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Temperature and airglow brightness oscillations in the polar mesosphere and lower thermosphere
Phrt.
Pergamon
Sp/w Sci.. Vol. 33. No. 9, pp. I 171 I 130. 1995 Copyright t 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0032 #f33’95 $9.50+n.nn
0032-0633(95)00029-1
Temperature and airglow brightness oscillations in the polar mesosphere and lower thermosphere I.Oznovich,’
D.
J. McEwen’ and G. G. Sivjee’
Network for Space Research, ISAS, University of Saskatchewan. Saskatoon. Saskatchewan S7N 5E’. Canada Space Physics Research Laboratory, Embry Riddle Aeronautical University. 600 S. Clyde Morris Blvd.. Daytona Beach. FL 321 l4-.1900. II.S.A.
! Canadian
Received 32 August
1994; revised 30 November
1994; accepted
Abstract. Large amplitude oscillations in airglow brightness and OH rotational temperature were observed on Dec. 31, 1993, 00 UT to Jan. 1, 1994, 18 UT aver Eureka (80.O”N). The airglow brightnesses of atomic oxygen O[‘S] emission at 5577 8, and sodium Na (2P3,2,,j2)emission at 5890 and 58% 8, were measured by a multi-channel, meridian scanning photometer. The Meinel OH (3,l) band brightness and rotational temperature were monitored by a Michelson interferometer. The period of the observed oscillations was 8.4 + 1 h. Supporting evidence for a non-migrating tide (zero zonal wave number) explanation of these oscillations are an observed period close to 8 h, a large vertical wavelength as derived from the airglow emissions, and wave persistence for five complete cycles. On the other hand, the small amplitude and negative phase of Krassovsky’s ratio are not consistent with a tidal source for the observed oscillations. Similar 8.5+ 1 h variations were observed on Dec. 2 1,1993. The case for a non-migrating tide as a source. for these oscillations is further weakened by a 1.5 h difference in local time when these oscillations were observed to peak during the two events (which occurred 10 days apart). Explanation of the above observations in terms of an inertiogravity wave is favored by the relatively infrequent appearance of these oscillations at all altitudes monitored, the incoherency between the two events, and the fact that temperature variations lead OH airglow brightness variations, in qualitative agreement with gravity wave theory. The observed lower limit to the horizontal wavelength of the wave Gompares with its value predicted by the dispersion relation of inertiogravity waves. Forced planetary waves, observed to peak in amplitude on Dec. 21, 1993, and Jan. 1, 1994, as well as the stratospheric warming of the middle of December and end of December/beginning of January, were likely driven by the same tropospheric disturbance or directly related to the source of the inertia-gravity wave observed over Eureka on Dec. 21 and Dec. 31Jan. 1, respectively.
25 January
1995
1. Introduction The winter polar cap is a unique laboratory for studying waves in the Mesosphere and Lower Thermosphere (MLT). Continuous darkness at high latitudes (101 >, X0 ) affords uninterrupted 24 h ground-based measurements of optical nightglow emissions, emanating from different height regimes of the MLT, for over three months. Such measurements permit investigations of the characteristics of planetary, tidal and gravity wave disturbances propagating through the atmospheric region extending from about 83 km to roughly 100 km. The only hindrances are weather conditions and difficulties associated with logistics and equipment maintenance in the harsh polar environment. Void of local solar heating. theoretical considerations suggest that the winter polar region is relatively weakly disturbed by tides. All the modes. except the zonally symmetric non-migrating tide (zonal wave number s = 0). at-e expected to vanish at the poles (Forbes. lY82a,b). Indeed. recent observations of 6 h oscillations of the Meinel OH (3,l) band brightness and rotational temperature over the south pole were interpreted as possible manifestations of the effects of a zonally symmetric tide (Sivjee and Walterscheid, 1994). Superimposed hourly means of the polar mesopause temperature for a 19 day period around winter solstice at 78’ N revealed a semi-diurnal trend with an amplitude of 5 K (Myrabo, 1984). Walterscheid c’t al. (1986) measured larger semi-diurnal oscillations of the mesopause temperature (I 3 K) at 78 N in winter. Their theory of a semidiurnal pseudotide. generated by gravity wave momentum fluxes modulated by the solar semi-diurnal tide, predicted mesopause temperature variations of the order of 10 K. An interpretation of a semi-diurnal pseudotide was also given to the Na lidar observation of a 12 h variation in the sodium density profile over the winter south pole (Collins et al., 1992). Hernandez et rll. (1992) suggested that some 12 h variations in the mesopause neutral wind and temperature over Antarctica were caused by ducting of lower-latitude tidal oscillations. Variations in the nightglow OH band emission in the north polar region with a period of 3.7 h (Sivjee it ul.. 19X7) and over the south
1122
pole with a period of 12 h (Hernandez et al.. 1993) were interpreted as manifestations of inertio-gravity wave activity. In the absence of obvious forcing mechanisms, the interpretation of the observed variations in polar MLT density and temperature with periods shorter than those of planetary waves is far from clear. Temporal information is necessary to identify the wave frequency. Vertical and horizontal information can be integrated to assess the propagation direction and help identify the nature of the wave. Both kinetic temperature fluctuations and airglow brightness variations must be observed in order to build a reliable theoretical model of the dynamics and chemical processes of the MLT region. In this study of wave activity over Eureka (80.O”N) on new year’s eve 1994, an effort has been made to synthesize observations of parameters of the MLT region that relate to all of the above. Section 2 describes the Eureka observatory, instruments used, their mode of operation, and the data reduction techniques. Section 3 presents 8.4 h oscillations of mesopause temperature and airglow brightness at three altitudes. Allsky scans of the airglow brightness are utilized to assess an observational lower limit on the horizontal wavelength of the wave. Similar oscillations observed on Dec. 21, 1993, as well as Krassovsky’s ratio (1972), are used in Section 4 to test critically the identification of the wave as either a non-migrating ter-diurnal tide or as an inertiogravity wave. The horizontal wavelength of the wave is deduced via the dispersion relation of inertio-gravity waves and compared with its observed lower limit. Possible relations of the waves reported here to forced planetary waves and minor stratospheric warming events of the middle of December and end of December/beginning of January are noted.
2. Eureka station, detectors, and data reduction An observatory very close to the north magnetic pole has been in operation since the winter of 1990-91 at Eureka, Northwest Territories, Canada. Eureka is located at geographic coordinates SO.O’N, 274.1 “E, PACE geomagnetic coordinates 88.9”N, 284.1 ME.Airglow instrumentation in the polar observatory of the Canadian Network for Space Research was run by the Polar Environment Project. One of the major objectives of the project was a half-solar cycle study of the polar MLTregion with much higher temporal and spatial resolution than could be achieved from space or by sporadic ground-campaigns. The Eureka instrumentation used in this study included a multi-channel Meridian Scanning Photometer (MSP). a Michelson Interferometer (MI), and an All-Sky Camera (ASC). The MSP was used to monitor oxygen O[‘S] and sodium Na (‘Pj/,,,:,) airglow emissions. The MI afforded measurement of the Meinel OH (3,l) band brightness and rotational temperature. ASC images were used to monitor cloud coverage and aurora] contamination of the airglow data. The instantaneous field-of-view (TFOV) of the MSP was 1 The measured spectral full width at half maximum of the filters was 4.1 A at 5577 A and 10.1 8, at 5896 A.
1. Oznovich
ct a!. : Temperature and airglow brightness
During the 1993-94 winter season the scan direction of the MSP was rotated 5 every 20 min to align it perpendicular to the sun+arth direction. The period of one scan (usable 79 to -79” zenith angle in steps of I ) was 32 s. with an additional mirror fly-back time of 6 s. The identification of wave activity over Eureka was based on zenithal airglow brightness measurements. The full scans were used in the investigation ofthe horizontal wavelength of the wave. Of the six channels recorded, O[‘S] at 5577 A (green) and Na (‘7Pj,2,,,Z) at 5896 and 5890 8, (unresolved sodium doublet) were utilized in this study. A filter 100 p\ wide at 5145 A was used to monitor background emission levels. The red oxygen line O[‘D] at 6300 A served as a monitor of dissociative recombination of 0; ions in the F-region. The raw count rate was converted to absolute brightness using photometer sensitivities derived from field calibrations and corrected for the Van Rhijn effect, ozone extinction, and ground albedo (Ashburn, 1954). First-order background correction was applied to both O[‘S] and Na-D emissions using the background channel brightness adjusted by the appropriate Rayleigh scattering factor. In order to reduce noise and match the temporal resolution of the MI, approximately 16 zenithal measurements were averaged to produce one data point every 10 min. The MSP filter characteristics and photometer sensitivities were published by Steele and McEwen (1990). The Eureka MI was a BOMEM MB160 with a low noise (NEP _ 2 x lOp’5 W) InGaAs detector modified fol emission measurements in the 1.0-1.7 pm wavelength region (Sivjee and Walterscheid, 1994). Its resolution could be set at any one of eight different values in the range l-128 cm-’ (in binary steps). At 2 cm-’ resolution its IFOV was 1.7 and the corresponding throughput 3.7 x IO-’ cm’ s. The MI completed one interferogram scan every 3.5 s. Approximately 170 such interferograms were coadded to derive a real-time spectrum by FFT every 10 min. The spectra were corrected for variations in the spectral sensitivity of the MI using a variable aperture black body source operating at 1273 K and illuminating a Lambertian surface placed 1 m away from the black body. This calibration also allowed inversion of the MI detector signal to absolute spectral brightness of the airglow emission. Correction for atmospheric extinction was applied with the USAF’s FASCODZ using the procedure described by Hammond and Espy (1994). The brightness and rotational temperature values of the airglow Meinel OH (3,l) band were determined through detailed least-squares fit of the OH (3,l) band rotational lines to their synthetic profiles constructed for different rotational distribution temperatures. The OH vibrational-rotational transition probabilities of Nelson et al. (1990) and the rotational term values of OH (2.’ = 3) of Coxon (1980) were used. Procedures for the inversion of OH rotational line emissions were detailed by Sivjee (1992). The chosen transition probabilities determine the absolute rotational temperature deduced from the OH emissions. However, different transition probabilities result in the same relative temperature change ATiT pre. sented here. The MI was pointed 25’ above the northern horizon and recorded mesopause airglow emissions from a region corresponding to 81.6 N latitude. Temporally-
I Oznovich PI ul. : Temperature and airglow brightness coincident MSP and MI data presented here were spatially separated (in the horizontal direction) by - 180 km. The ASC used a low light level intensified siliconintensifier target with an automatic gain control. Analog TV fields were recorded in a time-lapse mode. registering one filter sequence approximately every 18 s. The broad-band visual, green line (5577 WI. and red line (6300 A) filters nere alternated every 6 s. The detection threshold of the ASC at broad-band visual light was approximately 0.3 kR. The ASC system and its data reduction algorithms were described in detail by Oznovich er al. (1994). The temporally-coincident data of the Eureka MSP and MI for the 1993-94 season span three months (November 1993-January 1994). A search was performed for continuous (24 h) good viewing conditions during that period using weather observations and ASC images. Snow, fog. ice crystals, and cloud coverage were reported by the Eureka weather officer on duty on an hourly basis. Discrete aurora1 arcs were readily identified in the broad-band visual and green line images of the ASC. This study concentrates on the discovery of significant variations in temperature and airglow brightness around new year’s eve 1994. Excellent viewing conditions prevailed during the period Dec. 31. 1’193. 00 UT to Jan. 1. 1994. 18 UT. Aurora] arcs drifted in less than 1 min over the zenith of the Eureka MSP on Jan. I. 1994. at 08 : 43,09 : 14. and 15 : 33 UT. These three samples were removed from the MSP data-set. Other than discrete arcs. diffuse aurora1 emissions are also likely to contribute to the measured green line brightness. Low brightness (d 500 R) emissions that cover a major fraction of the sky will not be detected by the Eureka ASC. and are not necessarily discernible in the MSP scan across the sky. One can hypothesize that feeble emissions ol‘the green line (O[‘S] at 5577 A) detected in the polar cap are primarily due to polar rain precipitation of low energy (-:200 eV). low energy flux (
600 r
q()$-------
10” lo’* Energy Flux [ergs/s cm?
