spring period of 2003–2004

ARTICLE IN PRESS

Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) 2452–2464 www.elsevier.com/locate/jastp

Upper ionosphere variability over Alma-Ata and Observatorio Del Ebro using the DfoF2 data obtained during the winter/spring period of 2003–2004 G.I. Gordienkoa,, I.N. Fedulinaa, D. Altadillb,1, M.G. Shepherdc a

Institute of Ionosphere, Ministry of Education and Science, Almaty 050020, Kazakhstan Center for Atmospheric Research, University of Massachusetts Lowell, 600 Suffolk Street, 3rd Floor, Lowell, MA 01854, USA c CRESS, York University, 4700 Keele Street, Toronto, Ont., Canada M3J IP3

b

Received 25 January 2007; received in revised form 13 May 2007; accepted 23 May 2007 Available online 31 May 2007

Abstract The foF2 data obtained at Alma-Ata and Observatorio Del Ebro during the winter/spring of 2003–2004 are analyzed to compare and investigate the upper ionosphere variability at the two selected sites. The geomagnetic activity and the middle stratosphere dynamics, involving planetary wave (PW) activity, are analyzed for understanding the physical conditions and processes that can explain the observed ionospheric variability. By applying the same method of wavelet analysis to the data sets and doing a direct comparison of the results, two types of foF2 disturbances were found. The first type is 2–7-day oscillations, which appeared during periods of increased geomagnetic activity. The second type is oscillations arising from PW activity in the lower atmosphere. These consist of (1) 6–11-day oscillations arising from PW activity in lower atmospheric regions developed during the final stratosphere warming and indicating the timing of the transition from the winter to the summer circulation and (2) 9–13-day and 8–10-day oscillations mostly during the quiet level of geomagnetic activity, indicating a likely close relation with those in the geopotential height at the 1 hPa level for westward-propagating waves at 401N, which strengthened during stratosphere warming events in January 2004. The time delay of the oscillations in the DfoF2 with respect to those in the geopotential height is about 10 days and it seems that the assumed ionosphere response can occur under weakened eastward zonal wind or relatively weak westward zonal wind (Vo30 m s1). r 2007 Elsevier Ltd. All rights reserved. Keywords: Ionosphere variability; Ionosphere/atmosphere interactions via planetary waves; Planetary wave signatures in the ionosphere

1. Introduction

Corresponding author. Tel.: +327 2548074;

fax: +327 2540636. E-mail address: [email protected] (G.I. Gordienko). 1 Permanent address: Observatorio del Ebro, Universidad Ramon Llull, Carretera de I ‘Observatori 8, E43520 Roquetes, Spain. 1364-6826/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2007.05.008

Many experimental results show a significant presence of planetary wave signatures (PWS) in the mid-latitude F-region ionosphere (e.g., Altadill et al., 2004; Lastovicka, 2006; Pancheva et al., 2002; Gordienko et al., 2005; and references therein). The most pronounced quasi-periodic bands for the PWS are 2–3, 5–6, 9–10, 13–14 and 16 days, and the PWS

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occur as burst events with a typical duration of about 3–4 cycles of oscillation (Lastovicka et al., 2003). It is assumed that the source of the planetary waves (PW) in the ionosphere may arise from the PW activity in lower atmospheric regions through coupling from below (Kazimirovsky et al., 2003). It is now well accepted that the geomagnetic activity variations play an important role in the generation of the PWS in the ionosphere. Recent investigations have shown that geomagnetic activity can drive PWS in the ionospheric F-region (Xiong et al., 2006; Jarvis, 2006), supporting previous results which indicate that geomagnetic activity variations can be one of the main sources of the PWS in the ionosphere (Altadill and Apostolov, 2001, 2003). In this study, we extend our previous investigation of ionosphere variability (Gordienko et al., 2005; Aushev et al., 2006). The goal is to ascertain the features of the ionospheric effects produced in response to the geomagnetic storms and/or to the disturbances of atmospheric dynamics during winter/spring 2003–2004, which is rich in geomagnetically active periods. In addition, we examine the annual changes of the stratospheric circulation and wave activity in the geopotential height at 1 hPa and their possible coupling with these ionospheric effects. The data and method of analysis are presented in Section 2. The results of the dominant PWS events in the ionospheric variability, geomagnetic activity conditions and the middle stratosphere activity associated with the ionospheric events are presented in Sections 3–5, respectively. A discussion of the results obtained is given in Section 6. The paper ends with a summary and conclusion section. 2. Data and methods of analysis Ionosphere variability is examined employing hourly values of the critical frequency foF2 for Alma-Ata station (43.251N, 76.921E) and Observatorio Del Ebro (40.81N, 0.51E) obtained during the time interval of winter/spring 2003–2004. The foF2 value is a measure of the peak electron density in the F2-layer (N ¼ 1.24f2  1010 m3, where f is the critical frequency expressed in MHz). The AlmaAta foF2 data are retrieved from PARUS2 semi-automatic ionogram scaling using software developed at the Institute of Ionosphere. The source of the Observatorio Del Ebro data is a digital 2

