SH41A-1600
Horizontal Electric Currents in the Photosphere
We propose a proxy for horizontal electric currents in the solar photosphere based on spectral and spatial distributions of magnetic energy dissipation. For a set of evolving active regions (ARs) observed with SOHO/MDI in the high resolution mode, the dissipation spectrum, k2E(k), and the spatial structure of dissipation, i.e., the Stokes dissipation function ε(x,y), were calculated from the observed line-of-sight component of the magnetic field. These functions allowed us to calculate (a part of) the horizontal electric current density in the photosphere. We found that as an active region emerges, large-scale horizontal electric currents are gradually generated and they determine the bulk of dissipation. When an active region decays, the large-scale horizontal currents decay faster than the small-scale currents. The density of transverse currents in active regions is in the range of 〈 jh 〉 ~ (0.008 - 0.028) A/m2, that is comparable with the density of vertical currents in active regions. We estimated the upper boundary of the plasma conductivity to be σ < 5× 10-8 s/m2, which is four orders of magnitude lower than the classical ohmic conductivity for the photosphere. We suggest two possible mechanisms for generation of these horizontal currents in the photosphere. One of them is the horizontal drift of charged particles in the medium of varying plasma pressure gradient at the periphery of a sunspot. Such drift can produce quasi-circular closed horizontal currents around sunspots. Another possibility could be the existence of horizontal axial current within a highly twisted horizontal magnetic structure laying in the photosphere along the magnetic neutral line. The horizontal currents may contribute significantly to the dynamics of the photosphere/corona coupling, as well as the estimation of non-potentiality of ARs.
SH41A-1601
Investigation on the Possible Coupling Between Coronal Variability and Photospheric Dynamics
Density fluctuations are observed with the Ultraviolet Coronagraph Spectrometer – UVCS – operating onboard the SOHO spacecraft in the solar corona plasma of the regions of the low-latitude streamers and where the slow solar wind is accelerated. The results of the Fourier analysis, performed on solar minimum data, show that the coronal density fluctuations are dominated by discontinuities with a minimum temporal scale consistent with the lifetime of the photospheric supergranulation cells, thus suggesting a possible coupling of the variability of the solar corona and the dynamics of photospheric supergranulation. The spectral power derived on the basis of a Fourier analysis decreases with frequency as ν-2 from 10 to 100 μ Hz, in the frequency range where the supergranular motions mainly contribute to the photospheric intensity background, as observed for instance with VIRGO/SOHO. The fit of the low-frequency coronal spectra, performed according to the Harvey (1985) approach, yields a time constant of about 9×104 s, consistent with the time scales of the supergranulation regime in the solar noise. Furthermore, the degree of persistency of the density variations observed in the outer corona, derived by applying the Hurst analysis, is consistent with that inferred by Nesis et al. (1994), who investigated the dynamics of the solar granulation, by analyzing the photospheric intensity oscillations. The present results thus imply that the processes underlying the photospheric dynamics and coronal variability show common characteristics.
SH41A-1602
Comparison of Non-Linear Force-Free Field Extrapolation with Magnetohydrodynamics Simulations
We investigate how well non-linear force-free (NLFF) fields are able to match the coronal component of a more complex magnetohydrodynamic (MHD) simulation of flux emergence. To this end, we use the photospheric vector magnetic field obtained from an MHD simulation of flux emergence in order to specify the lower bound for a NLFF field extrapolation. We then compare the resulting NLFF field with the field given by the MHD simulation, in order to address the important question of how well NLFF fields can reproduce the coronal fields associated with solar active regions. We examine how even small departures from a force-free state in an active region field cause discrepancies with a NLFF reconstruction, and we examine how these discrepancies evolve with time. We discuss possible explanations for the interesting and perhaps very significant differences between the MHD results and the NLFF results that occur at times when the MHD results are expected to be force-free.
SH41A-1603
Understanding the correlation between X-ray Jets, CMEs and the Solar Wind
Hinode, STEREO, SoHO, and TRACE have detected several instances of filament eruptions, jets and mini- CMEs. These events are co-spatial and the temporal relationship is outlined. Recent observations show a direct link between jets and plume formation, and may also show a link between jets and mini-CMEs. The results of this investigation will be presented.
