Observations of shock impact, disturbance dynamo effect, and a midlatitude large‐density depletion at 600 km altitude on the 17 April 2002 storm day

Abstract

ROCSAT‐1 orbiting at 600 km altitude has observed a series of interesting topside ionospheric responses to the 17 April 2002 magnetic storm. After the impingement of the interplanetary shock, a Pi‐1 pulsation event in the plasma flow and ion density oscillations with a period of 19 s in the postnoon sector has been observed for less than 2 min. The storm induced disturbance dynamo effect is then observed an hour later with the postmidnight enhancement of the upward flow pattern described in the model of Scherliess and Fejer [1997] and the enhanced zonal flow pattern described in the models of Heelis and Coley [1992] and Scherliess et al. [2001]. However, the dynamo induced vertical electric field is noticed to be much weaker and decays faster than the induced eastward electric field in the postmidnight sector. A large‐density depletion is then observed at the end of this disturbance dynamo effect. It occurs at latitude 34°N (dip latitude 27°N) at the sunrise terminator. The density decreases about two orders of magnitude from the background level and the depletion width is about 500 km wide. An outward expanding flow up to 725 m/s is found inside the depletion. The NO+ ions found inside the depletion imply that the depletion was lifted‐up from the bottomside ionosphere in agreement with the observed large outward expanding flow. Large and rapid east‐west fluctuating flows up to 250 m/s are also observed together with large parallel flow variations of similar magnitude. The temperature inside the depletion is found increased to a peak value of 2050 K from the background temperature of 1200 K outside the depletion. However, this hottest temperature does not occur at the deepest density depletion. The observed temperature increase does not seem to fit in a proposed model of Oyama et al. [1988] where the heating comes from energy deposit of the fast electrons that are produced during the storm‐time and transported from the sunlit apex height at the equatorial region. The unfavorable reason is based on the ineffective energy exchanges between electrons and ions due to large ion mass and infrequent collisions between them at topside ionosphere. Furthermore, the observation fails to support the proposed inverse correlation between the temperature and the density. Instead, the temperature increase could come from the adiabatic heating by compressing ions along the flux tube from the equatorial region to high latitudes in the models of Heelis et al. [1978] and Bailey and Heelis [1980] in which the required large parallel flow has been observed.