A description of the digital ionosonde PARUS is available at the web site http://www.izniiran.rssi.ru.

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ionosonde model DGS 256 from the Center for Atmospheric Research of the University of Massachusetts, Lowell, USA. These ionograms are edited by an operator in order to avoid spurious data from the automatic scaling.3 The main criterion for the data to be suitable for analysis is ‘‘no-gap’’ in the ionospheric data sets. These gaps are due to the presence of spread echo traces (Fsp), complete blanketing with sporadic traces (Es), absorption in the vicinity of the critical frequency or the lack of measurements due to any non-ionospheric reasons. If the records indicate that conditions slowly vary, the missing values are replaced by interpolated values. When the gaps in the data do not exceed one-third of a day, the gaps are approximated by linear or quadratic functions within the day. Finally, when the gaps in the data exceed the time interval, they are approximated by a quadratic function within the whole time interval considered. In order to estimate the ionospheric state and describe the day-to-day ionospheric variability, we analyze the hourly values of the fluctuating component of the critical frequency (DfoF2) obtained as a relative deviation of the hourly foF2 values from their corresponding background level, namely  DfoF2 ¼

 ðfoF2  foF2med Þ foF2med

ðin % according to foF2med Þ,

ð1Þ

where the background level (foF2med) used in the study is the 13-day running median computed for each hour with an averaging interval of 13 days. Due to equipment problems the ionosphere measurements at Alma-Ata for winter/spring 2003–2004 are available only in two intervals, from 1 January to 29 February and from 16 March to 30 April 2004. Hence, the DfoF2 data are available in the intervals 7 January–23 February (48 days) and 22 March–24 April (34 days). The DfoF2 data for Ebro station are available for the entire interval from 28 November 2003 to 24 April 2004. The 1-min values of the X-component measured at the Alma-Ata geomagnetic observatory in January–April 2004 are used to characterize the local geomagnetic activity. In addition, the hourly Dst- and 3-hourly Kpindices (ftp://ftp.ngdc.noaa.gov/; http://sec.noaa. gov/) provide information on the planetary geomagnetic activity. 3

Information on the data is available online at the web site http://www.obsebre.es/php/ionosfera.php.

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Fig. 1. DfoF2 values obtained at Alma-Ata (a) and Ebro (b) stations. Shaded solid lines—hourly DfoF2 values; thick solid lines—25-h running means.

The dynamical regime in the stratospheric region during the period of interest is examined using the NCEP/NCAR4 data on re-analysis temperature and zonal wind (Kalnay et al., 1996) at 421N at l0 and 30 hPa pressure levels, STRATALERT5 Report descriptions, GDAS-CPC6 zonal mean wind time series (http://www.cpc.ncepnoaa.gov) and the UKMO7 assimilated geopotential heights at 1 hPa (Swinbank and O’Neill, 1994). All data sets have been subjected to Morlet wavelet analysis (Torrence and Compo, 1998) to investigate the changing composition of the PW field in the ionospheric data employing an approach similar to the space–time analysis of Pogoreltsev et al. (2002) (for details see Gordienko et al., 2005).

