On the possibility of detecting solar flare effects in the zodiacal light

To evaluate possible effects of solar flares on the brightness of the inner zodiacal light, it is necessary to consider the brightness contribution along the line of sight and as a function of Sun-particle distance. For this purpose, models of the brightness contribution along the line of sight are presented for both dielectric and metallic particles with a spatial distribution of the form r^?ν, ν = 0, 1, 2. These models are discussed in terms of the geometry of shock front interaction. A reported zodiacal light enhancement following a solar flare (Blackwell and Ingham, 1961) is analyzed on the basis of the shock front geometry.     

Source of the solar flare energy

Recent Skylab and magnetograph observations indicate that strong photospheric electric currents underlie small flare events such as X-ray loops and surges. What is not yet certain, because of the non-local dynamics of a fluid with embedded magnetic field, is whether flare emission derives from the energy of on-site electric currents or from energy which is propagated to the flare site through an intermediary, such as a stream of fast electrons or a group of waves. Nevertheless, occurrences of: (1) strong photospheric electric currents beneath small flares; (2) similar magnetic fine structure inside and outside active regions; (3) eruptive prominences and coronal white light transients in association with big flares; and, (4) active boundaries of large unipolar regions suggest the possibility that all phenomena of solar activity are manifestations of the rapid ejection and/or gradual removal of electric currents of various sizes from the photosphere. The challenge is to trace the precise magnetofluid dynamics of each active phenomenon, particularly the role of electric current build-up and dissipation in the low corona.     

On the possibility of a bimodal solar dynamo

A simple way to couple an interface dynamo model to a fast tachocline model is presented, under the assumption that the dynamo saturation is due to a quadratic process and that the effect of finite shear layer thickness on the dynamo wave frequency is analogous to the effect of finite water depth on surface gravity waves. The model contains one free parameter which is fixed by the requirement that a solution should reproduce the helioseismically determined thickness of the tachocline. In this case it is found that, in addition to this solution, another steady solution exists, characterized by a four times thicker tachocline and 4–5 times weaker magnetic fields. It is tempting to relate the existence of this second solution to the occurrence of grand minima in solar activity. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The possibility of energy accumulation in a current sheet above the NOAA 9077 active region prior to the flare on 14 July 2000

Current sheet (CS) creation and energy accumulation above the NOAA 9077 active region have been numerically simulated. The magnetic spots are approximated by vertical dipoles placed under the photosphere, and the system of resistive 3D MHD equations is solved for compressible plasma with anisotropic thermal conduction. Two neutral magnetic lines are present in the corona above the NOAA 9077 active region, and a vertical CS emerges in the vicinity of one of them. The energy accumulated in this CS is about 5×10³² erg. The j×B/c force in it accelerates plasma upward. The other neutral line is not suitable for CS creation.     

On Doppler shifts of the Fe XXV ion resonance line in solar flare X-ray spectra

Doppler shifts of the Fe xxv line in three solar flares show prevalently downward motions with velocities up to 200 km s^–1.     

A non-LTE analysis of solar flare K line

We carried out a non-LTE calculation of the simultaneous solution of equations of radiative transfer and statistical equilibrium for hydrogen and calcium. Based on the calculated results of a series of flare models, we investigated the effect of the Call K line by mircoturbulence, the temperature minimum region and the transition region. We also constructed a semi-empirical flare model that reproduced both the observed Hα and Call K line profiles.     

On the origin of solar flare X-rays

The origin of X-ray solar bursts is investigated on the basis of the theoretical model developed by Syrovatskii. According to this model (i) one of the most important manifestations of flares is the acceleration of charged particles (mainly of electrons) to subrelativistic and relativistic energies, and (ii) the two flare phases: stationary (soft) and nonstationary (hard) should be distinguished. The first phase is accompanied by the generation of the soft (2–8 Å) thermal X-rays and the second one by the generation of hard thermal and nonthermal X-rays in the 10 keV range. The thermal X-rays arise in both phases due to the heating of the ambient gas by accelerated particles. The possible mechanisms of non-thermal X-rays are investigated. Simple models of the emitting region are considered, taking into account the simultaneous observations in different regions of the electromagnetic spectrum.     

