Multiwavelength analysis of a well observed flare from SMM

We describe and analyse observations of an M1.4 flare which began at 17: 00 UT on 12 November, 1980. Ground based H and magnetogram data have been combined with EUV, soft and hard X-ray observations made with instruments on-board the Solar Maximum Mission (SMM) satellite. The preflare phase was marked by a gradual brightening of the flare site in Ov and the disappearance of an H filament. Filament ejecta were seen in Ov moving southward at a speed of about 60 km s^–1, before the impulsive phase. The flare loop footpoints brightened in H and the Caxix resonance line broadened dramatically 2 min before the impulsive phase. Non-thermal hard X-ray emission was detected from the loop footpoints during the impulsive phase while during the same period blue-shifts corresponding to upflows of 200–250 km s^–1 were seen in Ca xix. Evidence was found for energy deposition in both the chromosphere and corona at a number of stages during the flare. We consider two widely studied mechanisms for the production of the high temperature soft X-ray flare plasma in the corona, i.e. chromospheric evaporation, and a model in which the heating and transfer of material occurs between flux tubes during reconnection.     

Reconnection driven by an erupting filament in the May 14, 1981 flare

We present observations of the flare of May 14, 1981, which can be classified as a three-ribbon flare. After a detailed analysis in metric, decimetric, microwave, optical, and X-ray ranges we propose that the event was caused by a reconnection process driven by erupting filament. The energy was liberated in the current sheet above the filament in the region between the erupting flux and the overlying field. It is shown that plasma microinstabilities develop as the plasma enters the current sheet. The observations indicate that during the precursor phase a certain low-frequency turbulence, such as ion-accoustic turbulence had to be present.The reconnection rate was growing due to the increasing tension of the stretched overlying field. It is shown that the reconnection proceeded in the Sonnerup-Petschek regime during the precursor, and changed to the pile-up regime in the fast reconnection phase, when the maximal lateral expansion (50 km s^–1) of the H ribbons was observed. The proposed process of reconnection driven by an erupting filament can be applied to three- and four-ribbon flares.     

He I 10830 observations of the 3N/M4.0 flare of 4 September, 1982

Hei 10830 Å spectroheliograms of a major 3N two-ribbon flare occurring in Boulder Region 3885/3886 early on 4 September, 1982 are discussed and compared with H and soft X-ray observations of the event. This flare, observed for more than 60 hr in Hei 10830, was associated with the eruption of a large filament in the active region complex, the formation of coronal holes, a long-duration soft X-ray event, and was the probable source of a earthward coronal mass ejection and the largest geomagnetic storm of this solar cycle. The results of this study suggest the Hei flare is a chromospheric manifestation of the X-ray coronal loop structures associated with flares.Visitor, National Solar Observatory, operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Structures of chromospheric magnetic field in the vicinity of the filament of active region 5669

In this paper, the chromospheric magnetic structures and their relation to the photospheric vector magnetic field in the vicinity of a dark filament in active region 5669 have been demonstrated. Structural variations are shown in chromospheric magnetograms after a solar flare. Filament-like structures in the chromospheric magnetograms occurred after a solar flare. They correspond to the reformation of the chromospheric dark filament, but there is no obvious variation of the photospheric magnetic field. We conclude that (a) some of the obvious changes of the chromospheric magnetic fields occurred after the flare, and (b) a part of these changes is perhaps due to flare brightening in the chromospheric H line.During the reforming process of the dark filament, a part of its chromospheric velocity field shows downward flow, and it later shows upward flow.     

