Post-flare Loop System Research Articles

When Hot Meets Cold: Post-flare Coronal Rain

Abstract Most solar flares demonstrate a prolonged, hour-long post-flare (or gradual) phase, characterized by arcade-like, post-flare loops (PFLs) visible in many extreme ultraviolet (EUV) passbands. These coronal loops are filled with hot (∼30 MK) and dense plasma that evaporated from the chromosphere during the impulsive phase of the flare, and they very gradually recover to normal coronal density and temperature conditions. During this gradual cooling down to ∼1 MK regimes, much cooler (∼0.01 MK) and denser coronal rain is frequently observed inside PFLs. Understanding PFL dynamics in this long-duration, gradual phase is crucial to the entire corona–chromosphere mass and energy cycle. Here we report a simulation in which a solar flare evolves from pre-flare, over the impulsive phase all the way into its gradual phase, which successfully reproduces post-flare coronal rain. This rain results from catastrophic cooling caused by thermal instability, and we analyze the entire mass and energy budget evolution driving this sudden condensation phenomenon. We find that the runaway cooling and rain formation also induces the appearance of dark post-flare loop systems, as observed in EUV channels. We confirm and augment earlier observational findings, suggesting that thermal conduction and radiative losses alternately dominate the cooling of PFLs.

Stellar flare oscillations: evidence for oscillatory reconnection and evolution of MHD modes

Here, we report on the detection of a range of quasi-periodic pulsations (20–120 s; QPPs) observed during flaring activity of several magnetically active dMe stars, namely AF Psc, CR Dra, GJ 3685A, Gl 65, SDSS J084425.9+513830, and SDSS J144738.47+035312.1 in the GALEX NUV filter. Based on a solar analogy, this work suggests that many of these flares may be triggered by external drivers creating a periodic reconnection in the flare current sheet or an impulsive energy release giving rise to an avalanche of periodic bursts that occur at time intervals that correspond to the detected periods, thus generating QPPs in their rising and peak phases. Some of these flares also show fast QPPs in their decay phase, indicating the presence of fast sausage mode oscillations either driven externally by periodic reconnection or intrinsically in the post-flare loop system during the flare energy release.

OBSERVATIONS OF POST-FLARE PLASMA DYNAMICS DURING AN M1.0 FLARE IN AR11093 BY THESOLAR DYNAMICS OBSERVATORY/ATMOSPHERIC IMAGING ASSEMBLY

We observe the motion of cool and hot plasma in a multi-stranded post flare loop system that evolved in the decay phase of a two ribbon M1.0 class flare in AR 11093 on 7 August 2010 using SDO/AIA 304 \AA\ and 171 \AA\ filters. The moving intensity feature and its reflected counterpart are observed in the loop system at multi-temperature. The observed hot counterpart of the plasma that probably envelopes the cool confined plasma, moves comparatively faster ($\sim$34 km s$^{-1}$) to the later (29 km s$^{-1}$) in form of the spreaded intensity feature. The propagating plasma and intensity reflect from the region of another footpoint of the loop. The subsonic speed of the moving plasma and associated intensity feature may be most likely evolved in the post flare loop system through impulsive flare heating processes. Complementing our observations of moving multi-temperature intensity features in the post flare loop system and its reflection, we also attempt to solve two-dimensional ideal magnetohydrodynamic equations numerically using the VAL-IIIC atmosphere as an initial condition to simulate the observed plasma dynamics. We consider a localized thermal pulse impulsively generated near one footpoint of the loop system during the flare processes, which is launched along the magnetic field lines at the solar chromosphere. The pulse steepens into a slow shock at higher altitudes while moving along this loop system, which triggers plasma perturbations that closely exhibit the observed plasma dynamics.

