Impulsive Phase Of Flare Research Articles

Investigation of Quasi-periodic Variations in Hard X-Rays of Solar Flares. II. Further Investigation of Oscillating Magnetic Traps

In our recent paper (Solar Physics 261, 233) we investigated quasi-periodic oscillations of hard X-rays during impulsive phase of solar flares. We have come to conclusion that they are caused by magnetosonic oscillations of magnetic traps within the volume of hard-X-ray (HXR) loop-top sources. In the present paper we investigate four flares which show clear quasi-periodic sequences of HXR pulses. We also describe our phenomenological model of oscillating magnetic traps to show that it can explain observed properties of HXR oscillations. Main results are the following: 1. We have found that low-amplitude quasi-periodic oscillations occur before impulsive phase of some flares. 2. We have found that quasi-period of the oscillations can change in some flares. We interpret this as being due to changes of the length of oscillating magnetic traps. 3. During impulsive phase a significant part of the energy of accelerated (non-thermal) electrons is deposited within the HXR loop-top source. 4. Our analysis suggests that quick development of impulsive phase is due to feedback between pulses of the pressure of accelerated electrons and the amplitude of magnetic-trap oscillation. 5. We have also determined electron number density and magnetic filed strength for HXR loop-top sources of several flares. The values fall within the limits of $N \approx (2 -15) \times 10^{10}$ cm$^{-3}$, $B \approx (45 - 130)$ gauss.

THE SOLAR DECIMETRIC SPIKE BURST OF 2006 DECEMBER 6: POSSIBLE EVIDENCE FOR FIELD-ALIGNED POTENTIAL DROPS IN POST-ERUPTION LOOPS

A 1.4 GHz solar radio burst associated with a 3B/X6 eruptive flare on 2006 December 6 had the highest peak flux density ({approx}10{sup 6} sfu) of any event yet recorded at this frequency. The decimetric event characteristics during the brightest emission phase (numerous intense, short-lived, narrow-band bursts that overlapped to form a continuous spectrum) suggest electron cyclotron maser (ECM) emission. The peak 1.4 GHz emission did not occur during the flare impulsive phase but rather {approx}45 minutes later, in association with post-eruption loop activity seen in H{alpha} and by the Hinode EUV Imaging Spectrometer. During the Waves/LASCO era, three other delayed bursts with peak intensities >10{sup 5} sfu in the 1.0-1.6 GHz (L-band) frequency range have been reported that appear to have characteristics similar to the December 6 burst. In each of these three cases, high-frequency type IV bursts were reported in a range from {approx}150 to {approx}1500 MHz. Assuming a common ECM emission mechanism across this frequency range implies a broad span of source heights in the associated post-eruption loop systems. Difficulties with an ECM interpretation for these events include the generation of the lower frequency component of the type IVs and the long-standing problem of escape of themore » ECM emission from the loops. Magnetic-field-aligned potential drops, analogous to those observed for Earth's auroral kilometric radiation, could plausibly remove both of these objections to ECM emission.« less

2011 FEBRUARY 15: SUNQUAKES PRODUCED BY FLUX ROPE ERUPTION

We present an analysis of the 15 February 2011 X-class solar flare, previously reported to produce the first sunquake in solar cycle 24 (Kosovichev 2011). Using acoustic holography, we confirm the first, and report a second, weaker, seismic source associated with this flare. We find that the two sources are located at either end of a sigmoid which indicates the presence of a flux rope. Contrary to the majority of previously reported sunquakes, the acoustic emission precedes the peak of major hard X-ray (HXR) sources by several minutes. Furthermore, the strongest hard X-ray footpoints derived from RHESSI data are found to be located away from the seismic sources in the flare ribbons. We account for these discrepancies within the context of a phenomenological model of a flux rope eruption and accompanying two-ribbon flare. We propose that the sunquakes are triggered at the footpoints of the erupting flux rope at the start of the flare impulsive phase and eruption onset, while the main hard X-ray sources appear later at the footpoints of the flare loops formed under the rising flux rope. Possible implications of this scenario for the theoretical interpretation of the forces driving sunquakes are discussed.

