Thermospheric and mesospheric temperatures during geomagnetic storms at 23°S

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Journal of Atmospheric and Terrestrial Physics,VoL 58, No. 16, pp. 1963-1972, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved S0021-9169(96)00001-3 0021-9169/96 $15.00+0.00

Thermospheric and mesospheric temperatures during geomagnetic storms at 23°S ]?. R. Fagundes, Y. Sahai, H. Takahashi, D. Gobbi and J. A. Bittencourt Instituto Na.cional de Pesquisas Espaciais-INPE-CP 515-CEP 12.201-970, S~o Jos6 dos Campos SP Brazil (Received in final form 8 November 1995; accepted 10 November 1995)

Abstract--Night-time thermospheric temperatures, T630,and mesospheric rotational temperatures, T(OH) and T(O2), have been measured at Cachoeira Paulista (23°S, 45°W, 16°S dip latitude), located in both the equatorial ionospheric anomaly and the South Atlantic Geomagnetic Anomaly, with a Fabry-Perot interferome~:er and a multi-channel tilting filter-type photometer, respectively. The thermospheric temperatures are obtained from the Doppler line broadening of the OI 630.0 nm emission and the mesospheric rotational temperatures from the OH(9,4) and O2A(0,1) band emissions. Measurements made during three geomagnetic storms showed that the nocturnal mean values of 7"63oduring the recovery phase of the storms were lower than those observed during quiet time and from model predictions. Also, the nocturnal mean value of the T630soon after the SSC event on 27 June 1992 was higher than the quiet time and model predictions. The observed mesospheric nocturnal mean rotational temperatures, T(OH) and T(O2), were unaffected by the storms. A comparison of the night-time observed temperatures T~30,T(OH) and T(O2) with those calculated using the MSIS-86 model is also presented. Copyright © 1996 Elsevier Science Ltd

INTRODUCTION Geomagnetic storms have been a subject of intense study and debate since the nineteenth century; some details on geomagnetic storm research can be found in the extensive review presented by Gonzalez et al. (1994). During geomagnetic storms, the upper atmosphere at high latitudes is strongly affected by the energy deposited Jn the auroral region. Auroral processes, such as auroral electric currents and charged particle precipitation, produce Joule heating, travelling ionospheric: disturbances (TIDs) and gravity waves (Fagundes et al., 1995a), which can produce wind and temperature fluctuations as well as temperature enhancements. These waves propagate to mid and lower latitudes, producing a global scale dynamic response related to geomagnetic storms. Several investigators have studied the response of the thermosphere at high and mid latitudes during geomagnetic storms (Hernandez and Roble, 1976; Hernandez and Roble, 1978; Jacka et al., 1979; Rees et aL, 1984a; Rees et al., 1984b; Yagi and Dyson, 1985; Fagundes et al., 1995a). The response of the thermosphere at low and equatorial latitudes to geomagnetic storms has also been recently investigated, but the behaviour in these regions is much less studied (Biondi and Meriwether, 1985; Tinsley et al., 1988; Burnside et al., 1991; Burns and Killeen, 1992; Wu et al., 1994; Fagundes et al., 1996a).

It is usually accepted that, during geomagnetic storms, the thermospheric temperatures are enhanced in magnitude relative to those during quiet conditions. Biondi and Meriwether (1985) detected a substantial rise in the thermospheric temperatures observed at Arequipa, Peru (16.4°S, 71.5°W, 4.4°S magnetic) during geomagnetic storms and suggested that these higher temperatures were the result of energetic neutral particle precipitation at low latitudes from the ring current or from energy carried to equatorial regions from high latitudes. Later, Tinsley et al. (1988) introduced corrections for instrumental drift in the thermospheric temperature calculations presented by Biondi and Meriwether (1985) and showed they were overestimated, but still higher than those observed during quiet conditions. Hernandez and Roble (1976, 1978) studied the behaviour of the thermospheric winds and temperatures during geomagnetic storms at mid-latitudes and found a temperature increase and general concurrence with the global model based on the Ogo 6 satellite temperature observations. However, on 6 July 1974, during a geomagnetic storm, the observed temperatures were lower than those from model predictions; it was suggested that the temperatures measured during this night were not the exospheric temperatures but were rather measured at a lower altitude and therefore less than the temperatures

