F. Chiuderi Drago

Energy balance in the prominence-corona transition region

The prominence-corona transition region can be observed both at UV and radio wavelengths. The physical parameters needed to explain one set of observations are, however, in disagreement with those consistent with the other one. A solution of the problem is proposed, based on the proper consideration of the dependence of the thermal conduction on the angle between the magnetic field and the direction of the local temperature gradient.

Microwave emission above steady and moving 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.

The quiet-sun differential emission measure from radio and UV measurements

In the present work we combine UV and radio observations of the quiet Sun to determine the differential emission measure (DEM) of the average quiet solar atmosphere from the photosphere (5600 K) to the corona. UV line intensities have been used to constrain the DEM above 30,000 K, and the radio spectrum from 1.5 to 345 GHz has been used to extend the DEM determination down to 5600 K. Radio observations are shown to provide a much more reliable diagnostic tool for DEM determination than UV and EUV lines at T < 30,000 K. The resulting average quiet-Sun DEM that we found is in excellent agreement with curves from the literature for temperatures larger than 60,000 K, but is lower than previous determinations by more than 1 order of magnitude in the 10,000-30,000 K temperature range. The present work determines the DEM below 10,000 K for the first time, in a temperature region where UV and EUV lines cannot be used.

Flaring and quiescent radio emission of UX arietis : a time-dependent model

Two different interpretations of the quiescent radio emission of UX Ari are considered and compared with available observations. It is assumed that the radiation mechanism is gyrosynchrotron and that the emitting electrons are distributed according to (1) a Maxwellian distribution at the same temperature deduced from X-ray observations (T≃5×10 7 K); and (2) a power-law distribution with different exponents. Both electron populations can reproduce the observed spectrum, provided that a suitable configuration of the magnetic field is assumed. The time evolution of a population of electrons following an initial power-law distribution is also studied

Solving the Discrepancy between the Extreme-Ultraviolet and Microwave Observations of the Quiet Sun

The aim of the present work is to understand the origin of the long-standing discrepancy between the EUV/UV-based predictions of the quiet-Sun microwave spectrum and the observed one. We compare accurate measurements of the quiet-Sun microwave brightness temperature (Tb) with theoretical calculations obtained by using the differential emission measure (DEM) of the plasma derived from UV and EUV spectral line intensities observed by the SUMER and CDS instruments on board the Solar and Heliospheric Observatory (SOHO). No agreement can be found between the observed Tb and calculations carried out using the standard DEM curves obtained from the EUV/UV observations. In order to obtain agreement, it is necessary (1) to modify the temperature range in which the DEM is usually defined in order to take into account the presence of an isothermal corona, (2) to separate the contribution of the cell and the network structures in the transition region, and (3) to substitute the EUV/UV-based DEM values at very low temperature (log T ≤ 4.3) with values based on the Vernazza, Avrett, & Loeser model. In the present work we are able to solve a long-standing discrepancy between microwave and EUV/UV results, and we demonstrate the great potential of the simultaneous use of observations in these two spectral ranges.

SOHO CDS and SUMER observations of quiescent filaments and their interpretation

Three quiescent filaments located at different positions on the solar disk were selected from the SOHO CDS data archive: one of them was also observed by SUMER in the raster mode. We investigate the filament-corona transition region (PCTR) emission, to determine whether it is indeed negligible, as found in one previously-analysed case. The observations are interpreted on the basis of two different models: an isothermal (cool) prominence located above the quiet sun transition region (TR) with a portion of the corona below it, and a model composed of several cool threads embedded in the hot coronal plasma without any quiet sun TR below it. The first model indicates that, for all filaments, the PCTR emission at the top of the filament is indeed negligible, and that the chromosphere-corona TR emission under the filament is lower than the average. All filaments have similar column densities, ranging from ∼ 2t o 7× 10 17 cm −2 according to model A, and from 5 to 17 × 10 17 cm −2 according to Model B. It is not possible to determine which model better accounts for the observations, on the basis of the two prominences observed above and below the Lyman continuum limit. Model B predicts absorptions that are generally less consistent with the observations, and produces higher column densities. The comparison between the line intensities observed above and below the He I ionization limit provides an estimate of the relative neutral helium abundance N(He I)/N(H I) in the prominences.