Fig. 1. Polar cap green line (circles) and blue line (triangles) brightness measured by the Eureka MSP and the coinciding electron energy flux measured by the electrostatic analyzer aboard the DMSP satellite. The brightness of’ the blue line emission was multiplied by 10
I I’3
coincidences of the MSP scan and DMSP ground track that occurred between Dec. 1990 and Dec. 1991 near Eureka. The linear coefficient of correlation between the green line (marked as circles) and blue line (marked as triangles) brightness and the electron energy flux is - 0.16 and -0.02. respectively. The energy flux that is associated with these emissions is predominantly less than 0.03 erg cm-?sml, Therefore the expected green line brightness due to electron precipitation is at most 50 R (using a green line excitation efficiency of 1.73 kR (erg cm ’ s ‘) _’ (Steele and McEwen. 1990)). The green line brightness presented here is in the range 100-450 R. Thus one is led to the conclusion that diffuse aurora1 emissions did not significantly contaminate the green airglow data shown here.
3. Results Most of the green line emission originates from the lower thermosphere at 95-100 km where the concentration of atomic oxygen is at maximum (Greer ri trl.. 1986; Yee and Abreu. 1987). A small fraction of it originates from the F-region at 250- 300 km and is discussed at the end ot this section. The Na doublet emission is produced in an oxidation-reduction cycle with ozone and atomic oxygen in the altitude range 83.-95 km and peaks near X9 km (Hunten. 1967; von Zahn rral., 19X9; Collinsrl N/., 1993). The Meinel OH (3.1) band emission originates from the mesosphere in the altitude range 82-90 km (Rogers c’t ~1.. 1973 ; Lopez-Moreno et ui., 1987 : Sivjee and Hamwey, 1987). The distribution among the rotational lines of OH affords a calculation of its rotational temperature T,,,,. Since the OH radicals undergo - 10 collisions before radiating their excess energy, their distribution among the rotational states is thermalized to the ambient kinetic temperature (Walterscheid ~‘1r/l.. 1986). In summary, the optical measurements presented here provide mesopause temperature information (T,,,) and airglow brightness peaking approximately at 85 (l,,,). X9 (IN,,). and 97 km (I&. It should be noted that most of the altitude profile measurements of airglow emission rate and constituent density were performed at latitudes lower than the polar region. There are differences in peak OH emission height for different vibrational-rotational bands (Sivjee and Hamwey. 1987). More importantly. the vertical distribution of atomic oxygen concentration effects the peak emission height of all lines and bands discussed here in both production and deactivation mechanisms. Atomic oxygen is particularly variable in the polar region. However. it is the difference in peak emission height that determines the vertical phase speed and wavelength of the wave. and this could be maintained approximately constant for a general expansion or contraction of the upper atmosphere. Moreover. the usage of three airglow emissions (two peak emission height differences) reduces the uncertainty in the derived wave characteristics. Figure 2 shows large amplitude variations in airglow brightness (a) and OH rotational temperature (b) observed on Dec. 31, 1993,OO UT to Jan. 1. 1994, IX UT. Local midnight in Eureka is at 05:49 UT. Figure ?a depicts the Meinel OH (3.1) band brightness IO,, (solid
I. Oznovich er al. : Temperature and airglow brightness
1124
50~,.,.,,.,,,,..,,“_,~“’
z
0
260
0
s! 2 230 g
F
200
I
L)
00 Dee 31
L
’
06
12
1,.
18
UniversalTime
[hr]
‘.
1
1,
I
00
06
12
18
Jan I
Fig. 2. Oscillations in airglow brightness (a) and OH rotational temperature (b) observed over Eureka (SO’N) on Dec. 31, 1993, 00 UT to Jan. 1, 1994, 18 UT. The Meinel OH (3,1) band brightness Iou is marked by a solid line. The Na (‘P,,& line brightness I,, is traced by a dotted line. The O[‘S] green line
brightness I,, is shown by a dashed line. Airglow brightness is in Rayleighs for I,, and I, and in kilo-Rayleighs for I,,
2
INa (dotted line), and P3~2.,~2 ) line brightness O[‘S] line brightness IO (dashed line). Airglow brightness is in Rayleighs for I,, and Io and in kilo-Rayleighs for IOn. Figure 2b shows the OH rotational temperature T,,,. Five complete cycles of both airglow brightness and temperature variation as measured by the Eureka MSP and MI are evident from the data. The variations were measured by two independent instruments, at three different wavelengths, and are exhibited in both the brightness and the temperature data. Therefore it is highly unlikely that these variations were due to random statistical fluctuations. The variations in brightness and temperature shown in Fig. 2 reflect the modulation in concentration of major and minor gas constituents and of temperature by wave disturbances propagating through the atmosphere in the 80-100 km altitude region (Sivjee et al., 1987). The vertical phase propagation is apparently downward as the variations appear first at 95-100 km (as manifested by Zo), then at 85-90 km (as manifested by the sodium and hydroxyl emissions). The peak-to-peak change in IoH is about a third of the average brightness ; the peak-to-peak change in T,,, is - 10% of the average temperature. Therefore the amplitude of Krassovsky’s ratio ( 1972) is approximately 3. Changes in T,,, slightly precede those in Z,,, indicating a small negative phase for Krassovsky’s ratio. A quantitative LombScargle analysis (Press and Rybicki, 1989, and references therein) was performed in order to derive the frequency content of the data shown in Fig. 2. The resultant normalized power spectrum of in Fig. 3 by a bold T,,,, IOH, INS, and I0 is represented solid line, a solid line, a dotted line, and a dashed line. respectively. The horizontal long-dashed line indicates a normalized power level of 10.2, corresponding to a significance level of P = 0.01 (99% confidence level) ; a peak that lies above this line has a less than 1% probability P that it is due to random Gaussian noise. Three groups of statistically significant (> 99% confidence level) peaks line),
$
Na
(
Fig. 3. Normalized LombScargle power spectra of r,,, (bold solid line), lou (solid line), ZNa(dotted line), and Io (dashed line) for the data shown in Fig. 2. Peaks that lie above the horizontal long-dashed line have less than 1% probability to be due to random noise
appear in Fig. 3. The highest frequency group corresponds to a period of 6.6 h in I,, 5.9 h in IN,. and 5.9 h in IoH. This variation is most likely due to a 6 h tidal oscillation similar to that reported by Sivjee and Walterscheid (1994). The lowest frequency group corresponds to a period of 20 h in lo and 26 h in Z,,. This daily oscillation is due to imperfect background subtraction as it appears only in the visual channels and varies in phase with lunar elevation (maximum 1 h past local midnight). The highest peak in all three emissions and the temperature data corresponds to a period of 8.4 h. This result was also confirmed by a Fourier analysis. A least-squares analysis of the data revealed the average brightness (0 and temperature (T), its peak-to-peak variation (AI and AT). and the absolute phase of the wave, denoted by the local time of the wave maxima nearest midnight in Table 1. The wave characteristics were determined after removing a linear trend from the data. The observed average temperature i= = 228 K and its peak-topeak variation AT = 19 K are higher than those of Myrabo (1984) (F= 220 K and AT = 10 K) and those of Walterscheid et al. (1986) (T= 219 K and AT= 13 K). In fact, the data shown in Fig. 2 represents the largest cyclic variation in mesopause temperature observed over Eureka during the 1993-94 winter season. The relatively high sodium brightness is noted (&,, = 286 R). Such high levels of Na-D emission (200300 R) prevailed throughout the winter till February 1994, when it dropped to 50-100 R. Similar high levels of sodium brightness were observed over Eureka during the winters of 1991-92 and 1992293. The level of sodium emission over the winter polar cap may be high due to a combination of latitudinal (Takahashi et ~1.. 1989) and seasonal (Gardner et al., 1988) effects. Table 1 also shows the frequency corresponding to the peak power of the three emissions and the OH rotational temperature. The equivalent period of I,,. INil, I,. and T,,, is 8.5, 8.7, 8.3, and 8.1 h, respectively. The resolving limit in periodicity is 1-2 h. The change in I0 preceded the change in IN;, by 53 min and the change in lNa preceded the change in IOH by 24 min indicating a downward propagating phase. The vertical phase velocity of the wave is -2.5 m s-’ as deduced from IO and I,, and -2.8 m ss’ as deduced from I,, and IOn. Assuming a 2 km uncertainty in the peak emission height difference results in an average
1 Oznovich
ct cl/. Temperature
and airglow brightness
I125
Table I. Wave characteristics of airglow brightness and rotational temperature observed over Eureka during Dec. 3 1. 1993,OO UT to Jan. 11 1994, 18 UT. The
average brightness (n and temperature (F), its peak-to-peak variation (A/ and AT). the frequency at peak spectral power, and the wave phase (local time of maximum nearest midnight) (‘P, >, >), and O[‘S] Average IT
are for the airglow
Variation AI, AT
emissions
Frequency in h-’
of OH (3.1), Nn
Local time of maximum
_
179 kR 286 R 340 R 218 K
42+6 kR 63k4 R 118k14 R 19i2 K
0.117+_0.017 0.115+0.015 0.121~0.010 0.123_t0.012
01:39-&00:25 01:15f00:08 00:22f00:16 01:32-J00:l6
vertical phase velocity rZ = - 2.6kO.5 m s-‘. The horizontal spacing between the regions sampled by the MSP and MI are not expected to be important here because of the large horizontal wavelength of the wave (to be discussed later). Using the definition of the observed vertical phase speed r’: = W,,%;
(1)
where w,, is the observed frequency and k, the vertical wave number. we calculate the vertical wavelength i, to be 79& 18 km. Krassovsky’s ratio (I 972) q is a complex quantity summarizing all the dynamical and chemical processes that connect the wave forcing. as manifested by kinetic temperature fluctuations. to radiative energy dissipation, as manifested by variations in airglow brightness (Sivjee and Walterscheid, 1994). Its amplitude Iil( is given by
4oo
___/,___j E__/-----------.-_...__
300 200 ; ,uu’
(d) SE-NW
1 1 / -400 -300 -200 -100
L_u_1i 0 100
i---1 200
300
400
Distance from Eureka [km] 7zEi
(2) and its phase 4 by the relative phase of the fractional brightness and temperature oscillations. The wave characteristics of the Meinel OH (3.1) band brightness and rotational temperature summarized in Table 1 indicate that 1~) = 2.8$0.7 and 4 = - 13”+20’. The quantitative values of 111)and C/Jare in agreement with those estimated above by a visual inspection of Fig. 2. An investigation of the horizontal wavelength of the wave was conducted. It was facilitated by the fortuitous fact that the observed frequency was not a harmonic of the MSP rotation frequency and by the observation of five full cycles of the wave. Figure 4 shows two green line brightness scans of the sky for each of the four principal directions south-north (a). east-west (b), southwest-northeast (c). and southeast-northwest (d). The coverage of four look directions assures that the angle between the horizontal propagation direction of the wave and one of these directions is at most 22.5 The two scans correspond to times near maxima (solid line) and minima (dotted lint) in the green lint brightness plot of Fig. 2. The abscissa represents great circle distance from Eureka at an altitude of 97 km. The variation in all the green line brightness scans across the 850 km distance is less than half the total variation between maxima and minima, indicating that the MSP observed less than a quarter of the horizontal wavelength of the wave at any one time. The conclusion
Minima j
Fig. 4. Two O[‘S] green line (5577 A) brightness scans ofthe sky at look directions southhnorth (a), west-east (h). southwest northeast (c). and southeast-northwest (d). The two scans carrespond to times near maxima (solid line) and minima (dotted line) in the green line brightness plot of Fig. 2. Variations due to the 8 h wave should be manifested in changes from a minima to a maxima in one or at most two of the look directions
is that the horizontal wavelength of the wave i., was greater than 3400 km. This result was further confirmed by similar scans of the sodium line brightness. One technique to correct for the contribution from Fregion dissociative recombination of 0: ions to the column integrated green line emission is to subtract a certain fraction of the red line brightness from that of the green line (Takahashi et al., 1977). This fraction could be as much as 20% in tropical regions (Silverman, 1970). The green line brightness shown in Fig. 2 was compared with that subtracted by IO and 20% of the concurrent red line brightness. The resultant variations in green line brightness were 30-60 R. The corresponding green line power spectra were statistically identical to that shown in Fig. 3, implying that dissociative recombination of 0, ions in the F-region did not significantly contribute to the green line data shown here.
