Solar and stellar flares

Short-lived increases in the brightness of many red dwarfs have been observed for the last 30 yr, and a variety of more or less exotic models have been proposed to account for such flares. Information about flares in the Sun has progressed greatly in recent years as a result of spacecraft experiments, and properties of coronal flare plasma are becoming increasingly better known. In this paper, after briefly reviewing optical, radio and X-ray observations of stellar flares, we show how a simplified model which describes conductive plus radiative cooling of the coronal flare plasma in solar flares has been modified to apply to optical and X-ray stellar flare phenomena. This model reproduces many characteristic features of stellar flares, including the mean UBV colors of flare light, the direction of flare decay in the two-color diagram, precursors, Stillstands, secondary maxima, lack of sensitivity of flare color to flare amplitude, low flux of flare X-rays, distinction between so-called spike flares and slow flares, Balmer jumps of as much as 6–8, and emission line redshifts up to 3000 km s^–1. In all probability, therefore, stellar flares involve physical processes which are no more exotic (and no less!) than those in solar flares. Advantages of observing stellar flares include the possibilities of (i) applying optical diagnostics to coronal flare plasma, whereas this is almost impossible in the Sun, and (ii) testing solar flare models in environments which are not generally accessible in the solar atmosphere.     

Solar flares and magnetic topology

This article is a very brief review and comparison of the observational properties of flares and theoretical concepts of models of flares, especially the concepts of magnetic topology and its evolution. We examine the environmental aspects of flare behavior. Some of these aspects must be consequences of unknown processes occurring below the photosphere. Other aspects involve structures--such as filaments--that are closely related to flares. We then examine properties of flares to try to distinguish the different phases of energy release that can occur in the course of a flare. Finally we offer a schematic scenario and attempt to interpret these phases in terms of this scenario.     

Solar flares and magnetic topology

Solar Physics - This article is a very brief review and comparison of the observational properties of flares and theoretical concepts of models of flares, especially the concepts of magnetic...     

Solar flares at submillimeter wavelengths

We discuss the implications of the first systematic observations of solar flares at submillimeter wavelengths, defined here as observing wavelengths shorter than 3 mm (frequencies higher than 0.1 THz). The events observed thus far show that this wave band requires a new understanding of high-energy processes in solar flares. Several events, including observations from two different observatories, show during the impulsive phase of the flare a spectral component with a positive (increasing) slope at the highest observable frequencies (up to 405 GHz). To emphasize the increasing spectra and the possibility that these events could be even more prominent in the THz range, we term this spectral feature a “THz component”. Here we review the data and methods, and critically assess the observational evidence for such distinct component(s). This evidence is convincing. We also review the several proposed explanations for these feature(s), which have been reported in three distinct flare phases. These data contain important clues to flare development and particle acceleration as a whole, but many of the theoretical issues remain open. We generally have lacked systematic observations in the millimeter-wave to far-infrared range that are needed to complete our picture of these events, and encourage observations with new facilities.     

Solar flares controlled by helicity conservation

The energy release in a class of solar flares is studied on the assumption that during burst events in highly conducting plasma the magnetic helicity of plasma is approximately conserved. The available energy release under a solar flare controlled by the helicity conservation is shown to be defined by the magnetic structure of the associated prominence. The approach throws light on some solar flare enigmas: the role of the associated prominences; the discontinuation of the reconnection of magnetic lines long before the complete reconnection of participated fields occurs; the existence of quiet prominences which, in spite of their usual optical appearance, do not initiate any flare events; the small energy release under a solar flare in comparison with the stockpile of magnetic energy in surrounding fields. The predicted scale of the energy release is in a fair agreement with observations.Presently guest at Stanford Linear Acceleraton Center, Stanford University, P.O. Box 4349, Stanford, CA 94305, U.S.A.Work done at the Space Environment Laboratory, NOAA, ERL, Boulder, CO 80303, U.S.A.     

