The Response of the D and E Regions of the Equatorial Ionosphere to Solar Flare Events

Abstract

AbstractUsing measurements from a ground‐based magnetometer and an in‐house developed quasi‐two‐dimensional theoretical ionospheric model (QTD), the differential response of D and E regions of the equatorial ionosphere to the solar flare events which occurred on 20 February 2002 (M1.5 class), and 24 September 2011 (X1.9 class) has been investigated. The ground‐based magnetometer at Tirunelveli (8.71°N, 77.75°E, 2.9°S dip latitude) was used to monitor variations in the magnetic field (ΔH) during the two events. An abrupt increase in ΔH [positive Solar Flare Effect (SFE)] was noted during both the flare events, a signature of the prompt response to the flare. The ΔH, however, was seen to get weakened (reversed SFE) on 20 February 2002 at a time delay of about 10 min post flare peak. The weakening in ΔH happened during local noon (∼11:30 LT), a period when there was no sign of the counter electrojet (CEJ). The QTD model was used to simulate the ratio of field line integrated Hall to Pedersen conductivity and variations in the east–west currents during the flare events. Model simulations show differential response in the conductivity ratio at the D and E‐regions of the ionosphere during the period when the ionospheric current system was undergoing alternations manifesting as ΔH variations in the ground‐based magnetometer observations. We show that in addition to the changes in conductivity during a solar flare, the electric field variations also modulate ionospheric currents to produce differential footprints in ΔH as positive/negative SFE, SFE*, or reversed SFE.