I. Oznovich et al. : Temperature and airglow brightness
1126
4. Discussion It is somewhat problematical to attribute the large amplitude variations in the MLT temperature and airglow brightness observed over Eureka to low-order tidal oscillations of the atmosphere since their predicted amplitudes are very small in the polar region at winter solstice (Forbes, 1982a,b). High-order solar-driven modes potentially have significant amplitudes at high latitudes. but the forcing problem remains-the high-order modes are only weakly excited by the global solar forcing, and local solar forcing is absent in winter. It is also difficult to assign the observed oscillations to a ducted high-order ter-diurnal tide, similar to the ducting of a semi-diurnal wave from low latitudes to the south pole (Hernandez et ul., 1992), because very high-order modes of the tide are strongly absorbed by dissipative processes in the lower thermosphere. The period of the observed oscillations (8.4+ 1 h) is sufficiently different from the inertial period (12.2 h at 80 N) to exclude a free inertial wave explanation. Interactions between gravity waves and the mean flow are modulated by harmonics of the solar-driven tide. Therefore it is possible that the observed oscillations were due to a ter-diurnal pseudotide. The observed large-amplitude variations in the polar MLT region can be explained by a pseudotide only if there is a large factor for the amplification of the tide, since the amplitude of the solardriven tide itself is very small at 80-N. For example, the amplification factor r is 23 for a semi-diurnal tide at 78’~N (Walterscheid et al.. 1986). The amplification factor is given by
co2 x=---If2_J I where w is the frequency
of the tide and
f’= 20sin8
(4)
is the Coriolis parameter. R is the angular velocity of Earth and 0 the latitude. In our case tc) = 3Q and the amplification factor is 1.8, making the argument for a pseudotide a weak one. The two most likely explanations of the -8 h oscillations presented above are therefore a non-migrating tide and an inertio-gravity wave. We use the term inertiogravity wave rather than a pure internal gravity wave because the large horizontal wavelength of the wave (1, > 3400 km) dictates that it will be influenced by the rotation of the Earth. The only mode of the classical, solar-driven tide that has a significant amplitude at 80”N is the non-migrating tide (zero zonal wave number). The variations in temperature, divergence. vertical velocity, and major gas density of a non-migrating tide have their maximum amplitude at the poles (Longuet-Higgins, 1968). Supporting evidence for a non-migrating tide explanation of these oscillations are an observed period close to 8 h, a large vertical wavelength (i., = 79 + 18 km), and wave persistence for five complete cycles. However, in the absence of local solar heating there remains the problem of the forcing mechanism. Moreover. the theoretical calculations of Sivjee and Walterscheid (1994) show that for
a non-migrating tide the OH airglow brightness modulations lead the mesopause temperature modulations (positive phase for Krassovsky’s ratio). In contrast a negative phase (4 = - 12“ +_20”) was found here. If a non-migrating ter-diurnal tide prevails in the winter polar region, one might expect to observe it at times other than new year’s eve 1994. If other events that exhibit 8 h oscillations were indeed observed, a phase coherency between the different events would support a tidal explanation. A day-by-day search for - 8 h oscillations in airglow brightness and temperature was performed for the 1993-94 mid-winter season (November 1993- January 1994). During the 83 days of coincident MSP-MI data. approximately 60% of the time clear weather conditions prevailed over Eureka. During these clear days. only one additional instance of unambiguous 8 h oscillations in temperature and all airglow emissions was found. This fact by itself implies that a ter-diurnal oscillation in the polar MLT region is not a frequent phenomenon at all altitudes and weakens the case for a non-migrating tide. Variations in airglow brightness (a) and OH rotational temperature (b) are shown in Fig. 5 for the period Dec. 20, 19 UT to Dec. 21, 18 UT, 1993. Figure 5a shows the Meinel OH (3,l) band brightness I,, (solid line), Na (‘P 3,Z,,.,) line brightness INa (dotted line), and O[‘S] line brigh‘tness I, (dashed line). The data shown was smoothed with a running 3 h window to emphasize the relative phase between the different emissions. Three complete cycles of airglow brightness and temperature variation are evident from Fig. 5. The main characteristics of the Dec. 21 event, as summarized in Table 2, are very similar to those of the new year’s eve event. The period corresponding to the peak power in the Dec. 31 LombScargle spectrum (Fig. 6) of I,,,, INd, I,,, and T,,, is 9.0, 8. I, 9.0, and 8.1 h, respectively. The vertical phase propagation is downward as the change in I(, (97 km) leads the change in Z,, (89 km) by 2.5 h and the change in I,, (89 km) leads the change in ZOH(8.5 km) by 1.3 h. The vertical phase speed of the wave as deduced from the relative phase between I, and ZNais in excellent agreement with that deduced from the relative phase
I’
1
”
1
1
I
,-~
z2,0/,/ / 18
__I
1
00 Dec21 06 Universal Time
12
18
[hr]
Fig. 5. Same as Fig. 2 for the period Dec. 20. 19 UT to Dec. 2 1, 18 UT, 1993. The sodium brightness I,, was divided by 2. The data shown was smoothed with a running 3 h window to emphasize the relative phase between
the different
emissions
I. Oznovich
PI ol. Temperature
and airglow brightness
Table 2. Same as Table 1993
I OH I ‘r.8 I r::,
1127
1 for the period
Average - I, T
Variation AI, AT
156 241 141 223
80+3 kR 81&-8R 49k IO R 13k2 K
kR R R K
50 5 40 5
30
w Yi 20 E P
P = 0.01
10
0.05
0.00
0.10
0.15
0.20
0.25
Frequency [hi']
Fig. 6. Same as Fig. 3 for the period Dec. 20, I9 UT to Dec. 2 I. 18 UT. 1993
hetween I,, and IOH (II = -0.9kO.3 m s-l). The resultant vertical wavelength (equation (I)) is ;I; = 28 + 11 km. The average temperature (T = 223 K) and its peak-topeak variation (AT = 13 K) are very similar to those of Walterscheid it al. ( 1986) (T = 219 K and AT = 13 K). It is evident from Fig. 5 that changes in T,,, precede those a negative phase for Krassovsky’s ratio. in IOH. indicating Indeed, the wave characteristics of the Meinel OH (3,l) band brightness and rotational temperature of Table 2 indicate that 1~1= 8.8f 1.7 and 4 = - 107’+9 ‘. Although $I is negative for the Dec. 31-Jan. 1 event, its large uncertainty (due to statistically-significant peaks in the IoH power spectrum other than that at 8.5 h) makes it difficult to compare with theoretical predictions for Krassovsky’s ratio. The values of 1’11and r#~for the Dec. 21 event are certainly closer to those predicted by theory for gravity waves than those predicted by theory for nonmigrating tides (Sivjee and Walterscheid. 1994). Unless non-migrating tides are triggered by transient phenomena, the case for non-migrating tides here is somewhat weakcned by the incoherency of the two events. The local time of maximum nearest midnight for the Dec. 21 event precedes that of the Dec. 31-Jan. 1 event by 1.3, 1.5. and - 1.1 h for T,,,. lo. and IOH, respectively. This corresponds to a maximum phase shift of 68,’ for a ter-diurnal wave. It is unlikely that a seasonal drift of the phase of the tide caused such a large shift, as the two events are separated by only 10 days. The tidal hypothesis can be further explored by observing that the solution for the inhomogeneous vertical structure equation of the tide in the regime 0 < h < 4N2H2/,q is 7 kf =g--
1 (5)
4H’ where
h is the
equivalent
depth,
N
the
buoyancy
Dec. 20. 19 UT to Dec. 21. 18 UT.