Characteristics of Type-II Radio Bursts Associated with Flares and CMEs

We present a statistical study of the characteristics of type-II radio bursts observed in the metric (m) and deca-hectometer (DH) wavelength range during 1997–2008. The collected events are divided into two groups: Group I contains the events of m-type-II bursts with starting frequency ≥ 100 MHz, and group II contains the events with starting frequency of m-type-II radio bursts < 100 MHz. We have analyzed both samples considering three different aspects: i) statistical properties of type-II bursts, ii) statistical properties of flares and CMEs associated with type-II bursts, and iii) time delays between type-II bursts, flares, and CMEs. We find significant differences in the properties of m-type-II bursts in duration, bandwidth, drift rate, shock speed and delay between m- and DH-type-II bursts. From the timing analysis we found that the majority of m-type-II bursts in both groups occur during the flare impulsive phase. On the other hand, the DH-type-II bursts in both groups occur during the decaying phase of the associated flares. Almost all m-DH-type-II bursts are found to be associated with CMEs. Our results indicate that there are two kinds of shock in which group I (high frequency) m-type-II bursts seem to be ignited by flares whereas group II (low frequency) m-type-II bursts are CME-driven.

NEW SOLAR EXTREME-ULTRAVIOLET IRRADIANCE OBSERVATIONS DURING FLARES

New solar extreme-ultraviolet (EUV) irradiance observations from the NASA Solar Dynamics Observatory (SDO) EUV Variability Experiment provide full coverage in the EUV range from 0.1 to 106 nm and continuously at a cadence of 10 s for spectra at 0.1 nm resolution and even faster, 0.25 s, for six EUV bands. These observations can be decomposed into four distinct characteristics during flares. First, the emissions that dominate during the flare's impulsive phase are the transition region emissions, such as the He II 30.4 nm. Second, the hot coronal emissions above 5 MK dominate during the gradual phase and are highly correlated with the GOES X-ray. A third flare characteristic in the EUV is coronal dimming, seen best in the cool corona, such as the Fe IX 17.1 nm. As the post-flare loops reconnect and cool, many of the EUV coronal emissions peak a few minutes after the GOES X-ray peak. One interesting variation of the post-eruptive loop reconnection is that warm coronal emissions (e.g., Fe XVI 33.5 nm) sometimes exhibit a second large peak separated from the primary flare event by many minutes to hours, with EUV emission originating not from the original flare site and its immediate vicinity, but rather from a volume of higher loops. We refer to this second peak as the EUV late phase. The characterization of many flares during the SDO mission is provided, including quantification of the spectral irradiance from the EUV late phase that cannot be inferred from GOES X-ray diagnostics.

Hinode/EIS plasma diagnostics in the flaring solar chromosphere

Context: The impulsive phase of solar flares is a time of rapid energy deposition and heating in the lower solar atmosphere, leading to changes in the temperature, density, ionisation and velocity structure of this region. Aims: We aim to study the lower atmosphere during the impulsive phase of a flare using imaging and spectroscopic data from Hinode/EIS, RHESSI and TRACE. We place these observations in context by using a wide range of temperature observations from each instrument. Methods: We analyse sparse raster data from the Hinode/EIS spectrometer to derive the density and line-of-sight velocity in flare footpoints, in a GOES C6.6 flare observed on 05-June-2007. The raster duration was 150s across the centre of a small active region, allowing multiple exposures of the flare ribbons and footpoints. Using RHESSI and Hinode/XRT we test both non-thermal and thermal models for the HXR emission. Results: During the flare impulsive phase, we find evidence from XRT for flare footpoints at temperatures exceeding 7 MK. We measure the electron number density increasing up to a few ×10¹⁰ cm⁻³ in the footpoints, at temperatures of \~1.5-2 MK, accompanied by small downflows at temperatures below Fe XIII and upflows of up to \~140 km s⁻¹ at temperatures above. This is reasonable in the context of HXR diagnostics of the flare electron beam. The electrons inferred have sufficient energy to affect the chromospheric ionisation structure. Conclusions: EIS sparse raster data coupled with RHESSI imaging and spectroscopy prove useful here in studying the lower atmosphere of solar flares, and in this event suggest heat deposition relatively high in the chromosphere drives chromospheric evaporation while increasing the observed electron densities at footpoints. However, from RHESSI spectral fitting it is not possible to say whether the data are more consistent with a model including a non-thermal beam, or purely thermal model.

