SH41A-0297
Computational Study of Particle Cross-Field Diffusion Caused by Interaction With Interplanetary Magnetic Decreases
In the last ~ 3 decades, magnetic field observations have shown the existence of isolated regions in interplanetary space and even into the Earth's magnetosphere where the magnitude of the magnetic field drops to few tenths, or less, of its ambient values. These regions are now called magnetic decreases (MDs). Particles within MDs are heated preferentially perpendicular to B0, the ambient magnetic field. MDs are believed to be caused by a diamagnetic effect from these perpendicularly accelerated protons. The decrease of the magnetic field is accompanied by an increase of particle density in such a way that the balance pressure (magnetic plus plasma thermal) is sustained. The proton acceleration is associated with the dissipation of phase-steepened Alfvén waves, presumably through the ponderomotive force. In this paper we are interested in investigating the non-resonant particles cross-field diffusion due to their interactions with MDs at large heliospheric latitudes. To do such a study, we start from distribution functions obtained from analytical fits to experimental distributions, for an ensemble of high heliospheric latitude MDs observed by Ulysses. The distribution functions are dependent on the ratio between the magnitude values of the magnetic field inside the MD and around it, and on the space dimension of the MD. These distribution functions are them statistically represented by points, obtained using Monte Carlo method. The particle diffusion is analyzed using a theoretical model presented by Tsurutani et al. (Nonlinear Processes in Geophys., 6, 235, 1999). For the simulations, we let a particle with specific energy interacts with an MD of a field decrease and thickness given by the selected random points from the distribution functions. This process can be continued until all particles have been scattered. This computational simulation has other applications besides the high heliospheric latitude, such as interplanetary/magnetospheric diffusion of energetic particles, cosmic rays and other astrophysical problems
SH41A-0298
Shock Acceleration and Heavy ion Spectral Variability in Large SEP Events
Solar energetic particle (SEP) events can roughly be divided into gradual and impulsive events, when considering measurements in the energy range of a few MeV / nucleon. The gradual events typically have much larger particle intensities and coronal elemental abundances, while impulsive events typically show higher mean ionic charges, enhanced 3He/4He ratio and enhanced heavy ion abundances. However, above energies of a few tens of MeVs / nucleon, large gradual SEP events are highly variable in their heavy ion characteristics and some of the differences between gradual and impulsive events become somewhat blurred. For instance, some gradual events show enhanced Fe/O and 3He/4He ratios and elevated mean Fe charge states -- characteristics which are typically associated with impulsive events. It has been proposed that this variability results from a compound seed population, consisting of suprathermal ions from the solar corona and suprathermal ions from flares, and the evolution of the shock oblique angle, as a coronal mass ejection-driven shock moves outwards from the Sun. We present results from test-particle simulations which confirm that diffusive shock acceleration can produce most of the observed features if the seed population contains ions pre-accelerated at flares. We also discuss the features of the produced energy spectra and phenomena which affect them.
SH41A-0299
The Role of Modified Two-Stream Instability for Self-Reformation and Ion Acceleration in Quasi-Perpendicular Shocks
A number of one-dimensional full particle simulations of high Mach number quasi-perpendicular shocks are performed to investigate roles of modified two-stream instability (a microscopic process) in a self-reformation process (a macroscopic process). Spatial and temporal scales of self-reformation processes significantly increase with amplitude of small scale waves generated by modified two-stream instatility in the foot. Furthermore, an influence of the increase of the scales on maximum ion energy attained during one reformation cycle is not negligible. Based on the simulation results a mechanism for the above mentioned scale variation of the self-reformation process and the resulting variation of the maximum attainable ion energy in the foot region is proposed. This mechanism is based on scattering of some of the reflected ions by the waves produced by the modified two-stream instability in the foot and subsequently reflected at the ramp and/or downstream.
SH41A-0300
Toward a better understanding on particle acceleration in solar flares within the framework of the GEMSIS project
GEMSIS (Geospace Environment Modeling System for Integrated Studies) is one of projects in Solar-Terrestrial Environment Laboratory, Nagoya University. Its final goal is to build a geospace-environment model based on various (satellite and ground-based) observational facts in order to understand the dynamic energy-transport- processes taking place in geospace. In the first 3-years from this year, we set a few individual scientific targets as fundamental elements/information for the final model. One of them is to know how the accelerated particles are accelerated and lose their energies in solar flares. Here, we briefly introduce our approach to this scientific target. Thanks to Yohkoh and RHESSI observations, some essentials (energy spectrum of accelerated electrons, acceleration site, time-scale of acceleration) for revealing the acceleration mechanism are somehow obtained. However, we don"t have any direct observations about pitch-angle distribution of accelerated electrons even though it is important information to identify the acceleration mechanism. In some flares, most of accelerated electrons are trapped in a magnetic loop system, and stay in the loop for a few minutes until they precipitate into the footpoint region due to pitch-angle scattering via collision. At that time, hard X-ray thin-target emission and microwave gyrosynchrotron emissions from trapped electrons are observed in the corona and mainly hard X-ray thick target emissions are observed at the footpoint region. The temporal behavior of spectrum and intensity of each source depends on the initial pitch-angle distribution of accelerated electrons. However, it also depends on other two parameters, i.e., the magnetic mirror efficiency and the plasma density in the loop. First we must determine these two parameters. In this paper, some preliminary results are introduced.
