Solar Physics Research at Tufts University

The Tufts solar physics group has long been involved in the analysis and interpretation of radio, X-ray and optical wavelength observations of the Sun and active stars. The main objective of this research is an improved understanding of physical processes that occur within active regions that produce intense solar flares and other eruptive phenomena.

We have used the world's most sensitive radio telescope, the Very Large Array (VLA) to study the structure and evolution of radio emission from coronal loops and solar bursts. The VLA is unique in its ability to provide two-dimensional images of bursts and other long-lived structures with high spatial and temporal resolution. High resolution VLA observations of the Sun at 20 and 91 cm have made it possible to locate and resolve sites of impulsive energy release at two different heights in the solar corona, thereby providing insights to acceleration mechanisms and plasma parameters of the energetic particles. Investigations of microwave burst emission have also yielded insights to the pre- flare heating of coronal loops, the successive interaction of adjacent loops and the role of nonthermal particles during all phases of flares. Our VLA data have also been compared with X- ray observations from the Yohkoh Soft X-ray Telescope and with Extreme Ultraviolet observations from the Solar and Heliospheric Observatory (SOHO), the TRansition Region and Coronal Explorer (TRACE) and the Reuven Ramaty High Energy Solar and Spectroscopic Imager (RHESSI) satellites. These instruments sample solar emission from different temperature regimes and at different heights in the solar atmosphere and complement the VLA observations and enhance its scientific return. In the picture above, we compare EUV and soft X-ray images taken with the SOHO Extreme Ultraviolet Imaging Telescope (EIT) and the Yohkoh SXT with a VLA snapshot map at 91 cm wavelength. The VLA map shows intense Type I noise storm emission above the active region at the limb. This time-variable radio emission followed, by a few minutes, a solar flare and coronal mass ejection detected by SOHO and TRACE, This suggests that these evolving EUV sources may have played a role in accelerating electrons that were eventually seen in the noise storm at greater heights.

Recent Publications

Very Large Array, SOHO and RHESSI Observations of Magnetic Interactions and Particle Propagation ACross Large-SCale Coronal Loops

R.F. Willson and T.D. Groff Department of Physics and Astronomy, Tufts University, Medford, MA 02155 USA

to appear in Solar Physics, 2008

Collaborative VLA, SOHO and RHESSI Observations of Evolving Sources of Energy Release in the Corona Above Active Regions

R.F. Willson Department of Physics and Astronomy, Tufts University, Medford, MA 02155 USA

Paper presented at the 34th Scientific Assembly of COSPAR, Paris, July 2004

Multi-wavelength VLA and Spacecraft Observations of Evolving Coronal Structures Outside Flares

R.F. Willson Department of Physics and Astronomy, Tufts University, Medford, MA 02155 USA

Paper presented at IAU Symposium 223 "Multi-Wavelength Investigations of Solar Activity, St. Petersburg, Russia, June 2004

VLA and Spacecraft Imaging of Nonthermal Energy Release in the Solar Corona

R.F. Willson Department of Physics and Astronomy, Tufts University, Medford, MA 02155 USA

NRAO Newsletter, No. 94, p 10, 2003

Very Large Array Observations of Evolving Type I Noise Storms and Nonthermal Bursts In Association With Flares and Coronal Mass Ejections

R.F. Willson Department of Physics and Astronomy, Tufts University, Medford, MA 02155 USA

in Solar Physics, 211, 289-313 (2002)

Abstract:

Very Large Array (VLA) observations of the Sun at 20, 91 and 400 cm have been combined with data from the SOHO, TRACE and Wind solar missions to study the properties of long-lasting Type I noise storms and impulsive metric and decimetric bursts during solar flares and associated coronal mass ejections. These radio observations provide information about the acceleration and propagation of energetic electrons in the low and middle corona as well as their interactions with large-scale magnetic structures where energy release and transport takes place. For one flare and its associated CME, the VLA detected impulsive 20 and 91 cm bursts that were followed about ten minutes later by 400 cm burst emission that appeared to move outward into the corona. This event was also detected by the WAVES experiment on Wind which showed intense, fast-drifting interplanetary Type III bursts following the metric and decimetric bursts detected by the VLA. For another event, impulsive 91 cm emission was detected about a few minutes prior to impulsive bursts at 20.7 cm, suggesting an inwardly propagating beam of electrons that excited burst emission at lower levels and shorter wavelengths We also find evidence for significant changes in the intensity of Type I noise storms in the same or nearby active region during impulsive decimetric bursts and CMEs. These changes might be attributed to flare-initiated heating of the Type I radio source plasma by outwardly-propagating flare ejecta or or to the disruption of ambient magnetic fields by the passage of a CME.