Frequency inh-’ 0.11 I * 0.020 0.124~0.021 0. I I I f 0.020 0.11410.017
Local time of maximum 02:42 i 00:08 01:22~00:lh 22:55fOO:lh 00:10f00:08
frequency, and H the scale height (Andrews et al., 1987). For i., = 79+ 18 km (Dec. 31--Jan. 1) it is found that h=3.1~0.7km.Fori,=38f11km(Dec.21)itisfound that 11= 0.8+0.5 km. These are surprisingly low values for the equivalent depth. more typical of the middle rather than upper atmosphere. The large vertical wavelengths do not necessarily rule out a gravity wave explanation because the distribution of vertical wavelengths of gravity waves shifts to larger scales with altitude (due to preferential saturation of small-scale upward propagating waves). Murayama et a/. ( 1992). for example. found that the dominant vertical scale of the gravity wave wind held in the mesosphere is 15-60 km. The case for an inertio-gravity wave explanation of the above observations is enhanced by the relatively infrequent appearance of these oscillatrons at all MLT heights and the incoherency between the two events. Furthermore. the negative phase between OH airglow brightness and temperature variations observed in the polar MLT region on Dec. 31, 1993.00 UT to Jan. 1. 1994. and on Dec. 20. 19 UT to Dec. 21. 18 UT. 1993. is in qualitative agreement with gravity wave theory. The gravity wave hypothesis can be tested by comparing the lower limit to the horizontal wavelength obtained using the full MSP scans (Fig. 4) with the horizontal wavelength derived from the dispersion equation for gravity waves. The dispersion relation of an inertio-gravity wave is given by
(6) where(r), is the intrinsic frequency,_f’the Coriolis parameter (equation (4)), and k, and k, the horizontal and vertical wave numbers. respectively (Andrews et trl.. 1987). The horizontal wave number can be derived from (7) Assuming (0, = (u,, and using the frequency and vertical phase speed obtained observationally for the Dec. 3 1 -Jan. km. which is not in 1 event, one gets i, = 7200+2700 disagreement with the observational limit of i., > 3400 km. The intrinsic frequency is Doppler-shifted by the background mean wind velocity u such that o, = w-k-u. A preliminary analysis of Fabry-Perot interferometer measurements of green line wind velocity over Eureka showed that the mean horizontal wind speed was 30 m s-. ’ during the above period (N. Rolheiser. personal communication. 1994). Thus the difference
1128
between the intrinsic and observed frequency is at most AW,,, = 2.9 x 10e5 rad s-‘. According to equation (7), the uncertainty in /b., due to Am,,, is no more than 1800 km. This also implies that the intrinsic period differs from the observed period by 1.5 h at most. The problem of the excitation mechanism remains. It is an especially acute issue for this study because five and three complete cycles were observed during the two events discussed here while gravity waves tend to be short-lived, i.e. one rarely observes more than one or two cycles at a time. Aurora1 zone activity was high during Dec. 31, 1993 (Anz= 37, ZK,,=26-) and Jan. 1, 1994 (Am = 39, CK, = 3 1). This suggests Joule and particle heating in the lower thermosphere as a possible source of the observed oscillations. Somewhat reduced aurora1 activity on Dec. 21, 1993 (Am = 30, CK, = 27 +) may have caused the smaller temperature variations on that day. Since no optical signatures of particle precipitation were observed over Eureka during either period, a forcing mechanism involving particle heating would require waves excited in the aurora1 zone to travel poleward into the polar cap. This could have triggered an inertio-gravity wave that was observed 1000-3000 km poleward of its excitation source. Unless ducting was involved, this is an unlikely source of the observed oscillations because the forcing of a gravity wave in the lower thermosphere should manifest itself in the downward/upward propagation of the energy/phase. The opposite was observed-the relative phase between the airglow brightness oscillations presented in Fig. 2 indicate a downward phase propagation. A few minor stratospheric warmings took place during the winter of 1993-94 (Naujokat et al., 1994). The strongest occurred at the end of December/beginning of January. During that event the temperature over the north pole at 30 hPa increased by more than 10°C relative to the daily 28 year mean between Dec. 31, 1993, and Jan. 5, 1994. The second-strongest stratospheric warming during that winter occurred in the middle of December. During that event the temperature over the north pole at 30 hPa increased by more than 10’ C relative to the daily 28 year mean between Dec. 16 and Dec. 2 1, 1993. All the warming pulses started over northeastern Asia and moved towards North America. Current theories suggest that the dynamical mechanism responsible for stratospheric warmings involves vertically propagating planetary waves that are forced by large disturbances in the troposphere. Indeed, during the strongest stratospheric warming of the season the planetary wave 2 peaked to a geopotential height amplitude of 600 m (at 60-N. 30 hPa) on Jan. 1, 1994. During the second-strongest stratospheric warming the planetary wave 1 peaked to a geopotential height amplitude of 1000 m on Dec. 21, 1993. The largest amplitude wave (AT = 19 +2 K) observed in the polar MLT region over Eureka during the 1993-94 winter occurred on Dec. 31, 1993, to Jan. 1, 1994. The second-strongest wave (AT = 13 i 2 K) occurred on Dec. 21. 1993. The temporal and relative amplitude match between the inertio-gravity waves reported here and the stratospheric warmings and forced planetary waves reported by Naujokat et al. (1994) is not likely accidental. Although the chain of cause and effect in planetary wave
I. Oznovich
et al. : Temperature
and airglow brightness
amplification and stratospheric warming development is not yet clear, simulations of sudden stratospheric warmings in the winter polar region indicate that these events are associated with a burst of gravity wave activity (Mahlman and Umscheid, 1987) and increased gravity wave forcing and drag (Rind rt [I/.. 1988). The very large horizontal scale of the Dec. 31-Jan. 1 wave (I,, b 7 x lo3 km) is in line with the large spatial scale of the planetary wave (&on;ll _ 1O4km at 60 ‘N) and stratospheric warming, which covered a major part of the Canadian Arctic.
5. Conclusions An important objective of the polar environment observatory in Eureka (80.O”N, 274.1”E) is to study the polar MLT region with high temporal and spatial resolution. The Eureka instrumentation used in this study included a Meridian Scanning Photometer measuring oxygen O[‘S] and sodium Na-D airglow brightness and a Michelson Interferometer monitoring the Meinel OH (3.1) band brightness and rotational temperature. The optical measurements provided mesopause temperature information (T,,,) as well as airglow brightness at approximately 85 (IOH). 89 (I,,), and 97 km (I,). The following observational results are noted : (1) Large amplitude (AT = 19 K) variations in airglow brightness and OH rotational temperature were observed on Dec. 31, 1993.00 UT to Jan. 1, 1994, 18 UT. The periods of Zou, INa. I,, and T,,, oscillations were 8.5, 8.7, 8.3, and 8.1 h, respectively, with a l-2 h resolving limit. It is highly unlikely that these variations were due to random fluctuations as they were measured at three different wavelengths by two independent instruments. (2) The change in IO preceded the change in IN;! by 0.9 h and the change in INa preceded the change in IOH by 0.4 h, indicating a downward phase speed of -2.6f 0.5 m ss’ and a vertical wavelength of i., = 79$18 km. The wave characteristics of the Meinel OH (3,1) band brightness and rotational temperature indicate that ]q] = 2.8kO.7 and 4 = - 12”+20’ for Krassovsky’s ratio. (3) An investigation of the horizontal wavelength of the wave was performed using all-sky scans of the rotating MSP. Two green line brightness scans of the sky for each of four look directions were processed, corresponding to peaks and valleys of the wave. The flatness of all the brightness scans across the 850 km distance indicate that the horizontal wavelength of the wave was greater than 3400 km. (4) Similar 8.5 h variations in airglow brightness and temperature were observed on Dec. 21, 1993. The Dec. 21 event was characterized by V. = -0.920.3 m s-‘, &=28$11 km, ]q] = 8.8k1.7, and $ = -107”f9”. There was a phase shift of 68” between this wave and that of Dec. 31-Jan. 1. The interpretation of airglow brightness and temperature oscillations based on analysis of the above observations is as follows : (1) Inertio-gravity
waves are rendered
most plausible
I Oznovich
c~fol. : Temperature
and airglow brightness
by the relatively infrequent appearance of these oscillations at all MLT heights and the incoherency between the two events. Temperature variations lead OH airglow brightness variations (negative phase for Krassovsky’s ratio), in qualitative agreement with predictions of the gravity wave theory. (2) The gravity wave hypothesis is further supported by the fact that the observed lower limit to the horizontal wavelength of the wave (i, > 3400 km) compares with its value predicted by the dispersion relation of inertio-gravi(y waves (2, = 7200+2700 km). (3) The observed periods (- 8 h), large vertical wavelengths, and persistence for three and five complete cycles all point to a non-migrating tide (zero zonal wave number) esplanation. On the other hand, the small amplitude and negative phase of Krassovsky’s ratio distract from such an explanation. The case for a non-migrating tide is further weakened by a large phase shift between the two waves. (4) The observed oscillations are not likely due to a tcr-diurnal pseudotide because the amplification factor of the tide at X0 N IS small (x = 1.X). Finally. there is a temporal and relative amplitude coincidence bet%,ern the inertjo-gravity waves reported here and forced planetary waves and stratospheric warming events over the Arctic (Naujokat et al., 1994). The planetary wave I of Dec. 2 1, 1993. and the planetary wave I! of Jan. I. 1994. as well as the stratospheric warming of the middle of December and end of December/beginning 01‘ January. were likely driven by the same tropospheric disturbance or directly related to the source of the gravit] \\ave obserl.ed over Eureka on Dec. 21 and Dec. 31.-Jan. I. respectively .3~.l
References Andrews, D. G., Ho&on, J. R. and Leovy, C. B., Middle atmospheric dynamics. in Intmmtional Geophysics Series. 40. Academic Press. New York, 1987. Ashburn, E. V., The effect of Rayleigh scattering and ground reflection upon the determination of the height of the night airglow. J. atmos. terr. PItys. 5, 83, 19.54. Collins, R. L., Senft, D. C. and Gardner, C. S., Observations of a 12-hr wave in the mesopause region at the south pole. Grophw. Res. Lett. 19, 57. 1992. Coxon, J. A., Optimum molecular constants and term values for X’IJ(r d 5) and A?Z+(r ,< 3) state of OH. Can. J. Phj,.v. 58, 933. 1980. Forbes, .J. M., Atmospheric tides 1. Model description and
I13 results for the solar diurnal component. J. yoph~\. Rcs. 87, 5327, 198%. Forbes, J. M., Atmospheric tides 3. The solar and lunar semidiurnal components. .I. geoph~~s. Res. 87, 5241. 198-h. Gardner, C. S., Senft, D. C. and Kwon, H. K., Lidar observations of substantial sodium depletion in the summertime Arctic mesosphere. Nature 332, 142, 1988. Greer, R. G. H., Murtagh, D. P., McDade, 1. C., Dickinson, P. H. G., Thomas, L., Jenkins, D. B., Stegman, J., Llewellyn, E. J., Witt, G., Mackinnon, D. J. and Williams, E. R., ETON 1 : a data base pertinent to the study of energy transfer in the oxygen nightglow. Planet. S~UL.PSci. 34, 771. 19X6. Hammond, M. R. and Espy, P. J., Optimal OH transitions in the near-infrared for rotational temperature determination. .I Q. Spectrosc..Rudiat. Tramffer. 1994. Hernandez, G., Smith, R. W. and Conner, J. F.. Neutral wind and temperature in the upper mesosphere aho~,r 40~1th pole. Antarctica. Gq$rs. Rrs. Lrtr. 19, 53. 1997. Hernandez, G., Fraser, G. J. and Smith, R. W., Mesospherlc I? hour oscillation near south pole. Antarctica. (;c+~,l~,. Kc,\. Letr. 20, 1787. 1993. Hunten, D. M.. Spectroscopic studie\ of the twilight a~rglow. Spucr .%i. Rer. 6, 493. 1967. Krassovsky, V. I., Infrasonic variations of OH emission In the upper atmosphere. .4nrz. Gropl~r.\. 28, 739. 1071. Longuet-Higgins, M. S., The eigenfunctions of Laplace‘s tidal equations over a sphere. Phil. Trtrm. R. SOC A262, 5 I I, 1468. Lopez-Moreno, J. J.. Rodrigo, R., Moreno, F.. Lopez-Puertas, M. and Molina, A., Altitude distribution of vibralionally excited states ofatmospheric hydrosyl at levels 1’= 2 to 1’= 7. PImet. Spew Sd. 35, 1039. 1987. Mahlman, J. D. and L’mscheid, L. J., Comprehensive modelling of the middle atmosphere : the influence of horizontal resolution. in Trun.vport Procr.ssr.s iv //IV Mid& .A/mu. (edited by G. Visconti and R. R. Garcia). p. 253. D. Reidel. Hingham. Massachusetts. 19S7. Murayama, Y., Tsnda, T., Yamamoto, M., Nakamura, T., Sato. T., Kato, S. and Fukao, S., Dominant vertical scales ofgravity waves in the middle atmosphere observed with the Ml’ radar and rocketsondes. J. trtn~os. (err. Ph!,r. 54. 330. lY92. Myrabo, H. K., Temperature variation at mesopause levels during winter solstice at 78 N. Plorwt. Spwc Sci. 32, 74Y. 19X4. Naujokat, B., Labitzke, K., Lenschow, R., Rajewski, B.. Wiesner, M. and Wohlfart, R.-C., The stratospheric winter 1993:Y4 : :I winter with some minor warmings and an early tinal warming. Bril. Bedirrrr W’ettKurte SO 24 94. I9Y4. Nelson, Jr, D. D. D., Schiffman, A. and Mesbit, D. J.. H +O, Fourier-transform infrared emission and laser absorption studies of OH(X%) radical : an experimental dipole moment function and state-to-state Einstein A coeffictents. ./. C’hc,r~. P/I\‘?. 93, 7003. 1940. Oznodich, I., Yee, R.. Schiffler, A.. McEwen. D. J. and Sofko, G. J., The all-sky camera revitalized. ,JIJ,& Opri~,.\ 33. 7141, 1994. Press, W. H. and Rvbicki, G. B., Fast algorithm for spectral analysis of unevenly sampled data. ,411../. 338, 277. 19X9. Rind, D., Suozzo, R. and Balachandran, N. K., The GISS global chmate+middle atmosphere model, part II : Model variability due to interactions between planetary waves. the mean circulation. and gra\Ity wave drag. J. .ilmo.~. SC,;. 45. 371. 1988. Rogers, J. W., Murphry, R. E., Stair, A. T., Ulwick, J. C., radiometric Baker, K. D. and Jensen, L. L., Rocket-borne measurements of OH in the aurora1 Tone. ./. qc,op/~,~.\. Km. 72. 7023. 1973. Silverman, S. M.. Night airglow phrnomenolopy. S/JU~.OSr,i. Rer. 11, 341. 1970. Sivjee, G. G., Nightglow hydroxyl emissions. Pllr~c,r. S/U!<(’ .%i. 40, 235, 1991. Sivjee, G. G. and Hamwey, R. M., Temperature and chemistry of the polar mesopause OH. J. ,qyrw~p/~~~,~ Rc.c. 92. 4663. 1987.
