Non-thermal hydrogen Lyman line and continuum emission in solar flares generated by electron beams

Aims. Hydrogen Lyman continuum emission is greatly enhanced in the impulsive kernels of solar flares, with observations of Lyman lines showing impulsive brightening and both red and blue wing asymmetries, based on the images with low spatial resolution. A spate of proposed instruments will study Lyman emission in more detail from bright, impulsive flare kernels. In support of new instrumentation we aim to apply an improved interpretation of Lyman emission with the hydrodynamic radiative code, HYDRO2GEN, which has already successfully explained Hα emission with large redshifts and sources of white light emission in solar flares. The simulations can interpret the existing observations and propose observations in the forthcoming missions. Methods. A flaring atmosphere is considered to be produced by a 1D hydrodynamic response to injection of an electron beam, defining depth variations of electron and ion kinetic temperatures, densities, and macro-velocities. Radiative responses in this flaring atmosphere affected by the beams with different parameters are simulated using a fully non-local thermodynamic equilibrium (NLTE) approach for a five-level plus continuum model hydrogen atom with excitation and ionisation by spontaneous, external, and internal diffusive radiation, and by inelastic collisions with thermal and beam electrons. Integral radiative transfer equations for all optically thick transitions are solved using the L2 approximation simultaneously with steady state equations. Results. During a beam injection in the impulsive phase there is a large increase of collisional ionisation and excitation by non-thermal electrons that strongly (by orders of magnitude) increases excitation and the ionisation degree of hydrogen atoms from all atomic levels. These non-thermal collisions combined with plasma heating caused by beam electrons lead to an increase in Lyman line and continuum radiation, which is highly optically thick. During a beam injection phase the Lyman continuum emission is greatly enhanced in a large range of wavelengths resulting in a flattened distribution of Lyman continuum over wavelengths. After the beam is switched off, Lyman continuum emission, because of its large opacity, sustains, for a very long time, the high ionisation degree of the flaring plasma gained during the beam injection. This leads to a long enhancement of hydrogen ionisation, occurrence of white light flares, and an increase of Lyman line emission in cores and wings, whose shapes are moved closer to those from complete redistribution (CRD) in frequencies, and away from the partial ones (PRD) derived in the non-flaring atmospheres. In addition, Lyman line profiles can reflect macro-motions of a flaring atmosphere caused by downward hydrodynamic shocks produced in response to the beam injection reflected in the enhancements of Ly-line red wing emission. These redshifted Ly-line profiles are often followed by the enhancement of Ly-line blue wing emission caused by the chromospheric evaporation. The ratio of the integrated intensities in the Lyα and Lyβ lines is lower for more powerful flares and agrees with reported values from observations, except in the impulsive phase in flaring kernels which were not resolved in previous observations, in which the ratio is even lower. These results can help observers to design the future observations in Lyman lines and continuum emission in flaring atmospheres.

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Soft X-ray line profiles in the impulsive phase of electron-heated solar flares

Solar Physics ◽

10.1007/bf00206425 ◽

1987 ◽

Vol 110 (2) ◽

Cited By ~ 27

Author(s):

A.Gordon Emslie ◽

David Alexander

Keyword(s):

Solar Flares ◽

Impulsive Phase ◽

Line Profiles ◽

X Ray

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Electron Beam as Origin of White-Light Solar Flares

International Astronomical Union Colloquium ◽

10.1017/s025292110003178x ◽

1989 ◽

Vol 104 (1) ◽

pp. 19-30

Author(s):

J. Aboudarham ◽

J. C. Henoux

Keyword(s):

Electron Beam ◽

White Light ◽

Solar Flares ◽

Light Emission ◽

Continuum Emission ◽

Continuum Radiation ◽

Photospheric Level ◽

Hydrogen Ionization ◽

Semi Empirical ◽

Flare Model

AbstractWe 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 photo-spheric 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.

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HYDRO2GEN: Non-thermal hydrogen Balmer and Paschen emission in solar flares generated by electron beams

Astronomy and Astrophysics ◽

10.1051/0004-6361/201731053 ◽

2018 ◽

Vol 610 ◽

pp. A68 ◽

Cited By ~ 7

Author(s):

M. K. Druett ◽

V. V. Zharkova

Keyword(s):