3. Ionosphere variability Fig. 1a shows the variations of the DfoF2 values obtained at Alma-Ata station in January 2003–February 2004 and March–April 2004. Fig. 1b shows 4 National Centers for Environmental Prediction/National Center for Atmospheric Research. 5 A warning system. 6 Ground Data Analysis Software-Climate Prediction Center. 7 United Kingdom Meteorological Office.

the DfoF2 values obtained at Ebro station for the time interval of 28 November 2003–24 April 2004 encompassing the events observed at Alma-Ata station allowing comparison of the ionosphere variability with the two stations using the corresponding DfoF2 data. The shaded solid lines in the plots of Fig. 1 depict variations of the hourly DfoF2 data, while the thick solid lines describe the 25-h running means, which reduces the noise in the hourly DfoF2 time series. Hereafter, the 25-h running average of the DfoF2 is called ‘‘mean DfoF2’’. The inspection of the plots of Fig. 1 shows that there are PWS or oscillation events observed in the ionosphere above both stations. Some of these events seem to be quasi-simultaneous above both stations. The wavelet transforms of the mean DfoF2 (Fig. 2) show that at Alma-Ata the time interval January–February had three dominant events and the interval 22 March–24 April had two events (marked by arrows) (Fig. 2a). These events are (1) the occurrence of a 9–13-day oscillation in the DfoF2 with amplitude maximum close to the middle of January 2004, which extends throughout almost the entire time considered; (2) the signature of a 6-day wave in the DfoF2 between 27 January and 3 February; (3) short-period oscillations in the

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2–3-day periodic band existing from about 12 January to about 18 February; (4) a very strong 2–7-day oscillation between 27 March and 11 April; and (5) a significant 11-day oscillation that extends from the end of March to the middle of April. The same type of analyses is applied to Ebro DfoF2 data and the results are presented in Fig. 2b. Considering the time intervals for which the DfoF2 data are available for both stations, we noticed four events in the Ebro DfoF2: (1) a strong 8–10-day oscillation with amplitude maximum on 5 February; (2) a strong 3–7-day oscillation (around a 6-day period) at the end of March; (3) a very strong oscillation around the 2.5-day period found around 3–5 April; and (4) a strong oscillation around a 6-day period found in the middle of April. Moreover, some short-period (2–3 days) oscillations are also evident in the plot, these being more intensive

around 22 January and between 5 and 16 February 2004. Analyzing the ionosphere variability above both stations, Alma-Ata and Ebro, for the corresponding time intervals (7 January–23 February and 22 March–24 April) following the major ionospheric effects observed in the behavior of the F-region during the winter/spring period of 2003–2004 can be summarized: (1) short-period oscillations (2–3 days) in the DfoF2 data are observed above both stations in January with amplitude maximum around 22 January, and in February with amplitude maximum around mid-February; (2) very strong 3–7-day oscillations in the DfoF2 data of Alma-Ata and Ebro were simultaneously observed between 27 March and 11 April; (3) 9–13-day oscillations observed in the DfoF2 under mostly quiet geomagnetic activity at the beginning of January appear

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only above Alma-Ata; (4) DfoF2 oscillations are observed over both stations at the end of January/ early February (strong 8–10-day oscillations above Ebro station and 6-day oscillations above AlmaAta station); and (5) strong 6-day DfoF2 oscillations are present in the data from Ebro station after 16 April 2004.

