Corona VI Posters
Presiding: S Bradshaw, Imperial College; D Falconer, Marshall Space Flight Center
SP41A-01 0830h
Heating of the Quiet Solar Corona
A major goal in solar physics has during the last five decades been to find how energy flux generated in the solar convection zone is transported and dissipated in the outer solar layers. Progress in this field has been slow and painstaking. However, advances in computer hardware and numerical methods, vastly increased observational capababilities and growing physical insight seem finally to be leading towards understanding. We present numerical simulations of quiet sun heating that span the entire solar atmosphere from the upper convection zone to the lower corona. These models include non-grey, non-lte radiative transport in the photosphere and chromosphere, optically thin radiative losses as well as magnetic field-aligned heat conduction in the transition region and corona. The relation between the mean magnetic field strength and structure and the heating of the corona is discussed.
SP41A-02 0830h
Investigating the Coronal Heating Models at High Resolution
Active regions are an excellent testing ground for coronal heating models because of their complex magnetic-field topology. Due to their distinctive parametric dependences on the magnetic field and plasma properties, each heating mechanism tends to deposit energy in preferential locations. The thermal structure of the atmosphere is further made distinctive by the local magnetic field through the highly anisotropic thermal conductivity. As a result, each heating model gives rise to unique radiation signatures, including EUV and soft X-ray. The observed EUV and soft X-ray emissions can then be used to test the validity of the models. Unfortunately, computing the thermal structure in 3D encounters tremendous difficulty because of the extremely steep gradients in temperature and density in the transition region, even using a variable-size mesh. In our previous investigations, we were forced to use an approximated thermal conductivity and compute the structure at low resolution. We have developed a new, and improved, method to treat the transition region so that the radiation signatures are not affected by the approximations. Quantitative comparison with observations becomes possible and will be presented. Using this highly efficient method, we have also studied the coronal responses to time dependent heating. Work supported by The Sun-Earth Connection Theory Program of NASA
SP41A-03 0830h
Are there two coronal heating mechanisms?
The source of coronal heating remains one of the most significant unknowns in solar physics. In this poster, we present analysis of two types of active region structures - relatively long loops that are bright in EUV images and short, hot loops that are bright in X-ray images. We compare the temporal evolution of these loops in multiple filters to the evolution derived from hydrodynamic simulations with various heating function to determine the most likely heating function for each structure.
SP41A-04 0830h
Coronal Heating Through Reduced MHD Turbulence
We present 3D reduced-MHD simulations modeling the heating of coronal loops in the solar atmosphere via the tangling of coronal field lines by random photospheric footpoint motions, which we represent as eddies having a finite correlation time. The overall behaviour of the system is sensitive to the intrinsic time-scale present, namely Alfvén propagation time along the loop, dynamical transverse time and photospheric forcing correlation time. The line-tying effect associated with the Alfvén 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 increases 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.
SP41A-05 0830h
Coronal Loop Heating by Nanoflares: The Influence of the Field-aligned Distribution of the Heating on Observables
We investigate the effect of the spatial distribution of nanoflare heating on loop observables. We perform 1D time-dependent hydrodynamic simulations of nanoflares occurring at sub-resolution strands, that make up the observed coronal loops. The simulations use different spatial forms for the nanoflare heating (randomly localized, footpoint, uniform loop top). The outputs of the simulations are then used to calculate diagnostics from synthetic TRACE and SXT observations. We find that the diagnostics depend only weakly on the spatial distribution of the heating, and therefore are not especially useful for distinguishing among the different possibilities. We propose that the best way to study the field-aligned spatial distribution of nanoflare heating is to observe the very high temperature plasmas that are present only in the earliest stages of an event. Research supported by NASA and ONR.
SP41A-06 0830h
Coronal Loop Heating by Nanoflares: Non-thermal Velocities
Spectroscopic observations show non-negligible non-thermal velocities under coronal conditions. These motions place tight constraints on any coronal heating mechanism that should be able to reproduce them. We calculate the non-thermal velocities predicted by the nanoflare model. We perform 1D time-dependent hydrodynamic simulations of nanoflares occurring at sub-resolution strands, that make up the observed coronal loops and calculate profiles for representative spectral lines. We show that: (1) the calculated non-thermal velocities compare favorably with observations of cool and warm spectral lines and (2) the profiles of hot lines, that would be available in observations from the Extreme Ultraviolet Imaging Spectrometer (EIS) spectrometer onboard the SOLAR-B mission, can exhibit significant blue-wing asymmetries which can be used as a monitor of nanoflare properties. Research supported by NASA and ONR.
