ar060.willson01 Posted: 06-Nov-93 Updated: 7-Aug-94, 13-May-95, 24-Feb-96 Events specified: SXT FFIs on 19 and 21 October 1993
Investigators: Robert F. Willson, Kenneth R. Lang, Hugh Hudson
Category: Active Regions - Coronal Phenomena
Motivation: Full-disk VLA observations of the quiet Sun at 91 cm wavelength have revealed large-scale (5' - 10') structures that appear to connect widely-spaced active regions on the solar surface. VLA observations at 20 cm wavelength delineate shorter (1' - 2') loops above individual active regions that evolve on timescales of tens of minutes to hours. VLA observations at 20 cm wavelength also show that solar bursts can occur in the corona on opposite sides of the solar equator and that energetic electrons accelerated during a burst in one active region move along large-scale magnetic conduits,thereby triggering sympathetic flares in the other active region. Comparisons of full-disk soft X-ray and VLA images will give further information about the evolution and physical parameters, (such as electron temperature, electron density and magnetic field strength) of these large-scale structures.
Data and Method to be Used: We will use full-disk SXT and VLA (20 and 91 cm) images and compare them with centimeter wavelength (1.8 - 30 cm) spectral polarization scans taken by the RATAN 600. The SXT and VLA data will be used
to study the structure and evolution of compact and large-scale loops and to establish their plasma parameters. The RATAN data will be used to detect inversions of radio circular polarization that can provide estimates of the magnetic field strength inthe loops detected by the SXT and VLA.
Update 24-Feb-96
Essentially, we have completed this project, using Yohkoh SXT images images obtained from the public-domain archives at Goddard. We have written a paper, to be submitted to Solar Physics that describes these results. The abstract of this paper is given below.
"Very Large Array Observations of Evolving Noise Storm Sources on the Sun"
Robert F. Willson, James N. Kile and Barry Rothberg Dept. of Physics and Astronomy Tufts University Medford, MA 02155 USA
ABSTRACT:
The presence of coronal magnetic fields connecting active regions is inferred from decimetric observations of solar noise storms with the Very Large Array (VLA) and from soft X-ray images taken by Yohkoh. Temporal changes in the noise storms appear to be correlated with soft X-ray bursts detected by both Yohkoh and the GOES satellite. Combined analysis of the radio and X-ray data suggests a re-arrangement of the coronal magnetic field during the onset of impulsive noise storm burst emission. On one day, resolved 91 cm VLA observations show anti-correlated fluctuations in two widely-separated which appear to be connected by a faint trans-equatorial 91 cm source. On another day, a series of 91 cm noise storm bursts are observed along the major axis of the associated noise storm continuum. Time sequences of Yohkoh soft X-ray images show a contraction of coronal loops prior to the onset of this series bursts and a corresponding increase in the X-ray flux in the apparent footpoint of the overarching loop containing the noise storm. These observations imply that energy from a realignment of the magnetic field is being transferred, possibly by accelerated particles, along loops connecting separated active regions on the sun.
Update 13-May-95
In the past year we have compared the daily Yohkoh SXT images available on the Internet with VLA and RATAN radio data and have writted two papers that have been accepted for publication. The abstracts of these papers are given below.
"Large-Scale Coronal Magnetic Fields: Nosie Storms, Soft X-rays and Inversion of Radio Polarization"
R.F. Willson, J.N. Kile, K.R. Lang, S. Donaldson, V.M. Bogod, G.B. Gelfreikh, B.I. Ryabov and S.R. Hafizov
(to appear in the Proceedings of the COSPAR Meeting held in Hamburg, Germany, July 1994).
