Abstract
An impact of a magnetic cloud on the Earth’s magnetosphere occurred at 1636 UT on 25 June, 1998, associated with a sudden increase of the solar wind density and velocity, as well as a sudden increase of the zenithal component of the interplanetary magnetic field (IMF-Bz). Following the impact of the magnetic cloud, IMF-Bz was northward (10 nT) and remained steadily strong (about 15 nT) for the next six hours. IMF-Bz turned southward at 2330 UT on 25 June, 1998, and remained strongly southward (−15 nT) for the next four hours. During the positive phase of IMF-Bz, both the Auroral index and ring current index SYM/H remained steadily low indicating complete isolation of the Earth’s magnetosphere from the solar wind and no significant changes were observed in the equatorial ionosphere. After the southward turning, the steady southward IMF-Bz permitted solar wind energy to penetrate the magnetosphere and caused the generation of a magnetic storm associated with strong auroral electrojet activity (AE index). Strong southward IMF-Bz corresponds to the dawn-dusk interplanetary electric field (eastward on the dayside and westward on the night side). The ionograms at Jicamarca (night side) showed strong spread-F and at Thumba (dayside) showed an absence of equatorial type of sporadic-E, indicating a dusk-to-dawn electric field. Thus, the observations point to an electric field opposite in direction to that expected by the prompt penetration of the interplanetary electric field. An abnormally-large Auroral index (AE) associated with the start of the storm suggests that the cause of the equatorial electric field changes is due to the disturbance dynamo effect.
Highlights
The equatorial ionosphere is of special interest in studies of solar-terrestrial physics since the discovery by Chapman (1951) of the Equatorial Electrojet (EEJ), a band of eastward-flowing current in the ionospheric E-region during the daytime hours
Tulasi Ram et al (2008) studied the response of post sunset equatorial spread-F during five geomagnetic storms based on data in the Indian, and Japanese sectors, and Kwajalein, and showed the development of plasma bubbles due to the prompt penetration of the eastward electric field in the dusk sector
If one examines the interplanetary magnetic field, it does not show any sudden reversal of IMF from a southward to a northward direction, which is a signature for an over shielding effect and, as a result, this possibility has to be abandoned
Summary
The equatorial ionosphere is of special interest in studies of solar-terrestrial physics since the discovery by Chapman (1951) of the Equatorial Electrojet (EEJ), a band of eastward-flowing current in the ionospheric E-region during the daytime hours. Tulasi Ram et al (2008) studied the response of post sunset equatorial spread-F during five geomagnetic storms based on data in the Indian, and Japanese sectors, and Kwajalein (covering about 90◦ longitude), and showed the development of plasma bubbles due to the prompt penetration of the eastward electric field in the dusk sector. The occurrence of the counter electrojet ( Hequatorial − Hnon-equatorial becoming negative) is associated with the disappearance of Es-q at Thumba or the reversal of the ionospheric electric field to the westward direction, sometimes due to the reversal of IMF-Bz. In Fig. 3 are shown a set of ionograms at Thumba, near Trivandrum, during the daytime on a normal quiet day, June, 1998, and on a storm day June, 1998. These features are consistent with the presence of a westward electric field in the nightside of the Earth due to the positive interplanetary electric field
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