SH22B-01 INVITED
Solar Wind Acceleration and Heating
The heating of the solar corona and acceleration of the solar wind represent one of the fundamental problems in all of space science. While different scenarios have been proposed to explain the heating of magnetically confined and open regions of the corona, they all rely on the transfer, storage and dissipation of the abundant free energy present in photospheric convection. In this talk I will focus specifically on models involving Alfvén waves which lead to the solar wind as observed at 1 AU, discussing the difficulties in deriving consistently both wave- spectrum evolution and solar wind distribution functions. Using the example of reflection driven Alfvénic turbulence in coronal holes and the solar wind, I will describe which kinds of measurements are most likely to help advance our understanding of this crucial problem.
SH22B-02 INVITED
UVCS Empirical Constraints on Theoretical Models of Solar Wind Source Regions
Spectroscopic observations from the Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO) have resulted in empirical models of polar coronal holes, polar plumes, coronal jets and streamers. These findings have been used to make significant progress toward identifying and characterizing the physical processes that produce extended heating in the corona and accelerate fast and slow solar wind streams. The UVCS scientific observations, which began in April 1996 and continue at this writing, have provided determinations of proton and minor ion temperatures (including evidence for anisotropic microscopic velocity distributions in coronal holes and quiescent equatorial streamers), outflow velocities, and elemental abundances. The variations in these quantities over the solar cycle also have been determined. For example, observations of large polar coronal holes at different phases of the solar cycle indicate that line width is positively correlated with outflow speed and anti-correlated with electron density. This paper will review these results, and present new results from measurements taken as the current solar activity cycle approaches solar minimum. The results regarding preferential ion heating and acceleration of heavy ions (i.e., O5+) in polar coronal holes have contributed in a major way to the advances in understanding solar wind acceleration that have occurred during the past decade. It is important to verify and confirm the key features of these findings. Hence, the results from a new analysis of an expanded set of UVCS data from polar coronal holes at solar minimum by S. R. Cranmer, A. Panasyuk and J. L. Kohl will be presented. This work has been supported by the National Aeronautics and Space Administration (NASA) under Grants NNG06G188G and NNX07AL72G and NNX06AG95G to the Smithsonian Astrophysical Observatory.
SH22B-03
Acceleration of the Solar Wind: a Review of Kinetic Collisionless (Exospheric) Models
One of the basic properties of the solar wind, that is the high speed of the fast wind, is still not self-consistently explained. This is mainly due to the theoretical difficulty of treating weakly collisional plasmas. The fluid approach implies that the medium is collision dominated and that the particle velocity distributions are close to Maxwellians. However the electron velocity distributions observed in the solar wind depart significantly from Maxwellians, indicating the limited validity of this hypothesis. In this work, we present a review of exospheric models that assume electron velocity distributions in the corona with suprathermal tails, but make no assumption on the heat flux, which is calculated self-consistently. We show how the recent developments of these models achieve to predict the fast solar wind without assuming an unreasonably large corona temperature and without additional heating of the outer region of the corona. We finally show the limitations of the present models and outline the basic perspectives for future research.
SH22B-04
Remote Sensing of the Electron Temperature and the Solar Wind Speed Near the Sun
In-situ observations provide detailed information on the state of the solar wind at a particular observation site. But observation of the global state of the solar wind would require many observation points strategically placed throughout the heliosphere. In this paper we report the results of a new optical experiment (the Multi-Aperture Coronal Spectrograph, MACS) to obtain the electron temperature and flow speed in the solar corona by observing the visible K-coronal spectrum during the total solar eclipse on 29 March 2006 in Libya. Results show electron temperatures of 1.10 ± 0.05, 0.98 ± 0.12, and 0.70 ± 0.08 MK, at 1.1 R\odot in the solar north, east and west, respectively, and 0.93 ± 0.12 MK, at 1.2 R\odot in the solar east. The corresponding speeds obtained are 103.0 ± 92.0, 0.0 + 10.0, 0.0 + 10.0, and 0.0 + 10.0 km-s-1. The outer corona, where the solar wind speed is high, is too faint to observe during an eclipse from the ground but it is easily observed from space. Using the technique demonstrated by MACS, the next generation of solar coronagraphs will be able to provide not just density, but also the temperature and flow speed of the solar wind in the inner corona. This information will improve our models of the heliosphere, and significantly increase our understanding of the solar wind.
SH22B-05
On the puzzle of heavy ion properties near the Sun
Heavy ions can be considered as passive tracers of the solar wind which probe the solar wind heating and acceleration processes in the low corona and throughout the heliosphere. The dynamic properties and ionic charge state distributions of heavy ions are therefore a crucial test case for candidate processes related to coronal heating and solar wind acceleration. This paper will discuss recent measurements of the dynamic properties of heavy elements and put them in context with our current understanding of the dynamic state of heavies near the Sun. The results of this analysis are surprising: There is no coronal heating candidate process that successfully combines our knowledge gained from in situ measurements, and the remote sensing data from observations such as UVCS. Similarly, there appear to be disagreements of the ionic charge states between these observations. These puzzles will likely only be resolved by in situ measurements near the Sun.
SH22B-06
Hybrid simulations of anisotropic proton distributions in solar coronal holes
The plasma in solar coronal holes is likely to be energized by the resonant damping of proton cyclotron waves. In this case, the protons can develop considerable temperature anisotropy in the region where the solar wind becomes collisionless. The temperature anisotropy can give us important information about the processes in the solar corona based on in situ observations. The extrapolation of the values of the anisotropy from one heliocentric distance to another is not necessarily a valid procedure because the plasma heating and expansion, which contribute to the anisotropy, may operate differently there. However, the observations suggest that the mean anisotropy in the fast wind as a function of the plasma beta obeys the same scaling law at different distances. This can provide a link between widely separated regions of the solar wind. We will carry out hybrid simulations to determine how the proton anisotropy is affected by the interplay of the proton energization (perhaps by more than one mechanism), plasma instabilities self-driven by the distribution, and the solar wind expansion. We will verify if the observed beta scaling can be reproduced in a numerical experiment for typical coronal hole parameters.
SH22B-07 INVITED
Solar Probe and the Ongoing Low Cost Study
(*On behalf of the Solar Probe STDT and Engineering Teams.) Solar Probe will make the first and only planned direct measurements in the solar atmosphere, which shapes the harsh inner heliospheric environment and ultimately impacts our entire solar system. It will be humanity's first visit to a star and will explore this previously inaccessible region of the inner heliosphere. The 2003 Space Science Enterprise Strategy called for study of a Solar Probe to "fly through the solar atmosphere to answer fundamental questions that can be answered in no other way." The mission received highest priority in the National Academy of Sciences' decadal research strategy in solar and space physics in 2002. These strong endorsements led to a new Science and Technology Definition Team (STDT) study [http://solarprobe.gsfc.nasa.gov/], carried out in concert with engineering support from an Applied Physics Laboratory (APL)-led engineering team. This study made use of the significant advances in solar and solar wind science and instrument and spacecraft technology that had occurred since various earlier studies. Recently, NASA's Associate Administrator for Science challenged the STDT (which has remained empanelled for just such an eventuality) and APL engineering team to try and find a solution that requires no RTGs and can be accomplished within a New Frontiers mission cost (significantly less than $1B). This talk summarizes the latest Solar Probe study and presents an update on the current STDT study of this lower cost option.