scholarly journals Hydrodynamic Models of Solar and Stellar Flares

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.

X-ray Spectral Synthesis in Hydrodynamic Flare Models

1990 ◽  
Vol 115 ◽  
pp. 126-131
Author(s):  
S. Serio ◽  
E. Antonucci ◽  
M.A. Dodero ◽  
G. Peres ◽  
F. Reale
Keyword(s):  
Solar Flares ◽  
Energy Deposition ◽  
Electron Beams ◽  
Spectral Synthesis ◽  
Theoretical Models ◽  
Hydrodynamic Models ◽  
X Ray ◽  
Relativistic Electron Beams ◽  
Stellar Flares ◽  
Magnetically Confined

AbstractCompact solar flares are triggered by sudden energy release in magnetically confined plasma. This class of flares is well suited to be studied with numerical hydrodynamic models. In particular, one can compare the evolution of observed and synthetic X-ray spectra, computed under various assumptions for the mechanism of impulsive energy deposition, to constrain theoretical models and their parameter space. We discuss recent results on solar flares along this line, non thermal to models of energy depositions by relativistic electron beams. We shall also discuss possible applications of X-ray spectral synthesis to stellar flares.

scholarly journals Preflare very long-periodic pulsations observed in Hα emission before the onset of a solar flare

2020 ◽  
Vol 639 ◽  
pp. L5
Author(s):  
Dong Li ◽  
Song Feng ◽  
Wei Su ◽  
Yu Huang
Keyword(s):  
Solar Flare ◽  
Light Curve ◽  
Solar Flares ◽  
Extreme Ultraviolet ◽  
Light Curves ◽  
Trigger Mechanism ◽  
X Ray ◽  
Hα Emission ◽  
Plasma Loop ◽  
Full Disk

Context. Very long-periodic pulsations during preflare phases (preflare-VLPs) have been detected in the full-disk solar soft X-ray (SXR) flux. They may be regarded as precursors to solar flares and may help us better understand the trigger mechanism of solar flares. Aims. In this Letter, we report a preflare-VLP event prior to the onset of an M1.1 circular-ribbon flare on 2015 October 16. It was simultaneously observed in Hα, SXR, and extreme ultraviolet (EUV) wavelengths. Methods. The SXR fluxes in 1−8 Å and 1−70 Å were recorded by the Geostationary Operational Environmental Satellite (GOES) and Extreme Ultraviolet Variability Experiment, respectively; the light curves in Hα and EUV 211 Å were integrated over a small local region, which were measured by the 1 m New Vacuum Solar Telescope and the Atmospheric Imaging Assembly (AIA), respectively. The preflare-VLP is identified as the repeat and quasi-periodic pulses in light curves during preflare phase. The quasi-periodicity can be determined from the Fourier power spectrum with Markov chain Monte Carlo-based Bayesian. Results. Seven well-developed pulses are found before the onset of an M1.1 circular-ribbon flare. They are firstly seen in the local light curve in Hα emission and then discovered in full-disk SXR fluxes in GOES 1−8 Å and ESP 1−70 Å, as well as the local light curve in AIA 211 Å. These well-developed pulses can be regarded as the preflare-VLP, which might be modulated by LRC-circuit oscillation in the current-carrying plasma loop. The quasi-period is estimated to be ∼9.3 min. Conclusions. We present the first report of a preflare-VLP event in the local Hα line and EUV wavelength, which could be considered a precursor of a solar flare. This finding should therefore prove useful for the prediction of solar flares, especially for powerful flares.

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 Balmer and soft X-ray emission from solar and stellar flares

1990 ◽  
Vol 137 ◽  
pp. 153-157
Author(s):  
C. J. Butler
Keyword(s):  
Solar Flares ◽  
The Sun ◽  
X Ray ◽  
Stellar Flares ◽  
Basic Similarity ◽  
Good Agreement

Integrated soft X-ray (8-12A) fluxes for solar flares have been scaled to the equivalent EXOSAT fluxes using spectra obtained from a variety of rocket-based experiments. The data show good agreement with the soft X-ray - Hγ correlation established by Butler et al. (1988) for stellar flares and confirm the basic similarity, in this respect, of flares on the Sun to those on dMe stars.

