Studies through the University of Saskatchewan. References Bremer, J., Schminder, R., Greisiger, K.M., HoEmann, P., KMurschner, D., Singer, W., 1997. Solar cycle dependence and long-term trends in the wind 9eld of the mesosphere=lower thermosphere. Journal of Atmospheric and Solar-Terrestrial Physics 59, 497–509. Cevolani, G., 1991. Strato-meso-thermospheric coupling at midlatitudes in the course of mid-winter strat-warmings during Dyana. Geophysical Research Letters 18, 1987–1990. Chandra, S., 1985. Solar-induced oscillations in the stratosphere: a myth or reality? Journal of Geophysical Research 90, 2331–2339. Chandra, S., 1986. The solar and dynamically induced oscillations in the stratosphere. Journal of Geophysical Research 91, 2719–2734. Dameris, M., Ebel, A., Jakobs, H.J., 1986. Three-dimensional simulation of quasiperiodic perturbations attributed to solar activity eEects in the middle atmosphere. Annales de Geophysicae A4, 287–296. Ebel, A., Schwister, B., Labitzke, K., 1981. Planetary waves and solar activity in the stratosphere between 50 and 10 mbar. Journal of Geophysical Research 86, 9729–9738. Ebel, A., Dameris, M., Hass, H., Manson, A.H., Meek, C.E., Petzoldt, K., 1986. Vertical change of the response to solar activity oscillations with periods around 13 and 27 days in the middle atmosphere. Annales de Geophysicae A4, 271–280. Ebel, A., Dameris, M., Jakobs, H.J., 1988. Modeling of the dynamical response of the middle atmosphere to weak external forcing: in/uence of the stationary and transient waves. Annales de Geophysicae 6, 501–512. Floyd, L.E., Reiser, P.A., Crane, P.C., Herring, L.C., Prinz, D.K., Brueckner, G.E., 1998. Solar cycle 22 UV spectral irradiance variability: current measurements by SUSIM UARS. Solar Physics 177, 79–87. Hagan, M.E., Burrage, M.D., Forbes, J.M., Hackney, J., Randel, W.J., Zhang, X., 1999. GSWM-98: results for migrating solar tides. Journal of Geophysical Research 104, 6813–6828. Hocke, K., Schlegel, K., 1996. A review of atmospheric gravity waves and traveling ionospheric disturbances: 1982–1995. Annales de Geophysicae 14, 917–940. Igarashi, K., Nishimuta, I., Murayama, Y., Tsuda, T., Nakamura, T., Tsutsumi, M., 1996. Comparison of wind measurements between
Yamagawa MF radar and the MU radar. Geophysical Research Letters 23, 3341–3344. Jacobi, Ch., Schminder, R., KMurschner, D., 1998. Planetary wave activity obtained from long-period (2–18 days) variations of mesopause region winds over Central Europe (52◦ N, 15◦ E). Journal of Atmospheric and Solar-Terrestrial Physics 60, 81–93. Labitzke, K., van Loon, H., 2000. The QBO eEect on the solar signal in the global stratosphere in the winter of the Northern Hemisphere. Journal of Atmospheric and Solar-Terrestrial Physics 62, 621–628. Luo, Y., Manson, A.H., Meek, C.E., Meyer, C.K., Forbes, J.M., 2000a. The quasi 16-day oscillations in the mesosphere and lower thermosphere at saskatoon (52◦ N, 107◦ W), 1980 –1996. Journal of Geophysical Research 105, 2125–2138. Luo, Y., Manson, A., Meek, C., MacDougall, J., Hocking, W., Hall, C., Fritts, D., Vincent, R., Fraser, G., 2000b. The planetary wave in the mesosphere and lower thermosphere by multi MF radar observations from the Arctic to Equator. Presented at 2000 Western Paci9c Geophysics Meeting, American Geophysical Union, Tokyo, Japan, 27–30 June 2000. Manson, A.H., Gregory, J.B., Meek, C.E., 1981. Atmospheric waves (10 min-30 days) in the mesosphere and thermosphere at Saskatoon (52◦ N, 107◦ W) October 1978–September 1979. Planetary Space Science 29, 615–625. Manson, A.H., Meek, C.E., 1986. Comparisons between neutral winds near 100 km at Saskatoon (52◦ N, 107◦ W, L = 4:4) and variations in the geomagnetic 9eld (1979 –1983). Annales de Geophysicae 4, 281–286. Manson, A.H., Meek, C.E., 1991. The eEects of geomagnetic disturbances and atmospheric tides upon middle atmosphere winds and MF radar scatter at Saskatoon (52◦ N, 107◦ W, 61◦ Geomagnetic). Journal of Geophysical Research 96, 915–926. Pancheva, D., Schminder, R., Lastovicka, J., 1991. 27-day /uctuations in the ionospheric D-region. Journal of Atmospheric and Terrestrial Physics 53, 1145–1150. Namboothiri, S.P., Manson, A.H., Meek, C.E., 1993. Variations of mean winds and tides in the upper middle atmosphere over a solar cycle, Saskatoon, Canada, 52◦ N, 107◦ W. Journal of Atmospheric and Terrestrial Physics 55, 1325–1334. Scargle, J.D., 1982. Studies in astronomical time series analysis, II, Statistical aspects of spectral analysis of unevenly spaced data. Astrophysics Journal 263, 835–853. Schminder, R., KMurschner, D., 1994. Permanent monitoring of the upper mesosphere and lower thermosphere wind 9elds (prevailing and semidiurnal tidal components) obtained from LF D1 measurement in 1991 at the Collm Geophysical Observatory. Journal of Atmospheric and Terrestrial Physics 56, 1263–1269. Zhou, Q.H., Sulzer, M.P., Tepley, C.A., 1997. An analysis of tidal and planetary waves in the neutral winds and temperature observed at low-latitude E region heights. Journal of Geophysical Research 102, 11,491–11,505.
