Electron beam as origin of white-light solar flares

We study the effect of chromospheric bombardment by an electron beam during solar flares. Using a semi-empirical flare model, we investigate energy balance at temperature minimum level and in the upper photosphere. We show that non-thermal hydrogen ionization (i.e., due to the electrons of the beam) leads to an increase of chromospheric hydrogen continuum emission, H^– population, and absorption of photospheric and chromospheric continuum radiation. So, the upper photosphere is radiatively heated by chromospheric continuum radiation produced by the beam. The effect of hydrogen ionization is an enhanced white-light emission both at chromospheric and photospheric level, due to Paschen and H^– continua emission, respectively. We then obtain white-light contrasts compatible with observations, obviously showing the link between white-light flares and atmospheric bombardment by electron beams.     

Radiative hydrodynamic simulations of the spectral characteristics of solar white-light flares

As one of the most violent activities in the solar atmosphere,white-light flares(WLFs)are generally known for their enhanced white-light(or continuum)emission,which primarily originates in the solar lower atmosphere.However,we know little about how white-light emission is produced.In this study,we aim to investigate the response of the continua at 3600?and 4250?and also the Hαand Lyαlines during WLFs modeled using radiative hydrodynamic simulations.We take non-thermal electron beams as the energy source for the WLFs in two different initial atmospheres and vary their parameters.Our results show that the model with non-thermal electron beam heating clearly shows enhancements in the continua at 3600?and 4250?as well as in the Hαand Lyαlines.A larger electron beam flux,a smaller spectral index,or an initial penumbral atmosphere leads to a stronger emission increase at 3600?,4250?and in the Hαline.The Lyαline,however,is more obviously enhanced in a quiet-Sun initial atmosphere with a larger electron beam spectral index.It is also notable that the continua at 3600?and 4250?and the Hαline exhibit a dimming at the start of heating and reach their peak emissions after the peak time of the heating function,while the Lyαline does not show such behaviors.These results can serve as a reference for the analysis of future WLF observations.     

The role of non-thermal electrons in the optical continuum of stellar flares

The continuum emission of stellar flares in UV and visible bands can be enhanced by two or even three orders of magnitude relative to the quiescent level and is usually characterized by a blue colour. It is difficult for thermal atmospheric models to reproduce all these spectral features. If the flaring process involves the acceleration of energetic electrons which then precipitate downwards to heat the lower atmosphere, collisional excitation and ionization of ambient hydrogen atoms by these non-thermal electrons could be important in powering the continuum emission. To explore such a possibility, we compute the continuum spectra from an atmospheric model for a dMe star, AD Leo, at its quiescent state, when considering the non-thermal effects by precipitating electron beams. The results show that if the electron beam has an energy flux large enough (for example, ℱ₁∼10¹² erg cm⁻² s⁻¹), the U -band brightening and, in particular, the U − B colour are roughly comparable with observed values for a typical large flare. Moreover, for electron beams with a moderate energy flux ℱ₁≲10¹¹ erg cm⁻² s⁻¹, a decrease of the emission at the Paschen continuum appears. This can explain at least partly the continuum dimming observed in some stellar flares. Adopting an atmospheric model for the flaring state can further raise the continuum flux, but it yields a spectral colour incomparable with observations. This implies that the non-thermal effects may play the chief role in powering the continuum emission in some stellar flares.     

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.     

1974年10月11日白光耀斑的光谱分析

１９７４年１０月１１日耀斑的光谱分析表明，连续发射和微波爆发几乎同时达到极大。在连续辐射极大时刻出现了高项巴耳末发射线和微弱的巴耳末跳跃，所有这些说明这个耀斑是Ｉ型白光耀斑。我们发现，在这个耀斑连续发射的极大时刻，ＣａＩＩＫ线的Ｋ１处强度大大增强，约达到连续辐射强度的一半，对应的辐射温度达到５７１６Ｋ。这个反常增强的持续时间小于４分钟。现有的半经验模型似不能给出这样的结果。色球压缩区也不能给出解释     

RHESSI Microflares: II. Implications for Loop Structure and Evolution

We present simple analytic models which predict the peak X-ray emission measure and temperature attained in flares in which the chromospheric evaporation process takes place either in a single ‘monolithic’ loop or in a loop consisting of several filaments that are created successively as the energy release process proceeds in time. As possible mechanisms driving chromospheric evaporation we consider both classical heat conduction from the loop top and non-thermal electron beams. The model predictions are tested for a set of 18 well studied RHESSI microflares. The results suggest beam driven evaporation in filamented loops as being capable of accounting for the observed emission measures and temperatures though there are issues with the very high beam densities needed. On the other hand, estimates of the emission measures achieved by conductive evaporation which are derived by using the Rosner – Tucker – Vaiana (RTV) scaling law are much larger than the observed ones. Possible reasons for this discrepancy are discussed.     

