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

Abstract

Abstract. The work describes an intensive study of storm sudden commencement (SSC) impulses in horizontal (H), eastward (Y) and vertical (Z) ®elds at four Indian geomagnetic observatories between 1958±1992. The midday maximum of DH 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 ®eld at equatorial and low latitudes. The impulses in Y ®eld are shown to be linearly and inversely related to DH 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 eect of the orientation of shock front plane of the solar plasma with respect to the geomagnetic meridian. The DZ at SSC have a positive impulse as in DH. The ratio of DZ/DH are abnormally large exceeding 1.0 in most of the cases at Trivandrum. The latitudinal variation of DZ shows a tendency towards a minimum over the equator during the nighttime hours. These eects are explained as (1) resulting from the electromagnetic induction eects 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.

Analysis of geomagnetic sudden impulses at low latitudes

“… Tsunomura [1998], who analyzed events from low and middle latitudes in Japan ( λ ≈ 21°–43°), determined a smaller ΔH change during local morning with respect to the rest of the day. Rastogi [1999], at equatorial latitudes ( λ ≈ −1.2° ÷ 9.5°), observed a sharp ΔH maximum around noon and minimum values at dawn and dusk.…”

Sudden impulses at geosynchronous orbit and at ground

“…In addition, in any time sector, the H variations are accompanied by explicit negative variations of the D component (DD, perpendicular to H in the horizontal plane; positive eastward; paper 2). In some cases, moreover, the main field jump is preceded by transient phenomena which do not appear in the magnetospheric and SW measurements: for example, the event in panel c is characterized by the occurrence of a preliminary positive impulse (PPI) on the H component; in the D component, in this case, a similar impulse represents the main feature of the geomagnetic response: as for the long term D variation, these transient aspects are currently interpreted as ground manifestations of the ionospheric current systems (Tsunomura, 1998(Tsunomura, , 1999Rastogi, 1999Rastogi, , 2005.…”

On the discrimination between magnetospheric and ionospheric contributions on the ground manifestation of sudden impulses

“…So far, quantitative analysis of the SI characteristics mostly focused on the behavior of the H component alone [ Siscoe et al , ; Nishida , ; Russell et al , , , ; Russell and Ginskey , ; Francia et al , ]. By contrast, the behavior of the D component needs to be considered to infer a reliable representation of the ionospheric current pattern [ Rastogi , , ; Tsunomura , ; Villante and Piersanti , ].…”