SH13A-1129 1340h
Solar X-rays and Energetic Electrons Escaping from the Sun
The Sun frequently accelerates electrons in solar flares and type III radio bursts. Some of the accelerated electrons lose their energy by collisions in the denser, lower solar atmosphere producing hard X-ray (HXR) emissions and heat the corona, while others escape into interplanetary space. Whether the HXR producing and the escaping electrons are accelerated by the same mechanism is not known. Combining RHESSI X-ray observations with in-situ observations of energetic electrons from the WIND spacecraft allows for the first time a detailed temporal, spatial, and spectral study. Statistical results of 16 events with a close temporal agreement between the HXR and the in-situ detected electrons (taking the time of flight of the escaping electrons into account) show a correlation between the HXR photon spectral index and the electron spectral index observed in-situ thus indicating a common acceleration mechanism. Furthermore, the solar X-ray source structure of these events look similar showing hot loops with HXR footpoints plus an additional HXR source separated from the loop by typically ~15". This source structure can be explained by a simple magnetic reconnection model with newly emerging flux tubes that reconnect with previously open field lines, so-called interchange reconnection. Events with a delayed timing between the HXRs and the solar release of escaping electrons (Krucker et al. 1999, Haggerty & Roelof 2002) are presently investigated. If these delayed events are indeed accelerated later in the event by shocks, no correlation between the HXR photon spectrum and the in-situ observed electron spectrum is expect to be found.
SH13A-1130 1340h
Spectra of Solar Energetic Electrons in Flares and near Earth
Successful operation of Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) allows us to observe hard X-ray spectrum of many solar flares with unprecendent energy resolution. X-ray spectra provide us with vital information about the spectral properties of highly energetic electrons at the Sun. The mean electron spectrum of the solar flare can be recovered through a newly developed constraint regularization analysis of the bremsstrahlung photon spectra that energetic electrons produce. We emphasize the use of non-square inversion techniques combined with the correction of the observed spectrum for the effect of Compton photon back-scatter to achieve the most meaningful solution to the problem. For 16 solar flares the electron spectrum of temporally related solar energetic electron events has been measured using WIND/3DP allowing us simultaneous analysis of electron flux spectral properties at the Sun and near the Earth. Electron spectra from solar flares show strong correlations with the spectrum of solar energetic electrons, though vary with energy. Weaker correlation at lower energies can be viewed as a propogation and/or escape effect. Results suggest that the commonly used model of a collisional transport (thick-target) for flare electrons plus free streaming for interplanetary particles cannot explain the observed spectra.
SH13A-1131 1340h
Coronal Hard X-ray Emission in Occulted Flares
We use RHESSI to investigate coronal sources in six behind-the-limb flares with occulted or partially occulted footpoints. The major findings are as follows: (1) Coronal hard X-ray ($>$ 20 keV) emission is observed above thermal soft X-ray ($<$ 12 keV) and EUV (195 $\AA$) emission (which confirms Masuda's above-the-loop-top source); (2) Coronal HXR sources show impulsive (on a scale of tens of seconds) variation in their HXR time profiles; (3) The data can be fitted with a power law component with an index of 5 to 7 or with several thermal components with very high temperatures up to 100 MK. Presently, using the derived physical parameters, we test various heating models to determine whether the coronal HXR emission is thermal or non-thermal.
SH13A-1132 1340h
RHESSI Observations of Hard X-ray Footpoint Motions in Solar Flares
Solar HXR bremsstrahlung from energetic electrons accelerated in the impulsive phase of a flare is observed to be primarily from the footpoints of magnetic loops. Standard magnetic reconnection models predict increasing separation of the footpoints during the flare as longer and larger loops are produced. If the reconnection process results in accelerated electrons, the HXR footpoints should show this motion. The motion is only apparent; it is due to the HXR emission shifting to footpoints of neighboring newly reconnected field lines. The speed of footpoint separation reflects the rate of magnetic reconnection and should be roughly proportional to the total energy deposition in the footpoints. We studied footpoint motions in several large flares with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and compared the motion with the energy deposition rate in the footpoints. A correlation between the motion and the energy deposition rate is found during some periods in these flares, but not always.
