scholarly journals Radiation Hydrodynamics in Solar Flares

1986 ◽  
Vol 89 ◽  
pp. 53-74
Author(s):  
George H. Fisher
Keyword(s):  
Heating Rate ◽  
Transition Region ◽  
Solar Flares ◽  
Thermal Model ◽  
Thermal Conduction ◽  
Thick Target ◽  
Pressure Increase ◽  
Electromagnetic Spectrum ◽  
X Ray ◽  
Upper Limit

AbstractSolar flares are currently understood as the explosive release of energy stored in the form of stressed magnetic fields. In many cases, the released energy seems to take the form of large numbers of electrons accelerated to high energies (the nonthermal electron “thick target” model), or alternatively plasma heated to very high temperatures behind a rapidly moving conduction front (the “thermal” model). The transport of this energy into the remaining portion of the atmosphere results in violent mass motion and strong emission across the electromagnetic spectrum. Radiation processes play a crucial role in determining the ensuing plasma motion.One important phenomenon observed during flares is the appearance in coronal magnetic loops of large amounts of upflowing, soft X-ray emitting plasma at temperatures of 1−2×107 [K]. It is believed that this is due to chromospheric evaporation, the process of heating cool (T - 104[K]) chromospheric material beyond its ability to radiate. Detailed calculations of thick target heating show that if nonthermal electrons heat the chromosphere directly, then the evaporation process can result in explosive upward motion of X-ray emitting plasma if the heating rate exceeds a threshold value. In such a case, upflow velocities approach an upper limit of roughly 2.35 cs as the heating rate is increased beyond the threshold, where cs is the sound speed in the evaporated plasma. This is known as explosive evaporation. If the flare heating rate is less than the threshold, evaporation takes place indirectly through thermal conduction of heat deposited in the corona by the energetic electrons. Upflows in this case are roughly 10 to 20% of the upper limit. Evaporation by thermal model heating always takes place through thermal conduction, and the computed upflow speeds seem to be about 10% to 20% of the upper limit, independent of the energy flux.The pressure increase in the evaporated plasma for either the thick target or thermal model leads to a number of interesting phenomena in the flare chromosphere. The sudden pressure increase initiates a downward moving “chromospheric condensation”, an overdense region which gradually decelerates as it accretes material and propagates into the gravitationally stratified chromosphere. Solutions to an equation of motion for this condensation shows that its motion decays after about one minute of propagation into the chromosphere. When the front of this downflowing region is supersonic relative to the atmosphere ahead of it, a radiating shock will form. If the downflow is rapid enough, the shock strength should be sufficient to excite UV radiation normally associated with the transition region, and furthermore, the radiating shock will be brighter than the transition region. These results lead to a number of observationally testable relationships between the optical and ultraviolet spectra from the condensation and radiating shock.

Electron Acceleration in Collapsing Magnetic Traps during the Solar Flare on July 19, 2012: Observations and Models

2017 ◽  
Vol 13 (S335) ◽  
pp. 90-93
Author(s):  
P. A. Gritsyk ◽  
B. V. Somov
Keyword(s):  
Point Source ◽  
Solar Flares ◽  
High Efficiency ◽  
Reverse Current ◽  
Electron Acceleration ◽  
Thick Target ◽  
Two Dimensions ◽  
Magnetic Traps ◽  
Thin Target ◽  
X Ray

AbstractUsing the appropriate kinetic equation, we considered the problem of propagation of accelerated electrons into the solar corona and chromosphere. Its analytical solution was used for modelling the M7.7 class limb flare occurred on July 19, 2012. Coronal above-the-loop-top hard X-Ray source was interpreted in the thin-target approximation, the foot-point source - in the thick-target approximation with account of the reverse-current electric field. For the foot-point source we found a good accordance with the RHESSI observations. For the coronal source we also got very accurate estimate of the power-law spectral index, but significant differences between the modelled and observed hard X-ray intensities were noticed. The last discrepancy was solved by adding the coronal magnetic trap model to the thin target model. The former one implies that the trap collapses in two dimensions, locks and accelerates particles inside itself. In our report, we confirm an existence and high efficiency of the electron acceleration in collapsing magnetic traps during solar flares. Our new results represent (e.g. for RHESSI observations) the theoretical prediction of the double step particle acceleration in solar flares, when the first step is the acceleration in reconnection area and the second one – the acceleration in coronal trap.