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P. R. Fagundes et al.

predicted by the model (Hernandez and Roble, 1976). The mesospheric temperature response to geomagnetic storms at mid and high latitudes has been studied by only a few investigators. Shefov (1968, 1969) found variations of the mesospheric temperature deduced from measurements of the rotational temperature of the OH bands (T(OH)), associated with geomagnetic storms at mid and high latitudes. More recently, Wand (1983) discussed the dependence of the daily mean temperature as a function of geomagnetic activity by separating the temperature observations into quiet and disturbed periods on the basis of the Kp index values. Wand (1983) found that the magnitude of the temperature variations in the lower thermosphere (between 105 km and 135 km), due to Kp changes from 0 to 5, were between 18 K and 47 K. In this paper we present and analyze significant features occurring in simultaneous observations of thermospheric and mesospheric temperatures using an OI 630.0 nm Fabry-Perot interferometer and a multi-channel tilting filter-type zenith photometer, respectively. These observations have been carried out at Cachoeira Paulista (23°S, 45°W, 16°S dip latitude), Brazil, located in the equatorial ionospheric anomaly and in the South Atlantic Geomagnetic Anomaly. We present results obtained during three geomagnetic storms (viz. 6-10 July, 1991, 4-9 August 1991 and 25 June-1 July 1992).

is a 15 cm diameter etalon, with wavelength scanning controlled by three optically contacted piezoelectric spacer pads. A 64-channel digital analyzer is used to record the interferometer scan in wavelength and measurements are made at an elevation angle of 30 ° in four directions (N, S, E and W). The thermospheric temperature is determined from the OI 630.0 nm emission Doppler line broadening and it was possible to retrieve the temperature (through Doppler broadening) within an error range of + 40 K, for an intensity level of the OI 630.0 nm emission of about 200 R.

RESULTS AND DISCUSSION In what follows the nocturnal variations of the thermospheric and mesospheric temperatures observed at Cachoeira Paulista during geomagnetic storms are analyzed and compared with the MSIS-86 model (Hedin, 1987) predictions. The data analyzed include the night-time thermospheric temperatures observed during three geomagnetic storm periods and with the mesospheric rotational temperatures (T(OH)) and T(O2)) during two geomagnetic storm periods. The equatorial Dst parameter was used in this paper to classify the geomagnetic storms according to the following convention: intense storms are those with peak Dst < - 100 nT, moderate storms are those with - 5 0 nT >_ Dst >- - 100 nT and weaker storms are those with - 30 nT > Dst > - 50 nT (Gonzalez et al., 1 9 9 4 ) . Thermospheric temperatures

INSTRUMENTATION During the months of July-August 1991 and JuneJuly 1992, the Fabry-Perot interferometer and multichannel tilting filter-type zenith photometer were simultaneously in operation at Cachoeira Paulista. The multi-channel zenith photometer provides near simultaneous measurement of T(OH) and T(O2) (as well as of several other mesospheric emissions; for details see Takahashi and Batista (1981); Takahashi et al. (1986)). The photometer has a field of view of 2 ° full angle and uses a cooled Hamamatsu R953-02 (GaAs) photomultiplier and takes about 3 min to complete one sequence of observations with 5 filters, including background and dark noise checks. Estimated errors in the determination of the rotational temperatures of T(OH) and T(O2) are approximately + 5 K and + 10 K, respectively. The characteristics and other operational details of the Fabry-Perot interferometer at 630.0 nm, used to measure the night-time thermospheric temperatures presented in this study, are described by Sahai et al. (1992a,b). The core of the Fabry-Perot interferometer

The observed night-time thermospheric temperature variations for the three geomagnetic storms are shown in Figs 1-3, and are annotated with letters which denote the corresponding direction of observation (N,S,E,W) for each measurement. The thermospheric temperatures calculated using the MSIS86 model at 240 km altitude are indicated by a dashed line. Observations were not possible continuously during these three periods, as they were interrupted by daylight and by cloudy conditions on some nights. Also, there were some occasions on which large eastwest temperature gradients were observed (e.g. 2526 June, at around 22:00 U.T.). Such temperature gradients have been discussed in detail by Fagundes et al. (1996b). Figure 4 shows the temporal variation of Dst and Kp values for the three geomagnetic storms studied in this paper. The Fabry-Perot observation periods are indicated by thick black lines and the nocturnal mean values of T630, T(OH) and T(02) are also presented in Table 1 . The first observation period included an intense

