STORM-TIME VARIATIONS OF ELECTRON TitleCONCENTRATION IN THE EQUATORIAL TOP IONOSPHERE.

Transcription

1 STORM-TME VARATONS OF ELECTRON TitleCONCENTRATON N THE EQUATORAL TOP ONOSPHERE Author(s) NOUE, Takayoshi; CHO, Tegil Citation Contributions of the Geophysical n (197), : 9-7 ssue Date 197- URL Right Type Departmental Bulletin Paper Textversion publisher Kyoto University

2 Contributions, Geophysical nstitute, Kyoto University, No., 197 REPORT STORM-TME VARATONS OF ELECTRON CONCENTRATON N THE EQUATORAL TOPSDE ONOSPHERE By Takayoshi NOUE and Tegil CHO (Received September, 197) Abstract Storm-time variations of the topside ionosphere in equatorial latitudes over the American one are examined, by using Alouette sounding data. Three typical types of variations of electron concentration during geomagnetic storms are recognied. First type is characteried by the development of the equatorial anomaly, and second one by the disappearance of the anomaly. The occurrence of electron concentration enhancement ithout the anomaly over ide range of latitudes is third type. 1. ntroduction Variations of electron concentration in the equatorial topside ionosphere during geomagnetic storms have been studied by several orkers. Analysing Alouette topside sounding data over the Asian one, King et al. (197) reported that during the geomagnetic storm on 15 September 193 the depth of electron concentration trough and the latitudinal extent of equatorial anomaly crests decrease as compared ith those observed during a magnetically quiet day. Sato (19), using Alouette data over the American one, analysed the variations of electron concentration in the topside ionosphere and shoed that during geomagnetic storms the formation of the equatorial anomaly in the daytime appears to be inhibited. On the other hand, Raghavarao and Sivaraman (1973) examined the latitudinal variations of the topside electron density during eight magnetic storm events and shoed that for seven of the events the equatorial anomaly is enhanced during magnetically disturbed conditions. With respect to the latitudinal variations of total electron content in the noontime during geomagnetic storms, Basu and Das Gupta (19) found that the peak of electron content anomaly in the equatorial region moves toard the magnetic equator on magnetically active days. On the contrary, Mendonca et al. (199) shoed that the position of the anomaly crest of total electron content moves aay from the magnetic equator immediately after a magnetic sudden commencement. As shon above, it seems that the morphology of the electron concentration variations in the equatorial latitudes has been far from being established until no.

3 7 T. NOUE AND T. CHO n addition, it is not clear ho the topside ionosphere behaves during various phases of geomagnetic storms. n this paper, e report storm-time variations of the topside ionosphere in equatorial latitudes during geomagnetic storms.. Data and results We used Alouette ionospheric data published by the Defence Research Board of Canada. Electron concentration profiles over the American one ere selected. Magnetically quiet days prior and close to the commencements of the geomagnetic storms ere used as control days. Days used as control days ere not ones immediately preceding the storm commencements hen satellite ionospheric data on those days ere not available. The data on quiet and disturbed days ere for nearly the same local time and longitude. Tenty-one storm events ere examined and typical types of events out of them are shon in Figs. to 5. Fig. 1 shos the geomagnetic variations at Huancayo, Peru. Geomagnetic storms occurred on 1 November 19, October 193, 9 October 193 and 7 November 193, respectively. Arros shon in this figure represent the time at hich the ionospheric data ere taken. n Fig. latitudinal electron concentration profiles at the height of 5 km at different phases of the geomagnetic storm on October 193 (dotted line) are compared ith the corresponding profiles under undisturbed conditions on 3 October (solid line) and October (chain line). As shon in this figure, the equatorial anomaly that have existed under undisturbed conditions disappears during NOV 19 j J 1 3 > l OCT 193.tNt.l 1.r- > :cr OCT 193 A l " NOV 193 UT.,.,..._1.-_j_ 7 " 1 Fig.. Geomagnetic field data at Huancayo, Peru in November 19, October and November 193. The time at hich the ionospheric data ere acquired is shon by arros.

4 STORM-TME VARATONS OF THE EQUATORAL ONOSPHERE 71 ;;;- E (])., >- 1- V) :: 1- u _j 9'W 5 OCT 193 1'W OCT 193 'W OCT 193 3UT UT 3 3UT 19 LT 19?LT 19 OLT j'v.' : /'"'.. \ 1/ \ -/ \ (/ f\.... "\ n ----\ J\ /,l'>..... j / \,...,,. ' / il... P... ' o -' -' ' ' o -' -' ' ' o -' -' Fig.. Comparison of the latitudinal electron density profiles at the height of 5 km at different phases of the magnetic storm on October (dotted line) ith the corresponding profiles under undisturbed conditions at 37 UT (193 LT) on 3 October (solid line) and at 1 UT (193 LT) on October (chain line). The dates, UT, LT and longitudes at hich the satellite passed over the dip equator are also indicated. "' E u :::::, (]) "'o >- 1- V5 g-: u _j s' 31 OCT W 31 OCT UT 3 UT 1 37LT 1 31 LT 1'W NOV93 3 UT 1 LT ' o -' -' ' o o -o -' ' o -o -' Fig. 3. The version is the same as Fig.. The storm occurred on 9 October 193. The control day profile is taken at 31 UT (155 L T) on October (solid line).