Solar flares in the extreme ultraviolet

Measurements of flare-related impulsive enhancements in solar emission lines in the extreme ultraviolet, observed from the satellite OSO-III, are reported. The enhancement of a line, expressed in percent of the total disk intensity in the line, is of the same order of magnitude as the flare area, expressed in heliocentric square degrees. Rise-times and decay-times of impulsive enhancements average about 2 min and 5 min, respectively. The maximum enhancements of radiation from ions in the chromosphere-corona transition region precede the H maximum by an average of 2 min, and occur in the same period of time as the hard component of solar X-rays and the impulsive microwave bursts. Coronal lines in the extreme ultraviolet are less impulsive than the transition region lines in flare-related enhancements and their maxima follow the H maximum.     

Solar flares in the extreme ultraviolet

Solar-flare observations in the extreme ultraviolet (300–1350 Å) are reported. Some 269 flares observed by the Harvard College Observatory (HCO) experiment on OSO 4 and 211 flares observed by the HCO experiment on OSO 6 have been analyzed. The flares were observed in spectral lines and continua emitted by many ionic species over a temperature range from 10⁴ to 3.5 × 10⁶ K. The EUV data have been correlated with X-ray, H, and radio observations, and a significant number of EUV bursts not associated with reported H, X-ray, or radio bursts have been iden tified and investigated. The results indicate that these latter EUV events are less energetic by about a factor of 2 than EUV bursts associated with — F subflares.     

Solar flares: The impulsive phase

Only during the previous solar cycle have systematic observations begun to be made with the sensitivity and time resolution, and the continuous coverage required to catch the impulsive phase and measure the rapid variations present in many wavelength ranges. Observations in X-rays, gamma-rays, UV, H, and radio wavelengths all reveal rapid variations during the impulsive phase and have contributed to our understanding of the different phenomena involved. Results have been obtained from several spacecraft, from rocket and balloon flights, and from ground-based observations. These are reviewed in the context of a simple single loop flare model with a view to showing what results are consistent with this model and what the major problems are in our understanding of the impulsive phase. New instrumentation planned for observations during the present Cycle 22 will provide a concerted attack on the impulsive phase as part of the Max '91 program.     

Solar flares: The gradual phase

One has to distinguish between two kinds of the gradual phase of flares: (1) a gradual phase during which no energy is released so that we see only cooling after the impulsive phase (a confined flare), and (2) a gradual phase during which energy release continues (a dynamic flare).The simplest case of (1) is a single-loop flare which might provide an excellent opportunity for the study of cooling processes in coronal loops. But most confined flares are far more complicated: they may consist of sets of unresolved elementary loops, of conglomerates of loops, or they form arcades the components of which may be excited sequentially. Accelerated particles as well as hot and cold plasma can be ejected from the flare site (coronal tongues, flaring arches, sprays, bright and dark surges) and these ejecta may cool more slowly than the source flare itself.However, the most important flares on the Sun are flares of type (2) in which a magnetic field opening is followed by subsequent reconnection of fieldlines that may continue for many hours after the impulsive phase. Therefore, the main attention in this review is paid to the gradual phase of this category of long-decay flares. The following items are discussed in particular: The wide energy range of dynamic flares: from eruptions of quiescent filaments to most powerful cosmic-ray flares. Energy release at the reconnection site and modelling of the reconnection process. The post-flare loops: evidence for reconnection; observations at different wavelengths; energy deposit in the chromosphere, chromospheric ablation, and velocity fields; loops in emission; shrinking loops; magnetic modelling. The gradual phase in X-rays and on radio waves. Post-flare X-ray arches: observations, interpretation, and modelling; relation to metric radio events and mass ejections, multiple-ribbon flares and anomalous events, hybrid events, possible relations between confined and dynamic flares.     

Subphotospheric current systems and flares

Subphotospheric current systems inferred from recent vector magnetograph observations (e.g. Gary et al., 1987) imply the existence of electric currents penetrating the photosphere and thus flowing deep in the solar convection zone. These currents presumably originate in an internal dynamo that supplies the observed photospheric magnetic fields through the buoyant motions of the initially deeply-buried flux tubes. The coronal fields resulting from this process therefore must carry slowly-varying currents driven by emf's remote from the surface. These currents may then drive solar-flare energy release. This paper discusses the consequences of such a deep origin of the coronal parallel currents. Simple estimates for a large active region suggest a mean current-closure depth 10,000 km, with a subphotospheric inductance 100 H and a subphotospheric stored energy 10³³ ergs.     