RE-FLARING OF A POST-FLARE LOOP SYSTEM DRIVEN BY FLUX ROPE EMERGENCE AND TWISTING

In this letter, we study in detail the evolution of the post-flare loops on 2005 January 15 that occurred between two consecutive solar eruption events, both of which generated a fast halo CME and a major flare. The post-flare loop system, formed after the first CME/flare eruption, evolved rapidly, as manifested by the unusual accelerating rise motion of the loops. Through nonlinear force-free field (NLFFF) models, we obtain the magnetic structure over the active region. It clearly shows that the flux rope below the loops also kept rising accompanied with increasing twist and length. Finally, the post-flare magnetic configuration evolved to a state that resulted in the second CME/flare eruption. This is an event in which the post-flare loops can re-flare in a short period of $\sim$16 hr following the first CME/flare eruption. The observed re-flaring at the same location is likely driven by the rapid evolution of the flux rope caused by the magnetic flux emergence and the rotation of the sunspot. This observation provides valuable information on CME/flare models and their prediction.

Calcium‐to‐Argon and Nickel‐to‐Argon Abundance Ratios as Tracers of the Source Region of Postflare Loop System Material

Intensities of abundance diagnostic lines of Ca XV, Ca XVI, Ni XVII, Ar XIII, and Ar XV have been derived for a classic flare loop system observed during the Skylab mission. These have been used to test for photospheric or coronal origin of the flare loop material. The resulting FIP-bias factors are between 1.7 and 4.6 with a majority of the values around 4.5 indicating a source with material modified by the FIP effect. The loop system bias factors are similar to those observed in a sample of Skylab prominences, suggesting that the disrupted mass of the preflare embedded filament provided the loop system material.

Photon Spectroscopy with Imaging X-Ray Instruments

Individual X-ray photons in the keV energy range produce hundreds of photoelectrons in a single pixel of a CCD array detector. The number of photoelectrons produced is a linear function of the photon energy, allowing the measurement of spectral information with an imaging detector system. Most solar X-ray telescopes, such as Yohkoh/SXT and Hinode/XRT, use CCD detectors in an integrating mode and are designed to make temperature estimates from multiband filter photometry. We show how such instruments can be used in a new way to perform a limited type of this photon spectroscopy. By measuring the variance in intensity of a series of repeated images through a single filter of an X-ray source, the mean energy per detected photon can be determined. This energy is related to the underlying coronal spectrum, and hence it can be used to deduce the mean plasma temperature. We apply this technique to data from the Yohkoh Soft X-Ray Telescope and compare the temperatures obtained with this technique with the temperatures derived using the standard filter ratio method for a postflare loop system. Given the large dynamic range of the soft X-ray flux observed from the Sun, we describe the requirements for a future instrument that would be better suited to performing photon spectroscopy.

Loop top nonthermal emission sources associated with an over-the-limb flare observed with NoRH and RHESSI

We studied the M3.7 class flare which occurred on 2005 July 27, in the active region NOAA 10792. This flare is an over-the-limb flare, and the footpoints are entirely occulted by the solar disk. The microwave and the hard X-ray images obtained with the Nobeyama Radioheliograph and the RHESSI satellite, respectively, clearly showed emission sources above the post-flare loop system. We examined the emission sources in detail spatially, temporally, and spectroscopically. As a result, one of the hard X-ray emission sources and the microwave emission source are nonthermal.

The GLE on Oct. 28, 2003 – radio diagnostics of relativistic electron and proton injection

Timing discrepancies between signatures of accelerated particles at the sun and the arrival times of the particles at near-earth detectors are a matter of fundamental interest for space-weather applications. The solar injection times of various components of energetic particles were derived by Klassen et al. (2005, JGR, 110, A09S04) for the October 28, 2003, X-class/ γ -ray flare in NOAA AR 10486. This flare occured in connection with a fast halo coronal mass ejection and a neutron monitor-observed ground level event (GLE). We used radio (Astrophysikalisches Institut Potsdam, $\emph{WIND}$, Nancay Multifrequency Radio Heliograph), H α (Observatorium Kanzelhohe), $\emph{RHESSI}$, $\emph{SOHO}$ (EIT, LASCO, MDI), and $\emph{TRACE}$ data to study the associated chromospheric and low coronal phenomena. We identify three source sites of accelerated particles in this event. Firstly, there is a source in projection 0.3 $R_\odot$ away from AR 10486, which is the site of the reconnection outflow termination, as revealed by a termination shock signature in the dynamic radio spectrum. Secondly, there is the extended current sheet above a giant coronal postflare loop system in the main flare phase. Thirdly, there is a source situated on a magnetic separatrix surface between several magnetic arcades and neighbouring active regions. This source is 0.2 $R_\odot$ away from AR 10486 and acts during onset and growth of high energy proton injection in space. It is not clear if this source is related to the acceleration of protons, or if it only confirms that energetic particles penetrate a multistructure magnetic loop system after being previously accelerated near the main HXR- and γ -ray sources. The result is in favour of energetic particle acceleration in the low corona (<0.5 $R_\odot$ above the photosphere) and in contrast to acceleration of the relativistic particles at remotely propagating shock waves.