scholarly journals The Magnetic Corona: Magnetic Reconnection in Solar Flares

2004 ◽  
Vol 219 ◽  
pp. 91-102
Author(s):  
Harry P. Warren
Keyword(s):  
Spatial Resolution ◽  
Solar Flares ◽  
Light Curves ◽  
Small Scale ◽  
Flare Loop ◽  
X Ray ◽  
Stellar Flares ◽  
Emission Models ◽  
Reconnection Model ◽  
High Cadence

The ability of the Transition Region and Coronal Explorer (TRACE) to image the Sun at high spatial resolution and high cadence over a very broad range of temperatures makes it a unique instrument for observing solar flare plasma. TRACE observations have confirmed the reconnection model for solar flares, at least qualitatively. TRACE flare observations show impulsive footpoint brightenings that are followed by the formation of high-temperature loops in the corona. These loops then cool to lower temperatures, forming post-flare loop arcades. Comparisons between TRACE and lower spatial resolution Yohkoh Soft X-Ray Telescope (SXT) observations have revealed that solar flares are composed of a multitude of fine coronal loops. Detailed hydrodynamic modeling of flare light curves shows that this fine scale structuring is crucial to understanding the evolution of the observed emission. Models based on single, isothermal loops are not consistent with the TRACE observations. Models based on the sequential heating of small-scale loops, in contrast, are able to reproduce many of the salient features of the observed light curves. We will discuss the implication of these results for more energetic stellar flares as well as smaller-scale events that may be responsible for the heating of solar active region loops.

scholarly journals Ultraviolet Observations of Stellar Coronae: Early Results from HST

1992 ◽  
Vol 9 ◽  
pp. 657-658
Author(s):  
J.L. Linsky
Keyword(s):  
High Resolution ◽  
Spectral Resolution ◽  
Solar Flares ◽  
Extreme Ultraviolet ◽  
The Sun ◽  
X Ray ◽  
Spectroscopic Diagnostics ◽  
Early Results ◽  
Active Stars ◽  
Ultraviolet Observations

Although coronae for stars other than the Sun have previously been detected only in the X-ray and radio portions of the spectrum, the HST and future spacecraft sensitive to ultraviolet (UV) and extreme ultraviolet (ETIV) light will have the spectral resolution to study the dynamics and spectroscopic diagnostics of hot coronal plasmas. In the UV region accessible to HST, forbidden lines of FeXII at 1242 and 1349Å, of FeXXI at 1354Å, and other species seen in solar flares, are predicted to be present in the spectra of active stars. Upcoming observations with the Goddard High Resolution Spectrograph (GHRS) by S. Maran will search for these lines in the dM2e star AU Mic and other stars.

scholarly journals Physical Processes During Impulsive Solar and Stellar Flares

1995 ◽  
Vol 151 ◽  
pp. 176-184 ◽  
Author(s):  
Maria Katsova ◽  
Moissei Livshits
Keyword(s):  
Line Emission ◽  
Indirect Evidence ◽  
Impulsive Phase ◽  
Balmer Line ◽  
Time Interval ◽  
The Sun ◽  
Radio Bursts ◽  
X Ray ◽  
Stellar Flares ◽  
Optical Continuum

Investigations of impulsive flares on both the Sun and red dwarf stais during more than 30 years allow us to arrive at quite definite conclusions. Here we will consider impulsive events; on the Sun the impulsive phase of a flare is observed as a hard X-ray burst with the emission of photons with energies E > 30 keV up to the γ-ray range. At the same time microwave radio bursts, and sometimes UV and optical continuum bursts are registered. Typical durations of these processes are ∼l-3 min. In this time interval other kinds of flare emission like soft X-ray (2-10 keV) emission, meter radio bursts and Balmer line emission begin to rise, but their maxima occur later on, in the gradual (thermal) phase of the flare.Impulsive stellar flares are often observed as a significant increase in optical continuum, especially in the U-band, of similar duration (1-3 min), and this time interval is, like in the solar case, the rise phase of the soft X-ray emission.Modern observations demonstrate that both the impulsive phase of a flare or an impulsive flare develops in low-lying loops. Earlier only indirect evidence existed in optical and radio data. Recently, however, the heights of the hard X-ray sources in impulsive solar events were determined directly from YOHKOH’s HXT (Kosugi 1994, Masuda 1994) (Fig. 1a). Statistically, the height of the hard X-ray source in the 14-23 keV range is 9700 ± 2000 km above the photosphere, and this height reduces to 6500 km in the 53-93 keV range. Besides two hard X-ray sources in the loop footpoints, a third hard X-ray source exists at the top of the loop at least in some cases. The authors of this experiment suppose that the appearance of this loop-top source is due to reconnection in the impulsive phase. Note that the reconnection begins close to the apex of the loop, when this loop is filled by hot plasma that evaporated from both footpoints.