VLA, SOHO and TRACE Observations of Nonthermal Burst Activity, Coronal Mass Ejections and EUV Transient Events

R.F. Willson Department of Physics and Astronomy, Tufts University, Medford, MA 02155 USA

Solar Physics,197, 399, 2000

Abstract:

Very Large Array (VLA) observations of the Sun at 91 and 400 cm wavelength have been used to investigate the radio signatures of EUV heating events and coronal mass ejections (CMEs) detected by SOHO and TRACE. Our 91 cm observations show the onset of Type I noise storm emission about an hour after an EUV ejection event was detected by EIT and TRACE. The EUV event also coincided with the estimated start time of a CME detected by the LASCO C2 coronagraph, suggesting an association between the production of nonthermal particles and evolving plasma-magnetic field structures at different heights in the corona. On another day, our VLA 400 cm observations reveal weak, impulsive "microbursts" that occurred sporadically throughout the middle corona. These low brightness temperature (Tb = 0.7 -2.2 x 106 K) events may be weak Type III bursts produced by beams of nonthermal electrons which excite plasma emission at a height where the local plasma frequency or its first harmonic equals the observing frequency of 74 MHz. For one microburst, the emission was contained in two sources separated by 0.7Ro, indicating that the electron beams had access to widely-divergent magnetic field lines originating at a common site of particle acceleration. Another 400 cm microburst occurred in an arc-like source lying at the edge of EUV loops that appeared to open outward into the corona, possibly signaling the start of a CME. In most instances the 400 cm microbursts were not accompanied by detectable EUV activity, suggesting that particles that produce the microbursts were independently accelerated in the middle corona, perhaps as the result of some quasi- continuous, large-scale process of energy release.

VLA Decimetric Observations of Transient Events Detected by SOHO and TRACE

R.F. Willson Department of Physics and Astronomy, Tufts University, Medford, MA 02155 USA

C.C. Kankelborg Department of Physics, Montana State University, Bozeman, MT 59717, USA

in the Proceedings of the 8th SOHO Workshop, Paris, France, June, 1999, ESA Publication

Abstract

We discuss the results of recent collaborative investigations of the Sun that ombine Very Large Array (VLA) images at 91.6 cm wavelength with SOHO EIT and TRACE EUV spectroheliograms at several wavelengths. The main objective of these observations was to investigate the spatial and temporal relationship between nonthermal decimetric burst activity and small-scale transient EUV events in active regions. Our 91 cm observations show changes in the intensity of overlying Type I noise storms a few minutes after five of seven transient brightenings observed by SOHO and TRACE. This suggests that these evolving EUV sources may have played a role in the production of nonthermal electrons eventually seen in the noise storm emission at greater heights.

The First VLA Observations of Nonthermal Metric Bursts Associated With Coronal Mass Ejections Detected By SOHO

R. F. Willson, S. L. Redfield, and K.R. Lang, Department of Physics and Astronomy, Tufts University, Medford, MA 02155

B. J. Thompson, SOHO Experiment Operations and Analysis Facility, NASA Goddard Space Flight Center, Greenbelt, MD 20771

O.C. St. Cyr, CPI/NRL, Code 682.3, NASA Goddard Space Flight Center, Greenbelt, MD 20771