Current Sheet Evolution in the Aftermath of a CME Event

We report on SOHO UVCS observations of the coronal restructuring following a coronal mass ejection (CME) on 2002 November 26, at the time of a SOHO-Ulysses quadrature campaign. Starting about 1.5 hr after a CME in the northwest quadrant, UVCS began taking spectra at 1.7 R, covering emission from both cool and hot plasma. Observations continued, with occasional gaps, for more than 2 days. Emission in the 974.8 A line of [Fe XVIII], indicating temperatures above 6 x 10(exp 6) K, was observed throughout the campaign in a spatially limited location. Comparison with EIT images shows the [Fe XVIII] emission to overlie a growing post-flare loop system formed in the aftermath of the CME. The emission most likely originates in a current sheet overlying the arcade. Analysis of the [Fe XVIII] emission allows us to infer the evolution of physical parameters in the current sheet over the entire span of our observations: in particular, we give the temperature versus time in the current sheet and estimate its density. At the time of the quadrature, Ulysses was directly above the location of the CME and intercepted the ejecta. High ionization state Fe was detected by the Ulysses SWICS throughout the magnetic cloud associated with the CME, although its rapid temporal variation suggests bursty, rather than smooth, reconnection in the coronal current sheet. The SOHO-Ulysses data set provided us with the unique opportunity of analyzing a current sheet structure from its lowest coronal levels out to its in situ properties. Both the remote and in situ observations are compared with predictions of theoretical CME models.

Structure and evolution of the April 27, 1981 post-flare loop

A time series of H α filtergrams and two-dimensional H α spectral data of the well-known post-flare loop system of April 27, 1981 are obtained at the Yunnan Observatory. The two-dimensional spectral data are analyzed quantitatively by means of the `multi-cloud model' method. The line-of-sight velocity fields of the post-flare loop system are derived for three different times. The post-flare loop system is composed of a loop and an ejection. The loop almost faces the observer and the ejection is along the line-of-sight. A `double-loop' model is proposed in order to explain the structure and evolution of the post-flare loop system. The distributions of the line-of-sight velocity and the evolution time of the post-flare loop system derived from a free-fall model are similar to those obtained from the observations.

X-ray and radio observations of the activation stages of an X-class solar flare

We report interesting developments prior to the impulsive phase of an X-class solar flare that occurred on September 24, 2001. Our multiwavelength study makes use of X-ray data from the Yohkoh satellite, the Ondrejov radio spectral observations in the decimetric band, and the new Hard X-Ray Spectrometer instrument (HXRS) on board the MTI satellite. The GOES time history of this event showed a precursor phase starting as early as two hours prior to the impulsive phase, and we have used various data sets to identify what parts of this development could be associated with the flare itself. The most interest- ing time interval was identified roughly one hour before the main peak when an unusual drifting radio continuum was observed together with two radio sources (at 327 and 164 MHz) in positions corresponding to expanding loops seen in Yohkoh/SXT and SOHO/EIT images, accompanied by a filament disappearence during the same period. Hard X-ray observations revealed a soft spectrum that we interpret as non-thermal, located within loop structures observed in soft X-rays along the magnetic neutral line. The hard X-ray emission continued for more than one hour, as observed in turn by the two spacecraft. In the initial phase of the flare itself, the hard X-ray emission arose in structures closely identifiable with the early soft X-ray loops, which appeared to evolve smoothly into the post-flare loop system of the flare maximum. The decimeter spectra showed loosely-correlated spiky emission at frequencies consistent with the densities inferred from soft X-rays, but with rapid drifts implying motions along field lines. From all these data we infer that the initiation of the flare involved non-thermal processes extending along the neutral line in the photosphere, systematically including open magnetic field lines as shown by the occurrence of interplanetary Type III bursts observed by the WAVES spectrometer on board the WIND spacecraft.