Coronal magnetic fields from microwave polarization observations

The solar active region (AR) 7530 was observed at 6 cm on July 3 and 4, 1993 with the Westerbork Synthesis Radio Telescope, using a multi-channel receiver with very narrow bandwidth. We compare the radio data with Yohkoh SXT observations and with the magnetic field extrapolated from the Marshall vector magnetograms in the force-free and current-free approximations. The comparison with soft X-rays shows that, although a general agreement exists between the shape of the radio intensity map and the X-ray loops, the brightness temperature, Tb, obtained using the parameters derived from the SXT is much lower than that observed. The comparison with the extrapolated photospheric fields shows instead that they account very well for the observed Tb above the main sunspots, if gyroresonance emission is assumed. In the observation of July 4 an inversion and strong suppression of the circular polarization was clearly present above different portions of the AR, which indicates that particular relationships exist between the electron density and the magnetic field in the region where the corresponding lines of sight cross the field quasi-perpendicularly. The extrapolated magnetic field at a much higher level (∼ 1010 cm), satisfies the constraints required by the wave propagation theory all over the AR. However, a rather low electron density is derived.

Coronal magnetic fields from faraday rotation observations

In the first part of this communication we briefly summarize the results of the first observation of linear polarization in the microwave emission above a solar active region obtained with the Westerbork Synthesis Radio Telescope, taking advantage of the very narrow bandwidths of a multi-channel spectral line receiver. The intensity of the Stokes parameterU, measured at several points close to the line of zero circular polarization, showed a clear sinusoidal trend as a function of λ2, in accordance to what is expected from Faraday rotation (Alissandrakis and Chiuderi Drago, 1994). Combining the measured period of the Faraday rotation with the observed deplacement of the depolarization line with respect to the photospheric neutral line, the height above the photosphere of the depolarization point and the value of the electron density and the magnetic field at this point are computed. Although the calculations are done in the very simplified assumptions of a bipolar magnetic field and of a density following hydrostatic equilibrium, they represent the first estimate of the coronal magnetic field in an active region, far from sunspots.

Network to cell contrast at microwaves

Using different models deduced from EUV lines for the cell and for various network components, the corresponding radio brightness temperature in the millimetric and centimetric range of wavelengths are computed. The contrast C = [Tb (network)]/[Tb (cell)] and the difference ΔT = Tb (network) - Tb (cell) are compared with the few available observations of the quiet Sun inhomogeneities performed with sufficient angular resolution. The comparison shows a satisfactory agreement with most of the observations.

THE PROMINENCE – CORONA AND THE FILAMENT – CORONA TRANSITION REGION: IS THERE ANY DIFFERENCE?

The ratio between the Extreme Ultraviolet emission of the prominence–corona transition region and that of the quiet Sun (QS) transition region is measured using observations from the CDS and SUMER instruments on board the SOHO Satellite. These results are compared with those obtained in an earlier paper, analysing the same prominence as a filament on the disk. Theoretical models predict a difference in the emission of the prominence–corona transition region when it is observed at the limb and on the disk as a filament; the aim of the present work is to provide an observational check of this difference. SUMER and CDS data provide fairly good agreement if the prominence intensity measured by SUMER is compared with the average quiet-Sun intensity, measured near the disk center; the prominence intensity relative to the average quiet-Sun level measured on the same rasters results in disagreement with CDS, due to the smaller size of the disk portion and to the very strong limb brightening present in SUMER rasters. The relative prominence to quiet-Sun intensity ratio varies from 0.2 to 0.4, depending on the line formation temperature. This value leads to a discrepancy with the results obtained in a previous study when the same prominence was observed as a filament. This discrepancy indicates that the prominence–corona transition region emission is different when emitted by different sides of the prominence.