SPA-Solar and Heliospheric Physics General Contributions IV
Presiding: I Richardson, NASA Goddard Space Flight Center; C A de Koning, Los Alamos National Laboratory
SH34A-01 15:30h
Ion Abundances in Large Solar Energetic Particle Events
Large solar energetic particle (SEP) events are believed to occur as a result of solar wind ions being accelerated by coronal or interplanetary shocks driven by fast coronal mass ejections or CMEs. Although such events have been observed routinely over the last four decades, the origin of the seed population has remained highly controversial. Composition measurements in SEP events can provide vital clues regarding the seed population as well as provide insights into the physical processes that govern their acceleration and transport through the interplanetary medium. We will present results of a survey of the ~0.1-5.0 MeV/n He and heavy ion abundances measured during 64 large SEP events by the Ultra-Low Energy Isotope Spectrometer (ULEIS) on board the Advanced Composition Explorer (ACE) spacecraft from November 1997 through December 2004. In particular we investigate the event-to-event variability observed in the SEP ion abundances in terms of properties of the associated flare and CME. We also survey the ~1 MeV/n 3He/4He abundance ratio during the events and compare the heavy ion abundances with those measured in various particle populations such as the solar wind, and gradual and impulsive SEP events. We discuss our new results in terms of the origin of the seed population and current models of SEP acceleration and transport.
SH34A-02 15:45h
Anomalous Solar Wind Composition Observed With SWICS on ACE in January 2005
During a ~20 day period in January 2005 the Sun released some of the most unusual solar wind, characterized at times by periods of high densities of 3He++ and at other times by extremely low or very high ionization states of heavy ions. Such episodes seem to be rare, with the last reported occurrence in May 1998 (Gloeckler et al., 1999). In the January 2005 events the solar wind 3He++ density increased by ~40 during a ~10 hour interval on day 8 and again on days 16, 19 and 27. Very low charge states of all detectable minor ions (C, N, O, Ne, Si and Fe) were observed during short intervals on day 9. For example, during these periods all charge states of oxygen, from 1 to 8 were seen. At the time around the big blast of the ~1400-1500 km/s solar wind (from day ~18.27 to ~18.35) a very hot solar wind (e. g. almost all C+6, O+8) was observed. We will report on these and other aspects of this unusual solar wind and compare them to the May 1998 observations. Implications of these results will be discussed. Gloeckler, G., et al., Unusual composition of the solar wind in the 2-3 May 1998 CME observed with SWICS on ACE, Geophys. Res. Lett. 26,157-160, 1999.
SH34A-03 16:00h
Energetic 3He in the Inner Heliosphere: 1997 to 2005
ACE observations of helium isotopes in solar energetic particles have shown that 3He was present in the interplanetary medium at 1 AU more than half of the time during the early portion of solar cycle 23 (specifically from August 1997 to April 2002). The observed 3He particles, with energies in the range 0.2 to 16 MeV/nuc, are believed to be accelerated in numerous small impulsive solar flare events. The suprathermal ions resulting from such events have been shown to provide a seed population which can be further accelerated by shocks in the solar corona or interplanetary medium. We have extended our previous investigation to include the time interval from early 2002 to early 2005, a period during which solar activity declined significantly. In addition to presenting the time dependence of 3He in the interplanetary medium over a 7-year period, we examine correlations between this energetic particle population and various measures of solar flare activity in order to assess, on a statistical basis, the characteristics of the sources of these particles and to determine the extent to which flare activity can provide a useful indication of the presence of flare suprathermals in the interplanetary medium.
SH34A-04 16:15h
Signatures of Differential Rotation in Low-Frequency Spectra of Heliospheric Fields
We use Ulysses data to examine low-freqeuncy power spectra of the components and magnitude of the solar wind magnetic and velocity fields to search for signatures of photospheric differential rotation. Previous work has shown evidence for both the rigid coronal hole rotation period and for the photospheric rotation period associated with the likely source region of the fast wind at high latitidue. Here we show that while the differential rotation is seen in"compressive" quantities such as the magnitude of the magnetic field and density, the components of the field tend not to show the same differential rotation period peaks. Comparing the results to predictions of a version of the "Fisk field" model, while not ruling out the model, raises important questions about the explanations for rigid coronal hole rotation. We will discuss various alternative scenarios.
SH34A-05 16:30h
Folds in the Interplanetary Magnetic Field in High Speed Streams Observed with Ulysses and ACE
Large rotations observed in the IMF can be due to true field polarity reversals, i.e. crossings of the heliospheric current sheet. But very often they are instead encounters with fields that are locally folded back upon themselves. Within coronal hole-related high-speed streams, we have found that folds can be readily distinguished from true polarity reversals by using the property of differential streaming between alpha particles and protons. The vector difference between the alpha particle velocity and the proton velocity lies parallel (or anti-parallel) to the magnetic field, but always points in the field-aligned direction away from the sun. At times, folded fields have been discussed in terms of static structures that are simply carried along with the solar wind flow. However in high-speed stream events, every fold we have examined to date shows correlations between the field turnings and variations in the solar wind velocity vector, indicating the folds are more likely propagating Alfvenic features. We present a comprehensive survey of large field folds within Ulysses and ACE observations of high-speed streams. We report how the frequency and duration of folded fields varies with radial distance, as well as how they differ between flow from large polar coronal holes at solar minimum and smaller coronal holes found at all latitudes around solar max.