V. M. Bogod

RATAN-600 radio telescope in the 24th solar-activity cycle. II. Multi-octave spectral and polarization high-resolution solar research system

Radio astronomy studies of the solar atmosphere possess a very important, not duplicated by other methods, place in the study of solar activity at all stages—from the birth of an active region until its collapse. A significant progress in these studies can be achieved in the implementation of new technical possibilities, such as an increase in the sensitivity of radio telescopes, a detailed spectral analysis over a wide frequency range, high temporal resolution and a broad coverage range in time. We report about the implementation of regular observations with a new spectral and polarization high-resolution system SPHRS, installed at the radio telescope RATAN-600.We describe the concept of the new system and the methods of its implementation.

VERY LARGE ARRAY-RATAN 600 OBSERVATIONS OF A SOLAR ACTIVE REGION

The Very Large Array (VLA) and the RATAN 600 were used to observe a solar active region on two consecutive days around the time of a partial solar eclipse in July 1990. VLA synthesis maps at 2.0, 3.5, and 6.2 cm wavelength reveal bright (Tb = 0.2 − 2.2 × 106 K), compact (θ = 10″–40″) sources above the penumbra of the leading sunspot while maps at 20 cm wavelength reveal an extended (θ ≈ 4.5′) looplike structure (Tb ≈ 106 K) between the dominant spots. Total flux and brightness temperature spectra of both components were obtained by the RATAN at nine wavelengths between 1.7 and 21 cm. The relatively-flat spectrum of the extended emission is attributed to the optically thin thermal brems Strahlung of electrons trapped in a magnetic loop at coronal temperatures. Step-spectrum sunspot-associated emission is attributed to thermal gyroresonance radiation at different heights along the leg of a loop joining regions of opposite magnetic polarity. Comparisons with predicted distributions of gyroresonance radiation indicate that the compact sunspot-associated sources lie at heights of h = 2500–17500 km above the photosphere. Although potential fields of sufficient strength appear to exist at coronal heights, differences n the observed and predicted brightness distributions suggest some role for non-potential fields or for an inhomogeneous distribution of electron density or temperature above the sunspot.

The altitude structure of the coronal magnetic field of AR 10933

The magnetic fields of solar active regions are analyzed using a method based on comparing the spatial structures of the reconstructed magnetic field and of the radio emission of the active region. Two approaches are used: comparing the radio size of the active region and the corresponding size calculated using the reconstructed magnetic field, and comparing the radio spectra that are observed and calculated using the reconstructed magnetic field. Overall, the calculated sizes and spectra correspond fairly well to the observational data, making it possible to estimate physical parameters of the emitting region, such as the electron density and temperature.

On the comparison of radio-astronomical measurements of the height structure of magnetic field with results of model approximations

The results of microwave observations of the polarized emission of active regionsmade with the RATAN-600 radio telescope are used to develop the method for determining the structure of the magnetic field of these regions at coronal heights. About 1000-G-strong magnetic fields are observed in the solar atmosphere at rather high altitudes (from 10 to 25 Mm). This result is confirmed fairly well by the ultraviolet observations of magnetic loops, it is consistent with earlier radio-astronomical observations of the magnetic field at the height of the transition region, and it corresponds as well, if interpreted in terms of the dipole magnetic field model, to the vertical gradients of the photospheric magnetic field.

RATAN-600 radio telescope in the 24th solar-activity cycle. I. New opportunities and tasks

Regular upgrade of the RATAN-600 radio telescope has been completed in order to prepare the instrument for the studies of the active Sun during the 24th solar-activity cycle. The upgrade included the improvement of the parameters of the multi-octave solar spectral and polarization analyzer, realization of regular multiple observations, and the use of modern capabilities for the representation and processing of observational data. In this connection, there appear to be sufficient reasons to review the results obtained using the previous version of the receiving equipment and discuss the new capabilities in the study of the physics of the solar atmosphere provided by the new analyzer.

