The seed population for energetic ions accelerated at CME-driven interplanetary shocks: Myth versus Reality

Sun-Earth Connections Seminar NASA/GSFC Jan 24 2003

M. I. Desai
University of Maryland

Energetic ions above ~0.05 MeV nucleon-1 are accelerated routinely at 
interplanetary (IP) shocks driven by fast coronal mass ejections (CMEs).  On 
the basis of a very limited set of ion composition measurements, these 
observations were generally interpreted in terms of diffusive shock 
acceleration of ions originating either from the bulk solar wind or its 
suprathermal tail.  Recently, however, composition measurements from ACE have 
provided new insights into the question of the origin of the source population 
for IP shocks.  In particular, we will present ACE observations that have 
unexpectedly shown large enrichments of the rare isotope 3He over the solar 
wind value in a number of IP shock events (Desai et al. 2001, ApJ 553 L89).  
The acceleration of tracer ions like 3He in such events is clear evidence that 
the energetic particle population is not accelerated out of the thermal solar 
wind, but rather out of a suprathermal pool whose energy region lies above the 
bulk solar wind speed.  A more recent survey of the elemental composition and 
energy spectra of heavy ions (from C-Fe) has further shown that the IP 
shock-accelerated ions could not have originated from a suprathermal tail that 
is composed mainly of solar wind ions.  Instead, our results indicate that the 
IP shocks accelerated a suprathermal population that is composed predominantly 
of ions left over from prior impulsive solar flares and large solar energetic 
particle events or SEPs.  Finally, we will highlight some of the future efforts
that are required to fully understand and model the injection and acceleration 
of energetic particles at CME-driven interplanetary shocks.