Scientific Justification
Introduction
VLA-HESSI Investigations of the Sites of Particle Acceleration in Flares
Over the past two decades, hard X-ray and -ray observations of the Sun have shown that the acceleration of
high-energy bremsstrahlung-producing electrons and -ray line-producing ions is a common signature of the
impulsive phase of powerful solar flares. Observations have revealed that there is a rough correlation between the
hard-X-ray electron bremsstrahlung fluence and the -ray line one, suggesting that electrons and ions may be
accelerated during all flares (Vestrand 1988; Cliver et al. 1994). The relative variability of the bremsstrahlung
components, however, is different for different flares, reflecting the relative efficiency of electron and ion
acceleration as well as the different physical conditions in the magnetic structures in which particle acceleration
takes place.
The time profiles of hard X-ray and microwave bursts are generally well correlated (Cornell et al. 1984; Kai and
Nakajima 1987; Lee and Gary 1994) and this suggests that the emission in the two wavebands has a common
origin. It is generally thought that the hard X-ray emission is produced by nonthermal electrons which precipitate
from the injection site (usually assumed to be near the apex of the coronal loops) to the feet of magnetic loops by
so-called thick-target bremsstrahlung (Lu and Petrosian 1989), while the short cm (( 6 cm) microwave
emission is thought to be produced by the gyrosynchrotron radiation of the nonthermal electrons within the same
volume. Radio emission at longer wavelengths (  6 cm) may also be detected near the tops of coronal loops
where the lower electron densities can render these parts of the loop optically thick.
This proposal involves coordinated observations of solar flares and microflares using data from the VLA and
the Reuven Ramaty High Energy Spectroscopic Imager Mission. The goal of these investigations will be to combine
the hard X-ray and microwave data to provide information about the evolving magnetic field environment in the
corona where the energy release takes place. Although the hard X-ray spectral data of RHESSI provide information
about the energy distribution of the thermal an nonthermal emitting electrons, they cannot provide quantitative
information about the magnetic fields because this radiation is not affected by the magnetic field.
Radio observations provide the only means to obtain information about the magnetic field intensity at different
levels in the corona and this is because the radio brightness temperature and polarization strongly depend on the
intensity and direction of the magnetic field where the emission is produced. Although the quiescent microwave
emission from solar active regions may, in principle, be produced by either thermal bremsstrahlung or thermal
gyroresonance radiation,( the latter depending critically on the local magnetic field strength and direction) solar
microwave bursts are most-likely due to gyrosynchrotron radiation from either a thermal or nonthermal (power-law)
distribution of electrons (see Dulk 1985 for a review). These radio emission mechanisms depend on the local plasma
parameters including the electron temperature, the thermal and nonthermal electron density and the size of the
emitting region, upon which the hard X-ray emission also depends. Using the hard X-ray-derived plasma
parameters, one can calculate the expected microwave brightness temperature at different wavelengths and compare
it to the observed brightness temperature obtained from the VLA observations, thereby specifying the microwave
emission mechanism as well as the magnetic field strength.
For example, it is possible to relate the energy spectral indices of the centimeter-wavelength and hard
X-ray-emitting electrons, R and X, to the hard X-ray photon spectral index, , for different models of particle
propagation (see Trottet et al. 1998 and references therein). Our proposed VLA-RHESSI observations will also be
used to test such models, but will have the unique advantage of high angular resolution images that can be used to
directly examine the co-spatiality of the microwave and hard X-ray sources.
The VLA and RHESSI data will additionally be used to test magnetic reconnection models that have been
developed to explain particle acceleration during flares. Recent studies indicate, for example, that solar flares can be
triggered by interacting loops (Hanaoka 1997, 1998; Manoharan et al, 1996). Current two-dimensional models,
summarized by Hanaoka (1998), suggest that reconnection takes place between two nearly-oppositely directed, or
anti-parallel, magnetic loops, while other three-dimensional models include twisted loops and current-loop
interactions that can produce magnetic reconnection in nearly parallel fields. VLA snapshot maps of total intensity
and circular polarization at 1.2,2 and 3.5 cm may be used to locate the sites of particle acceleration and to infer the
magnetic configurations of the coronal loops involved, thereby providing tests of these different magnetic
reconnection models.
