MAGSAT for geomagnetic studies over Indian region

Software was prepared for removal of the external field due to the ring current and the associated induced part. Fast Fourier transformation was used to analyse the field of lithosphere with and without external current (+ induced internal current) component. A vertical component in the equatorial electrojet current system was identified from MAGSAT records

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

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

Signatures of storm sudden commencements in geomagnetic H, Y and Z fields at Indian observatories during 1958?1992

International audienceThe work describes an intensive study of storm sudden commencement (SSC) impulses in horizontal (H), eastward (Y) and vertical (Z) fields at four Indian geomagnetic observatories between 1958?1992. The midday maximum of ?H has been shown to exist even at the low-latitude station Alibag which is outside the equatorial electrojet belt, suggesting that SSC is associated with an eastward electric field at equatorial and low latitudes. The impulses in Y field are shown to be linearly and inversely related to ?H at Annamalainagar and Alibag. The average SC disturbance vector is shown to be about 10?20°W of the geomagnetic meridian. The local time variation of the angle is more westerly during dusk hours in summer and around dawn in the winter months. This clearly suggests an effect of the orientation of shock front plane of the solar plasma with respect to the geomagnetic meridian. The ?Z at SSC have a positive impulse as in ?H. The ratio of ?Z/?H are abnormally large exceeding 1.0 in most of the cases at Trivandrum. The latitudinal variation of ?Z shows a tendency towards a minimum over the equator during the nighttime hours. These effects are explained as (1) resulting from the electromagnetic induction effects due to the equatorial electrojet current in the subsurface conducting layers between India and Sri Lanka, due to channelling of ocean currents through the Palk Strait and (2) due to the concentration of induced currents over extended latitude zones towards the conducting graben between India and Sri Lanka just south of Trivandrum

On long-wavelength magnetic anomalies over Indian region

A data set composed of vector magnetic measurements obtained by MAGSAT and very accurate altitude determinations made using Sun sensors and star cameras was used to obtain data for very quiet days over the Indian region at 10 S to 40 N and 60 E to 110 E in an effort to determine the validity of quantitative estimates made from aeromagnetic data obtained by removing the core field. To further account for the external effects, the ring current contributions estimated using both X and Z variations were subtracted from the observed values. Before this, the core contribution was eliminated through a spherical harmonic expansion with terms up to N=13. Analysis of the residual measurements using Fast Fourier techniques indicates that the anomalies contain substantial power for wavelengths of about 1500 kms. Because the ring current effect has a spatial structure of this dimension over India, efforts are being made to exactly eliminate these two interfering effects from the data

Geomagnetic field variations at the equatorial electrojet station in Sri Lanka, Peredinia

The paper discusses the variations of the horizontal (&lt;i&gt;H&lt;/i&gt;), vertical (&lt;i&gt;Z&lt;/i&gt;) and eastward (&lt;i&gt;Y&lt;/i&gt;) components of the geomagnetic field at Peredinia (PRD), an electrojet station in Sri Lanka, with the time of the day, season, sudden commencement (SSC) and during geomagnetic storms. The daily variation of &lt;i&gt;H&lt;/i&gt; showed a large peak around midday. The daily variation of &lt;i&gt;Z&lt;/i&gt; appeared to be almost a time gradient curve of the daily variation of &lt;i&gt;H&lt;/i&gt;, showing a maximum around 09:00&amp;nbsp;LT (75&amp;deg; EMT) when the &lt;i&gt;H&lt;/i&gt; field was increasing fastest and not at noon when &amp;Delta; &lt;i&gt;H&lt;/i&gt; was the maximum. Storm time variation of &lt;i&gt;H&lt;/i&gt; resembled the variation of the &lt;i&gt;D&lt;sub&gt;st&lt;/sub&gt;&lt;/i&gt; index but that of &lt;i&gt;Z&lt;/i&gt; showed a large minimum about 2-3h before the time of minimum &lt;i&gt;D&lt;sub&gt;st&lt;/sub&gt;&lt;/i&gt; or at the time of maximum time gradient of &lt;i&gt;D&lt;sub&gt;st&lt;/sub&gt;&lt;/i&gt; variation. These features are compared with corresponding variations at the equatorial stations Trivandrum (TRD) in India, and remarkable similarity in all observations is noticed at PRD and TRD. It is suggested that the observed abnormal features of &lt;i&gt;Z&lt;/i&gt; variations at electrojet stations in India-Sri Lanka are due to (i) direct effect of the ionospheric electrojet current (ii) the induction effect of the image current by the average spatially extended conductivity region and (iii) the induction current in the local subsurface conductor. It is suggested that the conductor responsible for the observed features in &lt;i&gt;Z&lt;/i&gt; in India and Sri Lanka has to have extended spatial domain to latitudes well south of India, rather than confined to narrow Palk Strait