4. Geomagnetic variability In order to understand the geophysical background in which the ionospheric effects have occurred, the geomagnetic activity is analyzed for the same time interval as the ionospheric variability analysis as it is well known that the geomagnetic activity plays an important role in the ionosphere. Fig. 3 shows the Dst- and Kp-indices for the time interval of consideration. The asterisks of Fig. 3 indicate four significant events in the geomagnetic field observed for the time interval of interest: (1) a severe geomagnetic storm (Dsto150 nT, Kp ¼ 7), observed on 22 January, which is associated with the coronal mass ejection (CME) and the longduration C5 flare observed on 20 January; (2) isolated major storm levels (Dstp100 nT, Kp ¼ 6) observed on 11 February, decreasing to active levels on 13–14 February; (3) active-to-major storm conditions (Dstp75 nT, Kp ¼ 6), which became prevalent from midday of 9 March to early of 12 March due to a coronal hole-driven high-speed solar wind stream; (4) active-to-minor storm levels (Dstpl00 nT, Kp ¼ 4–5) observed from midday of 3 April to early of 4 April as a result of the fullhalo8 CME from 31 March. The rest of the time interval is mostly geomagnetically quiet (Kpo3+), with some time intervals of unsettled activity levels (3opKpo4+) and a few isolated intervals with significant activity (4oKpp5+) recorded at high latitudes. The 1-min values of the geomagnetic field X-component measured at Alma-Ata observatory for January–April 2004 were used to study the local geomagnetic field variability for the time period. All gaps present in the data sets were interpolated before hourly values of the X-component were selected for analysis. The Morlet wavelet transform 8 Coronal mass ejections can carry up to 10 billion tons of electrified gas. ‘‘Halo events’’ are CMEs aimed in the general direction of the Earth. As they loom larger and larger, they appear to envelop the Sun, forming a halo around our star (http://spacescience.com).

was applied to the Dst and smoothed X-component time series (see Fig. 4) in order to compare the oscillation activity of the geomagnetic field with that observed in the ionospheric variations. The 25-h running average is used in the wavelet analysis to reduce the noise of the hourly X-component time series. The direct comparison between Figs. 2 and 4 shows coincident oscillation events of the geomagnetic field with those PWS events observed in the ionosphere, suggesting geomagnetic activity forcing of the ionospheric PWS events. These are the PWS events observed above both ionospheric stations in late March–early April 2004 and the PWS event observed above Ebro for the first half of March. We will return to this issue later in the discussion. However, there are other PWS events that do not coincide with similar oscillation events of the geomagnetic field, which are possibly associated with other interactions. 5. Stratospheric dynamics The above results show coincident oscillation events in the ionospheric and geomagnetic activity variations, some of which might be coupled. To further examine this possibility, the middle stratosphere activity in the winter/spring of 2003–2004 is analyzed to identify possible ionospheric responses related to the disturbances in stratosphere dynamics. Fig. 5 presents the UKMO assimilated temperature (Fig. 5a, b) and zonal wind (Fig. 5c, d) fields at 421N and at 10 and 30 hPa pressure levels for the time interval of October 2003–April 2004. Fig. 5a shows a remarkable stratosphere warming at the beginning of December 2003 over Central Asia, Eastern Asia and Siberian sectors including the Alma-Ata location. The increase of temperature centered at the Siberian sector in the first decade of December 2003 was up to 240–245 K. The intense minor stratospheric warming, observed mostly in the middle and upper stratosphere levels (STRATALERT Report), gradually decreased by the middle of December 2003. The lower stratosphere shows mostly cold conditions in the early winter (Fig. 5b). At the same time, the eastward zonal wind with a maximum velocity of 50 m s1 at 401N and a l0 hPa pressure level over Central Asia decreased to the zero-wind line by the end of December (Fig. 5c). A second area of high temperature (240 K) above the Western Asia/Eastern Europe sector developed

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into a major warming at the beginning of January, after which the middle stratosphere returned to normal winter conditions (Fig. 5a). Highly disturbed circulation patterns can be seen in the middle and lower stratosphere over the Asia sector as a consequence of the major warming (Fig. 5c, d). At the beginning of January, the winds decreased to zero and reversed from eastward to westward for a short time with a maximum velocity of about 20 m s1 over Central Asia. Simultaneously, the circulation above Europe was dominated by strong eastward winds—with a maximum velocity of 60 m s1 at the 10 hPa pressure level. The global scale character of the event can be seen in the latitude–time section of zonal mean winds at 1 and 10 hPa pressure levels for the period given in Fig. 6a (using the NCEP/NCAR reanalysis data) and Fig. 6b (using the GDAS-CPC zonal mean wind

time series). Fig. 6a, b shows that the zonal mean winds have reversed from eastward to westward at all levels in the middle and upper stratosphere. Figs. 5 and 6 show that the averaged westward jets occurred at latitudes of 50–901N. Nevertheless, we consider that the daily winds represent the actual winds more closely and thus the wind reversal at 401 latitudes is real. An additional warming pulse has developed in the middle stratospheric level at the end of January (Fig. 5a). The disturbed temperature pattern lasted for almost 4 weeks, with weak eastward zonal winds of 0–20 m s1 at 60–751E and less than 30 m s1 over Europe. Manney et al. (2005) reported that the 2003–2004 winter was remarkable in the 50-year record of meteorological analysis of the Arctic stratosphere; a major warming beginning in early January 2004 led

