Daniel B. Seaton

Evolving Active Region Loops Observed with the Transition Region and Coronal Explorer. I. Observations

Observations made with TRACE have detected a class of persistent active region loops that have flat 195/171 A filter ratios. The intensity of these loops implies a density that is as much as 3 orders of magnitude larger than the densities of static solutions to the hydrodynamic equations. It has recently been suggested that these loops are bundles of impulsively heated strands that are cooling through the TRACE passbands. This scenario implies that the loops would appear in the hotter (Fe XV 284 A or Fe XII 195 A) TRACE filter images before appearing in the cooler (Fe IX/X 171 A) TRACE filter images. In this paper, we test this hypothesis by examining the temporal evolution of five active region loops in multiple TRACE EUV filter images. We find that all the loops appear in the hotter filter images before appearing in cooler filter images. We then use the measured delay to estimate a cooling time and find that four of the five loops have lifetimes greater than the expected lifetime of a cooling loop. These results are consistent with the hypothesis that each apparent loop is a bundle of sequentially heated strands; other explanations will also be discussed. To facilitate comparisons between these loops and hydrodynamic simulations, we use a new technique to estimate the loop length and geometry.

THREE-DIMENSIONAL MORPHOLOGY OF A CORONAL PROMINENCE CAVITY

We present a three-dimensional density model of coronal prominence cavities, and a morphological fit that has been tightly constrained by a uniquely well-observed cavity. Observations were obtained as part of an International Heliophysical Year campaign by instruments from a variety of space- and ground-based observatories, spanning wavelengths from radio to soft X-ray to integrated white light. From these data it is clear that the prominence cavity is the limb manifestation of a longitudinally extended polar-crown filament channel, and that the cavity is a region of low density relative to the surrounding corona. As a first step toward quantifying density and temperature from campaign spectroscopic data, we establish the three-dimensional morphology of the cavity. This is critical for taking line-of-sight projection effects into account, since cavities are not localized in the plane of the sky and the corona is optically thin. We have augmented a global coronal streamer model to include a tunnel-like cavity with elliptical cross-section and a Gaussian variation of height along the tunnel length. We have developed a semi-automated routine that fits ellipses to cross-sections of the cavity as it rotates past the solar limb, and have applied it to Extreme Ultraviolet Imager observations from the two Solar Terrestrial Relations Observatory spacecraft. This defines the morphological parameters of our model, from which we reproduce forward-modeled cavity observables. We find that cavity morphology and orientation, in combination with the viewpoints of the observing spacecraft, explain the observed variation in cavity visibility for the east versus west limbs.

On-Orbit Degradation of Solar Instruments

We present the lessons learned about the degradation observed in several space solar missions, based on contributions at the Workshop about On-Orbit Degradation of Solar and Space Weather Instruments that took place at the Solar Terrestrial Centre of Excellence (Royal Observatory of Belgium) in Brussels on 3 May 2012. The aim of this workshop was to open discussions related to the degradation observed in Sun-observing instruments exposed to the effects of the space environment. This article summarizes the various lessons learned and offers recommendations to reduce or correct expected degradation with the goal of increasing the useful lifespan of future and ongoing space missions.

Field Line Shrinkage in Flares Observed by the X-Ray Telescope on Hinode

The X-Ray Telescope on Hinode has observed individual loops of plasma moving downward in a manner that is consistent with field line shrinkage in the aftermath of reconnection at higher altitudes. An on-disk B3.8 flare observed on 2007 May 2 has loops that clearly change in shape from cusp-shaped to more rounded. In addition, bright loops are observed that decrease in altitude with a speed of approximately 5 km s−1, and fainter, higher loop structures shrink with a velocity of 48 km s−1. A C2.1 flare observed on 2006 December 17 also has loops that change shape. Many bright features are seen to be moving downward in this event, and we estimate their speed to be around 2-4 km s−1. We measure the shrinkage in both of these events, and find that it is 17%-27%, which is consistent with theoretical predictions.

OBSERVATIONAL EVIDENCE OF TORUS INSTABILITY AS TRIGGER MECHANISM FOR CORONAL MASS EJECTIONS: THE 2011 AUGUST 4 FILAMENT ERUPTION

Solar filaments are magnetic structures often observed in the solar atmosphere and consist of plasma that is cooler and denser than their surroundings. They are visible for days—even weeks—which suggests that they are often in equilibrium with their environment before disappearing or erupting. Several eruption models have been proposed that aim to reveal what mechanism causes (or triggers) these solar eruptions. Validating these models through observations represents a fundamental step in our understanding of solar eruptions. We present an analysis of the observation of a filament eruption that agrees with the torus instability model. This model predicts that a magnetic flux rope embedded in an ambient field undergoes an eruption when the axis of the flux rope reaches a critical height that depends on the topology of the ambient field. We use the two vantage points of theSolar Dynamics Observatory (SDO) and the Solar TErrestrial RElations Observatory to reconstruct the three-dimensional shape of the filament, to follow its morphological evolution, and to determine its height just before eruption. The magnetograms acquired by SDO/Helioseismic and Magnetic Imager are used to infer the topology of the ambient field and to derive the critical height for the onset of the torus instability. Our analysis shows that the torus instability is the trigger of the eruption. We also find that some pre-eruptive processes, such as magnetic reconnection during the observed flares and flux cancellation at the neutral line, facilitated the eruption by bringing the filament to a region where the magnetic field was more vulnerable to the torus instability.

