Comparisons of solar flare X-ray producing and escaping electrons from ∼ 2 TO 100 keV

Using observations from the ISEE-3 spacecraft, we compare the X-ray producing electrons and escaping electrons from a solar flare on 8 November, 1978. The instantaneous 5 to 75 keV electron spectrum in the X-ray producing region is computed from the observed bremsstrahlung X-ray spectrum. Assuming that energy loss by Coulomb collisions (thick target) is the dominant electron loss process, the accelerated electron spectrum is obtained. The energy spectrum of the escaping electrons observed from 2 to 100 keV differs significantly from the spectra of the X-ray producing electrons and of the accelerated electrons, even when the energy loss which the escaping electrons experienced during their travel from the Sun to the Earth is taken into account. The observations are consistent with a model where the escaping electrons come from an extended X-ray producing region which ranges from the chromosphere to high in the corona. In this model the low energy escaping electrons (2–10 keV) come from the higher part of the extended X-ray source where the overlying column density is low, while the high energy electrons (20–100 keV) come from the entire X-ray source.     

Thick target X-ray bremsstrahlung from partially ionised targets in solar flares

The effect of partial ionisation of a thick target bremsstrahlung source on the emitted X-ray intensity is analysed. It is shown that a totally ionised target produces an X-ray burst only about one third as intense as that from an unionised target.In the case of a solar flare plasma target, the ionisation decreases with increasing depth in the flare. Thus, in an X-ray flare model in which electrons are continuously accelerated down into the chromosphere, high energy photons are produced with increased efficiency in the deeper layers of the flare plasma with consequent hardening of the X-ray spectrum. As a result, the spectra of nonthermal electrons in flares, inferred from X-ray spectra, are steepened and their total energy correspondingly increased.     

Interpreting Yohkoh hard and soft X-ray flare observations

A simple model is presented to account for theYohkoh flare observations of Feldmanet al. (1994), and Masuda (1994). Electrons accelerated by the flare are assumed to encounter the dense, small regions observed by Feldmanet al. at the tops of impulsively flaring coronal magnetic loops. The values of electron density and volume inferred by Feldmanet al. imply that these dense regions present an intermediate thick-thin target to the energised electrons. Specifically, they present a thick (thin) target to electrons with energy much less (greater) thanE _c , where 15 keV <E _c < 40 keV. The electrons are either stopped at the loop top or precipitate down the field lines of the loop to the footpoints. Collisional losses of the electrons at the loop top produce the heating observed by Feldmanet al. and also some hard X-rays. It is argued that this is the mechanism for the loop-top hard X-ray sources observed in limb flares by Masuda. Adopting a simple model for the energy losses of electrons traversing the dense region and the ambient loop plasma, hard X-ray spectra are derived for the loop-top source, the footpoint sources and the region between the loop top and footpoints. These spectra are compared with the observations of Masuda. The model spectra are found to qualitatively agree with the data, and in particular account for the observed steepening of the loop-top and footpoint spectra between 14 and 53 keV and the relative brightnesses of the loop-top and footpoint sources.     

Collective plasma effects and the electron number problem in solar hard X-ray bursts

Due to the relatively high stream densities involved, collective interactions with the ambient plasma are likely to be important for the electrons producing solar hard X-ray bursts. In thick- and thin-target bremsstrahlung models the most relevant process is limitation of the invoked electron beams by ion sound wave generation in the neutralizing reverse current established in the atmosphere. For the thick target model it is shown that typical electron fluxes are near the maximum permitted by stability of the reverse current so that ion-sound wave generation may be the process which limits the electron injection rate. On the other hand the chromospheric reverse current is sufficient to supply the large total number of electrons which have to be accelerated in the corona. For the thin target the low density of the corona severely limits the possible reverse current so that the maximum upward flux of fast electrons is probably much too small to explain X-ray bursts but compatible with observations of interplanetary electrons.A distinct class of model postulates a small number of electrons confined by resonant scattering in a dense coronal slab surrounding a current sheet with continuous stochastic acceleration offsetting collisional losses. The energetic aspects of such a situation described by Hoyng (1975) are developed here by addition of equations describing the slab geometry in terms of electron diffusion by whistler scattering and of the collisional damping of the accelerating Langmuir waves. Solution of these equations results in values for the fieldB(70–350 G), densityn ₀(2–5 × 10¹² cm ^–3), slab dimensions (10¹⁸ km² × 0.3–3 km) and relative Langmuir energy density (10^–3 – 10^–2) required to produce the observed range of bursts. It is pointed out, however, that there may be no real gain in electron number requirements since the fast electrons in the emitting slab would be constantly swept out along with the frozen-in plasma as dissipation proceeds so that a large total number of electrons is still required. It could in fact be that just such a coronal region is the injection mechanism for the thick-target model.On leave from Department of Astronomy, University of Glasgow, Scotland.     