SH13A-1133 1340h
The Solar Active Region Differential Emission Measure from 1 to 20 MK
In this work we determine the differential emission measure (DEM) for solar active regions during non-flaring times, using a combination of RHESSI and GOES12-SXI data. Approximately 3000 measurements of non-flare RHESSI spectra were obtained between January 2003 and September 2004. Most of these spectra were taken close enough to times of SXI images to allow for calculation of the DEM. Each DEM is calculated using the same process as used with YOHKOH data from solar flares as presented by McTiernan, Fisher and Li, (1999). We will use the data to compare with predictions of the nanoflare model of solar heating (Cargill and Klimchuk, 2004). We will also check for correlations between the high temperature emission measure and total X-ray brightness and also solar flare activity.
SH13A-1134 1340h
The Red-Asymmetry Distribution at H$\alpha$ Flare Kernels Observed in the 2001 April 10 Solar Flare
We report a detailed examination about the evolution of the H$\alpha$ flare kernels during an X2.3 solar flare which occurred on 2001 April 10. The H$\alpha$ red-asymmetry, that is, the red-shifted H$\alpha$ emission, is observed at almost all H$\alpha$ flare kernels, during the impulsive phase of the flare. At H$\alpha$ kernels nonthermal particles and/or thermal conduction precipitate into the chromospheric plasma, and this is thought to lead the downward compression of the chromospheric plasma, which is observed as the reddening of H$\alpha$ emission (e.g. Ichimoto & Kurokawa 1984). We examined the evolution of the flare kernels inside the flare ribbons by using the H$\alpha$ images obtained with the Domeless Solar Telescope at Hida Observatory, Kyoto University. We also examined the spatial distribution of the H$\alpha$ kernels which show the red-asymmetry and their relationship with the intensity of the H$\alpha$ kernels. We found that the stronger the red-asymmetry is, the brighter the H$\alpha$ kernel is. Then, we compared the strengthes of the H$\alpha$ red-asymmetry at hard X-ray emitting sources with those at the H$\alpha$ kernels without the hard X-ray emissions.
SH13A-1135 1340h
Accelerated Particle Spectra and Abundances in the 2003 October 28 and November 2 Solar Flares
We shall discuss analysis of the $\gamma$-ray spectra of the 2003 October 28 and November 2 solar flares. We use flux ratios in nuclear lines such as 6.13 MeV ($^{16}$O)/ 1.63 MeV ($^{20}$Ne) and 2.223 MeV (n-capture)/4.43 MeV ($^{12}$C) to determine the power law spectral index of accelerated protons and $\alpha$-particles. We also determine the accelerated $\alpha$/p ratio by comparing fluxes in the $\alpha$-$^4$He fusion lines between 400 and 500 keV with those in nuclear de-excitation lines. We then discuss measurement of highly broadened nuclear lines that provide information on the accelerated abundances of $^{12}$C, $^{16}$O, and $^{56}$Fe. These measurements of the accelerated particles impacting the Sun can be compared with those observed in space. This work is supported by NASA DPR NNG04ED181 and by ONR.
SH13A-1136 1340h
Investigation of the Solar Neutron Event at Tsumeb on 28 October 2003
During the 28 October 2003 solar energetic particle event, the high rigidity cutoff (low latitude) neutron monitor located at Tsumeb in Namibia recorded a flux increase of 3-4 percent roughly 10 minutes before the onset of a Ground Level Event (GLE) observed by many low rigidity cutoff (high latitude) neutron monitors. An analysis of the properties of this neutron event is presented, based on direct neutrons produced in the lower solar atmosphere by hadronic interactions. After deriving the yield function for neutrons at Tsumeb and using a typical spectrum for solar neutrons, we determine the start time and emission duration, which are compared with other observations. This work is supported by NSF grant ATM-0000315, the Rachadapisek Sompoj Fund of Chulalongkorn University, and the Thailand Research Fund.