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Thermospheric and mesospheric temperatures during geomagnetic storms at 23°S

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Table 1. The observed and MSIS-86 mean nocturnal thermosphere and mesosphere temperatures (K), with standard deviations in parentheses Date

Observed

T630 6 July 1991 7 July 9 July 10 July 4 August 1991 5 August 6 August 9 August 25 June 1992 26 June 27 June 29 June 30 June 1 July

]006 (80) 994 (115) 752 (101) 783 (130) 769 (127) 876 (112) 883 (124) 799 (104) 805 (124) 785 (117) 949 (114) 811 (103) 721 (103) 784 (101)

MSIS-86 240 km

Observed T(OH)

MSIS-86 87 km

Observed

T(O2)

MSIS-86 94 km

1090 (57) 1073 (57) 1040 (57) 1039 (57) 1019 (63) 985 (53) 975 (63) 942 (60) 847 ( 4 3 ) 847 ( 4 2 ) 843 ( 4 2 ) 844 ( 4 2 ) 866 ( 4 3 ) 868 ( 4 2 )

205 (6) 197 (5) 197 (4) 202 (5) 205 (8) 200 (8) 208 (8) 199 (8) . . . . . .

202 (4) 202 (4) 202 (4) 202 (4) 201 (5) 200 (5) 201 (5) 200 (4) . . . . . .

168 (5) 164 (6) 167 (5) 165 (5) 170 (7) 166 (8) 177 (5) 170 (9)

185 (3) 185 (3) 185 (3) 185 (3) 185 (3) 186 (3) 186 (3) 186 (3)

geomagnetic storm, when the Dst and Kp three-hour values reach - 198 nT and 8, respectively (9 July 1991, at 15:00 U.T.). The observed night-time thermospheric temperatures on 6-7 July and 7-8 July, presented in Fig. 1,. constitute a good example of thermospheric temperatures during the winter months, for high solar activity and quiet conditions (see Sahai et al., 1992b for more details about seasonal thermospheric temperature variations). It is noted that the MSIS-86 model predictions agree quite well with the observed temperatures in this case. However, on 9-10 July and 10-11 July 1991, during the recovery phase of the geomagnetic storm, the observed temperatures were lower than 1:hose from the earlier nights and model predictions. Figure 4, upper panel, shows full details of this storm period. In Table 1, the nocturnal mesospheric and thermospheric temperatures mean values are given for each night in which there were observations along with MSIS mean temperature. During the quiet period before the storm, the nocturnal temperature mean values were 1006 K (6-7 July) and 994 K (7-8 July) and during tlhe recovery phase they were 752 K (9-10 July) and 783 K (10-11 July). A thermospheric temperature increase would normally be expected during the storm (Hernandez and Roble, 1976; Tinsley et al., 1988; Biondi and Meriwether, 1985; Burns and Killeen, 1992; Burnside et al., 1991), but relatively low temperatures were observed during the two consecutive nights of the recovery phase of this storm. Figure 5 shows the night-time f o F 2 and h'F values at Cachoeira Paulista (23°S, 16°S dip latitude) for the nights of 9-10 July and 10-11 July 1991, during the recovery phase, and for 24-25 July 1991, for quiet

. . . . . .

