Source Surface Models and Their Impact on Solar Wind Research I Posters
Presiding: H H Hudson, Space Sciences Laboratory, University of California, Berkeley; S E Gibson, NCAR High Altitude Observatory; K Balasubramaniam, National Solar Observatory
SH13A-01 1330h
Conical Current Sheets in a Source-Surface Model of the Heliosphere
Different methods of modeling the coronal and heliospheric magnetic field are conveniently visualized and intercompared by applying them to idealized axisymmetric field models. Thus, for example, a dipolar B field with its moment parallel to the Sun's rotation axis leads to a flat heliospheric current sheet. More general solar B fields (still axisymmetric about the solar rotation axis for simplicity) typically lead to cone-shaped current sheets beyond the source surface (and presumably also in MHD models). As in the dipolar case [Schulz et al., Solar Phys., 60, 83-104, 1978], such conical current sheets can be made realistically thin by taking the source surface to be non-spherical in a way that reflects the underlying structure of the Sun's B field. A source surface that seems to work well in this respect [Schulz, Ann. Geophysicae, 15, 1379-1387, 1997] is a surface of constant F = r-kB, where B is the scalar strength of the Sun's main magnetic field and k (~ 1.4) is a shape parameter. This construction tends to flatten the source surface in regions where B is relatively weak. Thus, for example, the source surface for a dipolar B field is shaped somewhat like a Rugby football, whereas the source surface for an axisymmetric quadrupolar B field is similarly elongated but somewhat flattened (as if stuffed into a cone) at mid-latitudes. A linear combination of co-axial dipolar and quadrupolar B fields generates a somewhat pear-shaped source surface. If the region surrounded by the source surface is regarded as current-free, then the source surface itself should be (as nearly as possible) an equipotential surface for the corresponding magnetic scalar potential (expanded, for example, in spherical harmonics). The solar wind should then flow not quite radially, but rather in a straight line along the outward normal to the source surface, and the heliospheric B field should follow a corresponding generalization of Parker's spiral [Levine et al., Solar Phys., 77, 363-392, 1982]. In particular, heliospheric current sheets (of which there are two if the underlying solar B field is mainly quadrupolar) should emanate from neutral lines on the corresponding source surface. However, because the source surface is relatively flattened in regions where such neutral lines tend to appear, the radial component of the heliospheric B field at r ~ 1 AU and beyond is much more nearly latitude-independent in absolute value than one would expect from models based on a spherical source surface.
SH13A-02 1330h
3-D structure of the complexes of solar activity
The typical complex of the solar activity persists on all levels of the solar atmosphere and consists of sunspots and surrounding plages in photosphere and chromosphere with arcades of loops visible in corona in Extreme Ultraviolet Emission and Soft X-ray. These coronal loops are the tracers of the lines of the magnetic field strength connected the magnetic areas with opposite polarities because of the complexes of solar activity are bipolar. The results of investigations of the evolution and the topology of the long-lived complexes of solar activity using SOHO/MDI and SOHO/EIT data for the period from 1996 to 2004 years are presented . We are discussing the nature of the existence of the long-lived complexes of solar activity.
SH13A-03 1330h
Global Structure of the Out-of-ecliptic Solar Wind
We use the observed photospheric field maps and the wind speed observed from Ulysses to study the out-of-ecliptic solar wind. The model calculates the wind speed from the rate of magnetic flux-tube expansion factor using a conversion function that is determined by least-squares fit of all currently available data from Ulysses. Using the best-fit conversion function we investigate the global solar wind at all latitudes, from 90 south to 90 north, covering a 36-year period from 1968 through 2003. The results complement and expand upon earlier studies conducted with IPS and other in situ spacecraft observations. The rotationally averaged wind speed is a function of two parameters: the heliolatitude and the phase of the solar cycle. The out-of-ecliptic solar wind has a recurrent stable structure, the average wind speed varies like a sine square of latitude profile spanning more than 5 years during the declining phase and solar minimum in each solar cycle. Near solar maximum the structure of the out-of-ecliptic solar wind is in a transient state lasting 2 to 3 years when the stable structure breaks down during the disappearance and reappearance of the polar coronal holes. We also report the Ulysses observations of the stable and transient structure.
