Soon after the first detection of radio emission from the sun two components of the solar radio radiation were identified: The emission related to active centres on the disk and the radiation of the undisturbed, static solar atmosphere, in which the active regions are embedded. The undisturbed component is observed to vary only slightly during the solar sunspot cycle, it is called the emission of the quiet sun. A theoretical estimate of this component was first given by Martyn (1946) and subsequently developed in more detail by many other authors. The basic observations were performed with poor angular resolution. Still at present most experimental data are taken with angular resolutions of about 1 to 4 arc min, too low to discriminate between the different solar atmospheric fine structures, clearly seen in various spectral lines. The quiet component of the solar radio radiation therefore represents the average emission of an inhomogenous solar atmosphere.

In 1950, a few years after the detection of the Sun at radiowavelengths by Southworth (1945), two important points were already established:

1. 1 — The theory (Martyn 1946, Ginzburg, 1946) indicates that the thermal bremmstrahlung is dominant.

2. 2 — The transfer calculations by Smerd (1950) and the total flux measurements (Pawsey 1946, Pawsey and Yabsley, 1949) lead to a coronal temperature of one million degrees.

Solar radio emission at centimeter and millimeter wavelengths originates in the chromosphere and transition region and is a useful probe for the temperature and density in these regions. High spatial resolution observations of the quiet sun provide valuable information on the structure of the solar atmosphere. We have performed high resolution (~ 6″ (E-W) x 15″ (N-S)) observations at 6 cm with the Westerbork Synthesis Radio Telescope (WSRT) in June 1976 in order to search for the radio analog of the supergranulation network and to study the extent and symmetry of limb brightening. The use of the WSRT for high spatial resolution solar mapping has been described by Bregman and Felli (1976), Kundu et al. (1977), and others.

The perturbing effects of a shock wave transit through the transition region are evaluated, using a simplified method of solution of the system of partial differential equations describing the fluid motion behind the shock passage. Calculated temperatures and densities are given at successive times during the shock transit. Based on these shocked models, time dependent brightness temperatures are derived at several decimetric wavelengths and compared with the unperturbed values to evaluate the amplitude of the induced brightness fluctuations. Finally, the possibility of detecting such shock induced phenomena is discussed.

The radial intensity profile at the extreme solar limb was measured with high angular resolution during the October, 1977 partial solar eclipse using the Owens Valley solar interferometer at 2.8 cm and the VLA at 6 cm. Substantial limb brightening was observed at 6 cm, with a peak brightness temperature corresponding to an 80% increase over the disk value. Much less limb brightening was observed at 2.8 cm. In each case an extended “tail” to the brightness distribution was detected. The results at both wavelengths are consistent with a simple model in which a rough chromosphere is overlaid by a corona in hydrostatic equilibrium with a density of 5 × 108 cm−3 at its base.

We discuss the relatively broad spectrum of fluctuations in flux which is observed at cm-wavelengths when tracking a small area on the Sun's disk. The distribution of power P over the frequencies ω of fluctuations can be approximated by a power-law where α ≃ 2; often a continuous “curvature” is superimposed and a “turnover” frequency is indicated at about ω/2π ≃ 10−3 Hz. Considering the phases of different Fourier components of the time series measured, a significant linear correlation is found between phases of neighbouring frequencies.

In the early years of centimetric radio astronomy, even before high resolution techniques were available, it was found that the apparent brightness temperature of the full disk is made up of a background level (the so-called quiet sun temperature) added to which is a contribution roughly proportional to the sum of the sunspot areas on the disk (Smerd 1964, Pawsey and Smerd 1953). Eclipse observations in 1946 at a wavelength of 10.7 cm (Covington 1947) and in 1948 at 10 cm (Piddington and Hindman 1949) and at 3.2 cm (Hagen et al. 1948) showed that the average bright area occupied about 4-thousandths of the disk, and had brightness temperatures of about 5 million degrees. Subsequent interferometric observations have extended and amplified these early studies in several ways. Kundu (1965) has reviewed the literature up to the early 1960's.