Spectral development of a solar X-ray burst observed on OSO-7

The UCSD solar X-ray instrument on the OSO-7 satellite observes X-ray bursts in the 2–300 keV range with 10.24 s time resolution. Spectra obtained from the proportional counter and scintillation counter are analyzed for the event of November 16, 1971, at 0519 UT in terms of thermal (exponential spectrum) and non-thermal (power law) components. The energy content of the approximately 20 × 10⁶K thermal plasma increased with the 60 s duration hard X-ray burst which entirely preceded the 5 keV soft X-ray maximum. If the hard X-rays arise by thick target bremsstrahlung, the nonthermal electrons above 10 keV have sufficient energy to heat the thermally emitting plasma. In the thin target case the collisional energy transfer from non-thermal electrons suffices if the power law electron spectrum is extrapolated below 10 keV, or if the ambient plasma density exceeds 4 × 10¹⁰ cm^–3.Formerly at UCSD.     

On the role of reverse current on the hard X-ray production in solar flares and time-lags between high- and low-energy photons

The evolution of energy and angular distributions of electrons has been studied accounting for the reverse current effect by combining analytically treated small angle multiple scatterings with large angle Monte-Carlo calculations. Reverse current and potential variations as function of column density have been computed. It is found that the reverse current decreases steeply with increase in electron energy. However, it becomes significant for low-energy electrons. By use of these distributions and bremsstrahlung crosssection, the X-ray energy spectrum has been calculated. The nature of the resulting X-ray spectrum integrated over all column depths is similar to the one without reverse current. The time-lag between high-and low-energy photon production has been calculated. It is found that there is a small difference between time-lags as function of observation angles. This fact can be used to test the validity of the beamed thick target model.     

The directivity and polarisation of thick target X-ray bremsstrahlung from solar flares

The directivity and polarisation of solar hard X-ray bursts is discussed in terms of two bremsstrahlung source models. These involve continuous and impulsive injection of electrons respectively, as described widely in the literature.A detailed analysis is made of the continuous injection model in which electrons are accelerated downward into the dense chromosphere where their velocity distribution is greatly modified by collisions. This thick target scattering is described by a simple analytical model. Directivity and polarisation of the bremsstrahlung emission are calculated in detail for a thick target model in which the guiding magnetic field is vertical.It is predicted for this model that X-ray sources should brighten from the centre to the limb of the solar disk, while the degree of polarisation should rise from zero to around 30 % near the limb. The plane of maximum intensity for any source is that containing the source and the disk centre. Both the directivity and polarisation should increase with increasing photon energy.Very recent observations agree with these predictions though they suggest that the field is significantly non-vertical. If scattering is not included in the model, or if an impulsive injection model is taken, agreement with observations is not obtained.Temporarily at the Lehrstuhl für Theoretische Astrophysik der Universität Tübingen, West Germany.     

