S. A. Matthews

A catalogue of white-light flares observed by Yohkoh

The aspect camera of the Soft X-ray Telescope (SXT) on Yohkoh provided the first systematic survey of white-light flares from an observatory in space. The observations were made in the Fraunhofer g-band at a pixel size of 2.46 arcsec and a typical sample interval on the order of ten seconds. A total of 28 flares with clear white-light signatures were detected, corresponding to GOES events down to the C7.8 level in one case. Above the X-class threshold, all 5 events observed by SXT were observed in white light, and the maximum average contrast observed was 30% relative to the pre-flare continuum brightness of the flare location. We have made comprehensive comparisons of Yohkoh soft X-ray and hard X-ray data for this list of flares. In addition we compare the properties of the WLF sample to a sample of 31 flares that showed no white-light emission. These comparisons show that while white-light continuum emission has a strong association with hard X-ray emission it is also strongly related to coronal overpressure, as determined from the soft X-ray spectrum, indicating a component with a thermal, rather than non-thermal origin.

Coronal Magnetic Reconnection Driven by CME Expansion—the 2011 June 7 Event

Coronal mass ejections (CMEs) erupt and expand in a magnetically structured solar corona. Various indirect observational pieces of evidence have shown that the magnetic field of CMEs reconnects with surrounding magnetic fields, forming, e.g., dimming regions distant from the CME source regions. Analyzing Solar Dynamics Observatory (SDO) observations of the eruption from AR 11226 on 2011 June 7, we present the first direct evidence of coronal magnetic reconnection between the fields of two adjacent active regions during a CME. The observations are presented jointly with a data-constrained numerical simulation, demonstrating the formation/intensification of current sheets along a hyperbolic flux tube at the interface between the CME and the neighboring AR 11227. Reconnection resulted in the formation of new magnetic connections between the erupting magnetic structure from AR 11226 and the neighboring active region AR 11227 about 200 Mm from the eruption site. The onset of reconnection first becomes apparent in the SDO/AIA images when filament plasma, originally contained within the erupting flux rope, is redirected toward remote areas in AR 11227, tracing the change of large-scale magnetic connectivity. The location of the coronal reconnection region becomes bright and directly observable at SDO/AIA wavelengths, owing to the presence of down-flowing cool, dense (1010 cm(-3)) filament plasma in its vicinity. The high-density plasma around the reconnection region is heated to coronal temperatures, presumably by slow-mode shocks and Coulomb collisions. These results provide the first direct observational evidence that CMEs reconnect with surrounding magnetic structures, leading to a large-scale reconfiguration of the coronal magnetic field.

2011 FEBRUARY 15: SUNQUAKES PRODUCED BY FLUX ROPE ERUPTION

We present an analysis of the 2011 February 15 X-class solar flare, previously reported to produce the first sunquake in solar cycle 24. Using acoustic holography, we confirm the first, and report a second, weaker, seismic source associated with this flare. We find that the two sources are located at either end of a sigmoid, which indicates the presence of a flux rope. Contrary to the majority of previously reported sunquakes, the acoustic emission precedes the peak of major hard X-ray (HXR) sources by several minutes. Furthermore, the strongest HXR footpoints derived from RHESSI data are found to be located away from the seismic sources in the flare ribbons. We account for these discrepancies within the context of a phenomenological model of a flux rope eruption and accompanying two-ribbon flare. We propose that the sunquakes are triggered at the footpoints of the erupting flux rope at the start of the flare impulsive phase and eruption onset, while the main HXR sources appear later at the footpoints of the flare loops formed under the rising flux rope. Possible implications of this scenario for the theoretical interpretation of the forces driving sunquakes are discussed.

Multi-wavelength observations of an X-class flare without a coronal mass ejection.