www.elsevier.nl/locate/jastp
Extra long period (20 – 40 day) oscillations in the mesospheric and lower thermospheric winds: observations in Canada, Europe and Japan, and considerations of possible solar in/uences Y. Luoa;∗ , A.H. Mansona , C.E. Meeka , K. Igarashib , Ch. Jacobic a Institute
of Space and Atmospheric Studies, University of Saskatchewan, Saskatoon, Canada S7N5E2 b Communications Research Laboratory, Tokyo, Japan c Institute for Meteorology, University of Leipzig, Leipzig, Germany
Received 10 January 2000; received in revised form 16 August 2000; accepted 1 September 2000
Abstract An extra long period (20 – 40 day) oscillation has been identi9ed in the mesospheric and lower thermospheric (60 –100 km) winds observed simultaneously by radars (MF, LF) at four sites from 70◦ N to 30◦ N in the northern hemisphere during the winter of 1995=1996. A long-term (1980 –1999) investigation of this oscillation at Saskatoon and Collm is also carried out to obtain climatological and statistical characteristics. Spectral analysis has shown that this oscillation is a common feature of the winter (November–March) atmosphere, having strong amplitudes throughout the mesosphere (∼10 m=s) and lower thermosphere (∼5 m=s), and being much stronger at mid-low latitudes. Although the oscillation has a climatology similar to the long period normal mode planetary waves (10 –16 day), the phases at the various sites are very similar, and not consistent with a freely propagating wave. Comparisons with geomagnetic=solar wind parameters and solar UV radiation suggest that the oscillation could be related to the short-term solar rotation period (ca. 27 days) in some way. However the range of observed wind periods is very broad and this raises questions about this interpretation. Nevertheless the inter-annual variations of this 20 – 40 day oscillation indicate a weak 11-year solar cycle correlation in the mesosphere (positive) and the lower thermosphere (negative). Also, the cross-correlation between the winds and solar radiation shows signi9cant quasi 27-day correlation and the wind lags behind the solar radiation a few days in the mesosphere. In general it is implied that the atmosphere could react c 2001 Elsevier Science Ltd. All rights reserved. to the solar activity in an indirect way due to certain dynamical mechanisms. Keywords: Mesospheric and lower thermospheric dynamics; 27-day oscillation
1. Introduction When analyzing the long-period oscillations of the wind in the mesosphere and lower thermosphere (MLT), apart from periods related to the well-known normal mode 2–16 day planetary waves, there also exist much longer oscillations with periods ranging from 20 to 40 days (Luo et al., 2000a). In fact there have been a few other reports of such ∗ Corresponding author. Tel.: 1-306-9662337; fax: 1-3069666400. E-mail address: [email protected] (Y. Luo).
long oscillations in the mesosphere. Ebel et al. (1986) investigated the response of the middle atmosphere to solar activity oscillations of quasi-27 day. They used coherence estimates relating solar radiation /ux and atmospheric oscillations from the lower thermosphere down to the middle stratosphere (30–120 km), which were inherent in winds from Saskatoon MF radar and from radiosondes, and concluded that main source of the 27-day oscillations induced by solar ultraviolet (UV) radiation may be near the stratopause. A 30-day wind oscillation of as large as 10 m=s in winter in the meteor region (80 –110 km) was also displayed when studying the strato-meso-thermosphere coupling (Cevolani,
c 2001 Elsevier Science Ltd. All rights reserved. 1364-6826/01/$ - see front matter PII: S 1 3 6 4 - 6 8 2 6 ( 0 0 ) 0 0 2 0 6 - 6
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1991). Pancheva et al. (1991) showed some /uctuations of 24 –30 day in the ionospheric absorption of radio waves at 80 –95 km altitudes and related them to variations in the neutral atmosphere at those periods. They conjectured that they might be of solar origin in some way. Unlike the normal mode (free travelling) planetary waves which have been comprehensively reported and studied, both observationally and theoretically by a great number of authors, the 20 – 40 day oscillation has not yet been clearly identi9ed because of its complexity. The oscillations have, on the one hand, periods comparable to the solar short-term variation, i.e. the solar 27-day rotation eEect, naturally implying a solar-induced oscillation. On the other hand, it will be shown that their latitudinal structure and seasonal variation often resemble that of some stationary or transient planetary waves, which are strongest at mid-high latitude during fall-spring epoch. This contradictorily suggests an excitement of, or linkage with, the planetary waves (Ebel et al., 1981; Chandra, 1985). Anyway it has been rather suggested that solar activity can modulate existing planetary waves or even generate wave-like circulation perturbations, probably via changing UV and corpuscular radiation /uxes in the middle atmosphere. Speci9cally, one likely cause of such a solar-terrestrial link, or source of forcing, is via the absorption of the solar UV radiation by ozone around the stratopause (40 –50 km). This solar forcing or generation of perturbations by solar activity seems capable of playing a signi9cant role in the entire middle atmosphere (10–100 km). For example, a number of modelling and observational studies have shown that a 27-day oscillation near the stratopause can propagate downward or aEect the lower atmospheric structure, possibly through dynamic processes such as planetary waves (Ebel et al., 1986; Dameris et al., 1986), and even penetrate into the middle atmosphere. Otherwise there have been few studies concerning its in/uence on the upper atmosphere, e.g. the mesosphere and lower thermosphere. Preliminary assessments of planetary wave responses to solar forcing at periods near 30 day, within the global scale wave model (GSWM), have produced very small amplitudes (C. Meyer, 1999, private communication). Another practicable energy and momentum transport from the sun to the earth atmosphere is through the interaction of the solar wind with the earth’s magnetosphere, i.e. the reaction of the neutral wind to particle precipitation and ionospheric current variations probably recurring with a quasi-period of 27 day at high latitudes (Hocke and Schlegel, 1996). In this paper we will make a detailed investigation of the “extra long period” wind oscillations (by that term we will be referring to the period ranging from 20 to 40 days) and of comparisons with the solar parameters. Firstly, the mesospheric winds observed by ground-based radars simultaneously at four northern hemispheric sites (from 70◦ N to 30◦ N) during the winter of 1995=1996 are displayed. After that the continuous daily winds from 1980 to 1999 at Saskatoon and Collm are used to obtain statistical charac-