Radiative Transfer ◽

Electron Beams ◽

Inelastic Collisions ◽

Continuum Emission ◽

Strong Increase ◽

Line Profiles ◽

Hydrogen Line ◽

Hydrogen Atoms ◽

Transfer Equations ◽

Radiative Transfer Equations

Aim. Sharp rises of hard X-ray (HXR) emission accompanied by Hα line profiles with strong red-shifts up to 4 Å from the central wavelength, often observed at the onset of flares with the Specola Solare Ticinese Telescope (STT) and the Swedish Solar Telescope (SST), are not fully explained by existing radiative models. Moreover, observations of white light (WL) and Balmer continuum emission with the Interface Region Imaging Spectrograph (IRISH) reveal strong co-temporal enhancements and are often nearly co-spatial with HXR emission. These effects indicate a fast effective source of excitation and ionisation of hydrogen atoms in flaring atmospheres associated with HXR emission. In this paper, we investigate electron beams as the agents accounting for the observed hydrogen line and continuum emission. Methods. Flaring atmospheres are considered to be produced by a 1D hydrodynamic response to the injection of an electron beam defining their kinetic temperatures, densities, and macro velocities. We simulated a radiative response in these atmospheres using a fully non-local thermodynamic equilibrium (NLTE) approach for a 5-level plus continuum hydrogen atom model, considering its excitation and ionisation by spontaneous, external, and internal diffusive radiation and by inelastic collisions with thermal and beam electrons. Simultaneous steady-state and integral radiative transfer equations in all optically thick transitions (Lyman and Balmer series) were solved iteratively for all the transitions to define their source functions with the relative accuracy of 10−5. The solutions of the radiative transfer equations were found using the L2 approximation. Resulting intensities of hydrogen line and continuum emission were also calculated for Balmer and Paschen series. Results. We find that inelastic collisions with beam electrons strongly increase excitation and ionisation of hydrogen atoms from the chromosphere to photosphere. This leads to an increase in Lyman continuum radiation, which has high optical thickness, and after the beam is off it governs hydrogen ionisation and leads to the long lasting orders of magnitude enhancement of emission in Balmer and Paschen continua. The ratio of Balmer-to-other-continuum head intensities are found to be correlated with the initial flux of the beam. The height distribution of contribution functions for Paschen continuum emission indicate a close correlation with the observations of heights of WL and HXR emission reported for limb flares. This process also leads to a strong increase of wing emission (Stark’s wings) in Balmer and Paschen lines, which is superimposed on large red-shifted enhancements of Hα-Hγ line emission resulting from a downward motion by hydrodynamic shocks. The simulated line profiles are shown to fit closely the observations for various flaring events.

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The Expected behaviour of the Hydrogen Lyman Lines in Solar Flares

International Astronomical Union Colloquium ◽

10.1017/s0252921100089995 ◽

1972 ◽

Vol 14 ◽

pp. 824-824

Author(s):

Z. Švestka ◽

L.D. De Feiter

Keyword(s):

Electron Density ◽

Solar Flares ◽

Line Profiles ◽

Stark Broadening ◽

Hydrogen Atoms ◽

Measured Intensity ◽

Balmer Series ◽

Model Calculations ◽

Intensity Measurements

While the merging of the higher lines of the Balmer series emitted by solar flares is solely determined by the electron density, the merging of the high Lyman lines is determined both by the electron density, through Stark broadening of the line absorption coefficient, and by the total number of hydrogen atoms in the flare, through the effect of self-absorption. Preliminary results of model calculations indicate that two or more intensity measurements, each midway between two consecutive lines of the Lyman series (lines 3–4, 4–5, 5–6) allow the determination of the column density of hydrogen atoms in the ground state provided that the electron density is known. One can believe that in between the Lyman lines only the flare elements contribute to the measured intensity since excited interflare matter of much lower electron density produces line profiles of substantially smaller width. Thus the data in between the lines can be reasonably compared to the Ne values deduced from high members of the Balmer series.

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Space Observations Needed for Improving Our Knowledge of Solar Flares

Highlights of Astronomy ◽

10.1017/s1539299600001702 ◽

1968 ◽

Vol 1 ◽

pp. 530-532

Author(s):

A.B. Severny

Keyword(s):

Solar Flares ◽

Ultraviolet Spectrum ◽

Careful Examination ◽

Physical Parameters ◽

Spectral Interval ◽

Hydrogen Atoms ◽

Uv Spectrum ◽

Electronic Temperature ◽

Lyman Continuum ◽

Made In

Up to the present time the technical possibilities of space research were not quite adequate for obtaining the ultraviolet spectrum of flares that occupy only a small portion (less than 10″ of arc) of the solar disk. Up to now all observations have been made in integrated sunlight. The importance of the knowledge of UV spectrum of flares is hardly necessary to emphasize. For example we just could mention that if ultraviolet spectra were available for a flare say from 850 Å up to 2000 Å, we would be able to estimate such extremely important parameters as the number of hydrogen atoms N1 in the first quantum state (by using L-α, L-β, etc.), electronic temperature and density (from Lyman continuum) and other physical parameters of a flare which at the present time we try to derive by different indirect and inadequate methods. Several resonance (ultimate) lines are concentrated in this spectral interval and their careful examination in flares can bring additional important information about conditions prevailing in flares and in the underlying chromosphere. The same applies of course to the whole UV spectrum below 800 Å, and in particular to the resonance lines of HeI (λ 512 Å) and HeII (λ 304 Å), as well as to a number of lines of highly ionized atoms.