SH41A-0301
Suprathermal particles in CIRs at 1 AU
Because of their already high energy, suprathermal particles likely serve as the source population for further acceleration at interplanetary shocks. The very low solar activity in 2007 and the recurrent corotating particle events make 2007 an ideal time period to study the abundance of non-flare associated, i.e. quiet-time, suprathermal particles. Using a maximum-likelihood method based on Poissonian statistics which is well adapted to deriving fluxes from small count numbers, we determine the fluxes of suprathermal particles in 2007. We will present results for CIRs, high-speed streams, slow wind, and solar wind dwells.
SH41A-0302
Long-Term Test Results and Implementation of Radiation Storm Forecasting with SOHO/COSTEP's Relativistic Electrons
A method of forecasting the intensity of prompt solar energetic protons of hazardous energies (~40 MeV) with relativistic electrons has been developed but, initially, tested only for 2003. It has been shown that forecasting of sudden intensity increases of such protons from solar energetic particle events is relevant for radiation protection of humans and technology in space. The method utilizes the speed advantage of electrons over 30-50 MeV protons and newly discovered correlations of inverse rise time and intensity between the two particle species. Its main advantage over other forecasting attempts is that electrons act as test particles by probing the ever- changing heliospheric transport conditions that act on the slower moving protons. The purpose of this presentation is to inform of forecasting attempts in the period from 2004-2007 under near solar minimum conditions with its unusually high occurrence frequency of major radiation storms, and to give an update on the implementation of the method with the SOHO/COSTEP team and the SOHO project.
SH41A-0303
Energetic Particles in the Quiet Corona
We address the fates of energetic particles in typical solar minimum coronal magnetic fields. A baseline density of such particles will be set by cosmic ray albedo neutron decay. Other, electromagnetic mechanisms may augment this. PFSS extrapolation is combined with synoptic magnetogram data to constrain the coronal fields, and particle trajectories are determined in the guiding center approximation. We discuss the extent to which solar analogues of terrestrial magnetospheric phenomena (radiation belts, ring current) may exist near solar minimum.
SH41A-0304
3D Visualization of Solar Disk: Martian Radiation Assessment of Solar Particle Events
During 2002 and 2003, MARIE (Martian Radiation Environment Experiment) instrument onboard the 2001 Mars Odyssey spacecraft provided some unique data from the Martian orbit. The orbit alignment of Mars-Sun-Earth provided an opportunity between 180 degrees (August 2002) and 0 degrees (October 2003). During this time, the MARIE data included the background GCR (Galactic Cosmic Rays) and several SPE (Solar Particle Events) enhanced radiation dose-rate measurements at Mars. Nearly 40 times increase in the quiet-time GCR dose-rate was noted from about 25 mrad/day to nearly 1000 mrad/day at Mars. Understanding the active regions on the Sun that are likely to result into SPE on the far side will also be of concern for future deep space explorations beyond LEO. We present our approach in depicting SPE with 3D visualization of solar disks facing Mars and Earth. We present the assessment of SPE activity between 2004 and 2006 towards Mars along with an estimated dose-rate during an SPE at Mars along with heliosphere distribution.
SH41A-0305
Temporary topological trapping and escape of charged particles in a flux tube
The scenario of temporary trapping of magnetic field lines and their subsequent suppressed diffusive escape from topological magnetic structures embedded in turbulence has been offered as a way to understand the persistence of "dropouts", or sharp gradients in observed heliospheric energetic particle intensities. Here we present a set of numerical experiments to show the basic physics of this process: charged test particles can be temporarily trapped in flux tubes and then escape due to slab turbulence of magnetic field. The overall effect is a delay in the onset of time-asymptotic transport. We thus confirm that previous arguments based on field line transport are also applicable to test particle transport.