ARTICLE IN PRESS G.I. Gordienko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) 2452–2464 Temperature (K), 42 N, 30 hPa

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to nearly 2 months of vortex disruption with highlatitude westward winds in the middle to lower stratosphere. The minimum temperatures observed in the lower stratosphere over the Arctic region were above average throughout the winter 2003–2004 and only rarely fell below 78 1C. The temperatures in the upper stratosphere increased drastically in December, with subsequent significant warming throughout the Arctic stratosphere (Fig. 7) (The Climate Prediction Center, NOAA/National Weather Service, National Centers for Environmental Prediction, 2003–2004). The UKMO assimilated geopotential heights at the 1 hPa pressure level for the time interval of

October 2003–April 2004 have also been used to analyze the large-scale PW activity. An example of wavelet analysis of geopotential height at the 1 hPa level for westward-propagating waves at 401N and for the time interval of our interest is presented in Fig. 8. As can be seen, a strong westwardpropagating wave with zonal wave number m ¼ 2 and period around 12 days (in the band from 11 to 15 days) developed from the second half of December to the first half of January, reaching an amplitude of 200 m by 3–4 January. Other dominant periodicities indicate the existence of westward-propagating waves with m ¼ l and 2 with amplitude maximum near the middle of February;

ARTICLE IN PRESS G.I. Gordienko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) 2452–2464 GDAS-CPS Zonal Zonal Wind Time Series 2004

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both of them have characteristic periods in the band of about 6–10 days and amplitude about 120 m for m ¼ 2 and up to 280 m for m ¼ l. The comparison of Fig. 5 with Fig. 8 leads to the conclusion that the periodic strengthening of the PW activities of both wave numbers 1 and 2 is in good correlation with the events of stratospheric warming described above (see Fig. 5a). Moreover, these wave events observed in the stratosphere occurred quasi-simultaneously with some of the PWS observed in the ionosphere and without significant oscillation activity of the geomagnetic field. The latter may be an indication

of the dynamic coupling between the lower–middle atmosphere and ionosphere by PW (Kazimirovsky et al., 2003). 6. Discussion Direct comparison of the results obtained in Sections 3 and 4 led to the conclusion that the simultaneous presence of the strong 3–7-day oscillations in the DfoF2 data at Alma-Ata and Ebro stations can be associated with the global-scale response of the ionosphere to geomagnetic forcing

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and we cannot assess for similar PWS in the ionosphere above Alma-Ata station, which makes impossible further comparisons with the Ebro observations. It is very likely that the strong 2–3-day oscillations in the ionospheric data observed over both stations in January (with amplitude maximum around 22 January) and February (with amplitude maximum around mid-February) are generated by an increase in the geomagnetic activity during the period, as can be seen from Fig. 3. A remarkable feature in Fig. 2 is that the amplitude maxima of the oscillations are located at the days of the two geomagnetic storms, the severe geomagnetic storm observed on 22 January and major storm observed on 11 February (marked by asterisks in Fig. 3). This indicates that there may be a dynamical link between the ionospheric and geomagnetic events. Fig. 4 shows clear similarities in the 2–3-day oscillations in the geomagnetic field on the days close to 22 January and 11 February, which are more clearly reflected in the Dst data. On the other hand, the quasi-2-day (QTD) oscillations are often observed in the troposphere and stratosphere and are associated with global scale, resonant, normal, Rossby modes (Salby, 1984). Forbes et al. (1997) showed the relationship between QTD oscillations in the neutral wind near a 90 km altitude from three different locations and the critical plasma frequency foF2 of the ionospheric F-region from 24 ionosonde stations. Altadill et al. (1998), for the first time, presented results of QTD oscillation of the electron density at fixed heights between 170 and 220 km, while Altadill and Apostolov (1998) provided evidences for an upward propagation of the QTDO in the F-region electron density. However, the fact