SH13A-04 1330h
Determining the Sources of Solar Wind using Potential Field Models and In Situ Measurements
This paper summarizes our studies of solar wind sources, both near solar maximum and solar minimum, using potential field models and ACE, Wind, and Ulysses measurements. In these studies, both MHD mapping and two-step (ballistic plus potential magnetic field) reverse mapping of solar wind from ACE, Wind, and Ulysses to the solar surface were used to determine the solar source regions of the sampled wind. At solar minimum, the wind mapped to both polar and near-equatorial coronal holes. At solar maximum, both coronal holes (as seen in Helium 10830 A) and active regions were sources of solar wind. A source was attributed to an active region if the mapped footpoints matched open flux in a region of strong magnetic field with no nearby coronal hole visible in the NSO He 10830 Å synoptic coronal hole maps. We found that the solar wind from these active regions had distinct signatures, e.g., it generally had a higher oxygen charge state than wind associated with Helium-10830Å coronal hole sources, indicating a hotter source region. We also found correlations of the magnetic topology of the open field lines of these active region sources with images of the hot corona in EUV and soft X-ray images. The active-region flows appeared to be organized into several substreams, unlike the more monolithic structure of flows from coronal holes. The boundaries between plasma flows from neighboring sources were marked by large magnetic holes, plasma sheets, and low entropy, independent of whether the sources had the same or opposite magnetic polarities.
SH13A-05 1330h
A Comparison of Solar Wind Speeds from a Source Surface Model and Comet Ion-Tail Observations
During February 2003 Comet C/2002 V1 (NEAT) passed through the field-of-view (FOV) of the LASCO C3 coronagraph onboard SOHO. The comet passed within 0.1 AU (about 20 solar radii) of the Sun and displayed complex dust and ion tails. Observations of the comet's ion tail orientation have been used to estimate the solar wind speed while in the C3 FOV. We have used the Wang-Sheeley model to estimate the solar wind speed in the vicinity of the comet for comparison with the ion-tail results. The comet's orbit combined with solar rotation produced a comet track along the source surface at nearly constant Carrington longitude and heliographic latitudes ranging from 70 North to 40 South. Photospheric magnetic field maps from Carrington Rotation 1999 measured at three observatories (Wilcox, Kitt Peak, and Mt. Wilson) were used as inputs to the Wang-Sheeley model and each gave different placements of the current sheet. Two of the model results (Wilcox and Kitt Peak) placed the current sheet at similar latitudes (40-45 degrees North) while the third (Mt. Wilson) placed the current sheet at lower latitudes (20 degrees) and appeared to agreed with the current sheet placement implied by the ion-tail results. In this presentation we will discuss the methods of solar wind speed determination from ion-tail observations, present the comparison of solar wind speeds derived from the ion tail measurements with values derived from magnetic field observations, and discuss differences in photospheric magnetic field maps that could affect the location of the current sheet.
SH13A-06 1330h
The MHD simulation on the coronal plasma in radially shrinking open magnetic flux tubes
Open field regions in the corona, or coronal holes, have been believed to be rapidly expanding with their flux tube expansion factors, FTEs, greater than 1. We present the MHD simulation results of three-dimensional solar corona, focusing on the opend field regions with FTEs less than 1. Most of such radially shrinking magnetic flux tubes are found to be rooted on the weak open field region sandwiched between two strong open field regions with FTEs greater than 1 and with same magnetic polarity as the weak region. The flow speeds at the shrinking magnetic flux tubes obtained with our MHD simulation are less dependent on FTEs, while the inverse relationship between the flow speed and FTE is obtained at the usual expanding magnetic flux tubes. This finding increases the complex in using the FTE to predict the solar wind speed, and may be used to better understand the coronal plasma expansion.
SH13A-07 1330h
Numerical Heliospheric Simulation of Evolving Ambient Solar Wind and Interplanetary Magnetic Field
Recent advances in numerical methods and computer systems make it possible to tackle complicated, more realistic ambient and transient solar wind flows. Essential to this effort are the boundary conditions that drive 3-D MHD heliospheric models. We have used the improved source-surface model developed at NOAA/SEC Boulder [Arge et al., 2003, 2004] to derive solar wind parameters at 21.5 Rs. Various approaches and data sources are used and the numerical results obtained are compared with spacecraft observations at the Earth. Particular attention is given to the incorporation of daily updated source surface maps, which can be used to simulate an evolving ambient solar wind and interplanetary magnetic field.