Revealing the transition from post-AGB stars to planetary nebulae: non-thermal and thermal radio continuum observations

Midcourse Space eXperiment and Infrared Astronomical Satellite colour diagnostics as well as OH maser profile characteristics were used to select a sample of post-asymptotic giant branch (pAGB) candidates for a radio continuum detection experiment with the Australia Telescope Compact Array. Seven out of 28 sources, six of which are new detections, show a continuum. A planetary nebula serendipitously detected in the field of an undetected pAGB candidate also reveals radio continuum. The radio continuum properties of these eight sources are described. Almost half have non-thermal emission. dusty modelling of the infrared spectral energy distributions (SEDs) of the three strongest detections reveals that they all have central stars with temperatures substantially lower than that required for significant photoionization, leading us to infer that the radio continuum has arisen from wind–shock interactions. This hypothesis is consistent with the detection of non-thermal radio emission in one of these three objects.     

Thermal evolution of flare plasma

The evolution of hot thermal plasma in solar flares is analyzed by a single-temperature model applied to continuum emission in the 5 keV < E ? 13 keV spectral range. The general trend that the thermal plasma observed in soft X-rays is heated by the non-thermal electrons that emit as the hard X-ray bursts is confirmed by the observation of an electron temperature increase at the time interval of hard X-ray spikes and a quantitative comparison between thermal energy content and hard X-ray energy input. Non-thermal electrons of 10 keV < E < 30 keV energy may play an important role in pre- and post-burst phases.     

Cosmic ray propagation and the star formation history of NGC 1961

We present new radio continuum data at four frequencies for the supermassive, peculiar galaxy NGC 1961. These observations allow us to separate the thermal and non-thermal radio emission and to determine the non-thermal spectral index distribution. This spectral index distribution in the galactic disc is unusual: at the maxima of the radio emission the synchrotron spectrum is very steep, indicating aged cosmic ray electrons. Away from the maxima the spectrum is much flatter. The steep spectrum of the synchrotron emission at the maxima indicates that a strong decline of the star formation rate has taken place at these sites. The extended radio emission is a sign of recent cosmic ray acceleration, probably by recent star formation. We suggest that a violent event in the past, most likely a merger or a collision with an intergalactic gas cloud, has caused the various unusual features of the galaxy.     

Non-thermal emission from old supernova remnants

We study the non-thermal emission from old shell-type supernova remnants (SNRs) on the frame of a time-dependent model. In this model, the time-dependent non-thermal spectra of both primary electrons and protons as well as secondary electron/positron (e^±) pairs can be calculated numerically by taking into account the evolution of the secondary e^± pairs produced from proton–proton (p–p) interactions as accelerated protons collide with the ambient matter in an SNR. The multiwavelength photon spectrum for a given SNR can be produced through leptonic processes such as electron/positron synchrotron radiation, bremsstrahlung and inverse Compton scattering as well as hadronic interaction. Our results indicate that the non-thermal emission of the secondary e^± pairs is becoming more and more prominent when the SNR ages in the radiative phase because the source of the primary electrons has been cut off and the electron synchrotron energy loss is significant for a radiative SNR, whereas the secondary e^± pairs can be produced continuously for a long time in the phase due to the large energy-loss time for the p–p interaction. We apply the model to two old SNRs, G8.7−0.1 and G23.3−0.3, and the predicted results can explain the observed multiwavelength photon spectra for the two sources.     