http://www.bartol.udel.edu/~neutronm/
SH13A-1137 1340h
TRACE Observations of Many Active Regions with X-flares and Rotating Sunspots in the Current Solar Cycle
The TRACE instrument in its 7 UV and EUV wavelength channels has observed approximately 55 X-flares throughout its mission so far in solar cycle 23 from launch in April 1998 through August 2004. About 40 of those X-flares (73%) can be associated with 17 solar active regions that contain rotating sunspots, which have been observed by TRACE in its white light channel. These sunspots, rotating about the center of their umbras, appear to be manifestations of the twist and/or writhe, i.e., the helicity, instilled in the rising active region omega loops as they travel through the convection region below the photosphere. Of the 40 X-flaring active regions observed, 26 (65%) can be further associated with large, fast Coronal Mass Ejections (CMEs) leaving the sun, as recorded by the LASCO instrument on SoHO. Furthermore, 24 of these 26 events appear to be geo-effective because the energetic proton fluxes, observed by instruments on the GOES geosynchronous spacecraft, increased by several orders of magnitude shortly after the solar events occurred. The data and the importance of such a high percentage of rotating sunspots being associated with X-flaring active regions and CMEs will be discussed. This work was supported by NASA under the TRACE contract NAS5-38099.
SH13A-1138 1340h
A Search for Extreme Ultraviolet Signatures of CME Initation
Present models for the initiation of a CME involve magnetic reconnection either in a multipolar magnetic field overlying a filament channel or between emerging and existing magnetic flux in the vicinity of a filament channel. We have searched for evidence in EUV (195 Angstrom) images recorded by the EIT on SOHO and in other EUV images obtained by earlier instruments that might characterize the sites of such reconnection. No transient emission above a filament channel that might be indicative of magnetic reconnection has been found to date. We have, however, observed rapid changes in brightness or morphology of new emission regions, which represent the presence of newly emerged bipolar magnetic fields adjacent to and below filaments, just prior to and during filament eruption. In the SOHO observations these changes precede the acceleration phase of CMEs and the rapid rise of the GOES soft X-ray flux. Detecting short-lived transients with EIT is problematic because of its poor image-taking cadence (12-min) in the CME Watch mode. One instance of a transient EUV brightening preceding CME initiation has been reported (Dere et al., 1997) and earlier higher cadence observations provide several examples of short duration (1-2 min) EUV bursts in the immediate vicinity of filaments just prior to lift-off.
SH13A-1139 1340h
A Search for Solar Energetic Particel Events with Radio-Silent Coronal Mass Ejections
Shock waves are well-known accelerators of energetic particles in the interplanetary space. It is widely believed that the shock waves presumably driven by fast coronal mass ejections (CMEs) are the main accelerators of large solar energetic particle events. But the ability of shock waves to accelerate ions and electrons rapidly to high energies is still subject to debate. On the observational side, the large solar energetic particle events are associated with fast CMEs, but also with flares, i.e with particle acceleration in and around active regions. Statistical association hence does not allow us to constrain the accelerator. Using COSTEP and LASCO on SoHO, we look for particle signatures (deka-MeV protons and electrons above 100 keV) in carefully selected CMEs which are fast, but not accompanied by radio emission revealing electron acceleration in the low or middle corona, and where EUV and optical images show that activity occurs on the disk. Only three events were found between 1996 and 1998. In one of them weak particle signatures are detected by COSTEP, but not in the two others. We discuss the relevance of this observation to the origin of solar energetic particle events.
