David E. McKenzie

Sigmoidal morphology and eruptive solar activity

Soft X-ray images of solar active regions frequently show S- or inverse-S (sigmoidal) morphology. We have studied the Yohkoh Soft X-Ray Telescope video movie for 1993 and 1997. We have classified active regions according to morphology (sigmoidal or non-sigmoidal) and nature of activity (eruptive or non-eruptive). As well, we have used NOAA sunspot areas for each region as a measure of size. We find that regions are significantly more likely to be eruptive if they are either sigmoidal or large.

RECONNECTION OUTFLOWS AND CURRENT SHEET OBSERVED WITH HINODE/XRT IN THE 2008 APRIL 9 'CARTWHEEL CME' FLARE

Supra-arcade downflows (SADs) have been observed with Yohkoh/SXT (soft X-rays (SXR)), TRACE (extreme ultraviolet (EUV)), SOHO/LASCO (white light), SOHO/SUMER (EUV spectra), and Hinode/XRT (SXR). Characteristics such as low emissivity and trajectories, which slow as they reach the top of the arcade, are consistent with post-reconnection magnetic flux tubes retracting from a reconnection site high in the corona until they reach a lower-energy magnetic configuration. Viewed from a perpendicular angle, SADs should appear as shrinking loops rather than downflowing voids. We present X-ray Telescope (XRT) observations of supra-arcade downflowing loops (SADLs) following a coronal mass ejection (CME) on 2008 April 9 and show that their speeds and decelerations are consistent with those determined for SADs. We also present evidence for a possible current sheet observed during this flare that extends between the flare arcade and the CME. Additionally, we show a correlation between reconnection outflows observed with XRT and outgoing flows observed with LASCO.

Observations of Separator Reconnection to an Emerging Active Region

Extreme-ultraviolet (EUV) observations of an emerging active region are used to study separator reconnection in the corona. We identify each EUV loop connecting the emerging polarity to a nearby existing active region over the 41 hr period beginning at emergence onset. Their geometrical resemblance to post-reconnection field lines from a magnetic model of the active region pair implicates separator reconnection in their production. While some reconnection is evident within 7 hr of emergence onset, the most intense period occurs after a 1 day delay. The sum of cross sections of all observed loops accounts for only one-fifth of the transferred magnetic flux predicted by the model. We suggest that the remaining loops remain at temperatures too high, or at densities too low, to be detected in our EUV data. The most intense reconnection requires as much as 109 V along the coronal separator; however, the observed loops suggests that the flux is transferred as discrete bundles of ~4 × 1018 Mx each. The reconnection appears to directly dissipate only a small fraction of the energy released, while the rest is dissipated within the post-reconnection flux over the ensuing 6 or more hours the loops remain visible. The net energy released, and ultimately dissipated, is consistent with the amount that could be stored magnetically during the 24 hr delay between emergence and reconnection.

Hinode XRT observations of a long-lasting coronal sigmoid

Aims. Coronal sigmoids are important sources of eruptions into interplanetary space, and a handful of models have been proposed to explain their characteristic S shape. However, the coronal X-ray images available to date have generally not had sufficient resolution to distinguish between these models. The goal of the present investigation is to determine whether the new observations from Hinode can help us to make such a distinction. Methods. We present the first observations of a persistent coronal sigmoid obtained with the Hinode X-Ray Telescope (XRT). The excellent angular resolution of XRT (1 arcsec per pixel) and the sigmoid’s location near disk center combined to provide an unprecedented view of the formation and eruption of this phenomenon. We compared the observed morphology with expectations inferred from two popular models of sigmoid formation, the bald-patch separatrix surface model and the kinking flux rope model. Results. The images during the pre-eruptive phase show that the overall S shape of the sigmoid comprises two separate J-shaped bundles of many loops. The straight sections of the two J patterns lie anti-parallel to one another in the middle of the S, on opposite sides of the magnetic polarity inversion line. The images during the eruptive phase reveal that, before any soft X-ray flaring begins, a diffuse linear structure almost as long as the sigmoid lifts off from the middle of the S. It shows slight clockwise rotation. The X-ray flare begins with the appearance of a sheared arcade of short loops, in the area centered between the two J-shaped patterns of the sigmoid. Conclusions. Taken together, the observational findings provide strong support for the bald-patch separatrix surface model for this sigmoid.

QUANTITATIVE EXAMINATION OF SUPRA-ARCADE DOWNFLOWS IN ERUPTIVE SOLAR FLARES

Downward motions above post-coronal mass ejection flare?arcades are an unanticipated discovery of the Yohkoh mission, and have subsequently been detected with TRACE, SOHO/LASCO, SOHO/SUMER, and Hinode/XRT. These supra-arcade downflows are interpreted as outflows from magnetic reconnection, consistent with a three-dimensional generalization of the standard reconnection model of solar flares. We present results from our observational analyses of downflows, which include a semiautomated scheme for detection and measurement of speeds, sizes, and?for the first time?estimates of the magnetic flux associated with each shrinking flux tube. Though model dependent, these findings provide an empirical estimate of the magnetic flux participating in individual episodes of patchy magnetic reconnection, and the energy associated with the shrinkage of magnetic flux tubes.

