The emission and propagation of ∼ 40keV solar flare electrons

Observations of prompt 40 keV solar flare electron events by the IMP series of satellites in the period August, 1966 to December, 1967 are tabulated along with prompt energetic solar proton events in the period 1964–1967. The interrelationship of the various types of energetic particle emission by the sun, including relativistic energy electrons reported by Cline and McDonald (1968) are investigated. Relativistic energy electron emission is found to occur only during proton events. The solar optical, radio and X-ray emission associated with these various energetic particle emissions as well as the propagation characteristics of each particle species are examined in order to study the particle acceleration and emission mechanisms in a solar flare. Evidence is presented for two separate particle acceleration and/or emission mechanisms, one of which produces 40 keV electrons and the other of which produces solar proton and possibly relativistic energy electrons. It is found that solar flares can be divided into three categories depending on their energetic particle emission: (1) small flares with no accompanying energetic phenomena either in particles, radio or X-ray emission; (2) small flares which produce low energy electrons and which are accompanied by type III and microwave radio bursts and energetic ( 20 keV) X-ray bursts; and (3) major solar flare eruptions characterized by energetic solar proton production and type II and IV radio bursts and accompanied by intense microwave and X-ray emission and relativistic energy electrons.     

Non-thermal processes in large solar flares

We analyze particle acceleration processes in large solar flares, using observations of the August, 1972, series of large events. The energetic particle populations are estimated from the hard X-ray and γ-ray emission, and from direct interplanetary particle observations. The collisional energy losses of these particles are computed as a function of height, assuming that the particles are accelerated high in the solar atmosphere and then precipitate down into denser layers. We compare the computed energy input with the flare energy output in radiation, heating, and mass ejection, and find for large proton event flares that:

1. The ～10–10² keV electrons accelerated during the flash phase constitute the bulk of the total flare energy.
2. The flare can be divided into two regions depending on whether the electron energy input goes into radiation or explosive heating. The computed energy input to the radiative quasi-equilibrium region agrees with the observed flare energy output in optical, UV, and EUV radiation.
3. The electron energy input to the explosive heating region can produce evaporation of the upper chromosphere needed to form the soft X-ray flare plasma.
4. Very intense energetic electron fluxes can provide the energy and mass for interplanetary shock wave by heating the atmospheric gas to energies sufficient to escape the solar gravitational and magnetic fields. The threshold for shock formation appears to be ～10³¹ ergs total energy in >20 keV electrons, and all of the shock energy can be supplied by electrons if their spectrum extends down to 5–10 keV.
5. High energy protons are accelerated later than the 10–10² keV electrons and most of them escape to the interplanetary medium. The energetic protons are not a significant contributor to the energization of flare phenomena. The observations are consistent with shock-wave acceleration of the protons and other nuclei, and also of electrons to relativistic energies.
6. The flare white-light continuum emission is consistent with a model of free-bound transitions in a plasma with strong non-thermal ionization produced in the lower solar chromosphere by energetic electrons. The white-light continuum is inconsistent with models of photospheric heating by the energetic particles. A threshold energy of ～5×10³⁰ ergs in >20 keV electrons is required for detectable white-light emission.

The highly efficient electron energization required in these flares suggests that the flare mechanism consists of rapid dissipation of chromospheric and coronal field-aligned or sheet currents, due to the onset of current-driven Buneman anomalous resistivity. Large proton flares then result when the energy input from accelerated electrons is sufficient to form a shock wave.     

Stellar and solar flares

Some ideas are developed concerning solar flares which have been presented earlier by the author (Schatzman, 1966a). Emphasis is laid on the problem of energy transport; from the energy supply to the region of the optical flare, on the storage of low energy cosmic ray particles in a magnetic bottle before the beginning of the optical flare, and the mechanism which triggers both the optical flare, and the production of high-energy cosmic rays. The relation between solar and stellar flares is considered.Lecture given at Goddard Space Flight Center, November 4, 1966.     

