SH51C-01 INVITED
Understanding Eruptive Phenomena in the Hinode Era
The magnetohydrodynamic (MHD) equations are frequently used to investigate coronal mass ejections, eruptive prominences, and solar flares. A key goal of such studies is to deduce how energy stored in the magnetic field is suddenly released to drive these phenomena, for which the proposed mechanism(s) is (are) still under vigorous debate. Because most MHD models use relatively simple energy equations, the discussion often centers on the interpretation and comparison of magnetic field evolution in the models with corresponding features observed in emission. With new capabilities to study X-ray and EUV emission from Hinode, as well as complementary observations from STEREO and SOHO, it now becomes imperative that models advance to more quantitative comparisons with emission measurements. We have developed MHD models that include energy transport (radiative losses, anisotropic thermal conduction, and coronal heating) in the transition region and solar corona. This more accurate representation of energy flow allows us to compute simulated EUV and X-ray emission and compare directly with observations. In this talk we will show examples of this modeling approach for specific events and describe the magnetic field evolution associated with commonly observed emission features such as dimming regions and postflare loops. Work supported by NASA, NSF and the Center for Integrated Space Weather Modeling (an NSF Science and Technology Center).
SH51C-02
Can we Improve the Preprocessing of Photospheric Vectormagnetograms by the Inclusion of Chromospheric Observations?
The solar magnetic field is key to understanding the physical processes in the solar atmosphere. Unfortunately, we can measure the magnetic field vector routinely with high accuracy only in the photosphere with, e.g., Hinode/SOT and in future with SDO/HMI. These measurements are extrapolated into the corona under the assumption that the field is force-free. That condition is not fulfilled in the photosphere, but is in the chromosphere and corona. In order to make the observed boundary data consistent with the force-free assumption, we therefore have to apply some transformations before nonlinear force-free extrapolation codes can be legitimately applied. We develop a minimization procedure that uses the measured photospheric field vectors as input to approximate a more chromospheric-like field. The procedure includes force-free consistency integrals, spatial smoothing, and - newly included in the version presented here - an improved match to the field direction as inferred from fibrils as can be observed in, e.g., chromospheric H-alpha images. We test the procedure using a model active-region field that included buoyancy forces at the photospheric level. We apply the combined preprocessing and nonlinear force-free extrapolation method to compute the coronal magnetic field in an active region measured with the Hinode/SOT instrument.
SH51C-03
Multiple Flux Ropes and Helicity Bounds in a Force-Free Corona
Magnetic flux ropes are twisted, elongated bundles of magnetic flux that serve as repositories of magnetic energy in the solar corona. Flux ropes are prevalent over solar active regions and in the cavities that underlie coronal streamers. As such, they may provide at least some of the energy that powers a range of solar events, including flares, prominence eruptions, and coronal mass ejections. We have shown previously that energy storage in coronal flux ropes is enhanced when a potential magnetic field overlies a region of sheared fields that includes a flux rope. In the present work we demonstrate solutions for a force-free corona that include multiple flux ropes whose locations extend high into the solar atmosphere. We also derive an upper bound on the magnetic helicity associated with these multiple-rope solutions, and show that the actual helicity approaches closer to the upper bound as the shearing of magnetic footpoints is confined to ever-smaller regions at the coronal base. This work was supported by NASA grant NNG04GB91G to Middlebury College.
SH51C-04
Onset of coronal mass ejections due to loss of confinement of coronal flux ropes
Using MHD numerical simulations in a three-dimensional spherical geometry, we model the loss of confinement and eruption of a flux rope emerging quasistatically into a pre-existing coronal arcade field. Our numerical experiments have investigated two distinct triggering mechanisms that led to the eruption of the flux rope. In one case, the overlying arcade field declines with height more slowly such that the emerging flux rope remains confined until a high amount of internal twist is built up, with the rope self-helicity normalized by the square of the rope flux reaching about -1.4, and the flux rope becomes significantly kinked. The kinking motion causes rotation of the tube to an orientation that makes it easier for it to rupture through the arcade field, leading to an eruption. In the second case, the overlying field is made to decline more rapidly with height and the emerging flux rope is found to lose equilibrium and erupt via the torus instability when the flux rope self-helicity normalized by the square of the rope flux is only -0.63, before it becomes kinked. The values of the total relative magnetic helicity normalized by the square of the total anchored flux are, on the other hand, quite close for the two cases when the eruption takes place. We study the eruptive properties resulting from the two mechansisms and compare them with observations.