1130 Sivjee, G. G. and Walterscheid, R. L., Six-hour zonally symmetric tidal oscillations of the winter mesopause over the south pole station. Planet. Space Sci. 43,447, 1994. Sivjee, G. G., Walterscheid, R. L., Hecht, J. H., Hamwey, R. M. and Schubert, G., Effects of atmospheric disturbances on polar mesopause airglow OH emissions. J. geophy_s. Res. 92, 7651, 1987. Steele, D. P. and McEwen, D. J., Electron aurora1 excitation efficiencies and intensity ratios. J. geophys. Res. 94, 10,321, 1990. Takahashi, H., Sahai, Y., Clemesha, B. R., Batista, P. P. and Teixeira, N. R., Diurnal and seasonal variations of the OH (8,3) airglow band and its correlation with 01 5577 A. Planet. Space Sci. 25,541, 1977. Takahashi, H., Sahai, Y., Clemesha, B. R., Simonich, D. M., Teixeira, N. R., Lobo, R. M. and Eras, A., Equatorial meso-
I. Oznovich
et nl. : Temperature
and airglow brightness
spheric and F-region airglow emissions observed from latitude 4” south. Planet. Space Sci. 37,649, 1989. von Zahn, U., Goldberg, R. A., Stegman, J. and Witt, G., Doublepeaked sodium layers at high latitudes. Planet. Space Sci. 37, 657. 1989. Walterscheid, R. L., Sivjee, G. G., Schubert, G. and Hamwey, R. M., Large-amplitude semi-diurnal temperature variations in the polar mesopause : evidence of a pseudotide. Nuture 324, 347. 1986. Winningham, J. D. and Heikkila, W. J., Polar cap aurora1 electron fluxes observed with ISIS I. J. geop1tj.s. Res. 79, 949. 1974. Yee, J. H. and Abreu, V. J., Mesospheric 5577 8, green line and atmospheric motions--Atmospheric E.uplorer satellite observations. P/unrr. Space Sci. 35, 1389, 1987.
Pergamon
Sp/w Sci.. Vol. 33. No. 9, pp. I 171 I 130. 1995 Copyright t 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0032 #f33’95 $9.50+n.nn
0032-0633(95)00029-1
Temperature and airglow brightness oscillations in the polar mesosphere and lower thermosphere I.Oznovich,’
D.
J. McEwen’ and G. G. Sivjee’
Network for Space Research, ISAS, University of Saskatchewan. Saskatoon. Saskatchewan S7N 5E’. Canada Space Physics Research Laboratory, Embry Riddle Aeronautical University. 600 S. Clyde Morris Blvd.. Daytona Beach. FL 321 l4-.1900. II.S.A.
! Canadian
Received 32 August
1994; revised 30 November
1994; accepted
Abstract. Large amplitude oscillations in airglow brightness and OH rotational temperature were observed on Dec. 31, 1993, 00 UT to Jan. 1, 1994, 18 UT aver Eureka (80.O”N). The airglow brightnesses of atomic oxygen O[‘S] emission at 5577 8, and sodium Na (2P3,2,,j2)emission at 5890 and 58% 8, were measured by a multi-channel, meridian scanning photometer. The Meinel OH (3,l) band brightness and rotational temperature were monitored by a Michelson interferometer. The period of the observed oscillations was 8.4 + 1 h. Supporting evidence for a non-migrating tide (zero zonal wave number) explanation of these oscillations are an observed period close to 8 h, a large vertical wavelength as derived from the airglow emissions, and wave persistence for five complete cycles. On the other hand, the small amplitude and negative phase of Krassovsky’s ratio are not consistent with a tidal source for the observed oscillations. Similar 8.5+ 1 h variations were observed on Dec. 2 1,1993. The case for a non-migrating tide as a source. for these oscillations is further weakened by a 1.5 h difference in local time when these oscillations were observed to peak during the two events (which occurred 10 days apart). Explanation of the above observations in terms of an inertiogravity wave is favored by the relatively infrequent appearance of these oscillations at all altitudes monitored, the incoherency between the two events, and the fact that temperature variations lead OH airglow brightness variations, in qualitative agreement with gravity wave theory. The observed lower limit to the horizontal wavelength of the wave Gompares with its value predicted by the dispersion relation of inertiogravity waves. Forced planetary waves, observed to peak in amplitude on Dec. 21, 1993, and Jan. 1, 1994, as well as the stratospheric warming of the middle of December and end of December/beginning of January, were likely driven by the same tropospheric disturbance or directly related to the source of the inertia-gravity wave observed over Eureka on Dec. 21 and Dec. 31Jan. 1, respectively.
25 January
1995
1. Introduction The winter polar cap is a unique laboratory for studying waves in the Mesosphere and Lower Thermosphere (MLT). Continuous darkness at high latitudes (101 >, X0 ) affords uninterrupted 24 h ground-based measurements of optical nightglow emissions, emanating from different height regimes of the MLT, for over three months. Such measurements permit investigations of the characteristics of planetary, tidal and gravity wave disturbances propagating through the atmospheric region extending from about 83 km to roughly 100 km. The only hindrances are weather conditions and difficulties associated with logistics and equipment maintenance in the harsh polar environment. Void of local solar heating. theoretical considerations suggest that the winter polar region is relatively weakly disturbed by tides. All the modes. except the zonally symmetric non-migrating tide (zonal wave number s = 0). at-e expected to vanish at the poles (Forbes. lY82a,b). Indeed. recent observations of 6 h oscillations of the Meinel OH (3,l) band brightness and rotational temperature over the south pole were interpreted as possible manifestations of the effects of a zonally symmetric tide (Sivjee and Walterscheid, 1994). Superimposed hourly means of the polar mesopause temperature for a 19 day period around winter solstice at 78’ N revealed a semi-diurnal trend with an amplitude of 5 K (Myrabo, 1984). Walterscheid c’t al. (1986) measured larger semi-diurnal oscillations of the mesopause temperature (I 3 K) at 78 N in winter. Their theory of a semidiurnal pseudotide. generated by gravity wave momentum fluxes modulated by the solar semi-diurnal tide, predicted mesopause temperature variations of the order of 10 K. An interpretation of a semi-diurnal pseudotide was also given to the Na lidar observation of a 12 h variation in the sodium density profile over the winter south pole (Collins et al., 1992). Hernandez et rll. (1992) suggested that some 12 h variations in the mesopause neutral wind and temperature over Antarctica were caused by ducting of lower-latitude tidal oscillations. Variations in the nightglow OH band emission in the north polar region with a period of 3.7 h (Sivjee it ul.. 19X7) and over the south
1122
pole with a period of 12 h (Hernandez et al.. 1993) were interpreted as manifestations of inertio-gravity wave activity. In the absence of obvious forcing mechanisms, the interpretation of the observed variations in polar MLT density and temperature with periods shorter than those of planetary waves is far from clear. Temporal information is necessary to identify the wave frequency. Vertical and horizontal information can be integrated to assess the propagation direction and help identify the nature of the wave. Both kinetic temperature fluctuations and airglow brightness variations must be observed in order to build a reliable theoretical model of the dynamics and chemical processes of the MLT region. In this study of wave activity over Eureka (80.O”N) on new year’s eve 1994, an effort has been made to synthesize observations of parameters of the MLT region that relate to all of the above. Section 2 describes the Eureka observatory, instruments used, their mode of operation, and the data reduction techniques. Section 3 presents 8.4 h oscillations of mesopause temperature and airglow brightness at three altitudes. Allsky scans of the airglow brightness are utilized to assess an observational lower limit on the horizontal wavelength of the wave. Similar oscillations observed on Dec. 21, 1993, as well as Krassovsky’s ratio (1972), are used in Section 4 to test critically the identification of the wave as either a non-migrating ter-diurnal tide or as an inertiogravity wave. The horizontal wavelength of the wave is deduced via the dispersion relation of inertio-gravity waves and compared with its observed lower limit. Possible relations of the waves reported here to forced planetary waves and minor stratospheric warming events of the middle of December and end of December/beginning of January are noted.
2. Eureka station, detectors, and data reduction An observatory very close to the north magnetic pole has been in operation since the winter of 1990-91 at Eureka, Northwest Territories, Canada. Eureka is located at geographic coordinates SO.O’N, 274.1 “E, PACE geomagnetic coordinates 88.9”N, 284.1 ME.Airglow instrumentation in the polar observatory of the Canadian Network for Space Research was run by the Polar Environment Project. One of the major objectives of the project was a half-solar cycle study of the polar MLTregion with much higher temporal and spatial resolution than could be achieved from space or by sporadic ground-campaigns. The Eureka instrumentation used in this study included a multi-channel Meridian Scanning Photometer (MSP). a Michelson Interferometer (MI), and an All-Sky Camera (ASC). The MSP was used to monitor oxygen O[‘S] and sodium Na (‘Pj/,,,:,) airglow emissions. The MI afforded measurement of the Meinel OH (3,l) band brightness and rotational temperature. ASC images were used to monitor cloud coverage and aurora] contamination of the airglow data. The instantaneous field-of-view (TFOV) of the MSP was 1 The measured spectral full width at half maximum of the filters was 4.1 A at 5577 A and 10.1 8, at 5896 A.
