Johns Hopkins University Applied Physics Laboratory
Energetic particle activity at 1 AU during the current Solar Minimum
The current solar minimum is unprecedented since the start of space age with sunspot count and similar measures of solar activity reaching low levels not seen for about a century. Although the last significant solar energetic particle events took place in December 2006, nevertheless there has been a good deal of transient energetic particle activity at 1 AU. This activity has been observed from new vantage points with the twin STEREO spacecraft, and the continued operation of instruments on the Advanced Composition Explorer (ACE) mission. During this period, high speed solar wind streams continued to generate Corotating Interaction Regions (CIRs) with great regularity, and at a significantly higher level than the previous solar minimum in 1996-97. This unexpected development was caused by the nearly continuous presence of coronal holes near the solar equator and may be related to the weak solar magnetic fields at the poles during the current minimum. The combined capabilities of ACE and STEREO made it possible to characterize new features of CIRs since single events moved past the three spacecraft in sequence, revealing a surprising degree of variability. We will discuss these features of the 2007-2010 solar minimum, and will also comment on a couple of examples of small, 3He-rich solar energetic particle events whose source regions could be identified without problems of source confusion generally present during active periods. These events lacked typical features such as type III radio bursts generally associated with impulsive events, and may give new insights into possible acceleration mechanisms.