TRANSITION REGION EMISSION FROM SOLAR FLARES DURING THE IMPULSIVE PHASE

There are relatively few observations of UV emission during the impulsive phases of solar flares, so the nature of that emission is poorly known. Photons produced by solar flares can resonantly scatter off atoms and ions in the corona. Based on off-limb measurements by the Solar and Heliospheric Observatory/Ultraviolet Coronagraph Spectrometer, we derive the O vi λ1032 luminosities for 29 flares during the impulsive phase and the Lyα luminosities of 5 flares, and we compare them with X-ray luminosities from GOES measurements. The upper transition region and lower transition region luminosities of the events observed are comparable. They are also comparable to the luminosity of the X-ray emitting gas at the beginning of the flare, but after 10–15 minutes the X-ray luminosity usually dominates. In some cases, we can use Doppler dimming to estimate flow speeds of the O vi emitting gas, and five events show speeds in the 40–80 km s−1 range. The O vi emission could originate in gas evaporating to fill the X-ray flare loops, in heated chromospheric gas at the footpoints, or in heated prominence material in the coronal mass ejection. All three sources may contribute in different events or even in a single event, and the relative timing of UV and X-ray brightness peaks, the flow speeds, and the total O vi luminosity favor each source in one or more events.

SOLAR SOURCE AND HELIOSPHERIC CONSEQUENCES OF THE 2010 APRIL 3 CORONAL MASS EJECTION: A COMPREHENSIVE VIEW

We study the solar source and heliospheric consequences of the 2010 April 3 coronal mass ejection (CME) in the frame of the Sun-Earth connection using observations from a fleet of spacecraft. The CME is accompanied by a B7.4 long-duration flare, dramatic coronal dimming, and EUV waves. It causes significant heliospheric consequences and space weather effects such as radio bursts, a prominent shock wave, the largest/fastest interplanetary CME at 1 AU since the 2006 December 13 CME, the first gradual solar energetic particle (SEP) events in solar cycle 24, and a prolonged geomagnetic storm resulting in a breakdown of the Galaxy 15 satellite. This event, together with several following periods of intense solar activities, indicates awakening of the Sun from a long minimum. The CME EUV loop begins to rise at least 10 minutes before the flare impulsive phase. The associated coronal wave forms an envelope around the CME, a large-scale three-dimensional structure that can only be explained by a pressure wave. The CME and its preceding shock are imaged by both STEREO A and B almost throughout the whole Sun-Earth space. CME kinematics in the ecliptic plane are obtained as a function of distance out to 0.75 AU by a geometric triangulation technique. The CME has a propagation direction near the Sun-Earth line and a speed that first increases to 1000-1100 km s–1 and then decreases to about 800 km s–1. Both the predicted arrival time and speed at the Earth are well confirmed by the in situ measurements. The gradual SEP events observed by three widely separated spacecraft show time profiles much more complicated than suggested by the standard conceptual picture of SEP event heliolongitude distribution. Evolving shock properties, the realistic time-dependent connection between the observer and shock source, and a possible role of particle perpendicular diffusion may be needed to interpret this SEP event spatial distribution.

PLASMA HEATING IN THE VERY EARLY AND DECAY PHASES OF SOLAR FLARES

In this paper we analyze the energy budgets of two single-loop solar flares under the assumption that non-thermal electrons are the only source of plasma heating during all phases of both events. The flares were observed by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and Geostationary Operational Environmental Satellite (GOES) on September 20, 2002 and March 17, 2002, respectively. For both investigated flares we derived the energy fluxes contained in non-thermal electron beams from the RHESSI observational data constrained by observed GOES light-curves. We showed that energy delivered by non-thermal electrons was fully sufficient to fulfil the energy budgets of the plasma during the pre-heating and impulsive phases of both flares as well as during the decay phase of one of them. We concluded that in the case of the investigated flares there was no need to use any additional ad-hoc heating mechanisms other than heating by non-thermal electrons.

RADIO EVIDENCE OF BREAK-OUT RECONNECTION?

We reconsider the 2003 October 28 X17 flare/coronal mass ejection (CME), studying the five minutes immediately before the impulsive flare phase (not discussed in previous work). To this aim we examine complementary dynamic radio spectrograms, single frequency polarimeter records, radio images, space-based longitudinal field magnetograms, and ultraviolet images. We find widely distributed faint and narrowband meter wave radio sources located outside active regions but associated with the boundaries of magnetic flux connectivity cells, inferred from the potential extrapolation of the observed photospheric longitudinal field as a model for coronal magnetic field structures. The meter wave radio sources occur during the initial decimeter wave effects, which are well known to be associated with filament destabilization in the flaring active region (here NOAA 10486). Antiochos et al. predict in their break-out model for CME initiation that huge phenomena ... may be controlled by detailed plasma processes that occur in relatively tiny regions. They suggest that the expected faint energy release on long field lines far away from any neutral line ... may be detectable in radio/microwave emission from nonthermal particles... In this paper, we describe meter wave sources whose properties correctly coincide with the quoted predictions of the break-out reconnection model of the CME initiation.

