We propose that when all sources on the solar disc are taken into account, the S component at 10.7 cm wavelength is dominated by thermal free-free (bremsstrahlung) emission. It is not produced only in the vicinity of sunspots; more than 60% of the total flux may be due to a widely-distributed emission associated with the hot complexes of activity. Using a model for the solar atmosphere based upon an assumption of weak (or vertical) magnetic fields, the spectrum of the S-component is calculated and its sensitivity to changes in the model parameters investigated. Variation of the thicknesses of the chromosphere, transition region and mixed zone cause only small changes in the S-component spectrum; there is a much stronger dependence upon the plasma density, particularly at the base of the corona. The behaviour of the S-component at 10.7 cm wavelength is examined in more detail. We find that the largest contribution to the 10.7 cm flux originates in the low corona, that structural changes affect it only slightly, but that it is strongly density-related. This dependence upon few quantities, together with its relative localization in the low corona, contributes to the usefulness of the 10.7 cm flux as an index of solar activity.

The origin of the 10.7 cm flux

Recent measurements of the relative temporal variations of solar UV radiation are reviewed. The long-term increase in solar UV irradiance during the rise of solar cycle 22 through April 1990 is comparable to the decrease from the solar cycle 21 peak in late 1981 to the 1986 minimum. Although several marked differences occur in the short-term or solar-rotational variations of the UV and Ottawa 10.7cm radio flux (F10), the shape of the long-term solar-cycle increase of F10 is quite similar to that of UV enhancements.

Solar UV Spectral Irradiance Variations

Using Yohkoh SXT and Nobeyama 17 GHz data, we have studied the soft X-ray and microwave emission above several stable, large sunspots near central meridian passage. Our study confirms the well-known fact that soft X-ray emission is depressed above sunspots. It also shows that the distribution of their soft X-ray intensity is not uniform; usually the darkest pixels are associated with the umbra or the far edges of the leading part of the penumbra while the following part of the penumbra may contain higher intensity pixels associated with brighter loops. For the first time, we present a systematic survey of the temperatures and emission measures of the soft X-ray material above sunspots. Sunspots always contain the lowest temperatures and emission measures in the active regions. The mean umbral temperature is 1.8 × 106 K, and the mean penumbral temperature is 2.4 × 106 K. The mean umbral and penumbral emission measures are log EM = 26.60 cm-5 and log EM = 27.00 cm-5, respectively. The differences between the umbral and penumbral plasma temperatures are physically significant. The higher penumbral values imply that the loops associated with the penumbrae are generally hotter and denser than the loops associated with the umbrae. The highest sunspot temperatures and emission measures are still lower than the average active region parameters but higher than the quiet-Sun plasma parameters. The coronal radiative energy loss rate above the umbrae is 15% higher than the radiative loss rate of the quiet-Sun plasma but a factor of 8.3 lower than the typical active region radiative loss rate. The radio emission comes from the gyroresonance mechanism, and, as expected, it is sensitive to the magnetic field rather than the soft X-ray-emitting plasma.

Soft X-Ray and Gyroresonance Emission above Sunspots

Two-dimensional maps of radio brightness temperature and polarization, computed assuming thermal emission with free-free and gyroresonance absorption, are compared with observations of active region 2502, performed at Westerbork at λ = 6.16 cm during a period of 3 days in June 1980. The computation is done assuming a homogeneous model in the whole field of view (5′ × 5′) and a force-free extrapolation of the photospheric magnetic field observed at MSFC with a resolution of 2″.34. The mean results are the following:

(a)

A very good agreement is found above the large leading sunspot of the group, assuming a potential extrapolation of the magnetic field and a constant conductive flux in the transition region ranging from 2 × 106 to 107 erg cm−2s−1.




(b)

A strong radio source, associated with a new-born moving sunspot, cannot be ascribed to thermal emission. It is suggested that this source may be due to synchrotron radiation by mildly relativistic electrons accelerated by resistive instabilities occurring in the evolving magnetic configuration. An order-of-magnitude computation of the expected number of accelerated particles seems to confirm this hypothesis.

