SH53B-01 INVITED 

Characteristics of ³He-rich Solar Energetic Particle Events Derived from in situ Observations

* Wiedenbeck, M E (mark.e.wiedenbeck@jpl.nasa.gov), Jet Propulsion Laboratory, California Institute of Technology, M.S. 169-327 4800 Oak Grove Dr., Pasadena, CA 91109, United States

Extensive in situ observations of ³He-rich solar energetic particle (SEP) events were made during cycle 23 using instruments on a variety of spacecraft including ACE, Wind, and SoHO. The combined data set, which has essentially no data gaps over a period of >10 years, has proven useful for both statistical and case studies of these events. The modern instrumentation flown on these spacecraft made it possible to measure: spectra over a broad energy range, abundances of rare elements and of element groups in the upper 2/3 of the periodic table, isotopic composition of several heavy elements, and timing and three-dimensional anisotropy distributions of arriving particles. The advances that have been made in our understanding of ³He-rich SEP event characteristics as the result of these observations will be discussed. In addition, the expected contributions from the STEREO mission, which was launched in late 2006, will be mentioned. This work was supported by NASA under grants NAS5-12929 and NAS5-03131.

SH53B-02 

Solar wind structures associated with multiday Helium-3 rich periods of solar energetic particles

* Kocharov, L (kocharov@utu.fi), Space Environment Center, 325 Broadway, Boulder, CO 80305, Laivola, J (jaolla@utu.fi), University of Turku, Departmet of Physics, Turku, 20014, Finland Mason, G M (glenn.mason@jhuapl.edu), Johns Hopkins University, Applied Physics Laboratory, Laurel, MD 20723-6099, Didkovsky, L (leonid@usc.edu), University of Southern California, Space Sciences Center, Los Angeles, CA 90089-1341, Judge, D L (judge@usc.edu), University of Southern California, Space Sciences Center, Los Angeles, CA 90089-1341,

Survey of helium isotope abundance data (1997--2005) measured with the Ultra–Low-Energy Isotope Spectrometer (ULEIS) on the Advanced Composition Explorer ( ACE) has revealed multiday periods of ³He-rich solar energetic particles (SEPs) in the energy range of ~0.4--10 MeV nucleon^-1 with ³He/⁴He~0.1--1. We analyze 17 periods, where there were good counting statistics, using also in situ measurements of solar wind at ACE and Solar and Heliospheric Observatory ( SOHO) along with solar observations by SOHO telescopes, to study coronal and interplanetary magnetic field structures associated with multiday periods of ³He-rich SEPs. Then we employ a simple model of corotating compression in solar wind to estimate possible effect of observed solar wind profiles on confinement of ³He- rich SEPs in interplanetary magnetic field structures, which indicates that the large-scale compressions in solar wind is one of significant factors contributing to occurrence of multiday periods of ³He-rich SEPs.

SH53B-03 INVITED 

Wave Diffusion and Stochastic Particle Acceleration in 3He-Rich Events

* Liu, S (liusm@lanl.gov), Los Alamos National Laboratory, TA-3, SM123 Room 275, MS B227, Los Alamos, NM 87545, Petrosian, V (vahe@astronomy.stanford.edu), Department of Physics, Stanford University, Varian Room 310 382 Via Pueblo Mall, Stanford, CA 94305, Jiang, Y (arjiang@stanford.edu), Department of Physics, Stanford University, Varian Room 310 382 Via Pueblo Mall, Stanford, CA 94305,

