SH23C-01
A Study of Solar Energetic Particle Events of Cycle 23
We examine the properties and associations of more than 200 solar proton events that extended above 25 MeV and occurred in the years 1997 through 2006. The properties include early peak intensities of e, H, He, O, and Fe, each at two different energies. The data are obtained from instruments on the IMP-8, SOHO, Wind, and ACE spacecraft. Solar event associations are made for as many events as possible and corresponding solar parameters are determined. These include Coronal Mass Ejection (CME) and flare properties and radio emissions from the metric through kilometric wavelength range. The events are divided into 5 major groups based on the relative abundances and particle profiles. Examples of events and basic correlations are illustrated. Tables containing our results will be made available via the internet.
SH23C-02
Inferred Ionic Charge States in 3He-rich Solar Energetic Particle Events
Ionic charge states can provide important indications of the physical processes and environments associated with the acceleration and transport of solar energetic particles (SEPs). To date, direct measurements of charge states in 3He-rich SEP events have not been made at energies above an MeV/nuc because the combination of high energy and low intensity is not compatible with present measurement technology employing either electrostatic deflection instruments or geomagnetic filtering techniques. Using data from the Solar Isotope Spectrometer (SIS) on ACE, we infer charge states at high energies in a number of 3He-rich events using an indirect technique introduced by Cohen et al. (1999) that is based on mass and charge fractionation as a power law in Q/M. In the best-measured event (20 Aug 2002), the derived Q values are found to have a simple dependence on atomic number, Z, with step increases in the mean number of attached electrons between Z=6 and 7, 8 and 10, and 16 and 18. This pattern, in conjunction with the known masses of the dominant isotopes, is sufficient to account for much of the observed fine structure in elemental abundance enhancement patterns in 3He-rich events relative to solar wind composition. We also have considered the alternative possibility that the fractionation depends on the parameter Q2/M, which affects the time scale for Coulomb losses in the acceleration region. In this case the inferred charge states are close to values found in the corona and solar wind. Comparison with charge states that have been directly measured below ~0.5 MeV/nuc indicates that the fractionation occurs before the particles have been accelerated beyond ~0.1 MeV/nuc and undergone collisional stripping. We present the charge state composition patterns obtained using these two assumptions about the form of the fractionation and then discuss and contrast the implications of the inferred charge states for the origin of 3He-rich SEP events. This work was supported by NASA at Caltech (under grants NAG5-12929 and NNX08AI11G), JPL, and GSFC.
SH23C-03
Solar Cycle Variations of Suprathermal Fe/O and He3/He4 Abundance Ratios Associated With CME-driven Interplanetary Shocks
CME-driven interplanetary (IP) shocks are presently believed to accelerate material out of a complex and dynamic pool of suprathermal material that is apparently ubiquitous in the inner heliosphere under all types of solar activity conditions. We survey the elemental abundances of suprathermal (200 keV/nucleon) heavy ions from 3He through Fe associated with ~80 such CME-driven IP shocks observed by the Ultra-Low-Energy Isotope Spectrometer (ULEIS) on board the Advanced Composition Explorer (ACE) and by the SupraThermal-through-Energetic Particle (STEP) instrument on board the Wind spacecraft from October 1997 through June 2008. In particular, we examine the dependences of these abundances on the phase of the solar cycle. Finally, we discuss our results in terms of the origin of the suprathermal seed populations through the various phases of the solar cycle and their relationship with various shock acceleration mechanisms.
SH23C-04
On the Location of the Acceleration Site for Energetic Helium-3 and Implications for Flare Models
The wide variation in the 3He/4He ratio in solar energetic particle events is most plausibly interpreted in terms of two distinct acceleration mechanisms for helium nuclei, one of which favors 3He. 3He-rich events are associated with impulsive energetic electron events which have (at least) two distinct acceleration mechanisms. Logically one of these should accelerate both 3He and electrons. Based on the electron energy spectrum from 3He-rich events Kahler et al (1987) proposed that the acceleration region should be the high corona, possibly as high as 2 solar radii. Two further observations provide information on where the 3He acceleration might occur. First is the remarkable upper limit on the total 3He fluence (Ho et al, 2005, 2008) which strongly suggests a model where the acceleration process accelerates the majority of 3He ions within a finite reservoir, which can only be a high coronal loop structure. The second is that at quiet times, at 1AU, the 3He/4He ratio is 6-60 times enhanced over the corresponding slow solar wind value (Desai et al, 2006). Both observations indicate that the acceleration, which favors 3He, is occurring quasi-continuously in the high coronal structure, with some leakage into the interplanetary medium. Eventually the coronal structure is disrupted and the trapped population is either released into the interplanetary medium or dumped into the "flare" site to provide both energy and seed particles for further acceleration, but by a different process which does not preferentially accelerate 3He. In the former case there will not be a significant flare, but a 3He-rich event, an impulsive electron event, and perhaps a fast jet from one of the footpoints of the trapping structure. In the latter case there will be a significant flare, with a major coronal mass ejection, probably driving a shock, which produces a non-3He-rich event and a much longer and larger energetic particle event.