caused by the events on 27 March and 3 April (see Figs. 3e–h). The wavelet analysis applied to the Dst data and the X-component measured at Alma-Ata station in January–April 2004 shows analogous periodicities in the geomagnetic activity during the period (see Fig. 4a, b). The latter fact supports our previous assumption that the geomagnetic activity could be a driver for these PWS in the ionosphere. In addition, the global geomagnetic variations can also drive the strong 2-day and longer-period oscillations in the DfoF2 observed above Ebro station in the first half of March (Fig. 2b, dotted lines; maximum amplitude is also observed between 6 and 11 March). Figs. 4a, b show that an analogous oscillation is evident in the X-component over Alma-Ata and Dst-index, which coincides with the event in geomagnetic field on 9 March (Figs. 3e, f). Unfortunately, there are no DfoF2 data from Alma-Ata station during 24 February–21 March -45 -50

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that Fig. 8 does not show high-frequency (less than a 5-day period) oscillations suggests that the 2–3day oscillations in the DfoF2 data are generated by the increased geomagnetic activity. For example, see the strong oscillation with an 2-day period, which peaks on 20 November, the day of a well-known strong geomagnetic storm in 2003. The 9–13-day oscillations in the DfoF2 data observed over Alma-Ata in mid-January are not unlikely generated by geomagnetic activity variations. Fig. 4a shows a similar 9-day oscillation in the X-component at the beginning of January with a maximum amplitude around 5 January. However, this 9-day oscillation is located in the corner of Fig. 4, and because of the wavelet-edge effect, the high density around the 9-day period might not be reliable. The phase difference between the oscillations of the geomagnetic field and those of DfoF2 was estimated to be about 10 days. This phase difference is too large to explain the oscillations in the DfoF2 by the ionospheric response to the geomagnetic forcing. Furthermore, the wavelet spectrum for the Dst planetary index (Fig. 4b) does not show the 9-day oscillation during the period considered. In this case, it is more likely that the 9–13-day oscillations in the DfoF2 observed under mostly quiet geomagnetic activity above Alma-Ata station (but not above Ebro) at the beginning of January is related to the dynamical generation of the oscillation by another source, for example, stratosphere dynamics. The 9–13-day oscillations in the DfoF2 were observed a little later in the oscillations of the UKMO geopotential height at the 1 hPa pressure level (Fig. 8 shows a strong westward-propagating wave with zonal wave number m ¼ 2 and period around 12 days with an amplitude maximum on 3–4 January). All this suggests that the strengthened stratosphere dynamics during the stratosphere warming events is more likely the driver for the observed long-term periodicities in the ionosphere. If we assume that there is a relationship between both regimes (stratosphere–ionosphere) with its source in the stratosphere, the ionospheric response must be delayed with respect to the stratospheric driver. The latter is clearly shown by the results: the amplitude maximum of the 9–13-day oscillations in DfoF2 is delayed by about 10 days from the oscillation activity in geopotential height at the 1 hPa level. The results obtained are consistent with earlier estimates of the time delay between the oscillation in the ionosphere and in the zonal wind of the meso-

