SH44C-01
Investigation of Ion Acceleration in Small RHESSI Flares
Solar flares accelerate both ions and electrons, and the energy content in accelerated ions may be comparable to that in accelerated electrons. The signature of ion acceleration is the emission of gamma-ray lines that result from nuclear interactions in the ambient solar atmosphere, and the gamma-ray line most easily observed by the high-resolution detectors on the Reuven Ramaty High-Energy Solar Spectroscopic Imager ( RHESSI) is the 2.223~MeV neutron-capture line. Unfortunately, gamma-ray lines are often too weak to observe in all but the largest flares, but it is of interest to be able to put constraints on ion acceleration in the more frequently occurring smaller flares. We combine the spectra of many small flares to obtain limits on the average neutron-capture line flux for small flares and discuss the implications of these limits relative to previous studies of larger flares. The work at the University of California, Berkeley, was supported by NASA contract NAS 5-98033.
SH44C-02
Backstreaming Electrons Associated With Solar Electron Bursts
Solar electron bursts are frequently observed in the ACE/SWEPAM suprathermal electron measurements at energies below 1.4 keV. A significant fraction of such events show backscattered electrons, beginning after the burst onset and traveling back towards the Sun along the magnetic field direction. Such backscattered particles imply a scattering mechanism beyond the spacecraft location. Some bursts also show backstreaming conic distributions, implying mirroring at magnetic field enhancements beyond the spacecraft. Here we present a study of these backstreaming particles during solar electron events. We examine the occurrence of backstreaming electrons and their relationship to other burst characteristics such as pitch angle width, duration, and energy range. We also investigate the time delay between burst onset and the appearance of backscattered electrons, including energy and pitch-angle dispersion. We examine the pitch angle distribution and energy dependence of backstreaming electrons, and consider possible origins of these electron distributions and their relationship to solar wind structure beyond the spacecraft.
SH44C-03
Modeling of Particle Acceleration and Transport in Gradual SEP Events Using the PATH Model
We discuss Particle Acceleration and Transport in the Heliosphere (PATH) numerical model developed at University of California at Riverside and present current progress on modeling of energetic particle acceleration at a traveling quasi-parallel CME-driven shock. We initiate the code by modeling a quiet-time solar wind background and then follow the propagation and evolution of an MHD shock from a distance of ~0.1 AU to the Earth's orbit. The model utilizes the solar wind parameters measured in situ by ACE. A semi-analytical approach is applied to simulate particle acceleration at the shock by injecting solar wind suprathermal ions locally. The diffusive shock acceleration mechanism due to the ion scattering by Alfvenic turbulence in the vicinity of the shock is adopted in the code. Monte-Carlo approach is used to follow transport of the energetic particles after they escape from the shock. The output of the PATH model includes time-dependent energetic particle fluxes, spectra and compositional ratios (Fe/O) for proton and heavy ions. We model specific SEP events and compare our modeling results with ACE measurements.
SH44C-04
Evidence for Mass-per-charge dependent acceleration of a multiple-component seed population by CME-driven interplanetary shocks near 1 AU
Using measurements from the ULEIS and SEPICA instruments on board the Advanced Composition Explorer between November 1997 and October 2000 we have surveyed the abundances of 0.25-0.8 MeV nucleon-1 He+, 3He, and heavy ions from C to Fe during 18 CME-driven interplanetary (IP) shocks observed near 1 AU. Our results show that each of the 18 IP shocks is accompanied by enhancements in the intensities of both 3He and He+ ions. In addition we find that, on a case-by-case basis, the abundances of He+ and the heavier elements such C to Fe (but not 3He) are depleted systematically as a function of the ion's M/Q ratio when compared with those measured in the ambient suprathermal ion population upstream of the IP shocks. These results show for the first time that individual IP shocks routinely accelerate ions from multiple seed populations such as multiples SEP events, CIRs, pick-up ions, etc., via systematic rigidity-dependent acceleration processes where ions with higher rigidity or M/Q ratios are accelerated less efficiently than those with lower M/Q ratios. We also compare the M/Q-dependent depletion of these abundances with the locally measured shock parameters and explore why the 3He abundance does not fit into the systematic M/Q-charge dependent fractionation processes.