SP41A-07 0830h
Investigation of Solar Coronal Heating Using a Time Dependent MHD Model with Full Conductivity Tensor
The transition region and lower corona is investigated using a newly developed time dependent MHD model that includes gravity and a self consistently computed conductivity tensor that depends on temperature, magnetic field, and density. The model is tested by its ability to preserve FAL equilibrium profiles, and to generate MHD waves with dispersion relations similar to those predicted by linear theory for the general types of MHD waves. The model is then used to examine solar atmospheric heating by Pedersen and magnetic field aligned current dissipation. Numerical experiments are conducted in which MHD waves are launched from either the transition region upward, or from the lower corona downward. Results from parametric studies of the evolution of these waves as a function of wavelength and amplitude are presented. In particular, the heating rate due to wave dissipation is compared with the FAL cooling rate, and with analytic results presented in M. Goodman [1,2]. % . The relative importance of physical dissipation due to the conductivity tensor, and numerical dissipation is estimated using Von Neumann stability analysis (VNSA) and numerical experiments with and without physical dissipation. It is then attempted to extrapolate from the simulation data the waves which could potentially lead to the correct heating rate, assumed to be the FAL net radiative loss rate. Realistic solar atmospheric data is used throughout the numerical investigations. This work was supported in part by NSF grant ATM-0242820 to the Institute for Scientific Research.
SP41A-08 0830h
Comparison Between Coronal Loop Solutions and Coronal Scaling Laws
During the past several years we have developed an accurate and efficient numerical technique for solving the steady-state energy and momentum balance equations in the solar corona, given an assumed variation of the coronal heating rate with the magnetic field strength and coronal loop length. By solving the energy equation for thousands of magnetic field lines in a given active region, one can build up a global solution for the thermodynamic structure of an active region, given its magnetic configuration. In contrast with other published results, our solutions explicitly allow for the arbitrary variation of field strength along a loop, and self-consistently determine the steady flows that will result from asymmetries in the field variation. While the steady-state energy balance assumption is probably not correct for most of the loops in an active region, our hope is that we will still gain insights as to how global properties of the corona are related to one another from the large number of loop solutions we can obtain. In this poster, we use the large number of field-line solutions to empirically test and correct the well-known scaling laws relating loop pressure, temperature, loop length, and magnetic field strength. We also present new results relating the variation of steady-state loop flow speeds with asymmetries in the loop geometry.
SP41A-09 0830h
How does Background Subtraction Affect SXT Loop Temperatures?
We have chosen a sample of 10 coronal loops that were visible on the limb and disk using SXT data. Our analysis was limited to Al1 and AlMg image observations taken when the instrument cycles through different filters during routine operations. The structures of the loop did not appear to change significantly during the cycle. We chose a range of twenty to thirty pixels along each loop and background pixels to correspond with the loop pixels. Temperature analysis was performed in three different ways: (1) standard SXT analysis of the loop pixels with no background subtraction; (2) constant background subtraction for each SXT image; (3) pixel pair background subtraction. Each method produced a temperature estimate for the selected pixels. We are interested specifically in how these results may differ from the EIT and TRACE loop temperature analysis that we have already completed. These results showed that background subtraction did not affect the EIT or TRACE temperatures. Solar physics research at the University of Memphis is supported by NASA grants NAG5-9783 and NAG5-12096.
SP41A-10 0830h
Connectivity of Quiet Sun Magnetic Features
We have examined quiet sun photospheric magnetic elements to assess their interelationships as part of a study of the connection between photospheric bipoles and transient coronal brightenings. The spatial relationship of elements in quiet sun have been studied in order to compare observed properties to a hypothetical uniform well-mixed distribution of sources. Quiet sun MDI magnetograms were used to identify photospheric magnetic elements, and the overall distribution of these sources on the sun was tested for uniform density. From the sources we generated distributions of nearest neighbor distances and signs. The distributions were then compared to those expected for a uniform well-mixed distribution. The nearest neighbors of opposite sign then serve as a population of bipoles defined spatially (spatial bipoles). Another set of bipoles was then selected using Magnetic Charge Topology (MCT) to predict connectivity among sources (MCT bipoles). These two sets of bipoles, spatial and MCT, were then compared to cotemporal EIT images to search for evidence of actual connectivity between sources predicted to be bipolar. This work was supported by NASA grant NAG5-10489.
SP41A-11 0830h
3D MHD Simulations of Magnetic Flux Emergence in Active Regions
We report on the progress of 3D simulations of active region magnetic flux emergence (and decay) through the stratified, sub-photospheric layers of the upper convection zone into the solar atmosphere and low corona. We use a recently-developed 3D semi-implicit MHD code (with a non-uniform, adaptive mesh) to address the inherent stiffness of the system of equations, and will compare our results with similar studies using second-order accurate, fully explicit numerical schemes.