scholarly journals Flares on the Sun: Selected Results from SMM

1983 ◽  
Vol 71 ◽  
pp. 289-305
Author(s):  
G.M. Simnett
Keyword(s):  
Solar Flares ◽  
Solar Maximum ◽  
Electromagnetic Spectrum ◽  
Significant Progress ◽  
X Ray ◽  
Spatially Resolved ◽  
Red Dwarfs ◽  
Important Diagnostic Tool ◽  
Flare Stars ◽  
Made In

Observationally the study of solar flares has reached the stage where intensity-time distributions of emission over broad and resolved regions of the electromagnetic spectrum are obtained for spatially resolved parts of the flare. Polarization measurements add an important diagnostic tool in some wavebands but we shall not report on these here. In the optical band good ground based observations have been available for many years, whereas in the UV, soft X-ray and hard X-ray (> 5 keV) bands recent spacecraft have greatly extended the data base. Good high resolution maps are being made in the microwave region with the ground based VIA. We are now at the point where significant progress into understanding the flare problem has been made, and will continue to be made, during the current solar maximum. This coincides with the development of soft X-ray instruments sensitive enough to detect transient and quiescent emission from flare stars, particularly red dwarfs in the solar neighbourhood (e.g. Kahn et al,1979, Haisch et al, 1980) which previously had only been detected in the optical and radio wavebands.

scholarly journals Simultaneous Kepler/K2 and XMM-Newton observations of superflares in the Pleiades

2019 ◽  
Vol 622 ◽  
pp. A210 ◽  
Author(s):  
M. G. Guarcello ◽  
G. Micela ◽  
S. Sciortino ◽  
J. López-Santiago ◽  
C. Argiroffi ◽  
...  
Keyword(s):  
Time Evolution ◽  
Energy Budget ◽  
Solar Flares ◽  
Light Curves ◽  
Integrated Analysis ◽  
Electromagnetic Spectrum ◽  
The Sun ◽  
X Rays ◽  
X Ray ◽  
Different Ages

Context. Flares are powerful events ignited by a sudden release of magnetic energy which triggers a cascade of interconnected phenomena, each resulting in emission in different electromagnetic bands. In fact, in the Sun flares are observed across the whole electromagnetic spectrum. Multi-band observations of stellar flares are instead rare. This limits our ability to extend what we learn from solar flares to the case of flares occurring in stars with different properties. Aims. With the aim of studying flares in the 125-Myr-old stars in the Pleiades observed simultaneously in optical and X-ray light, we obtained new XMM-Newton observations of this cluster during the observations of Kepler K2 Campaign 4. The objective of this paper is to characterize the most powerful flares observed in both bands and to constrain the energy released in the optical and X-ray, the geometry of the loops, and their time evolution. We also aim to compare our results to existing studies of flares occurring in the Sun and stars at different ages. Methods. We selected bright X-ray/optical flares that occurred in 12 known members of the Pleiades from their K2 and XMM-Newton light curves. The sample includes ten K-M stars, one F9 star, and one G8 star. Flare average properties were obtained from integrated analysis of the light curves during the flares. The time evolution of the plasma in the magnetic loops is constrained with time-resolved X-ray spectral analysis. Results. Most of the flares studied in this work emitted more energy in optical than in X-rays, as in most solar flares, even if the Pleiades flares output a larger fraction of their total energy in X-rays than typical solar flares do. Additionally, the energy budget in the two bands is weakly correlated. We also found comparable flare duration in optical and X-rays and observed that rapidly rotating stars (e.g., with rotation period shorter than 0.5 days) preferentially host short flares. We estimated the slope of the cooling path of the flares in the log(EM)-vs.-log(T) plane. The values we obtained are affected by large uncertainties, but their nominal values suggest that the flares analyzed in this paper are mainly due to single loops with no sustained heating occurring during the cooling phase. We also observed and analyzed oscillations with a period of 500 s during one of the flares. Conclusions. The flares observed in the Pleiades can be classified as “superflares” based on their energy budget in the optical, and share some of the properties of the flares observed in the Sun, despite being more energetic. For instance, as in most solar flares, more energy is typically released in the optical than in X-rays and the duration of the flares in the two bands is correlated. We have attempted a comparison between the X-ray flares observed in the Pleiades and those observed in clusters with different ages, but to firmly address any evolutionary pattern of flare characteristics, similar and uniform multi-wavelength analyses on more complete samples are necessary.