SH13A-1140 1340h
Observations of Separator Reconnection to an Emerging Active Region
Extreme ultraviolet (EUV) observations of an emerging active region are used to study separator reconnection in the corona. We follow each EUV loop connecting the emerging polarity to a nearby existing active region. Their geometrical resemblance to post-reconnection field lines from a magnetic model of the active region pair implicates separator reconnection in their production. While some reconnection is evident within 7 hours of emergence onset, the most intense period occurs after a one-day delay. The sum of cross sections of all observed loops accounts for only one-fifth of the magnetic flux whose transfer the model predicts. We suggest that the remaining loops remain at temperatures too high, or at densities too low, to be detected in our EUV data. The most intense reconnection requires as much as 260 MV along the coronal separator, however, the observed loops suggests that the flux is transfered as discrete bundles of 1.0e18 Mx each. The reconnection appears to directly dissipate only a small fraction of the energy released, while the rest is dissipated within the post-reconnection flux over the ensuing 6 or more hours, during which the flux remains visible. The net energy released, and ultimately disiipated, is consistent with the amount which could be stored during the 24-hour delay between emergence and reconnection. This work was supported by NASA grant NAG5-10489
SH13A-1141 1340h
Temporal Variations in the Evershed Flow
We present results of an analysis of the temporal Doppler variations of the Evershed flow. Doppler shifts were calculated from a unique, high-resolution data set of 39 spectral lines, spanning the photosphere and chromosphere. Our results indicate a quasi-periodic structure of the Evershed flow with a typical period between 18-24 minutes in the photosphere and 12-18 minutes in the chromosphere. We discuss the implications of these results for both the siphon flow and the moving flux tube models.
SH13A-1142 1340h
Photospheric Oscillations in the Solar Atmosphere: Driving Chromospheric Spicules and Coronal Waves
There are now many observations of oscillations and waves with periods around 5 minutes in the solar transition region and corona. We provide an observational overview of 5 minute periodicity in upper transition region moss, the low legs of coronal loops, and chromospheric spicules in active region plage. The source of the 5 minute periodicity is unclear, since photospheric p-modes are evanescent in the upper photosphere which should prevent them from propagating into the chromosphere, transition region and corona. Using a numerical model we show that photospheric oscillations can propagate into the low atmosphere as long as they are guided along a magnetic flux tube that is inclined away from the vertical. The leaked photospheric oscillations develop non-linearly into shocks at low chromospheric heights because of the density decrease with height. The upward traveling shocks and resulting rebound shocks of the chromosphere lead to periodic upward chromospheric flows, which in a recent paper we have identified as the periodic spicules that we observe in active region plage. After passage through the spicule, these shocked photospheric oscillations propagate into the corona. We suggest that TRACE observations of propagating acoustic waves in the corona are shocked and tunneled photospheric oscillations. We also explore whether these coronal waves can be exploited to determine the connectivity between photosphere and corona, and thus perform seismology of the lower solar atmosphere.
SH13A-1143 1340h
Large-Scale Flow Fields Beneath Major Solar Active Regions
By use of MDI dynamic campaign data and by employing time-distance helioseismology, we have continued our efforts to map synoptic flow charts in the solar interior to deeper depths, especially under some major active regions, such as AR9393 and AR10486. Two dimensional horizontal flow fields, as well as vorticity and kinetic helicity distributions, are presented for a few Carrington rotations. Flow fields, vorticity and kinetic helicity distributions are studied in more details under major solar active regions, in particular, before and after major solar flares. We try to identify connections between solar flares with these subsurface dynamical properties.
SH13A-1144 1340h
Diagnostics of Subphotospheric Sources of Solar Variability
Local helioseismology provides new tools for studying subphotospheric processes that are related to solar variability of various spatial and temporal scales. Large-scale flow patterns beneath active regions and sunspots affect the solar energy transport in the upper convection zone. Smaller-scale shearing and twisting flows may trigger instabilities of magnetic configurations that lead to flares and CMEs. We discuss the recent progress in the local helioseismology diagnostic tools and in our understanding of the subphotospheric dynamics and sources of variability.