Temporal variation of 2D multi-parameter fields of a post-flare loop system

The 2D H β spectral data of the post-flare loop system (PFLs) of August 17, 1989 are obtained and analyzed quantitatively for three different times. Three physical quantities (i.e., the column number density of hydrogen atoms at the second level along the line-of-sight direction N 2, the excitation temperature T ex and micro-turbulence velocity V t ) and their 2D fields are derived during the three times. The time variations of the 2D field are given for the three quantities more than 1 h after the maximum of the H α flare. Our analyses show that the average values of N 2 and V t decrease with time, while T ex is nearly unchanged except for the top part of the PFLs where it is increasing slightly with time. A new evolution property is found in which the regions with the maxima of T ex and N 2 move from the middle of the southern leg towards the top part of the PFLs, while the position of V t maximum shifts from the top part towards the northern leg of the system. This scenario may be a result of successive formation of new loops at higher heights while under continuous cooling. The emission measure (EM), electron density n e and pressure P e in the PFLs are also estimated.

Evidence of a Flux‐Rope Model for Corona Mass Ejections Based on Observations of the Limb Prominence Eruption on 2002 January 4

We report on a prominence eruption as seen in Kanzelhöhe Solar Observatory Hα images, Solar and Heliospheric Observatory (SOHO) EUV Imaging Telescope (EIT) 195 Å images, and coronal SOHO Large Angle Spectrometric Coronograph C2 images. Our data favor the flux-rope model for coronal mass ejections (CMEs), which suggests that a flux rope is formed long before the eruption. Our conclusion is based on a three-part structure of the pre-erupted configuration of the magnetic field and on the fact that the first Hα, SOHO EIT 195 Å brightenings occurred some 15 minutes after the filament began to ascend. The data also clearly demonstrate two rarely observed components of the standard flare model: (1) magnetic loops that overlay the pre-erupted filament and (2) magnetic field lines stretched vertically by the ascending filament. These field lines are compressed horizontally and move toward each other where they reconnect to form an apparently growing post-flare loop system.

RHESSI and Trace Observations of the 21 April 2002 X1.5 Flare

Observations of the X1.5 flare on 21 April 2002 are reviewed using the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and the Transition Region and Coronal Explorer (TRACE). The major findings are as follows: (1) The 3–25 keV X-rays started < 4 min before the EUV (195 A) emission suggesting that the initial energy release heated plasma directly to ≳20 MK, well above the 1.6 MK needed to produce the Fexu (195 A) line. (2) Using coaligned 12–25 keV RHESSI and TRACE images, further evidence is found for the existence of hot (15–20 MK) plasma in the 195 A passband. This hot, diffuse emission is attributed to the presence of the Fe xxiv (192 A) line within the TRACE 195 A passband. (3) The 12–25 keV source centroid moves away from the limb with an apparent velocity of ~ 9.9 km s−1, slowing to ~ 1.7 km s−1 after 3 hours, its final altitude being ~ 140 Mm after ~ 12 hours. This suggests that the energy release site moves to higher altitudes in agreement with classical flare models. (4) The 50–100 keV emission correlates well with EUV flare ribbons, suggesting thick-target interactions at the footpoints of the magnetic arcade. The 50–100 keV time profile matches the time derivative of the GOES light curve (Neupert effect), which suggests that the same electrons that produced the thick-target hard X-ray emission also heat the plasma seen in soft X-rays. (5) X-ray footpoint emission has an E−3 spectrum down to ~ 10 keV suggesting a lower electron cutoff energy than previously thought. (6) The hard X-ray (25–200 keV) peaks have FWHM durations of ~ 1 min suggesting a more gradual energy release process than expected. (7) The TRACE images reveal a bright symmetric front propagating away from the main flare site at speeds of ≥ 120 km s−1. This may be associated with the fast CME observed several minutes later by LASCO. (8) Dark sinuous lanes are observed in the TRACE images that extend almost radially from the post-flare loop system. This ‘fan of spines’ becomes visible well into the decay phase of the flare and shows evidence for both lateral and downward motions.

