International audienceThe geomagnetic horizontal (H) field from the chain of nine observatories in India are used to study the storm-time and disturbance daily variations. The peak decrease in storm-time variation in H showed significant enhancements at the equatorial electrojet stations over and above the normally expected decrease due to the ring current effects corrected for geomagnetic latitudes. The disturbance daily variation of H at equatorial stations showed a large decrease around midday hours over and above the usual dawn-maximum and dusk-minimum seen at any mid-latitude stations around the world. These slow and persistent additional decreases of H of disturbance daily variation at equatorial latitudes could be the effect of a westward electric field due to the Disturbance Ionospheric dynamo coupled with abnormally large electrical conductivities in the E region over the equator

Rastogi, R. G.

HAL-INSU

Letter to the Editor: Geomagnetic storm effects at lowlatitudesR. G. RastogiTo cite this version:R. G. Rastogi. Letter to the Editor: Geomagnetic storm effects at low latitudes. AnnalesGeophysicae, European Geosciences Union, 1999, 17 (3), pp.438-441. <hal-00316562>HAL Id: hal-00316562https://hal.archives-ouvertes.fr/hal-00316562Submitted on 1 Jan 1999HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.Letter to the EditorGeomagnetic storm eects at low latitudesR. G. RastogiSchool of Physics, University of New South Wales, Sydney 2052, Australiaand Department of Physics, Gujarat University, Ahmedabad 380 009, IndiaReceived: 30 April 1998 / Accepted: 3 September 1998Abstract. The geomagnetic horizontal (H) field fromthe chain of nine observatories in India are used to studythe storm-time and disturbance daily variations. Thepeak decrease in storm-time variation in H showedsignificant enhancements at the equatorial electrojetstations over and above the normally expected decreasedue to the ring current eects corrected for geomagneticlatitudes. The disturbance daily variation of H atequatorial stations showed a large decrease aroundmidday hours over and above the usual dawn-maximumand dusk-minimum seen at any mid-latitude stationsaround the world. These slow and persistent additionaldecreases of H of disturbance daily variation at equa-torial latitudes could be the eect of a westward electricfield due to the Disturbance Ionospheric dynamocoupled with abnormally large electrical conductivitiesin the E region over the equator.Key words. Ionosphere (electric fields andcurrents) á Magnetospheric physics (electric fields;storms and substorms).1 IntroductionMoos (1910) from his monumental study of mag-netic data collected at Bombay, India from 1846 to1905, identified two distinct components of magneticdisturbance in the horizontal component of the geo-magnetic horizontal field (H). After removing thenormal daily variation and averaging over a numberof events, the magnetic storm was found to start with asudden increase lasting a couple of hours, followed bya rapid decrease continuing for few hours and later therecovery to normal conditions in a day or two. Thisfeature of the storm occurring at same universal timeat dierent observatories round the world is known asstorm-time variation, Dst(H). The second componentdepending on the local solar time was identified asadditional solar daily variation imposed on normaldaily variation during disturbed days. Moos had founda maximum around dawn and a minimum arounddusk period. This is now known as disturbance dailyvariation, SD(H), and is computed by subtracting themean solar daily variation on five international days ofthe month (SqH) from the corresponding mean vari-ation on five international disturbed days of the month(SdH).Vestine et al. (1947) described the results of acomprehensive study of the solar daily variations ofthe three components of the geomagnetic field at a largenumber of observatories on quiet as well as disturbeddays. The disturbance daily variation of H, SD(H) wasfound to be a single sine wave with a dawn-maximumand a dusk-minimum at any of the stations in low andmiddle latitudes. Chapman (1951) concluded that themagnetic disturbance at equatorial stations does notpresent any abnormal features characteristic of theequatorial electrojet. Sugiura and Chapman (1960)similarly concluded, after their extensive studies of Dstand SD variations of D, H and Z components at anumber of observatories, that the eect of magneticstorms at equatorial stations are similar to those at otherlow and middle latitude stations. Recently Rastogi(1998a) has shown that disturbance daily variation ofequatorial electrojet current has a distinct minimumnear noon, the amplitude of which increases near themagnetic equator.Since 1975, a unique network of geomagnetic obser-vatories is in operation in India confined within anarrow longitude zone, but extending in latitude fromthe centre of the electrojet to the focus of Sq currentsystem. The list of these observatories with theircoordinates (for the year 1976) are given in Table 1.The present paper examines the storm-time anddisturbance daily variations of H at the unique set ofstations in India to check any possible abnormal eectsat