1. Oznovich
ct a!. : Temperature and airglow brightness
During the 1993-94 winter season the scan direction of the MSP was rotated 5 every 20 min to align it perpendicular to the sun+arth direction. The period of one scan (usable 79 to -79” zenith angle in steps of I ) was 32 s. with an additional mirror fly-back time of 6 s. The identification of wave activity over Eureka was based on zenithal airglow brightness measurements. The full scans were used in the investigation ofthe horizontal wavelength of the wave. Of the six channels recorded, O[‘S] at 5577 A (green) and Na (‘7Pj,2,,,Z) at 5896 and 5890 8, (unresolved sodium doublet) were utilized in this study. A filter 100 p\ wide at 5145 A was used to monitor background emission levels. The red oxygen line O[‘D] at 6300 A served as a monitor of dissociative recombination of 0; ions in the F-region. The raw count rate was converted to absolute brightness using photometer sensitivities derived from field calibrations and corrected for the Van Rhijn effect, ozone extinction, and ground albedo (Ashburn, 1954). First-order background correction was applied to both O[‘S] and Na-D emissions using the background channel brightness adjusted by the appropriate Rayleigh scattering factor. In order to reduce noise and match the temporal resolution of the MI, approximately 16 zenithal measurements were averaged to produce one data point every 10 min. The MSP filter characteristics and photometer sensitivities were published by Steele and McEwen (1990). The Eureka MI was a BOMEM MB160 with a low noise (NEP _ 2 x lOp’5 W) InGaAs detector modified fol emission measurements in the 1.0-1.7 pm wavelength region (Sivjee and Walterscheid, 1994). Its resolution could be set at any one of eight different values in the range l-128 cm-’ (in binary steps). At 2 cm-’ resolution its IFOV was 1.7 and the corresponding throughput 3.7 x IO-’ cm’ s. The MI completed one interferogram scan every 3.5 s. Approximately 170 such interferograms were coadded to derive a real-time spectrum by FFT every 10 min. The spectra were corrected for variations in the spectral sensitivity of the MI using a variable aperture black body source operating at 1273 K and illuminating a Lambertian surface placed 1 m away from the black body. This calibration also allowed inversion of the MI detector signal to absolute spectral brightness of the airglow emission. Correction for atmospheric extinction was applied with the USAF’s FASCODZ using the procedure described by Hammond and Espy (1994). The brightness and rotational temperature values of the airglow Meinel OH (3,l) band were determined through detailed least-squares fit of the OH (3,l) band rotational lines to their synthetic profiles constructed for different rotational distribution temperatures. The OH vibrational-rotational transition probabilities of Nelson et al. (1990) and the rotational term values of OH (2.’ = 3) of Coxon (1980) were used. Procedures for the inversion of OH rotational line emissions were detailed by Sivjee (1992). The chosen transition probabilities determine the absolute rotational temperature deduced from the OH emissions. However, different transition probabilities result in the same relative temperature change ATiT pre. sented here. The MI was pointed 25’ above the northern horizon and recorded mesopause airglow emissions from a region corresponding to 81.6 N latitude. Temporally-
I Oznovich PI ul. : Temperature and airglow brightness coincident MSP and MI data presented here were spatially separated (in the horizontal direction) by - 180 km. The ASC used a low light level intensified siliconintensifier target with an automatic gain control. Analog TV fields were recorded in a time-lapse mode. registering one filter sequence approximately every 18 s. The broad-band visual, green line (5577 WI. and red line (6300 A) filters nere alternated every 6 s. The detection threshold of the ASC at broad-band visual light was approximately 0.3 kR. The ASC system and its data reduction algorithms were described in detail by Oznovich er al. (1994). The temporally-coincident data of the Eureka MSP and MI for the 1993-94 season span three months (November 1993-January 1994). A search was performed for continuous (24 h) good viewing conditions during that period using weather observations and ASC images. Snow, fog. ice crystals, and cloud coverage were reported by the Eureka weather officer on duty on an hourly basis. Discrete aurora1 arcs were readily identified in the broad-band visual and green line images of the ASC. This study concentrates on the discovery of significant variations in temperature and airglow brightness around new year’s eve 1994. Excellent viewing conditions prevailed during the period Dec. 31. 1’193. 00 UT to Jan. 1. 1994. 18 UT. Aurora] arcs drifted in less than 1 min over the zenith of the Eureka MSP on Jan. I. 1994. at 08 : 43,09 : 14. and 15 : 33 UT. These three samples were removed from the MSP data-set. Other than discrete arcs. diffuse aurora1 emissions are also likely to contribute to the measured green line brightness. Low brightness (d 500 R) emissions that cover a major fraction of the sky will not be detected by the Eureka ASC. and are not necessarily discernible in the MSP scan across the sky. One can hypothesize that feeble emissions ol‘the green line (O[‘S] at 5577 A) detected in the polar cap are primarily due to polar rain precipitation of low energy (-:200 eV). low energy flux (
600 r
q()$-------
10” lo’* Energy Flux [ergs/s cm?
Fig. 1. Polar cap green line (circles) and blue line (triangles) brightness measured by the Eureka MSP and the coinciding electron energy flux measured by the electrostatic analyzer aboard the DMSP satellite. The brightness of’ the blue line emission was multiplied by 10
I I’3
coincidences of the MSP scan and DMSP ground track that occurred between Dec. 1990 and Dec. 1991 near Eureka. The linear coefficient of correlation between the green line (marked as circles) and blue line (marked as triangles) brightness and the electron energy flux is - 0.16 and -0.02. respectively. The energy flux that is associated with these emissions is predominantly less than 0.03 erg cm-?sml, Therefore the expected green line brightness due to electron precipitation is at most 50 R (using a green line excitation efficiency of 1.73 kR (erg cm ’ s ‘) _’ (Steele and McEwen. 1990)). The green line brightness presented here is in the range 100-450 R. Thus one is led to the conclusion that diffuse aurora1 emissions did not significantly contaminate the green airglow data shown here.
3. Results Most of the green line emission originates from the lower thermosphere at 95-100 km where the concentration of atomic oxygen is at maximum (Greer ri trl.. 1986; Yee and Abreu. 1987). A small fraction of it originates from the F-region at 250- 300 km and is discussed at the end ot this section. The Na doublet emission is produced in an oxidation-reduction cycle with ozone and atomic oxygen in the altitude range 83.-95 km and peaks near X9 km (Hunten. 1967; von Zahn rral., 19X9; Collinsrl N/., 1993). The Meinel OH (3.1) band emission originates from the mesosphere in the altitude range 82-90 km (Rogers c’t ~1.. 1973 ; Lopez-Moreno et ui., 1987 : Sivjee and Hamwey, 1987). The distribution among the rotational lines of OH affords a calculation of its rotational temperature T,,,,. Since the OH radicals undergo - 10 collisions before radiating their excess energy, their distribution among the rotational states is thermalized to the ambient kinetic temperature (Walterscheid ~‘1r/l.. 1986). In summary, the optical measurements presented here provide mesopause temperature information (T,,,) and airglow brightness peaking approximately at 85 (l,,,). X9 (IN,,). and 97 km (I&. It should be noted that most of the altitude profile measurements of airglow emission rate and constituent density were performed at latitudes lower than the polar region. There are differences in peak OH emission height for different vibrational-rotational bands (Sivjee and Hamwey. 1987). More importantly. the vertical distribution of atomic oxygen concentration effects the peak emission height of all lines and bands discussed here in both production and deactivation mechanisms. Atomic oxygen is particularly variable in the polar region. However. it is the difference in peak emission height that determines the vertical phase speed and wavelength of the wave. and this could be maintained approximately constant for a general expansion or contraction of the upper atmosphere. Moreover. the usage of three airglow emissions (two peak emission height differences) reduces the uncertainty in the derived wave characteristics. Figure 2 shows large amplitude variations in airglow brightness (a) and OH rotational temperature (b) observed on Dec. 31, 1993,OO UT to Jan. 1. 1994, IX UT. Local midnight in Eureka is at 05:49 UT. Figure ?a depicts the Meinel OH (3.1) band brightness IO,, (solid
I. Oznovich er al. : Temperature and airglow brightness
1124
50~,.,.,,.,,,,..,,“_,~“’
z
0
260
0
s! 2 230 g
F
200
I
L)
00 Dee 31
L
’
06
12
1,.
18
UniversalTime
[hr]
‘.
