Solar flares present outstanding theoretical and observational challenges. At issue is by what processes does the Sun first store and then release large quantities of energy? How is the release triggered? How is the free energy subsequently converted into and transported by hot plasma, fast particles, electromagnetic radiation, and mass motions?
X-ray and microwave observations have played key rôles in the study of solar flares because the fast electrons which emit hard X-rays (HXRs) and microwaves carry a significant fraction of the energy liberated during the impulsive phase of a solar flare. HXR emission is produced by thermal bremsstrahlung, as fast electrons are stopped by cool, dense material in the low corona and chromosphere. Microwaves are produced by mildly relativistic electrons gyrating in strong magnetic fields (gyrosynchrotron emission). Both emission mechanisms are well-understood and can be exploited to deduce physical conditions in the source.
HXR spectrometers have been flown on space-based platforms for many years, but HXR imagers are still immature. While groundbased interferometric imaging of flare-associated radio emission has been employed effectively for many years, microwaves remain under-exploited do to a lack of appropriate instruments or instrumental capabilities. In order to fully exploit microwaves as a flare diagnostic the source must be spatially, temporally, and spectrally resolved over a sufficiently large range of frequencies; in addition, the Stokes parameters I and V must be measured since, unlike HXRs, gyrosynchrotron emission depends sensitively on the magnetic field vector. No instrument presently exists which satisfies all of these requirements. While the VLA currently approaches the requirements of high time resolution (it can snapshot image at a rate of ) and high spatial resolution, it typically samples only one or two frequencies during the course of a flare. The spectrum of flare-associated microwave emission is therefore only crudely constrained, or is wholly unconstrained. It is therefore extremely important to expand the spectral coverage of the VLA on short timescales.
Spectral coverage at the Cassegrain focus may be acquired in a number of ways:
The first option is preferred, but may be unworkable if it compromises performance in other bands. Some combination of the remaining possibilities may be feasible.
Another important requirement for observations of highly time variable radio emissions from the Sun is a robust total power system. A fast, linear, high-dynamic-range, and accurate measurement of the system temperature is critical to flare observations since the flare-associated emission often dominates the system temperature and its variation in time.
Solar radio bursts at meter and decameter wavelengths were among the first cosmic radio emissions to be studied. Yet many of them defy a detailed explanation. For example, type III bursts are due to mildly relativistic electron beams traversing the solar corona. The beam excites Langmuir waves; the Langmuir waves either scatter or coalesce to produce plasma radiation at the fundamental or harmonic of the electron plasma frequency. Yet the details remain poorly understood. For other burst types (e.g., the millisecond spike bursts) the emission mechanism has not even been unambiguously identified!
With the launch of the Japanese solar-dedicated satellite, Yohkoh, the Compton Gamma-Ray Observatory, and the availability of broadband digital radio spectrometers on the ground, there has been a resurgence of interest in solar radio bursts. In particular, joint observations carried out by the VLA and the aforementioned instrumentation has yielded new insights into radio bursts; e.g., the relationship between soft X-ray jets and type III bursts.
The enhanced VLA can have a major impact on our understanding of solar radio bursts. Of key interest is the installation of one or more broadband UHF feeds at the prime focus of each antenna (e.g., 150-300 MHz and/or 300-600 MHz). It will be possible to perform imaging spectroscopy of radio bursts for the first time. Two examples of the importance of such a system must suffice:
Figure 2.1: Example of type III U-burst. The burst type was identified in a spectrographic record. The longitudinal magnetogram is shown in greyscale. White represents positive magnetic polarity while black represent negative magnetic polarity. Magnetic lines of force resulting from a potential magnetic field extrapolation are shown. The 333 MHz source, imaged by the VLA, is shown in contours.
While these two examples are especially vivid, significant insights are sure to come from imaging spectroscopy of the entire ``zoo'' of radio burst types, including type I bursts, millisecond spike bursts, and a variety of phenomena associated with decimetric emissions (e.g., ``patches'', quasi-periodic pulsations, ``sudden reductions'', etc.).
Imaging spectroscopy in the UHF will require the full capabilities of the new correlator and then some. While the requirements on spectral resolution are comparable to those required for applications described elsewhere in this document ( channels with dual polarization), the need for roughly an octave (or more) of instantaneous bandwidth, and the need to process this bandwidth over very short timescales ( ms for some experiments), present substantial technical challenges to the project. Given the extremely high data rates involved in the most demanding experiments, it is worth considering a ``burst mode'', where data are recorded at high rates and later correlated.