Initial phase of chromospheric evaporation in a solar flare

In this paper we discuss the initial phase of chromospheric evaporation during a solar flare observed with instruments on the Solar Maximum Mission on May 21, 1980 at 20:53 UT. Images of the flaring region taken with the Hard X-Ray Imaging Spectrometer in the energy bands from 3.5 to 8 keV and from 16 to 30 keV show that early in the event both the soft and hard X-ray emissions are localized near the footpoints, while they are weaker from the rest of the flaring loop system. This implies that there is no evidence for heating taking place at the top of the loops, but energy is deposited mainly at their base. The spectral analysis of the soft X-ray emission detected with the Bent Crystal Spectrometer evidences an initial phase of the flare, before the impulsive increase in hard X-ray emission, during which most of the thermal plasma at 10⁷ K was moving toward the observer with a mean velocity of about 80 km s^-1. At this time the plasma was highly turbulent. In a second phase, in coincidence with the impulsive rise in hard X-ray emission during the major burst, high-velocity (370 km s^-1) upward motions were observed. At this time, soft X-rays were still predominantly emitted near the loop footpoints. The energy deposition in the chromosphere by electrons accelerated in the flare region to energies above 25 keV, at the onset of the high-velocity upflows, was of the order of 4 × 10¹⁰ erg s^-1 cm^-2. These observations provide further support for interpreting the plasma upflows as the mechanism responsible for the formation of the soft X-ray flare, identified with chromospheric evaporation. Early in the flare soft X-rays are mainly from evaporating material close to the footpoints, while the magnetically confined coronal region is at lower density. The site where upflows originate is identified with the base of the loop system. Moreover, we can conclude that evaporation occurred in two regimes: an initial slow evaporation, observed as a motion of most of the thermal plasma, followed by a high-speed evaporation lasting as long as the soft X-ray emission of the flare was increasing, that is as long as plasma accumulation was observed in corona.     

21 May 1980 flare review

A review is given of observations and theories relevant to the solar flare of 21 May, 1980, 20 ∶ 50 UT, the best studied flare on record. For more than 30 hr before the flare there was filament activation and plasma heating to above 10 MK. A flare precursor was present ≥6 min before the flare onset. The flare started with filament activation (20 ∶ 50 UT), followed by thick-target heating of two footpoints and subsequent ablation and convective evaporation involving energies of 1 to 2 × 10³¹ erg. Coronal explosions occurred at 20 ∶ 57 UT (possibly associated with a type-II burst) and at 21 ∶ 04 UT (associated with an Hα spray?). Post-flare loops were first seen at 20 ∶ 57 UT, and their upward motion is interpreted as a manifestation of successive field-line reconnections. A type-IV radio burst which later changed into a type-I noise storm was related to a giant coronal arch located just below the radio noise storm region. Some implications and difficulties these observations present to current flare theories are mentioned.     

Hα flare of 14 March, 1984 - evidence for reconnection?

Kodaikanal H monochromatic and white-light observations are used to study the circular flare of 14 March, 1984. We report here the dynamic activity of the H filament, which attained a severe twist before erupting as a 4B flare. We feel that the relative motion between the emerging spot field and its neighbouring field is responsible for the field line reconnection, which triggered the flare.     

Stereoscopic Analysis of the 19 May 2007 Erupting Filament

A filament eruption, accompanied by a B9.5 flare, coronal dimming, and an EUV wave, was observed by the Solar TERrestrial Relations Observatory (STEREO) on 19 May 2007, beginning at about 13:00 UT. Here, we use observations from the SECCHI/EUVI telescopes and other solar observations to analyze the behavior and geometry of the filament before and during the eruption. At this time, STEREO A and B were separated by about 8.5°, sufficient to determine the three-dimensional structure of the filament using stereoscopy. The filament could be followed in SECCHI/EUVI 304 Å stereoscopic data from about 12 hours before to about 2 hours after the eruption, allowing us to determine the 3D trajectory of the erupting filament. From the 3D reconstructions of the filament and the chromospheric ribbons in the early stage of the eruption, simultaneous heating of both the rising filamentary material and the chromosphere directly below is observed, consistent with an eruption resulting from magnetic reconnection below the filament. Comparisons of the filament during eruption in 304 Å and Hα? show that when it becomes emissive in He II, it tends to disappear in Hα?, indicating that the disappearance probably results from heating or motion, not loss, of filamentary material.     