SH23C-05
Ion Acceleration During Magnetic Reconnection
We explore the mechanism for ion acceleration during magnetic reconnection to understand the energetic particle spectra produced during flares and in the solar wind. Reconnection driven ion acceleration is initiated as particles move from upstream into the Alfvenic exhaust. In the case of a weak guide field, protons and higher mass particles behave like pickup particles in that they abruptly cross a narrow boundary layer and find themselves in a region of Alfvenic outflow. Their motion then mimics that of a classic pickup ion, gaining an Alfvenic ExB flow in the jet and a thermal speed close to the Alfven speed. In the case of a strong guide field particle acceleration is strongly enhanced for ions with high mass-to-charge (m/q) ratio since these particles act as pick-up particles while small m/q particles are adiabatic. Once ions become super-Aflvenic their acceleration is, like electrons, dominated by Fermi reflection during island contraction and their energy increases until it is limited by firehose marginal stability. The ion distribution function for super-Alfvenic ions then takes the form of a v-5 distribution. During reconnection in a multi-island environment, as in flares, strong enhancements in high m/q ions are expected. This picture is consistent with several observations related to flare and local solar wind ion acceleration: (1) the ubiquitous observations of energy proportional to mass; (2) strong enhancements in high m/q ions during impulsive flares; and (3) the temperature increments of solar wind exhausts.
SH23C-06
Role of Self-Amplified Waves in Coronal Shock Acceleration
Solar-energetic-proton (SEP) acceleration at a parallel shock is modeled with self-consistent Alfvén-wave excitation and shock transmission. 18 - 50 keV seed protons at 0.001 plasma proton density are accelerated in 10 minutes to a power-law intensity spectrum rolling over at ~ 300 MeV by a 2500 km/s shock launched from 3.5 solar radii in typical coronal condition with low ambient wave intensity. Scattering of large pitch-angle high-energy SEPs by waves amplified by small pitch-angle low-energy SEPs bootstraps the acceleration. The rapid acceleration is inappreciably retarded by wavenumber shift of shock-transmitted Alfvén waves or by the interaction of sunward streaming SEPs with downstream waves. There is no significant second-order Fermi acceleration. SEP acceleration beyond the 'knee' energy proceeds from large to small pitch-angles, for gyroresonant wave-particle interaction at k ~ B/(μ P). B is magnetic field, k wavenumber, P rigidity and μ pitch-cosine. Classical mean-free-path is inapplicable in this time-dependent process. Using the above μ-dependent resonant condition to calculate wave-growth rates but the 'sharpened' condition k ~ B/P to calculate μ-scattering rates underestimates scattering by self-amplified waves, giving little acceleration (Berezhko et al 1998). Using k ~ B/P to calculate wave-growth and scattering rates (Vainio & Laitinen 2007) is consistent but incorrectly requires the SEPs to amplify unique waves from the weak ambient state. In contrast, true gyroresonant interaction must 'relay' the influence of amplified waves from low-P large-μ SEPs to high-P low-μ SEPs, leading to fast acceleration. Model results will be presented and dependence on model parameters will be discussed.
SH23C-07
Modeling the transport of protons and heavy ions in SEP events
In Solar Energetic Particle (SEP) events, particles are accelerated at flares and/or shocks and propagate to 1 AU. Their propagation is affected by the MHD turbulence of the solar wind. Therefore a better understanding of these events (e.g. to obtain the injection profile at the Sun in impulsive SEP events) requires that we understand how charged particles interact with solar wind MHD turbulence. We report in this work our recent modeling effort of ion transport in SEP events for ions with a range of charge-to-mass ratios. We describe particle-turbulence interaction using an extended Quasi Linear Theory. A Monte-Carlo simulation following single particle motion that is based on the stochastic differential equation method is developed. Among other advantages over traditional finite difference method, our approach allows us to easily investigate the effect of an radial dependent diffusion coefficient (Duu) on the observed time intensity profile and spectra. Individual events are selected for modeling. Results and implications from our modeling are shown and discussed.
SH23C-08
Hybrid Simulations of ion Acceleration at Interplanetary Shocks: Decoupling From the Wave Turbulence on Large Scales and Resulting Flux Profiles
Several different methods have been developed over the past years to make progress in calculating and predicting solar energetic particles (SEPs) that result from CME-driven shocks - also to forecast their associated hazards. Ion acceleration at interplanetary (IP) shocks involves complex processes that happen on vastly different scales, from the thermal ion scales of the kinetic shock transition to those of the transport and radial plasma changes in the solar wind environment. We have adopted an approach that separates these scales by assuming energization takes place in a limited region, outside of which the particles are assumed to more or less freely stream along the interplanetary magnetic field (IMF). In this approach, both the shock strength and IMF can be obtained from MHD simulations, while the energetic ion fluxes may be derived form the known shock parameters via heuristic methods, from shock acceleration theory, or from particle simulations. In the past, however, such particle simulations were very local. That is, they only described the local enhancement of energetic ions at the shock, what corresponds to observed ESP events, when the CME-driven IP shock passes the spacecraft. We have previously shown that for parallel and slightly oblique shocks, the resulting peak fluxes at moderate energies agree well with observations at undisturbed, isolated events. However, modeling of the arriving SEPs at Earth (or other locations of interest) from remote shocks also requires knowledge of the freely streaming fluxes and their pitch angle distributions, once the energetic ions are decoupled from the wave turbulence surrounding the shock. Here we present results of sufficiently large hybrid simulations (kinetic ions, electron fluid) that allow us to study the decoupling of the energetic ions with distance from the shock, and its dependence on energy. We compare the calculated flux profiles with those derived from ACE observations, and illustrate how our results can be used in global SEP model calculations.