teristics, climatological patterns, and long-term variations of this ‘extra long period oscillation’. Finally comparisons with solar-related parameters are made. 2. Data sets and processing methods The winds are measured by the “spaced-antenna” technique in the medium-frequency (MF), and in the low-frequency (LF) ranges. Three MF radars used here are located at TromsH (70◦ N, 19◦ E), Saskatoon (52◦ N, 107◦ W), and Yamagawa (30◦ N, 130◦ E) with operating frequencies of 2.8, 2.2, and 2.0 MHz, respectively. They use the partially re/ected signals from the D- and lower E-region of the ionosphere to derive the wind pro9les. The Saskatoon MF radar and the spaced antenna “full correlation analysis” method have been described in detail elsewhere (Manson et al., 1981). The TromsH system is virtually identical to that at Saskatoon. For these two radars the winds are sampled every 5 min (post-integration time) with height samples at 3 km intervals from about 60 up to 110 km (virtual heights, which assume the radio wave propagates as it does in a vacuum). It is generally realized that the virtual height is equal to the real height up to ca. 100=95 km for the winter=summer season. Above these altitudes a certain amount of group retardation should be considered. Uniquely, the Saskatoon MF radar has been operated continuously since 1979, and the data can therefore be used for long-term (e.g. solar cycle) analysis. At Yamagawa the winds are detected with samples at intervals of 2 minutes and 2 km (Igarashi et al., 1996). In this paper the LF measurements carried out at Collm (52◦ N, 15◦ E) are also used. The Collm radar uses the re/ected ‘sky wave’ of commercial radio transmissions at 177, 225 and 270 kHz from the lower boundary of the E-region of the ionosphere, and supplies the night-time and twilight wind around 95(±5) km (Schminder and KMurschner, 1994). We are mainly concerned with two aspects of solar activity in this paper. First is the solar ultraviolet radiation represented by the Mg II index. In this paper the Mg II index (data version V19r2) is computed from the daily mid-resolution spectrum in the vicinity of 280 nm measured by the solar ultraviolet spectral irradiance monitor (SUSIM) on the upper atmospheric research satellite (UARS) (Floyd et al., 1998). As for the solar cycle (∼11 yr) variation, the solar 10.7 cm /ux can be used as the proxy indicator of the slowly varying component of the solar UV /ux in the 160 – 400 nm region. Another useful solar parameter is the solar wind. In this paper the hourly data from the solar wind experiment (SWE) on the WIND spacecraft are used. Also, there is interest in the eEect of magnetospheric particles upon the earth upper atmosphere and dynamo region, so for the purpose of assessing the short- (∼27 days) and long-term (solar cycle) variations, the Kp-index is a good representative. All wind analyses in this paper are based on daily mean data. For the MF radar the hourly mean wind is 9rstly
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obtained from the 5 (or 2)-min wind and is only considered valid when there are at least 2 (or 4) values within an hour. Secondly, the daily mean wind is derived from these hourly data and requires at least 3 values per day. Actually the real situations are far better than the above criteria, usually with the number of hourly values greater than 12 (and approaching 24 by 80 km), which can for the most part reduce the tidal=semi-tidal contamination. For the LF radar only night and twilight measurements can be derived due to the strong absorption of the sky wave during the day. Therefore a multiple regression analysis is used to handle these inhomogenously distributed data and to determine estimates of the daily prevailing winds (Jacobi et al., 1998). The data gaps in the daily-mean data set are 9lled with linear interpolation when the length of gap is less than 10 days (about 1=3 of the period of interest). Otherwise, when gaps are longer than 10 days but less than 1=3 of the length of the time window for spectral analysis (64 day long in this paper except in Fig. 2), they are 9lled with random values having the same means and standard deviations as the rest of the available data set. Lastly if gaps are longer than the above-mentioned 9t-length, analysis does not proceed. All data are detrended with polynomial 9tting of order 2 before any spectral analysis is applied. For detrending we have also tried polynomials of order 1 (linear) and 3. The order 1 process still leaves some variations of long-term comparable to the window width (∼50 days). On the other hand, the order 3 process suppresses a great deal of the components longer than about the 2=3 of the length of the window (∼40 days) and alters the position of spectral peaks within the range of 20 – 40 days. For the purpose of this paper, order 2 will be the compromise. The data are also modulated by a Hanning window before spectral analysis to reduce the eEect of spectral leakage. In the time domain, to get a 9lter of a band-pass nature, the simple and convenient method of the inverse-transform of appropriate components from the Fourier analysis is used for a short-term sequences. For long-term sequences an FIR (9nite impulse response) 9lter is operated. For spectral purposes a couple of techniques were employed. The 9rst is harmonic 9tting with the period sweeping through the range we are interested in. Also the Lomb–Scargle (LS) method (Scargle, 1982) and Fourier transform are used to get estimates of the spectral amplitudes and con9dence levels as well as the phase information. Although the LS method is suitable for unevenly time spaced data or data with gaps (in fact only for short gaps, to be exact, as we state next), we have tried to 9ll the gaps as possible as already described. Scargle’s de9nition of the time-translation invariance of the periodogram makes it exactly equivalent to a Least Square Fitting of a sine wave to data (Scargle, 1982). The nature of any harmonic 9tting is such that whether it is intentionally or not, one is making assumptions for the missing interval. By completely ignoring the missing interval, we are actually making the assumption that whatever happens in this interval is acceptable. When the missing interval becomes
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longer the spectral analysis may produce results appropriate to an enormous false burst within the gap interval. Therefore adding any a priori knowledge to the missing data is adding constraints and thus will potentially enable us to obtain more realistic results. Zhou et al. (1997) provided a detailed test for this problem. This was also tested by ourselves by using real data, and arti9cially creating gaps. Finally a cross-spectral method based on the Fourier transform, and a cross-correlation analysis, are applied to check the spectral relationship and phase diEerence between the winds and solar parameters. 