Author name code: chiuderi-drago ADS astronomy entries on 2022-09-14 author:"Chiuderi Drago, Franca" ------------------------------------------------------------------------ Title: The Quiet-Sun Differential Emission Measure from Radio and UV Measurements Authors: Landi, E.; Chiuderi Drago, F. Bibcode: 2008ApJ...675.1629L Altcode: 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. Title: The HeI abundance in Solar filaments Authors: Chiuderi Drago, F. Bibcode: 2005A&A...443.1055C Altcode: Three filaments observed with the CDS instrument on the SOHO satellite are analysed to determine the HeI/HI ratio. The HI and Hel bound-free absorptions are the major processes responsible for the lower intensity of transition region (TR) lines observed above filaments. One of the filaments was also observed by SUMER at λ > 912 Å, thus supplying the unabsorbed background intensity. The HI and Hel column densities are derived from several TR lines using a least squares method applied to two different models. The resulting HeI/HI ratio is independent of the model, while the column densities are different by about a factor of two. This difference enables us to discriminate between the two models by comparing the resulting value of the optical depth at the Lyman continuum limit, τ912, with previous observations and models. Title: On the Technique of Coronal Magnetography Through Quasi-Transverse Propagation of Microwaves Authors: Bezrukov, D. A.; Ryabov, B. I.; Bogod, V. M.; Gelfreikh, G. B.; Maksimov, V. P.; Drago, F.; Lubyshev, B. I.; Peterova, N. G.; Borisevich, T. P. Bibcode: 2005BaltA..14...83B Altcode: 2005OAst...14...83B The technique of coronal magnetography is illustrated and analyzed on the samples of four solar active regions (ARs). Observational bases for the coronal magnetography are the radio maps of the Sun taken with the Siberian Solar Radio Telescope (SSRT) at the wavelength 5.2 cm both in l I and l V Stokes parameters. The magnetic field strength is found using the theory of wave mode coupling in the region of quasi-transverse (QT) propagation. The proper values are found to be in the range of 10--30 G. The technique used is based on the fact that the circular polarization of a radio source is modified when the ray path crosses a region in the solar corona where the magnetic field is transverse to the line of sight. We have successfully applied this method to the whole period of transit on the solar disk of NOAA 9068, 9097 and NOAA 9339, while we have noticed some limitations of the coronal magnetography for the flare-productive active region NOAA 9415. The coronal magnetograms appear to be a series of 2D partial magnetograms, each day covering a part of the AR: the following part near the eastern limb and the leading one near the western limb. The technique has been verified to be reliable in the determination of coronal magnetic fields, but needs further improvements concerning the evaluation of their height, related to the heights of the QT-region. Title: Effect of suprathermal particles on the quiet Sun radio emission Authors: Chiuderi, C.; Chiuderi Drago, F. Bibcode: 2004A&A...422..331C Altcode: The bremsstrahlung emissivity and absorption coefficient in the radiofrequency range are derived under the assumption that the electron population is not purely thermal, but presents a tail of high energy particles. This population is approximated by a two-component Maxwellian distribution and by the kappa-functions of different (integer) index. It is shown that, if the temperature ratio of the two Maxwellians is larger than 10, the absorption coefficient and the effective temperature (the quantities entering the radio transfer equation) depend only on the fraction R of particles in the highest temperature Maxwellian. In the case of kappa-functions the above quantities depend on the index n of the functions.

The microwave radio spectrum is computed for different values of R and for 3 ≤ n ≤ 6, finding, in all cases, brightness temperatures lower than those computed with a pure thermal distribution. This could explain some inconsistencies found between radio and EUV observations.

