Analysis of the positive ionospheric response to a moderate geomagnetic storm using a global numerical model

TLDR: In this article, a case study of a moderate geomagnetic storm at low solar activity during the northern winter solstice exemplifies the complex storm phenomena, and the authors focused on positive iono- spheric storm eAects in relation to thermospheric disturbances in general and thermosphere composition changes in particular.

Abstract: Current theories of F-layer storms are dis- cussed using numerical simulations with the Upper Atmosphere Model, a global self-consistent, time de- pendent numerical model of the thermosphere-iono- sphere-plasmasphere-magnetosphere system including electrodynamical coupling eAects. A case study of a moderate geomagnetic storm at low solar activity during the northern winter solstice exemplifies the complex storm phenomena. The study focuses on positive iono- spheric storm eAects in relation to thermospheric disturbances in general and thermospheric composition changes in particular. It investigates the dynamical eAects of both neutral meridional winds and electric fields caused by the disturbance dynamo eAect. The penetration of short-time electric fields of magneto- spheric origin during storm intensification phases is shown for the first time in this model study. Compar- isons of the calculated thermospheric composition changes with satellite observations of AE-C and ESRO-4 during storm time show a good agreement. The empirical MSISE90 model, however, is less consis- tent with the simulations. It does not show the equatorward propagation of the disturbances and pre- dicts that they have a gentler latitudinal gradient. Both theoretical and experimental data reveal that although the ratio of (O)/(N2) at high latitudes decreases signif- icantly during the magnetic storm compared with the quiet time level, at mid to low latitudes it does not increase (at fixed altitudes) above the quiet reference level. Meanwhile, the ionospheric storm is positive there. We conclude that the positive phase of the ionospheric storm is mainly due to uplifting of ionospheric F2-region plasma at mid latitudes and its equatorward movement at low latitudes along geomagnetic field lines caused by large-scale neutral wind circulation and the passage of travelling atmospheric disturbances (TADs). The calcu- lated zonal electric field disturbances also help to create the positive ionospheric disturbances both at middle and low latitudes. Minor contributions arise from the general density enhancement of all constituents during geomagnetic storms, which favours ion production processes above ion losses at fixed height under day- light conditions.