A two-temperature model for the flare of 5 September, 1973

The energetics and mass transfer during the X-ray flare of 1831 GMT on 5 September, 1973 have been studied using the observations in the objective grating mode of the AS&E X-ray spectrographic telescope on Skylab. The flare was a moderately energetic one, Class M1 according to Solrad. In H, however, it was only a subflare of class - N. The data are approximately monochromatic images of the small X-ray source. They show a continued rise in the emission for several minutes followed by a decline. The size and temporal evolution are slightly different for ions associated with higher temperatures (Fe xxii, Si xiii) than with those of lower temperatures (Fe xvii, Mg xi). The time of maximum emission moves from one side of the flare to the other and peaks earlier for hotter temperature ions. The observations are analyzed using a two-temperature model in order to determine the changes in the distribution of emission measure and of the amount of material as a function of temperature. The development of the flare can be divided into three periods in each of which different mechanisms are operating. For the first 3–4 min, evaporation drives mass into the entire emitting region. Second, the evaporation ceases: Hot material loses energy, and we see a loss of hot material and a corresponding gain of cool material. Later, after 1838, we see a decline in the emission measure.     

Hα line profile observations of solar flares with high temporal resolution

H line profile observations of solar flares with high temporal resolution are an important tool for the analysis of the energy transport mechanism from the site of the flare energy release to the chromosphere. A specially designed instrument (imaging spectrograph) allows two-dimensional imaging of an active region simultaneously in 15 spectral channels along the H line profile with a temporal resolution of 5.4 s. Two flares have been observed in November 1982. The first one shows H signatures which one would typically expect in the case of explosive chromospheric evaporation produced by massive injection of non-thermal electrons. The observations of the other flare indicate that the heating of the upper chromosphere is dominated by thermal conduction, although during the impulsive hard X-ray burst there are also signatures of heating by non-thermal electrons.     

Temporal evolution of the chromospheric electron density in the flare of January 5, 1992

The analysis of the dynamic evolution of the chromospheric electron density during solar flares is fundamental for the testing of solar flare models. For this purpose we developed a digital imaging spectrograph for the observation of higher Balmer lines below 400 nm with a time resolution of 1 s and an algorithm for the determination of the electron density from the observed line profiles. On January 5, 1992 a M1/1N flare was observed in H, H and Caii H and the temporal evolution of the electron density was determined. The chromospheric electron density rises several times from less than 3 × 10¹⁹ to 1 × 10²⁰ m^–3 during the hard X-ray peaks.     

Chromospheric downflow velocities as a diagnostic in solar flares

We explore the dynamics of chromospheric condensations driven by evaporation during the impulsive phase of solar flares. Specifically, we find that the maximum chromospheric downflow speed obeys the approximate relation _d = 0.4 (F/ _ch )^1/3, where F is that part of the flare energy flux driving chromospheric evaporation, and _ch is the mass density in the preflare chromosphere just below the preflare transition region. This implies that chromospheric downflows as measured by H asymmetries may be a powerful probe of flare energetics.     

Yohkoh observations of the creation of high-temperature plasma in the flare of 16 December 1991

Yohkoh observations of an impulsive solar flare which occurred on 16 December, 1991 are presented. This flare was a GOES M2.7 class event with a simple morphology indicative of a single flaring loop. X-ray images were taken with the Hard X-ray Telescope (HXT) and soft X-ray spectra were obtained with the Bragg Crystal Spectrometer (BCS) on board the satellite. The spectrometer observations were made at high sensivity from the earliest stages of the flare, are continued throughout the rise and decay phases, and indicate extremely strong blueshifts, which account for the majority of emission in Caxix during the initial phase of the flare. The data are compared with observations from other space and ground-based instruments. A balance calculation is performed which indicates that the energy contained in non-thermal electrons is sufficient to explain the high temperature plasma which fills the loop. The cooling of this plasma by thermal conduction is independently verified in a manner which indicates that the loop filling factor is close to 100%. The production of superhot plasma in impulsive events is shown to differ in detail from the morphology and mechanisms appropriate for more gradual events.     