On the possibility of nuclear processes in atmospheres of flare stars

A resume of the main results obtained in the framework of fast electron hypothesis is presented. The fast electrons are possessed by strong constant properties, that is, nearly monoenergetic in spectrum and with the energy of 3. Fast electrons appear spontaneously in all kinds of cool dwarfs, young and old, and stimulate the generation of all types of flares, faint and powerful, in all ranges of electromagnetic waves — optical, ultraviolet, radio, X-ray. The general flare mechanisms are inverse Compton effect at the faint and moderate flares and nonthermal bremsstrahlung at powerfulness. This hypothesis explains an enormous number of observational facts and relationships — all these are illustrated by two tree-like diagrams in Figures 1 and 2.In the second part, the problem of the spontaneous appearance of fast electrons above the stellar atmosphere is discussed. The nature of the fast electrons in universal and is independent of the physical conditions of the star or its atmosphere. Their origin have nothing in common also with the well known acceleration processes, including the magnetic and hydrodynamic nature, in stellar atmospheres. At present nuclear decay processes seem to be most probable for the appearance of fast electrons in outer regions of stellar atmospheres. Some decay processes are examined with various probability of their realization. Creation of the radioactive nucleus¹³N as a result of capture of H by¹²C and its consequent decay with the escape of a positron seems most probable; as this fast positron will act just like a fast electron. It is outlined the high effectivity of this process in binary systems.     

Analysis of the emission line spectra of a solar flare observed from Skylab

The EUV emission spectra in the wavelength range 110–1900 Å of the 5 September 1973 flare observed with the NRL slit spectrograph on Skylab are studied. The results are: (1) The chromospheric and transition-zone lines are greatly enhanced during the flare. In particular, the allowed lines are enhanced more than the intersystem lines. The Ni ii and P ii lines show the greatest enhancement with a factor of 800 increase in intensity. Other lines such as O i, C i, Si iii, S iii, S iv, O iv, O v, and N v show increases in intensity 10–100 times during the flare. (2) The chromospheric lines, although greatly enhanced during the flare, maintain their sharp and gaussian profiles and are not appreciably broadened. The transition zone lines, on the other hand, show a red-shifted component during the initial phase of the flare. The deduced downward velocity in the transition zone is 50 km s^–1. In addition, there are large turbulent mass motions. The downward mass motion is probably caused by the pressure imbalance between the flare hot plasma at 13 × 10⁶ K and the cooler plasma at 10⁵ K. (3) The density of the 10⁵ K flare plasma, as deduced from density-sensitive lines, is greater than 10¹² cm^-3. The depth of the 10⁵ K plasma in the flare transition zone is only of the order of 0.1 km, giving a steep temperature gradient. Consideration of the energy balance between the conductive flux and the radiative energy losses shows that, indeed, the high density in the transition zone requires that its thickness be very small. This is a consequence of the maximum radiative efficiency at the temperature around 10⁵ K in the solar plasma.Ball Brothers Research Corporation.     

Heating of the solar flare plasma by high energy electrons

Heating of the ambient plasma by high energy electrons in solar flares is discussed. It is shown that for large flares the heating is enough to produce a thermal plasma of a temperature up to a few times of 10⁷K rapidly in the initial phase of the flares. Thus thermal bremsstrahlung in addition to non-thermal bremsstrahlung should be considered for the X-ray emission of solar flares in the initial phase.NAS-NRC Resident Research Associate.     

On the origin of solar flare microwave radio bursts

The broadband microwave radio bursts associated with solar flares are suggested to be synchrotron emission from relativistic positrons resulting from the decay of pions produced by the interaction of accelerated flare protons with the solar material.A detailed calculation is made of the positron yield and the resulting synchrotron spectrum, and good agreement is found between the calculated radioemission and the observed microwave flux density. It is suggested, therefore, that broadband microwave bursts are an indication of the presence of high density protons at the Sun even when such protons are not observable at the Earth.     

On the 24-day period observed in solar flare occurrence

Time series of daily numbers of solar Hα flares from 1955 to 1997 are studied by means of wavelet power spectra with regard to predominant periods in the range of ∼ 24 days (synodic). A 24-day period was first reported by Bai (1987) for the occurrence rate of hard X-ray flares during 1980–1985. Considering the northern and southern hemisphere separately, we find that the 24-day period is not an isolated phenomenon but occurs in each of the four solar cycles investigated (No. 19–22). The 24-day period can be established also in the occurrence rate of subflares but occurs more prominently in major flares (importance classes ≥ 1). A comparative analysis of magnetically classified active regions subdivided into magnetically complex (i.e., including a γ and/or δ configuration) and non-complex (α, β) reveals a significant relation between the appearance of the 24-day period in Hα flares and magnetically complex sunspot groups, whereas it cannot be established for non-complex groups. It is suggested that the 24-day period in solar flare occurrence is related to a periodic emergence of new magnetic flux rather than to the surface rotation of sunspots.     

