Chromospheric models of a solar flare including velocity fields (original) (raw)
Related papers
X-ray-heated models of stellar flare atmospheres - Theory and comparison with observations
The Astrophysical Journal Supplement Series, 1992
We compute a sequence of five model atmospheres consisting of the photosphere, chromosphere, and transition region. The models represent the response of the gas in a magnetically confined loop to intense flare energy release. We assume that the energy release is confined to the corona, and include the effects of chromospheric evaporation and indirect heating of the lower atmosphere by X-rays emitted from the coronal plasma. The models are computed in hydrostatic and energetic equilibrium and incorporate a detailed non-LTE solution of the radiative transfer and statistical equilibrium equations for a 6 level plus continuum hydrogen atom, a 5 level plus continuum Ca n ion, and a 3 level plus continuum Mg n ion. Complete tables of the depth-dependent model atmospheres are included in the Appendix. Line and continuum surface fluxes are presented in the wavelength range 1000-9000 Á and are compared with those observed during a giant flare on the M dwarf star AD Leo. Our conclusions are the following : 1. The structure of the flare transition region is consistent with conductive heating balancing optically thin cooling; we also find that some UV line fluxes (e.g., N v, C rv) can be used as a transition-region "pressure gauge" and can provide a constraint on the flare area. 2. Our models predict ratios of Ca n to hydrogen emission which are much greater than those observed; they also predict Balmer line profiles which are much narrower than those observed. This suggests that additional heating is taking place in the upper chromosphere beyond that assumed in the models. 3. The observed flare continuum is much bluer than that computed from the models; the observations fit a blackbody spectrum with T-8500-9500 K. We propose that the flare continuum is formed by photospheric reprocessing of intense ultraviolet to extreme ultraviolet (EUV) line emission from the upper chromosphere. We suggest that if the UV/EUV line emission is formed in response to the deposition of a large flux of nonthermal electrons, the continuum luminosity and color temperature can be used to determine both the energy flux and the flare area being bombarded by energetic electrons. The same reprocessing mechanism may be responsible for some solar "white fight" flares. 4. We use the Ca n and H7 fine fluxes from our chromospheric models to estimate the coronal evolution (temperature and emission measure) in the AD Leo flare. When we compare the result with the coronal evolution predicted from the loop evolution model of Fisher and Hawley, we find good agreement during the first half of the flare but poor agreement toward the end of the flare. This fact, coupled with the large discrepancy between the coverage factors for the fine and the continuum emission, suggests to us that the AD Leo flare evolves in a similar fashion to a solar two-ribbon flare; thus, it is not possible to describe all aspects of the flare using only a single evolving loop.
NUMERICAL SIMULATIONS OF CHROMOSPHERIC HARD X-RAY SOURCE SIZES IN SOLAR FLARES
The Astrophysical Journal, 2012
X-ray observations are a powerful diagnostic tool for transport, acceleration, and heating of electrons in solar flares. Height and size measurements of X-ray footpoint sources can be used to determine the chromospheric density and constrain the parameters of magnetic field convergence and electron pitch-angle evolution. We investigate the influence of the chromospheric density, magnetic mirroring, and collisional pitch-angle scattering on the size of X-ray sources. The time-independent Fokker-Planck equation for electron transport is solved numerically and analytically to find the electron distribution as a function of height above the photosphere. From this distribution, the expected X-ray flux as a function of height, its peak height, and full width at half-maximum are calculated and compared with RHESSI observations. A purely instrumental explanation for the observed source size was ruled out by using simulated RHESSI images. We find that magnetic mirroring and collisional pitch-angle scattering tend to change the electron flux such that electrons are stopped higher in the atmosphere compared with the simple case with collisional energy loss only. However, the resulting X-ray flux is dominated by the density structure in the chromosphere and only marginal increases in source width are found. Very high loop densities (>10 11 cm −3 ) could explain the observed sizes at higher energies, but are unrealistic and would result in no footpoint emission below about 40 keV, contrary to observations. We conclude that within a monolithic density model the vertical sizes are given mostly by the density scale height and are predicted smaller than the RHESSI results show.
Earth, Planets and Space, 2009
The chromosphere (the link between the photosphere and the corona) plays a crucial role in flare and CME development. In analogies between flares and magnetic substorms, it is normally identified with the ionosphere, but we argue that the correspondence is not exact. Much of the important physics of this interesting region remains to be explored. We discuss chromospheric flares in the context of recent observations of white-light flares and hard X-rays as observed by TRACE and RHESSI, respectively. We interpret key features of these observations as results of the stepwise changes a flare produces in the photospheric magnetic field.
Dynamics and evolution of an eruptive flare
Astronomy and Astrophysics, 2006
Aims. We study the dynamics and the evolution of a C2.3 two-ribbon flare, developed on 2002 August 11, during the impulsive phase as well as during the long gradual phase. To this end we obtained multiwavelength observations using the CDS spectrometer aboard SOHO, facilities at the National Solar Observatory/Sacramento Peak, and the TRACE and RHESSI spacecrafts.