Subphotospheric current systems and flares

Solar Physics - Subphotospheric current systems inferred from recent vector magnetograph observations (e.g. Gary et al., 1987) imply the existence of electric currents penetrating the photosphere...     

Solar flares and the cosmic ray intensity

The relationship between the cosmic ray intensity and solar activity during solar cycle 20 is discussed. A model is developed whereby it is possible to simulate the observed cosmic ray intensity from the observed number of solar flares of importance 1. This model leads to a radius for the modulation region of 60–70 AU. It is suggested that high speed solar streams also made a small contribution to the modulation of cosmic rays during solar cycle 20.     

Solar flares: High-energy radiation and particles

Due to the Sun's proximity flares can be investigated in the gamma-ray regime and flare generated particles can be measured in space and related to particular events. In this review paper we focus on the problem of particle acceleration by using as observational ingredients: the fluxes and spectra of particles inferred from gamma-ray measurements and observed in interplanetary space, the temporal characteristics of flares at high-energy X- and gamma-rays and the distribution of gamma-ray flares over the solar disc.     

Solar and stellar flares: Questions and problems

Although progress has been made in understanding certain aspects of the physics of solar and stellar flares, there are a number of topics which, in the author's opinion, still pose a problem. We summarize these topics here.     

The development of coronal electric current systems in active regions and their relation to filaments and flares

The equilibrium positions of coronal currents are determined. It is shown that the fact that the photosphere has a very large inertia as compared to the corona, poses an important boundary condition at the surface. Electric currents flowing in a coronal active region show a tendency to concentrate above a neutral line. Only here equilibria are possible, determined at low heights by the background field and at large heights by gravity. An instability may occur when the current at low heights exceeds a certain value. The model given is compared with observations of the corona, of prominences, and of fibril motions. Also, the relation with solar flares is discussed in general terms.     

Solar flares in the EUV observed from OSO-5

Solar flares in three broad EUV spectral bands have been observed from OSO-5 with a grating spectrophotometer. Results are given for three large flares of March 12, March 21 and April 21, 1969. In general the time dependence of flare intensity in each band is characterized by a slowly varying component with impulsive bursts superimposed. Bands 2 (465–630 Å) and 3 (760–1030 Å) are quite similar in their time variations, but band 1 (280–370 Å) shows less impulsive structure, and declines more slowly. Absolute EUV intensities for the flares are estimated, and a comparison made with the 2800 mc s^–1 radio emission. A flare model is proposed to account for the EUV time variations during a large flare.     

Solar wind,non-force-free magnetic field and two-ribbon flares

With the assumption that the magnetic field lines are radial at some quite high level in the solar corona, a non-constant shearing magnetic field is introduced into the magnetohydrostatic equations. It is found that a same critical amount of shearing a magnetic island is formed and then breaks out to form an open magnetic configuration in which resistive tearing-mode instability may occur, and may initiate a two-ribbon flare. In addition, high shearing magnetic fields are investigated. It is shown that high shearing magnetic configurations are weak two-dimensional neutral sheets, the instability of which has been studied by Janicke (1982).     

Solar flares,magnetic clouds,and geomagnetic storms

Firstly, semi-empirical distributions of solar wind proton number density and velocity ordered around the Heliospherical Current Sheet (HCS) of the inner heliosphere are considered. Then, the velocity profiles of flare-generated Coronal Mass Ejections (CMEs) running through the inhomogeneous heliosphere are calculated. They show that the velocities strongly depend on flare positions with respect to the HCS. Finally, a specific mutual flare-HCS-Earth location leading to a strong geomagnetic storm is deduced from calculations and supported by a few real events of solar-terrestrial physics.