SH13A-1145 1340h
Longitudinal Structure of Solar cycle 23
The non-axisymmetrical pattern of solar activity during the 1996-2004 years has been studied using the SOHO/MDI, SOHO/EIT and WSO data. These data reveal close correlation between the non-axisymmetrical magnetic field and coronal structures. The long-lived complexees of solar activity have clusterred in preferred (active) longitudes and have shown an impulsive nature of the solar cycle. The nature of active longitudes or active longotudinal zones is discussed.
SH13A-1146 1340h
Preliminary Results of Nitrate Analysis of Greenland Ice Cores
Previous studies have suggested that large solar proton events are followed several weeks later by nitrate spikes in ice samples. In an attempt to validate these initial results we have examined ice cores obtained from Summit, Greenland. To facilitate analysis of large sections of ice cores we have established a continuous flow analysis (CFA) system which allows for sub-yearly resultion of the nitrates. We present here our first results and compare with known solar events. Ultimately, such detailed analysis will allow us to probe historical solar events.
SH13A-1147 1340h
``Transition-Edge Sensors for Solar X-ray Spectral Observations - An Update''
ABSTRACT: The advent of cryogenic microcalorimeters (operating at temperatures of $\sim$ 0.1 K) in ground-based and space-based astronomy promises a revolution of new discoveries. Particularly, Transition-Edge Sensors (TES) have demonstrated high-energy resolution measurements of soft X-rays of up to {\it E/$\Delta$E} $\sim$ 1500 (at 6keV) and with high temporal resolution of a msec or less in photon pulse detections. Fabricated into multiplexed arrays of single detectors, or position sensing macropixels, and placed at the focus of a Wolter optic would further yield high spatial resolution capability of 2 arcsec or less, thus producing unprecedented "3-D" solar observations. We report herein, on progress to date in the development of these detectors and particularly, with on-going work with the sounding rocket payload, the Advanced Technology Solar Spectroscopic Imager, which will debut a TES instrument operating in the 300eV - 1.5keV range to study active region magnetic reconnection. Furthermore, as part of our larger effort, we discuss also current technical developments and plans at the Lockheed Martin Solar & Astrophysics Laboratory to design a TES instrument (3 - 8keV range) for realization into a dedicated Explorer-class solar observatory in the next 5 - 10 years.
SH13A-1148 1340h
The energy spectra of particles accelerated in a reconnecting current sheet with strong guiding field
Electron and proton acceleration by a super-Dricer electric field is investigated in the non-neutral reconnecting current sheet (RCS) with a non-zero longitudinal component of magnetic field ('a guiding field', GF) parallel to the electric field. The other two magnetic field components, transverse and tangential, are considered varying with distances from the X null-point of a RCS. The proton and electron energy spectra are calculated numerically from a motion equation using the test particle approach for model RCSs with constant and variable densities. In the presence of a strong or moderate guiding field protons were found fully or partially separated from electrons at ejection from a RCS into the opposite semiplanes, 'electron' and 'proton' ones. In the case of a weak guiding field both protons and electrons are ejected symmetrically in equal proportions as neutral beams. The particles ejected from a RCS with a weak guiding field have the power low energy spectra with spectral indices about 1.7 for protons and 2.2 for electrons. In case of strong guiding field spectra become slightly harder and the spectral indices decrease to 1.5 and 1.9 for protons and electrons, respectively.
SH13A-1149 1340h
A Unified Model of Particle Acceleration and Atmospheric Response in Solar Flares
We present initial results from a unified and self-consistent model of particle acceleration and atmospheric response in impulsive solar flares. In our model, electrons and ions are stochastically energized from thermal to relativistic energies on short timescales by cascading MHD turbulence, which is assumed to have been excited initially in the coronal region of a flare loop during the primary energy release phase. The accelerated particles then propagate to the denser transition region and chromosphere, where they can deposit a large fraction of their energy and drive the formation of a hydrodynamic shock that propagates back into the corona. The density enhancements that accompany this shock in turn modify the particle acceleration process in the corona. The two main components of the simulation are the NRL Dynamic Solar Flux Tube Model code and a spatially-dependent quasilinear particle acceleration/wave evolution code. We demonstrate through these realistic simulations that stochastic acceleration by MHD turbulence is able to account for all the major features of solar flare energetic particles. This work was supported by NASA grant NAG5-12794.
