X-rays and extreme ultraviolet (EUV) emissions from solar flares rapidly change the physical composition of the Earth’s thermosphere and ionosphere, thereby causing space weather phenomena such as ...communication failures. Numerous empirical and physical models have been developed to estimate the effects of flare emissions on the Earth’s upper atmosphere. We verified the reproduction of the flare emission spectra using a one-dimensional hydrodynamic calculation and the CHIANTI atomic database. To validate the proposed model, we used the observed EUV spectra obtained by the Extreme ultraviolet variability experiment (EVE) on board the Solar Dynamics Observatory (SDO). We examined the “EUV flare time-integrated irradiance” and “EUV flare line rise time” of the EUV emissions for 21 events by comparing the calculation results of the proposed model and observed EUV spectral data. The proposed model successfully reproduced the EUV flare time-integrated irradiance of the Fe VIII 131 Å, Fe XVIII 94 Å, and Fe XX133 Å, as well as the 55–355 Å and 55–135 Å bands. For the EUV flare line rise time, there was an acceptable correlation between the proposed model estimations and observations for all Fe flare emission lines. These results demonstrate that the proposed model can reproduce the EUV flare emission spectra from the emitting plasma with a relatively high formation temperature. This indicates that the physics-based model is effective for the accurate reproduction of the EUV spectral irradiance.
Abstract
We conduct a wide-band X-ray spectral analysis in the energy range of 1.5–100 keV to study the time evolution of the M7.6-class flare of 2016 July 23, with the Miniature X-ray Solar ...Spectrometer (MinXSS) CubeSat and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft. With the combination of MinXSS for soft X-rays and RHESSI for hard X-rays, a nonthermal component and three-temperature multithermal component—“cool” (
T
≈ 3 MK), “hot” (
T
≈ 15 MK), and “superhot” (
T
≈ 30 MK)—were measured simultaneously. In addition, we successfully obtained the spectral evolution of the multithermal and nonthermal components with a 10 s cadence, which corresponds to the Alfvén timescale in the solar corona. We find that the emission measures of the cool and hot thermal components are drastically increasing more than hundreds of times and the superhot thermal component is gradually appearing after the peak of the nonthermal emission. We also study the microwave spectra obtained by the Nobeyama Radio Polarimeters, and we find that there is continuous gyrosynchrotron emission from mildly relativistic nonthermal electrons. In addition, we conducted a differential emission measure (DEM) analysis by using Atmospheric Imaging Assembly on board the Solar Dynamics Observatory and determined that the DEM of cool plasma increases within the flaring loop. We find that the cool and hot plasma components are associated with chromospheric evaporation. The superhot plasma component could be explained by the thermalization of the nonthermal electrons trapped in the flaring loop.
In addition to X-rays, extreme ultraviolet (EUV) rays radiated from solar flares can cause serious problems, such as communication failures and satellite drag. Therefore, methods for forecasting EUV ...dynamic spectra during flares are urgently required. Recently, however, owing to the lack of instruments, EUV dynamic spectra have rarely been observed. Hence, we develop a new method that converts the soft X-ray light curve observed during large flare events into an EUV dynamic spectrum by using the Solar Dynamics Observatory/Atmospheric Imaging Assembly images, a numerical simulation, and atomic database. The simulation provides the solution for a coronal loop that is heated by a strong flare, and the atomic database calculates its dynamic spectrum, including X-ray and EUV irradiances. The coefficients needed for the conversion can be calculated by comparing the observed soft X-ray light curve with that of the simulation. We apply our new method to three flares that occurred in the active region 12673 on September 06, 2017. The results show similarities to those of the Flare Irradiance Spectral Model, and reconstruct some of the EUV peaks observed by the EUV Variability Experiment onboard the Solar Dynamics Observatory.
•A new method to convert a soft X-ray light curve into an EUV dynamic spectrum during flares is presented.•The conversion can be calculated via a numerical simulation and an atomic database.•It succeeds in reconstructing some EUV peaks in the observed dynamic spectrum.
