Evidence for a Suprathermal Seed Population of Heavy Ions Accelerated by Interplanetary Shocks near 1 AU

Abstract

We have surveyed the 0.1-10 MeV nucleon À1 elemental abundances at 72 interplanetary (IP) shocks observed by the Ultra-Low-Energy Isotope Spectrometer on board the Advanced Composition Explorer from 1997 October through 2002 September. We find the following: (1) The C/O ratios in IP shocks were substantially depleted (by more than $40%) relative to solar wind values. (2) The IP shock abundances were poorly correlated with those measured in the slow and fast solar wind. (3) Energetic ions above $0.1 MeV nucleon À1 from impulsive and gradual solar energetic particle events (SEPs) were present upstream of all the IP shocks in our survey. (4) The $1 MeV nucleon À1 Fe/O ratio in IP shocks was positively correlated with that measured upstream of the shocks. (5) The IP shock abundances were well correlated with the upstream abundances, with a negative dependence on mass/charge. (6) The mean Fe/O ratio in IP shocks exhibited a positive correlation with the level of solar activity, as measured by the occurrence rates of X-ray flares and sunspots. The above results are inconsistent with shock acceleration of ions originating mainly from the bulk solar wind or a suprathermal tail composed predominantly of solar wind ions. Instead, it appears that for the events surveyed here, the IP shocks accelerated a seed population predominantly comprising ions that were previously accelerated in impulsive and gradual SEPs and that the shock acceleration process accelerated higher rigidity ions less efficiently than lower rigidity ions.

The Origin of Element Abundance Variations in Solar Energetic Particles

“…Desai et al, 2003). Jets are also seen in solar-polar regions and, in principle, could contribute to gradual SEP events with cooler source plasma.…”

Element Abundances in Solar Energetic Particles and the Solar Corona

“…These events are magnetically well-connected to the shock when it is near the Sun and the Parker spiral of the field sweeps the Meyer (1985) or the averaged abundances of Reames (1995a). Other A/Qdependent enhancements, such as the impulsive-suprathermal ions in the seed populations (Desai et al, 2003(Desai et al, , 2006Tylka et al, 2005;, must also be considered, although their effects are greatest above ten MeV amu -1 .…”

“…Desai et al, 2003Desai et al, , 2004Desai et al, , 2006Slocum et al, 2003;Cohen et al, 2005Cohen et al, , 2007Tylka et al, 2005Leske et al, 2007;Reames and Ng, 2010 , in very large SEP events, intensities are strongly suppressed until the approach of the shock near one AU (e.g. Desai et al, 2006), so that abundances are more typical of accelerated local plasma (Desai et al, 2006) than of the corona.…”

Acceleration of Low-Energy Ions at Parallel Shocks With a Focused Transport Model

“…However, the sources of the energetic particles in the heliosphere are usually regarded as low-energy thermal or suprathermal ions of solar and interplanetary origin (e.g., Zank et al 2001;Mason et al 2005Mason et al , 2012Desai et al 2003Desai et al , 2006Desai & Burgess 2008). For example, there is strong evidence that a small portion of thermal and suprathermal particles from hot corona material or remnants of previous solar energetic particle (SEP) events serve as the source of large SEP events (Desai et al 2006).…”