equatorial electrojet stations.Ann. Geophysicae 17, 438–441 (1999) Ó EGS – Springer-Verlag 19992 Disturbance daily variation of the electrojetIn Fig. 1 are shown the disturbance daily variations ofH, SD(H) = Sd(H))Sq(H) at all Indian observatoriesaveraged over all months of the year 1980. The diurnalinequalities were properly removed from the Sq and Sdvariations. The SD variations at stations outside theelectrojet belt show primarily a single sine wave with adawn-maximum and a dusk-minimum, in conformitywith the results of Vestine (1947) and of Yacob andSastri (1966). At equatorial electrojet stations TRD,KOD and ANN a significant depression of H isobserved around midday hours. The magnitude of themidday depression in H is progressively reduced withincreasing distance from the equatorial station Trivan-drum.This suggests that a regular westward electric field isimposed on the equatorial latitudes during the middayhours of the geomagnetically disturbed days. Theamplitude of midday decreases of SD(H) showed anenhancement over the magnetic equator in a fashionsimilar to the daily range of Sq(H) at equatoriallatitudes. Rastogi (1998b) has compared SD(H) varia-tions at equatorial electrojet stations in Indian andAmerican longitudes. The SD(H) at Kodaikanal showeda prominent midday decrease during the years 1954–1960 but at Huancayo SD(H) did not show any evidenceof midday decrease beyond the dawn-maximum anddusk-minimum.3 Storm-time variation of the electrojetIn Fig. 2(a) are shown the storm-time variations of H atall of the nine Indian observatories for the magneticstorm which started at 2335 hours 75°EMT on 11September 1986. The variations of DST-H index and ofthe top frequency reflected from the equatorial E regioninstabilities, Es-q, over Kodaikanal are also shown inthe diagram. In Fig. 2(b) are shown the latitudinalvariations of the observed Dst-H at the peak of thestorm compared with the axial symmetric part of theDst-H corrected for the dipole latitude, km. The Dst-Hindex shows sudden increase of H at midnight of 11–12Sept. 1986 associated with SSC. The main phase startedat 0630 hours on 12 Sept. 1986 reaching the peakactivity at 1130 hours (Dst-H = )170 nT) followed by aslow recovery on 13 Sept. 1986. The storm changes in Hat all of the stations were very similar to the changes ofDst-H index. At Jaipur the maximum decrease of H was)170 nT almost the same as of the Dst index. Themagnitude of the maximum of H was )173 nT at UJJ,)181 nT at ABG, )195 nT at HYB, )202 nT at ANN,)202 nT at KOD and )214 nT at TRD. Thus Dstobserved at any of the stations was larger than theglobal average index and specially at the stations withinTable 1 List of geomagnetic observatories in IndiaObservatory Code Geog. Geog. Dip.Lat. Long. Lat.°N °E °NTrivandrum TRD 8.5 77.0 )0.3Kodaikanal KOD 10.5 77.5 1.6Annamalainagar ANN 11.4 79.5 2.8Hyderabad HYB 17.4 78.6 10.7Alibag ABG 18.6 72.9 12.8Ujjain UJJ 23.2 75.8 18.1Jaipur JAI 26.9 75.8 22.4Sabhawala SAB 30.4 77.8 28.9Gulmarg GUL 34.1 74.6 31.4Fig. 1. Disturbance daily variations on H field at Indian observato-ries averaged for all months of the year 1980Geomagnetic storm eects at low latitudes 439the electrojet belt. Around noon on 12 Sept. 1986 thevalue of H(TRD) )H(ABG) was )33 nT suggesting acounter electrojet event, which is confirmed by theabsence of Es-q reflections at the same time. The Eq wasabsent at Kodaikanal at each of the 15-min observationsfrom 1115 to 1230 hours 75°EMT (K. Karunakaran,private communication). The regular hourly ionogramsat Thumba showed Esq at all hours from 0730 hours to1530 hours 75°E on 11 September 1986; on 12 Septem-ber 1986 Esq was present from 0730 to 1130 hours, wasabsent at 1230 hours and present again from 1330 hoursto 1530 hours 75°E (B. V. Krishna Murthy, personalcommunication). This shows that a strong westwardelectric field was imposed over the equatorial ionospherewhich overbalanced the normal eastward Sq field duringthe peak of the storm. Referring to Fig. 2(b), the axiallysymmetric part of Dst increased from a value of )168nT at 25° latitude to )170 nT at 0° latitude. Theobserved Dst-H increased slowly with decreasing lati-tude and later increased very fast at latitudes close to theequator indicating the characteristics of the equatorialelectrojet. It is to be noted that no large substorms wereindicated during the storm. Similar increases of Dst-H atequatorial latitudes were not seen during the storms withthe main phase occurring during the local night timehours.4 DiscussionRastogi (1977) has shown the evidence of such a changeof large electric field imposed on the dayside of theelectrojet during a geomagnetic storm. The presentanalyses indicate a rather slow and persistent electricfield imposed on the equatorial ionosphere during themain phase of the storm. Blanc and Richmond (1980)have suggested, based on their numerical analyses, thatthe global distribution of ionospheric currents andelectric fields can be altered by the perturbations in thesolar wind magnetosphere dynamo and the ionosphericwind dynamo. These modifications in the neutralatmospheric circulation during geomagnetic distur-bances can decrease or even reverse the quiet timeelectric