The Impact of Solar Activity on Forecasting the Upper Atmosphere via Assimilation of Electron Density Data
<p>We present a comprehensive comparison of the impact of solar activity&#160;on forecasting the ionosphere and thermosphere. Here we investigate the&#160;response of physics-based TIE-GCM (thermosphere-ionosphere-electrodynamics general circulation model) in a data assimilation scheme through&#160;assimilating radio occultation (RO)-derived electron density (Ne) using an ensemble Kalman filter (KF). Constellation observations of Ne&#160;profiles offer opportunities to assess the accuracy of the model forecasted state on a global scale. In this study, we emphasise the&#160;importance of understanding how the assimilation results vary with solar activity, which is one of the primary drivers of thermosphere-ionosphere dynamics.</p><p>We validate the assimilation results with independent RO-derived GRACE&#160;(Gravity Recovery and Climate Experiment mission) Ne data. The main&#160;result is that the forecast Ne agree best with data during the solar&#160;minimum compared to solar maximum. The results also show that the&#160;assimilation scheme significantly adjusts both the nowcast and forecast&#160;states during the two solar activity periods. The results show that TIE-GCM significantly underestimate Ne in low altitudes below 250 km&#160;and the assimilation of Ne is not as effective in these lower altitudes compared to higher altitudes. The results demonstrate that assimilation&#160;of Ne significantly impacts the neutral mass density estimates via the KF state vector. This impact is larger during solar maximum than solar&#160;minimum relative to a control run. The results also demonstrate that the impact of assimilation of Ne on neutral mass density state persists&#160;through to forecast state better during solar minimum compared to solar&#160;maximum. The results are useful to explain the inherent model bias, to&#160;understand the limitations of the data, and to demonstrate the capability of the assimilation technique.</p>