Radio emission from solar flares offers a number of unique diagnostic tools to address long-standing questions about energy release, plasma heating, particle acceleration, and particle transport in magnetized plasmas. At millimeter and centimeter wavelengths, incoherent gyrosynchrotron emission from electrons with energies of tens of kilo electron volts to several mega electron volts plays a dominant role. These electrons carry a significant fraction of the energy released during the impulsive phase of flares. At decimeter and meter wavelengths, coherent plasma radiation can play a dominant role. Particularly important are type III and type III‐like radio bursts, which are due to upward- and downwarddirected beams of nonthermal electrons, presumed to originate in the energy release site. With the launch of Yohkohand the Compton Gamma-Ray Observatory , the relationship between radio emission and energetic photon emissions has been clarified. In this review, recent progress on our understanding of radio emission from impulsive flares and its relation to X-ray emission is discussed, as well as energy release in flare-like phenomena (microflares, nanoflares) and their bearing on coronal heating.

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Radio emission from solar flares

In this paper, we present a model of the plasma beta above an active region and discuss its consequences in terms of coronal magnetic field modeling. The β-plasma model is representative and derived from a collection of sources. The resulting β variation with height in the solar atmosphere is used to emphasize that the assumption that the magnetic pressure dominates over the plasma pressure must be carefully employed when extrapolating the magnetic field. This paper points out (1) that the paradigm that the coronal magnetic field can be constructed from a force-free magnetic field must be used in the correct context, since the force-free region is sandwiched between two regions which have β>1, (2) that the chromospheric Mg ii–C iv magnetic measurements occur near the β-minimum, and (3) that, moving from the photosphere upwards, β can return to ∼1 at relatively low coronal heights, e.g., R∼1.2 R
s.

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Plasma beta above a solar active region: rethinking the paradigm

A three-dimensional magnetohydrodynamic (MHD) model for the propagation and dissipation of Alfv?n waves in a coronal loop is developed The model includes the lower atmospheres at the two ends of the loop The waves originate on small spatial scales (less than 100?km) inside the kilogauss flux elements in the photosphere The model describes the nonlinear interactions between Alfv?n waves using the reduced MHD approximation The increase of Alfv?n speed with height in the chromosphere and transition region (TR) causes strong wave reflection, which leads to counter-propagating waves and turbulence in the photospheric and chromospheric parts of the flux tube Part of the wave energy is transmitted through the TR and produces turbulence in the corona We find that the hot coronal loops typically found in active regions can be explained in terms of Alfv?n wave turbulence, provided that the small-scale footpoint motions have velocities of 1-2?km?s?1 and timescales of 60-200?s The heating rate per unit volume in the chromosphere is two to three orders of magnitude larger than that in the corona We construct a series of models with different values of the model parameters, and find that the coronal heating rate increases with coronal field strength and decreases with loop length We conclude that coronal loops and the underlying chromosphere may both be heated by Alfv?nic turbulence

https://iopscience.iop.org/article/10.1088/0004-637X/736/1/3/pdf

Heating of the solar chromosphere and corona by alfvén wave turbulence

The Transition Region and Coronal Explorer (TRACE) – described in the companion paper by Handy et al. (1999) – provides an unprecedented view of the solar outer atmosphere. In this overview, we discuss the initial impressions gained from, and interpretations of, the first million images taken with TRACE. We address, among other topics, the fine structure of the corona, the larger-scale thermal trends, the evolution of the corona over quiet and active regions, the high incidence of chromospheric material dynamically embedded in the coronal environment, the dynamics and structure of the conductively dominated transition region between chromosphere and corona, loop oscillations and flows, and sunspot coronal loops. With TRACE we observe a corona that is extremely dynamic and full of flows and wave phenomena, in which loops evolve rapidly in temperature, with associated changes in density. This dynamic nature points to a high degree of spatio-temporal variability even under conditions that traditionally have been referred to as quiescent. This variability requires that coronal heating can turn on and off on a time scale of minutes or less along field-line bundles with cross sections at or below the instrumental resolution of 700 km. Loops seen at 171 A (∼1 MK) appear to meander through the coronal volume, but it is unclear whether this is caused by the evolution of the field or by the weaving of the heating through the coronal volume, shifting around for periods of up to a few tens of minutes and lighting up subsequent field lines. We discuss evidence that the heating occurs predominantly within the first 10 to 20 Mm from the loop footpoints. This causes the inner parts of active-region coronae to have a higher average temperature than the outer domains.

A new view of the solar outer atmosphere by the transition region and Coronal Explorer

The formation of wind-driven solar model based on a new hypothesis for nonlinear wave dissipation is described. The equations and solutions used to derive the model are examined. The corona and solar wind are analyzed. The model is applied to observations and it is detected that it displays the proper steep temperature rise to a maximum coronal temperature in excess of 10 to the 6th K, a solar wind flux in excess of 3.5 x 10 to the 8th/sq cm per sec at 1 AU, and nonthermal velocities are observed at the bases of coronal holes; however, the model does not produce enough detail.

Transition region, corona, and solar wind in coronal holes

An analytical model for the quiescent inhomogeneous solar corona is developed on the basis of the hypothesis that looplike structures are the basic coronal building blocks. By assuming that quiescent loop structures observed in X-rays are in hydrostatic equilibrium, it is demonstrated that such loops must have their temperature maximum located near their apex and that substantial nonradiative energy deposition must occur along most of their length. The calculations yield a unique relation among loop temperature, pressure, and size, which fits the X-ray observations of quiescent structures well and is consistent with the initial assumption of hydrostatic equilibrium. The results suggest that the coronal loops visible in X-rays represent a relatively steady-state equilibrium of the confined plasmas and that fluctuations in such quantities as the local heating rate can lead to dynamically unstable states in which the loop plasma does not attain a temperature sufficient for X-ray emission. A parameterization of various proposed coronal heating theories is also developed within the context of the analytical model.

W. H. Tucker

Papers

Dynamics of the quiescent solar corona.