A theoretical model of flare which explains observed quantities in Ha, EUV, soft X-ray and flare-associated solar wind is presented. It is assumed that large mass observed in the soft X-ray flare and the solar wind comes from the chromosphere by the process like evaporation while flare is in progress. From mass and pressure balance in the chromosphere and the corona, the high temperature in the soft X-ray flare is shown to be attained by the larger mass loss to the solar wind compared with the mass remained in the corona, in accord with observations. The total energy of 10 a2 erg, the electron density of l013'5 cm -a in Hc~ flare, the temperature of the X-ray flare of 107.3K and the time to attain maximum Ha brightness (600 s) are derived consistent with observations. It is shown that the top height of the Ha flare is located about 1000 km lower than that of the active chromosphere because of evaporation. So-called limb flares are assigned to either post-flare loops, surges or rising prominences. The observed small thickness of the Ha flare is interpreted by free streaming and/or heat conduction. Applications are suggested to explain the maximum temperature of a coronal condensation and the formation of quiescent prominences.

THEORETICAL MODEL OF FLARES AND PROMINENCES I: Evaporating Flare Model

The main challenge for the theory of solar eruptions has been to understand two basic aspects of large flares. These are the cause of the flare itself and the nature of the morphological features which form during its evolution. Such features include separating ribbons of H
 
$\alpha$
 emission joined by a rising arcade of soft x-ray loops, with hard x-ray emission at their summits and at their feet. Two major advances in our understanding of the theory of solar flares have recently occurred. The first is the realisation that a magnetohydrodynamic (MHD) catastrophe is probably responsible for the basic eruption and the second is that the eruption is likely to drive a reconnection process in the field lines stretched out by the eruption. The reconnection is responsible for the ribbons and the set of rising soft x-ray loops, and such a process is well supported by numerical experiments and detailed observations from the Japanese satellite Yohkoh. Magnetic energy conversion by reconnection in two dimensions is relatively well understood, but in three dimensions we are only starting to understand the complexity of the magnetic topology and the MHD dynamics which are involved. How the dynamics lead to particle acceleration is even less well understood. Particle acceleration in flares may in principle occur in a variety of ways, such as stochastic acceleration by MHD turbulence, acceleration by direct electric fields at the reconnection site, or diffusive shock acceleration at the different kinds of MHD shock waves that are produced during the flare. However, which of these processes is most important for producing the energetic particles that strike the solar surface remains a mystery.

The magnetic nature of solar flares

We present an overview of solar flares and associated phenomena, drawing upon a wide range of observational data primarily from the RHESSI era Following an introductory discussion and overview of the status of observational capabilities, the article is split into topical sections which deal with different areas of flare phenomena (footpoints and ribbons, coronal sources, relationship to coronal mass ejections) and their interconnections We also discuss flare soft X-ray spectroscopy and the energetics of the process The emphasis is to describe the observations from multiple points of view, while bearing in mind the models that link them to each other and to theory The present theoretical and observational understanding of solar flares is far from complete, so we conclude with a brief discussion of models, and a list of missing but important observations

An Observational Overview of Solar Flares

A study of gas-phase element abundances reported in the literature for 17 different elements sampled over 243 sight lines in the local part of our Galaxy reveals that the depletions into solid form (dust grains) are extremely well characterized by trends that employ only three kinds of parameters. One is an index that describes the overall level of depletion applicable to the gas in any particular sight line, and the other two represent linear coefficients that describe how to derive each element's depletion from this sight-line parameter. The information from this study reveals the relative proportions of different elements that are incorporated into dust at different stages of grain growth. An extremely simple scheme is proposed for deriving the dust contents and metallicities of absorption-line systems that are seen in the spectra of distant quasars or the optical afterglows of gamma-ray bursts. Contrary to presently accepted thinking, the elements sulfur and krypton appear to show measurable changes in their depletions as the general levels of depletions of other elements increase, although more data are needed to ascertain whether or not these findings are truly compelling. Nitrogen appears to show no such increase. The incorporation of oxygen into solid form in the densest gas regions far exceeds the amounts that can take the form of silicates or metallic oxides; this conclusion is based on differential measurements of depletion and thus is unaffected by uncertainties in the solar abundance reference scale.