SH13A-08 1330h
Validation of a Coupled Source Surface to MHD Model System at ACE and Ulysses
The Potential Field Source Surface model [Wang and Sheeley, 1988] combined with the Current Sheet modification [Schatten, 1971] is now in routine operation at the NWS Space Environment Center of NOAA. We use the sequence of source surface current sheet (SSCS) maps so produced. We developed a set of relatively simple empirical relationships to translate the SSCS map parameters into time-dependent MHD model lower boundary conditions at 0.1 AU. This system provides the 3D time-dependent slowly evolving background solar wind conditions in the inner heliosphere. To this system we add shock initiation perturbations to the lower boundary condition based on observed solar flares, CMEs and Type II solar radio bursts. The necessary shock descriptive parameters are generated in near real-time from these data. We compare simulated results with ACE solar wind observations. We have retrospectively adjusted the shock initiation parameters to maximize agreement with ACE observations, and extended the MHD model outer boundary to 10 AU. We will show results and comparisons of model results with Ulysses observations during the 2003 Halloween epoch. This work was partially funded by a NASA Living With a Star (LWS) TR&T grant through NOAA Work Order No.W-10,118 (ZS and TRD) and NASA Grant NAG-12527 (CDF and MD), by University Partnering for Operational Support program (UPOS) sponsored jointly by the U.S. Air Force and U.S. Army (CDF and MD), and by Carmel Research Center (DI).
SH13A-09 1330h
Source Surface Models and Their Impact on Solar Wind Research
To perform realistic modeling of the important processes in the solar corona, such as coronal mass ejections, flares, as well as the acceleration of solar particles, one needs to incorporate into the physical models any complicated pattern of the coronal magnetic field. The coronal magnetic field topology is determined by the helmet streamers (with closed field lines), the coronal holes (with open field lines) as well as the fine, but crucially important, details of the small-scale active regions. The standard practice to recover the global 3-D structure of the solar magnetic field from observations is to use the source surface model, in which the field is assumed to be potential, i.e., current-free. This approach ignores any volumetric current there may be present in the corona, and also neglects the existence of the equatorial current sheet, which starts from a height of 3-5 Rs above the solar surface. The fully potential solar magnetic field would have only closed field lines, not allowing for the solar wind to exist. In our Solar Corona model, incorparated into the Space Weather Modelling Framework, the solar magnetic field is split into two constituitive parts: one potential part which is recovered from the magnetic field data (e.g., from WSO, MWO, or MDI data) using the source surface method; and, one other non-potential part. For the potential field, we keep only the spherical harmonics decreasing with distance from the Sun or, equivalently, we use a very large value of the source surface radius. For the non-potential field, we solve the time-dependent induction equation with zero boundary condition at the solar surface. The full set of conservation laws for the MHD system is solved numerically using the BATS-R-US code. To power the solar wind in our model, we use a phenomenological turbulence model described in an earlier paper. The resulting steady-state MHD solution includes the well-resolved current sheet and helmet streamers. The modeled structure of active regions is very close to that recovered from the same magnetic data by assuming the potential magnetic field with a smaller source surface radius (2.5 Rs). The important difference, however, is in theappearance of a strong current loop near the active region being studied,namely AR 10486 on Oct 27, 2003.
SH13A-10 1330h
Comparison of Photospheric Footpoints of Open Magnetic Field Regions using CSSS and PFSS models
Comparison of Photospheric Footpoints of Open Magnetic Field Regions using CSSS and PFSS models The Current--Sheet Source Surface (CSSS) model developed by Zhao and Hoeksema (1995) assumes a cusp surface at the cusp point of coronal streamers at around 2.5 Rsun, which divides the corona into three regions, one bounded by the photosphere and the cusp surface, the second, between the cusp surface and the source surface, and the third, the region beyond the source surface. In this model the source surface can be placed closer to the Alfven critical point which is a great advantage over the traditional Potential Field Source Surface (PFSS) models, where it is at 2.5 Rsun. The source surface magnetic field obtained by CSSS model exhibits little latitudinal variation, which is consistent with the Ulysses observation of the interplanetary magnetic field (IMF). On the other hand, the source surface field computed using the PFSS model shows a latitudinal structure. We have carried out a comparative study of the photospheric footpoints of open magnetic fields obtained dusing the two models. The magnetic neutral line was found to be coinciding in the two models, as expected. There are significant differences in the locations and sizes of the open field regions on the photosphere, and they are not always consistent with the observations of coronal holes. We present the results of the comparison.