Solar Flares and the Chromosphere
Eprint Arxiv 1011 4650, 2010
A white paper prepared for the Space Studies Board, National Academy of Sciences (USA), for its Decadal Survey of Solar and Space Physics (Heliophysics), reviewing and encouraging studies of flare physics in the chromosphere.
Chromospheric Bubbles in Solar Flares
The Astrophysical Journal, 2020
We analyse a grid of radiative hydrodynamic simulations of solar flares to study the energy balance and response of the atmosphere to non-thermal electron beam heating. The appearance of chromospheric bubbles is one of the most notable features that we find in the simulations. These pockets of chromospheric plasma get trapped between the transition region and the lower atmosphere as it is superheated by the particle beam. The chromospheric bubbles are seen in the synthetic spectra, appearing as an additional component to Balmer line profiles with high Doppler velocities as high as 200 kms −1. Their signatures are also visible in the wings of Ca II 8542 Å line profiles. These bubbles of chromospheric plasma are driven upward by a wavefront that is induced by the shock of energy deposition, and require a specific heating rate and atmospheric location to manifest.
Solar flare X-ray source motion as a response to electron spectral hardening
Astronomy & Astrophysics, 2013
Context. Solar flare hard X-rays (HXRs) are thought to be produced by nonthermal coronal electrons stopping in the chromosphere, or remaining trapped in the corona. The collisional thick target model (CTTM) predicts that more energetic electrons penetrate to greater column depths along the flare loop. This requires that sources produced by harder power-law injection spectra should appear further down the legs or footpoints of a flareloop. Therefore, the frequently observed hardening of the injected power-law electron spectrum during flare onset should be concurrent with a descending hard X-ray source. Aims. To test this implication of the CTTM by comparing its predicted HXR source locations with those derived from observations of a solar flare which exhibits a nonthermally-dominated spectrum before the peak in HXRs, known as an early impulsive event.
The Astrophysical Journal, 2009
Acceleration and transport of high-energy particles and fluid dynamics of atmospheric plasma are interrelated aspects of solar flares, but for convenience and simplicity they were artificially separated in the past. We present here selfconsistently combined Fokker-Planck modeling of particles and hydrodynamic simulation of flare plasma. Energetic electrons are modeled with the Stanford unified code of acceleration, transport, and radiation, while plasma is modeled with the Naval Research Laboratory flux tube code. We calculated the collisional heating rate directly from the particle transport code, which is more accurate than those in previous studies based on approximate analytical solutions. We repeated the simulation of Mariska et al. with an injection of power law, downward-beamed electrons using the new heating rate. For this case, a ∼10% difference was found from their old result. We also used a more realistic spectrum of injected electrons provided by the stochastic acceleration model, which has a smooth transition from a quasi-thermal background at low energies to a nonthermal tail at high energies. The inclusion of low-energy electrons results in relatively more heating in the corona (versus chromosphere) and thus a larger downward heat conduction flux. The interplay of electron heating, conduction, and radiative loss leads to stronger chromospheric evaporation than obtained in previous studies, which had a deficit in low-energy electrons due to an arbitrarily assumed low-energy cutoff. The energy and spatial distributions of energetic electrons and bremsstrahlung photons bear signatures of the changing density distribution caused by chromospheric evaporation. In particular, the density jump at the evaporation front gives rise to enhanced emission, which, in principle, can be imaged by X-ray telescopes. This model can be applied to investigate a variety of high-energy processes in solar, space, and astrophysical plasmas.
Proceedings of the International Astronomical Union
Initiation and development of a M 1.0 class flare of June 12, 2014, was observed by space and ground-based telescopes, including EUV and X-ray imaging spectroscopy by IRIS and RHESSI, and high-resolution optical imaging by 1.6 m New Solar Telescope (NST). Analyzing the NST data, we found small-scale loop-like structures in the region of the magnetic field Polarity Inversion Line (PIL), the emergence and interaction of which caused photospheric brightenings temporarily coinciding with hard X-ray impulses. Detailed studies of the PIL region reveal signatures of photospheric plasma downflows and dissipation of electric currents. The reconstructed magnetic field topology shows a bundle of lines connecting the PIL region with the flare ribbons which were places of chromospheric evaporation observed by IRIS. The observations suggest a scenario with the primary energy release processes located in the low atmospheric layers of the PIL, energizing the overlying large-scale magnetic structure...
Proceedings of the International Astronomical Union, 2016
While progress has been made on understanding how energy is released and deposited along the solar atmosphere during explosive events such as solar flares, the chromospheric and coronal heating through the sudden release of magnetic energy remain an open problem in solar physics. Recent hydrodynamic models allow to investigate the energy deposition along a flare loop and to study the response of the chromosphere. These results have been improved with the consideration of transport and acceleration of particles along the loop. RHESSI and Fermi/GBM X-ray and gamma-ray observations help to constrain the spectral properties of the injected electrons. The excellent spatial, spectral and temporal resolution of IRIS will also help us to constrain properties of explosive events, such as the continuum emission during flares or their emission in the chromosphere.