scholarly journals Return Current Instability in Flares

1989 ◽  
Vol 104 (2) ◽  
pp. 313-316
Author(s):  
D. Cromwell ◽  
P. McQuillan ◽  
J.C. Brown
Keyword(s):  
Solar Flares ◽  
Small Area ◽  
Thick Target ◽  
Target Electron ◽  
Return Current ◽  
Ion Acoustic Wave ◽  
X Ray ◽  
Acoustic Wave Generation ◽  
Bremsstrahlung Emission ◽  
Current Instability

AbstractWe consider the problem of ion-acoustic wave generation, and resultant anomalous Joule heating, by a return current driven unstable by a small-area thick-target electron beam in solar flares. We find that, contrary to the usual assumption, the hard X-ray bremsstrahlung emission may actually be enhanced in comparison to conventional thick-target models. This present paper is a summary of the work of Cromwell, McQuillan and Brown (1988).

Turbulence driven by chromospheric evaporations in solar flares

2021 ◽  
Author(s):  
Wenzhi Ruan ◽  
Chun Xia ◽  
Rony Keppens
Keyword(s):  
Numerical Simulation ◽  
Standing Wave ◽  
Solar Flares ◽  
Fast Electron ◽  
Thermal Conduction ◽  
High Speed ◽  
Helmholtz Instability ◽  
Flare Loop ◽  
X Ray ◽  
Flare Loops

<p>Chromospheric evaporations are frequently observed at the footpoints of flare loops in flare events. The evaporations flows driven by thermal conduction or fast electron deposition often have high speed of hundreds km/s. Since the speed of the observed evaporation flows is comparable to the local Alfven speed, it is reasonable to consider the triggering of Kelvin-Helmholtz instabilities. Here we revisit a scenario which stresses the importance of the Kelvin-Helmholtz instability (KHI) proposed by Fang et al. (2016). This scenario suggests that evaporations flows from two footpoints of a flare loop can meet each other at the looptop and produce turbulence there via KHI. The produced KHI turbulence can play important roles in particle accelerations and generation of strong looptop hard X-ray sources. We investigate whether evaporation flows can produce turbulence inside the flare loop with the help of numerical simulation. KHI turbulence is successfully produced in our simulation. The synthesized soft X-ray curve demonstrating a clear quasi-periodic pulsation (QPP) with period of 26 s. The QPP is caused by a locally trapped, fast standing wave that resonates in between KHI vortices.</p><div></div><div></div><div></div><div></div><div></div><div></div><div></div><div></div><div></div><div></div>

scholarly journals Spectroscopic Diagnostics of the UV Emitting Plasmas in Solar Flares Observed from SMM

1984 ◽  
Vol 86 ◽  
pp. 147-150 ◽  
Author(s):  
Chung-Chieh Cheng ◽  
E. Tandberg-Hanssen
Keyword(s):  
Transition Region ◽  
Solar Flares ◽  
Transport Processes ◽  
Spatial Correlations ◽  
Dynamic Changes ◽  
X Ray ◽  
Physical Conditions ◽  
Uv Emission ◽  
Temporal And Spatial ◽  
Ultraviolet Spectrometer

The transition region exhibits dynamic changes during solar flares. To understand the energy release and transport processes in solar flares, it is important to determine observationally the physical conditions in the flare transition region plasmas, which emit in the UV wavelength ranges. During the Solar Maximum Mission (SMM) we observed many flares in the UV emission lines of Si IV (1402 Å) and 0 IV (1401 Å) using the Ultraviolet Spectrometer and Polarimeter (Woodgate et al. 1983). In this paper we present the Si IV/O IV flare observations, in particular the dynamic evolution of the transition zone plasmas (~ 105 K). We also studied the temporal and spatial correlations between the impulsive UV and hard X-ray bursts.

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