Particle Acceleration and Propagation in Strong Flares without Major Solar Energetic Particle Events

Solar energetic particles (SEPs) detected in space are statistically associated with flares and coronal mass ejections (CMEs). But it is not clear how these processes actually contribute to the acceleration and transport of the particles. The present work addresses the question why flares accompanied by intense soft X-ray bursts may not produce SEPs detected by observations with the GOES spacecraft. We consider all X-class X-ray bursts between 1996 and 2006 from the western solar hemisphere. 21 out of 69 have no signature in GOES proton intensities above 10 MeV, despite being significant accelerators of electrons, as shown by their radio emission at cm wavelengths. The majority (11/20) has no type III radio bursts from electron beams escaping towards interplanetary space during the impulsive flare phase. Together with other radio properties, this indicates that the electrons accelerated during the impulsive flare phase remain confined in the low corona. This occurs in flares with and without a CME. Although GOES saw no protons above 10 MeV at geosynchronous orbit, energetic particles were detected in some (4/11) confined events at Lagrangian point L1 aboard ACE or SoHO. These events have, besides the confined microwave emission, dm-m wave type II and type IV bursts indicating an independent accelerator in the corona. Three of them are accompanied by CMEs. We conclude that the principal reason why major solar flares in the western hemisphere are not associated with SEPs is the confinement of particles accelerated in the impulsive phase. A coronal shock wave or the restructuring of the magnetically stressed corona, indicated by the type II and IV bursts, can explain the detection of SEPs when flare-accelerated particles do not reach open magnetic field lines. But the mere presence of these radio signatures, especially of a metric type II burst, is not a sufficient condition for a major SEP event.     

Stochastic forces in space plasmas with ion-acoustic and lower-hybrid-drift turbulence

A Langevin equation for charged particles in a plasma with electrostatic turbulence is developed from first principles and in consistency with the kinetic theory in polarization approximation. For the case of ion-acoustic and electrostatic lower-hybrid-drift turbulence approximate expressions for the space-time spectral density of the wave energy are given and estimates of the intensities of the stochastic wave forces are made. The application is done for the plasmas of the earth's magnetosphere, the solar wind and solar flares. It seems, that ion-acoustic and electrostatic lower-hybrid-drift waves can contribute to electron chaotization in different regions of the space plasma.     

Distribution of temperature and emission measure in a steadily heated solar atmosphere

The distribution of temperature and emission measure in the stationary heated solar atmosphere was found for the limiting cases of slow and fast heating, when either the gas pressure or the concentration are constant through the layer depth. Results are relevant to the conditions when the energy injected by waves or by non-thermal particles or in some different way quickly transforms into a thermal flux. Under these conditions the temperature distribution with depth is determined by radiation loss and thermal conductivity, and at any values of energy flux and plasma concentration it is characterized by two universal functions. One of them gives the relation between the energy flux and temperature at the region boundary: the other - the temperature run with the depth. This run is such that a considerable part of the energy is radiated by a thin transition region with a very large temperature gradient.The results may be applied for calculation of the temperature and the emission measure both for the high temperature region of a flare, and for the quiet corona. The dimensionless structure of the transition region is the same for any value of the energy flux. These results concerning solar flares can help to explain the identity of optical spectra for flares of different types, the emission in a wide temperature interval from nearly the same region of space and the very small thickness of the region emitting optical lines. The latter is due to the shell structure of the flare as opposed to the usually assumed filamentary one.     

Statistics and investigation of white light solar flares

We discuss the properties of white light flares on the basis of the published accounts of these events, together with the associated H flares, radio bursts, X-ray bursts, proton events and ionosperic distrubances. In addition, spectral plates taken at Purple Mountain Observatory since 1962 have been examined. We found that 5% of the spectrograms of solar flares show variable white-light emission. A minority of the white light flares are associated with H flare of small importance classes. We think these may be caused by perturbations originating in the convective zone below, while the majority accompanied by high-energy events are caused by the bombardment of energetic particles from above.     