Astrophysical Journal Letters 504,L116,1998

Abstract

We present the first observations of nonthermal decimetric burst emission of the Sun using the new 400 cm (74 MHz) system at the VLA. Our VLA observations were carried out in collaboration with the Large Angle Spectroscopic Coronagraph (LASCO) and the Extreme-ultraviolet Imaging Telescope (EIT) on board the SOlar and Heliospheric Observatory (SOHO) spacecraft. Full-disk observations at 400 and 91 cm were used to study the spatial and temporal variations of nonthermal radio bursts during Coronal Mass Ejections (CMEs) detected by LASCO as well as transient Extreme Ultraviolet (EUV) brightenings detected by the EIT. VLA snapshot maps at 400 cm revealed impulsive burst emission in the low corona that began near the estimated start time of the CME activity; that beginning also coincided with a C1.1 GOES X-ray burst and an EIT flare-like brightening. The nonthermal metric bursts then continued sporadically during the next several hours, which included the ejection of spatially separated CME components. The 400 cm bursts are contained within curved, or arch-like, sources at a fixed radial distance, but with a varying position between two active regions detected by the EIT near the limb, suggesting that they were emitted within large-scale magnetic loops. Our 91 cm observations also show the onset of a long-lasting Type I noise storm following the initiation of CME activity, again suggesting an intimate relationship between the production of nonthermal particles and large-scale evolving plasma-magnetic field structures in the corona.

The Radio and EUV Signatures of Small-Scale Coronal Magnetic Reconnection Events

R.F. Willson

(paper presented at the Coronal Plumes and Solar X-ray jets, Guadeloupe, February, 1998)

ESA-SP-421. 349, 1998

Abstract

We discuss the results of recent collaborative investigations of evolving solar active regions that combine Very Large Array (VLA)images at 3.5, 6.2, 20.7 and 91.6 cm with SOHO EIT and CDS EUV spectroheliograms at several wavelengths. The main objective of these observations was to study the spatial and temporal relationship of transient events in both the radio and EUV domains and to constrain physical parameters such as electron temperature, electron density and magnetic field strength in emitting regions. Observations made on eight different days in 1997 show that only some EUV events have counterparts in microwaves. The EUV transient events seem to occur throughout the target active regions while the evolving microwave sources predominantly overlie regions of high magnetic field strength near sunspots. Our VLA 91 cm observations also show changes in the brightness of Type I noise storm and decimetric Type III-like burst sources following some EUV transient events. In one case, significant changes in the structure of 20 cm coronal loops and 91 cm noise storm sources occurred a few tens of minutes prior to the appearance of an EUV ejection event or jet suggesting interaction between large and small-scale loops at different levels in the corona.

Microwave Spectral-Polarization Structure of Type I Noise Storm-Producing Solar Active Regions

V.M. Bogod, V.I. Garaimov, G.B Gelfreikh, R.F. Willson, J.N Kile and K.R. Lang

(paper presented at the 10th Cool Stars, Stellar Systems and the Sun Workshop, Cambridge, MA)

Abstract

RATAN-600 spectral-polarization observations of solar active regions have been compared with with VLA and Nancay Radioheliograph observations of noise storms and Yohkoh soft X-ray images of coronal loops. The RATAN data were used to identify active regions which show narrow-band polarization inversions of sunspot-associated microwave sources. Several different types of spectral and temporal changes of the microwave polarization of the local sources associated with noise storm-producing active regions were detected.In contrast to some previous observations, our data show that the time at which the polarization inversion occurs does not always depend on the location of the active region on the disc with respect to central meridian passage. For one active region the inverted polarity of a sunspot-associated source persists for several days and then reverts to what one would normally expect from the polarity of the underlying sunspot. For other microwave sources, a double inversion in circular polarization was observed over a relatively narrow frequency range. The observed inversion of the sign of circular polarization was, in some cases, impossible to explain in terms of the propagation of radio waves through a quasi-transverse (QT) region in the corona. Instead, these narrow-band polarization changes require more complicated magnetic configurations, such as current sheets, that may also produce the accelerated particles responsible for noise storms at longer wavelengths

