Nighttime enhancement of the amplitude of geomagnetic sudden commencements and its dependence on IMF-Bz

T Araki,H Yang,T Kamei,S Alex,K Keika

doi:10.1186/bf03351912

Abstract

Abstract We present a statistical study of the diurnal variation of the occurrence frequency of geomagnetic sudden commencements (SCs) observed at Kakioka (geomagnetic latitude, θ = 27.4°). SCs with an H-component amplitude (ΔH) larger than 40 nT occur more frequently in the nighttime than the daytime, while those with smaller amplitudes (ΔH < 39 nT) occur more frequently in the daytime. Three large amplitude SCs (ΔH = 85, 117 and 145 nT at Kakioka) were analyzed in detail. All three exhibited larger amplitudes during the nighttime at all low latitudes except those near the dayside equator. A statistical study reveals that the averaged amplitudes are slightly larger in the daytime at Alibag (θ = 10.2°) but considerably larger in the nighttime at three higherlatitude Japanese stations, Kanoya (θ = 21.9°), Kakioka and Memambetsu (θ = 35.4°). Case studies of two moderate amplitude SCs which occurred at the same UT indicate that nighttime SC amplitudes at low latitudes are slightly (considerably) larger than daytime amplitudes when the interplanetary magnetic field (IMF) points northward (southward). We suggest that the diurnal variation of SC amplitudes can be explained by a combination of field aligned and resultant ionospheric currents produced during the main impulse of SCs.

Highlights

Geomagnetic sudden commencements (SCs) are observed globally everywhere on the ground, the amplitude and waveform greatly change depending upon latitude and local time
According to Russell et al (1992, 1994) the SC amplitude takes the largest values around noon during the northward interplanetary magnetic field (IMF) and near midnight during the southward IMF
Clauer et al (2001) reported an SC event with the largest amplitude in nighttime which occurred during a northward IMF turning

Summary

Introduction

Geomagnetic sudden commencements (SCs) are observed globally everywhere on the ground, the amplitude and waveform greatly change depending upon latitude and local time. The list includes 140 SCs with the amplitude larger than 40 nT and the SC on March 24, 1991 was second largest Looking at this list, we noticed that large amplitude SCs occur more frequently in the nighttime than the daytime and began to study the local time dependence of the SC amplitude observed at low and middle latitude stations. The enhancement of the magnetotail current associated with SCs produces the negative H -component in low latitudes on the ground It is largest near midnight and smallest near noon and so contributes to make the SC amplitude larger in the daytime. The AU and AL indices exhibited large disturbances before and after the 3 SCs suggesting the southward IMF

Station Geomagnetic Latitude θ Data period Number of SC

Discussion