SH13A-1150 1340h
On the Relation Between Reconnected Flux and Parallel Electric Fields in the Solar Corona
The complicated magnetic topology in the solar corona often does not lend itself to the ready differentiation of topologically different regions. Instead, magnetic reconnection often happens in coronal fields that do not vanish, and for which no separator or separatrix is readily identifiable. For this, rather generic, situation, the question of how to relate observed magnetic flux changes to parallel electric fields remains unanswered. In order to shed light on this question, we apply to this problem the theory of General Magnetic Reconnection. We prove that a unique relation exists between the time evolution of the reconnected magnetic flux on one hand, and the integral along magnetic field lines of the parallel electric field on the other. Furthermore, we will use two explicit examples to illustrate how parallel electric fields are related to the formation of closed loops by the reconnection process, and how flux rope-like topologies result from reconnection if photospheric electric fields are negligible on flare time scales.
SH13A-1151 1340h
Simulating The Breakout Model In An Asymmetric Configuration
In a recent paper, MacNeice et al (Ap.J. v614) presented the first complete MHD simulation of the `Breakout' model for CME initation. They used an idealized magnetic topology in 2.5D, with an initial symmetry plane at the equator. We have modified this configuration to test the breakout model in an initial topology without this symmetry. We examine the differences in the two simulations, experiment with applying the driving shear to the different flux systems in this complex configuration, and study the implications for the evolution of magnetic helicity.
SH13A-1152 1340h
Dynamics of Forced MHD Turbulence and Coronal Heating
We present 3D MHD simulations modeling the heating of coronal loops in the solar atmosphere via the tangling of coronal field lines by photospheric footpoint motions. The overall behaviour of the system is sensitive to the intrinsic time-scale present, namely Alfv\'en propagation time along the loop, dynamical transverse time and photospheric forcing correlation time. The line-tying effect associated with the Alfv\'en wave propagation along the loop and the reflective photospheric boundary conditions limit the extent of the inverse cascade of magnetic energy when compared to 2D approximations and increase intermittency in both kinetic and magnetic energy absorption and dissipation. The simulations show that the corona self-organizes in response to the forcing in what we conjecture to be a state of minimal dissipation compatible with the driving.
SH13A-1153 1340h
A Comparison of Coronal Magnetic Field and Heliospheric Current Sheet Derived From the MAS and WSA Models
The coronal magnetic field is exceptionally complex and exceedingly difficult to measure. Several models have been put forth which generate solutions for the coronal fields based on photospheric field measurements. Here we examine and compare the underlying structures and topologies of the solar coronal magnetic field and the heliospheric current sheet derived from two very different coronal models, SAIC's MAS model and the coupled potential field source surface+Schatten current sheet (PFSS+SCS) model used in the Wang-Sheeley-Arge (WSA) model. The MAS model solves the three-dimensional resistive MHD equations using a semi-implicit finite difference scheme utilizing staggered meshes. The WSA model uses the values at the outer boundary or "source surface" (R=2.5Rs) as the input for the SCS model. The same synoptic maps derived from the line-of-sight photospheric magnetic field for selected rotations during solar minimum and solar maximum periods of the solar cycle are used as the boundary conditions for both models. We compare the two magnetic field solutions along with the shape and size of the two regions of open solar magnetic field lines and the total open flux. We also compare the location and shape of the inner heliospheric current sheet obtained by the two models.