Reverse-current effect in present-day models of solar flares: Theory and high-accuracy observations

We propose an accurate analytical model for the source of hard X-ray emission from a flare in the form of a “thick target” with a reverse current to explain the results of present-day observations of solar flares onboard the GOES, Hinode, RHESSI, and TRACE satellites. The model, one-dimensional in coordinate space and two-dimensional in velocity space, self-consistently takes into account the fact that the beam electrons lose the kinetic energy of their motion along the magnetic field almost without any collisions under the action of the reverse-current electric field. Some of the electrons return from the emission source to the acceleration region without losing the kinetic energy of their transverse motion. Based on the observed hard X-ray bremsstrahlung spectrum, the model allows the injection spectrum of accelerated electrons to be reconstructed with a high accuracy. As an example, we consider the white-light flare of December 6, 2006, which was observed with a high spatial resolution in the optical wavelength range at the main maximum of hard X-ray emission. Within the framework of our model, we show that to explain the hard X-ray spectrum, the flux density of the energy transferred by electrons with energies above 18 keV was ～3 × 10¹³ erg cm^?2 s^?1. This exceeds the habitual values typical of the classical model of a thick target without a reverse current by two orders of magnitude. The electron density in the beam is also very high: ～10¹¹ cm^?3. A more careful consideration of plasma processes in such dense electron beams is needed when the physical parameters of a flare are calculated.     

激光驱动亥姆霍兹电容线圈靶磁重联实验的数值模拟

激光驱动亥姆霍兹电容线圈靶的磁重联实验已经提出并进行了多年.当实验中的金属板被强激光照射时产生自由电子,这些自由电子的运动在连接两金属板的两个平行线圈中产生电流,由两个平行线圈内部电流产生的磁场之间随即发生重联.该实验不同于其他直接由Biermann电池效应所产生高β(等离子体热压与磁压的比值)环境下的磁重联实验.对该类实验进行了3维磁流体动力学数值模拟,首次展示了亥姆霍兹电容器线圈靶如何驱动磁重联的过程.数值模拟结果清楚地表明,磁重联的出流等离子体在线圈周围发生与实验结果相一致的堆积现象.线圈电流产生的磁场可高达100 T,使得磁重联区域周围的等离子体β值达到10^-2.与实验室结果进行比较,数值模拟重复了实验展示的大多数特征,可有助于深入认识和理解实验结果背后的物理学原理.     

The inter-relationship of hard X-ray and EUV bursts during solar flares

A comparison is made between the flux-versus-time profile in the EUV band and the thick target electron flux profile as inferred from hard X-rays for a number of moderately large solar flares. This complements Kane and Donnelly's (1971) study of small flares. The hard X-ray data are from ESRO TD-1A and the EUV inferred from SFD observations.Use of a ₂ minimising method shows that the best overall fit between the profile fine structures obtains for synchronism to 5 s which is within the timing accuracy. This suggests that neither conduction nor convection is fast enough as the primary mechanism of energy transport into the EUV flare and rather favours heating by the electrons themselves or by some MHD wave process much faster than acoustic waves.The electron power deposited, for a thick target model, is however far greater than the EUV luminosity for any reasonable assumptions about the area and depth over which EUV is emitted. This means that either most of the power deposited is conducted away to the optical flare or that only a fraction 1–10% of the X-ray emitting electrons are injected downwards. Recent work on H flare heating strongly favours the latter alternative - i.e. that electrons are mostly confined in the corona.     

The effect of wave generation on HXR bremsstrahlung spectra from flare thick-target beams

Fast electrons in the solar atmosphere are detected by their hard X-ray bremsstrahlung and by type III radio bursts caused by ‘bump-on-tail’ plasma wave generation. This paper investigates empirically the effect of wave generation on the HXR spectrum. Purely collisional propagation of an electron beam generates a bump in the distribution function, due to stopping of low-velocity electrons. The consequent positive gradient means there is a possibility of wave generation, production of type III radio bursts, and energy redistribution of the electron beam. We have represented this relaxation parametrically and calculated the global bremsstrahlung HXR emission spectrum. We show that for a range of relaxed forms, with different local electron spectral shapes, the bremsstrahlung spectrum integrated over the whole target is identical in shape to the purely collisionally evolved beam. Our results show that spatially integrated HXR spectral measurements would be unable to distinguish between the presence or absence of relaxation effects. Only spatially resolved hard X-ray spectra, such as anticipated from the HESSI mission, will be able to remove this ambiguity in HXR diagnostics of beam relaxation.     

