The effects of Corotating interaction region/High speed stream storms on the thermosphere and ionosphere during the last solar minimum

Geomagnetic storms at solar minimum are driven by the interaction between high speed streams and low speed streams (Corotating Interactions regions/High Speed Streams-CIR/HSSs- this includes both the CIR part of the storm and the HSS part as both has effects on the thermosphere and ionosphere), rather than by Coronal Mass Ejections (CMEs). Solar minimum storms are generally of smaller amplitude, but they also have other characteristics that affect the response of the thermosphere/ionosphere (TI) system to them. We explore both the background upper atmosphere and the characteristics of these CIR/HSS events in 2008 using both models and data. The model data comparisons were good, showing mid-latitude positive storm effects on NmF2 on the day of the storm and long, extended periods of storm induced changes on the following days. Generally, the version of the NCAR-TIEGCM (National Center for Atmospheric Research-Thermosphere Ionosphere Electrodynamics Model) run with the Heelis convection pattern was in better agreement with the data than the version run with the Weimer convection pattern. We attribute this difference in the response of the two versions of the NCAR-TIEGCM to the fact that the precipitation we associate with the Heelis model has been tuned to better represent observed precipitation over many years, whereas such tuning has just begun for the version of the NCAR-TIEGCM that uses the Weimer model. Physically, the most important conclusion that we draw is the CIR/HSS events can have significant effects on the ionosphere and thermosphere for several days after the CIR has ended. While CIR/HSS events are normally weaker than CME events, the effects of the latter normally disappear in a day or two. Thus the effects of CIR/HSS events may be comparable to those of CME events for some fields, notably neutral density changes. (C) 2012 Elsevier Ltd. All rights reserved.