1
1,
I
00
06
12
18
Jan I
Fig. 2. Oscillations in airglow brightness (a) and OH rotational temperature (b) observed over Eureka (SO’N) on Dec. 31, 1993, 00 UT to Jan. 1, 1994, 18 UT. The Meinel OH (3,1) band brightness Iou is marked by a solid line. The Na (‘P,,& line brightness I,, is traced by a dotted line. The O[‘S] green line
brightness I,, is shown by a dashed line. Airglow brightness is in Rayleighs for I,, and I, and in kilo-Rayleighs for I,,
2
INa (dotted line), and P3~2.,~2 ) line brightness O[‘S] line brightness IO (dashed line). Airglow brightness is in Rayleighs for I,, and Io and in kilo-Rayleighs for IOn. Figure 2b shows the OH rotational temperature T,,,. Five complete cycles of both airglow brightness and temperature variation as measured by the Eureka MSP and MI are evident from the data. The variations were measured by two independent instruments, at three different wavelengths, and are exhibited in both the brightness and the temperature data. Therefore it is highly unlikely that these variations were due to random statistical fluctuations. The variations in brightness and temperature shown in Fig. 2 reflect the modulation in concentration of major and minor gas constituents and of temperature by wave disturbances propagating through the atmosphere in the 80-100 km altitude region (Sivjee et al., 1987). The vertical phase propagation is apparently downward as the variations appear first at 95-100 km (as manifested by Zo), then at 85-90 km (as manifested by the sodium and hydroxyl emissions). The peak-to-peak change in IoH is about a third of the average brightness ; the peak-to-peak change in T,,, is - 10% of the average temperature. Therefore the amplitude of Krassovsky’s ratio ( 1972) is approximately 3. Changes in T,,, slightly precede those in Z,,, indicating a small negative phase for Krassovsky’s ratio. A quantitative LombScargle analysis (Press and Rybicki, 1989, and references therein) was performed in order to derive the frequency content of the data shown in Fig. 2. The resultant normalized power spectrum of in Fig. 3 by a bold T,,,, IOH, INS, and I0 is represented solid line, a solid line, a dotted line, and a dashed line. respectively. The horizontal long-dashed line indicates a normalized power level of 10.2, corresponding to a significance level of P = 0.01 (99% confidence level) ; a peak that lies above this line has a less than 1% probability P that it is due to random Gaussian noise. Three groups of statistically significant (> 99% confidence level) peaks line),
$
Na
(
Fig. 3. Normalized LombScargle power spectra of r,,, (bold solid line), lou (solid line), ZNa(dotted line), and Io (dashed line) for the data shown in Fig. 2. Peaks that lie above the horizontal long-dashed line have less than 1% probability to be due to random noise
appear in Fig. 3. The highest frequency group corresponds to a period of 6.6 h in I,, 5.9 h in IN,. and 5.9 h in IoH. This variation is most likely due to a 6 h tidal oscillation similar to that reported by Sivjee and Walterscheid (1994). The lowest frequency group corresponds to a period of 20 h in lo and 26 h in Z,,. This daily oscillation is due to imperfect background subtraction as it appears only in the visual channels and varies in phase with lunar elevation (maximum 1 h past local midnight). The highest peak in all three emissions and the temperature data corresponds to a period of 8.4 h. This result was also confirmed by a Fourier analysis. A least-squares analysis of the data revealed the average brightness (0 and temperature (T), its peak-to-peak variation (AI and AT). and the absolute phase of the wave, denoted by the local time of the wave maxima nearest midnight in Table 1. The wave characteristics were determined after removing a linear trend from the data. The observed average temperature i= = 228 K and its peak-topeak variation AT = 19 K are higher than those of Myrabo (1984) (F= 220 K and AT = 10 K) and those of Walterscheid et al. (1986) (T= 219 K and AT= 13 K). In fact, the data shown in Fig. 2 represents the largest cyclic variation in mesopause temperature observed over Eureka during the 1993-94 winter season. The relatively high sodium brightness is noted (&,, = 286 R). Such high levels of Na-D emission (200300 R) prevailed throughout the winter till February 1994, when it dropped to 50-100 R. Similar high levels of sodium brightness were observed over Eureka during the winters of 1991-92 and 1992293. The level of sodium emission over the winter polar cap may be high due to a combination of latitudinal (Takahashi et ~1.. 1989) and seasonal (Gardner et al., 1988) effects. Table 1 also shows the frequency corresponding to the peak power of the three emissions and the OH rotational temperature. The equivalent period of I,,. INil, I,. and T,,, is 8.5, 8.7, 8.3, and 8.1 h, respectively. The resolving limit in periodicity is 1-2 h. The change in I0 preceded the change in IN;, by 53 min and the change in lNa preceded the change in IOH by 24 min indicating a downward propagating phase. The vertical phase velocity of the wave is -2.5 m s-’ as deduced from IO and I,, and -2.8 m ss’ as deduced from I,, and IOn. Assuming a 2 km uncertainty in the peak emission height difference results in an average
1 Oznovich
ct cl/. Temperature
and airglow brightness
I125
Table I. Wave characteristics of airglow brightness and rotational temperature observed over Eureka during Dec. 3 1. 1993,OO UT to Jan. 11 1994, 18 UT. The
average brightness (n and temperature (F), its peak-to-peak variation (A/ and AT). the frequency at peak spectral power, and the wave phase (local time of maximum nearest midnight) (‘P, >, >), and O[‘S] Average IT
are for the airglow
Variation AI, AT
emissions
Frequency in h-’
of OH (3.1), Nn
Local time of maximum
_
179 kR 286 R 340 R 218 K
42+6 kR 63k4 R 118k14 R 19i2 K
0.117+_0.017 0.115+0.015 0.121~0.010 0.123_t0.012
01:39-&00:25 01:15f00:08 00:22f00:16 01:32-J00:l6
vertical phase velocity rZ = - 2.6kO.5 m s-‘. The horizontal spacing between the regions sampled by the MSP and MI are not expected to be important here because of the large horizontal wavelength of the wave (to be discussed later). Using the definition of the observed vertical phase speed r’: = W,,%;
(1)
where w,, is the observed frequency and k, the vertical wave number. we calculate the vertical wavelength i, to be 79& 18 km. Krassovsky’s ratio (I 972) q is a complex quantity summarizing all the dynamical and chemical processes that connect the wave forcing. as manifested by kinetic temperature fluctuations. to radiative energy dissipation, as manifested by variations in airglow brightness (Sivjee and Walterscheid, 1994). Its amplitude Iil( is given by
4oo
___/,___j E__/-----------.-_...__
300 200 ; ,uu’
(d) SE-NW
1 1 / -400 -300 -200 -100
L_u_1i 0 100
i---1 200
300
400
Distance from Eureka [km] 7zEi
(2) and its phase 4 by the relative phase of the fractional brightness and temperature oscillations. The wave characteristics of the Meinel OH (3.1) band brightness and rotational temperature summarized in Table 1 indicate that 1~) = 2.8$0.7 and 4 = - 13”+20’. The quantitative values of 111)and C/Jare in agreement with those estimated above by a visual inspection of Fig. 2. An investigation of the horizontal wavelength of the wave was conducted. It was facilitated by the fortuitous fact that the observed frequency was not a harmonic of the MSP rotation frequency and by the observation of five full cycles of the wave. Figure 4 shows two green line brightness scans of the sky for each of the four principal directions south-north (a). east-west (b), southwest-northeast (c). and southeast-northwest (d). The coverage of four look directions assures that the angle between the horizontal propagation direction of the wave and one of these directions is at most 22.5 The two scans correspond to times near maxima (solid line) and minima (dotted lint) in the green lint brightness plot of Fig. 2. The abscissa represents great circle distance from Eureka at an altitude of 97 km. The variation in all the green line brightness scans across the 850 km distance is less than half the total variation between maxima and minima, indicating that the MSP observed less than a quarter of the horizontal wavelength of the wave at any one time. The conclusion
Minima j
Fig. 4. Two O[‘S] green line (5577 A) brightness scans ofthe sky at look directions southhnorth (a), west-east (h). southwest northeast (c). and southeast-northwest (d). The two scans carrespond to times near maxima (solid line) and minima (dotted line) in the green line brightness plot of Fig. 2. Variations due to the 8 h wave should be manifested in changes from a minima to a maxima in one or at most two of the look directions
is that the horizontal wavelength of the wave i., was greater than 3400 km. This result was further confirmed by similar scans of the sodium line brightness. One technique to correct for the contribution from Fregion dissociative recombination of 0: ions to the column integrated green line emission is to subtract a certain fraction of the red line brightness from that of the green line (Takahashi et al., 1977). This fraction could be as much as 20% in tropical regions (Silverman, 1970). The green line brightness shown in Fig. 2 was compared with that subtracted by IO and 20% of the concurrent red line brightness. The resultant variations in green line brightness were 30-60 R. The corresponding green line power spectra were statistically identical to that shown in Fig. 3, implying that dissociative recombination of 0, ions in the F-region did not significantly contribute to the green line data shown here.
I. Oznovich et al. : Temperature and airglow brightness
1126
4. Discussion It is somewhat problematical to attribute the large amplitude variations in the MLT temperature and airglow brightness observed over Eureka to low-order tidal oscillations of the atmosphere since their predicted amplitudes are very small in the polar region at winter solstice (Forbes, 1982a,b). High-order solar-driven modes potentially have significant amplitudes at high latitudes. but the forcing problem remains-the high-order modes are only weakly excited by the global solar forcing, and local solar forcing is absent in winter. It is also difficult to assign the observed oscillations to a ducted high-order ter-diurnal tide, similar to the ducting of a semi-diurnal wave from low latitudes to the south pole (Hernandez et ul., 1992), because very high-order modes of the tide are strongly absorbed by dissipative processes in the lower thermosphere. The period of the observed oscillations (8.4+ 1 h) is sufficiently different from the inertial period (12.2 h at 80 N) to exclude a free inertial wave explanation. Interactions between gravity waves and the mean flow are modulated by harmonics of the solar-driven tide. Therefore it is possible that the observed oscillations were due to a ter-diurnal pseudotide. The observed large-amplitude variations in the polar MLT region can be explained by a pseudotide only if there is a large factor for the amplification of the tide, since the amplitude of the solardriven tide itself is very small at 80-N. For example, the amplification factor r is 23 for a semi-diurnal tide at 78’~N (Walterscheid et al.. 1986). The amplification factor is given by
co2 x=---If2_J I where w is the frequency
of the tide and
f’= 20sin8
(4)
is the Coriolis parameter. R is the angular velocity of Earth and 0 the latitude. In our case tc) = 3Q and the amplification factor is 1.8, making the argument for a pseudotide a weak one. The two most likely explanations of the -8 h oscillations presented above are therefore a non-migrating tide and an inertio-gravity wave. We use the term inertiogravity wave rather than a pure internal gravity wave because the large horizontal wavelength of the wave (1, > 3400 km) dictates that it will be influenced by the rotation of the Earth. The only mode of the classical, solar-driven tide that has a significant amplitude at 80”N is the non-migrating tide (zero zonal wave number). The variations in temperature, divergence. vertical velocity, and major gas density of a non-migrating tide have their maximum amplitude at the poles (Longuet-Higgins, 1968). Supporting evidence for a non-migrating tide explanation of these oscillations are an observed period close to 8 h, a large vertical wavelength (i., = 79 + 18 km), and wave persistence for five complete cycles. However, in the absence of local solar heating there remains the problem of the forcing mechanism. Moreover. the theoretical calculations of Sivjee and Walterscheid (1994) show that for
a non-migrating tide the OH airglow brightness modulations lead the mesopause temperature modulations (positive phase for Krassovsky’s ratio). In contrast a negative phase (4 = - 12“ +_20”) was found here. If a non-migrating ter-diurnal tide prevails in the winter polar region, one might expect to observe it at times other than new year’s eve 1994. If other events that exhibit 8 h oscillations were indeed observed, a phase coherency between the different events would support a tidal explanation. A day-by-day search for - 8 h oscillations in airglow brightness and temperature was performed for the 1993-94 mid-winter season (November 1993- January 1994). During the 83 days of coincident MSP-MI data. approximately 60% of the time clear weather conditions prevailed over Eureka. During these clear days. only one additional instance of unambiguous 8 h oscillations in temperature and all airglow emissions was found. This fact by itself implies that a ter-diurnal oscillation in the polar MLT region is not a frequent phenomenon at all altitudes and weakens the case for a non-migrating tide. Variations in airglow brightness (a) and OH rotational temperature (b) are shown in Fig. 5 for the period Dec. 20, 19 UT to Dec. 21, 18 UT, 1993. Figure 5a shows the Meinel OH (3,l) band brightness I,, (solid line), Na (‘P 3,Z,,.,) line brightness INa (dotted line), and O[‘S] line brigh‘tness I, (dashed line). The data shown was smoothed with a running 3 h window to emphasize the relative phase between the different emissions. Three complete cycles of airglow brightness and temperature variation are evident from Fig. 5. The main characteristics of the Dec. 21 event, as summarized in Table 2, are very similar to those of the new year’s eve event. The period corresponding to the peak power in the Dec. 31 LombScargle spectrum (Fig. 6) of I,,,, INd, I,,, and T,,, is 9.0, 8. I, 9.0, and 8.1 h, respectively. The vertical phase propagation is downward as the change in I(, (97 km) leads the change in Z,, (89 km) by 2.5 h and the change in I,, (89 km) leads the change in ZOH(8.5 km) by 1.3 h. The vertical phase speed of the wave as deduced from the relative phase between I, and ZNais in excellent agreement with that deduced from the relative phase
I’
1
”
1
1
I
,-~
z2,0/,/ / 18
__I
1
00 Dec21 06 Universal Time
12
18
[hr]
Fig. 5. Same as Fig. 2 for the period Dec. 20. 19 UT to Dec. 2 1, 18 UT, 1993. The sodium brightness I,, was divided by 2. The data shown was smoothed with a running 3 h window to emphasize the relative phase between
the different
emissions
I. Oznovich
PI ol. Temperature
and airglow brightness
Table 2. Same as Table 1993
I OH I ‘r.8 I r::,
1127
1 for the period
Average - I, T
Variation AI, AT
156 241 141 223
80+3 kR 81&-8R 49k IO R 13k2 K
kR R R K
50 5 40 5
30
w Yi 20 E P
P = 0.01
10
0.05
0.00
0.10
0.15
0.20
0.25
Frequency [hi']
Fig. 6. Same as Fig. 3 for the period Dec. 20, I9 UT to Dec. 2 I. 18 UT. 1993
hetween I,, and IOH (II = -0.9kO.3 m s-l). The resultant vertical wavelength (equation (I)) is ;I; = 28 + 11 km. The average temperature (T = 223 K) and its peak-topeak variation (AT = 13 K) are very similar to those of Walterscheid it al. ( 1986) (T = 219 K and AT = 13 K). It is evident from Fig. 5 that changes in T,,, precede those a negative phase for Krassovsky’s ratio. in IOH. indicating Indeed, the wave characteristics of the Meinel OH (3,l) band brightness and rotational temperature of Table 2 indicate that 1~1= 8.8f 1.7 and 4 = - 107’+9 ‘. Although $I is negative for the Dec. 31-Jan. 1 event, its large uncertainty (due to statistically-significant peaks in the IoH power spectrum other than that at 8.5 h) makes it difficult to compare with theoretical predictions for Krassovsky’s ratio. The values of 1’11and r#~for the Dec. 21 event are certainly closer to those predicted by theory for gravity waves than those predicted by theory for nonmigrating tides (Sivjee and Walterscheid. 1994). Unless non-migrating tides are triggered by transient phenomena, the case for non-migrating tides here is somewhat weakcned by the incoherency of the two events. The local time of maximum nearest midnight for the Dec. 21 event precedes that of the Dec. 31-Jan. 1 event by 1.3, 1.5. and - 1.1 h for T,,,. lo. and IOH, respectively. This corresponds to a maximum phase shift of 68,’ for a ter-diurnal wave. It is unlikely that a seasonal drift of the phase of the tide caused such a large shift, as the two events are separated by only 10 days. The tidal hypothesis can be further explored by observing that the solution for the inhomogeneous vertical structure equation of the tide in the regime 0 < h < 4N2H2/,q is 7 kf =g--
1 (5)
4H’ where
h is the
equivalent
depth,
N
the
buoyancy
Dec. 20. 19 UT to Dec. 21. 18 UT.