The role of electrostatic instabilities in the critical ionization velocity mechanism

The role of electrostatic instabilities in the critical ionization velocity mechanism is investigated. The analysis is based on the theory developed by Sherman, which interprets Alfvén's critical velocity in terms of a circular process. This process involves the acceleration of electrons by a two-stream instability modified by the presence of a magnetic field. A general expression for the energy and momentum of ions and electrons associated with an electrostatic mode is derived in terms of the plasma dielectric constant. This is used in the case of the modified two-stream instability to determine the distribution of energy between ions and electrons. An extrapolation from the linear phase then gives an estimate of the energy delivered to the electrons which is compared to that required to ionize the neutral gas.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May, 1978.     

Collective effects of stellar winds and unidentified gamma-ray sources

We study collective wind configurations produced by a number of massive stars, and obtain densities and expansion velocities of the stellar wind gas that is to be target, in this model, of hadronic interactions. We study the expected γ-ray emission from these regions, considering in an approximate way the effect of cosmic ray modulation. We compute secondary particle production (electrons from knock-on interactions and electrons and positrons from charged pion decay), and solve the loss equation with ionization, synchrotron, bremsstrahlung, inverse Compton, and expansion losses. We provide examples where configurations can produce sources for GLAST satellite, and the MAGIC, HESS, or VERITAS telescopes in non-uniform ways, i.e., with or without the corresponding counterparts. We show that in all cases we studied no EGRET source is expected.     

Thermal electrons runaway from a hot plasma during a flare in the reverse-current model and their X-ray bremsstrahlung

The behaviour of the thermal electrons escaping from a hot plasma to a cold one during a solar flare is investigated. We suppose that the direct current of fast electrons is compensated by the reverse current of the thermal electrons in ambient plasma. It is shown that the direct current strength is determined only by the regular energy losses due to Coulomb collisions. The reverse-current electric field and the distribution function of fast electrons are found in the form of an approximate analytical solution to the self-consistent kinetic problem of the dynamics of a beam of escaping thermal electrons and its associated reverse current.The reverse-current electric field in solar flares leads to a significant reduction of the convective heat flux carried by fast electrons escaping from the high-temperature plasma to the cold one. The spectrum and polarization of hard X-ray bremsstrahlung, and its spatial distribution along flare loops are calculated and can be used for diagnostics of flare plasmas and escaping electrons.Send offprint requests to B. V. Somov.     

Evidence for thin-target X-ray emission in a small solar flare on 26 February 1972

We compare solar X-ray observations from the UCSD experiment aboard OSO-7 with high resolution energetic electron observations from the UCAL experiment on IMP-6 for a small solar flare on 26 February 1972. A proportional counter and NaI scintillator covered the X-ray energy range 5–300 keV, while a semiconductor detector telescope covered electrons from 18 to 400 keV. A series of four non-thermal X-ray spikes were observed from 1805 to 1814 UT with average spectrum dJ/d (hv) (hv)^–4.0 over the 14–64 keV range. The energetic electrons were observed at 1 AU beginning 1840 UT with a spectrum dJ/dE E ^–3.1. If the electrons which produce the X-ray emission and those observed at 1 AU are assumed to originate in a common source, then these observations are consistent with thin target X-ray production at the Sun and inconsistent with thick target production. Under a model consistent with the observed soft X-ray emission, we obtain quantitative estimates of the total energy, total number, escape efficiency, and energy lost in collisions for the energetic electrons.     

Numerical simulation of electromagnetic emissions in the solar wind plasma with non-thermal electron distribution

Dynamics of fundamental and second harmonic electromagnetic emissions are simulated in the solar wind plasma in the presence of non-thermal electron distribution function in which primary Langmuir waves are driven by an electron beam. The electron velocity distribution function is separated into two distributions representing the distribution of the ambient electrons (Maxwellian) and the suprathermal electrons (non-thermal electrons). The effects of the non-thermal electrons on the generation of primary Langmuir waves, emission rates of the fundamental (F) and harmonic waves (H) and their distributions are investigated. The both of the F and H emissions are sensitive to the characterizes of the non-thermal electrons. It is found that in the presence of non-thermal electrons the production of the Langmuir waves decreases and consequently the levels of fundamental and second harmonic waves are reduced. The emission rate of the fundamental transverse waves decreases and its peak moves slightly toward smaller wave-numbers.     