Two-dimensional multi-parameter fields of a limb flare loop system

The 2D H-beta spectral data of the post-flare loop system (PFLs) of August 17, 1989 are obtained and analyzed quantitatively. Four physical parameters (the line-of-sight velocity V-//, column number density of the hydrogen atoms at the second level along the line-of-sight direction N-2, the excitation temperature T-ex and the micro-turblence velocity V-t) and their 2D distributions are derived. The 2D fields of V-//, N-2, T-ex and V-t exhibit inhomogeneous and asymmetric properties in two loop legs. Red-shift velocities in the south leg are smaller than those of blue shift in the north leg where the velocity reaches 60 km s(-1). The values of N-2 and T-ex in the south leg are all much higher than those in the north leg, N-2 ranges between 0.5 to 4.5 x 10(13) cm(-2), while T-ex ranges from 0.8 to 2.2 x 10(4) K, lower at the top part than in the two legs. A high temperature region is located in the middle of the south leg. V-t at the top part is greater than that in the two legs, with the average being 30 similar to 40 km s(-1). Electron density n(e) and electron pressure P-e are estimated at 0.2 similar to 0.6 x 10(11) cm(-3) and at 0.04 similar to 0.12 dyn cm(-2), respectively. These results are compared with those obtained from the PFLs of 1984 February 18.

Internal and external reconnection in a series of homologous solar flares

Using data from the extreme ultraviolet imaging telescope (EIT) on SOHO and the soft X‐ray telescope (SXT) on Yohkoh, we examine a series of morphologically homologous solar flares occurring in National Oceanic and Atmospheric Administration (NOAA) active region 8210 over May 1–2, 1998. An emerging flux region (EFR) impacted against a sunspot to the west and next to a coronal hole to the east is the source of the repeated flaring. An SXT sigmoid parallels the EFR's neutral line at the site of the initial flaring in soft X rays. In EIT each flaring episode begins with the formation of a crinkle pattern external to the EFR. These EIT crinkles move out from, and then in toward, the EFR with velocities ∼20 km s−1. A shrinking and expansion of the width of the coronal hole coincides with the crinkle activity, and generation and evolution of a postflare loop system begins near the time of crinkle formation. Using a schematic based on magnetograms of the region, we suggest that these observations are consistent with the standard reconnection‐based model for solar eruptions but are modified by the presence of the additional magnetic fields of the sunspot and coronal hole. In the schematic, internal reconnection begins inside of the EFR‐associated fields, unleashing a flare, postflare loops, and a coronal mass ejection (CME). External reconnection, first occurring between the escaping CME and the coronal hole field and second occurring between fields formed as a result of the first external reconnection, results in the EIT crinkles and changes in the coronal hole boundary. By the end of the second external reconnection, the initial setup is reinstated; thus the sequence can repeat, resulting in morphologically homologous eruptions. Our inferred magnetic topology is similar to that suggested in the “breakout model” of eruptions [Antiochos, 1998], although we cannot determine if our eruptions are released primarily by the breakout mechanism (external reconnection) or, alternatively, primarily by the internal reconnection.

A typical example of the application of the 'Multi-Cloud Model' method to the asymmetric profiles processing

Multi-Cloud Model(MCM) is an effective method for dealing with asymmetric profiles of spectral line formed by active objects on the sun. A typical example is given for the application of the MCM method in this paper. The Hβ spectral data of a large post-flare loop (PFL) system which occurred on the solar western limb are obtained with MW-SSHG at the Yunnan Observatory and analysed based on the MCM method. The line-of-sight velocity fields observed in different times are derived and discussed.

