SH43B-01 INVITED
Connecting the Sun and the Heliosphere: The Slow Solar Wind Problem
The inner heliosphere and the space environment of all planets are dominated by the inner boundary, the solar corona, which has been largely elusive to our investigations. The main reason for this lack of knowledge is that we are not currently able to observe the driving agent in the corona, its magnetic field. We report on the results of a TRT focus team dealing with that particular aspect. First, we discuss key results of the team's research focused on the following four questions: 1) What is the spatial distribution of open magnetic flux in the corona? 2) How does the coronal magnetic flux change over time? 3) What are the physical processes governing these changes? 4) How can we use our knowledge of coronal and photospheric processes to predict physical conditions in the heliosphere? Second, we show how these questions culminate in a research topic currently unresolved: The question about the origin of the slow solar wind. We argue how this question leads to the need for new and improved theoretical, experimental and modeling techniques well beyond our current scope.
SH43B-02 INVITED
Observed Properties of the Slow Solar Wind
The source of the slow solar wind is poorly understood, in part because observations of its properties have led to conflicting phenomenological views. To provide a sound basis for a synthesis view and, ultimately, for a successful physical model, the observed properties are reviewed with minimal reference to interpretation. Slow wind properties can be sorted into three categories according to scale size. At large scales they include depressed temperature and helium abundance and elevated density, charge-state ratios, and elemental abundance ratios. At medium scales these parameters are highly variable. At small scales the magnetic field magnitude plays a major role in defining structures with high plasma beta and isotropic suprathermal electron distributions. Other slow wind properties of interest include evidence for a boundary layer and the relationship of slow wind structures to the heliospheric current sheet (HCS). Against expectations, a relatively large fraction of spacecraft passages through the slow wind lack HCS crossings. Seen as a whole, the observed properties of the slow wind suggest a synthesis view with both steady-state and transient aspects.
SH43B-03
Quiescent Current Sheets in the Solar Wind and Origins of Slow Wind
Solar wind near the heliospheric current sheet is investigated using Ulysses and ACE data, in a superposed epoch analysis for several days on either side of the current sheets. Only data near sunspot minima are used, minimizing the influence of transients. New results are shown for composition and ionization state. Existing results showing a ~2 day wide depletion in He/H at the current sheet are confirmed, although the depletion is generally more narrow. A recent finding of a broad 5-10 day wide reduction in He/H around the current sheet is also confirmed. An important result is that the narrow depletion is not a real phenomenon, but is instead a statistical consequence of the superposition of transient depletions that also create the broad reduction in the averages. These transient depletions last from a few hours up to several days, come from the core of streamers, and are embedded in a quasi-steady flow from streamers legs. Most depletions contain a current sheet just inside one edge, leading to the apparent narrow depletion at the current sheet in the superposed epoch analysis. These results lead us to a hypothesis for how the He/H depletions form with a current sheet just inside one edge. Fe/O fluctuations associated with the He/H fluctuations further show that mixing of plasma from coronal holes adjacent to streamer brightness boundaries into outflow inside the brightness boundary is not an important process.
SH43B-04
Source Regions of Helium Variations in the Slow Solar Wind
The relative abundances of helium and hydrogen in the solar wind vary on solar cycle time scales as well as with solar wind speed. Kasper et al., 2007 demonstrated further that, during solar minimum, the relative helium abundance correlated with the heliographic latitude of the observer. They also found a linear relationship between the solar wind speed and relative helium abundance for slow (less than 550 km/s) solar wind. Mapping the in situ measurements back to the solar wind source region allows us to relate helium abundance to conditions at the sun. We investigate the coronal sources of the relative helium abundance variations using the WSA-ENLIL model. Improvements to the WSA-ENLIL model have led to better solar wind predictions. Using these improved simulations of the connection between the corona and interplanetary space, we determine the source regions for several time periods during solar minimum when the predicted solar wind speed yields good results as compared with observations. We investigate helium abundance as a function of source region properties such as distance to the current sheet, magnetic expansion factor and the distance to the edge of a coronal hole.
SH43B-05 INVITED
Remote Sensing of the Slow Solar Wind
We are using SECCHI (Sun Earth Connection Coronal and Heliospheric Imager) observations from the STEREO (Solar Terrestrial Relations Observatory) spacecraft to construct elongation/time maps of material moving outward from about 2 solar radii to distances beyond the orbit of Earth. These maps span all position angles within the low-latitude fields of the Heliospheric Imagers and provide synoptic coverage of the motions observed since April 2007. These motions include the gradual acceleration of streamer blobs to solar wind speeds in the range 300-400 km/s. They also include streamer detachments and eruptions, in which the inflating streamers stretch until they separate into collapsing loops and outgoing arches that reach terminal speeds in the range 300-500 km/s. During 2008, the two STEREO spacecraft have separated by 68 degrees, and are now providing complementary views of these ejecta. Recent observations show that some of the streamer blobs and ejections have the helical topology expected for magnetic flux ropes produced by field line reconnection in the corona and solar wind.
SH43B-06
Characterizing Solar Wind and its Source Regions as Empirical Constraints for Investigating Solar Wind Formation
We use ACE solar wind data during years 2004-2007 and characterize the properties of the solar wind that originates from equatorial and low-latitude coronal holes. Such coronal holes have been observed to produce fast solar wind streams often followed by a smooth transition into slow wind streams. We investigate solar wind properties of such origin, particularly the ion charge composition and elemental abundances, and their relation to the properties at their source regions. These types of coronal holes and the associated solar wind have frequent occurrences in the past and current solar cycles, and have great advantages in investigating a wide variety of the solar wind properties and its sources at the Sun because of the diversity in their size, location, as well as the magnetic and plasma properties in the coronal hole and the surrounding closed field structures.
SH43B-07
Topological Origins of the Slow Solar Wind
Although the slow solar wind has been studied for decades with both in situ and remote sensing observations, its origin is still a matter of intense debate. In the standard quasi-steady model, the slow wind is postulated to originate near coronal hole boundaries that define topologically well-behaved separatrices between open and closed field regions. In the interchange model, on the other hand, the slow wind is postulated to originate on open flux that is dynamically diffusing throughout the seemingly closed-field corona. We argue in favor of the quasi-steady scenario and propose that the slow wind is due to two effects: First, the open-closed boundary is highly complex due to the complexity of the photospheric flux distribution. Second, this boundary is continuously driven by the transport of magnetic helicity from the closed field region into the open. The implications of this model for the structure and dynamics of the corona and slow wind are discussed, and observational tests of the model are presented. This work has been supported, in part, by the NASA LWS, HTP, and SR&T programs.
SH43B-08
Is Disconnection Necessary?
A number of diverse observations indicate that at least a portion of the slow solar wind plasma must originate from regions that were at one time magnetically closed (e.g., within the helmet streamer belt). The release of this material could result from closed fields expanding outward, balanced by disconnection of previously open fields, or it could occur through interchange reconnection, where open field lines reconnect with previously closed field lines. The scarcity of evidence for disconnection in interplanetary measurements has led to the idea that reconfiguration of coronal fields must occur entirely through interchange reconnection (e.g. Fisk and Schwadron 2001 ApJ 560, 425). We have performed several time-dependent MHD simulations of coronal evolution in the presence of surface flows such as differential rotation and the motion of small bipoles. We find that all three processes (disconnection, interchange reconnection, and opening of previously closed loops) occur. We discuss the implications of our work for the origin of the slow solar wind. Research supported by NASA, NSF (through CISM and the Strategic Capabilities Program) and AFOSR.