A model for active region emission at centimeter wavelengths

We present multi-frequency observations and model computations of the microwave emission of a solar active region. The radio observations were obtained with the RATAN-600 at several wavelengths between 0.8 and 31.6 cm and with the VLA at 6 and 20 cm. The active region was also observed in the EUV O Iv lines by the HRTS instrument aboard the Space Shuttle Spacelab-2 mission. These lines are formed in the chromosphere-corona transition region and their intensity ratio is sensitive to pressure. Photospheric magnetograms provided both the longitudinal and the transverse component of the magnetic field. The microwave observations were checked against model computations taking into account both the free-free and the gyro-resonance emission mechanisms and using the pressure data from the O IV lines. The magnetic field was computed through constant-α force-free extrapolations of the longitudinal photospheric field. We computed both the flux from 2 to 20 cm and the spatial structure of the microwave emission at 6 and 20 cm. The comparison of the computed and observed flux spectra allowed us to estimate the magnetic field strength at the base of the transition region and in the low corona, as well as the values of the conductive flux and the height of the base of the transition region. The model maps at 6 cm and 20 cm showed that α was not constant above the active region; the same conclusion was reached on the basis of the photospheric observations. The use of pressure measurements allowed us to identify microwave structures which were determined by pressure enhancements. At 6 cm the computations confirmed the fact that the magnetic field is the principal factor that determines the structure of sunspot-associated sources and showed that the effect of pressure variations was small. Pressure variations were more important at 20 cm, where the peak of the emission was associated with the sunspot and a diffuse component was associated with the plage which had an average pressure higher by a factor of 1.54 than the sunspot.

Peculiarities of the microwave emission from active regions generating intense solar flares

RATAN-600 multiwavelength observations of the Sun reveal sharp spectral inhomogeneities in the polarized radiation from active regions that produce intense flares. These events occur in a wide range of radio fluxes (0.05–10 s.f.u.) in a relatively narrow wavelength range (2–5 cm). They are detected on time scales from several hours to several days before and during an intense flare. We analyze the detected events and their relationship to the preliminary phase of intense flares. Significant statistical material was obtained in 2001. The new flare-plasma properties can be used to test existing solar-flare models and to develop new criteria of flaring activity.

Two-dimensional mapping of the Sun with the RATAN-600

We present two-dimensional solar maps at 2.7, 3.2, 4, and 8.2 cm computed from one-dimensional observations with the RATAN-600, using Earth rotation aperture synthesis techniques. Before the calculation of maps, the position of each scan was corrected with respect to the center of the solar disk and the scans were calibrated. The circular polarization scans were corrected for polarization cross-talk between the I and V channels. Subsequently, the quiet-Sun background emission was subtracted. After all corrections, a dirty map was computed by combining the scans at different position angles. The last step of the processing was an attempt to free the dirty map of the sidelobes, using the standard CLEAN procedure. The resolution of the clean maps at 2.7 cm was 0.5′ by 6′. Both active regions which were present on the solar disk were mapped. We studied the flux spectra of different types of sources: one was associated with a sunspot, the second was located over the neutral line of an active region, and the other was associated with the plage. The emission mechanism of the former was attributed to the gyroresonance process, while the short wavelength emission of the others was attributed to the free-free process. For the sunspot-associated source we estimated the magnetic field strengths at the base of the transition region and in the low corona.

On Properties of Microwave Sources Located above the Neutral Line of Radial Magnetic Field

In this study we continue our investigation of the radio sources located above the neutral line of the radial magnetic field in solar active regions, i.e., the so-called neutral line associated sources (NLS). The nature of NLS is still far from being understood. To study it, we use the spectroscopic capabilities of the new broadband polarimetric facility of the RATAN-600 radio telescope. We study the radio spectra of NLS sources in several solar active regions over a wide range of variations of their sizes. We find the NLS radio emission fluxes to be related to the gradient of the quasi-longitudinal magnetic field in the photosphere. We estimate the vertical positions of NLS relative to the cyclotron radio sources. We find fine spectral features in the NLS emission, which confirm the presence of a current sheet in their sources. We associate the appreciable lack of polarization in such sources with their location near the tops of the coronal arches.

RATAN-600 Observations of Small-Scale Structures with High Spectral Resolution

We present observations of quiet-Sun small-scale structures (SSS) in the microwave range with the Radio Astronomical Telescope of the Academy of Sciences 600 (RATAN-600) spectral-polarization facility in a wide range of frequencies. The SSS are regularly recorded in routine observations of the large reflector-type radio telescope and represent manifestations in the radio range of various structures of the quiet Sun: supergranulation network, bright points, plage patches, and so on. A comparison with images from the Solar Dynamics Observatory (SDO) showed that the microwave emission comes from a region extending from the chromosphere to the low transition region. We measured the properties of the SSS as well as the degree of circular polarization averaged over the beam of the radio telescope, and from this we estimated the magnetic field at the formation level of the radiation.