Chromospheric and Coronal Observations of Solar Flares with the Helioseismic and Magnetic Imager
TL;DR: In this article, the authors report observations of white-light ejecta in the low corona, for two X-class flares on the 2013 May 13, using data from the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics Observatory.
Abstract: We report observations of white-light ejecta in the low corona, for two X-class flares on the 2013 May 13, using data from the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics Observatory. At least two distinct kinds of sources appeared (chromospheric and coronal), in the early and later phases of flare development, in addition to the white-light footpoint sources commonly observed in the lower atmosphere. The gradual emissions have a clear identification with the classical loop-prominence system, but are brighter than expected and possibly seen here in the continuum rather than line emission. We find the HMI flux exceeds the radio/X-ray interpolation of the bremsstrahlung produced in the flare soft X-ray sources by at least one order of magnitude. This implies the participation of cooler sources that can produce free-bound continua and possibly line emission detectable by HMI. One of the early sources dynamically resembles "coronal rain", appearing at a maximum apparent height and moving toward the photosphere at an apparent constant projected speed of 134 $\pm$ 8 $\mathrm{km s^{-1}}$. Not much literature exists on the detection of optical continuum sources above the limb of the Sun by non-coronagraphic instruments, and these observations have potential implications for our basic understanding of flare development, since visible observations can in principle provide high spatial and temporal resolution.
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TL;DR: In this paper, two processing pipelines have been implemented at NASA's Solar Dynamics Observatory (SDO) Joint Science Operations Center (JSOC) at Stanford University to compute these observables from calibrated Level-1 filtergrams, one that computes line-of-sight quantities every 45 seconds and the other, primarily for the vector magnetic field, that compute averages on a 720-second cadence.
Abstract: NASA’s Solar Dynamics Observatory (SDO) spacecraft was launched 11 February 2010 with three instruments onboard, including the Helioseismic and Magnetic Imager (HMI). After commissioning, HMI began normal operations on 1 May 2010 and has subsequently observed the Sun’s entire visible disk almost continuously. HMI collects sequences of polarized filtergrams taken at a fixed cadence with two $4096 \times 4096$
cameras, from which are computed arcsecond-resolution maps of photospheric observables that include line-of-sight velocity and magnetic field, continuum intensity, line width, line depth, and the Stokes polarization parameters [
$I, Q, U, V$
]. Two processing pipelines have been implemented at the SDO Joint Science Operations Center (JSOC) at Stanford University to compute these observables from calibrated Level-1 filtergrams, one that computes line-of-sight quantities every 45 seconds and the other, primarily for the vector magnetic field, that computes averages on a 720-second cadence. Corrections are made for static and temporally changing CCD characteristics, bad pixels, image alignment and distortion, polarization irregularities, filter-element uncertainty and nonuniformity, as well as Sun–spacecraft velocity. We detail the functioning of these two pipelines, explain known issues affecting the measurements of the resulting physical quantities, and describe how regular updates to the instrument calibration impact them. We also describe how the scheme for computing the observables is optimized for actual HMI observations. Initial calibration of HMI was performed on the ground using a variety of light sources and calibration sequences. During the five years of the SDO prime mission, regular calibration sequences have been taken on orbit to improve and regularly update the instrument calibration, and to monitor changes in the HMI instrument. This has resulted in several changes in the observables processing that are detailed here. The instrument more than satisfies all of the original specifications for data quality and continuity. The procedures described here still have significant room for improvement. The most significant remaining systematic errors are associated with the spacecraft orbital velocity.
153 citations
TL;DR: In this article, the authors considered the non-thermal fraction of electron energies (Re) in above-the-loop-top (ALT) and derived the kappa distribution.
Abstract: The presentation begins by considering the non-thermal fraction of electron energies (Re) in 'above-the-looptop' (ALT). Several spectral models area considered: isothermal model, power-law, and the newly derived kappa distribution. Technically, all 3 non-thermal models can fit the data. In the discussion of flare scenario, energy partition, energization mechanism, and collisionality are considered. It is concluded that the kappa distribution works (Re ≲ 50%), magnetic reconnection scenario works, and Coulomb collisions may reduce the non-thermal fraction of electron energies.