Analysis of the August 7, 1972 white light flare: Light curves and correlation with hard X-rays

Cinematic, photometric observations of the 3B flare of August 7, 1972 are described in detail. The time resolution was 2 s; the spatial resolution was 1–2″. Flare continuum emissivity at 4950 Å and at 5900 Å correlated closely in time with the 60–100 keV non-thermal X-ray burst intensity. The observed peak emissivity was 1.5 × 10¹⁰ erg cm^?2 s^?1 and the total flare energy in the 3900–6900 Å range was ～10³⁰ erg. From the close temporal correspondence and from the small distance (3″) separating the layers where the visible emission and the X-rays arose, it is argued that the hard X-ray source must have had the same silhouette as the white light flare and that the emission patches had cross-sections of 3–5″. There was also a correlation between the location of the most intense visible emissions near sunspots and the intensity and polarization of the 9.4 GHz radio emission. The flare appeared to show at least three distinct particle acceleration phases: one, occurring at a stationary source and associated with proton acceleration gave a very bluish continuum and reached peak intensity at ～ 1522 UT. At 1523 UT, a faint wave spread out at 40 km s^?1 from flare center. The spectrum of the wave was nearly flat in the range 4950–5900 Å. Association of the wave with a slow drift of the microwave emission peak to lower frequencies and with a softening of the X-ray spectrum is interpreted to mean that the particle acceleration process weakened while the region of acceleration expanded. The observations are interpreted with the aid of the flare models of Brown to mean that the same beam of non-thermal electrons that was responsible for the hard X-ray bremsstrahlung also caused the heating of the lower chromosphere that produced the white light flare.     

Peculiar oscillations in the microwave emission of Jupiter during the impact of K fragment of comet Shoemaker-Levy 9

In this article we report the peculiar oscillations in the intensity of microwave (4.15 GHz) emission seen during the impact of K fragment of comet Shoemaker-Levy 9 on July 19, 1994. The oscillations begin at 10h 13m 25s UT suddenly with a frequency of ~0.3 Hz and gradually the frequency of these oscillations increases to ~ 1 Hz. The oscillations are not due to local atmosphere or the radio interference from signals of geostationary satellite. They are intrinsic to the microwave emission from Jupiter during the impact of K fragment. Peak-to-peak amplitude of the oscillations is about 34% of the total microwave emission from Jupiter. If we assume that only 50% of the microwave emission is non-thermal and only the non-thermal emission suffered oscillations, then the oscillations are about 68% of the non-thermal emission from Jupiter. The observations also indicate that there are three continuum enhancements during this event and periodic oscillations almost all through. The third enhancement was the largest and during this enhancement there were some additional aperiodic variations. The aperiodic variations were of the order of few minutes and were possibly generated by the gravity waves. The periodic oscillations could be synchrotron emission modulated by the plasma oscillation in the outer magnetosphere. kg]Key words     

A Special Radio Spectral Fine Structure Used for Plasma Diagnostics in Coronal Magnetic Traps

A specific combination of spectral fine structures in meter –  decimeter dynamic spectra of solar radio burst emission is reported in observations carried out at the Astrophysical Institute Potsdam. We describe and interpret the occurrence of zebra patterns in fast drifting (type III burst-like) envelopes of absorbed continuum emission. A possible mechanism of the origin of such an involved spectral pattern is put forward, leading to a necessarily multinonequlibrium component coronal plasma. The suggested mechanism is based on the fact that during the passage of a fast electron beam through the corona the loss cone instability (which is caused by electrons captured in a magnetic trap generating the continuum) is quenched. As result, a fast drift burst appears in absorption, and the zebra pattern becomes visible on the low background emission. This zebra pattern is generated by a group of electrons with a nonequilibrium distribution over transverse velocities. In the absence of the beam the pattern is invisible against the background of the stronger continuum. It is shown that the mechanism is sensitive to the distribution parameters of the different electron ensembles. Therefore the effect in dynamic radio spectra is comparatively rare but its proper existence underlines that the simultaneous presence of different ensembles of electrons in the flaring corona can be quite a frequent situation. This can explain some problems in deconvolving X-ray photon spectra to electron energy spectra.     

Radio observations of the planetary nebula around the OH/IR star OH 354.88-0.54 (V1018 Sco)

We present radio observations of the unique, recently formed, planetary nebula (PN) associated with a very long-period OH/IR variable star V1018 Sco that is unequivocally still in its asymptotic giant branch phase. Two regions within the optical nebula are clearly detected in non-thermal radio continuum emission, with radio spectral indices comparable to those seen in colliding-wind Wolf–Rayet binaries. We suggest that these represent shocked interactions between the hot, fast stellar wind and the cold nebular shell that represents the PN's slow wind moving away from the central star. This same interface produces both synchrotron radio continuum and the optical PN emission. The fast wind is neither spherical in geometry nor aligned with any obvious optical or radio axis. We also report the detection of transient H₂O maser emission in this nebula.     

