Deep Solar Activity Minimum 2007 – 2009: Solar Wind Properties and Major Effects on the Terrestrial Magnetosphere
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We discuss the temporal variations and frequency distributions of solar wind and interplanetary magnetic field parameters during the solar minimum of 2007 – 2009 from measurements returned by the IMPACT and PLASTIC instruments on STEREO-A. We find that the density and total field strength were significantly weaker than in the previous minimum. The Alfvén Mach number was higher than typical. This reflects the weakness of magnetohydrodynamic (MHD) forces, and has a direct effect on the solar wind–magnetosphere interactions. We then discuss two major aspects that this weak solar activity had on the magnetosphere, using data from Wind and ground-based observations: i) the dayside contribution to the cross-polar cap potential (CPCP), and ii) the shapes of the magnetopause and bow shock. For i) we find a low interplanetary electric field of 1.3±0.9 mV m−1 and a CPCP of 37.3±20.2 kV. The auroral activity is closely correlated to the prevalent stream–stream interactions. We suggest that the Alfvén wave trains in the fast streams and Kelvin–Helmholtz instability were the predominant agents mediating the transfer of solar wind momentum and energy to the magnetosphere during this three-year period. For ii) we determine 328 magnetopause and 271 bow shock crossings made by Geotail, Cluster 1, and the THEMIS B and C spacecraft during a three-month interval when the daily averages of the magnetic and kinetic energy densities attained their lowest value during the three years under survey. We use the same numerical approach as in Fairfield’s (J. Geophys. Res. 76, 7600, 1971) empirical model and compare our findings with three magnetopause models. The stand-off distance of the subsolar magnetopause and bow shock were 11.8 R E and 14.35 R E, respectively. When comparing with Fairfield’s (1971) classic result, we find that the subsolar magnetosheath is thinner by ∼1 R E. This is mainly due to the low dynamic pressure which results in a sunward shift of the magnetopause. The magnetopause is more flared than in Fairfield’s model. By contrast the bow shock is less flared, and the latter is the result of weaker MHD forces.
KeywordsFlare Solar Wind Carrington Rotation Slow Solar Wind Interplanetary Coronal Mass Ejection
We are grateful to the referee for many helpful suggestions. We thank Marc Hairston, Jana Safrankova and Zdenek Nemecek for helpful comments. For the data used in the geomagnetic response section, we thank the World Data Center for Geomagnetism at the Kyoto web site, Japan; the OMNI website at Goddard Space Flight Center, USA; and the Defense Meteorological Satellite Program (DMSP) program, NOAA. Work at UNH was supported by NASA grants NNX10AQ29G and NAS5-03131. This work has received funding from the European Commissions’s Seventh Framework Programme (FP7/2007 – 2013) under the grant agreement No. 263253 [COMESEP]. V.O. was supported by NASA Grant NNXO9AR80G. M.T. greatly acknowledges the Austrian Science Fund (FWF): FWF V195-N16. This research was supported by a Marie Curie International Outgoing Fellowship within the 7th European Community Framework Program.
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