The correlation of solar flare production with magnetic energy in active regions

An investigation of 531 active regions was made to determine the correlation between energy released by flares and the available energy in magnetic fields of the regions. Regions with magnetic flux greater than 10²¹ maxwell during the years 1967–1969, which included sunspot maximum, were selected for the investigation. A linear regression analysis of flare production on magnetic flux showed that the flare energy is correlated with magnetic energy with a coeificient of correlation of 0.78. Magnetic classification and field configuration also significantly affect the production of flares.This work was supported by the Aerospace Sponsored Research Program.     

A solar flare in the Fe I 5324 line on 24 June, 1993

A solar flare with both H and Fe i 5324 emissions was observed in AR 7529 (S13, E65) on 24 June, 1993 at the Bejing Astronomical Observatory. Our calculations show that the Fe i 5324 emission region of the flare was located in the low photosphere at a height of about 180 km above ₅₀₀₀ = 1, which is lower than many previous studies of white-light flares. To study a Fe i 5324 flare, which represents a kind of extreme case in solar flares, would be useful for clarifying some arguments in the researches of white-light flares as well as for understanding the mechanism of solar flares.The synthetic analyses from vairous features of the flare lead to the following possible exciting mechanism of the Fe i 5324 flare: owing to the flow of energetic electrons from the corona and probably also the thermal conduction downward into the lower atmosphere, a condensation with a temperature higher than that below it was formed near the transition region. Then the low photosphere was heated through backwarming. The Fe i 5324 flare occurred as an indicator of the excitation in the low photosphere.     

A kinematic model of a solar flare

Hyder advocated the idea that the optical (H) flares can be identified with the response of the solar chromosphere to an infalling material stream resulting from the disparition brusque of a prominence. Since some flares are observed without any apparent association with infalling streams, in this paper we examine the possibility of identifying the optical flare with the response of the chromosphere to a supersonic disturbance, i.e., a shock, propagating downward. The undisturbed chromosphere is represented by the Harvard-Smithsonian Reference Atmosphere and the evolution of the shock is evaluated with the use of the CCW (Chisnell, Chester, Whitham) approximation based on the theory of characteristics. It is shown that the chromosphere is heated by the shock and that radiation is enhanced, and that the enhanced radiation terminates the shock around the height of the temperature minimum. Numerical results obtained and possible future improvements of this type of study are discussed.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Propagation of low energy protons associated with the 24 January 1969 solar flare

This paper presents directional low energy solar proton measurements together with inter-planetary magnetic field measurements. Propagation of 1 to 13 MeV solar protons is discussed in terms of the relative importance of field-aligned streaming compared to convection of the proton population in the solar wind. Evidence is presented to show that protons associated with the January 24, 1969 solar flare were stored near the Sun for at least 90 minutes. It is also shown that under favourable conditions solar protons can be accelerated near the Earth's bow shock. The decay of solar protons is shown to be mainly convective; however, there are indications that in smooth field regimes convection of 1 MeV solar protons can be greatly reduced. Finally, it is pointed out that the effect of adiabatic deceleration can be quite important.     

Puzzles and potential for gamma-ray line observations of solar flare ion acceleration

The best tool for understanding ion acceleration in solar flares is gamma-ray line emission from nuclear de-excitation, positron annihilation, and neutron capture. These techniques have not yet come close to reaching their potential due to limited counting statistics in the lines. Instruments with focusing optics and large effective areas promise real breakthroughs in understanding high-energy solar processes. I discuss what can be learned from the various lines and the instrumental requirements for future focusing observations.“Mama always told me not to look into the sights of the Sun; oh, but mama, that’s where the fun is...” [1]     

Energy storage and deposition in a solar flare

High temporal and spatial resolution solar X-ray pictures of a flare at 1827 UT on 5 September 1973 were taken with the S-056 Aerospace Corporation/Marshall Space Flight Center telescope on the Apollo Telescope Mount. Photographs taken at 9 s intervals allow detailed information to be obtained about the site of the energy release, as well as about the evolution of the flare itself. Observations show that the flare occured in an entire arcade of loops rather than in any single loop. Sequential brightening of different X-ray features indicates that some excitation moved perpendicular to the magnetic field of the arcade at velocities of 180–280 km/s. The most intense X-ray features were located in places where the magnetic field composing the arcade had a small radius of curvature with horizontal field gradients higher than the surrounding region and where the axis of the arcade changed direction. We feel that the arcade geometry strongly influenced the propagation of the triggering disturbance, as well as the storage and site of the subsequent deposition of energy. A magnetosonic wave is suggested as the propagating mechanism triggering instabilities that may have existed in the preflare structure. This event demonstrates that all energy emitted during a flare need not be released immediately nor in the same location, thereby eliminating some problems encountered in many flare theories. Conditions for energy release are discussed.