SH13A-1154 1340h
Force-Free Magnetic Flux Ropes in the Solar Corona
Magnetic flux ropes offer a venue for storage of magnetic energy in the solar corona, energy that can contribute to eruptive events such as coronal mass ejections (CMEs). When a flux rope encircles the Sun, the associated magnetic field can store more energy than is needed to open the field fully---one of three energy-demanding tasks required for a CME. This work explores conditions necessary for the formation of energetic force-free flux ropes. We show that flux-rope formation is encouraged in sheared, force-free fields when (1) the shear is highly concentrated near the photospheric neutral line (taken to be the equator in our simple models) and (2) a strong potential field overlies the sheared field. Furthermore, the energy stored in the flux rope increases as the latitudinal extent of the sheared region diminishes. We demonstrate these properties with a new computational technique that follows force-free solution sequences through and beyond their states of maximum energy.
SH13A-1155 1340h
Can the Roll Effect in Prominence Eruption Be Explained by Anisotropic Electrical Conductivity?
When a prominence erupts, it generally rises non-radially and the top part of the prominence ribbon bends in one direction to make the ribbon horizontally flat. This phenomenon is very recently discovered and named the ``role effect'' by S. F. Martin. This effect can hardly be understood in the framework of ideal MHD or MHD with isotropic conductivity. Such breaking of a geometrical symmetry in the evolution of an initially symmetric system can take place when the electrical conductivity is anisotropic. We perform simulations of a solar magnetic field evolution with anisotropic conductivity. Now the magnetic field does not only move together with plasma bulk flows, but also with electric currents. Thus the rising motion of the field is skewed to one direction. When magnetic reconnection takes place in a magnetic loop or arcade, the reconnected field under the reconnection region is swept by the reconnecting current and is consequently bent over as observed in prominence eruptions.
SH13A-1156 1340h
Time-dependent Three-dimensional MHD Simulation of Solar Corona and Solar Wind at Solar Minimum Activity Phase
We will present results of time-dependent three-dimensional MHD simulation of solar corona and solar wind at the solar minimum activity phase using the boundary treatment that can limit mass flux flowing outward through the subsonic/Alfvenic solar surface. The aim of limiting mass flux is for obtaining simulated solar wind well matching Ulysses data and good contrast of density near the sun. The MHD simulation code we have developed is based on the concept of TVD-MUSCL method and the treatments of boundary at 1 $R_s$ is based on the normal projected characteristic method. To specify the solar surface boundary magnetic field, the potential field model and the synoptic map format data of WSO and MDI solar photospheric magnetic field data are used.
SH13A-1157 1340h
MHD Modeling of Differential Rotation
In the Fisk model differential rotation on the solar photosphere is thought to cause large excursions of magnetic field lines in the upper corona and in the heliosphere. This is in contrast with the apparently rigid rotation of some coronal holes. The model of Fisk et al.\ (1999) consists of a dipole field with the magnetic axis distinct from the rotation axis. The field from the northern and southern polar holes superexpands into the solar wind. In order to reconcile the pattern of velocity in the upper corona with the photospheric differential flow, magnetic reconnection is invoked between open field lines and low-lying loops. This mechanism releases the plasma trapped in the closed field lines into the corona and originates the slow solar wind. We have used our MHD model in spherical coordinates to study this mechanism. We have imposed a magnetic flux distribution as in Fisk et al.\ (1999). After relaxing the system to steady state, we have applied differential rotation for the equivalent of 5 rotations. We will describe the changes in the coronal magnetic field in response to the photospheric flows.