SH44C-05
Simulations of Diffusion in Solar Energetic Particle Events
New observations by high-sensitivity instruments onboard the ACE spacecraft show that Fe and O may share similar injection profiles close the solar surface, and that diffusion dominates the transport of these particles (Mason et al 2006). Multi-spacecraft observations by Helios and IMP-8 also confirm the spatial diffusion is important (Wibberenz & Cane 2006). The "reservoir" phenomenon or "spatial invariance" states that during the decay phase of individual gradual solar energetic particle events, the intensities measured by different spacecraft are nearly equal, even if these spacecraft are separated by several AU in radius and by 70 degrees in latitude. Results from our multidimensional numerical model, based on Parker's transport equation, with reasonable values of κ\perp and κ\| are compared with observations from Ulysses, IMP-8, and ACE. We demonstrate that most of the features of the "reservoir" phenomenon can be reproduced by a transport model which includes drift, energy loss, and spatial diffusion.
SH44C-06
Instabilities in the Foot Region of Quasi-Perpendicular Shocks: Full Particle Electromagnetic Simulations
We have investigated quasi-perpendicular collisionless shocks by one-dimensional full particle simulations in order to determine the instabilities in the foot region in various parameter regimes. Due to specularly reflected ions in the foot there is free energy in the relative streaming of incoming and reflected ions, of incoming electrons and reflected ions, and of incoming ions and incoming electrons. The main instabilities expected are the Buneman instabilty (BI) and the modified two-stream instability (MTSI). In addition, phase standing whistler waves may be radiated upstream from the ramp and stand in the foot. The problem with identifying instabilities occuring in the foot region of collisionless shocks in PIC simulations is that low ratios of the electron plasma to electron gyrofrequency suppress the BI, whereas low ion to electron mass ratios suppress the MTSI. Having both ratios at the physical value is presently computationally not feasable. By making an appropriate compromise we can investigate both instabilities as well as the phase standing whistler waves at the same time and we determine their influence on the shock reformation process at different Mach numbers and shock normal - magnetic field angles. In Particular we will investigate the consequences for self-reformation at very high Mach numbers (MA>20).
SH44C-07
The Trapping of Low-Energy Particles by Interplanetary Shocks
Using ~0.045-10 MeV/nucleon ion data from ACE/ULEIS, we have found that a substantial number of shock- associated solar energetic particle events (20 events) have significant delays in the arrival of the low-energy component beyond what is expected from the travel time of energetic particles from the sun to the earth at 1 AU. Indeed, for some events, after correcting for the velocity dispersion, the low energy component (E < 0.1 MeV/nucleon) is almost completely absent while the high-energy component (E > 1 MeV/nucleon) has very large enhancements. SEP events with the most dramatic initial depletion of low-energy particles are accompanied by large proton fluxes and have large enhancements of the low-energy particles later, in coincidence with the arrival of the interplanetary shock, a day or two after the start of the event. In addition, these events show Fe/O enhancements during the periods in which the low-energy component is depleted and lower Fe/O values once the shock arrives. These new observations appear to be explained by the trapping of particles with low energy-to-charge (E/Q) ratios in the vicinity of the shock by magnetohydrodynamic waves, possibly generated by high energy protons streaming along the magnetic field lines.
SH44C-08
Consequences for Ground Level Enhancements (GLEs) from a Reduced Heliomagnetic Field
A recent assessment of the long-term records of solar energetic particle (SEP) events by McCracken (Space Weather, 2007) has concluded that the frequency of occurrence of large SEP events is smallest for the most active solar cycles. In particular, the E > 4 GeV fluences were ~ 10 times larger and 4 times more frequent in the 20 years preceding the onset of the space era in 1958. A return in the near future to the solar activity level characteristic of the Gleissberg minima of 1800-1820 and 1875-1910 could introduce an era of much more energetic SEP events with longer fluences than those observed since 1958. The working explanation of this variation of SEP event intensity is that the lower solar activity produces weaker magnetic fields, which decrease the coronal Alfven speeds, which reduce the compression ratios of shocks driven by coronal mass ejections (CMEs). This explanation implicitly assumes not only that other variables such as solar wind density and CME speeds are unchanged through a declining heliospheric magnetic field, but also assumes the existence of the controversial long-term increase in the heliospheric field. We examine these assumptions from two points of view: 1) the relationship of solar magnetic fields to CME and solar wind characteristics, and 2) our best knowledge of the long-term variations in solar wind properties.