We explore the formation of the He 10 830 Å line in a flaring atmosphere, with special attention to the nonthermal effects of an electron beam. Using non-LTE calculations we obtain the line profiles from different model atmospheres. Without the nonthermal effects, the line changes from weak absorption in a cool atmosphere to emission in a hot and condensed atmosphere, as expected. However, the presence of an electron beam can significantly change the line strength, producing much stronger absorption and emission in these two cases. We find that in the nonthermal case, the collisional ionization of He followed by recombinations becomes an important process in populating the triplet levels corresponding to the He 10 830 Å line. These results suggest that the He 10 830 Å line is also a potential diagnostic tool for nonthermal effects in solar or stellar flares.

The Ha line profile in moustaches is characterized by enhanced wings and a deep central absorption. We explore the possibility that such a profile may be clue to the effect of energetic particles bombarding the atmosphere. Computations show that the characteristics of moustache line profiles can be qualitatively reproduced in two extreme cases, either injection from the corona of high energy particles (60keV electrons or 3MeV protons) or injection in a low-lying site, in middle cthromosphere or deeper, of less energetic particles (～20keV electrons or～400keV protons). The requirements on the energy and on the depth of the injection site of energetic particles are reduced in the case of observations close to the solar limb. The rSle of protons of energies below 1MeV is slightly less ignificant than that of deka-keV electrons in the case of a high particle injection site, but such protons remain to be viable candidates in the case of lower particle injection sites and of observations at larger heliocentric angles. Observations at various wavelengths are needed to find which of these hypotheses is convenient for explaining a given event.

Observationally, there is a small fraction of solar white-light flares (WLFs), the so-called type II WLFs, showing an increased visible continuum but no significant Balmer jump and less strong chromospheric line emission in comparison with type I WLFs. The classical point of view, that the flare energy is initially released in the corona and then transported downward, can hardly explain WLFs of this kind. In this paper we explore the possibility that type II WLFs originate from a deeper layer. Assuming an in situ energy release, in particular in the form of high-energy particles, in a region around the temperature minimum, the continuum emission is computed in diferent time stages during the

The continuum emission of stellar flares in UVand 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-2s-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-2s-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.

There is observational evidence showing that stellar and solar flares occur with a similar circumstance, although the former are usually much more energetic. It is expected that the bombardment by high-energy electrons is one of the chief heating processes of the flaring atmosphere. In this paper we study how a precipitating electron beam can influence the line profiles of Lya, Ha, CaII K and l8542. We use a model atmosphere of a dMe star and makenon-LTE computations taking into account the non-thermal collisional rates owing to the electron beam. The results show that the four lines can be enhanced to different extents. The relative enhancement increases with increasing formation height of the lines. Varying the energy flux of the electron beam has different effects on the four lines. The wings of Lya and Ha become increasingly broad with the beam flux; change of the Ca II K and l8542 lines, however, is most significant in the line centre. Varying the electron energy (i.e. the low-energy cut-off for a power-law beam) has a great influence on the Lya line, but little on the Ha and Ca II lines. An electron beam of higher energy precipitates deeper, thus producing less enhancement of the Lya line. The Lya/Ha flux ratio is thus sensitive to the electron energy.

Fine temporal structures in hard X-ray and microwave emissions of solar flares have been known for many years. Recent observations with high time and spatial resolution revealed that emissions in the wings of Ha could also exhibit fast (subsecond) fluctuations. We argue that such fluctuations are physically related to the small-scale injection of high-energy electrons. We explore this through numerical calculations. The energy equation and the equations for energy-level populations in hydrogen, in particular including the nonthermal collisional excitation and ionization rates, are solved simultaneously for an atmosphere impacted by a short-lived electron beam. We determine the temporal evolution of the atmospheric temperature, the atomic level populations, and the Ha line intensity. We find that although the background Ha wing emission is mainly formed in the photosphere, the fast fluctuations are probably produced in the chromosphere, which is penetrated by ～20keV electrons. To yield Ha wing fluctuations of amplitude comparable to the observations, a mean energy flux of ～(1-2)×1011 ergs cm-2 s-1 is required for the electron beam, if one adopts a Gaussian macrovelocity of 25km s-1. Such a burst contains a total energy of 1025-1026 ergs. These parameters are compatible with elementary flare bursts.

The Ni I 676.8nm line is used by the Solar and Heliospheric Observatory Michelson Doppler Imager to measure the magnetic field and velocity field in the solar atmosphere. We make non-LTE calculations of this line in an atmosphere that is bombarded by an energetic electron beam. This case is associated with the occurrence of solar flares. The electron beam produces nonthermal ionization and excitation of the hydrogen atoms and redistributes the level populations. This results in an enhanced opacity near the Ni i line and an upward shift of its formation height, as well as an increased line source function. We find that the Ni i line may appear in emission in the presence of a fairly strong electron beam and preferentially in a cool atmosphere (i.e., sunspot umbrae/penumbrae). On the other hand, if there is no bombarding electron beam, the profile can hardly turn to emission even though the atmosphere may be heated to higher temperatures through other ways. This result implies that the sign reversal of the longitudinal magnetic field observed in some flare events may not be a true reversal but just an artifact associated with the production of an emission profile.

We explain the origin of the infrared continuum in the solar flare of 10 March 2001 which shows a positive contrast in the maximum phase but possibly a negative contrast in the early phase. The general feature is consistent with a flare model heated by an electron beam. By making non-LTE model calculations, we find that in the early phase, when the electron beam bombards an unheated atmosphere, the non-thermal ionization by the beam results in an increased Hopacity which then reduces the emergent intensity. With the flare development, the atmosphere is gradually heated. In particular, radiative backwarming plays a chief role in the heating of the temperature minimum region and upper photosphere. We estimate the temperature rise in these regions for a fully heated atmosphere in which energy balance is attained. In this case, the continuum emission rises above the quiescent value. Therefore, this corresponds to the flare maximum phase. We further find that the energy flux of the electron beam deduced from the hard X-ray emission is large enough to account for the continuum contrast.

An X1.6 two-ribbon flare was observed on 2001 October 19 using the imaging spectrograph at the Solar Tower of Nanjing University. We obtained a time series of two-dimensional H line spectra. Combining the observations by the Nobeyama Radioheliograph (NoRH), the Transition Region and Coronal Explorer (TRACE), the Yohkoh soft X-ray telescope (SXT) and hard X-ray telescope (HXT), and the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO), we performed a multiwavelength analysis for this flare in detail. The hard X-ray (HXR) and radio time profiles for this event have a double-peaked structure. Four H bright kernels exist, two of which are identified as the conjugate footpoints of a loop, while the other two are probably the footpoints of two different loops that are closely related. We compared the spatial distribution of the HXR sources with that of the H kernels at the two peak times. The results show quite asymmetric HXR emission in the H kernels. We ascribe this asymmetric behavior to the magnetic mirroring effect. In addition, we found that different heating mechanisms (nonthermal electron beam vs. heat conduction) may apply to different H kernels. We also derived the velocity field and found that the maximum velocity tends to occur at the outer edge of the flare ribbons. This is consistent with the general scenario of two-ribbon flare models.