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conditions. It is noted that the bottomside of the Flayer was never below 180 km and the F-layer motion during the recovery phase is similar to that during the quiet period. During the period 1 to 7 August 1991, there were three closely spaced geomagnetic storms. The first one (2 August) was an intense storm when the Dst and Kp values reached - 1 1 3 nT and 6 + , respectively, at 14:00 U.T. The second one (4 August) was a moderate storm when the Dst and Kp values reached - 9 1 nT and 6, respectively, at about 05:00 U.T. The last one (6 August) was also a moderate storm for which the Dst and Kp values were - 87 nT and 6, respectively, at 04:00 U.T. Figure 4, middle panel, gives full details of this storm period (hourly Dst and Kp values) and the nocturnal temperature mean values are also given for each night in which there were observations in Table 1. The observed nocturnal variation of the temperatures during August 1991 are shown in Fig. 2, as well as the MSIS-86 model predicted thermospheric temperatures. The observed temperatures during 4-5 August 1991, were lower than those predicted by the model during almost the whole night, except on two occasions (see Fig. 2). On 5-6 August 1991, the observed temperatures were lower than the model predictions during 19:00 LT to 22:00 LT, but after 22:00 LT the model results agree well with the observations. On 6-7 August and 9-10 August, the observed temperatures agree well with the model predictions during the whole night, except during the early observation hours (18: 30-20:15 LT) on 9 August. It can be noticed in Fig. 4, middle panel, that this is not a classical storm period, since the Dst index was recovering from

P. R. Fagundes et al.

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the first intense storm when the second one (moderate) occurred and it was again recovering from the second storm when the third one (moderate) occurred. The nocturnal temperature mean value, during the recovery phase of the second storm, was 769 K, whereas during the main phase of the third storm it was 876 K and during the recovery phase it was 883 K. During the period 27 June to 1 July 1992, a moderate geomagnetic storm occurred. The nocturnal thermospheric temperatures were also observed during two nights just before the storm, which are good examples of quiet nights. On 27 June there was a Sudden Storm Commencement (SSC) at 20:34 U.T. On 30 June the Dst and Kp values reached - 9 6 nT and 7 - , respectively, at 04:00 U.T. It can be noted from Fig. 3 that the observed temperatures agree well with those from the model predictions during the quiet period 25-26 June and 2627 June 1992. On 27-28 June 1992, the majority of the observed temperatures showed higher values than the model predictions, which could be associated with the Sudden Storm Commencement (SSC) event. On 29-30 June, during the main phase of the storm, the

observed temperatures agree well with the model results. However, during the recovery phase the majority of the observed temperatures were lower than those from the model predictions. Only the temperatures observed in the west directions, during 21:00 LT to 01:00 LT, were higher than or comparable with the model predicted values. On 1-2 July the observed temperatures agree well with the model predicted values. In Fig. 4, bottom panel, full details of this storm period (hourly Dst and Kp values) and the nocturnal temperature mean values are indicated for each night in which there were observations; see Table 1. This is one of the most complete sets of thermospheric temperature observations during a geomagnetic storm recorded at Cachoeira Paulista. During the quiet nights the temperature mean values were 805 K (2526 June) and 785 K (26-27 June), but soon after the SSC the temperature mean value went up to 949 K (27-28 June). During the main phase (minimum Dst) the temperature mean value observed was 811 K (30 June), which is a value comparable with that for quiet days. However, during the recovery phase, the tem-

Thermospheric and mesospheric temperatures during geomagnetic storms at 23°S perature mean values went down to 721 K (30 June1 July). These observations suggest that there was thermospheric heating soon after the SSC and a reduced thermospheric temperature during the recovery phase. These storm-time thermospheric temperature observations, particularly reduced temperatures during recovery phase,, indicate that there are still unresolved questions related to the response of the equatorial thermosphere during geomagnetic storms. One possibility could be that the reduced temperatures are not exospheric temperatures but rather temperatures measured at lower altitudes, and therefore smaller than model temperature predictions, as suggested by Hernandez and Roble (1976). Using the MSIS-86 model we carried out a simulation for nighttime temperature wtriations as a function of altitude to investigate the Hernandez and Roble (1976) hypothesis, and we :found that for the night 9-10 June 1991 (intense storm) the observed temperature (T630) agreed well with the model prediction at 170 km altitude (emission layer peak during quiet days is around 240 km). However, it does not seem reasonable that the emission layer moved 70 km downwards during this night, because the h'Fvalue is higher than 200 km for the whole night (Fig. 5). The storm response of the equatorial thermosphere temperature and of the neutral density have been investigated by Burns and Killeen (1992), using measurements from the Dynamics Explorer-2 (DE-2) satellite; they showed both compressional heating and adiabatic cooling (about 3-4 hours period) associated with waves launched during geomagnetic storms. However, the present observations showed thermospheric reduced temperatures over an extended period during the recovery phase, suggesting that these reduced temperatures might not be generated by waves. Thermospheric h,~ating might be expected after the SSC event, possibly associated with the transport of heat from the ring current or from high latitude Joule heating. However, the unexpected reduced thermospheric temperatures over extended periods observed during the recovery phase need further investigation for a better understanding of the physical processes involved. The observed reduced thermospheric temperatures might be associated with a global modification of the spatial temperature distribution during post-geomagnetic storm conditions, in such a way that regions of lower temperature moved over the observation site. Thermospheric temperature observations providing some spatial coverage (e.g. satellite or multi-site ground-based observations) during and after geomagnetic disturbances will be important for a better understanding of the storm-time