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.

Gamma-Ray and High-Energy-Neutron Measurements on CORONAS-F during the Solar Flare of 28 October 2003

The solar flare of 28 October 2003 (X17.2/4B) was recorded by the SONG instrument onboard the CORONAS-F satellite. A description of the SONG instrument, its in-orbit operation and the principal data reduction methods used to derive the flare gamma-ray properties are presented. Appreciable gamma-ray emission was observed in the 0.2 – 300 MeV energy range. Several time intervals were identified which showed major changes in the intensity and spectral shape of the flare gamma-ray emission. The primary bremsstrahlung proves to be extended to 90 MeV and dominates during 11:02:11 – 11:03:50 UT time interval, i.e. at the beginning of the flare impulsive phase. Afterwards, the SONG response was consistent with detection of the pion-decay gamma emission. A sharp increase in the pion-decay-generated gamma-ray emission was observed at 11:03:51±2 s UT, implying a substantial change in the spectrum of accelerated ions, which testified the appearance of protons with energies of >300 MeV on the Sun. This emission lasted at least 8 – 9 min until the end of our measurements. The ion acceleration to high energies was also proved by the detection of neutrons with energies >500 MeV. It was found that the most efficient acceleration of high-energy protons coincides in time with the highest rate of the magnetic-flux change rate. The maximum gamma-ray flux at 100 MeV was 1.1×10−2 photons cm−2 s−1 MeV−1, exceeding all the fluxes that have ever been recorded.

Dynamics and energetics of the thermal and nonthermal components in the solar flare of January 20, 2005, based on data from hard electromagnetic radiation detectors onboard the CORONAS-F satellite

Based on data from the SONG and SPR-N multichannel hard electromagnetic radiation detectors onboard the CORONAS-F space observatory and the X-ray monitors onboard GOES satellites, we have distinguished the thermal and nonthermal components in the X-ray spectrum of an extreme solar flare on January 20, 2005. In the impulsive flare phase determined from the time of the most efficient electron and proton acceleration, we have obtained parameters of the spectra for both components and their variations in the time interval 06:43–06:54 UT. The spectral index in the energy range 0.2–2 MeV for a single-power-law spectrum of accelerated electrons is shown to have been close to 3.4 for most of the time interval under consideration. We have determined the time dependence of the lower energy cutoff in the energy spectrum of nonthermal photons Eγ0(t) at which the spectral flux densities of the thermal and nonthermal components become equal. The power deposited by accelerated electrons into the flare volume has been estimated using the thick-target model under two assumptions about the boundary energy E0 of the electron spectrum: (i) E0 is determined by Eγ0(t) and (ii) E0 is determined by the characteristic heated plasma energy (≈5kT (t)). The reality of the first assumption is proven by the fact that plasma cooling sets in at a time when the radiative losses begin to prevail over the power deposited by electrons only in this case. Comparison of the total energy deposited by electrons with a boundary energy Eγ0(t) with the thermal energy of the emitting plasma in the time interval under consideration has shown that the total energy deposited by accelerated electrons at the beginning of the impulsive flare phase before 06:47 UT exceeds the thermal plasma energy by a factor of 1.5–2; subsequently, these energies become approximately equal and are ∼(4–5) × 1030 erg under the assumption that the filling factor is 0.5–0.6.

Radio fine structures in dm–cm wavelength range associated with magnetic reconnection processes

Radio bursts with fine structures in decimetric–centimetric wave range are generally believed to manifest the primary energy release process during flare/CME events. By spectropolarimeters in 1–2 GHz, 2.6–3.8 GHz, and 5.2–7.6 GHz at NAOC/Huairou with very high temporal (1.25–8 ms) and spectral (4–20 MHz) resolutions, the zebra patterns, spikes, and new types of radio fine structures with mixed frequency drift features are observed during several significant flare/CME events. In this paper we will discuss the occurrence of radio fine structures during the impulsive phase of flares and/or CME initiations, which may be connected to the magnetic reconnection processes.