Current state of the problem of solar flares: New observations and new models

A review of current questions related to the problem of large solar flares is given. The basic physical principles applied in numerical simulation of flares are presented and illustrated. The main attention is given to the phenomenon of magnetic reconnection in large-scale current layers at separators of magnetic field in the corona. This phenomenon is demonstrated within the framework of the Rainbow topological model. The model provides the possibility of explaining specific features of large-scale reconnection as a physical process that makes it possible to accumulate large energy in the form of the magnetic energy of current layers before a flare and to quickly transform this energy to the kinetic energy of particles during a flare. The secondary effects in the solar atmosphere caused by energy fluxes from reconnecting current layers are also discussed. These consequences of the primary energy release are responsible for the flare pattern observed in X-ray, optical, UV, and other spectral ranges.     

Gamma-ray emission and neutrons from solar flares recorded by the SONG instrument in 2001–2004

The SONG instrument onboard the CORONAS-F satellite recorded gamma-ray emission with energy above 500 keV in 28 solar flares over three years of its in-orbit operation. According to the GOES classification, the X-ray importance of these flares lay within the range M1.4-X28. The gamma-ray energy recorded by SONG exceeded 4 MeV in 16 flares. Gamma-ray emission with energy up to 100 MeV was recorded in three events, more specifically, on August 25, 2001, October 28, 2003, and November 4, 2003. Increases in the count rate in the SONG channels that recorded neutrons with energies above 20 MeV were found during these three events. The energies of the recorded neutrons were estimated for the neutron increases. The time dependence of the neutron increases was compared with data from high-altitude ground-based neutron monitors that could, in principle, record the arrival of high-energy neutrons from the Sun. It should be noted that we detected series of flares with gamma-ray emission generated by the same active region (AR). The series in the last decade of August 2002 (AR NOAA 0069), the end of May 2003 (AR NOAA 0365), and the famous period of extreme solar activity in October–November 2003 associated with AR NOAA 0486 and AR NOAA 0501 are quite revealing. The catalog can be of use for future statistical and correlation analyses of solar flares.     

Thick-target processes and white-light flares

Observations indicate that fast electrons in solar flares, which cause the hard X-ray burst and the impulsive microwave burst, lose energy predominantly by collisional processes. This requires a thick-target theory of the emission, for which the electron spectrum inferred from the X-ray spectrum becomes 1.5 powers steeper than in the usual thin-target theory.The low-energy end of this spectrum contains enough energy above about 5 keV to supply the white-light continuum emission occasionally observed in major flares. The penetration of the nonthermal electrons creates long-lived excess ionization which enhances the free-free and free-bound continuum in the heated medium. The emission will occur high above the photosphere at small optical depth in the visible continuum. Thus its spectrum will extend into the infrared and ultraviolet.     

Long term storage of relativistic particles in the solar corona

A model is presented which shows that large numbers of energetic electrons (0.3-> 10 MeV) and protons (1–30 MeV) can be stored in the solar corona at altitudes around 3 × 10⁵ km for periods in excess of 5 days. Specific reference is made to the time period July 6–16 1968 as an excellent example of energetic solar particle storage. Time histories of interplanetary charged particle intensities observed by the IMP-4 and Pioneer 8 satellites are used to substantiate this contention. Detailed reference is also made to solar X-ray, optical and radio data obtained during the period in question, in addition to interplanetary magnetometer data. This model provides a unique solution to many hitherto unexplained solar particle events, and can also account for the lack of prompt particle emission from certain large solar flares recorded in the past.     

Prolonged nonthermal emission from solar flares and the Neupert effect

We analyze the observations of the hard (ACS SPI, > 150 keV) and soft (GOES, 1–8 Å) X-ray emissions and the microwave (15.5 GHz) emission in the solar flares on September 7, 2005 and December 6 and 13, 2006. The time profiles of the nonthermal emission from these flares had a complex structure, suggesting that active processes in the flare region continued for a long time (more than an hour). We have verified the linear relationship between the nonthermal flux and the time derivative of the soft X-ray flux (the Neupert effect) in the events under consideration. In the first two cases, the Neupert effect held at the time of the most intense nonthermal emission peak, but not at the decay phase of the soft X-ray emission, when the intensity of the nonthermal emission was much higher than the background values. At the same time, the hard X-ray emission was suppressed compared to the main peak, while the microwave emission remained approximately at the same level. In the December 13, 2006 event, the prolonged hard X-ray emission was difficult to observe due to the fast arrival of solar protons, but the Neupert effect did not hold for its main peak either. At comparable intensities of the microwave emission on December 6 and 13, the intensity of the hard X-ray emission on December 13 at the time of the main peak was suppressed approximately by an order of magnitude. These observational facts are indicative of several particle acceleration and interaction episodes under various physical conditions during one flare. When the Neupert effect did not hold, the interaction of electrons took place mainly in a low-density medium. An effective escape of accelerated particles into interplanetary space rather than their precipitation into dense layers of the solar atmosphere may take place precisely at this time.     