Coronal Mass Ejections (CMEs) have a major influence on the Earth and the interplanetary medium (IPM), and perhaps play a rôle in the initiation of solar flares. Currently, they are primarily observed by ground- or space-based white-light coronographs, although the SOHO mission will carry a UV coronograph and efforts are underway to design and build groundbased infrared coronographs.
CMEs are often associated with type II radio bursts and imaging spectroscopy of radio bursts is an important goal of solar radiophysics. However, radio bursts involve emission processes which depend on physical parameters in complex and often nonlinear ways. It is therefore also desirable to observe the incoherent (free-free and/or gyrosynchrotron) radio emission associated with CMEs. Because CMEs possess very low surface brightness, a configuration such as the ultra-compact E configuration is needed. Because the Sun itself is a major source of sidelobe confusion, differential techniques may have to be employed (as they are in CME studies at visible wavelengths) to detect incoherent, low-surface-brightness emission from CMEs.
Eruptive prominences are another example of a transient phenomenon, whereby cool prominence material erupts outward from the Sun with speeds up to several 100 km/s. Eruptive prominences are often accompanied by a CME. Prominence material should be easily detected in microwaves, given sufficient surface brightness sensitivity, because the prominence material is relatively cool and therefore very optically thick to free-free absorption. Because the VLA's field of view decreases with frequency, eruptive prominences will most likely be observed between 3-8 GHz in an ultra-compact array configuration.
Solar active regions are localized areas on the Sun where magnetic flux has erupted through the photosphere into the chromosphere and corona. They are characterized by the presence of sunspots in white light, enhanced line emission (e.g., H, CaII), and greatly enhanced soft X-ray (SXR) and radio emission. As their name implies, solar active regions are the sites of solar flares, a variety of radio bursts, enhanced coronal heating, and play a rôle in various mass ejections. A key goal of solar physics is to understand their birth, evolution, and decay, and their production of transient, energetic activity.
Two sources of opacity are relevant in active regions at cm and dm wavelengths. The first is thermal free-free absorption and the second is thermal gyroresonance absorption. Radio waves with frequencies which are low integer multiples of the the electron gyrofrequency (i.e., ) may be resonantly absorbed and emitted by electrons gyrating in the local magnetic field. For example, the electron gyrofrequency in a 580 G magnetic field is GHz, the third harmonic of which lies in the 4.9 GHz band. Given sufficient spectral and spatial resolution, one can exploit gyroresonance absorption to place unique constraints on the magnetic field at the base of the corona. It works as follows: consider a given frequency which is optically thick to gyroresonance absorption in the solar corona at . It therefore yields a brightness temperature which is similar to the effective temperature of the corona ( K). Now let increase - as increases, the resonance condition is matched at higher magnetic field strengths, which occur at lower heights in the corona. At some critical frequency , the resonance condition is matched at the base of the corona. For , drops precipitously as the resonance layer traverses the transition region and chromosphere where the effective temperature is much lower. A high resolution map of may therefore be converted into a map of at the base of the corona. With care, the field vector may also be constrained. Coronal magnetography provides the most direct means available of estimating the magnetic field at these heights. It would provide an invaluable tool for assessing departures of the field from potential configurations in active regions, and for assessing the rôle of electric currents. While the VLA currently provides sufficient angular resolution for this purpose, broadband spectral coverage has been absent. The frequency coverage and the large bandwidth ratios proposed as part of the VLA Development Plan go a long way toward remedying this shortcoming.
Figure 2.2: Joint radio and SXR images of a solar active region. The upper left frame shows SXR emission from hot (3-5 MK), dense plasma confined by strong magnetic fields. The upper right frame shows the corresponding photospheric magnetogram. The lower left frame shows the 1.6 GHz VLA map while the lower right shows the 4.5 GHz VLA map.
A technique somewhat analogous to coronal magnetography can be used at lower frequencies (1-4 GHz) to determine the temperature and mean density of coronal material. At frequencies GHz the corona above active regions becomes optically thick to free-free absorption. Since the optically-thick layer typically lies above the gyroresonance sources, the character of radio images of active regions changes dramatically between the 1.4 and 4.9 GHz bands (Fig. 2.2). Given sufficient spectral coverage in the range 1.3 - 4 GHz, the same technique of studying radio spectra as a function of position yields the mean temperature (from the optically-thick portion of the spectrum) and the density (from the location of the spectral break where the spectrum changes from the roughly -shape of an optically thick source to the flat spectrum of an optically-thin source). Again, it is not possible to exploit this technique with the VLA at present due to the gaps in frequency coverage, and the small bandwidth ratios, a shortcoming which is again addressed by the receiver upgrade.