Local re-acceleration and a modified thick target model of solar flare electrons

The collisional thick target model (CTTM) of solar hard X-ray (HXR) bursts has become an almost 'Standard Model' of flare impulsive phase energy transport and radiation. However, it faces various problems in the light of recent data, particularly the high electron beam density and anisotropy it involves.} {We consider how photon yield per electron can be increased, and hence fast electron beam intensity requirements reduced, by local re-acceleration of fast electrons throughout the HXR source itself, after injection.} {We show parametrically that, if net re-acceleration rates due to e.g. waves or local current sheet electric (${\cal E}$) fields are a significant fraction of collisional loss rates, electron lifetimes, and hence the net radiative HXR output per electron can be substantially increased over the CTTM values. In this local re-acceleration thick target model (LRTTM) fast electron number requirements and anisotropy are thus reduced. One specific possible scenario involving such re-acceleration is discussed, viz, a current sheet cascade (CSC) in a randomly stressed magnetic loop.} {Combined MHD and test particle simulations show that local ${\cal E}$ fields in CSCs can efficiently accelerate electrons in the corona and and re-accelerate them after injection into the chromosphere. In this HXR source scenario, rapid synchronisation and variability of impulsive footpoint emissions can still occur since primary electron acceleration is in the high Alfv\'{e}n speed corona with fast re-acceleration in chromospheric CSCs. It is also consistent with the energy-dependent time-of-flight delays in HXR features.

Hard X-ray emission from a flare-related jet

Aims. We aim to understand the physical conditions in a jet event which occurred on the 22nd of August 2002, paying particular attention to evidence for non-thermal electrons in the jet material. Methods. We investigate the flare impulsive phase using multiwavelength observations from the Transition Region and Coronal Explorer (TRACE) and the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) satellite missions, and the ground-based Nobeyama Radioheliograph (NoRH) and Radio Polarimeters (NoRP). Results. We report what we believe to be the first observation of hard X-ray emission formed in a coronal jet. We present radio observations which confirm the presence of non-thermal electrons present in the jet at this time. The evolution of the event is best compared with the magnetic reconnection jet model in which emerging magnetic field interacts with the pre-existing coronal field. We calculate an apparent jet velocity of ~500 km s -1 which is consistent with model predictions for jet material accelerated by the $\vec{J}\times\vec{B}$ force resulting in a jet velocity of the order of the Alfven speed (~100–1000 km s -1 ).

RAPID CHANGES OF ELECTRON ACCELERATION CHARACTERISTICS AT THE END OF THE IMPULSIVE PHASE OF AN X-CLASS SOLAR FLARE

We present a detailed spectral analysis of the X1.3 flare of 2005 January 19 using hard X-ray (HXR) spectra obtained with RHESSI. This flare exhibits HXR pulses during the impulsive phase, with a particularly pronounced peak at the end of the impulsive phase. This peak is associated with HXR emission up to high energies (>300 keV) but does not show any Neupert effect (i.e., no simultaneous rise in soft X-rays). Fitting the spatially integrated photon spectra with a Maxwellian plus a nonthermal thick-target component reveals that the data are consistent with a high low-energy cutoff (≈ 100 keV) of the energetic electrons during the late peak. The high low-energy cutoff straightforwardly explains the lack of a Neupert effect—while highly energetic electrons are produced efficiently, there is a lack of low-energy electrons that usually contain the bulk of the total energy. Hence, the energy input into the chromosphere remains too small to trigger chromospheric evaporation. This observation shows that the characteristics of electron acceleration can change dramatically and rapidly at the end of the impulsive phase of solar flares. This could be evidence for physically distinct acceleration processes acting in the same event, or alternatively for a sudden shift in the characteristic parameters of the accelerator. Using radio observations and comparing HXR images with magnetograms, we conclude that changes in the strength and the topology of the magnetic field in which the accelerator is working are responsible for the profound changes in the injected electron spectrum.