A radio study of the superwind galaxy NGC 1482

We present multifrequency radio continuum as well as H  i observations of the superwind galaxy NGC 1482, with both the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA). This galaxy has a remarkable hourglass-shaped optical emission-line outflow as well as bipolar soft X-ray bubbles on opposite sides of the galactic disc. The low-frequency, lower-resolution radio observations show a smooth structure. From the non-thermal emission, we estimate the available energy in supernovae, and examine whether this would be adequate to drive the observed superwind outflow. The high-frequency, high-resolution radio image of the central starburst region located at the base of the superwind bi-cone shows one prominent peak and more extended emission with substructure. This image has been compared with the infrared, optical red continuum, Hα, and soft and hard X-ray images from Chandra to understand the nature and relationship of the various features seen at different wavelengths. The peak of the infrared emission is the only feature that is coincident with the prominent radio peak, and possibly defines the centre of the galaxy.
The H  i observations with the GMRT show two blobs of emission on opposite sides of the central region. These are rotating about the centre of the galaxy and are located at ∼2.4 kpc from it. In addition, these observations also reveal a multicomponent H  i absorption profile against the central region of the radio source, with a total width of ∼250 km s⁻¹. The extreme blue- and redshifted absorption components are at 1688 and 1942 km s⁻¹, respectively, while the peak absorption is at 1836 km s⁻¹. This is consistent with the heliocentric systemic velocity of  1850 ± 20 km s⁻¹  , estimated from a variety of observations. We discuss possible implications of these results.     

On the relation of mid-infrared emission with the gamma-ray properties of Fermi-detected BL Lac objects

We present the Wide-field Infrared Survey Explorer (WISE) photometric data of 158 Fermi-detected BL Lacs and investigate the nature of their mid-infrared (MIR) continuum emission. In the [3.4]-[4.6]-[12] μm color–color diagram, nearly all their colors lie within the WISE Blazar strip (WBS), which is an effective diagnostic tool to separate sources dominated by non-thermal radiation from those dominated by thermal radiation. This feature indicates that their MIR emission is predominantly non-thermal. This argument is further supported by the strong radio-MIR flux correlation. We derive their MIR spectral indices and compare them with the near-infrared (NIR) spectral indices. We find that there is a prevalent steepening from MIR spectrum to NIR spectrum. The low-frequency-peaked BL Lacs (LBLs) have on average a larger MIR spectral index and a higher MIR luminosity than the high-frequency-peaked BL Lacs (HBLs), and the intermediate-frequency-peaked BL Lacs (IBLs) appear to bridge them. The MIR-γ-ray flux correlation is highly significant. A strong positive correlation is also found between the MIR and γ-ray spectral indices. The γ-ray-MIR loudness is significantly correlated with the synchrotron peak frequency. Finally we propose that the γ-rays are highly associated with the MIR emission from the jet, and the γ-ray emission is likely from the synchrotron self-Compton process for the Fermi-detected BL Lacs in our sample.     

Radiation mechanisms and geometry of Cygnus X-1 in the soft state

We present X-ray/ γ -ray spectra of Cyg X-1 observed during the transition from the hard to the soft state and in the soft state by ASCA , RXTE and CGRO /OSSE in 1996 May and June. The spectra consist of a dominant soft component below ∼2 keV and a power-law-like continuum extending to at least ∼800 keV. We interpret them as emission from an optically thick, cold accretion disc and from an optically thin, non-thermal corona above the disc. A fraction f ≳0.5 of total available power is dissipated in the corona.
We model the soft component by multicolour blackbody disc emission taking into account the torque-free inner-boundary condition. If the disc extends down to the minimum stable orbit, the ASCA RXTE data yield the most probable black hole mass of M _X≈10 M_⊙ and an accretion rate,     , locating Cyg X-1 in the soft state in the upper part of the stable, gas-pressure-dominated, accretion-disc solution branch.
The spectrum of the corona is well modelled by repeated Compton scattering of seed photons from the disc off electrons with a hybrid, thermal/non-thermal distribution. The electron distribution can be characterized by a Maxwellian with an equilibrium temperature of kT _e∼30–50 keV, a Thomson optical depth of τ ∼0.3 and a quasi-power-law tail. The compactness of the corona is 2≲ℓ_h≲7, and a presence of a significant population of electron–positron pairs is ruled out.
We find strong signatures of Compton reflection from a cold and ionized medium, presumably an accretion disc, with an apparent reflector solid angle, Ω/2π∼0.5–0.7. The reflected continuum is accompanied by a broad iron K α line.     