SH41A-1604
Nonlinear Force-Free Magnetic Field Modeling of the Solar Corona: A Critical Assessment
Nonlinear force-free field (NLFFF) modeling promises to provide accurate representations of the structure of the magnetic field above solar active regions, from which estimates of physical quantities of interest (e.g., free energy and helicity) can be made. However, the suite of NLFFF algorithms have so far failed to arrive at consistent solutions when applied to cases using the highest-available-resolution vector magnetogram data from Hinode/SOT-SP (in the region of the modeling area of interest) and line-of-sight magnetograms from SOHO/MDI (where vector data were not been available). It is our view that the lack of robust results indicates an endemic problem with the NLFFF modeling process, and that this process will likely continue to fail until (1) more of the far-reaching, current-carrying connections are within the observational field of view, (2) the solution algorithms incorporate the measurement uncertainties in the vector magnetogram data, and/or (3) a better way is found to account for the Lorentz forces within the layer between the photosphere and coronal base. In light of these issues, we conclude that it remains difficult to derive useful and significant estimates of physical quantities from NLFFF models.
SH41A-1605
Spatial and Temporal Relationships Between WL/UV Continuum and hard X-ray Footpoints in Solar Flares
Hard X-rays show the presence of energetic electrons in the impulsive phase of a solar flare. According to standard models, these electrons contain a large fraction of the total flare energy. We show that comparable amounts of energy are present in the compact, rapidly variable WL and UV bright points that constitute white-light flares. This suggests that these structures can be identified with each other, and indeed the image centroids and time variations match well. There are image differences that we believe mainly to be due to the different resolving powers of Hinode and TRACE WL/UV imaging on the one hand, and RHESSI hard X-rays on the other. We therefore also use RHESSI modeling software to simulate hard X-ray images using TRACE and Hinode data as templates to understand this relationship more precisely.
SH41A-1606
Three-Dimensional Magnetic Topology in the AR10930 based on the Non-Linear Force- Free Modeling
Three-dimensional (3D) magnetic field cannot be directly observed by ground observatories and space satellites, because the magnetic field is unfortunately obtained only solar surface. Therefore the reconstruction of 3D coronal magnetic field using only 2D data on the photosphere is the strong tool to help the understanding not only 3D structure but also the solar activities. We have also developed a new Non- linear Force-Free (NLFF) field extrapolation method, based on the extended magnetofrictional model and applied this NLFF solver to the AR NOAA 10930 observed by Hinode/SOT. As a result, we succeeded to reproduce the strong sheared structure on the neutral line before the flare and the post flare loop structure after the flare. However we have some problems such as narrow calculation domain caused by Hinode/SOT,imposed on the artificial sides and top boundary conditions, 180 degree ambiguity and so on. In this study, to overcome these problems, first, the calculation domain is extended 2 times for Hinode/SOT region in the East-West direction and 4 times in the North-South one by connecting between Hinode/SOT data and SOHO/MDI data. Sides and top boundaries are determined by global potential field model form SOHO/MDI (Shiota et al. 2008). From this result, the strong sheared region was reproduce as same as previous one, however, the field line connectivity is different. This result suggests the difference of separatrix structure between every each different boundary condition. Furthermore we will report the results of the 3D field line topology in detail by not only different boundary conditions but also 180 degree ambiguity. We will check the validity of these structures compared with Hinode/XRT.
SH41A-1607
Predicting the Amplitude of Alfvén Waves in the Coronal Holes
We present a new method for using fluctuations observed by Hinode spacecraft in solar coronal holes as a diagnostic tool for predicting the amplitudes of Alfvén waves. The method is based on a fundamental plasma physics problem concerning driven compressible waves in an inhomogeneous plasma. Alfvén waves propagating in divergent inhomogeneous coronal hole structures are expected to excite driven compressible waves. By analyzing the density fluctuations observed by Hinode, we can obtain information about the ultimate source of those fluctuations, i.e., the Alfvén waves. The obtained characteristics of the Alfvén waves can then be used as inputs to a global solar wind model to produce more accurate estimates of solar wind characteristics at large heliocentric distances, in particular in the near Earth environment.
SH41A-1608
MHD Simulations of Shock Wave Generation, Propagation, and Heating in the Photosphere and Chromosphere Using a Complete Electrical Conductivity Tensor
A complete anisotropic, inhomogeneous electrical conductivity tensor, which includes
Spitzer, Pedersen, and Hall conductivities is included in an MHD simulation to
describe how MHD shock waves may form, propagate, and resistively heat the atmosphere
from the photosphere through the chromosphere. The MHD model includes an energy equation.
