Abstract:
On 7 January 2005 (Ap = 40) prompt penetration electric field perturbations of opposite
polarities were observed over Thumba and Jicamarca on a few occasions during 13:45–16:30 UT. However,
the electric field was found to be eastward during 14:45–15:30 UT over both Thumba and Jicamarca
contrary to the general expectation wherein opposite polarities are expected at nearly antipodal points.
On closer scrutiny, three important observational features are noticed during 14:10–15:15 UT. First, during
14:10–14:45 UT, despite increasing southward interplanetary magnetic field (IMF) Bz condition, the already
westward electric field over Thumba weakened (less westward) while the eastward electric field over
Jicamarca intensified (more eastward). Second, the electric field not only became anomalously eastward
over Thumba but also got intensified further during 14:45–15:00 UT similar to Jicamarca. Third, during
15:00–15:15 UT, despite IMF Bz remaining steadily southward, the eastward electric field continued to
intensify over Thumba but weakened over Jicamarca. It is suggested that the changes in IMF By component
under southward IMF Bz condition are responsible for skewing the ionospheric equipotential patterns over
the dip equator in such a way that Thumba came into the same DP2 cell as that of Jicamarca leading to
anomalous electric field variations. Magnetic field measurements along the Indian and Jicamarca longitude
sectors and changes in high-latitude ionospheric convection patterns provide credence to this proposition.
Thus, the present investigation shows that the variations in IMF By are fundamentally important to
understand the prompt penetration effects over low latitudes.