sphere/lower thermosphere of 5–6 days (Pancheva et al., 2002). As to 6-day and 8–10-day oscillations in the DfoF2 variability observed at the end of January/ beginning of February, there is no analogy of an 6-day oscillation in the geomagnetic field during the period, but there is an 10-day oscillation in the Dst data centered at 29 January. Again, the phase difference is too large to explain the 8–10-day oscillations in the DfoF2 data observed over Ebro station around 5–6 February. Therefore, similar to the case of the 9–13-day oscillations, the 6- and 8–10-day oscillations in DfoF2 observed above both stations at the end of January/beginning of February can be explained by the dynamical generation of the oscillations by the stratosphere dynamics (Fig. 8 indicates the presence of westward-propagating waves in the 6–10-day period band; wave numbers are m ¼ l and 2). However, a different ionospheric response above both ionospheric stations observed in mid-January was observed. In order to explain this, it is assumed that the ionosphere response can occur under weakened eastward or not strong westward winds. The velocities for eastward zonal wind above 30 m s1 seem to be critical. The fact that there are no oscillations in the DfoF2 above Ebro station during the period of strong eastward zonal wind (V430 m s1) and that there are similar 6- and 8–10-day oscillations in DfoF2 above the two ionospheric stations at the end of January/beginning of February (Vo30 m s1) support this assumption. It is more difficult to explain the 6-day oscillations found in the DfoF2 data at Ebro in the middle of April. Again, due to the same edge effect of the wavelet spectral analysis, the confidence level of the spectral power density around the 6day period is low. However, we may note that Fig. 2 shows similar, although larger, period (around 11-day) oscillation in DfoF2 over Alma-Ata that extends practically over the entire spring time interval. There is no similar or significant oscillation in geomagnetic activity. The only subject of discussion is a global strengthening of the stratosphere dynamics during the spring days reflected in the geopotential height. Thus, for example, Fig. 8 shows a clear existence of a westward-propagating wave with zonal number m ¼ l and period around 9 days developed in the stratosphere in the spring, which is probably related to the oscillations present in the ionosphere. This westward-propagating wave

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was associated with the late final warming (Manney et al., 2005), which gives an indication on the timing for the transition from the winter to the summer circulation. The results presented suggest that the 6–11-day oscillations observed in the ionosphere may arise from PW activity in lower atmospheric regions developed during the springtime transition. In general, the difference in behavior patterns of the temperature and zonal mean winds over the Europe/Western Asia sector for the time period was small, and the 10 and 30 hPa zonal mean winds were weak, under 10 m s1. 7. Summary and concluding remarks Measurements of the mid-latitude F-region obtained simultaneously at Alma-Ata station (Kazakhstan, 43.251N, 76.921E) and Ebro station (Spain, 40.81N, 0.51E) during January–April 2004 with PARUS (at Alma-Ata) and DGS256 (at Ebro) are used to analyze PWS variability in foF2 at the two spaced locations. By applying the same method of wavelet analysis in the data sets and doing a direct comparison of the results, three types of disturbances were found in the foF2: (1) 2–7-day oscillations in the DfoF2 observed above both stations of possible geomagnetic origin; (2) 6–11day oscillations found in the ionosphere from the end of March to the beginning of April consistent with PW activity in lower atmospheric regions developed during the final stratosphere warming, which gives an indication of the timing for the transition from the winter to the summer circulation; and (3) 9–13-, 6- and 8–10-day oscillations occurring in winter for mostly quiet levels of geomagnetic activity, which indicate a likely close relation with those in the geopotential height at 1 hPa level for westward-propagating waves at 401N, strengthened during stratosphere warming events that occurred in January 2004. The time delay of the oscillations in the DfoF2 to these in the geopotential height is about 10 days. The assumed ionosphere response to the PW activity in the stratosphere can occur under weakened eastwardpropagating zonal wind or not very strong westward-propagating zonal winds (Vp30 m s1). Acknowledgments Portions of this work have been carried out with the support of INTAS Grant 03-51-6425. The work of D.A. has been partially supported by Spanish

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project CGL2006-12437-C02-02/ANT of MEC and 2006BE00112 of AGAUR, and also by USAF Grant FA8718-L-0072 of the AF Research Laboratory. We are grateful to the staff of the Alma-Ata geomagnetic observatory for providing the data used in the study.

