SH51C-01 INVITED
Advancing our understanding of the chromosphere
Recent progress has shown the solar chromosphere to be fundamentally dynamic, where non-linear techniques must be used to understand its nature. It is also the region where the magnetic field grows to dominate the plasma and where the coupling between radiation and matter becomes becomes quite tenuous. Understanding the workings of the chromosphere is vital if one is to understand the flow of energy between the solar surface and its outer atmosphere and wind. Recent numerical developments have shown that it is feasible to model the chromosphere, even to the extent that newly available high resolution observations sometimes can be reproduced in detail. We will discuss the challenges facing numerical chromospheric models and the observations needed to validate or refute them.
SH51C-02
A First Step Towards a Nonlinear and Self-consistent Modelling of the Interface Region Between Photosphere, Chromosphere and Corona
The interface region between the solar photosphere and corona is an interesting environment which is, however, difficult to model. Low and high plasma-beta regions are present in this layer side by side and require that the magnetic field and plasma are modelled self-consistently in one model. A popular simplification used to model the low beta solar corona is the assumption of force-free magnetic fields, but this approach is not justified in the mixed beta interface region. We can, however, generalize numerical schemes developed for nonlinear force-free magnetic field extrapolations. Here we report a first step towards such an approach based on an optimization principle which solves the non-force-free magneto-hydrostatic equations by minimizing a functional. As a result we get a self-consistent equilibrium of magnetic field, plasma density and plasma pressure. Measured quantities, e.g. the photospheric magnetic field vector, are used as boundary condition. We test our code with the help of a semi-analytic magneto-hydro-static equilibrium. The quality of the reconstruction was judged by comparing the exact and reconstructed solution qualitatively by magnetic field-line plots and artificial plasma-images and quantitatively by several different numerical criteria. Our code is able to reconstruct this semi-analytic test equilibrium with high accuracy. The strongly varying plasma-beta environment requires, however, a smaller time step and longer computing time compared with low beta force-free field extrapolations.
SH51C-03 INVITED
Physics of the chromosphere and the lower coronal boundary conditions
I will review observations and theoretical work concerning the role of the chromosphere as the lower boundary for the corona. I will highlight the need for measurements of the chromospheric magnetic field as the plasma regimes change from beta > 1 to < 1, using a striking observational example. I will review some of the important physical processes occurring within the partially ionized chromosphere which can greatly alter the conditions at the coronal base from those expected based only upon photospheric measurements. I will re-iterate the obvious conclusion, but one often ignored, that one must understand specific chromospheric processes if one is to have hope of addressing the nature of the supply of mass, momentum and energy into the corona.
SH51C-04
A RHESSI search for chromospheric evaporation in super-hot flares
Chromospheric evaporation (CE) - thought to occur when downward-accelerated coronal electrons impact
the denser chromosphere, heating the ambient material which then rises to fill the flaring loop - has often
been suggested as the primary source of hot thermal looptop plasma. Evidence for CE is given by crystal
spectrometer observations of blueshifted spectral lines from footpoints.
RHESSI observations show that peak flare temperatures generally occur soon after the hard X-ray (HXR;
>20~keV) peak, when the emission measure is still only ~20% of its later peak value. Imaging places
the thermal plasma at the looptop even for large (GOES X-class) flares that exceed super-hot (Te >
30~MK) temperatures. In such flares, non-thermal energy deposition can exceed ~1029~erg/s,
which for typical electron energies, chromospheric densities, and footpoint areas should yield CE
temperatures of ~10-20~MK with emission measures of ~1050~cm-3. Such bright
thermal emission has never been observed from footpoints, suggesting that while CE may contribute the bulk
loop material, heating occurs primarily in the corona.
We present an analysis of selected RHESSI M- and X-class flares, employing imaging spectroscopy to
determine the temperature and emission measure (or limits thereof) of spatially-separated footpoint and
looptop sources. We compare the time evolution of the thermal footpoint signatures to HXR lightcurves and to
the looptop thermal component. We calculate the energy contained in non-thermal electrons, the thermal
energies at the looptop and footpoints, and discuss the implications for heating by non-thermal electrons and
the contribution of CE to the thermal flare plasma.
http://sprg.ssl.berkeley.edu/~cepheid/agu2008/
SH51C-05 INVITED
Dynamics of the upper chromosphere
In the past few years, high-resolution observations with ground-based telescopes and the Broadband Filter Imager (BFI) and Narrowband Filter Imager (NFI) of the Solar Optical Telescope onboard Hinode have revolutionized our view of the dynamics and energetics of the chromosphere. We review some of these results, including the discovery of two different types of spicules and the finding that the chromosphere is riddled with strong Alfvenic waves. We describe how these observations, when combined with advanced numerical simulations, can help address important unresolved issues regarding the connection between the photosphere and corona, such as the role of waves and of reconnection in driving the dynamics and energetics of the upper chromosphere, and how chromospheric dynamics impact the transition region and corona.