Active-Region Filaments and X-ray Sigmoids

We use Yohkoh soft X-ray telescope data and H full-disk observations to study the evolution of chromospheric filaments and coronal sigmoids in 6 active regions in association with coronal mass ejections (CMEs). In two cases, CMEs are directly observed by the SOHO/LASCO C2 coronagraph. In four cases, other observations (magnetic clouds, geomagnetic storms, sigmoid-arcade evolution) are used as CME indicators. Prior to eruption, each active region shows a bright coronal sigmoidal loop and underlying H filament. The sigmoid activates, erupts and gets replaced by a cusp, or an arcade. In contrast, the H filament shows no significant changes in association with sigmoid eruption and CME. We explain these observations in a framework of the classical two-ribbon flare model.     

Analysis of X-ray observations of the 15 June 1973 flare in active region NOAA 131

Observations and analyses of the 1B/M3 flare of 15 June, 1973 in active region NOAA 131 (McMath 12379) are presented. The X-ray observations, consisting of broadband photographs and proportional counter data from the Skylab/ATM NASA-MSFC/Aerospace S-056 experiment, are used to infer temperatures, emission measures, and densities for the flaring plasma. The peak temperature from the spatially resolved photographs is 25 × 10⁶ K, while the temperature from the full-disk proportional counter data is 15 × 10⁶ K. The density is 3 × 10¹⁰cm^–3. The X-ray flare emission appears to come primarily from two low-lying curvilinear features lying perpendicular to and centered on the line where the photospheric longitudinal magnetic field is zero. Similarities in the preflare and postflare X-ray emission patterns indicate that no large-scale relaxation of the coronal magnetic configuration was observed. Also discussed are H and magnetic field observations of the flare and the active region. Finally, results of numerical calculations, including thermal conduction, radiative loss and chromospheric evaporation, are in qualitative agreement with the decay phase observations.Presently at NASA/Marshall Space Flight Center.     

The impulsive and gradual phases of a solar limb flare as observed from the solar maximum mission satellite

Simultaneous observations of a solar limb flare in the X-ray and ultraviolet regions of the spectrum are presented. Temporal and spectral X-ray observations were obtained for the 25–300 keV range while temporal, spectral, and spatial X-ray observations were obtained for the 30–0.3 keV range. The ultraviolet observations were images with a 10 spatial resolution in the lines of O v (T _e 2.5 × 10⁵ K) and Fe xxi (T _e 1.1 × 10⁷ K). The hard X-ray and O v data indicate that the impulsive phase began in the photosphere or chromosphere and continued for several minutes as material was ejected into the corona. Impulsive excitation was observed up to 30 000 km above the solar surface at specific points in the flare loop. The Fe xxi observations indicate a preheating before the impulsive phase and showed the formation of hot post-flare loops. This later formation was confirmed by soft X-ray observations. These observations provide limitations for current flare models and will provide the data needed for initial conditions in modeling the concurrent coronal transient.     