VLA-SOHO Observations of Evolving Coronal Structures on the Sun

R. F. Willson and K.R. Lang, B. Thompson, U. Schuehle and D. M. Zarro

(paper presented at the 10th Cool Stars, Stellar Systems and the Sun Workshop, Cambridge, MA)

abstract

We discuss preliminary results of collaborative VLA and SOHO observations of the Sun made during two Joint Observing Programs in 1996 and 1997. VLA images at 3.5, 6.2, 20.7 and 91.6 cm were combined with SOHO EIT, CDS and SUMER spectroheliograms to delineate the structure of magnetic loops in the transition region and corona and to constrain the emission mechanism and plasma parameters in these regions. VLA snapshot maps at 3.5 and 6.2 cm, together with sequences of EIT and CDS images (2' - 8.5' fd of view), were also used to study temporal variations in source structure that reflect changes in temperature and magnetic field strength of the transition region and corona. Full-disk VLA maps at 20 cm wavelength reveal compact (10" - 20") sources that may be the radio counterparts of EUV bright points. The 91 cm VLA maps show extended emission from the quiet Sun including brightness temperature depressions above coronal holes seen by EIT at EUV wavelengths. These radio maps also show large-scale (5' - 7') loop-like structures at the solar limbs that lie at the base of evolving coronal streamers detected by the LASCO coronagraphs

X-ray Jets and Their Radio Signatures at Metric and Centimeter Wavelenths

L. van Driel-Gesztelyi, R.F. Willson, J.N. Kile, A. Raoult L. Klein, N. Mein, P. Rudawy, B. Cader, B. Rompolt, B. Schmieder, P. Mein and J.M. Malherbe

(paper presented at the 10th Cool Stars, Stellar Systems and the Sun Workshop, Cambridge, MA)

Abstract

On October 19, 1995, two homologous X-ray jets were observed with the Yohkoh/SXT from AR 7912, a region having a reversed polarity group with high magnetic shear. The high shear is well-visible in the vortex-like H$\alpha$ fibril pattern observed with the MSDP instrument mounted on the German VTT on Tenerife as well as in twisted-looking X-ray loops on the Yohkoh images. The first event, between 10:25-10:32 UT, was observed with the Nancay radio heliograph at 167, 236 and 327 MHz. Type III activity superimposed on a noise-storm was clearly visible at 164 MHz and 327 MHz. At 164 MHz and 236 MHz the storm had a double structure, while at 327 MHz it consisted of a single component. Type III activity first appeared at 164 MHz and 236 MHz close to the storm position and in the direction of the X-ray jet. Between 10:28:40 UT and 10:28:50 UT a new group of sources appeared eastward of the former activity, which may correspond to remote footpoints of a large-scale loop linked to the active region or to a second jet event along a more eastern path. At 327 MHz burst activity was observed near the position of the storm, and close to the location of the first type III burst,along the X-ray jet. The second X-ray jet event occurred between 16:58-17:12 UT, and was also observed with the Very Large Array (VLA) at 6.2, 20.7 and 91.6 cm. For this event, VLA snapshot maps at 6.2 and 20.7 cm reveal low-brightness temperature changes in source structure at the site of the X-ray jet during the preburst, impulsive and decay phases. The VLA 91.6 cm observations also show noise storm emission above the active region but there is no clear temporal correlation between this later X-ray jet and the impulsive decimetric bursts that were observed during this period. Although the X-ray observations show that the two jets had similiar temperatures and emission measures, they appear to have had dissimiliar centimeter and metric responses to these events and the implications of these results will be discussed in our paper.

VLA-SOHO OBSERVATIONS OF THE SOLAR CORONA

K.R. Lang, R.F. Willson

(paper presented at the 5th SOHO Workshop, Oslo, Norway)

Abstract

Very Large Array (VLA) observations at 3.5, 6.2, 20.7 and 91.6 cm wavelength are compared with SOHO SUMER, CDS, EIT and LASCO data taken during three days of coordinated observations in June, 1996. VLA synthesis maps at these wavelengths have been combined with SUMER and CDS spectroheliograms to delineate the structure of a target active region from the chromosphere to corona and to constrain the emission mechanism and plasma parameters at these levels. Snapshot maps at 3.5 and 6.2 cm have also been used to study temporal variations in the emission that may reflect changes in the temperature or magnetic structure in the underlying active region. Full-disk VLA maps at 20 cm wavelength reveal a number of compact (10"-20") low brightness sources that may be the radio counterparts of EUV bright points detected by the EIT. The 91.6 cm VLA maps show extended emission across the solar disk including brightness temperature depressions that lie above coronal holes seen in the EIT images. These maps also show loop-like structures at the solar limbs that appear to lie at the base of evolving coronal streamers detected by the LASCO coronagraphs.