1969

thermospheric dynamics and spatial temperature variations.

Mesospheric rotational temperatures The night-time variations of the rotational temperatures from OH observations (T(OH)) and from 02 observations (T(O2)) for the two geomagnetic storms of July and August 1991, are shown in Fig. 6 and Fig. 7, respectively. The comparison with the MSIS-86 model predicted results is made considering that the peak height of the O2A(0-1) emission profile is located around 94 km and that for OH(9-4) at 87 km. It may be noted in Fig. 6 that before the storm (67 and 7-8 July) the observed night-time rotational temperature T(OH) and T(O2) showed a smaller nighttime variation than observed during the recovery phase (9-10 July). Also, the MSIS-86 model predictions showed better agreement with the observed T(OH) than with observed T(O2) for all nights studied during this period. In spite of the fact that the rotational temperature variations showed a larger night-time variation during the recovery phase, the nocturnal mean rotational temperature values remained almost constant for the four nights studied (see Table 1). Figure 7 shows the observed rotational temperature T(OH) and T(O2) variations for a period during which three consecutive storms occurred, and all nights showed similar night-time variations. The nocturnal mean rotational temperature values were again unaffected by the geomagnetic storm (see Table 1). The MSIS-86 model prediction again showed better agreement with the observed T(OH). The observed rotational temperatures T(OH) agree well with the MSIS-86 model values; however, T(O2) during all nights showed lower values than those from the model predictions. Nevertheless, the difference between the observation and model results are less than the estimated errors, but this systematic difference may be due to an overestimation of the mesopause altitude at low latitudes by the MSIS-86 model. Further investigations, possibly with other groundbased or satellite observations, will be important for a more conclusive study. CONCLUSIONS The principal features of the behaviour of the thermospheric and mesospheric temperatures at Cachoeira Paulista, are summarized below: 1. The nocturnal thermospheric temperature variations observed during quiet time conditions agree

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Thermospheric and mesospheric temperatures during geomagnetic storms at 23°S well with the MSIS-86 model predictions. However, during; the SSC event a n d recovery phase of the storm, the MSIS-86 model predictions do n o t show a good agreement with the observed temperatures. 2. The observed n o c t u r n a l t h e r m o s p h e r i c temp e r a t u r e variations d u r i n g the SSC event were generally higher t h a n those from the MSIS-86 model predictions. 3. D u r i n g the recovery phase of a n intense storm, all the observed t e m p e r a t u r e s were lower t h a n MSIS86 model predicted temperatures; for m o d e r a t e storms, the majority o f the observed temperatures

1971

were also lower t h a n those from the model predictions. 4. The n o c t u r n a l m e a n values o f the t h e r m o s p h e r i c t e m p e r a t u r e s were lower during the recovery phase t h a n those observed d u r i n g quiet time conditions. 5. The n o c t u r n a l m e a n values o f the mesospheric rotational temperatures T(OH) a n d T(O2) analyzed do n o t show any significant influence of the geomagnetic storms. Acknowledgements--Partial funding for this work was provided through the Conselho Nacional de Desenvolvimento Cientifico e tecnol6gico, processo CNPq N ° 300955/93-6.

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Burnside R. G., Tepley C. A., Sulzer M. P., Fuller-Rowell T. J., Torr D. G. and Roble R. G. Fagundes P. R., Aruliah A. L., Rees D. and Bittencourt J. A.

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