Developments in our knowledge of coronal mass ejections (CMEs) have shown that many of these transients occur in association with solar flares. On the occasions when there is a common occurrence of the eruption and the flare, it is most likely that the flare is of high intensity and/or long-duration (Burkepile, Hundhausen, and Webb, 1994; Munro etxa0al., 1979; Webb and Hundhausen, 1987). A model for the relationship between the long-duration event and eruption has been developed (Carmichael, 1964; Sturrock, 1966; Hirayama, 1974; Kopp and Pneuman, 1976), but not so for the high-intensity flares and eruptions. This work investigates the magnetic topology changes that occur for a X1.2 GOES classification flare which has no associated CME. It is found that the flare is likely to result from the interaction between two pre-existing loops low in the corona, producing a confined flare. Slightly higher in the corona, a loop is observed which exhibits an outward motion as a result of the reconfiguration during reconnection. The objective of this work is to gain insight on the magnetic topology of the event which is critical in order to determine whether a high-intensity flare is likely to be related to a CME or not.

Flows in the solar atmosphere due to the eruptions on the 15th July, 2002

Which kind of flows are present during flares? Are they compatible with the present understanding of energy release and which model best describes the observations? We analyze successive flare events in order to answer these questions. The flares were observed in the magnetically complex NOAA active region (AR) 10030 on 15 July 2002. One of them is of GOES X-class. The description of these flares and how they relate to the break-out model is presented in Gary and Moore (2004). The Coronal Diagnostic Spectrometer on board SOHO observed this active region for around 14 hours. The observed emission lines provided data from the transition region to the corona with a field of view covering more than half of the active region. In this paper we analyse the spatially resolved flows seen in the atmosphere from the preflare to the flare stages. We find evidence for evaporation occurring before the impulsive phase. During the main phase, the ongoing magnetic reconnection is demonstrated by upflows located at the edges of the flare loops (while downflows are found in the flare loops themselves). We also report the impact of a filament eruption on the atmosphere, with flows up to 300 km s −1 observed at transition-region temperatures in regions well away from the location of the pre-eruptive filament. Our results are consistent with the predictions of the break out model before the impulsive phase of the flare; while, as the flare progresses, the directions of the flows are consistent with flare models invoking evaporation followed by cooling and downward plasma motions in the flare loops.

Nonthermal Velocity Evolution in the Precursor Phase of a Solar Flare

We present observations of two solar flares occurring in Active Region 7590 on 1993 October 3 using data from the Yohkoh spacecraft. The hard X-ray bursts from the two flares occurred within an 18 minute interval, with the soft X-ray emission having a shorter separation of ≈5 minutes. Both flares occurred within one Yohkoh orbit, and hence we have continuous coverage of the soft X-ray line broadening at the peak of the first flare, reducing to the active region level of 33.5 km s-1 in S XV (66.1 km s-1 in Ca XIX) and then increasing to the peak in the second flare. The rise above the active region background level begins 11 minutes before the start of the second flare as defined by the start of the hard X-ray emission. During this extended rise time of Vnt, there is no increase in the light curves or the electron temperature. We suggest that this increase is an indicator of turbulent changes in the active region prior to the flare that are related to the flare trigger mechanism.

Investigating the dynamics and density evolution of returning plasma blobs from the 2011 June 7 eruption

This work examines infalling matter following an enormous Coronal Mass Ejection (CME) on the 2011 June 7. The material formed discrete concentrations, or blobs, in the corona and fell back to the sur- face, appearing as dark concentrations against the bright corona. In this work we examined the density and dynamic evolution of these blobs in order to formally assess the intriguing morphology displayed throughout their descent. The blobs were studied in five wave lengths(94,131,171,193and211A )usingtheSolarDynamicsObser- vatory Atmospheric Imaging Assembly (SDO/AIA), comparing background emission to attenuated emission as a function of wavelength to calculate column densities across the descent of four separate blobs. We found the material to have a column density of hydrogen of approximately 2 × 1019 cm−2, which is comparable with typical pre-eruption filament column densities. Repeated splitting of the returning material is seen in a manner consistent with the Rayleigh-Taylor instabil- ity. Furthermore, the observed distribution of density and its evolution are also a signature of this instability. By approximating the three-dimensional geometry (with data from STEREO-A), volumetric densities were found to be approximately 2 × 10−14 g cm−3, and this, along with observed dominant length-scales of the instability, was used to infer a magnetic field of the order 1 G associated with the descending blobs.