5 7 T. NOUE AND T. CHO the recovery phase of the geomagnetic storm (3 UT on 5 October). This type of variation is similar to Sate's D-type ones. Then, as geomagnetic conditions recover to quiet state the equatorial anomaly is formed again (33 UT on October). Fig. 3 also shos that the equatorial anomaly disappears on the disturbed days. During the recovery phase of the geomagnetic storm (15 UT on 31 October) the anomaly disappears ith the enhancement of electron density around the magnetic equator and ith the depression at dip angles above 13 o at the height of 5 km. "'E Ql "'o >< g: G:J 9 7 NOV 193 i! /1 i \ j : \ / i \." i i _, km 13UT 1737LT 1 NOV 193 9UT \ km 17\LT \ \ \ \ \ i \ \ ' \ NOV UT 175LT t''\ km i \ \, i o oo - - co oo - - oo NOV UT 1737 LT 5 km NOV 193 9UT 17 LT 5km W NOV UT 17 5LT 5km oo - - (\.,... J : J -r! \ i )/' oo -oo - \. :/ oo o -o -o Fig.. The version is the same as Fig.. n this figure profiles at the height of km and 5 km are shon. The storm occurred on 7 November.193. The control day profile is taken at UT (17 L T) on November (solid line).

6 t STORM-TME VARATONS OF THE EQUATORAL ONOSPHERE 73 At the end of the recovery phase of the geomagnetic storm (3 UT on 31 October) the disappearance of the anomaly is also seen, and the peak value of electron density at the equator decreases. Hoever, under quiet conditions (3 UT on 1 November) folloing the geomagnetic storm the electron concentration at the height of 5 km is depressed over all latitudes beteen dip angles of ± o as compared ith that on the control day and the anomaly does not exist. Fig. shos the variations at the height of km and 5 km during the geomagnetic disturbance on 7 November 193. On the control day ( November) prior to the commencement of the disturbance the equatorial anomaly does not exist. Under the disturbed conditions on 7 November the asymmetrical anomaly is ell developed both at km and 5 km and the anomaly crests are located at dip angle of about o. The enhancement of electron concentration is also recognied over the ide range of latitudes at both these heights. On the next day ( November) the anomaly crests move toard the magnetic equator and are situated at dip angle of about 15 o at the height of 5 km, and the electron density at both heights is further enhanced around the magnetic equator. As the geomagnetic field recovers to quiet state the equatorial anomaly is inhibited again. Fig. 5 shos a different type of variation from ones shon above. The equatorial anomaly is not formed both on the control day and on the disturbed one, and electron density is enhanced under disturbed conditions. 3. Discussions Three types of the variations shon in this report may be caused mainly by the "'E :::; Q) "'o >- :: LiJ ss Nov 19 3UT 1 LT W NOV UT 15 LT oo - - oo - - Fig. 5. The version is the same as Fig.. The storm occurred on 1 November 19. The control day profile is taken at 3 UT (1 LT) on 19 November (solid line).

7 7 T. NOUE AND T. CHO changes of electromagnetic drifts. The type shon in Fig. that is characteried by the disappearance of the equatorial anomaly during geomagnetically disturbed days may be explained by the electromagnetic drift associated ith an electric field for the DS magnetic variation as suggested by Sato (19). Other types of the variations shon in Figs. 3 to 5 might also be related to changes in electric fields in the ionosphere during geomagnetic storms (Raghavarao and Sivaraman, 1973), hile it appears that the depression of electron concentration on 1 November 193 (Fig. 3) is partly due to changes of neutral composition as suggested by Rush et al. (199). n order to establish the morphology of the variations in the equatorial topside ionosphere, more data of the topside ionosphere ill be needed, and to make clear the effects of geomagnetic storms on the electron concentration the observations of electric fields at the equatorial ionosphere ill be required, too. Acknoledgements The authors ish to express their thanks to Prof. H. Maeda for his encouragement, and also to Mrs. S. Maeda, Mr. S. Handa and Mr. Y. Muraoka for their helpful discussions and assistances. The authors are grateful to the Communications Research Centre, Ottaa, Canada for providing them ith the Alouette data. References Basu, S. and A. Das Gupta, 19; Latitude variation of electron content in equatorial region under magnetically quiet and active conditions, J. Geophys. Res., 73, King, J. W., K. C. Reed, E.. Olatunji and A. J. Legg, 197; The behaviour of the topside ionosphere during storm conditions, J. Atmos. Terrest. Phys., 9, Mendonca, F.,. J. Kantor and B. R. Clemesha, 199; Lo-latitude ionospheric electron content measurements during half a solar cycle, Radio Sci.,, 3-. Raghavarao, R. and M. R. Sivaraman, 1973; Enhancement of the equatorial anomaly in the topside ionosphere during magnetic storms, J. Atmos. Terrest. Phys., 35, Rush, C. M., S. V. Rush, L. R. Lyons and S. V. Venkatesaran, 199; Equatorial anomaly during a period of declining solar activity, Radio Sci.,, 9-1. Sato, T., 19; Electron concentration variations in the topside ionosphere beteen N and S geomagnetic latitude associated ith geomagnetic disturbances, J. Geophys. Res., 73, 5-1.