Showing papers by "George C. Ho published in 2005"

The Upper Limit on 3He Fluence in Solar Energetic Particle Events

Abstract: We investigated 201 3He-rich (3He/4He > 0.004 at 0.2-2.0 MeV nucleon-1) solar energetic particle (SEP) events from 1997 September through 2003 December using the Ultra Low Energy Isotope Spectrometer on the A dvanced Composition E xplorer. 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 6 years of continuous SEP data, we could not find any SEP event that has 3He fluence higher than 2.0 × 105 particles (cm2 sr MeV nucleon-1)-1, while the largest 4He fluence observed was 7.0 × 107 particles (cm2 sr MeV nucleon-1)-1. To the approximation that the event fluence is to first order proportional the number of particles released from the Sun, the observed upper limit for the 3He fluence seems to indicate that only a limited number of 0.2-2 MeV nucleon-1 3He can be released from the Sun in an SEP event.

Statistical properties of fast forward transient interplanetary shocks and associated energetic particle events: ace observations

Abstract: RESUME We investigate 191 fast forward transient shocks observed by the ACE spacecraft from 1998 February 1 to 2003 October 28 and classify the energetic particle response associated with the passage of these shocks. We were able to compute the parameters of about 150 shocks. We present the frequency distributions of the angle between the upstream magnetic field and the normal to the shock, the Alfvenic Mach number, the shock speed along the normal to the shock in the frame of reference of the ambient plasma, VS, and the plasma density and magnetic field ratios. We observe a trend for more quasi-perpendicular shocks. The Alfvenic Mach numbers show values lower than about 7, the shock speeds VS lower than about 350 km/s, and the density and magnetic field compression ratio values below 4.5. A few exceptions above these values were also observed. There is a trend for faster and stronger shocks to have greater effects on the energetic particle intensities. However, the parameters of the shock do not determine unequivocally the characteristics of the energetic particle event observed at the passage of the shock.

The Energetic Storm Particle Event on 2003 October 24: A Test of Diffusive Shock Acceleration Theory

Abstract: A strong interplanetary shock accompanied by low‐energy (<300 keV) ion intensity enhancements was observed by the ACE spacecraft at 14:48 UT on 2003 October 24. Energetic particle data collected by the EPAM instrument onboard ACE show that the time profiles of the 47–321 keV ion intensities, the ion anisotropies observed upstream and downstream of the shock, and the downstream energy spectra of the 47–321 keV ions agree with the predictions of the quasi‐linear theory of diffusive shock‐acceleration. This shock was also accompanied by an intense level of magnetic field fluctuations as observed during an extended (6 hours) fore‐shock region by ACE/MAG. Although many of the signatures of this event are consistent with the predictions of the diffusive shock‐acceleration theory acting on a low‐energy particle population, analyses of the elemental abundances of heavy ions from 4He to Fe measured by ACE/ULEIS indicate that the abundances of the accelerated ions are poorly correlated with those measured in the so...

Energetic Particle Responses to Interplanetary Shocks Observed by ACE

Abstract: At very strong shock passages, protons (and other ions) may be accelerated at or near a spacecraft resulting in substantial particle intensities increases. Such events may be a significant space weather hazard, yet we are still unable to accurately forecast their arrival time or the magnitude of the particle increase. Here, we classify the >10 MeV proton response (observed by ACE/SIS) to passing shocks (identified by ACE/MAG, ACE/SWEPAM, SOHO/PM), examine heavy ion properties, and relate them to the measured shock parameters in an effort to further our understanding of these events and our ability to predict them.