Equatorial Storm Sudden Commencements & Interplanetary Magnetic Field

Abstract

A comparison is made of the signatures of interplanetary (IP) shocks in the <i>B </i> and <i>θ</i> plots of interplanetary magnetic field (IMF) data of satellites Explorer 33, 34, and 35 and in the <i>H </i> magnetograms at ground observatories within the equatorial electrojet belt, Huancayo, Addis Ababa and Trivandrum associated with major storm sudden commencements during 1967-70. The IP shocks showing sudden increase of the scalar value of IMF, i.e. <i>B </i> without any change of the latitude <i>θ</i> or with the southward turning of <i>θ</i>, were followed by a purely positive sudden increase of <i>H, </i>at any of the magnetic observatories. either on the dayside or the nightside of the earth. The IP shocks identified by a sudden increase of <i>B </i>and with the northward turning of the latitude θ (positive <i>ΔB_Z </i> were associated with purely positive sudden commencement (SC) at the observatories in the nightside but at the equatorial observatories in the dayside of the earth the signature of the shock was a SC in <i>H </i> with a preliminary negative impulse followed by the main positive excursion (SC - +). It is suggested that the SC in <i>H </i> at low latitudes are composed of two effects, viz. (i) one due to hydromagnetic pressure on the magnetosphere by the solar plasma and (ii) the other due to the induced electric field associated with the solar wind velocity, V,and the Z-component of the IP magnetic field <i>(E </i>= - <i>V</i> x <i>B_z.). </i>The effect of magnetosphere electric field is faster than the effect due to the compression of the magnetosphere by the impinging solar plasma. The negative impulse of SC - + at low latitude is seen at stations close to the dip equator and only during daytime due to the existence of high ionospheric conductivities in the equatorial electrojet region.