SH51C-05
Understanding Warm Coronal Loops
One of the great mysteries of coronal physics that has come to light in the last few years is the discovery that warm (~ 1 MK) coronal loops are much denser than expected for quasi-static equilibrium. It has been shown that the excess density can be explained if loops are bundles of unresolved strands that are heated impulsively and quasi-randomly to very high temperatures. This picture of nanoflare heating predicts that neighboring strands of different temperature should coexist and therefore that loops should have multi-thermal cross sections. In particular, emission should be produced at temperatures hotter than 2 MK. Such emission is sometimes but not always seen, however. We offer two possible explanations for the existence of over-dense warm loops without corresponding hot emission: (1) loops are bundles of nanoflare heated strands, but a significant fraction of the nanoflare energy takes the form of a nonthermal electron beam rather then direct heating; (2) loops are bundles of strands that undergo thermal nonequilibrium that results when steady heating is sufficiently concentrated near the footpoints. We verify these possibilities with numerical hydro simulations. Time permitting, we will show FeXVII line profile observations from EIS/Hinode that support the existence of nanoflare heating. Work supported by NASA and ONR.
SH51C-06
Hydrodynamic Modelling of a Multi-stranded, Nanoflare-heated Coronal Loop
There is a growing body of evidence that the loops seen with current instrumentation (SOHO, TRACE and Hinode) actually consist of many sub-resolution elements. Thus, the total emission we observe is the cumulative result of the radiation from numerous evolving plasma strands. This paper presents a "global loop" as 125 individual strands where each is modelled independently by a one-dimensional hydrodynamic simulation. The energy release mechanism across the strands consists of localised, discrete heating events (nanoflares) and the strands are "coupled" together through the frequency distribution of this energy input which follows a power law distribution. The simulated data is then folded through the response functions for both TRACE and Hinode/XRT. Overall for the TRACE emission appears very uniform along the global loop as it is originating mainly from plasma cooling into the passband. However, the corresponding XRT emission is much more structured, responding to both the heating and the cooling phases. The implications of these results upon the emission measure, resolving the coronal heating problem and possible future observing campaigns for will be discussed. http://www.star.uclan.ac.uk/~as1/AGU.html
SH51C-07
Nonlinear Dynamics of the Parker Scenario for Coronal Heating
The Parker field line tangling problem for coronal heating is studied comprehensively via longtime high-resolution simulations of the dynamics of a coronal loop in cartesian geometry within the framework of reduced magnetohydrodynamics (RMHD). Slow photospheric motions induce a Poynting flux that injects energy in the loop at the scales of convective motions (~ 1,000 km). During the linear stage the magnetic field and the currents grow linearly in time, until they saturate by driving an anisotropic nonlinear turbulent cascade. Although the efficient turbulent cascade prevents the magnetic field lines from becoming strongly entangled, current sheets are continuously formed and dissipated. We show that the current sheets are the result of the nonlinear cascade that transfer energy from the scale of convective motions down to the dissipative scales, where it is finally converted to heat and/or particle acceleration. A picture is then realized, where both slightly entangled magnetic field lines and current sheets are present. Current sheets are the dissipative structure for this system, and the associated magnetic reconnection gives rise to impulsive "bursty" heating events. This picture is consistent with the slender loops observed recently by HINODE which, although apparently quiescent, present an X-ray emission and at the resolution scale (~ 800 km) do not seem to reveal entangled features. We also show how the different regimes of MHD turbulence in the system influence the scaling laws for the small-scale energy deposition. http://www.df.unipi.it/~rappazzo
SH51C-08
Methods of Analyzing Temperatures in Post-Flare Loops using the XRT on Hinode
The X-Ray Telescope on Hinode has unrivaled temperature coverage, with 9 X-Ray filters in the focal plane. Using 7 different filter combinations, XRT observed a C8.2 flare on July 10, 2007. We use two different methods to glean temperature information about the post-flare loop system in this event. First, we examine the flare loops using the combined filter ratio method, which is a ratio method that utilizes observations in multiple filters in order to optimize the signal quality. Secondly, we calculate temperatures based on a differential emission measure method, which is a forward fitting method of determining temperatures. The results of these two methods will be compared.