Flares in Sigmoidal Coronal Structures — A Case Study

We analyze radio observations, magnetograms and extrapolated field line maps, Hα filtergrams, and X-ray observations of two flare events (6 February 1992 in AR 7042 and 25 October 1994 in AR 7792) and study properties, evolution and energy release signatures of sigmoidal loop systems. During both events, the loop configuration seen in soft X-ray (SXR) images changes from a preflare sigmoidal shape to a relaxed post-flare loop system. The underlying magnetic field system consists of a quadrupolar configuration formed by a sheared arcade core and a remote field concentration. We demonstrate two possibilities: a sigmoidal SXR pattern can be due to a single continuous flux tube (the 1992 event). Alternatively, it can be due to a set of independent loops appearing like a sigmoid (the 1994 event). In both cases, the preflare and post-flare loops can be well reproduced by a linear force-free field and potential field, respectively, computed using preflare magnetograms. We find that thermal and non-thermal flare energy release indicators of both events become remarkably similar after applying spatial and temporal scale transformations. Using the spatial scaling between both events we estimated that the non-thermal energy release in the second event liberated about 1.7 times more energy per unit volume. A two-and-a-half times faster evolution indicates that the rate of the energy release per unit volume is more than four times higher in this event. A coronal type II burst reveals ignition and propagation of a coronal shock wave. In contrast, the first event, which was larger and released about a 10 times more energy during the non-thermal phase, was associated with a CME, but no type II burst was recorded. During both events, in addition to the two-ribbon flare process an interaction was observed between the flaring arcade and an emerging magnetic flux region of opposite polarity next to the dominant leading sunspot. The arcade flare seems to stimulate the reconnection process in an `emerging flux-type' configuration, which significantly contributes to the energy release. This regime is characterized by the quasiperiodic injection of electron beams into the surrounding extended field line systems. The repeated beam injections excite pulsating broadband radio emission in the decimetric-metric wavelength range. Each radio pulse is due to a new electron beam injection. The pulsation period (seconds) reflects the spatial scale of the emerging flux-type field configuration. Since broadband decimetric-metric radio pulsations are a frequent radio flare phenomenon, we speculate that opposite-polarity small-scale flux intrusions located in the vicinity of strong field regions may be an essential component of the energy release process in dynamic flares.

Observations of the Association of Prominences and the Surrounding Corona

AbstractEmission-line coronal images in Fe XIV (530.3 nm) and FeX (637.5 nm) show faint enhancements at the location of quiescent prominences. Such enhancements can appear in the outer portions of a prominence, similar to the high-temperature sheath surrounding prominences as inferred from UV and EUV observations. The observational evidence supports the interpretation that this enhanced coronal emission is due to energy carried by the prominence threads and dissipated in the adjacent coronal region. Also, observed coronal loop interactions involving partial magnetic reconnection have associated Hα structures. These structures can have the appearance of an active prominence, similar to those typically observed in post flare loop systems. It appears that the association of such active prominences with the adjacent corona is fundamentally different from that of quiescent prominences. The observations indicate that these types of active prominences arise simply as a consequence of reconnection processes in the corona itself.

Speeds of rising post-flare structures

There are basically two kinds of post-flare coronal structures: those rising with decreasing speed, and others which rise with constant speed for a long period of time. As a rule, those structures with decreasing speed are post-flare loop systems, while those rising with constant speed are postflare giant arches. However, there are exceptions. We demonstrate several cases of post-flare loop systems which rise with constant speed for many hours, three of them observed by Yohkoh. These observations imply that the Kopp and Pneuman interpretation of post-flare loops as sequentially reconnecting open field lines cannot be generally valid. The most likely interpretation is that all post-flare loop systems start with the Kopp and Pneuman process, but in some of them later-formed loops begin to be continuously heated; thus they cease to cool and begin to expand into the corona. This kind of post-flare loops might represent an intermediate stage between the ordinary post-flare loops and post-flare giant arches.