Acceleration of Relativistic Protons During the 20 January 2005 Flare and CME

The origin of relativistic solar protons during large flare/CME events has not been uniquely identified so far. We perform a detailed comparative analysis of the time profiles of relativistic protons detected by the worldwide network of neutron monitors at Earth with electromagnetic signatures of particle acceleration in the solar corona during the large particle event of 20 January 2005. The intensity – time profile of the relativistic protons derived from the neutron monitor data indicates two successive peaks. We show that microwave, hard X-ray, and γ-ray emissions display several episodes of particle acceleration within the impulsive flare phase. The first relativistic protons detected at Earth are accelerated together with relativistic electrons and with protons that produce pion-decay γ rays during the second episode. The second peak in the relativistic proton profile at Earth is accompanied by new signatures of particle acceleration in the corona within ≈1R ⊙ above the photosphere, revealed by hard X-ray and microwave emissions of low intensity and by the renewed radio emission of electron beams and of a coronal shock wave. We discuss the observations in terms of different scenarios of particle acceleration in the corona.

EPISODIC X-RAY EMISSION ACCOMPANYING THE ACTIVATION OF AN ERUPTIVE PROMINENCE: EVIDENCE OF EPISODIC MAGNETIC RECONNECTION

We present an X-ray imaging and spectroscopic study of a partially occulted C7.7 flare on 2003 April 24 observed by RHESSI that accompanied a prominence eruption observed by TRACE. (1) The activation and rise of the prominence occurs during the preheating phase of the flare. The initial X-ray emission appears as a single coronal source at one leg of the prominence and it then splits into a double source. Such a source splitting happens three times, each coinciding with an increased X-ray flux and plasma temperature, suggestive of fast reconnection in a localized current sheet and an enhanced energy release rate. In the late stage of this phase, the prominence displays a helical structure. These observations are consistent with the tether-cutting and/or kink instability model for triggering solar eruptions. (2) The eruption of the prominence takes place during the flare impulsive phase. Since then, there appear signatures predicted by the classical CSHKP model of two-ribbon flares occurring in a vertical current sheet trailing an eruption. These signatures include an EUV cusp and current-sheet-like feature (or ridge) above it. There is also X-ray emission along the EUV ridge both below and above the cusp, which in both regions appears closer to the cusp at higher energies in the thermal regime. This trend is reversed in the nonthermal regime. (3) Spectral analysis indicates thermal X-rays from all sources throughout the flare, while during the impulsive phase there is additional nonthermal emission which primarily comes from the coronal source below the cusp. This source also has a lower temperature, a higher emission measure, and a much harder nonthermal spectrum than the upper sources.

Response of optical hydrogen lines to beam heating

<i>Context. <i/>Observations of hydrogen Balmer lines in solar flares remain an important source of information on flare processes in the chromosphere during the impulsive phase of flares. The intensity profiles of optically thick hydrogen lines are determined by the temperature, density, and ionisation structure of the flaring atmosphere, by the plasma velocities and by the velocity distribution of particles in the line formation regions.<i>Aims. <i/>We investigate the role of non-thermal electrons in the formation regions of H<i>α<i/>, H<i>β<i/>, and H<i>γ<i/> lines in order to unfold their influence on the formation of these lines. We concentrate on pulse-beam heating varying on a subsecond timescale. Furthermore, we theoretically explore possibility that a new diagnostic tool exists indicating the presence of non-thermal electrons in the flaring chromosphere based on observations of optical hydrogen lines. <i>Methods. <i/>To model the evolution of the flaring atmosphere and the time-dependent hydrogen excitation and ionisation, we used a 1-D radiative hydrodynamic code combined with a test-particle code that simulates the propagation, scattering, and thermalisation of a power-law electron beam in order to obtain the flare heating and the non-thermal collisional rates due to the interaction of the beam with the hydrogen atoms. To not bias the results by other effects, we calculate only short time evolutions of the flaring atmosphere and neglect the plasma velocities in the radiative transfer.<i>Results. <i/>All calculated models have shown a time-correlated response of the modelled Balmer line intensities on a subsecond timescale, with a subsecond timelag behind the beam flux. Depending on the beam parameters, both line centres and wings can show pronounced intensity variations. The non-thermal collisional rates generally result in an increased emission from a secondary region formed in the chromosphere. <i>Conclusions. <i/>Despite the clear influence of the non-thermal electron beams on the Balmer line intensity profiles, we were not able on the basis of our simulations to produce any unambiguous diagnostic of non-thermal electrons in the line-emitting region, which would be based on comparison of individual Balmer line intensity profiles. However, fast line intensity variations, well-correlated with the beam flux variations, represent an indirect indication of pulsating beams.