Response of equatorial‐low latitude ionosphere to sudden expansion of magnetosphere

The response of the equatorial‐low latitude nightside ionosphere to the geomagnetic negative sudden impulse(si − ) on March 15, 1993 is studied using recordings of Doppler velocity of ionospheric echoes at vertical incidence at Kodaikanal(10.2°N) and Doppler shift of standard HF signals on oblique paths at Lunping(25.0°N) and Kure(34.25°N). The si − at 1541 UT is characterised by a simple decrease of H‐field at low latitude stations widely distributed in longitude, and by a double‐pulse structure at mid and high latitude stations on the dayside. The usual downward drift of F region plasma during the premidnight hours over Kodaikanal near the dip equator abruptly increased for ≈ 2.5 min coincident with the first pulse of the si − and immediately reversed direction to upward. Recordings at Lunping and Kure also showed short‐lived Doppler frequency deviations simulataneous with those at Kodaikanal and of the same polarity and sequence. These observations constitute the first and direct experimental evidence for vertical plasma motions due to si − associated electric fields in the nighttime equatorial‐low latitude ionosphere. The case study supports the view that si − can be explained by the physical model of sc/si + with a reversal in the direction of the global current systems responsible for the groundlevel magnetic field variations.