scholarly journals Coronal Response to Energy Release During Solar Flares

1986 ◽  
Vol 7 ◽  
pp. 731-738
Author(s):  
Ester Antonucci
Keyword(s):  
High Resolution ◽  
Energy Release ◽  
Solar Flares ◽  
Solar Maximum ◽  
Impulsive Phase ◽  
Primary Energy ◽  
Release Process ◽  
X Ray ◽  
The Past ◽  
Flare Region

AbstractThe observations obtained with high resolution spectrometers flown in the past solar maximum, in the years 1979-1981, have shown that the soft x-ray plasma during the impulsive phase of solar flares is characterized by upflows. as well as by isotropic flows, at velocities of a few hundred km s-1. Isotropic flows can be directly related to the primary energy release process. While, upflows are an indication of chromospheric material heated to coronal temperatures and evaporating along the magnetic fluxtubes; they are therefore related to the development of the coronal soft x-ray source in the flare region.

scholarly journals Solar extreme ultraviolet (EUV) flare observations and findings from the Solar Dynamics Observatory (SDO) EUV Variability Experiment (EVE)

2015 ◽  
Vol 11 (S320) ◽  
pp. 27-40
Author(s):  
Thomas N. Woods ◽  
Francis G. Eparvier ◽  
James P. Mason
Keyword(s):  
Extreme Ultraviolet ◽  
Late Phase ◽  
Impulsive Phase ◽  
The Sun ◽  
Solar Dynamics Observatory ◽  
X Ray ◽  
Gradual Phase ◽  
Stellar Flares ◽  
Solar Dynamics ◽  
Coronal Dimming

AbstractNew solar soft X-ray (SXR) and extreme ultraviolet (EUV) irradiance observations from NASA Solar Dynamics Observatory (SDO) EUV Variability Experiment (EVE) provide full coverage from 0.1 to 106 nm and continuously at a cadence of 10 seconds for spectra at 0.1 nm resolution. These observations during flares can usually be decomposed into four distinct characteristics: impulsive phase, gradual phase, coronal dimming, and EUV late phase. Over 6000 flares have been observed during the SDO mission; some flares show all four phases, and some only show the gradual phase. The focus is on the newer results about the EUV late phase and coronal dimming and its relationship to coronal mass ejections (CMEs). These EVE flare measurements are based on observing the sun-as-a-star, so these results could exemplify stellar flares. Of particular interest is that new coronal dimming measurements of stars could be used to estimate mass and velocity of stellar CMEs.

scholarly journals Detection of Flare Multiperiodic Pulsations in Mid-ultraviolet Balmer Continuum, Lyα, Hard X-Ray, and Radio Emissions Simultaneously

2021 ◽  
Vol 921 (2) ◽  
pp. 179
Author(s):  
Dong Li ◽  
Mingyu Ge ◽  
Marie Dominique ◽  
Haisheng Zhao ◽  
Gang Li ◽  
...  
Keyword(s):  
Low Frequency ◽  
Impulsive Phase ◽  
Light Curves ◽  
Release Process ◽  
X Ray ◽  
Nonthermal Electrons ◽  
Stellar Flares ◽  
Hot Plasma ◽  
Generation Mechanisms ◽  
Powerful Flare

Abstract Quasi-periodic pulsations (QPPs), which usually appear as temporal pulsations of the total flux, are frequently detected in the light curves of solar/stellar flares. In this study, we present the investigation of nonstationary QPPs with multiple periods during the impulsive phase of a powerful flare on 2017 September 6, which were simultaneously measured by the Hard X-ray Modulation Telescope (Insight-HXMT), as well as the ground-based BLENSW. The multiple periods, detected by applying a wavelet transform and Lomb–Scargle periodogram to the detrended light curves, are found to be ∼20–55 s in the Lyα and mid-ultraviolet Balmer continuum emissions during the flare impulsive phase. Similar QPPs with multiple periods are also found in the hard X-ray emission and low-frequency radio emission. Our observations suggest that the flare QPPs could be related to nonthermal electrons accelerated by the repeated energy release process, i.e., triggering of repetitive magnetic reconnection, while the multiple periods might be modulated by the sausage oscillation of hot plasma loops. For the multiperiodic pulsations, other generation mechanisms could not be completely ruled out.

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