The Evaporation Regime in a Confined Flare

We studied the evolution of a small eruptive flare (GOES class C1) from its onset phase using multi-wavelength observations that sample the flare atmosphere from the chromosphere to the corona. The main instruments involved were the Coronal Diagnostic Spectrometer (CDS) aboard SOHO and facilities at the Dunn Solar Tower of the National Solar Observatory/Sacramento Peak. Transition Region and Coronal Explorer (TRACE) together with Ramaty High-Energy Spectroscopic Imager (RHESSI) also provided images and spectra for this flare. Hα and TRACE images display two loop systems that outline the pre-reconnection and post-reconnection magnetic field lines and their topological changes revealing that we are dealing with an eruptive confined flare. RHESSI data do not record any detectable emission at energies ≥25 keV, and the observed count spectrum can be well fitted with a thermal plus a non-thermal model of the photon spectrum. A non-thermal electron flux F ≈ 5 × 10¹⁰ erg cm⁻² s⁻¹ is determined. The reconstructed images show a very compact source whose peak emission moves along the photospheric magnetic inversion line during the flare. This is probably related to the motion of the reconnection site, hinting at an arcade of small loops that brightens successively. The analysis of the chromospheric spectra (Ca II K, He I D₃ and Hγ, acquired with a four-second temporal cadence) shows the presence of a downward velocity (between 10 and 20 km s⁻¹) in a small region intersected by the spectrograph slit. The region is included in an area that, at the time of the maximum X-ray emission, shows upward motions at transition region (TR) and coronal levels. For the He I 58.4 and O v 62.97 lines, we determine a velocity of ≈−40 km s⁻¹ while for the Fe XIX 59.22 line a velocity of ≈−80 km s⁻¹ is determined with a two-component fitting. The observations are discussed in the framework of available hydrodynamic simulations and they are consistent with the scenario outlined by Fisher (1989). No explosive evaporation is expected for a non-thermal electron beam of the observed characteristics, and no gentle evaporation is allowed without upward chromospheric motion. It is suggested that the energy of non-thermal electrons can be dissipated to heat the high-density plasma, where possibly the reconnection occurs. The consequent conductive flux drives the evaporation process in a regime that we can call sub-explosive.     

X-ray bright point flares due to magnetic reconnection

Ground-based optical observations coordinated with Yohkoh/SXT X-ray observations of an old, disintegrating bipolar active region AR NOAA 7493 (May 1, 1993) provided a multiwavelength data base to study a flaring active region X-ray bright point (XBP) of about 16 hr lifetime, and the activity related to it in different layers of the solar atmosphere. The XBP appeared to be related to a new minor bipole of about 10²⁰ Mx. Superposed on a global evolution of soft X-ray brightness, the XBP displayed changes of brightness, lasting for 1–10 min. During the brightenings the XBP apparently had a spatial structure, which was (tiny) loop-like rather than point-like. The X-ray brightenings were correlated with chromospheric activity: (i) brightenings of underlying chromospheric faculae, and (ii) appearance of strong turbulent velocities in the arch filament system. We propose that the XBP brightenings were due to reconnection of the magnetic field lines (sketched in 3D) between the new bipole and a pre-existing plage field induced by the motion of one of the new pores (v = 0.2 km s^–1) towards the plage, and that the XBP itself was a reconnected hot loop between them.     

Coronal X-ray enhancements associated with Hα filament disappearances

A survey of soft X-ray images from Skylab has revealed a class of large-scale transient X-ray enhancements in the lower corona which are typically associated with the disappearance of H filaments away from active regions. Contemporary with the H filament disappearance, X-ray emitting structures appeared at or near the filament location with shape and size resembling the filament. Eventually these structures faded, but the filament cavity was no longer obvious. Typically the peak of the X-ray event lagged the end of the filament disappearance by tens of minutes. The durations of the coronal X-ray enhancements were considerably longer than the associated H filament disappearances. Major flare effects, such as chromospheric brightenings, typically were not associated with these X-ray events.One event analyzed quantitatively had a peak temperature between 1.8 and 2.7 × 10⁶ K, achieved a peak density of 10⁹ cm^–3 and resulted in an enhancement in the plasma pressure over the conditions of the preexisting coronal cavity of at least a factor of 7. The mass of the coronal X-ray emitting material was about 10% that of the preexisting filament and the thermal energy of the coronal event was on the order of 10²⁹ erg, about 10% of the mechanical energy of the H filament eruption. The event appeared to cool by radiative losses and not by thermal conduction. It is likely that the coronal enhancements are caused by heating of an excess of previously cooler material, either from the filament itself, or by compression of coronal material by a changing magnetic field.     

Mass motions associated with solar flares

Mass motions are a principal means by which components of solar flares can be distinguished. Typical patterns of mass motions in H are described for chromospheric flare ribbons, remote chromospheric flare patches, flare loops, flaring arches, surges, erupting filaments and some expanding coronal features. Interrelationships between these phenomena are discussed and illustrations of each are presented.