A Stable Filament Cavity with a Hot Core

We present observations of a long-lived solar filament cavity with soft X-ray sources along its axis. This structure appeared above the southern polar crown polarity-inversion line for approximately three rotations during 1997 June-August, centered at a west-limb passage on approximately July 3. At the limb, the Yohkoh soft X-ray data showed a bright region situated above and around the projected filament location but near the axis of the cavity. We describe measurements of the geometry of the cavity, which we interpret as a flux rope that is partially embedded in the photosphere, and use the Yohkoh data to describe the physical parameters of the structure. We find that the core consists of an unresolved mass of filamentary substructures, with a volume filling factor significantly less than unity for the soft X-ray telescope (SXT) resolution. The core has a higher temperature than the cavity surrounding it, ruling out explanations in terms of a transition region supported by thermal conduction. Transient activity occurred in the polar crown region, but no detectable destabilization or eruption of the cavity structure resulted from it. We suggest that the bright structure at the core of the cavity corresponds to higher altitude coronal segments of the field lines that support the filament material.

Quantitative Examination of a Large Sample of Supra-Arcade Downflows in Eruptive Solar Flares

Sunward-flowing voids above post-coronal mass ejection flare arcades were first discovered using the soft X-ray telescope aboard Yohkoh and have since been observed with TRACE (extreme ultraviolet (EUV)), SOHO/LASCO (white light), SOHO/SUMER (EUV spectra), and Hinode/XRT (soft X-rays). Supra-arcade downflow (SAD) observations suggest that they are the cross-sections of thin flux tubes retracting from a reconnection site high in the corona. Supra-arcade downflowing loops (SADLs) have also been observed under similar circumstances and are theorized to be SADs viewed from a perpendicular angle. Although previous studies have focused on dark flows because they are easier to detect and complementary spectral data analysis reveals their magnetic nature, the signal intensity of the flows actually ranges from dark to bright. This implies that newly reconnected coronal loops can contain a range of hot plasma density. Previous studies have presented detailed SAD observations for a small number of flares. In this paper, we present a substantial SADs and SADLs flare catalog. We have applied semi-automatic detection software to several of these events to detect and track individual downflows thereby providing statistically significant samples of parameters such as velocity, acceleration, area, magnetic flux, shrinkage energy, and reconnection rate. We discuss these measurements (particularly the unexpected result of the speeds being an order of magnitude slower than the assumed Alfv?n speed), how they were obtained, and potential impact on reconnection models.

Turbulent Dynamics in Solar Flare Sheet Structures Measured with Local Correlation Tracking

High-resolution observations of the Suns corona in extreme ultraviolet and soft X-rays have revealed a new world of complexity in the sheet-like structures connecting coronal mass ejections (CMEs) to the post-eruption flare arcades. This article presents initial findings from an exploration of dynamic flows in two flares observed with Hinode/XRT and SDO/AIA. The flows are observed in the hot ( 10?MK) plasma above the post-eruption arcades and measured with local correlation tracking. The observations demonstrate significant shears in velocity, giving the appearance of vortices and stagnations. Plasma diagnostics indicate that the plasma ? exceeds unity in at least one of the studied events, suggesting that the coronal magnetic fields may be significantly affected by the turbulent flows. Although reconnection models of eruptive flares tend to predict a macroscopic current sheet in the region between the CME and the flare arcade, it is not yet clear whether the observed sheet-like structures are identifiable as the current sheets or thermal halos surrounding the current sheets. Regardless, the relationship between the turbulent motions and the embedded magnetic field is likely to be complicated, involving dynamic fluid processes that produce small length scales in the current sheet. Such processes may be crucial for triggering, accelerating, and/or prolonging reconnection in the corona.

Large Doppler Shifts in X-Ray Plasma: An Explosive Start to Coronal Mass Ejection

We report observations, taken with the Solar Ultraviolet Measurements of Emitted Radiation spectrometer, of spatially resolved high red and blue Doppler shifts (up to 650 km s-1) from X-ray-emitting plasma in the corona above a flare. The high Doppler shifts are seen minutes after a fast, faint optical front is seen racing through the same part of the corona in images taken with the Mirror Coronagraph for Argentina. The association of the large-scale fast optical emission front with soft X-ray emission and high Doppler shifts suggests plasma heating and acceleration in the wake of a shock.

Periodic Modulation of X-Ray Intensity from Coronal Loops – Heating by Resonant Absorption?

We have applied time-series analysis to sequences of coronal loop images obtained with the Yohkoh soft X-ray telescope (SXT). Using images obtained in fast-cadence mode (δ t = 4 s), we have generated X-ray light curves and power spectra for multiple subregions of a number of coronal loops. Our analysis indicates that out of 544 spectra of loops which range in length L from 40 Mm to 150 Mm, there are 16 cases where the X-ray brightness is modulated periodically with ≥99.5% confidence. The periods τ range from 9.6 s to 61.6 s, and the amplitudes range from 0.4% to 1.8%. Our result is statistically significant at the 8σ level.The observed periods can hardly be due to fluctuations in gas pressure: temperatures estimated from image ratios indicate sound speeds of 200–280 km s-1, with loop crossing times of hundreds of seconds. On the other hand, the ratio 2 L/τ yields ‘velocities’ of 2–14 Mm s-1: these overlap with published estimates of Alfvén speeds vA in coronal loops, 3.5–37 Mm s-1 (Schmelz et al., 1994). Now, in the context of coronal loop heating by resonant absorption of MHD waves, the period τg ≈ 2L/vA plays a key role: loops which are oscillating in their global mode (with period τg) are maximally efficient at absorbing energy (Steinolfson and Davila, 1993). We suggest that the loops which we have found to be modulated periodically are oscillating in their global mode. With this interpretation, we find that the plasma beta in the loops lies in the range βp = 0.001–0.045. According to the oscillating loop model of Zaitsev and Stepanov (1989), loops with such βp values should be modulated with amplitudes of 1.1% or less: our data are consistent with at least the upper limit of this prediction.