Amy R. Winebarger

Evolving Active Region Loops Observed with the Transition Region and Coronal explorer. II. Time-dependent Hydrodynamic Simulations

Observations with the Transition Region and Coronal Explorer (TRACE) have revealed a new class of active region loops. These loops have relatively flat filter ratios, suggesting approximately constant temperatures near 1 MK along much of the loop length. The observed apex intensities are also higher than static, uniformly heated loop models predict. These loops appear to persist for much longer than a characteristic cooling time. Recent analysis has indicated that these loops first appear in the hotter Fe XV 284 A or Fe XII 195 A filters before they appear in the Fe IX/Fe X 171 A filter. The delay between the appearance of the loops in the different filters suggests that the loops are impulsively heated and are cooling when they are imaged with TRACE. In this paper we present time-dependent hydrodynamic modeling of an evolving active region loop observed with TRACE. We find that by modeling the loop as a set of small-scale, impulsively heated filaments we can generally reproduce the spatial and temporal properties of the observed loop. These results suggest that both dynamics and filamentation are crucial to understanding the observed properties of active region loops observed with TRACE.

Transition Region and Coronal Explorer and Soft X-Ray Telescope Active Region Loop Observations: Comparisons with Static Solutions of the Hydrodynamic Equations

Active region coronal loop observations with broadband X-ray instruments have often been found to be consistent with the predictions of static loop models. Recent observations in the EUV, however, have discovered a class of active region loops that are difficult to reconcile with static loop models. In this paper, we take a comprehensive look at how coronal loops compare with static models. We select 67 loops with a large range of apex temperatures and half-lengths observed with either the Transition Region and Coronal Explorer or the Soft X-Ray Telescope. We compare these observations to static loop models using both uniform and nonuniform heating. We find that only 2 of the 67 loops are fully consistent with static solutions with uniform heating and a filling factor of unity. We further find that long, cool ( 3 MK) loops are as much as 63 times underdense when compared to the static solutions with uniform heating. We then consider the possibility that the disparity in the density could be due to steady, nonuniform heating along the loop and find that footpoint heating can increase densities only by a factor of 3 over density solutions with uniform heating while loop-top heating results in density solutions that are, at most, a factor of 2.5 smaller than the density solutions with uniform heating. Only 19 of the 67 loops in this data set could be fully consistent with hydrodynamic solutions with steady heating. Hence, we conclude that static loop models are poor representations of most active region loops.

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.

Steady Flows Detected in Extreme-Ultraviolet Loops

Recent Transition Region and Coronal Explorer (TRACE) observations have detected a class of active region loops whose physical properties are inconsistent with previous hydrostatic loop models. In this Letter we present the first co-aligned TRACE and the Solar Ultraviolet Measurement of Emitted Radiation (SUMER) observations of these loops. Although these loops appear static in the TRACE images, SUMER detects line-of-sight flows along the loops of up to 40 km s-1. The presence of flows could imply an asymmetric heating function; such a heating function would be expected for heating that is proportional to (often asymmetric) footpoint field strength. We compare a steady flow solution resulting from an asymmetric heating function to a static solution resulting from a uniform heating function in a hypothetical coronal loop. We find that the characteristics associated with the asymmetrically heated loop better compare with the characteristics of the loops observed in the TRACE data.

Hydrodynamic Modeling of Active Region Loops

Recent observations with the Transition Region and Coronal Explorer (TRACE) have shown that many apparently cool (Te ~ 1-1.5 MK) active region loops are much brighter and have flatter temperature profiles than static loop models predict. Observations also indicate that these loops can persist much longer than a characteristic cooling time. Using time-dependent hydrodynamic simulations, we explore the possibility that these active region loops are actually a collection of small-scale filaments that have been impulsively heated and are cooling through the TRACE 171 A (Fe IX/X) and 195 A (Fe XII) bandpasses. We find that an ensemble of independently heated filaments can be significantly brighter than a static uniformly heated loop and would have a flat filter ratio temperature when observed with TRACE.

Cooling active region loops observed with sxt and trace

An impulsive heating multiple strand (IHMS) model is able to reproduce the observational characteristics of EUV (~1 MK) active region loops. This model implies that some of the loops must reach temperatures where X-ray filters are sensitive (>2.5 MK) before they cool to EUV temperatures. Hence, some bright EUV loops must be preceded by bright X-ray loops. Previous analyses of X-ray and EUV active region observations, however, have concluded that EUV loops are not the result of cooling X-ray loops. In this paper, we examine two active regions observed in both X-ray and EUV filters and analyze the evolution of five loops over several hours. These loops first appear bright in the X-ray images and later appear bright in the EUV images. The delay between the appearance of the loops in the X-ray and EUV filters is as little as 1 hr and as much as 3 hr. All five loops appear as single monolithic structures in the X-ray images but are resolved into many smaller structures in the (higher resolution) EUV images. The positions of the loops appear to shift during cooling, implying that the magnetic field is changing as the loops evolve. There is no correlation between the brightness of the loop in the X-ray and EUV filters, meaning that a bright X-ray loop does not necessarily cool to a bright EUV loop, and vice versa. The progression of the loops from X-ray images to EUV images and the observed substructure is qualitatively consistent with the IHMS model.