ABSTRACT : Large-scale coronal magnetic fields can link widely-separated activ eregions in opposite hemispheres of the Sun. The presence of such features is inferred from Very Large Array (VLA) observations of noise storms located within them; by Soft X-ray Telescope (SXT) images that delineate their magnetic structure; and by RATAN-600 spectral observations of inversions of the circularly-polarized radio emission that can be used to infer the coronal magnetic field strength. Extrapolations of photospheric magnetic fields suggest nonpotential fields may be required to explain the polarization inversions in some regions. These anomalously high magnetic fields are also located at the polarization inversions in the source of the noise storms: such storms require long-lasting (hours) nonthermal particle acceleration.
paper 2 "Noise Storms and the Structure of Microwave Emission of Solar Active Regions" - V.M. Bogod, V. Garimov, G.B. Gelfreikh, K.R.Lang, R.F. Willson, J.N. Kile - to appear in Solar Physics
ABSTRACT: Solar radio and microwave sources were observed with the Very Large ARray (VLA) an dthe RATAN 600, providing high spatial resolution at 91 cm (VLA) and detailed spectral and polarization data at microwave wavelengths (1.7 to 20 cm - RATAN). The radio observations have been compared with images from the Soft X-ray Telescope (SXT) aboard the Yohkoh satellite and with full disk photospheric magnetic field data from the Kislovodsk Station of the Pulkovo Observatory. The VLA observations at 91 cm show fluctuating nonthermal noise storm sources in the middle corona. The active regions that were responsible for the noise storms generally had weaker microwave emission, gainter thermal soft X-ray emission, as well as less intense coronal magnetic fields than those associated with other active regions on the solar disk. The noise storms did, however, originate in active regions whose magnetic fields and radiation properties were evolving on timescales of days or less. We interpret these noise storms in terms of accelerated particles trapped in radiation belts above or near active regions, forming a decimetric coronal halo. The particles trapped in the radiation belts may be the source of other forms of nonthermal emission, while also providing a reservoir from which energetic particles may drain down into lower-lying magnetic structures.
More recently, we observed the Sun with the VLA during the solar eclipse on November 3, 1994. From e-mail observing reports, I understand that there are several Japanese Yohkoh team members who might be interested in comparing the SXT results with radio data taken around the same time. Our VLA observations were made between 1500-1900 UT at 3.5, 6.1 20.7 amd 91 cm. The 20 and 91 cm observations covered the full disk while those at the two shorter wavelengths were made by pointing at the target active region, AR7794. Our VLA maps made on November 3 and previously on October 28, 30 and 31 showed that AR7794 was gradually decaying, as was a noise storm source associated with this region. However, it would be very interesting to compare the electron temperature and emission measure derived from the SXT data with the VLA brightness temperatures in order to infer the radio emission mechanism as well as the coronal magnetic field strength and other parameters. I was wondering if you would be kind enough to pass this information along to the relevant investigators who are analyzing the SXT data from November 3. I could also write a proposal requesting a formal collaboration, if you think that would be useful.
Update 7-Aug-94
Our project involves an analysis of full-disk Yohkoh SXT images and VLA snapshot maps at 20 and 91 cm in order to study possible correlations between between decimetric Type-III-like bursts, Type I noise storms, underlying coronal structures. The times for which we have VLA observations are the following: January 9 and 10, 1992; May 9, 10 and 11, 1992; May 6, 8 and 9, 1993; August 19 and 22, 1993; October 19 and 21, 1993; January 9 and 10, 1994. We have spent most of the past six months analzing the VLA data and identifying periods of decimetric burst activity. Our 1.67 snapshot maps of total intensity and circular polarization have been compared with daily SXT images available via FTP from the on-line Yohkoh database. Some preliminary results were presented at the recent COSPAR meeting in Hamburg, Germany. In our paper entitled "Large-Scale Coronal Magnetic Fields: Noise Storms, Soft X-rays and Inversion of Radio Polarization", we showed that some Type I noise storm sources appeared to be located along slowly-evolving large-scale soft X-ray loops that join active regions on opposite sides of the solar equator; during one brief interval (15 minutes), widely-spaced noise storm sources intensified nearly simultaneously, suggesting particle propagation along an arcade of trans-equatorial coronal loops.
During the next few months we will complete the analysis of our VLA data and compare them, when possible, with short-timescale (few seconds duration) SXT imges. These comparisons may give clues to the evolution of large-scale coronal loops in which bursts and noise storms occur. AT the COSPAR meeting, Dr Marylin Bruner promised to send us a video tape of all SXT images during our VLA time periods and these data will be examined for possible changes in X-ray loop structure that might accompany decimetric bursts and noise storms.