Flare Scenario Research Articles

Impulsive Flare Plasma Energization in the Light of Yohkoh Discoveries

Flare scenario, based on Yohkoh observations and magnetic reconnection (MR) hypotesis, has been elaborated recently (Tsuneta, 1996; Shibata, 1997). Using theoretical prediction that the fast MR results in a high-speed outflows from reconnection site, the reconnection downflow has been supposed to collide with the top of underlying magnetic loop, producing superhot plasma and high-energy particles.

Phenomena related to electron beams in solar flares

Starting from basic observational characteristics of solar flares the flare physical processes are briefly described. The role of electron beams in such a flare scenario is expressed. The observational manifestations of electron beams as, e.g., type III and U radio bursts are presented. Then results of numerical modelling of these processes are shown. Besides the magnetic field-line reconnection process which is considered to be a primary flare one, the main attention is devoted to the evolution of electron beams in flare loops, to the interpretation of U bursts, to the electron beam bombardment of deep and dense layers of the solar atmosphere and to accompanied processes as the evaporation process and the asymmetry of optical chromospheric lines.

Human exposure to large solar particle events in space.

Whenever energetic solar protons produced by solar particle events traverse bulk matter, they undergo various nuclear and atomic collision processes which significantly alter the physical characteristics and biologically important properties of their transported radiation fields. These physical interactions and their effect on the resulting radiation field within matter are described within the context of a recently developed deterministic, coupled neutron-proton space radiation transport computer code (BRYNTRN). Using this computer code, estimates of human exposure in interplanetary space, behind nominal (2 g/cm2) and storm shelter (20 g/cm2) thicknesses of aluminum shielding, are made for the large solar proton event of August 1972. Included in these calculations are estimates of cumulative exposures to the skin, ocular lens, and bone marrow as a function of time during the event. Risk assessment in terms of absorbed dose and dose equivalent is discussed for these organs. Also presented are estimates of organ exposures for hypothetical, worst-case flare scenarios. The rate of dose equivalent accumulation places this situation in an interesting region of dose rate between the very low values of usual concern in terrestrial radiation environments and the high dose rate values prevalent in radiation therapy.

Formation and cooling of the giant HXIS arches of November 6?7, 1980

Giant arches, first detected by the HXIS instrument aboard SMM, are still a poorly understood component of the flare scenario. Their origin remains uncertain and their behavior, quite different in separate events, has not yet been satisfactorily explained. The purpose of the present paper is to analyze the giant arches imaged on November 6–7, 1980, which, in contrast to that observed on May 21, 1980, were not stationary and had shorter cooling times. In particular, we use a procedure, already applied to the May 21 case, to compute the three-dimensional topology of the magnetic field which forms by reconnection over the active region containing the November arches. This technique allows us to verify that the observed structures are aligned with the computed field lines, lending support to the hypothesis that they originate through a reconnection process which occurs at progressively larger altitudes. Moreover, a calculation of the magnetic energy liberated by reconnection shows that enough energy may be thereby released to account for the observed thermal energy enhancement of the HXIS arches. Finally, the lifetime of the features is shown to be consistent with that predicted by cooling via radiation and field-aligned conduction to the underlying chromosphere.

Magnetic Shear and Flares

Magnetic tension posseses low values at regions of large magnetic “shear” on the polarity inversion line. The low tension allows for a non-force-free field that depends on the density of the medium. Instabilities which can modify the density may then possibly evolve into solar flares. Heuristic arguments are given to favour instability driven flare scenarios.

Do all flares occur within a hierarchy of magnetic loops?

X-ray events observed by the Hard X-ray Imaging Spectrometer on the Solar Maximum Mission frequently indicate the following scenario for solar flares: The initial energy release occurs in a compact magnetic loop and during the impulsive phase may spread rapidly to involve a larger structure. In later phases the soft X-ray emission is from a much larger structure encompassing these initial features and, overlying them all is a huge loop with footpoints separated by up to several hundred thousand km. In the light of these observations, we believe a flare model involving a single magnetic loop is rarely, if ever appropriate.

Development of flare morphology in X-rays, and the flare scenario

We define the impulsive phase of a flare as its first phase, characterized by: X-ray bursts of short (seconds to tens of seconds) duration, a patchy X-ray morphology, and injection of energy. It lasts some five to ten minutes. The gradual or diffuse phase starts virtually at the same time as the impulsive one and is characterized by a gradually varying X-ray flux from a larger, diffuse, area situated higher than the sources of the impulsive X-ray bursts. The diffuse cloud is initially (during the first five minutes) hotter by a few million degrees than the sources of the impulsive phase bursts and is assumed to be caused by convective motions with upward velocities of a few hundred km s−1. It contains about the same number of energetic electrons as the impulsive burst patches contained initially. It cools gradually down by radiative and conductive losses, a process that may last for about an hour.

IUE spectra of a flare in the RS Canum Venaticorum-type system UX ARIETIS

IUE spectra of UX Ari obtained during the large flare of 1979 January 1 exhibit chromospheric and transition-region emission-line fluxes about 2.5 and 5.5 times brighter than quiescent fluxes, respectively, and up to 1400 times brighter than the quiet sun. A high-dispersion spectrum of the 2000-3000 A region exhibits enhanced Fe II emission, which is probably associated mainly with the K0 IV star, and enhanced Mg II emission with asymmetric wings extending to +475 km/s. These line wings are interpreted as evidence for mass flow from the K0 IV star to the G5 V star. A speculative scenario of major long-lived RS CVn flares is proposed in which the component stars have very large corotating flux tubes, which occasionally interact. Magnetic reconnection results in flux tubes that temporarily connect the two stars.