The SWAP EUV Imaging Telescope. Part II: In-flight Performance and Calibration

The Sun Watcher with Active Pixel System detector and Image Processing (SWAP) telescope was launched on 2 November 2009 onboard the ESA PROBA2 technological mission and has acquired images of the solar corona every one to two minutes for more than two years. The most important technological developments included in SWAP are a radiation-resistant CMOS-APS detector and a novel onboard data-prioritization scheme. Although such detectors have been used previously in space, they have never been used for long-term scientific observations on orbit. Thus SWAP requires a careful calibration to guarantee the science return of the instrument. Since launch we have regularly monitored the evolution of SWAP’s detector response in-flight to characterize both its performance and degradation over the course of the mission. These measurements are also used to reduce detector noise in calibrated images (by subtracting dark-current). Because accurate measurements of detector dark-current require large telescope off-points, we also monitored straylight levels in the instrument to ensure that these calibration measurements are not contaminated by residual signal from the Sun. Here we present the results of these tests and examine the variation of instrumental response and noise as a function of both time and temperature throughout the mission.

Asymmetric Magnetic Reconnection in Solar Flare and Coronal Mass Ejection Current Sheets

We present two-dimensional resistive magnetohydrodynamic simulations of line-tied asymmetric magnetic reconnection in the context of solar flare and coronal mass ejection current sheets. The reconnection process is made asymmetric along the inflow direction by allowing the initial upstream magnetic field strengths and densities to differ, and along the outflow direction by placing the initial perturbation near a conducting wall boundary that represents the photosphere. When the upstream magnetic fields are asymmetric, the post-flare loop structure is distorted into a characteristic skewed candle flame shape. The simulations can thus be used to provide constraints on the reconnection asymmetry in post-flare loops. More hard X-ray emission is expected to occur at the footpoint on the weak magnetic field side because energetic particles are more likely to escape the magnetic mirror there than at the strong magnetic field footpoint. The footpoint on the weak magnetic field side is predicted to move more quickly because of the requirement in two dimensions that equal amounts of flux must be reconnected from each upstream region. The X-line drifts away from the conducting wall in all simulations with asymmetric outflow and into the strong magnetic field region during most of the simulations with asymmetric inflow. There is net plasma flow across the X-line for both the inflow and outflow directions. The reconnection exhaust directed away from the obstructing wall is significantly faster than the exhaust directed toward it. The asymmetric inflow condition allows net vorticity in the rising outflow plasmoid which would appear as rolling motions about the flux rope axis.

An Analytical Model for Reconnection Outflow Jets Including Thermal Conduction

We develop a model that predicts the velocity, density, and temperature of coronal reconnection jets in the presence of thermal conduction. Our model is based on the quasi-one-dimensional treatment of reconnecting current sheets developed by B. Somov and V. S. Titov using the magnetohydrodynamics nozzle equations. We incorporate thermal conduction into the Somov-Titov framework using slow-shock jump conditions modified to include losses due the conduction of thermal energy along field lines mapping to the chromosphere. We find that thermal conduction has a significant effect on the fast-mode Mach number of the reconnection outflow, producing Mach numbers possibly as high as 7 for some solar-flare conditions. This value is three times greater than previously calculated. We conclude that these termination shocks are considerably more efficient at producing particle acceleration than previously thought since the efficiency of particle acceleration at shocks increases dramatically with Mach number. We compare this model with numerical simulations by T. Yokoyama and K. Shibata and find good agreement.

STRUCTURE AND DYNAMICS OF THE 2010 JULY 11 ECLIPSE WHITE-LIGHT CORONA

The white-light corona (WLC) during the total solar eclipse on 2010 July 11 was observed by several teams in the Moons shadow stretching across the Pacific Ocean and a number of isolated islands. We present a comparison of the WLC as observed by eclipse teams located on the Tatakoto Atoll in French Polynesia and on Easter Island, 83?minutes later, combined with near-simultaneous space observations. The eclipse was observed at the beginning of the solar cycle, not long after solar minimum. Nevertheless, the solar corona shows a plethora of different features (coronal holes, helmet streamers, polar rays, very faint loops and radial-oriented thin streamers, a coronal mass ejection, and a puzzling curtain-like object above the north pole). Comparing the observations from the two sites enables us to detect some dynamic phenomena. The eclipse observations are further compared with a hairy-ball model of the magnetic field and near-simultaneous images from the Atmospheric Imaging Assembly on NASAs Solar Dynamics Observatory, the Extreme Ultraviolet Imager on NASAs Solar Terrestrial Relations Observatory, the Sun Watcher, using Active Pixel System Detector and Image Processing on ESAs PRoject for Onboard Autonomy, and the Naval Research Laboratorys Large Angle and Spectrometric Coronagraph on ESAs Solar and Heliospheric Observatory. The Ludendorff flattening coefficient is 0.156, matching the expected ellipticity of coronal isophotes at 2 R ?, for this rising phase of the solar-activity cycle.

OBSERVATIONAL CHARACTERISTICS OF CORONAL MASS EJECTIONS WITHOUT LOW-CORONAL SIGNATURES

Solar eruptions are usually associated with a variety of phenomena occurring in the low corona before, during, and after the onset of eruption. Though easily visible in coronagraph observations, so-called stealth coronal mass ejections (CMEs) do not obviously exhibit any of these low-coronal signatures. The presence or absence of distinct low-coronal signatures can be linked to different theoretical models to establish the mechanisms by which the eruption is initiated and driven. In this study, 40 CMEs without low-coronal signatures occurring in 2012 are identified. Their observational and kinematic properties are analyzed and compared to those of regular CMEs. Solar eruptions without clear on-disk or low-coronal signatures can lead to unexpected space weather impacts, since many early warning signs for significant space weather activity are not present in these events. A better understanding of their initiation mechanism(s) will considerably improve the ability to predict such space weather events.