The solar chromosphere is a highly inhomogeneous layer of the solar atmosphere lying between the relatively cool photosphere and the super-hot corona. Semi-empirical modeling of the chromosphere has largely relied on UV/EUV line measurements, and far-IR and sub-mm continuum brightness measurements. Interpretation of the UV and EUV lines is complex because most form under conditions of non-local thermodynamic equilibrium (non-LTE) . Radio emission, in contrast, occurs under conditions of LTE and therefore provides observers with a convenient, linear thermometer with which to probe the chromospheric medium.
While there is a general correspondence between the microwave brightness distribution and the supergranular network, point-by-point comparisons between frequencies often reveal striking differences. A general correspondence is also found between the network and decimetric emission, which arises from the transition region and low corona, but decimeter sources are more diffuse and the contrast between the network and cell interiors is significantly reduced in comparison to that found in microwaves. The situation is qualitatively similar to the finding that high-excitation EUV and SXR lines also display less contrast than C IV and other chromospheric lines.
Comparisons of the brightness increment observed between the network and cell interiors are found to be in general agreement with model calculations at centimeter wavelengths. However, the absolute brightness of measurements for wavelengths between roughly 2-20 cm are not in agreement with the semi-empirical models, with the models producing substantially more brightness than is observed.
The problem with VLA observations to date is that they have been conducted at one or two widely separated frequencies. It is difficult to assess and understand the structural differences observed in the brightness distribution. While information is available about the brightness temperature spectrum over the wavelength range relevant to the chromosphere and transition region, it represents a spatial average over complex structure. The proposed receiver upgrades will allow observers to image the chromosphere at many frequencies, sufficient to track the changing brightness with frequency due to the complex changes in opacity with height. In particular, the D configuration, with an accurate total power system, and the expanded 4.9, 8.4, and 15 GHz bands will provide an unprecedented view of the mid-to-upper chromosphere. A super-compact configuration will accomplish the same for the low-to-mid-chromosphere with comparable resolution between 18-50 GHz. Because of the small field of view for frequencies GHz, a robust total power system in support of mosaicing is especially desirable. And because multi-frequency imaging is critical, it must be possible to tune within a frequency band on a short timescale.
Prominences and filaments are manifestations of the same phenomenon: the appearance of cool, relatively dense material in the corona along magnetic neutral lines. They tend to be extremely long and filamentary - hence, the name. Filaments are almost always visible in H filtergrams of the Sun as features in absorption. They generally reside in chromospheric ``filament channels'' which appear as regions of somewhat diminished brightness at mm and sub-mm wavelengths. At coronal heights, filaments are embedded in a ``filament cavity'', a volume of lower coronal density. On the limb, filaments are seen in emission as prominences.
Observations of prominences and filaments in the millimeter and centimeter wavelength bands have most commonly been interpreted in terms of prominence/ corona transition region models wherein the cool, dense prominence is optically thick and is surrounded by a thin sheath through which the temperature increases to coronal values. While these models have been successful in accounting for the radio spectrum of H filaments between 3mm and 20cm, they have been inconsistent with UV observations to the extent that the models imply far less UV radiation than is observed. In this respect, prominence models suffer from a problem similar to that outlined for the solar chromosphere.
Like the quiet Sun, observations of filaments, prominences, filament channels, and filament cavities would benefit from the increased frequency coverage and the wide bandwidth ratios proposed for the VLA. To date, no high resolution images of filaments and their associated environment have been acquired at more than two widely disparate frequencies.
Coronal holes were discovered in the 1970's in SXRs by the ATM on board Skylab. They are macroscopic coronal structures whose distinguishing feature is that they are magnetically open. That is, magnetic field rooted in coronal holes does not close back onto the Sun - rather, it opens out into the IPM. The particle number density is 2-10 times lower in coronal holes than in the rest of the corona. Coronal holes therefore appear dark in SXRs. They play a key rôle in modulating the IPM because high-speed solar-wind streams originate in coronal holes. Prominent polar coronal holes appear during the decline to solar minimum, and then wane and disappear as the Sun returns to maximum levels of activity. Hence the structure and evolution of coronal holes, and the rôle they play in modulating the IPM throughout the solar cycle, is of great interest.
Coronal holes are dark at decimetric wavelengths for the same reason that they appear dark in SXRs, the contrast between the quiet Sun and a coronal hole being at 20cm. At shorter wavelengths the contrast decreases - near 1cm, however, contrast reappears in the opposite sense: coronal holes are brighter than the quiet Sun! For wavelengths shortward of a few mm, the contrast again disappears. The reason for the contrast behavior of the chromosphere in coronal holes is presently unknown. With the continuous frequency coverage between 12-50 GHz, the VLA will provide an outstanding opportunity to map the chromospheric structure in coronal holes in detail.