SH13A-11 1330h
Potential Field Source Surface analysis from high resolution synoptic and heliographic maps
The well established existence of an inverse correlation between the solar magnetic flux tube expansion factor (FTE) and the solar wind speed (SWS) at the Earth allows for the possibility to predict the SWS using observed solar photosphere magnetograms. A widely used method to compute the FTE is the potential field source surface (PFSS) model. Current PFSS models, however, do not always produce good agreement between the observed and predicted SWS. One complication is that traditional synoptic maps, used as input in the PFSS models, use the Carrington coordinates as the horizontal and vertical axes of the synoptic chart. Unfortunately, this choice leads to a mix of the time and space coordinates unless differential rotation is included in the tracking of magnetic features. We show here that the heliographic maps (longitude vs latitude or sine latitude) may be a better choice for the PFSS models. An additional problem is that the photospheric foot points and the FTE calculated from the PFSS models depend on Nmax, the number of multipole components used in the analysis. Current analyses use a combination of low resolution maps and small values of Nmax, typically around 30. We find that using a small Nmax produces fringing patterns in the reconstructed photospheric magnetic map. These fringing patterns alter the locations of the computed foot prints and the computed magnetic values on the photosphere as well as the predicted FTE. Increasing the Nmax value will then improve the reconstructed map and reveal more detail. In our analysis, we use high resolution synoptic and heliographic maps, which are 512 by 256, and Nmax up to 255. A typical simplification in previous analyses is to assume that the line-of-sight component of the solar magnetic field in the observed photospheric maps does not have a north-south contribution. This assumption may not be correct if one wants to reconstruct the north-south component map from the calculated coefficients using PFSS model. We use the Shrauner-Scherrer method to create both line-of-sight component and transverse component from the same observations. With the assumption (∇ × \vec{B}) = 0, we aim to obtain the north-south map from the transverse map and remove the north-south component from the line-of- sight component.
SH13A-12 1330h
Comparison of Heliospheric Magnetic Field Lines from PFSS Models with SEP Observations
Impulsive Solar Energetic Particle (SEP) events are thought to come locally from solar flares, in contrast with large gradual SEP events that are attributed to extended shocks driven by fast CMEs. For several impulsive SEP events, we identified the possible solar sources, using the timings of type III bursts. The solar sources thus indentified tend to be minor brightenings, sometimes not even detectable by the GOES X-ray Spectrometer. We found whether the source active region is open to the heliosphere, using potential field source surface (PFSS) models. We also traced field lines from the spacecraft observing SEPs to the source surface assuming constant solar wind speed, and then mapped them to the photosphere using PFSS models. In a number of cases, these traced field lines go close to the flare site. In other cases, their foot-points are far from the flare, or the source active region shows no open field lines. We interpret these various results in terms of different magnetograms for PFSS modeling, and assumptions used in the models.
SH13A-13 1330h
Implications of Solar Cycle Dependent Helium Modulation on Source Surface Models of Coronal Fields
Observations of the modulation of the solar wind helium abundance over the solar cycle provide evidence for a physical difference in the mechanism for coronal helium acceleration in the slow solar wind during solar minimum and maximum. During solar minimum, the relative abundance is a linear function of the solar wind speed. This process can be understood in terms of force-free models of the coronal field and the derived expansion factor of the field at the source surface. During solar maximum, however, this linear dependence vanishes. Recent studies of helium modulation using the solar wind measurements of the Faraday Cup instruments on the Wind spacecraft have resulted in additional information about the connection between the in-situ abundance and the coronal field topology. For example, a six-month periodic modulation of the helium abundance - at all solar wind speeds - is related to distance from the heliospheric current sheet. These observations suggest that there are two source regions for slow solar wind in which the coronal magnetic field plays a different role. We will present these observations in the context of source surface calculations and discuss how in-situ data may test coronal field and solar wind acceleration models.