SH13A-1158 1340h
MHD Modeling of the Solar Wind with with MH4D
MH4D (Magnetohydrodynamics on a TETRAhedral Domain) is a numerical code to solve the resistive and viscous MHD equations on an unstructured mesh of tetrahedra. MH4D is device independent and runs on desktop computers as well as massively-parallel systems. Thanks to the use of an unstructured grid, the computational domain can be of arbitrary shape and the resolution can be increased in the regions of physical interest. Consequently, a wide range of spatial scales can be studied at the same time, for example active regions can be embedded in the large scale corona. A variational formulation of the differential operators ensures accuracy and the preservation of the analytical properties of the operators ($\nabla \cdot \mathbf{B}=0$), and self-adjointness of the resistive and viscous operators. The combined semi-implicit treatment of the waves and implicit formulation of the diffusive operators can accommodate the wide range of time scales present in the solar corona. The capability of mesh refinement and coarsening is also included. We will present a model of the solar wind obtained with the full MHD algorithm on the IBM SP3.
SH13A-1159 1340h
Relationship Between Plasma Heating and Coronal Emissions
The physical mechanisms that deposit energy into the coronal plasma and dissipate it as heat have been speculated for decades, and yet remain inconclusive. Many theoretical models have been proposed and need to be validated by observation. To distinguish these models, we take advantage of the complex magnetic-field structure and the spatial inhomogeneity of an active region, and compute the thermal structure in 3-D using the heating mechanism from each of the proposed theories. Distinctive thermal structures are expected from different heating models. As a result, each model has a unique signature in its emissions in EUV and soft X-ray that are observable by EIT/SOHO and SXT/Yohkoh. In our study, we survey the available models and compare their predicted emissions with observations. Work supported by Sun Earth Connection Theory Program of NASA
SH13A-1160 1340h
Three Dimensional, Patchy Reconnection in a One Dimensional Current Sheet
Due to the highly complex nature of the 3D solar coronal magnetic field, it is likely that reconnection will often be patchy, consisting of many short-duration, highly localized events. To investigate this possibility and its implications, we have studied the dynamics of localized 3D reconnection in a large scale 1D current sheet. We impose the reconnection in an MHD simulation by enhancing the resistivity in a small region for a short time, thus allowing a finite amount of flux to reconnect. This forms a pair of 3D reconnected flux tubes piercing the current sheet. These tubes then retract from the reconnection region, pushing their way through the surrounding magnetic field. We will discuss the dynamics of these flux tubes, their possible relation to descending `tadpole'-like features sometimes observed during flares, and the evolution of the shocks which form during the reconnection and then propagate along the tubes. This work has been supported by NASA and ONR.
SH13A-1161 1340h
Energy Balance in the Corona Over the 22-Year Solar Cycle
A model for dynamic energy balance in the solar corona over the 11-year solar cycle is presented. The model predicts that the flaring rate in the corona should lag behind the solar-cycle related driving because of the coronal response time of order 9 months. This effect may explain hysteresis phenomena between solar activity indices. The model can also incorporate 22-year driving consistent with the Gnevyshev-Ohl rule. The modified model in principle accounts for the observed variation of the flaring time lag for odd and even solar cycles.
SH13A-1162 1340h
Mode Conversion in Magneto-Atmospheres
Numerical simulations of wave propagation in a simple magneto-atmosphere are employed to illustrate the complex nature of wave transformation and conversion taking place in solar and stellar atmospheres. An isothermal atmosphere threaded by a potential poloidal magnetic field, and a superposed uniform toroidal field, is treated in a local cartesian approximation. Spatial variations are restricted to the two poloidal dimensions, but the toroidal field ensures that all three MHD waves are present in the simulation. As in our previous purely two-dimensional simulations (Bogdan et al. ApJ 599, 626-60, 2003), mode mixing and transformation take place at surfaces where the magnetic and thermal pressures are equal. In the present case, the upward propagating acoustic-gravity (MAG) wave is converted into roughly equal parts transmitted fast, intermediate (Alfven), and slow magneto-acoustic-gravity waves in passing through this mixing layer. Unlike the fast and slow waves, the Alfven wave is weakly damped, and is able to deposit its energy and momentum in the upper chromosphere and corona. The fast and slow MAG waves are decoupled on either side of mixing layer owing to their disparate propagation speeds. Under certain fortuitous circumstances, the Alfven wave also decouples from the fast and slow MAG waves.