87 citations
TL;DR: In this paper, the authors investigate the source height of chromospheric footpoints in white light (WL) and hard X-rays (HXR) and find the WL and HXR (?30 keV) centroids to be largely co-spatial and from similar heights for all events, with altitudes around 800 km above the photosphere or 300?450 km above a limb height.
Abstract: We report analysis of three solar flares that occur within 1? of limb passage, with the goal to investigate the source height of chromospheric footpoints in white light (WL) and hard X-rays (HXR). We find the WL and HXR (?30 keV) centroids to be largely co-spatial and from similar heights for all events, with altitudes around 800 km above the photosphere or 300?450 km above the limb height. Because of the extreme limb location of the events we study, emissions from such low altitudes are influenced by the opacity of the atmosphere and projection effects. STEREO images reveal that for SOL2012-11-20T12:36 the projection effects are smallest, giving upper limits of the absolute source height above the nominal photosphere for both wavelengths of ?1000 km. To be compatible with the standard thick target model, these rather low altitudes require very low ambient densities within the flare footpoints, in particular if the HXR-producing electrons are only weakly beamed. That the WL and HXR emissions are co-spatial suggests that the observed WL emission mechanism is directly linked to the energy deposition by flare accelerated electrons. If the WL emission is from low-temperature ( K) plasma as currently thought, the energy deposition by HXR-producing electrons above ?30 keV seems only to heat chromospheric plasma to such low temperatures. This implies that the energy in flare-accelerated electrons above ?30 keV is not responsible for chromospheric evaporation of hot ( K) plasma, but that their energy is lost through radiation in the optical range.
72 citations
TL;DR: In this article, the European Research Council under the European Union Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement nr. 291058.PA.
Abstract: PA. GV are funded by the European Research Council under the European Union Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement nr. 291058
47 citations
TL;DR: In this paper, the authors present the analysis of MHD oscillations and kinematics of the coronal rain observed in chromospheric and transition region lines by IRIS, Hinode/SOT and SDO/AIA.
Abstract: Coronal rain composed of cool plasma condensations falling from coronal heights along magnetic field lines is a phenomenon occurring mainly in active region coronal loops. Recent high resolution observations have shown that coronal rain is much more common than previously thought, suggesting its important role in the chromosphere-corona mass cycle. We present the analysis of MHD oscillations and kinematics of the coronal rain observed in chromospheric and transition region lines by IRIS, Hinode/SOT and SDO/AIA. Two different regimes of transverse oscillations traced by the rain are detected: small-scale persistent oscillations driven by a continuously operating process and localised large-scale oscillations excited by a transient mechanism. The plasma condensations are found to move with speeds ranging from few km s−1 up to 180 km s−1 and with accelerations largely below the free fall rate, with the likely reasons being pressure effects and the ponderomotive force resulting from the loop oscillations. The observed evolution of the emission in individual SDO/AIA bandpasses is found to exhibit clear signatures of a gradual cooling of the plasma at the loop top. We determine the temperature evolution of the coronal loop plasma using regularised inversion to recover the differential emission measure (DEM) and by forward modelling the emission intensities in the SDO/AIA bandpasses using a two-component synthetic DEM model. The inferred evolution of the temperature and density of the plasma near the apex is consistent with the limit cycle model and suggests the loop is going through a sequence of periodically repeating heating-condensation cycles.
34 citations
References
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TL;DR: The CHIANTI database as mentioned in this paper is a set of atomic data and transition probabilities necessary to calculate the emission line spectrum of astrophysical plasmas, including atomic energy levels, atomic radiative data such as wavelengths, weighted oscillator strengths and A values, and electron collisional excitation rates.
Abstract: CHIANTI consists of a critically evaluated set of atomic data and transition probabilities necessary to calculate the emission line spectrum of astrophysical plasmas. The data consist of atomic energy levels, atomic radiative data such as wavelengths, weighted oscillator strengths and A values, and electron collisional excitation rates. A set of programs that use these data to calculate the spectrum in a desired wavelength range as a function of temperature and density is also provided. A suite of programs has been developed to carry out plasma diagnostics of astrophysical plasmas. The state-of-the-art contents of the CHIANTI database will be described and some of the most important results obtained from the use of the CHIANTI database will be reviewed.
2,116 citations
"Chromospheric and Coronal Observati..." refers methods in this paper
...We thank the CHIANTI consortium (Dere et al. 1997) for their excellent atomic-physics tools....