References Altadill, D., Apostolov, E.M., 1998. Vertical development of the 2-day wave in the midlatitude ionospheric F-region. Journal of Geophysical Research 103, 29,199–29,206. Altadill, D., Apostolov, E.M., 2001. Vertical propagating signatures of wave-type oscillations (2- and 6.5-days) in the ionosphere obtained from electron-density profiles. Journal of Atmospheric and Solar-Terrestrial Physics 63 (9), 823–834. Altadill, D., Apostolov, E.M., 2003. Time and scale size planetary wave signatures in the ionospheric F-region: role of the geomagnetic activity and mesosphere/lower thermosphere winds. Journal of Geophysical Research 108 (A11), 1403. Altadill, D., Sole, J.G., Apostolov, E.M., 1998. First observation of quasi 2-day oscillations in ionospheric plasma frequency at fixed heights. Annales de Geophysique 16, 609–617. Altadill, D., Apostolov, E.M., Boska, J., Lastovicka, J., Sauli, P., 2004. Planetary and gravity wave signatures in the F-region ionosphere with impact to radio propagation predictions and variability. Annales de Geophysique 47 (2/3), 1109–1119. Aushev, V.M., Fedulina, I.N., Gordienko, G.I., Lo´pez-Gonza´lez, M.J., Pogoreltsev, A.I., Ryazapova, S.Sh., Shepherd, M.G., 2006. Springtime effects in the mesosphere and ionosphere observed at northern midlatitudes. Planetary and Space Science 54, 559–571. Forbes, J.M., Guffe, R., Zang, X., Fritts, D., Riggin, D., Manson, A., Vincent, R.A., 1997. Quasi 2-day oscillation of the ionosphere during summer 1992. Journal of Geophysical Research 102, 7301–7305. Gordienko, G.I., Aushev, V.M., Fedulina, I.N., Ryazapova, S.Sh., Shepherd, M.G., 2005. Observation of the F2-layer variability from the ‘‘Alma-Ata’’ observatory. Journal of Atmospheric and Solar-Terrestrial Physics 67, 563–580. Jarvis, M.G., 2006. Planetary wave trends in the lower thermosphere—evidence for 22-year solar modulation of the quasi 5day wave. Journal of Atmospheric and Solar-Terrestrial Physics 68, 1902–1912. Kalnay, E., Kanamitsu, R., Kistler, R., et al., 1996. The NCEP/ NCAR 40-year reanalysis project. Bulletin of the American Meteorological Society 77, 437–471. Kazimirovsky, E.S., Herraiz, M., de la Morena, B.A., 2003. Effects on the ionosphere due to phenomena occurring below it. Surveys in Geophysics 24, 139–184. Lastovicka, J., 2006. Forcing of the ionosphere by waves from below. Journal of Atmospheric and Solar-Terrestrial Physics 68, 479–497. Lastovicka, J., Krizan, Sauli, P., Novotna, D., 2003. Persistence of the planetary wave type oscillations in foF2 over Europe. Annales de Geophysique 21, 1543–1552. Manney, G.L., Kriiger, K., Sabitus, J.L., Sena, S.A., Pawson, S., 2005. The remarkable 2003–2004 winter and other recent

ARTICLE IN PRESS 2464

G.I. Gordienko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) 2452–2464

warm winters in the Arctic stratosphere since the late 1990s. Journal of Geophysical Research 110, D04107. Pancheva, D., Mitchell, N., Clark, R.R., Drobjeva, J., Lastovicka, J., 2002. Variability in the maximum height of the ionospheric F2-layer over Millstone Hill (September 1998–March 2000); influence from below and above. Annales de Geophysique 20, 1807–1819. Pogoreltsev, A.I., Fedulina, I.N., Mitchell, N.J., Muller, H.G., Luo, Y., Meek, C.E., Manson, A.H., 2002. Global free oscillations of the atmosphere and secondary planetary waves in the mesosphere and lower thermosphere region during August/September time conditions. Journal of Geophysical Research 107 (D24), 1–12.

Salby, M.L., 1984. Survey of planetary-scale traveling waves: the state of theory and observations. Reviews of Geophysics and Space Physics 22, 209–236. Swinbank, R., O’Neill, A., 1994. A stratosphere–troposphere assimilation system. Monthly Weather Review 122, 686–702. Torrence, Ch., Compo, G.P., 1998. A practical guide to wavelet analysis. Bulletin of the American Meteorological Society 79, 61–78. Xiong, J., Wan, W., Ning, B., Liu, L., Gao, Y., 2006. Planetary wave-type oscillations in the ionosphere and their relationship to mesospheric/lower thermospheric and geomagnetic disturbances at Wuhan (30.611N, 114.5I1E). Journal of Atmospheric and Solar-Terrestrial Physics 68, 498–508.