Corona I
Presiding: R Moore, Marshall Space Flight Center; H Gilbert, High Altitude Observatory
SP13B-01 13:30h
Dissertation talk: The EUV Unresolved Corona and Coronal Loops
In this work, physical characteristics of the solar corona in the Extreme Ultra-Violet (EUV) regime of the electromagnetic spectrum are investigated. Regions of heightened coronal activity are generally found near the locations of sunspots. These regions are commonly called Active Regions and are the focus of the project. Multiple space based observing platforms have been deployed in the last decade and it is possible to use several of these observatories in combination to develop a more complete picture of the solar corona. Joint Observing Program 146 was created to collect spectroscopic intensities using the Coronal Diagnostic Spectrometer (CDS) on Solar and Heliospheric Observatory (SoHO) and EUV images using NASA's Transition Region and Coronal Explorer (TRACE). The emission line intensities are used to develop an understanding of the temperature and density of the active region coronal plasma. However, the performance of the CDS instrument in the spatial and temporal domains is limited and to compensate for these limitations data collected by the TRACE instrument provide a high spatial and temporal resolution set of observations. The combination of the information contained in the data from these two instruments is used to determine the temperature, density and dynamics of the solar coronal active regions. One of the most exciting unsolved problems in solar astrophysics is to understand why the corona maintains a temperature roughly two orders of magnitude higher than the underlying material. An detailed investigation of the coronal emission has provided constraints on models of the heating mechanism, since the temperature, density and evolution of emission rates for multiple ionic species are indicative of the mechanism(s) working to heat the corona. The observed corona appears to consist of multiple unresolved structures as well as resolved active region structures, called coronal loops. The purpose of the present work is to determine the characteristics of the unresolved background corona. This has two important applicatons. Any model of the corona must account for the emission comprising the unresolved background, and the study of coronal loops can be improved by subtracting the emission along the line-of-sight due to the background contribution to the total intensity. Using the characterizations of the coronal unresolved background, results for loops after background subtraction are also presented. This work demostrates the magnitude of the unresolved coronal emission with respect to the total emission along the line of sight, and how this compares to that of the resolved loops. It is apparent from this analyis that the unresolved corona is the dominant source of radiation in the active region corona.
SP13B-02 14:00h
The Elementary Structure of Coronal Loops
There is the question whether the plasma of coronal loops consists of unresolved strands or not, given the best spatial resolution of <1" we have today with EUV telescopes such as TRACE. Since the hypothetical loop strands would be thermally insulated from each other, due to the inhibited cross-field diffusion in the coronal plasma that has a plasma-beta parameter significantly smaller than unity, we expect that loop strands have arbitrarily different heating rates and temperatures. Consequently, both resolved as well as unresolved loop strands are expected not to be co-spatial in different temperature filters. We present the results of a triple-filter analysis of a larger number of coronal loop structures observed with TRACE in the 171, 195, and 284 A filters, where we measure the cospatiality of loop substructures with sub-arcsecond accuracy in near-simultaneous measurements with triple filters. In virtually all loop structures we identify multiple resolved substructures that are not cospatial in triple temperature filters, but are cospatial in dual temperature filters and consistent with a single temperatures over partial loop length segments. From these results we conclude that the cross-section of heated loop strands is resolved on ~1" spatial scale, but no thermal equilibrium is achieved over the entire length of coronal loops, and thus loops are always asymmetric in their temperature and density structure.
SP13B-03 14:15h
Why Does TRACE See So Many Isothermal Loops?
The Transition Region and Coronal Explorer (TRACE) has advanced our view of the dynamics of solar active regions. TRACE brings the highest spatial resolution and reasonable temporal coverage to bear on the evolution and structure of coronal plasma; temperature discrimination is achieved with three narrowband EUV filters and the filter ratio method. Many thin coronal loops have been observed to have near-constant filter ratios along their length, which has commonly been interpreted as evidence for isothermal structure. We discuss and quantify how the TRACE filter response ratios are biased to estimate relatively constant isothermal temperatures in the observed range when the plasma along the line-of-sight is multithermal.
SP13B-04 14:30h
EM Loci of CDS Loop Data
Our original analysis of the 1998 April 20 SOHO-CDS spectral line data for a coronal loop on the solar limb showed that the plasma was multi-thermal, both along the length of the loop as well as along the line of sight. Here we consider the effect of background subtraction on our analysis, and show EM Loci plots of three representative pixels, one near the loop apex, a second at the upper loop leg, and a third at the lower loop leg. Comparisons of the original and background-subtracted intensities show that the EM Loci are more tightly clustered after background subtraction, but that the plasma is still not well represented by an isothermal model. Possible explanations include a series of isothermal loops contributing along the line of sight, or multiple adjacent isothermal strands at different temperatures within the resolution element. Solar physics research at the University of Memphis is supported by NASA grants NAG5-9783 and NAG5-12096.
SP13B-05 14:45h
The Ability of Extreme Ultraviolet and Soft X-ray Imaging Telescopes to Constrain Coronal Differential Emission Measure
Using the Chianti database, we construct temperature response functions for the extreme ultraviolet and soft x-ray imaging channels on a variety of current and future solar observatories. Simulated observations in these channels are then inverted to recover the coronal differential emission measure. We investigate the effect of statistical error, and systematic uncertainties in the atomic physics and instrument calibration used to calculate the instrument temperature response functions, on the accuracy of the DEM recovery. By selecting different combinations of channels, we can characterize the ability of different instrument architectures to constrain the DEM, and identify an optimal combination of bandpasses for accurate DEM recovery.