Microwave emission above steady and moving sunspots

Neutral Line associated Sources (NLSs) are quasi-stationary microwave sources projected onto vicinities of the neutral line of the photospheric magnetic field. NLSs are often precursors of powerful flares, but their nature is unclear. We endeavor to reveal the structure of an NLS and to analyze a physical connection between such a source with a site of energy release in the corona above NOAA 10488 (October/November 2003). Evolution of this AR includes emergence and collision of two bipolar magnetic structures, rise of the main magnetic separator, and the appearance of an NLS underneath. The NLS appears at a contact site of colliding sunspots, whose relative motion goes on, resulting in a large shear along a tangent. Then the nascent NLS becomes the main source of microwave fluctuations in the AR. The NLS emission at 17 GHz is dominated by either footpoints or the top of a loop-like structure, an NLS loop, which connects two colliding sunspots. During a considerable amount of time, the emission dominates over that footpoint of the NLS loop, where the magnetic field is stronger. At that time, the NLS resembles a usual sunspot–associated radio source, whose brightness center is displaced towards the periphery of a sunspot. Microwave emission of an X2.7 flare is mainly concentrated in an ascending flare loop, initially coinciding with the NLS loop. The top of this loop is located at the base of a non-uniform bar-like structure visible in soft X-rays and at 34 GHz at the flare onset. We reveal i) upward lengthening of this bar before the flare onset, ii) the motion of the top of an apparently ascending flare loop along the axis of this bar, and iii) a non-thermal microwave source, whose descent along the bar was associated with the launching of a coronal ejection. We connect the bar with a probable position of a nearly vertical diffusion region, a site of maximal energy release inside an extended pre-flare current sheet. The top of the NLS loop is located at the bottom of this region. A combination of the NLS loop and diffusion region constitutes the skeleton of a quasi-stationary microwave NLS.

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Microwave Neutral Line Associated Source and a Current Sheet

Observations de la region active solaire AR 3804 les memes jours de juillet 1982 avec le RATAN 600 et le VLA

Structure of a solar active region from Ratan 600 and very large array observations

Recently recognized solar millimetre-wave off-limb sources are interpreted as a special phenomenon of long-duration post- and inter-flare emission at coronal altitudes. We present, for the first time, information about the brightness and polarization spectrum in the centimetre range for one such event of September 22, 1980 by means of RATAN-600 observations. The brightness temperatures observed favour the interpretation of the bulk of the emission by thermal optically thin bremsstrahlung. The degree of polarization measured (p ≈ 0.1–0.2 in the range 7.5–15 GHz) implies quite strong magnetic fields of about ≥300 ± 100 G at a height z > 3 × l04km above the photosphere and indicates a possible contribution of gyromagnetic radiation and/or optically thick bremsstrahlung at longer wavelengths.

The spectrum of coronal sources at millimetre and centimetre waves

We study pre-eruptive, eruptive, and post-eruptive phenomena related to a CME that occurred on November 23, 2000 by means of joint analyses of data from various spectral ranges. Almost all known CME-associated phenomena were observed during this event, i.e., a filament eruption, solar flare, dimmings, and a post-eruptive arcade formation. Following a chain of events observed in various spectral ranges, we find that the event occurred in an activity complex consisting of active regions 9231 and 9238, and that it was triggered by a magnetic flux emergence, which caused a flare in AR 9231. In turn, the flare triggered activation and eruption of the filament followed by the CME and the flare in AR 9238 in which the post-eruptive arcade was observed. We discuss some characteristics of the flare and CME and also estimate the magnetic field strength in the coronal arcade to be about 200 G from spatially resolved polarization measurements in microwaves with radio telescopes. In this particular case, the only significant emission mechanism is optically thin free-free emission, and the possible contribution of nonthermal emissions cannot change our estimate of the magnetic field strength in the corona. However, generally one should make sure that the nonthermal contribution cannot be important in similar cases; otherwise, the magnetic field can be well overestimated. Here, we specifically address the identification technique of the radio emission mechanism.

Observations of CME-related phenomena in a wide spectral range

This paper presents a short summary of observations of coronal structures at microwaves using an instrument with high spatial resolution and good wavelength coverage. The comparison of the RATAN-600 data with optical observations of coronal loops in the green line and with the Very Large Array maps at 21 cm has shown that the loops represent only a small part of coronal matter, although their role may be of great importance in the physics of the solar corona. Prominence (filament) associated sources, especially “peculiar” ones, are also reviewed.

V. N. Borovik

Papers

Structure of a solar active region from Ratan 600 and very large array observations

The spectrum of coronal sources at millimetre and centimetre waves

Observations of CME-related phenomena in a wide spectral range

Coronal loops and prominences as observed with RATAN 600

Cooperative magnetographic and radioastronomical investigations of the active solar region AR 3804 in July 1982.