一个宽带的太阳射电尖峰事件

根据不同的发射波模以及偏振态（偏振度与偏振方向）的快速时变特征,对1993年10月2日07：44：34-07：44：52：99UT期间的太阳射电事件进行了证认,认为这是一个由两群、总数约为40个尖峰（spike）结构组成的罕见的宽带事件,它的总带宽＞300MHz、相对带宽■5％．根据它们在2．545GHZ,2．645GHZ;2．695GHZ和2B40GHZ上的流量资料,首次对一些spike结构作了谱分析．发现spike结构谱的峰值频率,第一群呈现为随时间由高频向低频漂移的倾向,第二群基本上位于2．695GHZ上．Spike谱的这些观测特征,可定性地用非热电子束流平均能量的时变特性来解释．对于第二群具有相同峰值频率的spike辐射,可能来自同一电子回旋脉泽不稳定区域．     

Flares in Sigmoidal Coronal Structures – a Case Study

We analyze radio observations, magnetograms and extrapolated field line maps, Hα filtergrams, and X-ray observations of two flare events (6 February 1992 in AR 7042 and 25 October 1994 in AR 7792) and study properties, evolution and energy release signatures of sigmoidal loop systems. During both events, the loop configuration seen in soft X-ray (SXR) images changes from a preflare sigmoidal shape to a relaxed post-flare loop system. The underlying magnetic field system consists of a quadrupolar configuration formed by a sheared arcade core and a remote field concentration. We demonstrate two possibilities: a sigmoidal SXR pattern can be due to a single continuous flux tube (the 1992 event). Alternatively, it can be due to a set of independent loops appearing like a sigmoid (the 1994 event). In both cases, the preflare and post-flare loops can be well reproduced by a linear force-free field and potential field, respectively, computed using preflare magnetograms. We find that thermal and non-thermal flare energy release indicators of both events become remarkably similar after applying spatial and temporal scale transformations. Using the spatial scaling between both events we estimated that the non-thermal energy release in the second event liberated about 1.7 times more energy per unit volume. A two-and-a-half times faster evolution indicates that the rate of the energy release per unit volume is more than four times higher in this event. A coronal type II burst reveals ignition and propagation of a coronal shock wave. In contrast, the first event, which was larger and released about a 10 times more energy during the non-thermal phase, was associated with a CME, but no type II burst was recorded. During both events, in addition to the two-ribbon flare process an interaction was observed between the flaring arcade and an emerging magnetic flux region of opposite polarity next to the dominant leading sunspot. The arcade flare seems to stimulate the reconnection process in an `emerging flux-type' configuration, which significantly contributes to the energy release. This regime is characterized by the quasiperiodic injection of electron beams into the surrounding extended field line systems. The repeated beam injections excite pulsating broadband radio emission in the decimetric-metric wavelength range. Each radio pulse is due to a new electron beam injection. The pulsation period (seconds) reflects the spatial scale of the emerging flux-type field configuration. Since broadband decimetric-metric radio pulsations are a frequent radio flare phenomenon, we speculate that opposite-polarity small-scale flux intrusions located in the vicinity of strong field regions may be an essential component of the energy release process in dynamic flares.     

Synthetic Spectra of the Fe VIII – Fe XVI Emission Lines for Electron Non-Thermal Distributions

A certain class of non-thermal electron distributions can exhibit more mono-energetic shape and a higher peak than the Maxwellian distribution. This type of electron distribution can be observed mainly in flaring plasmas. We have studied the influence of this kind of electron energy distribution on the excitation equilibrium of Fe VIII – Fe XVI in the solar corona. The changes in synthetic spectra of the emission lines belonging to these ions due to this non-thermal distribution are shown. The possibilities of finding the shape of the energy distribution function of electrons from the Fe line ratios are also discussed. The results can be used for diagnostics of coronal plasmas where the deviations of particle energy distributions from the Maxwellian one can be significant.