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Plasma heating in the initial phase of solar flares

Proceedings of the International Astronomical Union ◽

10.1017/s1743921309992791 ◽

2009 ◽

Vol 5 (S264) ◽

pp. 282-284

Author(s):

P. Rudawy ◽

M. Siarkowski ◽

R. Falewicz

Keyword(s):

Solar Flares ◽

Energy Transport ◽

Electron Beams ◽

Plasma Heating ◽

Impulsive Phase ◽

Thermal Electron ◽

Flare Loop ◽

X Ray ◽

Heating Source ◽

Rise Phase

AbstractIn this paper we analyze soft and hard X-ray emission of the 2002 September 20 M1.8 GOES class solar flare observed by RHESSI and GOES satellites, where soft X-ray emission precedes the onset of the main bulk hard X-ray emission by ~5 min. This suggests that an additional heating mechanism may be at work at the early beginning of the flare. However RHESSI spectra indicate presence of the non-thermal electrons also before impulsive phase. So, we assumed that a dominant energy transport mechanism during rise phase of solar flares is electron beam-driven evaporation. We used non-thermal electron beams derived from RHESSI spectra as the heating source in a hydrodynamic model of the analyzed flare. We showed that energy delivered by non-thermal electron beams is sufficient to heat the flare loop to temperatures in which it emits soft X-ray closely following the GOES 1–8 Å light-curve.

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Abrupt Changes of the Photospheric Magnetic Field in Active Regions and the Impulsive Phase of Solar Flares (Preprint)

10.21236/ada566133 ◽

2012 ◽

Author(s):

E. W. Cliver ◽

G. J. Petrie ◽

A. G. Ling

Keyword(s):

Magnetic Field ◽

Solar Flares ◽

Active Regions ◽

Impulsive Phase ◽

Photospheric Magnetic Field ◽

Abrupt Changes

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Hydrodynamic Models of Solar and Stellar Flares

International Astronomical Union Colloquium ◽

10.1017/s0252921100031948 ◽

1989 ◽

Vol 104 (1) ◽

pp. 289-298

Author(s):

Giovanni Peres

Keyword(s):

High Resolution ◽

Light Curve ◽

Solar Flares ◽

Impulsive Phase ◽

Light Curves ◽

Physical Parameters ◽

The Sun ◽

Hydrodynamic Models ◽

X Ray ◽

Stellar Flares

AbstractThis paper discusses the hydrodynamic modeling of flaring plasma confined in magnetic loops and its objectives within the broader scope of flare physics. In particular, the Palermo-Harvard model is discussed along with its applications to the detailed fitting of X-ray light curves of solar flares and to the simulation of high-resolution Caxix spectra in the impulsive phase. These two approaches provide complementary constraints on the relevant features of solar flares. The extension to the stellar case, with the fitting of the light curve of an X-ray flare which occurred on Proxima Centauri, demonstrates the feasibility of using this kind of model for stars too. Although the stellar observations do not provide the wealth of details available for the Sun, and, therefore, constrain the model more loosely, there are strong motivations to pursue this line of research: the wider range of physical parameters in stellar flares and the possibility of studying further the solar-stellar connection.

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Response of chromospheric lines to different periodic non-thermal electron beams

Proceedings of the International Astronomical Union ◽

10.1017/s1743921316000429 ◽

2015 ◽

Vol 11 (S320) ◽

pp. 239-242

Author(s):

Jianxia Cheng ◽

Mingde Ding

Keyword(s):

Electron Beam ◽

Solar Flares ◽

Local Thermodynamic Equilibrium ◽

Response Times ◽

Electron Beams ◽

Lower Atmosphere ◽

Mass Motion ◽

Thermal Electron ◽

Hydrodynamic Simulations ◽

Beam Injection

AbstractSolar flares produce radiations in very broad wavelengths. Spectra can supply us abundant information about the local plasma, such as temperature, density, mass motion and so on. Strong chromospheric lines, like the most studied Hα and Ca II 8542 Å lines are formed under conditions of departures from local thermodynamic equilibrium in the lower atmosphere subject to flare heating. Understanding how these lines are formed is very useful for us to correctly interpret the observations. In this paper, we try to figure out the response of chromospheric lines heated by different periodic non-thermal electron beams. Our results are based on radiative hydrodynamic simulations. We vary the periods of electron beam injection from 1.25 s to 20 s. We compare the response times to different heating parameters. Possible explanations are discussed.

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