Various frequency spectra with the fine structure resulting from the thermal cyclotron radio emission from solar active regions are discussed. The conditions in sources (distribution of magnetic field and kinetic temperature over the height) are put forward which provide the frequency spectrum as a set of cyclotron lines and high frequency cut-offs. For each kind of distribution the frequency spectrum and polarization are of peculiar character. This permits one to find the conditions in the source through the properties of the observed microwave solar radio emission. To obtain reliable data on the fine structure and judge about conditions in the sources it is necessary to study microwave solar radio emission using the swept-frequency or multi-channel receivers combined with high directional antennae.

The influence of the magnetic field on the structure of sunspot-associated sources at cm-λ has been investigated through model calculations. The case of a source observed with the Westerbork SRT and the structure of a single sunspot source are considered.

We have used the Very Large Array of the NRAO to observe solar active regions at 2 and 6 cm wavelengths in May 1979. In this paper we present and discuss the fine structure components of a sunspot associated radio region observed with a resolution of 3″ arc on May 4 and 5, 1979. In this region (McMath 15974) there were two distinct groups of sunspots separated by about 3′ arc. Group I located towards the west limb contained two spots while Group II in the east was a complex containing several spots that emerged during the period of our observations. At 6 cm both the sunspot groups were within the primary beam whereas at 2 cm only Group I was within the field of view. The VLA observations were made at 6 and 2 cm alternating between them every 5 minuts. The region was mapped with a resolution of about 3″ x 3″ using 10 hours of synthesis data on each day.

Very Large Array (VLA) synthesis maps of the total intensity and the circular polarization of three active regions at 6 cm wavelength are presented. The radiation from each active region is dominated by a few intense cores with angular sizes of ~ 0.5′, brightness temperatures of ~ 106 K, and degrees of circular polarization of 30 to 90%. Some of the core sources within a given active region exhibit opposite senses of circular polarization, suggesting the feet of magnetic dipoles, and the high brightness temperatures suggest that these magnetic structures belong to the low solar corona. We also present comparisons between our VLA maps of circular polarization and Zeeman effect magnetograms of the lower lying photosphere. There is an excellent correlation between the magnetic structures inferred by the two methods, indicating that synthesis maps of circular polarization at 6 cm can be used to delineate magnetic structures in the low solar corona.

Coordinated high-resolution XUV and centimetric observations of active regions obtained from Skylab and Stanford are compared with the aim of determining the nature of the centimetric radiation. It is concluded that the thermal gyroresonance process is the most likely emission mechanism for the bright compact radio sources associated with sunspots.

We have constructed a 17GHz interferometer of a multi-correlator type at the Nobeyama Solar Radio Observatory. Novel features of the new interferometer are summarized as (i) high time-resolution up to 0.8 s and (ii) “real-time” calibration of the whole system with an accuracy of ~ 2% for amplitudes and ~ 2° for phases. With the aid of these advantages over an interferometer of a conventional drift-scan type we are able to detect and follow rapid time variations of even a faint source (say, ~ 0.5 s.f.u.) on the Sun with a spatial resolution of ~ 40″. The interferometer has been put in operation since July 1978. We have recorded hundreds of bursts at 17GHz in a year including some tens of rapidly changing sources which would not precisely be measured so far. We present here some preliminary results of observations such as polarization structures of both rapidly changing and GRF bursts.

Observational results of radio bursts at 35 GHz by the sixteen-element fanbeam interferometer in Nagoya are briefly discussed.

Four multi-source radio bursts observed with the new 17GHz interferometer at Nobeyama are analyzed and presented from the point of view of their time-variations.

Three of them (Sep. 6, 1978; Oct. 9, 1978; and Feb. 17, 1979) were essentially of an impulsive nature and all showed distinct double-source structures. Their polarization structures and time-variations are discussed and compared with type III and type II burst occurrences. The results of the comparison suggest either that electron accelerating regions moved across magnetic lines of force or that a rapid magnetic field rearrangement occurred near an accelerating region.

A quite different type of multi-source radio bursts was observed on Nov.10,1978. It had a long duration of several hours and was associated with type IV bursts at metric and decimetric wavelengths. At least five radio sources were observed to appear at 17GHz. A detailed description of this event is presented in comparison with the evolution of both Hα-flare and type IV bursts.

Currently planned modifications to the Very Large Array to facilitate solar observations are described. The performance characteristics of the instrument in this observing mode are outlined.