3. Results and analysis 3.1. Winter of 1995=1996 The spectra of the detrended daily mean wind, both zonal and meridional components, observed at TromsH, Saskatoon, Yamagawa and Collm during the year of 1995=1996 (October–April) are shown in Fig. 1a,b,c and d, respectively. We have an extended set of data for Saskatoon and Collm, but data only starting from November 1995 for TromsH and Yamagawa. For the MF radar sites, data from four selected altitudes (76, 82, 88 and 94 km) are displayed, but for the LF radar there is only one layer (∼95 km) available. The spectral amplitudes in the contour plots are calculated by harmonic 9ttings within a set of time-windows of 64 days length, whose center is chosen as the corresponding time for the plots. This window is shifted by 5 days each time to get a “continuous” distribution of the spectra with time. With this window the frequency resolution is 0.016 cycle-per-day; converting to periods, they are approximately 4.8–5.2 day at 5 day, 9.3–10.8 d at 10 d, 13.4 –17.0 d at 15 d, and 24.3– 39.2 d at 30 d. The well-known planetary wave periods can be clearly seen as bands at most altitudes and sites; such as the 6-, 12-, 17-day oscillation at TromsH, the 5-, 8-, 10- and 16-day at Saskatoon and the 6-, 10-, 16-day at Yamagawa. Besides the planetary waves, longer period oscillations with stronger amplitudes also appear at all altitudes and sites. They fall into the period range of 20 – 40 day, and the spectral peaks generally change in period a little with time, e.g., conspicuously, the zonal winds at TromsH, Saskatoon and Yamagawa at 76 km change periods from 30 to 25 day from February through March. The amplitude is as strong as 10 m=s or even more at lower altitudes (76 –82 km), but usually decreases to ∼5 m=s at greater heights (∼94 km). In general, oscillations in this band are richer and stronger in the zonal wind component than in the meridional one. Considering variations with time, at lower altitudes the oscillation displays a clear seasonal feature with stronger contours in winter-centered months, and with a tendency toward peaks during the late fall to early spring epoch. This character is very similar to that of the large-scale westward traveling planetary waves with phase speeds smaller than the background /ow, such as the 16-day wave of zonal wave number one which can only propagate in eastward /ow, i.e. in the
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Fig. 1. (a) The 1995=1996 amplitude spectral contours of the zonal and meridional winds at four altitudes (76, 82, 88, and 94 km) at TromsH (19◦ E, 70◦ N). The individual spectra are calculated by “swept harmonic 9tting” within a 64-day window shifted by 5 days; (b) The same as (a) but for Saskatoon (107◦ W, 52◦ N); (c) The same as (a) but for Yamagawa (130◦ E, 31◦ N); (d) The same as (a) but for one altitude at Collm (15◦ E, 52◦ N).
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Fig. 1. (continued).
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Fig. 1. (continued).
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Fig. 1. (continued).
“winter” season (Luo et al., 2000a). Also from Fig. 1 we note that strong 20 – 40 d oscillations frequently occur when the 16-day wave activity is strong. Considering the latitudinal diEerences, as for the four sites involved in this paper (31–70◦ N), it seems that at middle to low latitudes the amplitude becomes the largest. An extension of the 16-day wave study to include radars from 2 to 70◦ N is processing (Luo et al., 2000b), and those preliminary results show no evidence that the long-period (20 – 40 day) oscillation is larger at tropical sites than at Yamagawa. This is consistent with normal mode structure, and why the ‘extra long period oscillation’ initially looks like a normal mode wave. As mentioned before, the period of this oscillation varies with time. Besides the behavior at 76 km, it can also be seen that at 82 km, at all three sites, a strong zonal long-period oscillation begins in January 1996 with a period at about 35 – 40 days, then decreases with time: TromsH to 25 days in March, Saskatoon to as low as 20 days in March=April, and Yamagawa to 28 day in March=April. This may be one of the global characteristics of this kind of oscillation. We now present a detailed analysis for the interval from January to March of 1996 when the ‘extra long period oscillation’ is most conspicuous at all sites. Except for Collm, heights within the 76 –85 km layer are used, where the amplitudes are largest at all sites. The oscillations of winds as well as solar parameters are shown in Fig. 2. In Fig. 2a and b the daily zonal and meridional winds at four sites are displayed as time series on the left panels. The dotted lines are the FFT-based band-pass (∼23–32 day) 9ltered results consistent with the spectral peaks. For winds the 9ltered amplitudes are displayed twice as large as their actual values for easier inspection. Corresponding to these time series, on the right panels are their LS spectra with 95% con9dence level marked. Please note that the amplitude scales for each plot are diEerent. The data have only a few gaps so that should not aEect the determination
of the spectral features at longer periods. Meanwhile, the daily solar wind, Kp-index, solar 10.7 cm /ux and UV radiation are also shown in the same way in Fig. 2c. This is to explore any possible relationships between solar activity and the ‘extra long period oscillations’ in the winds. As expected, the UV and F10.7 sequences and spectra are quite similar, and in this case have almost a pure 25 day oscillation throughout the sequence. The oscillations in the solar-wind and Kp-index sequences, however, seem more complicated but have dominant periods at 14 days and subordinate ones at about 30 days. The wind ‘extra long period oscillations’ are prominent (well above the con9dence levels) and quite similar at diEerent sites for this time interval. In the zonal component three cycles of oscillation are clearly displayed and are almost in phase between sites. Among them the spectra for Saskatoon and Yamagawa have peak amplitudes at 30 days, and for TromsH at 25 days. The amplitude at Collm is smaller, probably because the wind is observed at higher altitude (∼95 km), but it also has a peak at 27 days. In the meridional component there is a manifested 30-day peak only at Saskatoon; at other sites the 25 –30 day oscillations, if there are any, are merged with unknown longer oscillations. But the band-pass outputs still indicate a tendency for in-phase variations between sites. Furthermore, the in-phase feature also applies to the comparison between the meridonal and zonal components. When comparing oscillations of the wind and solar parameters one should ask if the wind oscillations are correlated with the solar short-term variations. From Fig. 2 the phases of band-pass 9ltered outputs can be seen over almost 3 cycles and comparisons made. The wind oscillations are apparently nearly out of phase with the UV variations. The more quantitative correlations are shown in Fig. 3, where cross-spectral analysis has been used. The solid curves are the amplitude product and the cross symbols denote the phase diEerences. The largest response peaks between 25
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and 29 day period can be seen at all sites. Caution is needed in interpretation of the amplitude of cross-spectra in that the dominant spectral peak (here 25 days in the UV compared to 25 –30 days for the winds) can lead to a slight shift of the dominant cross-spectral peaks in the plots. The phase diEerences are fairly stable and /at around these peaks and this also indicate good correlation at these periods. Furthermore, the phase diEerences are all nearly 270◦ which means the wind oscillations at diEerent locations are in phase. In other words the oscillation is more like a symmetrically forced or standing oscillation instead of a free travelling one. When considering the correlation between winds and Kp-index (not shown) the cross-spectra do not show a convincing phase relationship.