Abstract
We performed statistical analysis on the flare emission data to examine parameters related to the flare extreme-ultraviolet (EUV) spectra. This study used the data from the Geostationary ...Operational Environmental Satellite X-ray Sensors to determine the fundamental flare parameters. The relationship between soft X-ray data and EUV emission data observed by the Extreme Ultraviolet Variability Experiment on board the Solar Dynamics Observatory (SDO) MEGS-A was investigated for 50 events. The results showed the hotter Fe line emissions have strong correlation with soft X-ray data in many cases. However, our statistical study revealed that EUV flare peak flux of Fe
xv
, Fe
xvi
and He
ii
lines have weak correlation with soft X-ray peak flux. In EUV line light curves, there was time difference in peak time, however the tendency to reach the peak in order from the hotter line to cooler line was not so clear. These results indicate that the temporal evolution of EUV emission can be roughly explained by soft X-ray data. However, the time changes of temperature and density distributions in the flare loop must be needed for accurate reproduction. Moreover, we compared the geometrical features of solar flares observed by the Atmospheric Imaging Assembly on board the SDO with the fundamental flare parameters for 32 events. The ribbon distance strongly correlated with both soft X-ray flare rise and decay times. This results indicate that the geometrical feature is essential parameter for predicting flare emission duration.
Abstract
We performed coordinated observations of AR 12205, which showed a C-class flare on 2014 November 11, with the Interface Region Imaging Spectrograph (IRIS) and the Domeless Solar Telescope ...(DST) at Hida Observatory. Using spectral data in the Si iv 1403 Å, C ii 1335 Å, and Mg ii h and k lines from IRIS and the Ca ii K, Ca ii 8542 Å, and Hα lines from DST, we investigated a moving flare kernel during the flare. In the Mg ii h line, the leading edge of the flare kernel showed an intensity enhancement in the blue wing and a smaller intensity of the blue-side peak (h2v) than that of the red-side one (h2r). The blueshift lasted for 9–48 s with a typical speed of 10.1 ± 2.6 km s−1, which was followed by a high intensity and a large redshift with a speed of up to 51 km s−1 detected in the Mg ii h line. The large redshift was a common property for all six lines, but the blueshift prior to it was found only in the Mg ii lines. Cloud modeling of the Mg ii h line suggests that the blue-wing enhancement with such a peak difference could have been caused by a chromospheric-temperature (cool) upflow. We discuss a scenario in which an upflow of cool plasma is lifted up by expanding hot plasma owing to the deep penetration of non-thermal electrons into the chromosphere. Furthermore, we found that the blueshift persisted without any subsequent redshift in the leading edge of the flare kernel during its decaying phase. The cause of such a long-lasting blueshift is also discussed.
The active region NOAA 11283 produced two X-class flares on 6 and 7 September 2011 that have been well studied by many authors. The X2.1 class flare occurred on September 6, 2011 and was associated ...with the first of two homologous white light flares produced by this region, but no sunquake was found with it despite the one being detected in the second flare of 7 September 2011. In this paper we present the first observation of a sunquake for the 6 September 2011 flare detected via statistical significance analysis of egression power and verified via directional holography and time–distance diagram. The surface wavefront exhibits directional preference in the north-west direction We interpret this sunquake and the associated flare emission with a combination of a radiative hydrodynamic model of a flaring atmosphere heated by electron beam and a hydrodynamic model of acoustic wave generation in the solar interior generated by a supersonic shock. The hydrodynamic model of the flaring atmosphere produces a hydrodynamic shock travelling with supersonic velocities toward the photosphere and beneath. For the first time we derive velocities (up to 140 km s−1) and onset time (about 50 s after flare onset) of the shock deposition at given depths of the interior. The shock parameters are confirmed by the radiative signatures in hard X-rays and white light emission observed from this flare. The shock propagation in the interior beneath the flare is found to generate acoustic waves elongated in the direction of shock propagation, that results in an anisotropic wavefront seen on the solar surface. Matching the detected seismic signatures on the solar surface with the acoustic wave front model derived for the simulated shock velocities, we infer that the shock has to be deposited under an angle of about 30° to the local solar vertical. Hence, the improved seismic detection technique combined with the double hydrodynamic model reported in this study opens new perspectives for observation and interpretation of seismic signatures in solar flares.