Evidence of flaring in a transequatorial loop on the Sun

We present evidence of flaring behavior in a transequatorial loop (TEL) that lights up in soft X-rays on 2000 July 13. The large loop structure connects NOAA Active Regions 9070/9066 in the northern hemisphere and AR 9069/9068 in the southern hemisphere. We follow the loop systems for 2 days and observe several pieces of evidence strongly suggesting flare behavior of the form seen in standard flaring in active regions. These include brightenings of the loop structure, cooling of plasma that is seen both in soft X-rays and in the transition region temperatures, morphological evidence of reconnection inflow, and blueshifts around the footpoint of the TEL suggestive of chromospheric evaporation. We present, to our knowledge for the first time, observations of TEL in the O V emission line.

An Investigation of the CME of 3 November 2011 and Its Associated Widespread Solar Energetic Particle Event

Multi-spacecraft observations are used to study the in-situ effects of a large coronal mass ejection (CME) erupting from the farside of the Sun on 3 November 2011, with particular emphasis on the associated solar energetic particle (SEP) event. At that time both Solar Terrestrial Relations Observatory (STEREO) spacecraft were located more than 90 degrees from Earth and could observe the CME eruption directly, with the CME visible on-disk from STEREO-B and off the limb from STEREO-A. Signatures of pressure variations in the corona such as deflected streamers were seen, indicating the presence of a coronal shock associated with this CME eruption. The evolution of the CME and an associated extreme-ultraviolet (EUV) wave were studied using EUV and coronagraph images. It was found that the lateral expansion of the CME low in the corona closely tracked the propagation of the EUV wave, with measured velocities of 240±19xa0kmu2009s−1 and 221±15xa0kmu2009s−1 for the CME and wave, respectively. Solar energetic particles were observed to arrive first at STEREO-A, followed by electrons at the Wind spacecraft at L1, then STEREO-B, and finally protons arrived simultaneously at Wind and STEREO-B. By carrying out a velocity-dispersion analysis on the particles arriving at each location, it was found that energetic particles arriving at STEREO-A were released first and that the release of particles arriving at STEREO-B was delayed by about 50 minutes. Analysis of the expansion of the CME to a wider longitude range indicates that this delay is a result of the time taken for the CME edge to reach the footpoints of the magnetic-field lines connected to STEREO-B. The CME expansion is not seen to reach the magnetic footpoint of Wind at the time of solar-particle release for the particles detected here, suggesting that these particles may not be associated with this CME.

Relative Timing of Soft X-Ray Nonthermal Line Broadening and Hard X-Ray Emission in Solar Flares

The time development of both hard X-ray emission and soft X-ray nonthermal line widths is important for an understanding of energy transport in the flaring solar corona. In this Letter, we investigate the relationship between the temporal behavior of these two phenomena for a number of flares detected by instruments on the Yohkoh spacecraft. We examine 10 flares, all occurring within 30° of the limb, using data from the Bragg Crystal Spectrometer (BCS) and the hard X-ray telescope (HXT). We find that the nonthermal velocity either (1) exhibits a maximum prior to the first significant burst of hard X-rays or (2) is already decaying from an earlier unobserved maximum at the time of the first significant burst of hard X-rays. The decay of the nonthermal velocity as it proceeds from its observed maximum shows little evidence for a direct association with individual hard X-ray bursts. These observations suggest that the nonthermal broadening may be a direct consequence of the flare energy release process rather than a by-product of the energy deposition. In addition, the attainment of a maximum in the nonthermal line width very early in the flare is more indicative that plasma turbulence is the source of the observed broadening rather than hydrodynamic flows, such as chromospheric evaporation.