So far we have considered the 76 –85 km altitude range for each site, but it is interesting to see the situations at other altitudes within the MF radar’s detecting range. The height pro9les, both of phase and amplitude, of the ‘extra long period oscillation’ are shown in Fig. 4. It presents the monthly wave from January to April of 1996 observed at three MF radar sites. For each month a 64-day long data set centered at the middle of that month was used in the spectral analysis, and the quasi 27-day (23–32 day) component is extracted. This band was chosen because it includes the range of spectral peaks shown in Fig. 2. The left-half of each diagram denotes the phase (all calibrated to January 1st) at each altitude calculated from the Fourier transform.
Fig. 2. (a) On the left are the time series of the daily mean zonal winds (m/s) at four sites from day 1 to 80 of 1996. The dotted lines are the band-pass (22–33 day) 9ltered outputs (values are doubled in the plots). On the right are their corresponding amplitude spectra by the Lomb–Scargle method with 95% con9dence level marked. (b) The same as (a) but for meridional winds. (c) The same as (a) but for the daily solar wind velocity, Kp-index, 10.7 cm /ux and UV radiation (MgII index).
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Fig. 2. (continued).
The right-half illustrates the wave amplitude (curve with diamonds), its percentage con9dence levels (dotted line) and the mean zonal wind (thick curve). Generally speaking, the large amplitudes almost always occur at lower latitude locations and lower altitudes, and with higher signi9cance levels. For example at Saskatoon and Yamagawa the peaks are as large as 15 m=s always below 85 km. But at TromsH the amplitudes are smaller and have no prominent peak values. When comparing the extra long period amplitude with the background (mean) wind, it seems that when there is a strong eastward /ow will there be a greater likelihood of a stronger ‘extra long period oscillation’. The phase pro9les, though usually varying through the height range, are very consistent and continuous with height. The exception is during April at TromsH, which looks sparse and random, consistent with the small amplitudes there. If we only consider the phase at
lower altitudes (¡ 85 km), the oscillations at the three sites are almost in phase in the 9rst two months of 1996, and then change to diEerent values of phase in the next two months probably due to the period shifting to diEerent values at different sites. It is therefore suggested that they have a common beginning but individual evolution, and this can also be veri9ed by checking the variation of period in Fig. 1. It is additionally noticeable that at Saskatoon the phase pro9le has a rapid change or even discontinuity at about 80 –85 km in most of months, and that meanwhile a subpeak in amplitude appears at a higher altitude near 95 km. Con9dence levels are high in these two regions. This probably implies diEerences of the wave characteristics, their sources or related dynamical processes, aEecting the propagation conditions for the higher (thermosphere) and lower (mesosphere) atmosphere.
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Fig. 2. (continued).
3.2. The Climatology and Inter-annual Variation (1980 –1999) One of the signi9cant conclusions reached through analyzing the 1995=1996 data, is that the wind’s ‘extra long period oscillation’ is more likely to be a winter phenomenon and is stronger at lower altitudes. Now the question is whether this is a common feature for all other years. Fortunately we have Saskatoon data continuously since 1979, so it is appropriate and ideal for investigating the so-called ‘climatology’ of this oscillation. A simple and convincing way is a statistical average because we have such a long history of data. At each altitude the same spectral method used for Fig. 1 was used again for each individual year to obtain totally 20 annual spectral contours (1980 –1999) of the zonal wind. They were then arithmetically averaged to acquire a
mean annual spectra. Fig. 5 shows these mean spectra at eight selected altitudes (58–100 km at 6 km increment) for the zonal wind oscillations. Obviously a strong oscillation of 20 – 40 day occurs in ‘winter’ (November to March), especially for the lower altitude ranges (58–76 km), and the amplitude decreases monotonically with height from ∼10 to ∼5 m=s. The amplitude-peak’s period of oscillation also changes gently with increasing altitude from longer to shorter. For example, in the November–December epoch the period of the main peak at 58 km is around 35 days, but changes to 30 days at 76 km, and further to 25 days at 88 km. Above 82 km the winter=summer distinction is not so prominent with oscillations appearing in most months. After the averaged situation is viewed another interesting question is whether the ‘extra long period oscillation’ in the MLT region has an inter-annual variation, or a long-term
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Fig. 3. Cross-spectra between the zonal winds and the solar UV sequences which are exactly displayed in Figs. 2a and b. The solid line is for the amplitude product, cross symbol for the phase diEerence.
