Source Surface Models and Their Impact on Solar Wind Research II
Presiding: S R Habbal, University of Wales at Aberystwyth; T L Duvall, NASA Goddard Space Flight Center
SH23C-01 INVITED 13:30h
Using the potential field source model to predict the ambient solar wind
The potential field source surface model (PFSS) was developed independently by Schatten et al. [1969] and Altschuler et al. [1969]. The model is magnetostatic in nature and makes two major simplifying assumptions about the large-scale corona: 1) there is zero current in the coronal region between the photosphere and an imaginary spherical surface (i.e., the source surface) positioned typically between 2-3 Rs from Sun center and 2) the field is radial at the source surface. The latter assumption is essentially an attempt to account for the fact that the solar magnetic field is frozen into the coronal plasma and is being dragged along as it flows radially out into the heliosphere. While both assumptions are clearly gross over-simplifications of the true state of the corona, the PFSS model nonetheless works well at describing the large-scale, quasi-steady state of the corona field. In fact, the model has been shown [Wang et al. 1992, 1996, and 2001] to reproduce reasonably well, over multiple solar cycles, the observed coronal hole patterns, and for 3-month running averages, the interplanetary magnetic field (IMF) strength and solar wind speed at L1. In addition, the PFSS model and the more advanced MHD coronal models have been shown [Neugebauer et al., 1998] to reproduce equally well the size, shape, and positions of observed coronal holes and the observed heliospheric field sector structure. Work by Arge et al. [2000, 2002, and 2004] demonstrates that the model can successfully predict, on time-scales of less than a day, the ambient solar wind speed and IMF polarity 3-5 days in advance. In this talk, we provide a brief overview of the PFSS model and how its predictions compare with observations. I then describe our efforts to use it to predict the ambient solar wind.
SH23C-02 INVITED 13:50h
How Good are Potential Field Source Surface Models? What the MHD Modelers Don't Want you to Know
The large-scale, steady-state magnetic field configuration of the solar corona is typically computed using boundary conditions derived from photospheric observations. The two most popular approaches in use today are: (1) potential field, source surface (PFSS) models; and (2) magnetohydrodynamic (MHD) models. The former have the advantage that they are: simple to develop and implement; require relatively modest computer resources; and can resolve global structure on spatial scales beyond those that can be handled by current MHD models. On the other hand, they have been criticized because their basic assumptions (that the field is potential and that a single, spherical source surface exists) are seldom, if ever, met. In addition, PFSS models cannot directly incorporate time dependent phenomena, such as magnetic reconnection. In this study, we assess how well PFSS models can reproduce the large-scale quasi-static magnetic structure of the corona by making detailed comparisons with MHD solutions at different phases in the solar activity cycle. Specifically, we: (1) compute the shape of the source surface as inferred from the MHD solutions; (2) compare the coronal hole boundaries as determined using the two models; and (3) correlate the open flux determined from the models with the magnetic flux observed at 1 AU. Our results suggest that PFSS models compare relatively well with MHD computations of untwisted coronal fields (matched to line-of-sight magnetograms). It remains an open question how well PFSS models compare with MHD models that match vector magnetograms. This question can be addressed once data from SOLIS and Solar-B are incorporated into the MHD models.
SH23C-03 INVITED 14:10h
Potential Field Source Surface Model and Solar Wind Prediction
Various magnetic activities of the Sun causes disturbances in the near-Earth enviornment as well as on the weather and technology on Earth. "Addressing these disturbances and predicting them well in advance are the main task of Space Weather research. Much of the solar side of Space Weather is concerned with the accurate prediction of solar wind and its properties which are closely related to the coronal magnetic field. Since a direct measurement of the coronal magnetic field is still limited to strong field regions, solar wind predictions are based on theoretical models of the corona. The primary prediction scheme of the solar wind speed currently used at SEC is based on the empirical relationship between the flux tube expansion (FTE) factor obtained using Potential Field Source Surface (PFSS) model of the corona and the solar wind speed near the Earth. Though successful, this scheme has significant discrepancies. We have studied, using the near-Earth saltellites data as well as near--Sun Helios data, the possible causes of these discrepancies. In our study, FTE at the source surface were obtained using two different coronal models: PFSS model and the Current--Sheet Source Surface (CSSS) model. We present the results of this investigation and a comparison of the two models.
SH23C-04 14:30h
Appropropriate Application of Source Surface Models
Magnetostatic source surface models provide useful and reasonably reliable indications of the large-scale structure of coronal and interplanetary magnetic and velocity fields. Long term studies of the solar magnetic field can reveal much about the structure of the heliosphere. Improving data availability allows for more frequent calculations with higher spatial and temporal resolution, but still under highly idealized assumptions. The models provide at best a snapshot of a highly dynamic atmosphere. Appropriate application of the models provides a global context for many studies, but care must be exercised to address appropriate questions.
SH23C-05 14:45h
Magnetograph Saturation: Comparison of WSO and SOLIS
The calculation of potential magnetic field models of the magnetic field in the higher corona and its extension into interplanetary space depends on accurate measurements of the magnetic field in the photosphere. To fit the observed interplanetary magnetic field and the observed sector boundaries various "corrections" have been applied: addition of extra polar fields to compensate for the difficulties in measuring the weak line-of-sight true polar fields, and correction for"magnetograph saturation" for Babcock-type double-slit magnetographs. The latter correction is typically either a factor between 4.5 and 2 depending on distance from disk center (Mt. Wilson-type correction) or a constant factor of about 1.8 (Wilcox Solar Observatory). The new NSO-SOLIS magnetograph now operating atop of Kitt Peak does not suffer from magnetograph saturation. Comparison between WSO mean field measurements and SOLIS disk averaged measurements show that SOLIS = 1.70 * WSO. This is close to the WSO saturation factor and destroys the agreement between potential field models and the IMF radial component calculated with a typical saturation factor of about 4 for disk-averaged fields. We are back to the old problem of the calculated flux beeing to small by a factor of two.