SH41A-0306
MHD-PIC interlocked simulation model in space plasma: application to collisionless shocks
A new simulation model has been developed to understand the multi-scale coupling in space plasmas. In this new model, so-called "macro-micro interlocked simulation", Hall-MHD and PIC/Hybrid simulations are simultaneously performed, and the mutual interaction between them is handled self-consistently. The model can treat MHD-scale dynamics including particle kinetic effects (Sugiyama and Kusano, JCP, 2007). Here we have applied this interlocked simulation model onto a collisionless shock problem. Hall-MHD simulation covers the whole system, and, only in the shock transition region, PIC/Hybrid simulation is embedded to incorporate the wave-particle interaction for dissipation processes at shock. It has been demonstrated that the interlocked model can work well for the shock-related multiscale coupling processes: (1) the whistler waves excited in the shock transition region in PIC model propagate into the upstream region and modify the incoming plasma flow, and (2) the reflected ions at the shock transition region in Hybrid model escape into the far upstream region and excite the upstream Alfven waves. By following the motions of the escaped ions both in Hybrid region and in Hall-MHD region self-consistently, we can see the diffusive acceleration process in the wide region of the simulation system. The results are a good indication of the validity and the effectiveness of the new model for the multiscale plasma processes.
SH41A-0307
Joint Fitting of the Gamma-Ray Spectrum and Neutron-Capture Line Decay Profile to Constrain Ion Acceleration in Large Flares
We use the RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager) to study solar flare gamma rays that are signatures of the accelerated ions. Specifically, we analyze the prominent 2.2 MeV line due to neutron capture whose time profile depends on the ambient 3He/H ratio, accelerated ion spectral index and incident ion angular distribution. For the first time we fit the 2.2 MeV time profile jointly with a full spectral fit to find a minimum in the total chi-square. This allows for self-consistent constraints on many of the flaring environment parameters, including the energetic particle spectral index, degree of pitch angle scattering, ambient abundances including 3He, accelerated abundances, magnetic field convergence, and the effective angle of the flare. The temporal model consists of a kernel (the response to a delta-function particle injection) convolved with the observed 4-7MeV flux, dominated by prompt nuclear emission. The spectral fitting includes a recently improved model of the nuclear de-excitation continuum.
SH41A-0308
Synoptic Solar Radio Burst Source Directions Derived by the Ulysses URAP Investigation
The Unified Radio and Plasma (URAP) investigation is one of 10 instruments on the Ulysses spacecraft. Ulysses, with its highly inclined orbit around the sun, provides URAP with a unique perspective on solar radio bursts, which are usually emitted at low heliolatitudes as the electron sources move outward from the sun. These radio bursts provide positional information relating to interplanetary coronal mass ejections (type II radio bursts), the initiation of CMEs (type III-L bursts), and solar flares (type III bursts). In this presentation, we use the routine radio direction-finding data from URAP to track radio bursts and locate their sources when Ulysses is near perihelion. Plots of these data are available on the URAP Goddard Space Flight Center web site (for example, http://urap.gsfc.nasa.gov/cgi/giffer?date=20070726&PLOT_TYPE= DIRFIND), as are ASCII data files. The results shown are derived from fitting the spin-plane antenna data only; we compare the source directions so derived to the more accurate determinations made by fitting to both URAP antennas. The accuracy of the radio source directions to identify flare locations, determine solar wind densities remotely, etc., will be compared to previously published determinations. Applications to Wind Waves and STEREO Waves data, for which the spacecraft are in-ecliptic, will be addressed briefly. http://urap.gsfc.nasa.gov
SH41A-0309
Statistics of Hot Plasmas in M/X Flares Using RHESSI Fe & Fe/Ni Line and Continuum Observations
Observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) have shown that "super-hot" (T > ~30~MK) thermal plasmas are a somewhat common feature of (large) flares of GOES class M or X. However, the origins and evolution of such super-hot plasmas are still poorly understood. RHESSI observes solar photons >3 keV with a spectral resolution of ~1~keV FWHM, and is especially sensitive to flare plasmas hotter than ~10~MK. RHESSI's rich data set allows an unprecedented level of analysis of thermal flare plasma through imaging and spectroscopic observations of the thermal bremsstrahlung continuum and the Fe and Fe/Ni line complexes at ~6.7 and ~8~keV. Accurately characterizing the thermal plasma provides information about flare heating and cooling mechanisms and improves our understanding of flare energy transport and release. We present early results of a statistical survey of hot thermal plasmas in ~300 RHESSI-observed M/X-class flares (2002-2005), utilizing an analytical method to compare the thermal continuum and line emission (Caspi & Lin 2007). We present flare energetics, size, and temperature distributions and discuss the implications for the origins of super-hot thermal plasmas. http://sprg.ssl.berkeley.edu/~cepheid/agu2007/