(such as the 11-year solar cycle) variation. We have applied the FIR 9lter with order length (window) of 64 days and band width of 20 – 40 day to the Saskatoon daily mean winds from 1980 through 1999. Three altitude ranges, 61– 67, 73–79 and 85 –91 km, in which the winds are altitudinally averaged before input to the 9lter, are displayed in Fig. 6. In order to make a comparison with the solar parameters easier, the same 9lter outputs but for the solar 10.7 cm /ux and the geomagnetic Kp-index are also given in Fig. 6, respectively. In Fig. 6 each tick on the x-axis denotes the beginning of that year. So at the lower altitudes the wind oscillation is a maximum in every mid-winter. This
annual variation, as already shown in Fig. 5, is predominant throughout the 20 years. But on the other hand, the peak-to-peak variation, or the envelope of the wave, gives an indication of a solar cycle variation; most of the larger wind oscillation bursts are seen during the solar maximum years. As a reference the solar cycle can be clearly seen in the 27-day oscillations of solar 10.7 cm /ux but not in the Kp-index. This is consistent with the notion that the extra long period wind oscillations might be associated with the solar UV radiation, at least at lower altitudes. The 9ltered wind amplitudes at higher altitudes, however, are smaller and irregular, and it is hard to see such long-term variations.
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Fig. 4. The monthly pro9les of phase and amplitude of the ‘extra long period oscillations’ at three sites. In each panel the left-half denotes the phase (all calibrated to January 1st). The right-half illustrates the wave amplitude (curve with diamonds), its signi9cance (dotted line) and the zonal mean wind (thick curve).
If anything, the largest extra long period wind oscillations above 70 km occurred during a solar cycle minima (1984 –1988). In order to have a more accurate or quantitative check, a linear regression analysis has been made for the 20 year variation of wind oscillations depending on the solar cycle. From the spectra of each individual year, which have been produced for the calculation of Fig. 5, the yearly ‘extra long period oscillations’ are extracted based on the average of each winter (November–March). Then the amplitudes of each year versus the F10.7 cm /ux are plotted in Fig. 7. The 12 altitudes from 61 to 94 km are shown in three divided diagrams; meanwhile the regression lines for each altitude are also plotted and the correlation coeQcients are denoted. According to the t-distribution test, for our cases the con9dence levels of 99, 95, 90 and 80% correspond to the coeQcients of 0.559, 0.442, 0.377 and 0.299, respectively. Therefore at lower altitudes (61– 67 km) they show weak positive correlation with con9dence levels between
90 and 95%. Above this altitude range until the mesopause there is not any hint of a signi9cant correlation. But going up again into the lower thermosphere just above the mesopause (∼82 km) negative correlation is displayed at 88–91 km with con9dence level greater than 95%. In addition, the same wind oscillations (20 – 40 days) at Collm from 1980 to 1999 versus F10.7 cm are also shown in Fig. 7, but they do not indicate a signi9cant correlation. Given this possible correlation between wind and solar radiation at the solar rotation period, it is natural to check their phase relationship in this period band. We have already done this for a particular event in Fig. 3, but here all available winters will be inspected with a statistical description. A cross-correlation technique has been applied to the wind and F10.7 cm /ux of all winters (November–March, 1979 – 1999), and the results are shown in Fig. 8. Cross-correlation analysis provides a set of time-lag-dependent correlation coeQcients of two signals. If the two time series have the same
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Fig. 5. The Saskatoon (1980 –1999) averaged annual spectra of the zonal daily mean wind from 58 to 100 km. The same spectral technique as in Fig. 1 is employed.
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Fig. 6. From the upper to the bottom are the FIR band-pass (20 – 40 day) 9lter outputs of the zonal daily mean winds (m/s) at Saskatoon at 85 –91 km, 73–79 km and 61– 67 km layers, and the daily solar F10.7 cm /uxes (10−22 w=m2 =Hz) and the Kp indexes.
periodical variations but diEerent phases, their correlation coeQcients will also have that periodical variation against the time lags, and the diEering lags at a particular phase of the oscillation represent their phase diEerences. Fig. 8 shows in most of years there are quasi-27-day (25 –30 day) variations with coeQcient peaks around ±0:2 (con9dence level ¿ 95%), but in a few years, such as 85=86; 86=87; 89=90 and 93=94, the correlation coeQcients are irregular and without a long-period oscillation presented. The latter situations seem to occur most often during the solar minimum years, but an exception is for the year 89=90. If we look back at Fig. 6 for this winter the amplitude of the wind’s ‘extra long period oscillation’ is abnormally small compared with those of other maximum years. Focusing on the lower altitude range (say, ¡ 82 km), if the lags (phases) of the quasi periodic cross-correlation are compared to zero lad, there is in general a negative lag of 0 to a few days. This means that the F10.7 cm variation leads the wind oscillation by a few days.