The initial state is defined by FAL density, pressure, and temperature profiles, and by
a magnetic field that decreases with height z. The initial magnetic field strength at the
photosphere is 500 G.
A harmonic magnetic field perturbation with amplitude 250 G and period 30 seconds is
applied at the photosphere. Smooth waves are generated at the photosphere that propagate
upward and begin to form shock waves near z=350 km. This is the height near which
electrons first become magnetized. The shocks become fully formed near the FAL temperature
minimum at z=500 km. This is the height where the product of the electron and proton
magnetizations first exceeds unity, causing the Pedersen resistivity to begin to rapidly
exceed the Spitzer resistivity by orders of magnitude with increasing height. This is also
the height at which heating by proton Pedersen current dissipation rapidly increases with
height, and rapidly becomes large enough to balance the radiative losses from the
chromosphere. The onset of this strong heating is triggered by the onset of electron
and proton magnetization near the temperature minimum.
The shock thicknesses are ~ ~ 5 km. The shocks are the sites of resistive heating
rates as large as 3-10 ergs-cm-3-sec-1 in the chromosphere. The time averaged
heating rate over an interval of 162 seconds corresponds to a chromospheric heating flux ~ 2-3 ×
106
ergs-cm-2-sec-1. The heating rate increases with driving frequency, and is
∝ B2.
These results support the proposition of Goodman (e.g. Goodman 2000, ApJ, 533, 501; Goodman
2004, A&A, 424,691; Kazeminezhad & Goodman 2006, ApJ, 166, 613) that the onset of electron
and proton magnetization near the local temperature minimum, and their rapid increase with
height causes the rate of proton Pedersen current dissipation to rapidly increase by orders
of magnitude with height, creating and maintaining the solar chromosphere, and the
chromospheres of solar type stars. This mechanism is not restricted to shock waves. It
operates on any current generating MHD process. Such a process must involve currents driven by a
combination of
induction and convection generated electric fields. Examples are linear waves, and steady
convection across magnetic field lines.
It is the weakly ionized, strongly magnetized nature of the chromosphere that allows this
heating mechanism to be so effective, and that distinguishes the chromosphere from the
weakly ionized, weakly magnetized photosphere, and the strongly ionized, strongly
magnetized corona. The dominance of proton-neutral H collisions in determining the proton
collision frequency is necessary for this Pedersen current dissipation mechanism to be an
effective heating mechanism in the chromosphere.
This work was supported by Grant ATM 0650443 from the National Science Foundation to the
West Virginia High Technology Consortium Foundation.
class="ab'>
SH41A-1609
Fine Scale, Rapid Dynamics of the Solar Atmosphere from Space-Based Versus Ground- Based Observations
We compare analyses of multi-wavelength, high-cadence sequences of high-resolution solar images that are derived from ground-based observations and from space-based observations. The original analyses aim to show the effects of magnetism on the propagation of wave energy from the photosphere into the solar atmosphere. Here we focus on differences that arise from the differing circumstances of the data acquisition. The ground-based data are a 9 hour sequence of Swedish Vacuum Solar Telescope filtergram images made on 1998 May 30 in the photospheric G-band and in the chromospheric CaII K-line with 21 s cadence. Atmospheric distortion was removed by phase diversity reconstruction, and the images were 4 x 4 square averaged to a spatial resolution of 0.24 Mm/px. A sequence of line-of-sight magnetograms had lesser resolution and longer cadence. The primary space-based data are a 6 hour sequence at 1 min cadence of Hinode SOT-FG images in G-band and CaII H-line and line-of-sight magnetic field, made on 2007 May 2. For space-based data phase reconstruction is irrelevant. The spatial scale is 0.08 Mm/px but can be averaged to lower resolutions. The relative phases of oscillations in the different data channels and the correlations between oscillation periods and spectral intensities show significant differences between the space- and ground-based cases. These differences may come partly from terrestrial atmospheric fluctuations that, in spite of phase reconstruction, act to artificially strengthen correlations among the ground-based data channels. For example, the photospheric and the chromospheric intensity fluctuations are more strongly correlated in the ground data than in the space data. The relative phases of oscillations in the three data channels show some different dependences on magnetic field strength between the two cases. This may be attributable to the higher quality of the available space magnetic data.