Resonant wave-particle interaction plays essential roles in the enrichment of high-energy elements of solar 3He-rich events, which are triggered by the energy release processes of magnetic field reconnection in the solar corona. In the context of stochastic particle acceleration by plasma waves, the released magnetic field energy drives the generation of large scale waves, which then cascade to small scales and accelerate some of the charged background particles to high energies in this process. The wave cascading from large scales to small scales can be described as a diffusion process in the wavenumber space along different wave-mode branches. In this talk, we show how the enrichment of 3He and heavy elements may be addressed self-consistently with the stochastic particle acceleration model and the diffusion approximation for the wave cascade. The model predicted results only depend on the properties of the background plasma and the energy release rate. Observations of 3He-rich events can be used to constrain the diffusion tensors of different wave-mode branches, which will lead to a better understanding the energy release processes of magnetic reconnection. http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v636n1/62242/62242.html

SH53B-04 

Upper Limit on ³He Fluence in Solar Energetic Particle Events

* Ho, G C (George.Ho@jhuapl.edu), Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, United States Mason, G M), Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, United States Roelof, E C), Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, United States

We have investigated more than 260 ³He-rich (³He/⁴He > 0.004 at 0.2-2.0 MeV/nucleon) solar energetic particle (SEP) events from September 1997 through December 2006 using the ULEIS instrument on ACE. Both "impulsive" (flare-related) and "gradual" (CME-related) events are included. The ³He fluences varied only by a factor of 100 above our instrument sensitivity threshold while the 4He fluences varied by factor of 10,000 above the same threshold. Moreover, there appears to be no significant correlation between the ³He and ⁴He fluences. We find it striking that with more than 9 years of continuous SEP data, we could not find any SEP event that has a ³He fluence higher than 2.0×10⁵/cm2-sr-MeV/nucleon, while the largest ⁴He fluence observed was 7.0×10⁷/cm2-sr-MeV/nucleon. In the approximation that the event fluence is proportional to the net particle outflow from the Sun, the observed upper limit for the ³He fluence seems to indicate that only limited outflow of 0.2-2 MeV/nucleon ³He can be released from the Sun in a SEP event.

SH53B-05 INVITED 

Solar Origin of ³He-rich Solar Energetic Particle Events

* Nitta, N V (nitta@lmsal.com), LMATC, Bldg 252, Dept ADBS 3251 Hanover Street, Palo Alto, CA 94304, United States Mason, G M), JHU/APL, Mailstop MP3-E128, Laurel, MD 20723, United States Wiedenbeck, M E), JPL, MS 169-327 4800 Oak Grove Dr, Pasadena, CA 91109, United States Cohen, C M), Caltech, MC 220-47, Pasadena, CA 91125, United States

For more than three decades, we have known solar energetic particle (SEP) events that are characterized by enrichment of ³He and heavy ions. In order to understand the phenomenon, we first need to know where these ions come from. It is not straightforward to identify the solar source of the ³He-rich SEP event, since it often leaves electromagnetic signatures too small to be detected as a flare in spatially-integrated GOES X-ray measurements. Another ambiguity arises from the fact that ³He-rich SEP events are more often observed at low energies, meaning large uncertainties in the particle injection time. In this work we concentrate on ³He-rich SEP events whose injection times are relatively well determined by the ULEIS and SIS experiments on ACE. We show the association of these ³He-rich events with type III radio bursts and non- or mildly-relativistic electron events, and with solar flares, jets and CMEs. We compare our results with those in the literature, and also discuss the possible origin of problem events where we fail to find any solar signatures in the corona.

SH53B-06 

Understanding tansport of electrons in electron/He3 rich events using a direct Monte-Carlo approach

Roth, I (ilan@ssl.berkeley.edu), Space Science Lab, University of California, Berkeley, CA 94720, United States * Li, G (ganli@ssl.berkeley.edu), Space Science Lab, University of California, Berkeley, CA 94720, United States * Li, G (ganli@ssl.berkeley.edu), IGPP, University of California, Riverside, CA 92521, United States Wang, L (windsound@ssl.berkeley.edu), Space Science Lab, University of California, Berkeley, CA 94720, United States Wang, L (windsound@ssl.berkeley.edu), Department of Physics, University of California, Berkeley, CA 94720, United States Lin, R (rlin@ssl.berkeley.edu), Space Science Lab, University of California, Berkeley, CA 94720, United States Lin, R (rlin@ssl.berkeley.edu), Department of Physics, University of California, Berkeley, CA 94720, United States