For example, in 79=80 winter the lag at the crest is almost 0 at 61 km and changes slightly to about −8 days at 79 km, meaning the oscillations are almost in phase in this range. In the next year the troughs are near −6 days up through to 82 km, which close to an out of phase situation. Again in the third year it (the crest) is at −8 days up to 73 km. It might be noted that in the above the crest and trough of cross-correlations appear alternatively close to zero lag. If one looks at other years (except those without quasi period oscillation) this point of view can often be seen, such as from 87=88–92=93 and 94=95–98=99, and therefore presents a quasibiennial oscillation (QBC). On the other hand, when the higher altitude range is involved, such variations cannot be seen. Instead the cross-correlation sequences are usually out of phase with those at lower altitudes when the latter has negative correlation (a trough near zero lag); for instance, in 83=84; 90=91; 92=93 and 95=96. In other words for these cases the correlations at zero lag at higher altitudes
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while in others the clear oscillation is restrained to the lower heights, e.g. 1987=1988. 4. Concluding remarks
Fig. 7. Scatter plots of the wind ‘extra long period oscillation’ (20 – 40 days) amplitudes averaged over each winter (November– March) from 1980 to 1999 at Saskatoon and Collm, versus the solar 10.7 cm /uxes (units of 10−22 w=m2 =Hz), and their linear regressive lines. In each diagram for the Saskatoon winds four altitude situations are displayed, from high to low, of amplitudes by circles, diamonds, triangles and squares symbols; regressive lines by solid, dashed, long dashes and dash dot lines, respectively. The correlation coeQcients are denoted on the right of each diagram.
are always positive. This kind of discrepancy between altitude ranges is also seen in Fig. 4 where the phase pro9le for 1995=1996 has a jump or discontinuity around 80 –85 km. It is interesting that in some years the cross-correlations are regularly oscillatory from 61 to 94 km, e.g. 1998=1999,
We have shown the 9rst detailed assessment of the MLT ‘extra long period oscillations’ and also compared these with solar parameters. The four stations (31◦ N–70◦ N) show generally similar behaviors and climatologies: there are winter-centered months of strongest amplitudes with responses being strongest at lower heights. In contrast to the spectral bands for the normal mode waves (5 –16 days), the periods of the extra long oscillation are in a quite broad band and often vary systematically with time (and heights). The coherence of the extra long period wind oscillations with the solar 10.7 cm /ux and the geomagnetic Kp-index quasi 27-day variations favors some physical relationship, and that the UV as likely to be dominant in the lower mesosphere. The behaviors of wind oscillations at four sites indicate a zonally stationary wave structure with strongest amplitude at mid-low latitudes. It is known that the UV forcing (mainly absorbed by ozone in the stratosphere) cannot directly aEect the wind 9eld in the mesosphere. The fact that the ‘extra long period oscillations’ at mesospheric levels are the strongest during winter (i.e. depends upon the background wind) suggests a plausible connection with some longer period planetary waves, which are only large in the winter, and which are also aEected by forcing from the troposphere=stratosphere. On the other hand, for long period planetary wave runs in a numerical planetary wave model (i.e. GSWM), there was no indication of a substantial peak near 27 days for a westward propagating zonal wave number one oscillation with the ‘winter’ (January and April) climatological wind 9elds (C. Meyer, 1999, private communication; for a description of the model see Hagan et al., 1999). Studies of the stratospheric response to solar radiation, however, give strong evidence that atmospheric dynamics play an important role in the response of the middle atmosphere to short-term (13–27 days) solar variability. This suggests that the response of the atmosphere is controlled and presumably modi9ed by stationary and transient planetary waves (Chandra, 1985; Ebel et al., 1988). Furthermore, the model calculation by Ebel et al., (1988) indicated that the 16-day wave proved to be a suitable one, of those planetary waves studied, for the generation of a seasonal variation in the magnitude of the quasi-27 day perturbations induced by weak external forcing (i.e. solar UV). We argue there may be a similar mechanism for the response of the mesospheric winds following the propagation of the wave into that region. This hypothesis is certainly consistent with the earlier 9nding (Section 3) that the seasonal variation and latitudinal structure of the extra long period “wave” is very similar to that of the 16-day wave, and that even the appearance of bursts of the oscillation is accompanied by a strong 16-day occurrence. Another possible
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Fig. 8. Cross-correlations of the Saskatoon zonal winds at 61–94 km and the solar 10.7 cm /uxes in 20 winters (November–March of 1979 –1999).
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dynamic eEect is the modulation of the 27-day periodicity by the background wind, which has annual and semiannual variations (Ebel et al., 1981). This can cause the oscillation period to depart from the 27-day to shorter or longer periods. A three-dimensional model used to explore the dynamical response of the atmosphere to possible forcing by solar activity eEects indicated that nonlinear dynamical responses are so important that the oscillation periods are diEerent from that of the most prominent periodicities of the solar activity signal (i.e. 27 day) (Dameris et al., 1986). For example, a nonlinear interaction between the 27-day and semiannual (∼180 day)=annual (∼360 day) variations would produce a 23–32 and 25 –29 day oscillations. This may explain the broad band features (25 –29 day) we have seen in the observations (e.g. Fig. 2), especially in the lower altitude layer where the annual variation of the background /ow is strong. This rather broad band reaction was also observed in the stratospheric temperature=ozone mixing ratio which is in a continuous state of oscillation with period varying between 3 and 7 weeks (Chandra, 1986). The tendency for small negative time-lags (phases) between the wind and F10.7 cm oscillations on many occasions indicate a certain delay of the atmospheric reaction on the solar sources, but otherwise an in-phase tendency. A similar feature was also reported for the stratospheric temperature and ozone mixing ratio responses to the solar F10.7 cm where a 5 –10 day delay was shown (Chandra, 1985). However, the range of phase diEerences for the quasi 27-day oscillation between the wind and the UV are not easily explained; it can either show the preferential “in-phase” or an obvious “out-of-phase” (after ignoring the delay of a few days) correlation, suggesting wave re/ections and standing waves. The positive and negative correlations in the correlograms (in phase, out of phase) appear in alternative winters and give a hint of a QBO variation. A very recent study using global stratospheric data by Labitzke and van Loon (2000) showed evidences that the correlations between the winter stratospheric geopotential heights, especially at high latitudes, and the solar 11-year cycle are aEected or connected with the QBO in the winds of the lower equatorial stratosphere. They found a positive correlation in the QBO westward-wind years and negative in the eastward-wind, and related this to the fact that the stratospheric mid-winter warmings in the westward phase of QBO primarily happen at peaks in the solar cycle. As for our own case, at present there is no generally accepted explanation of this behavior. In the lower thermosphere (above ∼85 km), however, a diEerent dynamical process is implied as mentioned earlier. Possibly magnetospheric activity could aEect the wind 9eld. Hocke and Schlegel (1996) suggested that particle precipitation and electric current variations in the ionosphere at high latitudes would generate gravity waves which propagate to mid-latitudes, namely providing a source of energy=momentum transfer from high to mid-latitudes. This kind of transfer is supported by the observational results
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that the tidal and gravity wave amplitudes in the lower thermosphere (¿ 90 km) increase during the geomagnetic disturbances (Manson and Meek, 1986, 1991). Because coronal holes are most prominent during the declining phase of the solar cycle, the tendency for the amplitude of ‘extra long period oscillations’ (20 – 40 days, but often with periods near the solar rotation period ∼27 day, e.g. Fig. 8), to be in anti-correlation with solar cycles above 80 km (cf. Fig. 7), also indicate a possible transfer of energy from the sun to the wind 9eld. Earlier work for the tidal variations based on the Saskatoon MF radar data suggested a negative correlation of the semidiurnal tidal amplitudes at 79–97 km layer with solar activity. (Namboothiri et al., 1993). As for the negative correlations, these are not unique, as the radio absorption /uctuations (at 80–95 km) of 30 days period which were claimed to correspond to the variations in the mesopause winds, are largest near solar activity minimum (Pancheva et al., 1991). Using multi-station data from radars (D1 and D2 wind measurements) the zonal prevailing wind as well as the semi-diurnal tidal amplitudes were shown to be weakly negatively correlated with solar activity (Bremer et al., 1997). In this paper we have explored the ‘extra long period oscillations’ of 20 – 40 day in the MLT region using both case and statistical studies. Their behavior has demonstrated a possible link to a solar source through dynamical mechanisms which apparently involves the stratosphere, mesosphere and thermosphere coupling. Thus MLT data alone are not suQcient to obtain a complete morphology and satisfying explanation. On the theoretical side there is a need to more carefully assess the possibility of normal-mode coupling with other planetary waves, and the possibility of quasi 27-day wave oscillations in some sophisticated models, e.g. the GSWM and TIME-GCM (thermosphere ionosphere mesosphere electrodynamics general circulation model). Advantageously, the second model involves the processes of dynamics, chemistry and energetics, with inputs including the solar /ux, auroral particles, high-latitude electric 9elds and upward propagating tides and gravity wave /uxes. We expect that models such as these can shed light on the generation and evolution of the ‘extra long period oscillations’ in middle and upper atmosphere. Returning to observational aspects, the analyses used here can also be repeated and improved using new data from the PSMOS (planetary scale mesopause observing system) campaigns (1998–2002). We will leave these to future studies. Acknowledgements We are grateful to the SUSIM experiment team at Naval Research Laboratory for their free supply of the solar UV data, and the NGDC for the well-formatted Kp-index in their database. We acknowledge Dr. K.W. Ogilvie (NASA GSFC) and CDAWeb for the solar wind data from the SWE on WIND available. The authors also thank Dierk
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KMurschner, Collm, for his work in maintaining the Collm measurements. The Saskatoon MF radar is supported by the Natural Sciences and Engineering Research Council of Canada, and the Institute of Space and Atmospheric Studies through the University of Saskatchewan. References Bremer, J., Schminder, R., Greisiger, K.M., HoEmann, P., KMurschner, D., Singer, W., 1997. Solar cycle dependence and long-term trends in the wind 9eld of the mesosphere=lower thermosphere. Journal of Atmospheric and Solar-Terrestrial Physics 59, 497–509. Cevolani, G., 1991. Strato-meso-thermospheric coupling at midlatitudes in the course of mid-winter strat-warmings during Dyana. Geophysical Research Letters 18, 1987–1990. Chandra, S., 1985. Solar-induced oscillations in the stratosphere: a myth or reality? Journal of Geophysical Research 90, 2331–2339. Chandra, S., 1986. The solar and dynamically induced oscillations in the stratosphere. Journal of Geophysical Research 91, 2719–2734. Dameris, M., Ebel, A., Jakobs, H.J., 1986. Three-dimensional simulation of quasiperiodic perturbations attributed to solar activity eEects in the middle atmosphere. Annales de Geophysicae A4, 287–296. Ebel, A., Schwister, B., Labitzke, K., 1981. Planetary waves and solar activity in the stratosphere between 50 and 10 mbar. Journal of Geophysical Research 86, 9729–9738. Ebel, A., Dameris, M., Hass, H., Manson, A.H., Meek, C.E., Petzoldt, K., 1986. Vertical change of the response to solar activity oscillations with periods around 13 and 27 days in the middle atmosphere. Annales de Geophysicae A4, 271–280. Ebel, A., Dameris, M., Jakobs, H.J., 1988. Modeling of the dynamical response of the middle atmosphere to weak external forcing: in/uence of the stationary and transient waves. Annales de Geophysicae 6, 501–512. Floyd, L.E., Reiser, P.A., Crane, P.C., Herring, L.C., Prinz, D.K., Brueckner, G.E., 1998. Solar cycle 22 UV spectral irradiance variability: current measurements by SUSIM UARS. Solar Physics 177, 79–87. Hagan, M.E., Burrage, M.D., Forbes, J.M., Hackney, J., Randel, W.J., Zhang, X., 1999. GSWM-98: results for migrating solar tides. Journal of Geophysical Research 104, 6813–6828. Hocke, K., Schlegel, K., 1996. A review of atmospheric gravity waves and traveling ionospheric disturbances: 1982–1995. Annales de Geophysicae 14, 917–940. Igarashi, K., Nishimuta, I., Murayama, Y., Tsuda, T., Nakamura, T., Tsutsumi, M., 1996. Comparison of wind measurements between
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