https://iopscience.iop.org/article/10.1088/0004-637X/700/2/1299/pdf

A unified representation of gas-phase element depletions in the interstellar medium*

This paper provides a short review of some of the basic concepts related to the origin of coronal mass ejections (CMEs). The various ideas which have been put forward to explain the initiation of CMEs are categorized in terms of whether they are force-free or non-force-free and ideal or nonideal. A few representative models of each category are examined to illustrate the principles involved. At the present time there is no model which is sufficiently developed to aid forecasters in their efforts to predict CMEs, but given the current pace of research, this situation could improve dramatically in the near future.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JA000005

A review on the genesis of coronal mass ejections

This is an entirely new edition of Harold Zirin's classic text on the solar atmosphere. Combining an introductory course in astrophysics with a comprehensive treatment of the theoretical and observational aspects of our present knowledge of the sun, the book has been completely updated. It includes a large number of spectacular new photographs, including many of the best solar pictures from the world's observatories. Professor Zirin is one of the leading scientists in his field. His lucid writing style, combined with considerable teaching experience, has resulted in a valuable and important textbook of astrophysics.

Astrophysics of the Sun

We have studied 8 yr of active region observations from the United States Air Force/Mount Wilson data set, supplied by the NOAA World Data Center, to confirm the relation between δ spots and large flares We found that after correcting some errors we were able to describe relationships among active region size, peak flare soft X-ray (SXR) flux (measured by GOES 1-8 A flux), and magnetic classification We found the Solar Optical Observing Network magnetic classification to be reasonably accurate but its area measures to be inaccurate for many of the regions This is due partly to transcription errors and partly to wrong correction for limb foreshortening Errors could, however, be repaired by intercomparison of multiple observations We confirm Kunzel's original idea that regions classified βγδ produce many more large flares than other regions of comparable size Almost all substantial flares occurred in regions classified βγδ by the Air Force sites Each region larger than 1000 μh and classified βγδ had nearly 40% probability of producing flares classified X1 or greater Yet only a half-dozen of those, showing the "island delta" configuration, produced great activity There is a general trend for large regions to produce large flares, but it is less significant than the dependence on magnetic class

https://iopscience.iop.org/article/10.1086/309303/pdf

The Dependence of Large Flare Occurrence on the Magnetic Structure of Sunspots

We present high-resolution transverse and longitudinal magnetic field measurements bracketing five X-class solar flares. We show that the magnetic shear, defined as the angular difference between the measured field and calculated potential field, actually increases after all of these flares. In each case, the shear is shown to increase along a substantial portion of the magnetic neutral line. For two of the cases, we have excellent time resolution, on the order of several minutes, and we demonstrate that the shear increase is impulsive. We briefly discuss the theoretical implications of our results.

Vector magnetic field changes associated with X-class flares

Observations of the August, 1972 flares at Big Bear and Tel Aviv, involving monochromatic movies, magnetograms, and spectra, are analyzed. The region (McMath 11976) showed inverted polarity from its inception on July 11; the great activity was due to extremely high shear and gradients in the magnetic field, as well as a constant invasion of one polarity into the opposite; observations in lambda 3835 show remarkable fast flashes in the impulsive flare of 18:38 UT on Aug. 2 with lifetimes of 5 sec, which may be due to dumping of particles in the lower chromosphere. Flare loops show evolutionary increases of their tilts to the neutral line in the flares of Aug. 4 and 7. Spectroscopic observations show red asymmetry and red shift of the H alpha emission in the flash phase of the Aug. 7 flare, as well as substantial velocity shear in the photosphere during the flare, somewhat like earthquake movement along a fault. Finally the total H alpha emission of the Aug. 7 flare could be measured accurately as about 2.5 x 10 to the 30th power erg, considerably less than coarser previous estimates for great flares.

Harold Zirin

Papers

Astrophysics of the Sun

The Dependence of Large Flare Occurrence on the Magnetic Structure of Sunspots

Vector magnetic field changes associated with X-class flares

The flares of August 1972

The solar atmosphere