Apparent Flows above an Active Region Observed with the Transition Region and Coronal Explorer

The Transition Region and Coronal Explorer (TRACE) observed Active Region 8395 on 1998 December 1 from 1:30:00 to 3:00:00 UT at high cadence in the Fe IX/Fe X channel (log Te ≈ 6.0). Throughout the observing time, brightness variations along a dense bundle of coronal field lines in the southwest corner of the active region were observed. Movies made of this region give the impression of continuous intermittent outflow in this bundle of coronal loops; such apparent outflow is often seen in the TRACE data. In this Letter, we present an analysis of four separate flow events occurring in three different loops. These events are used as tracers of the flow in order to characterize its physical properties, such as apparent velocity. The projected velocities of the intensity fronts of these flows (and hence lower limits of true velocities) are between 5 and 20 km s-1. Comparisons of the observed intensities with those predicted by a quasi-static model suggest that the events can be explained only by a mass flow from the chromosphere into the corona. The persistence of the flows, and their ubiquity in the TRACE observations, indicates that hydrostatic loops models are not applicable to this class of coronal structures.

An Investigation into the Variability of Heating in a Solar Active Region

Previous studies have indicated that both steady and impulsive heating mechanisms play a role in active region heating. In this paper, we present a study of 20 hours of soft X-ray and EUVobservations of solar active region NOAA AR 8731. We examine the evolution of six representative loop structures that brighten and fade first from X-ray images and subsequently from the EUV images. We determine their lifetime and the delay between their appearance in the different filters. We find that the lifetime in the EUV filters is much longer than expected for a single cooling loop. We also notice that the delay in the loops’ appearance in the X-ray and EUV filters is proportionaltothelooplength.Wemodeloneoftheloopsusingahydrodynamicmodelwithbothimpulsiveandquasisteady heating functions and find that neither of these simple heating functions can well reproduce the observed loop characteristics in both the X-ray and EUVimages. Hence, although this active region is dominated by variable emission and the characteristics oftheobservedloopsarequalitatively consistentwith acooling loop, the timescale of the heating in this active region remains unknown. Subject headingg Sun: corona Online material: mpeg animations

ENERGETICS OF EXPLOSIVE EVENTS OBSERVED WITH SUMER

Observations of solar chromosphere-corona transition region plasma show evidence of small-scale, short-lived dynamic phenomena characterized by significant nonthermal broadening and asymmetry in the wings of spectral line profiles. These impulsive mass motions (explosive events) are thought to be the product of magnetic reconnection and to be similar in driving mechanism (though larger in size) to nanoflares, the small-scale events proposed to heat the corona. In this paper, we present a statistical analysis of the energetics of explosive events to address the viability of the nanoflare heating theory. We consider high spectral, spatial, and temporal resolution spectra of the C III λ977, N IV λ765, O VI λ1032, and Ne VIII λ770 lines observed with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) telescope and spectrometer. Each line profile exhibiting explosive event characteristics was analyzed using the velocity differential emission measure (VDEM) technique. A VDEM is a measure of the emitting power of the plasma as a function of its line-of-sight velocity and hence provides a method of accurately measuring the energy flux associated with an explosive event. We find that these events globally release ~4 × 104 ergs cm-2 s-1 toward both the corona and chromosphere. This implies that explosive events themselves are not energetically significant to the solar atmosphere. However, the distribution of these explosive events as a function of their energy has a power-law spectral index of α = 2.9 ± 0.1 for the energy range 1022.7-1025.1 ergs. Since α is greater than 2, the energy content is dominated by the smallest events. Hence, if this distribution is representative of the size distribution down to lower energy ranges (~1022 ergs), such small and (currently) undetectable events would release enough energy to heat the solar atmosphere.

Density and Temperature Measurements in a Solar Active Region

We present electron density and temperature measurements from an active region observed above the limb with the Solar Ultraviolet Measurements of Emitted Radiation spectrometer on the Solar and Heliospheric Observatory. Density-sensitive line ratios from Si VIII and S X indicate densities greater than 108 cm-3 as high as 200 (or 145 Mm) above the limb. At these heights, static, uniformly heated loop models predict densities close to 107 cm-3. Differential emission measure analysis shows that the observed plasma is nearly isothermal with a mean temperature of about 1.5 MK and a dispersion of about 0.2 MK. Both the differential emission measure and the Si XI/Si VIII line ratios indicate only small variations in the temperature at the heights observed. These measurements confirm recent observations from the Transition Region and Coronal Explorer of overdense plasma at temperatures near 1 MK in solar active regions. Time-dependent hydrodynamic simulations suggest that impulsive heating models can account for the large densities, but they have a difficult time reproducing the narrow range of observed temperatures. The observations of overdense, nearly isothermal plasma in the solar corona provide a significant challenge to theories of coronal heating.