VLA-HESSI Observations of Microflares
Our VLA observations will also be used to study smaller-scale hard X-ray events, called microflares, which
mimic their more intense counterparts, releasing stored magnetic energy in the corona by magnetic reconnection.
Nitta (1997) has used the Yohkoh HXT and SXT to show that hard X-ray microflares are most noticeable in the
rising phase of soft X-ray events, suggesting an association with impulsive energy release and a non-thermal origin.
Because of its greater sensitivity, RHESSI should be able to detect the impulsive, nonthermal emission of similar,
less intense nonthermal microflares. Radio observations with the VLA have also shown that microflares can exhibit
nonthermal radiation spectra and other signatures of energetic, nonthermal electrons (Gary Hartl and Shimizu 1997;
Gopalswamy et al. 1997; Kundu et al. 1994).
Our proposed VLA-RHESSI observations will help to understand the radiation mechanisms of microflares in
active regions and areas of quiet Sun, as well as their relationship to the evolving magnetic environment in which
they occur. For example, Shimizu et al. (1994) found that soft X-ray microflares occur predominantly in loop
systems while Gopalswamy et al (1994) and Willson et al. (1998) found that 2 cm (VLA) microflares repeatedly
occurred near regions of high magnetic field strength. VLA snapshot maps will localize time-variable sources within
specific coronal loop structures whose magnetic geometry is described by VLA circular polarization maps as well as
photospheric magnetograms, thereby providing a better understanding of the magnetic environment in which
microflares occur.
Proposed Observations
We propose to observe the Sun for five hours per day for three (preferably consecutive or alternate) days when
the VLA is in the DnC or D configuration in 2003. These configurations are well-suited for mapping solar
structures at the short wavelengths that would otherwise be resolved out in more extended VLA configurations.
Our VLA observations will be part of a Max Millennium Campaign involving RHESSI as well as the SOHO
EIT and CDS and TRACE. HESSI will be used to detect hard X-ray emission from transient sources with a spatial
resolution down to ~2" and a timescale as good as a few tens of milliseconds. These observations will establish the
energy distribution of the emitting particles that give clues to the nature of the emission process (i.e., thermal or
nonthermal). Simultaneous observations of the same target region will be made with the VLA in the DnC or D
configuration alternately using two subarrays at 1.2, 2.0 or 3.5 cm or by frequent switching among these
wavelengths. Observations at these wavelengths will be used to study the evolution of small-scale loops at three
different heights and to provide information about their radiation spectra. Snapshot maps on timescales of11.2
seconds of total intensity and circular polarization will be used to investigate magnetic and/or temperature changes
in the low corona and transition region that may accompany hard X-ray bursts or smaller-scale events such as
microflares. Whenever possible, we will coordinate our VLA observations with those from the Owens Valley Solar
Array (OVSA) which provides spectral coverage of the Sun at 45 frequencies between 1 and 18 Ghz with a spatial
resolution as good as 7 seconds of arc.
Our VLA and HESSI observations will be combined with simultaneous observations from TRACE and the
SOHO EIT, CDS and MDI. The objective of the TRACE observations will be to study the transition region and
low-corona counterparts of transient brightenings and evolving microwave and hard X-ray sources. TRACE has
the unique ability to acquire images in chromospheric, transition region and coronal wavebands over relatively large
fields of view (up to 8.5') with a cadence of about 10 seconds and with an angular resolution of about 1". Our
VLA, HESSI, SOHO and TRACE observations will also be compared with Kitt Peak photospheric magnetograms in
order to examine the relationship between evolving microwave and EUV sources and the underlying photospheric
magnetic field structure.
References
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We propose VLA observations of the Sun at 1.2, 2 cm and 3.5 cm (DnC or D configuration 2002/3) in support of
the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) Mission. The proposed VLA and HESSI images
will be used to investigate the spatial location, magnetic environment, and temporal evolution of energetic,
nonthermal electrons that are produced in solar flares. We will additionally use combined VLA and HESSI data to
study nonthermal microflares that may play a role in coronal heating, both within active regions and outside them.
The VLA images will be used to test various models for flares and microflares, to provide information on
nonthermal energy release at different levels in the corona and to provide constraints on the strength and structure of
the coronal magnetic field where hard X-ray bursts ocur.