SPA-Solar and Heliospheric Physics General Contributions II
Presiding: B V Jackson, Center for Astrophysics and Space Sciences, University of California, San Diego; H H Hudson, Space Sciences Laboratory, University of California, Berkeley
SH32A-01 10:30h
Solar Polar Electron Density Lower Than Equator in 2002
In 2002, Cassini was in cruise to Saturn past Jupiter and was in solar conjunction on June 21. The radio signal from Cassini was occulted by the solar corona enabling measurement of its integrated electron density along the line of sight. The point of closest of approach of Cassini's radio signal passed beneath the south pole of the sun within 2 solar radii allowing measurement of the electron density as the latitude of the point of closest approach varied. Differenced Range Versus Integrated Doppler (DRVID) data is collected by transmitting a code to the spacecraft and measuring how long it takes for the code to return. We fit a longitudinally symmetric, latitudinally varying electron density model to the Cassini 2002 DRVID data and compare to the spherically symmetric electron density model. We find that the electron density is lower over the polar regions than over the equator in 2002.
SH32A-02 10:45h
Statistical Relationships in Characteristics of a Sample of Interplanetary Coronal Mass Ejections Detected Near Earth
Using in situ measurements of solar wind conditions near Earth, we compared the signatures of a sample of transients, which we tentatively identify as magnetic clouds. Images from several solar observatories were utilized to identify the source eruptions which launched each ICME from the Sun. The plasma conditions within these structures were compared, and trends were sought that would correlate with the type of solar progenitor structure that spawned the eruptions. Additionally, a magnetic model was fitted to the measurements of each ICME detected near Earth. We present findings that suggest a systematic difference in cloud structure, depending on progenitor type; also, we present results indicating a relationship between the speed of transit from Sun to Earth and the magnetic field strength in the ICME.
SH32A-03 11:00h
The Decay of Interplanetary Coronal Mass Ejections and Forbush Decrease Recovery Times
We investigate the relationship between Forbush cosmic ray decrease recovery time and coronal mass ejection transit time between the Sun and the L1 Lagrange point. We identify 17 Forbush decreases from ground based neutron count rates between 1978 and 2003 that occur during at same phase in the solar cycle and can be associated with single coronal mass ejections (CMEs) in the SOHO LASCO CME Catalog and specific interplanetary coronal mass ejections (ICMEs) crossing the vicinity of Earth. We find a negative correlation between Forbush recovery times and CME transit times that contradicts the predictions of simple diffusive barrier models. The anti-correlation suggests that the decay rate of ICMEs is anti-correlated with their travel speed. Forbush recovery times range from five times the transit time for the fastest disturbance to a fifth the Sun-Earth transit time for the slowest. To account for the large range of measured recovery times we infer that the slowest disturbances must decay rapidly with radius whereas the fastest events must remain strong. The longest recovery times suggest that the fastest disturbances in our sample decayed less rapidly with radius than the ambient solar wind magnetic field strength.
SH32A-04 11:15h
Thermodynamics of Collision-Dominated Expanding Plasma: Heating of ICMEs
A recent statistical study of interplanetary coronal mass ejections (ICMEs) between 0.3 and 5.4 AU (Liu et al. 2005) shows that ICMEs expand during their propagation through the heliosphere: the density drops faster than r-2, and the magnetic field magnitude also exhibits a steeper decrease with distance than the typical solar wind. The expansion, however, does not accelerate the cooling of ICMEs and seems to be governed by a polytrope with γ = 1.1-1.2. The ICME data reveal that the ratio of the expansion time to the Coulomb collision time is usually larger than unity inside ICMEs, so Coulomb collisions are important contributors to the ion-ion equilibration process. As expected for a collision-dominated plasma, the alpha-proton differential speed quickly drops below 10 km s-1. The temperature ratio of alpha particles to protons, in contrast, is even higher within ICMEs than in the ambient solar wind, suggestive of a preferential heating of alpha particles. Taking into account the expansion and energy transfer between protons and alpha particles via Coulomb collisions, we model the thermodynamics of ICMEs. The heating rate as a function of heliocentric distance required for the temperature profile is deduced. We also examine the role of turbulence dissipation in the local heating of ICMEs at 0.3-5.4 AU, using high time-resolution magnetic filed observations. The turbulence cascade rate is thereby inferred from the inertial range power spectrum of magnetic fluctuations, based on Kolmogoroff's law and its MHD equivalent, Kraichnan's formulation, respectively. We will compare the required heating rate with the turbulence dissipation rate. In addition, turbulence generated by micro-instabilities driven by temperature anisotropies will be investigated. Preliminary results show that the ICME plasma is not near the thresholds for instabilities, so these instabilities may not contribute to the ICME heating.
SH32A-05 11:30h
Space Weather Implications of the 20 January 2005 Solar Energetic Particle Event
The solar energetic particle event of January 20, 2005 has been called, by some measures, the most intense in 15 years, with a >100 MeV proton intensity comparable to that of the October 1989 event and the largest ground-level neutron monitor enhancement in many years. Using data from five instruments on the ACE, GOES, and SAMPEX spacecraft, we have measured the energy spectra of H, He, and heavier nuclei over the energy range from ~0.1 to several hundred MeV/nucleon. During the first 24 hours, when >90% of the ions >30 MeV/nucleon arrived, the energy spectra of species from H to Fe could be characterized by power laws, with a spectral index of -2.2 extending from 1 MeV/nucleon to at least 400 MeV/nucleon in the case of protons. Another surprising aspect of this event was the speed of its onset, reaching the intensity- maximum for >100 MeV protons within ~20 minutes of the first arriving particles - at essentially the same time as the maximum of the associated x-ray flare. On the other hand, the intensities of ions <1 MeV/nuc lagged many hours behind the expected arrival times, presumably as a result of trapping by the shock. The hard, power-law energy spectra and the rapid onset of the January 20 event make it a particular challenge for efforts to provide improved forecasts of solar particle radiation storms. We compare the time history, intensity, and energy spectra of the January 20 event to the largest events of solar cycle 23 and of the space era, and discuss the radiation hazard that events such as this pose to human spaceflight and space hardware.