field pattern in the vicinity of the dip equator.Fejer et al., (1983) showed evidence that the F regionvertical drift at Jicamarca are sometimes perturbed 16–24 h after the onset of geomagnetic storms. Fejer andScherliess (1955) using the radar measurements of Fregion vertical plasma drifts at Jicarmarca and AEindices, have shown that the high latitude electric fieldfrom the dynamo action of storm time winds producelargest perturbations a few hours after the onset ofmagnetic activity. Short period fluctuations in H atequatorial stations are known to occur during storms(Gonzales et al., 1983; Chandra and Rastogi 1997).These eects are often related to the directional changesin the Bz component of the interplanetary magnetic field(Rastogi and Kroehl 1978). Mazaudier and Venkates-waran (1990) have described variations of the E and Fregion meridional winds derived from the quadristaticradar at St Santin on 23 March 1979 with a time delay ofseveral hours after the substorm event of 22 March,when the state of magnetosphere was fairly quiet. TheyFig. 2. a Storm time variations of H field at Indian observatoriesduring the storm starting at 2335 hours 75°EMT on 11 Sep. 1986.b Latitudinal variations of the observed minimum of H at dierentstations during the peak disturbance activity compared with thelatitudinal variation of Dst(H) index ´ cosine geomagnetic latitude.440 Geomagnetic storm eects at low latitudesexplained the eect as due to the thermospheric andionospheric winds caused by disturbance dynamo elec-tric field.The additional storm time decrease of H at electrojetstations are in perfect time coincidence with the changesof ring current strength (= Dst H index) and hencecannot be attributed to any neutral wind modificationswhich are associated with a long time lag. Mostprobably this phenomenon is due to imposition of awestward electric field at the equatorial latitudes simul-taneously with the arrival and trapping of chargedparticles of solar wind origin.Unfortunately there is no equatorial chain of mag-netic observatories east or west of India where suchphenomenon can be studied for longitudinal inequalityif any.Acknowledgements. The author is thankful to the University ofNew South Wales for the facilities of the present study and to Prof.R. Stenning and Prof. D. E. Winch for useful discussions. Thanksare due to Indian National Science Academy, Gujarat Universityand Physical Research Laboratory for supporting the researchactivities of the author.Topical Editor D. Alcayde´ thanks M. A. Abdu for his help inevaluating this paper.ReferencesBlanc, M., and A. D. Richmond, The ionospheric disturbancedynamo. J. Geophys. Res., 85, 1699–1688, 1980.Chandra, H., and R. G. Rastogi, Geomagnetic storm eects onzonal and meridional equatorial electrojet currents. J. Geophys.Res., 102, 11611–11617, 1997.Chapman, S., The normality of geomagnetic disturbance atHuancayo, Geofisica, Pura e Applicata 14, 151–158, 1951.Fejer, B. G., and L. Scherliess, Time dependent response ofequatorial ionospheric electric fields to magnetospheric distur-bances, Geophys. Res. Lett., 22, 851–854, 1995.Fejer, B. G., M. F. Larsen, and D. T. Farley, Equatorial disturbancedynamo electric fields. Geophys. Res. Lett., 10, 5337–5340, 1983.Gonzales, C. A., M. C. Kelley, R. A. Behnke, J. F. Vikrey, R. Ward,and J. Holt, On the latitudinal variations of the ionosphericelectric field during magnetic disturbances, J. Geophys. Res., 88,9135–9144, 1983.Mazaudier, C., and S. V. Venkateswaran, Delayed ionosphericeects of the geomagnetic storms of March 22, 1979 studied bythe sixth co-ordinated data analysis workshop (CDAW-6 Ann.Geophysicae 8, 511–518, 1990.Moos, N. A. F., Magnetic observations made at the governmentobservatory Bombay for the period 1846–1905 and theirdiscussions, Part II The phenomenon and its discussions,Bombay. 1910.Rastogi, R. G., Geomagnetic storm and electric fields in theequatorial ionosphere, Nature 268, 422–424, 1977.Rastogi, R. G., Disturbance daily variation of equatorial electrojetcurrent, Current Sci., 74(5), 457–460, 1998a.Rastogi, R. G., Disturbance diurnal electric field in Indian andAmerican equatorial electrojet regions, J. Atmos. Solar Terr.Phys., 1998b (in press).Rastogi, R. G., and H. W. Kroehl, Interplanetary magnetic field andequatorial ionosphere, Ind. J. Rad. Space Phys., 7, 84–89, 1978.Sugiura, M., and S. Chapman, The average morphology ofgeomagnetic storms with sudden commencement, Abb. Akad.Wies. Gottingen. Maths-Phys. K. I. Sonderheft 4, 1960.Vestine, E. M., L. Laporte, I. Lange, and W. E. Scott, Thegeomagnetic field and its application and analysis, CarnegieInst. Washington, Publ. 580, 1947.Yacob, A, and N. S. Sastri, The morphology of great geomagneticstorms recorded at Alibag for the period 1905–1945: Part II.The disturbance local time inequality, DS and the disturbancedaily variation SD, Ind. J. Meteor. Geophys., 17, 187–198, 1966.Geomagnetic storm eects at low latitudes 441

<i>Letter to the Editor:</i> Geomagnetic storm effects at low latitudes

https://hal.archives-ouvertes.fr/hal-00316562

<i>Letter to the Editor:</i> Geomagnetic storm effects at low latitudes

Abstract

Similar works

Full text

Available Versions

HAL-INSU