Measuring abundance of iron-group elements (IGEs; Cr, Mn, Fe, and Ni) is essential for understanding astrophysical phenomena such as supernovae. The L-shell emissions from IGEs will soon be resolved ...by next-generation X-ray satellites, enabling more accurate measurements of abundance. However, theoretical calculations for these L-shell transitions have not been sufficiently validated using experimental work. Herein, large helical device (LHD) experiments that measured L-shell emission from Ni and Mn are reported. The temporal evolution of LHD plasma is characterized by three phases, each of which resolves different L-shell emissions. We modeled EUV-Short spectra considering LHD plasma structure with AtomDB, an atomic database widely used in the X-ray astronomy community. We discovered that the observed intensity ratios of the 3C (2p53d1 1P1 → 2p6 1S0) to 3D (2p53d1 3D1 → 2p6 1S0) transition of Ne-like ions are 0.4 - 0.6 times lower than the theoretically predicted ratios.
We attempted to reproduce the total electron content (TEC) variation in the Earth's atmosphere from the temporal variation of the solar flare spectrum of the X9.3 flare on September 6, 2017. The ...flare spectrum from the Flare Irradiance Spectral Model (FISM), and the flare spectrum from the 1D hydrodynamic model, which considers the physics of plasma in the flare loop, are used in the GAIA model, which is a simulation model of the Earth's whole atmosphere and ionosphere, to calculate the TEC difference. We then compared these results with the observed TEC. When we used the FISM flare spectrum, the difference in TEC from the background was in a good agreement with the observation. However, when the flare spectrum of the 1D-hydrodynamic model was used, the result varied depending on the presence or absence of the background. This difference depending on the models is considered to represent which extreme ultraviolet (EUV) radiation is primarily responsible for increasing TEC. From the flare spectrum obtained from these models and the calculation result of TEC fluctuation using GAIA, it is considered that the enhancement in EUV emission by approximately 15
–
35 nm mainly contributes in increasing TEC rather than that of X-ray emission, which is thought to be mainly responsible for sudden ionospheric disturbance. In addition, from the altitude/wavelength distribution of the ionization rate of Earth's atmosphere by GAIA (Ground-to-topside Atmosphere and Ionosphere model for Aeronomy), it was found that EUV radiation of approximately 15
–
35 nm affects a wide altitude range of 120
–
300 km, and TEC enhancement is mainly caused by the ionization of nitrogen molecules.
The X-rays and extreme ultraviolet (EUV) emitted during solar flares can rapidly change the physical composition of Earth’s ionosphere, causing space weather phenomena. It is important to develop an ...accurate understanding of solar flare emission spectra to understand how it affects the ionosphere. We reproduced the entire solar flare emission spectrum using an empirical model and physics-based model, and input it into the Earth’s atmospheric model, GAIA to calculate the total electron content (TEC) enhancement due to solar flare emission. We compared the statistics of nine solar flare events and calculated the TEC enhancements with the corresponding observed data. The model used in this study was able to estimate the TEC enhancement due to solar flare emission with a correlation coefficient greater than 0.9. The results of this study indicate that the TEC enhancement due to solar flare emission is determined by soft X-ray and EUV emission with wavelengths shorter than 35 nm. The TEC enhancement is found to be largely due to the change in the soft X-ray emission and EUV line emissions with wavelengths, such as Fe XVII 10.08 nm, Fe XIX 10.85 nm and He II 30.38 nm.
Graphical Abstract
The heating of the solar chromosphere and corona is a long-standing puzzle in solar physics. Hinode observations show the ubiquitous presence of chromospheric anemone jets outside sunspots in active ...regions. They are typically 3 to 7 arc seconds = 2000 to 5000 kilometers long and 0.2 to 0.4 arc second = 150 to 300 kilometers wide, and their velocity is 10 to 20 kilometers per second. These small jets have an inverted Y-shape, similar to the shape of x-ray anemone jets in the corona. These features imply that magnetic reconnection similar to that in the corona is occurring at a much smaller spatial scale throughout the chromosphere and suggest that the heating of the solar chromosphere and corona may be related to small-scale ubiquitous reconnection.
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