Evidence for accelerated electrons upstream from Jupiter's bow shock: Ulysses results

Several transient increases of electrons with energies in the range 40–100 keV have been detected upstream and immediately downstream from the Jovian bow shock (and only in these regions), by instruments on the Ulysses spacecraft during February 1992. The energy spectra of these electrons differ markedly from the energy spectrum of the trapped magnetospheric electrons measured by the same instrument. Two populations of the upstream electrons were identified. Type I electrons appear at times when the direction of the interplanetary magnetic field at the spacecraft could have been tangent to the Jovian bow shock surface thus paralleling, for the first time at another planetary bow shock, the rather well understood situation at Earth's bow shock. Type II electrons have the same energy spectrum as Type I electrons, but are not so clearly associated with the tangent field-line condition. They occur at high southerly latitudes only while the Type I electrons are detected both on the inbound and outbound passages. Type II electrons have never been reported at the Earth's bow shock or any other planetary bow shock. Under the assumption that the field line that goes through Ulysses connects to the bow shock in a straight line, two possible explanations for the Type II electrons may be: (1) very large distortions of the bow shock surface, perhaps caused by deformations of the magnetopause, may permit the tangent condition; and (2) upstream electrons are preferentially, but not necessarily, accelerated when the IMF is tangent to the bow-shock surface.     

Multiple compton scattering of electrons in a photon field

Consideration is given to the motion of electrons in a photon field of the monoenergetic or power-law spectrum under the conditions when the main mechanism of energy loss is the inverse Compton scattering by field photons. This process changes the primary spectrum of electrons and converts low-energy field photons to high-energy gamma-quanta for which the electron confinement region is assumed to be optically thin. The electron and gamma-ray spectra have been obtained in a wide energy interval including the Klein-Nishina and Thomson regions. A simple qualitative dependence of the solutions found on the field parameters and the primary spectrum of electrons has been established.The electron and gamma-ray spectra have been obtained by numerically solving the kinetic equation dependent on two variables: the energy of electrons and their path (or the time of motion) in a photon field. The results dramatically differ from the solution of the steady-state kinetic equation which depends only on the electron energy and is frequently used in the given problem.     

Ion-temperature-gradient driven modes in dust-contaminated plasma with nonthermal electron distribution and dust charge fluctuations

The effect of nonthermal electrons on ion-temperature-gradient (ITG) driven modes is investigated in the presence of variable dust charge and ion shear flow. The dust charge fluctuating expression is obtained in the presence of kappa distributed electrons. A dispersion relation is derived and analyzed numerically by choosing space plasma parameters of Jupiter/Saturn magnetospheres. It is found that the presence of nonthermal electrons population reduces the growth rate of ITG mode driven instability. The effects of ion temperature, electron density and magnetic field variation on the growth rate of ITG instability are presented numerically. It is also pointed out that the present results will be useful to understand the ITG driven modes with variable dust charge and kappa distributed electrons, present in most of the space plasma environments.     

Damping of electrostatic noise by warm auroral electrons

Cyclotron damping by warm electrons limits the amplitude of high frequency electrostatic waves propagating in discrete auroral arcs. The effect of this damping on whistler VLF hissupper hybrid noise and Bernstein modes is examined by calculating temporal growth rates and power flux intensities of amplified noise produced by precipitating electrons. The auroral electrons are described by a realistic distribution function. The effect of varying ionospheric conditions is also considered. Whistler mode noise is found to be less sensitive to the warm electron model than the upper hybrid mode. Bernstein modes are rapidly absorbed by the ionospheric and warm electrons. The difference in the peak power flux of the whistler and upper hybrid modes is indicative of the local value of the ratio of electron plasma frequency to electron gyrofrequency. For peak upper hybrid noise to exceed peak whistler noisethis ratio should be greater than 1. Ionospheric electron temperature has little effect on the spectrum, and intense narrow beams in the distribution function should be most effective at producing high noise levels for a given warm electron model.