Impulsive solar energetic particle (SEP) events are characterized by a high e/p ratio and over-abundance of He3. To decipher the underlying acceleration process of these events, it is crucial to deduce precisely the injection time of electrons and ions from the observed time intensity profiles at 1 AU. In a collisionless plasma like the solar wind, the propagation of electrons and ions follows the Parker spiral magnetic field lines, subject to pitch angle scattering due to the presence of solar wind magnetic turbulence. The effect of the pitch angle scattering is to alter particle pitch angles in a random manner during their propagation, leading to a prolonged propagation time and a less-focused pitch angle distribution. In this work, we study the transport of electrons in impulsive SEP events. The pitch angle scattering is investigated using a direct Monte-Carlo technique where the underlying Fokker-Planck transport equation is solved by casting it to a set of equations describing single particle's motion. By following the trajectories of individual particles, the time intensity profiles and pitch angle distributions at 1 AU are obtained. We discuss the comparison of our simulation results to observations by WIND/3DP and its implications on the interplanetary turbulence spectrum.

SH53B-07 INVITED 

MHD Simulations and Kinetic Analyses of Solar ³He-Rich Events

* Zhang, T (tianxi.zhang@aamu.edu), Alabama A & M University, 4900 Meridian Street, Normal, AL 35762, United States Wu, S (wus@cspar.uah.edu), University of Alabama in Huntsville, 301 Sparkman Drive, Huntsville, AL 35899, United States Tan, A (arjun.tan@aamu.edu), Alabama A & M University, 4900 Meridian Street, Normal, AL 35762, United States Winebarger, A (amy.winebarger@aamu.edu), Alabama A & M University, 4900 Meridian Street, Normal, AL 35762, United States

The origin and magnetic topology of solar impulsive (or ³He-rich) events are numerically simulated by using a three-dimensional axisymmetric time-dependent self-consistent magnetohydrodynamic (MHD) model. The results indicate that, when a magnetic flux of opposite polarity is emerged from the photosphere at the open field line region near a coronal hole boundary, the magnetic topology that leads to solar impulsive events is formed. Magnetic reconnections at the coronal base due to emergences of photospheric magnetic flux strongly disturb the magnetic fields in the solar corona and interplanetary space, and generate fast jet-like plasma outflows (or non- flux-rope coronal mass ejections). The magnetic field line disturbances scatter charged particles and therefore accelerate them to high energies through the Fermi acceleration mechanism. In terms of our previously developed two-stage acceleration model, the Fermi acceleration of a certain species (e.g. ³He) of ions can lead to the abundance enhancement of this species of ions in high-energy particles relative to the solar corona, if they are preferentially heated. The magnetic reconnections at the coronal base can generate the high-frequency Alfvén waves, which can heat heavy ions with low charge-mass ratios, especially ultraheavy ions with Z > 50. The electrostatic and electromagnetic ion-cyclotron waves including ⁴He-cyclotron waves and H-cyclotron waves generated by electron beams, electric currents, and the low-frequency global MHD modes can significantly heat ³He, electrons, and heavy ions with appropriate charge states via the harmonic cyclotron and Landau resonances. In this presentation, we will present our new MHD simulations for solar impulsive events including the origin; magnetic topology, reconnection, and fluctuations; jet-like fast plasma outflows and non-flux-rope coronal mass ejections (CMEs); properties of CME-driven quasi-parallel shocks; and mechanisms of acceleration. We will also overview the kinetic analyses in accord with the two-stage acceleration model for the impulsive (or ³He-rich) events including plasma wave excitations, heating of particles by waves, acceleration of particles via the Fermi acceleration mechanism; and abundance enhancements and energy spectra of high- energy particles.

Author(s) (2007), Title, Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract #####-##.