IPIM modeling of the cause of the ionospheric F2-layer depletion at high-latitude during High Speed Stream event

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Abstract

We intend to understand the impact of a High Speed Stream event on the high latitude ionosphere, and more specifically the observed decrease of the FoF2 frequency. First, we have selected one summertime event, for which a large dataset was available: SuperDARN and EISCAT IS radars, Tromso and Sodankyla ionosondes and the CHAMP satellite and we modeled with IPIM the dynamics of the ionosphere at Tromso and Sodankyla using inputs derived from the data. We obtained simulated results which nicely match the measurements made by IS radar and ionosondes at both locations, and we showed that the decrease of FoF2 is associated with a transition from F2 layer to F1 layer resulting from a decrease of neutral atomic oxygen concentration.

This modelling allowed showing that electrodynamics can explain the short term behavior on the scale of a few hours, but the long term behavior on the scale of a few days results from the perturbation induced on the atmosphere. The enhancement of the convection is responsible for a sharp increase of the ion temperature by Joule heating, which leads through chemistry to an immediate reduction of the F2 layer. Then, the ion drag on the neutral atmosphere is responsible for a rapid heating and expansion of the thermosphere. This expansion affects the atomic oxygen through a non thermal upward flow which results in a decrease of its concentration and amplifies the decrease of [O]/[N2] ratio due to thermal expansion. This thermosphere alteration explains the long term extinction of the F2 layer.