SH51C-06 INVITED
Effects of Partial Ionization: From Waves to Reconnection
The photosphere and chromosphere are sufficiently cold that there is a significant fraction of neutral hydrogen in these layers. Collisions with these neutrals change the nature of the plasma resistivity in such a way that cross-field currents experience a resistivity which may be many orders of magnitude larger than the classical Spitzer resistivity of a fully ionized plasma. This partially ionized layer can therefore act to damp MHD waves and dissipate cross field currents in emerging magnetic flux. We show that the neutral mediated resistivity can damp MHD waves so that Alfvén waves with frequencies above ~eq 0.5 Hz cannot travel from the photosphere into the corona. For flux emergence it is shown that the removal of cross field current changes the amount of material uplift associated and the rising flux thereby preventing its early fragmentation. The effect of neutrals on chromospheric reconnection will also be discussed. Based on these results we argue that the effects of neutral hydrogen on the energy equation and plasma resistivity ought to be essential ingredients in all simulations which aim to couple the photosphere to the overlying corona.
SH51C-07
MHD Model Estimates of the Contribution of Driven, Linear, Non-Plane Wave Dissipation to Chromospheric Heating Using a Complete Electrical Conductivity Tensor
Analytic solutions of an MHD model that includes an anisotropic, inhomogeneous electrical conductivity tensor containing Hall, Pedersen, and Spitzer conductivities are used to compute resistive heating rates as a function of height z from the photosphere to the lower corona due to dissipation of driven, linear, non- plane waves. The background state of the atmosphere is assumed to be an FAL atmosphere. This state is linearly perturbed by a harmonic perturbation of frequency ν. The height dependence of the perturbation in the presence of the inhomogeneous background state is determined by solving the MHD equations given the harmonic, horizontal, driving magnetic field Bx1 at the photosphere, the constant vertical magnetic field Bz, and the magnetic field strength Bcond(z) that enters the electrical conductivity tensor. The variation of the heating rates per unit volume and mass with ν, Bx1, and Bcond(0) are determined. The heating rates are found to be ∝ Bcond(0)2 Bx12, and to increase with ν. The Pedersen resistivity is ∝ Bcond(0)2. It is several orders of magnitude greater than the Spitzer resistivity in the chromosphere, and determines the rate of heating by Pedersen current dissipation in the chromosphere. The Pedersen current is essentially a proton current in the chromosphere. The onset of Pedersen current dissipation rates large enough to balance the net radiative loss from the chromosphere occurs near the height of the FAL temperature minimum, and is triggered by the product of the electron and proton magnetizations first exceeding unity. The magnetizations and heating rate increase rapidly with height beginning near the temperature minimum. For the special case of Bz = 200 G, Bx1=140 G, and 400 ≤ Bcond(0) ≤ 1500 G the driver frequency for which the period averaged chromospheric heating flux FCh = 5 × 106 ergs-cm-2-sec-1 has the corresponding range of 91 ≥ ν ≥ 25 mHz. Larger magnetic field strengths correspond to lower frequencies for a given heating rate. At magnetic field strengths < 400 G, this value of FCh is achieved only at higher frequencies corresponding to solutions that violate the linear approximation. For the similar special case of Bz = 200 G, Bx1=140 G, and 50 ≤ Bcond(0) ≤ 1500 G the range of the maximum allowed driver frequency that is consistent with the linear approximation is 100.25 ≥ ν ≥ 92.5 mHz. The corresponding range of FCh is 2 × 106 ≤ FCh ≤ 5.4 × 107 ergs-cm-2-sec-1. This raises the possibility that linear MHD waves with periods ~ 10 seconds might make a major contribution to chromospheric heating in regions where the photospheric magnetic field strength is moderate to high. 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 linear waves. It operates on any current generating MHD process. This work was supported by Grant ATM 0650443 from the National Science Foundation to the West Virginia High Technology Consortium Foundation.