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TL;DR: The Helioseismic and Magnetic Imager (HMI) as discussed by the authors was designed to measure the Doppler shift, intensity, and vector magnetic field at the solar photosphere using the 6173 A FeI absorption line.
Abstract: The Helioseismic and Magnetic Imager (HMI) investigation (Solar Phys. doi: 10.1007/s11207-011-9834-2, 2011) will study the solar interior using helioseismic techniques as well as the magnetic field near the solar surface. The HMI instrument is part of the Solar Dynamics Observatory (SDO) that was launched on 11 February 2010. The instrument is designed to measure the Doppler shift, intensity, and vector magnetic field at the solar photosphere using the 6173 A Fe i absorption line. The instrument consists of a front-window filter, a telescope, a set of waveplates for polarimetry, an image-stabilization system, a blocking filter, a five-stage Lyot filter with one tunable element, two wide-field tunable Michelson interferometers, a pair of 40962 pixel cameras with independent shutters, and associated electronics. Each camera takes a full-disk image roughly every 3.75 seconds giving an overall cadence of 45 seconds for the Doppler, intensity, and line-of-sight magnetic-field measurements and a slower cadence for the full vector magnetic field. This article describes the design of the HMI instrument and provides an overview of the pre-launch calibration efforts. Overviews of the investigation, details of the calibrations, data handling, and the science analysis are provided in accompanying articles.
1,997 citations
"Chromospheric and Coronal Observati..." refers background in this paper
...HMI observes the line via six pass bands spanning a range of about 345 mÅ around the target line and it also obtains full Stokes profiles (Schou et al. 2012)....
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384 citations
"Chromospheric and Coronal Observati..." refers methods in this paper
...…shrinkage of the magnetic structures during the dipolarization of the reconnected field volumes as suggested by the scenario of the coronal mass cycle (e.g. Švestka et al. 1987; Forbes & Acton 1996), but in this picture the density also decreases as the cooling proceeds (Serio et al. 1991)....
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TL;DR: In this paper, it was shown that the density at the top of the loops of a flare can reach 4 x 10 to the 12th/cu cm for a turbulent velocity of about 12 km/s.
Abstract: The dynamic flare of November 6, 1980 (max at about 15:26 UT) developed a rich system of growing loops which could be followed in H-alpha for 1.5 hr. Throughout the flare, these loops, near the limb, were seen in emission against the disk. Theoretical computations of deviations from LTE populations for a hydrogen atom reveal that this requires electron densities in the loops close to, or in excess of 10 to the 12th/cu cm. From measured widths of higher Balmer lines the density at the tops of the loops was found to be 4 x 10 to the 12th/cu cm if no nonthermal motions were present, or 5 x 10 to the 11th/cu cm for a turbulent velocity of about 12 km/s. It is now general knowledge that flare loops are initially observed in X-rays and become visible in H-alpha only after cooling. For such a high density, a loop would cool through radiation from 10 to the 7th to 10 to the 4th K within a few minutes so that the dense H-alpha loops should have heights very close to the heights of the X-ray loops. This, however, contradicts the observations obtained by the HXIS and FCS instruments on board SMM which show the X-ray loops at much higher altitudes than the loops in H-alpha. Therefore, it is suggested that the density must have been significantly lower when the loops were formed, and that the flare loops were apparently both shrinking and increasing in density while cooling.
139 citations
"Chromospheric and Coronal Observati..." refers methods in this paper
...…shrinkage of the magnetic structures during the dipolarization of the reconnected field volumes as suggested by the scenario of the coronal mass cycle (e.g. Švestka et al. 1987; Forbes & Acton 1996), but in this picture the density also decreases as the cooling proceeds (Serio et al. 1991)....
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Book•
01 Jan 1974
TL;DR: In this article, the authors present a survey of the role of radiation in high energy particle physics and cosmology, including the following: 1. Continuum Radiation. 2. Monochromatic (Line) Radiation. 3. Gas Processes. 4. Nuclear Astrophysics and High Energy Particles.
Abstract: 1. Continuum Radiation.- 2. Monochromatic (Line) Radiation.- 3. Gas Processes.- 4. Nuclear Astrophysics and High Energy Particles.- 5. Astrometry and Cosmology.- References.- Supplemental References to the Second Edition.- Author Index.
132 citations