Frequency inh-’ 0.11 I * 0.020 0.124~0.021 0. I I I f 0.020 0.11410.017
Local time of maximum 02:42 i 00:08 01:22~00:lh 22:55fOO:lh 00:10f00:08
frequency, and H the scale height (Andrews et al., 1987). For i., = 79+ 18 km (Dec. 31--Jan. 1) it is found that h=3.1~0.7km.Fori,=38f11km(Dec.21)itisfound that 11= 0.8+0.5 km. These are surprisingly low values for the equivalent depth. more typical of the middle rather than upper atmosphere. The large vertical wavelengths do not necessarily rule out a gravity wave explanation because the distribution of vertical wavelengths of gravity waves shifts to larger scales with altitude (due to preferential saturation of small-scale upward propagating waves). Murayama et a/. ( 1992). for example. found that the dominant vertical scale of the gravity wave wind held in the mesosphere is 15-60 km. The case for an inertio-gravity wave explanation of the above observations is enhanced by the relatively infrequent appearance of these oscillatrons at all MLT heights and the incoherency between the two events. Furthermore. the negative phase between OH airglow brightness and temperature variations observed in the polar MLT region on Dec. 31, 1993.00 UT to Jan. 1. 1994. and on Dec. 20. 19 UT to Dec. 21. 18 UT. 1993. is in qualitative agreement with gravity wave theory. The gravity wave hypothesis can be tested by comparing the lower limit to the horizontal wavelength obtained using the full MSP scans (Fig. 4) with the horizontal wavelength derived from the dispersion equation for gravity waves. The dispersion relation of an inertio-gravity wave is given by
(6) where(r), is the intrinsic frequency,_f’the Coriolis parameter (equation (4)), and k, and k, the horizontal and vertical wave numbers. respectively (Andrews et trl.. 1987). The horizontal wave number can be derived from (7) Assuming (0, = (u,, and using the frequency and vertical phase speed obtained observationally for the Dec. 3 1 -Jan. km. which is not in 1 event, one gets i, = 7200+2700 disagreement with the observational limit of i., > 3400 km. The intrinsic frequency is Doppler-shifted by the background mean wind velocity u such that o, = w-k-u. A preliminary analysis of Fabry-Perot interferometer measurements of green line wind velocity over Eureka showed that the mean horizontal wind speed was 30 m s-. ’ during the above period (N. Rolheiser. personal communication. 1994). Thus the difference
1128
between the intrinsic and observed frequency is at most AW,,, = 2.9 x 10e5 rad s-‘. According to equation (7), the uncertainty in /b., due to Am,,, is no more than 1800 km. This also implies that the intrinsic period differs from the observed period by 1.5 h at most. The problem of the excitation mechanism remains. It is an especially acute issue for this study because five and three complete cycles were observed during the two events discussed here while gravity waves tend to be short-lived, i.e. one rarely observes more than one or two cycles at a time. Aurora1 zone activity was high during Dec. 31, 1993 (Anz= 37, ZK,,=26-) and Jan. 1, 1994 (Am = 39, CK, = 3 1). This suggests Joule and particle heating in the lower thermosphere as a possible source of the observed oscillations. Somewhat reduced aurora1 activity on Dec. 21, 1993 (Am = 30, CK, = 27 +) may have caused the smaller temperature variations on that day. Since no optical signatures of particle precipitation were observed over Eureka during either period, a forcing mechanism involving particle heating would require waves excited in the aurora1 zone to travel poleward into the polar cap. This could have triggered an inertio-gravity wave that was observed 1000-3000 km poleward of its excitation source. Unless ducting was involved, this is an unlikely source of the observed oscillations because the forcing of a gravity wave in the lower thermosphere should manifest itself in the downward/upward propagation of the energy/phase. The opposite was observed-the relative phase between the airglow brightness oscillations presented in Fig. 2 indicate a downward phase propagation. A few minor stratospheric warmings took place during the winter of 1993-94 (Naujokat et al., 1994). The strongest occurred at the end of December/beginning of January. During that event the temperature over the north pole at 30 hPa increased by more than 10°C relative to the daily 28 year mean between Dec. 31, 1993, and Jan. 5, 1994. The second-strongest stratospheric warming during that winter occurred in the middle of December. During that event the temperature over the north pole at 30 hPa increased by more than 10’ C relative to the daily 28 year mean between Dec. 16 and Dec. 2 1, 1993. All the warming pulses started over northeastern Asia and moved towards North America. Current theories suggest that the dynamical mechanism responsible for stratospheric warmings involves vertically propagating planetary waves that are forced by large disturbances in the troposphere. Indeed, during the strongest stratospheric warming of the season the planetary wave 2 peaked to a geopotential height amplitude of 600 m (at 60-N. 30 hPa) on Jan. 1, 1994. During the second-strongest stratospheric warming the planetary wave 1 peaked to a geopotential height amplitude of 1000 m on Dec. 21, 1993. The largest amplitude wave (AT = 19 +2 K) observed in the polar MLT region over Eureka during the 1993-94 winter occurred on Dec. 31, 1993, to Jan. 1, 1994. The second-strongest wave (AT = 13 i 2 K) occurred on Dec. 21. 1993. The temporal and relative amplitude match between the inertio-gravity waves reported here and the stratospheric warmings and forced planetary waves reported by Naujokat et al. (1994) is not likely accidental. Although the chain of cause and effect in planetary wave
I. Oznovich
et al. : Temperature
and airglow brightness
amplification and stratospheric warming development is not yet clear, simulations of sudden stratospheric warmings in the winter polar region indicate that these events are associated with a burst of gravity wave activity (Mahlman and Umscheid, 1987) and increased gravity wave forcing and drag (Rind rt [I/.. 1988). The very large horizontal scale of the Dec. 31-Jan. 1 wave (I,, b 7 x lo3 km) is in line with the large spatial scale of the planetary wave (&on;ll _ 1O4km at 60 ‘N) and stratospheric warming, which covered a major part of the Canadian Arctic.
5. Conclusions An important objective of the polar environment observatory in Eureka (80.O”N, 274.1”E) is to study the polar MLT region with high temporal and spatial resolution. The Eureka instrumentation used in this study included a Meridian Scanning Photometer measuring oxygen O[‘S] and sodium Na-D airglow brightness and a Michelson Interferometer monitoring the Meinel OH (3.1) band brightness and rotational temperature. The optical measurements provided mesopause temperature information (T,,,) as well as airglow brightness at approximately 85 (IOH). 89 (I,,), and 97 km (I,). The following observational results are noted : (1) Large amplitude (AT = 19 K) variations in airglow brightness and OH rotational temperature were observed on Dec. 31, 1993.00 UT to Jan. 1, 1994, 18 UT. The periods of Zou, INa. I,, and T,,, oscillations were 8.5, 8.7, 8.3, and 8.1 h, respectively, with a l-2 h resolving limit. It is highly unlikely that these variations were due to random fluctuations as they were measured at three different wavelengths by two independent instruments. (2) The change in IO preceded the change in IN;! by 0.9 h and the change in INa preceded the change in IOH by 0.4 h, indicating a downward phase speed of -2.6f 0.5 m ss’ and a vertical wavelength of i., = 79$18 km. The wave characteristics of the Meinel OH (3,1) band brightness and rotational temperature indicate that ]q] = 2.8kO.7 and 4 = - 12”+20’ for Krassovsky’s ratio. (3) An investigation of the horizontal wavelength of the wave was performed using all-sky scans of the rotating MSP. Two green line brightness scans of the sky for each of four look directions were processed, corresponding to peaks and valleys of the wave. The flatness of all the brightness scans across the 850 km distance indicate that the horizontal wavelength of the wave was greater than 3400 km. (4) Similar 8.5 h variations in airglow brightness and temperature were observed on Dec. 21, 1993. The Dec. 21 event was characterized by V. = -0.920.3 m s-‘, &=28$11 km, ]q] = 8.8k1.7, and $ = -107”f9”. There was a phase shift of 68” between this wave and that of Dec. 31-Jan. 1. The interpretation of airglow brightness and temperature oscillations based on analysis of the above observations is as follows : (1) Inertio-gravity
waves are rendered
most plausible
I Oznovich
c~fol. : Temperature
and airglow brightness
by the relatively infrequent appearance of these oscillations at all MLT heights and the incoherency between the two events. Temperature variations lead OH airglow brightness variations (negative phase for Krassovsky’s ratio), in qualitative agreement with predictions of the gravity wave theory. (2) The gravity wave hypothesis is further supported by the fact that the observed lower limit to the horizontal wavelength of the wave (i, > 3400 km) compares with its value predicted by the dispersion relation of inertio-gravi(y waves (2, = 7200+2700 km). (3) The observed periods (- 8 h), large vertical wavelengths, and persistence for three and five complete cycles all point to a non-migrating tide (zero zonal wave number) esplanation. On the other hand, the small amplitude and negative phase of Krassovsky’s ratio distract from such an explanation. The case for a non-migrating tide is further weakened by a large phase shift between the two waves. (4) The observed oscillations are not likely due to a tcr-diurnal pseudotide because the amplification factor of the tide at X0 N IS small (x = 1.X). Finally. there is a temporal and relative amplitude coincidence bet%,ern the inertjo-gravity waves reported here and forced planetary waves and stratospheric warming events over the Arctic (Naujokat et al., 1994). The planetary wave I of Dec. 2 1, 1993. and the planetary wave I! of Jan. I. 1994. as well as the stratospheric warming of the middle of December and end of December/beginning 01‘ January. were likely driven by the same tropospheric disturbance or directly related to the source of the gravit] \\ave obserl.ed over Eureka on Dec. 21 and Dec. 31.-Jan. I. respectively .3~.l
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