Mechanics of Seismic Emission from Solar Flares

Instances of seismic transients emitted into the solar interior in the impulsive phases of some solar flares offer a promising diagnostic tool, both for understanding the physics of solar flares and for the general development of local helioseismology. Among the prospective contributors to flare acoustic emission that have been considered are: i) chromospheric shocks propelled by pressure transients caused by impulsive thick-target heating of the upper and middle chromosphere by high-energy particles, ii) heating of the photosphere by continuum radiation from the chromosphere or possibly by high-energy protons, and iii) magnetic-force transients caused by magnetic reconnection. Hydrodynamic modeling of chromospheric shocks suggests that radiative losses deplete all but a small fraction of the energy initially deposited into them before they penetrate the photosphere. Comparisons between the spatial distribution of acoustic sources, derived from seismic holography of the surface signatures of flare acoustic emission, and the spatial distributions of sudden changes both in visible-light emission and in magnetic signatures offer a possible means of discriminating between contributions to flare acoustic emission from photospheric heating and magnetic-force transients. In this study we develop and test a means for estimating the seismic intensity and spatial distribution of flare acoustic emission from photospheric heating associated with visible-light emission and compare this with the helioseismic signatures of seismic emission. Similar techniques are applicable to transient magnetic signatures.     

A review of stellar flares and their characteristics

We review the flaring activity of stars across the HR-diagram. Brightenings have been reported along the entire Main Sequence and in many stars off the Main Sequence. Some stars are decidedly young, others are in advanced stages of stellar evolution. Flares are common on stars with outer convection zones and outbursts have been reported also on other types of stars, although confirmations are needed for some of them.Analyses of flare occurrence sometimes find flares to be randomly distributed in time, and sometimes indicate a tendency for flares to come in groups. Preferred active longitudes have been suggested. Recent solar results, where the occurrence rate for flares is found to exhibit a periodicity of 152 days, suggest that stellar flare data should be reanalyzed over long time baselines to see if the present confusing situation can be resolved.The radiation from stellar flares is dominated by continuum emission and about equal amounts of energy have been recorded in the optical, UV, and X-ray regions of the spectrum. In solar flares strong continuum emission is rarely recorded and a large collection of bright emission lines takes prominence. Small flares occur more frequently than large ones and the latter have longer time-scales. Flare energies can exceed 10³⁷ erg. The most productive flare stars are those where the convective envelopes occupy large volumes. Slow stellar rotation rates are believed to reduce the level when the star has been braked significantly from its young rotation rate.     

Effect of solar corona conditions on flare particle propagation

Data on high energy electrons and protons in different energy windows are analyzed and compared for two solar flares which occurred at 37 W solar longitude on the 25th February 1969 and the 29th March 1970. While the data for the first of these flares can be interpreted in the framework of a diffusion model with suitable values of the parallel diffusion coefficient, in order to explain the time behaviour of the different particles after the second event, we are led to suppose that the coronal magnetic fields are such that particles of different rigidity are ejected at different longitudes and that there is no good magnetic connection of the Earth with the flare region.     

An electric-current description of solar flares

The presently prevailing theories of solar flares rely on the hypothetical presence of magnetic flux tubes beneath the photosphere and the two subsequent hypotheses, their emergence above the photosphere and explosive magnetic reconnection, converting magnetic energy carried by the flux tubes to solar flare energy. In this paper, we discuss solar flares from an entirely different point of view, namely in terms of power supply by a dynamo process in the photosphere. By this process, electric currents flowing along the magnetic field lines are generated and the familiar ‘force-free’ fields or the ‘sheared’ magnetic fields are produced. Upward field-aligned currents thus generated are carried by downward streaming electrons; these electrons can excite hydrogen atoms in the chromosphere, causing the optical Hα flares or ‘low temperature flares’. It is thus argued that as the ‘force-free’ fields are being built up for the magnetic energy storage, a flare must already be in progress.     