SH41A-1610
The Center-to-Limb Variation of TRACE Travel-Times
We explore the limb-to-limb behavior of multi-frequency Transition Region and Coronal Explorer (TRACE) travel-time measurements of magneto-atmospheric waves in the solar chromosphere. We establish that while the higher frequency acoustic travel-times (~ 7~mHz) show little or no limb-to-limb variation, the previously documented variations of travel-time measurements on the magnetic environment through which the waves propagate are evident: increased travel-times in coronal holes; decreased travel-times in strong closed magnetic concentrations. For frequencies approaching the classical acoustic cut-off frequency (5.2~mHz) and below there is an increasing dependence of the measured travel-time with viewing angle and decreasing frequency. In this paper we demonstrate, using supporting observations from the Solar Optical Telescope on Hinode, that the center-to-limb variation of the low-frequency travel-times is the signature of propagating waves on magnetic network structures at granular spatial scales [i.e., structures close the spatial Nyquist frequency of TRACE] whose signal is a result of sub-resolution UV emission line 'contamination' in the 1600Å passband. Further, these structures must have a line-of-sight extension normal to the solar surface that increases across the disk as we approach the limb. We deduce that the low- frequency travel-time signal is directly caused by spicule motions which are increasingly inclined to the TRACE line-of-sight. Similarly, using SOT support, we propose that the apparent TRACE travel-time enhancement in coronal holes from TRACE, at same granular network locations, is the result of a change in vertical stratification in the coronal hole compared to quiet Sun counterpart emission. This effort is of particular relevance to full-disk travel-time investigations from the Solar Dynamics Observatory.
SH41A-1611
Coupling Between the Solar Convection Zone and Corona by Shear Flows Driven by the Lorentz Force
It has long been know that magnetic field near polarity inversion lines (PILs) is nearly horizontal and parallel to the PIL. Observations are showing in increasing detail that vigorous shearing motions in active regions proceed CMEs and produce the build up of energy that drives the eruption. It has recently been shown that such shearing motions are driven by the Lorentz force that naturally arises when bipolar magnetic fields emerge through the photosphere. Here, we examine the degree to which these shear flows couple the convection zone to the corona with a series of larger-scale MHD simulations of emerging magnetic flux ropes. Shearing motions transport flux, energy and magnetic helicity from the submerged portion of the field to the expanding coronal portion, causing a build up which may cause coronal mass ejections.
SH41A-1612
The Spectroscopic Footprint of the Fast Solar Wind
We explore a large, complex equatorial coronal hole (ECH) and its immediate surroundings through the temperature dependence of the non-thermal line widths of three transition region emission lines observed by SOHO/SUMER, placing them in context with recent studies of the other spectroscopic measures taken. Using a recent semi-empirical model of the solar wind as a basis, we explore the structure of the solar wind during the observing period and seek to gain a better understanding of the interaction of this region with the nascent solar wind.
SH41A-1613
Fine-Structured Plasma Flows in Prominences
Plasmas in prominences (filaments against the disk) exhibit a very wide spectrum of different kind of motions. Here we analyze the plasma motions inside prominences observed by Hinode/SOT during 2006-2007 with focus on two spectacular examples from 25 April 2007 in Halpha line and 30 November 2006 in CaH line and then carry out some simulations of the possible dynamics. Most filaments are composed of fine threads of similar dimensions rooted in the chromosphere/photosphere. Recent observations of counter-streaming motions together with oscillations along the threads provide strong evidence that the threads are field aligned. To more correctly interpret the nature of observed downward flows of dense and cool plasma as well as the upward dark flows of less dense plasma, we take into account the geometry of the prominence structures and the viewing angle. The dark upflows exhibit turbulent patterns such as vortex formation and shedding that are consistent with the motions predicted by instabilities of the interchange type. Sometimes an appearance of dark motions is generated by dark voids opened in the prominence sheet after initiation of nearby downflow streams, implying mass drainage in the downflows. Based on 304 A observations, there is more filament mass in prominences than is visible in either the Halpha or CaH lines. The source of the downward moving plasma may be located either higher above the visible upper edge of the prominence or on the far end of the prominence spine. The bright downward motions of the more cool and dense plasma may be partly due to the counter-streaming motion along the magnetic fields lines and also to the presence of Rayleigh-Taylor type or ballooning/interchange instabilities in the upper regions of the prominence. Transverse motions of filament threads caused by magnetic instabilities constantly provide the conditions for reconnection in the low part of the corona and the chromosphere. We suggest that the combination of flows along field lines, shear, and unstable stratification may provide the answers to the intriguingly elegant motions seen in prominences.