The role of high-energy protons and electrons in powering the solar white-light flare emission

The temporal histories of three intense and impulsive gamma-ray flares, for which also white-light emission had been observed, are analyzed in order to test the role of high-energy particles- electrons and protons - in powering the optical continuum. By comparing the light curves at optical wavelengths and at X-ray and gamma-ray energies, we find a good correlation of the main peaks of emission, which confirms previous findings that the continuum emission is most likely associated with the energy loss of energetic particles. The power carried by the greater-than-50 keV nonthermal electrons may be sufficient to balance the optical emission. The power residing in protons or ions with energies greater than 1 MeV depends largely on the spectral shape of the particle distribution. Only if this is similar to a power law, may the energy carried by these high-energy particles be sufficient to balance the white-light flare emission.Operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. Partial support for the National Solar Observatory is provided by the USAF under a Memorandum of Understanding with the NSF.     

Energetic Particle Acceleration and Propagation in Strong CME-Less Flares

Flares and coronal mass ejections (CMEs) contribute to the acceleration and propagation of solar energetic particles (SEP) detected in the interplanetary space, but the exact roles of these phenomena are yet to be understood. We examine two types of energetic particle tracers related with 15 CME-less flares that emit bright soft X-ray bursts (GOES X class): radio emission of flare-accelerated electrons and in situ measurements of energetic electrons and protons near 1 AU. The CME-less flares are found to be vigorous accelerators of microwave-emitting electrons, which remain confined in low coronal structures. This is shown by unusually steep low-frequency microwave spectra and by lack of radio emission from the middle and high corona, including dm?–?m wave type IV continua and metre-to-hectometre type III bursts. The confinement of the particles accelerated in CME-less flares agrees with the magnetic field configuration of these events inferred by others. Two events produced isolated metric type II bursts revealing coronal shock waves. None of the seven flares in the western hemisphere was followed by enhanced particle fluxes in the GOES detectors, but one, which was accompanied by a type II burst, caused a weak SEP event detected at SoHO and ACE. Three of the CME-less flares were followed within some hours by SEP-associated flares from the same active region. These SEP-producing events were clearly distinct from the CME-less ones by their association with fast and broad CMEs, dm?–?m wave radio emission, and intense DH type III bursts. We conclude that radio emission at decimetre and longer waves is a reliable indication that flare-accelerated particles have access to the high corona and interplanetary space. The absence of such emission can be used as a signal that no SEP event is to be expected despite the occurrence of a strong soft X-ray burst.     

The solar flare catalog in the low-energy gamma-ray range based on the AVS-F instrument data onboard the CORONAS-F satellite in 2001–2005

The AVS-F apparatus onboard the CORONAS-F satellite (operated from July 31, 2001, to December 6, 2005) was intended for investigation of solar hard X-ray and gamma-ray radiation and for registration of gamma-ray bursts. The AVS-F apparatus constitutes a system for processing the data from two detectors: SONG-D (a CsI(Tl) scintillation detector 200 mm in diameter and 100 mm in height, fully surrounded by plastic anticoincidence shield) and RPS-1 (a solid state CdTe detector 4.9 mm × 4.9 mm in size). Over 60 solar flares stronger than M1.0 class by GOES classification were registered during the period from August 2001 to February 2005. Most flares showed gamma-ray emission during the periods when a rise in the X-ray flux was observed by the GOES instruments. Some flares produced gamma-rays only at maximum X-ray emission; for some flares, the durations of gamma-ray and X-ray emissions were the same. Up to six complexes of spectral lines were detected in some solar flares. The AVS-F instrument analyzes temporal profiles of low-energy gamma-ray emission with a temporal resolution of 1 ms within the first 4.096 seconds of solar flares. The preliminary analysis of such temporal profiles for seven solar flares revealed time regularities with scales from 7 to 35 ms in the 0.1-to 20-MeV energy range only for the flare of January 20, 2005, at a confidence level of 99%.