SH41A-1614
Leakage of Sub-Photospheric Hot Plasma through Magnetic Flux Tube
The solar corona has million degree temperature even though it is located above the cooler chromosphere and the photosphere. Also, coronal plasma is constantly flowing out as the solar wind. So the mechanism of coronal heating needs to include a mechanism of constant plasma supply. Most of the proposed coronal heating mechanisms are by waves or nano-flares. Both mechanisms are to carry energy of photospheric convection motion into the corona and dissipate there. Plasma supply is assumed to be through evaporation (or ablation) from the lower atmosphere due to steep temperature gradient. In these mechanisms, tenuous coronal plasma receives large amount of energy to heat and evaporate low temperature plasma into the corona. Hence the temperature of the initially heated plasma should be much higher than the million degree corona. Charged particles with such high temperature in the tenuous corona cannot reach the lower atmosphere due to their strong diamagnetic moment. They will be pushed upwards where magnetic field is weak rather than downwards where magnetic field is strong. Hence, the evaporation mechanism will not work. I will propose a quite different mechanism of million-degree plasma supply in more direct manner. The solar corona is filled with magnetic fields. They are generated at the base of the convection zone and lifted by the convection motion. The standard model of the Sun shows that the temperature at the base of the convection zone is about 2 million degree. So the plasmas trapped in the magnetic flux tube must have temperature there. Plasma particles have diamagnetic moments and they will be pushed along the flux tube toward weaker magnetic field region. As the result, the solar atmosphere which is filled with magnetic field, generated at the base of the convection zone, must have million degree temperature.
SH41A-1615
Neutral-plasma Interaction in the Chromosphere: Collisional Heating
The chromosphere is a weakly ionized region with an ionization fraction about one in ten thousand. As the neutral and plasma densities decrease with height, the collision frequencies between the two species decrease. In the upper half of the chromosphere, the Alfven time for a magnetic field perturbation to propagate to the transition region and the ion-neutral collision time are both much shorter than the neutral- ion collision time. Under these conditions, the bulk flow velocity of the plasma can differ from that of the neutrals; collisions between the neutrals and the plasma then become an important mechanism for transferring momentum between neutrals and plasma and converting energy of different types. We study the chromosphere on the basis of a three-fluid (electrons, ions, and neutrals) approach. The physical description includes the three-fluid generalized Ohm's law, Maxwell's equations, the plasma momentum equation, and the neutral momentum equation, thus taking into account collisions among all three species as well as electromagnetic coupling among the charged species. The geometrical configuration in this initial study, however, is highly simplified and is approximated as a localized one-dimensional incompressible flow region with vertical magnetic field but no vertical flow. The system is driven by an assumed change in the tangential (horizontal) flow of plasma at around 1000 km altitude. The plasma flow perturbation distorts the magnetic field, and the distortion propagates upward in the chromosphere to the transition region, while the neutrals respond much more slowly, in the neutral-ion collision time. The difference between plasma and neutral velocities produces collisions between the neutrals and the plasma, with the result that the neutrals are accelerated by the plasma flow and part of the plasma flow kinetic energy is converted to thermal energy of both neutrals and plasma. With a perturbation of ~ 1 km/s, a preliminary estimate gives a heating rate in the chromosphere of order 108 erg cm-2 s-1.
SH41A-1616
Height of Quiet Solar Chromosphere at the Limb
We present the result of observation of solar limb during the solar quiet phase in three different spectral lines: (i) He I 1083.0 nm, (ii) Hydrogen Paschen alpha 1281.8nm, and (iii) hydrogen Bracket Gamma 2166.1 nm. These spectral lines were observed using McMath-Pierce Telescope and near Infrared Camera. We focus our investigation to study the variation of the chromospheric height in the spectral lines at different positions on the limb.
SH41A-1617
A Magnetohydrodynamic (MHD) Analyses of Energy and Magnetic flux Transport from Sub-photosphere to the Corona
To understand how the particle (mass) flow transport across the magnetic boundary, a realistic example will be used to illustrate this process, which is to simulate the mass, magnetic flux, and energy transport from the sub-photosphere to the corona. The numerical simulation model that will be used in this paper is a newly developed data-driven three-dimensional global magnetohydrodynamic (MHD) model with the observed magnetic field and velocity field from GONG's data as the inputs at the photosphere. The difference between this new model and the model used in Wu, et al. 2005 is to include the effects of radiation and the transition region. Numerical simulation results to be presented are mass, total magnetic flux, and energy transport through photosphere to the corona, also the solar wind for the period of Halloween event.