Appendix is only available in electronic form at http://www.edpsciences.org Title: SOHO CDS and SUMER observations of quiescent filaments and their interpretation Authors: Del Zanna, G.; Chiuderi Drago, F.; Parenti, S. Bibcode: 2004A&A...420..307D Altcode: 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 ∼2 to 7× 1017 cm-2 according to model A, and from 5 to 17×1017 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. Title: Coronal magnetograms of solar active regions Authors: Ryabov, B. I.; Bogod, V. M.; Gelfreikh, G. B.; Maksimov, V. P.; Drago, F.; Lubyshev, B. I.; Peterova, N. G.; Borisevich, T. P.; Bezrukov, D. A. Bibcode: 2004IAUS..223..215R Altcode: 2005IAUS..223..215R A series of coronal magnetograms, related to the active region NOAA 9068 is derived using the effects of the quasi-transverse (QT-) propagation at microwaves. In the assumption that the product of the electron density N and the scale of magnetic divergence L_d is constant and equal to 10^{18} cm^{-2}, the sensitivity of the measurements peaks at B(G) approx 180 λ^{-4/3} (cm), where λ is the observational wavelength. The radio maps of the Sun taken with the Solar Siberian Radio Telescope (SSRT) at λ = 5.2 cm were sampled and converted into the 2D coronal magnetograms supplying absolute values of the total magnetic vector ěc B in the range 10-30 G. Each coronal magnetogram covers the portion of the microwave source in the active region affected by the QT-propagation conditions in the corona. Title: Effect of suprathermal particles on the quiet sun radio emission Authors: Chiuderi, C.; Chiuderi Drago, F. Bibcode: 2004cosp...35..289C Altcode: 2004cosp.meet..289C The bremsstrahlung emissivity and absorption coefficient in the radiofrequencies range are derived in the assumption that the electron population is not purely thermal, but presents a tail of high energy particles. This population is approximated by a bi-Maxwellian distribution and by kappa-functions of different (integer) index. It is shown that, if the temperature ratio of the two Maxwellians is larger than 10, the absorption coefficient and the effective temperature (the quantities entering the radio transfer equation) depend only on the fraction R of particles in the highest temperature maxwellian. In case of kappa-functions the above qunatities depend on the index n of the functions. The microwave radio spectrum is computed for different values of R and for 3 ≤ n ≤ 6, finding, in all cases, brightness temperatures lower than those computed with a pure thermal distribution. This fact could explain some inconsistencies found between radio and EUV observations. Title: Analysis of polarized microwave emission of Flare-Productive Active Region 9415 Authors: Bogod, V. M.; Gelfreikh, G. B.; Drago, F. Ch.; Maximov, V. P.; Nindos, A.; Kaltman, T. I.; Ryabov, B. I.; Tokhchukova, S. Kh. Bibcode: 2003astro.ph..9444B Altcode: The results of the microwave observations of the Sun made with the RATAN-600 have shown the existence of many types of spectral peculiarities in polarized emission of active regions, which produce powerful flares. These phenomena happen at microwaves and reflect inhomogeneous structure of magnetic field in magnetospheres of flaring active regions in wide range of heights above the photosphere. In this presentation we demonstrate an analysis of the AR 9415 during all the period of its passage across the solar disk. Results of the study point out to existence of different scenarios of circular polarization variations in the radio wave band. Here, we separated the phenomenon of the cyclotron emission passage through the quasi-transverse magnetic field (QT-region) and several effects connected with flare activity of active region. New observational data are presented and compared with the data of several observatories: SSRT, NoRH, MDI SOHO, GOES and MEES. The preliminary interpretation of the phenomena are given. Title: Solving the Discrepancy between the Extreme-Ultraviolet and Microwave Observations of the Quiet Sun Authors: Landi, E.; Chiuderi Drago, F. Bibcode: 2003ApJ...589.1054L Altcode: 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 (logT<=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. Title: The Solar Transition Region from UV and microwave observations Authors: Chiuderi Drago, F.; Landi, E. Bibcode: 2003MmSAI..74..691C Altcode: The quiet sun chromosphere-corona transition region is analyzed by comparing the ultraviolet line intensities (observed by the SOHO satellite) with the radio emission in the microwave range. Results from the two wavelength ranges seem to be in strong disagreement when standard techniques are applied to UV data. A more careful analysis of the line intensities done separately in the network and in the cell decreases the disagreement, but it does not remove it.

It is finally shown that the most important reason of disagreement comes from the lowest portion of the transition region, at log T < 4.5, where the plasma parameters derived from the UV lines (no more optically thin) are very uncertain. The radio emission puts therefore important constraints on the physical parameters of this portion of the solar atmosphere. Title: Effect of Superthermal Particles in the Quiet Sun Radio Emission Authors: Chiuderi, C.; Chiuderi Drago, F. Bibcode: 2003MmSAI..74..695C Altcode: The bremsstrahlung emissivity and absorption coefficient, in the radiofrequencies range, are derived in the assumption that the electron population is not purely thermal, but presents a tail of high energy particles. This population is approximated by a bi-Maxwellian distribution. It is shown that, if the temperature ratio of the two Maxwellians is larger than 10, the absorption coefficient and the effective temperature depend only on the fraction R of particles in the highest temperature Maxwellian. The microwave radio spectrum is computed for some values of R, finding brightness temperatures lower than those computed with a pure thermal distribution. This fact could explain some inconsistencies found between radio and EUV observations. Title: EUV observations of the same prominence at the limb and on the disk Authors: Chiuderi Drago, F. Bibcode: 2002ESASP.508..291C Altcode: 2002soho...11..291C The same prominence has been observed, using the Coronal Diagnostic Spectrometer (CDS) onboard the SOHO Satellite, at the eastern limb and, ten days later, on the disk as a filament. The intensity of the Transition Region lines above the prominence at the limb varies from 0.2 to 0.5 that of the Quiet Sun, depending on the line formation temperature, while the same quantity measured above the disk filament results negligible. The lack of emission observed in coronal lines above the filament indicates that the gas in between the Hα thin threads must be the hot coronal plasma. In this framework the observed difference between filament and prominence line intensity is ascribed to the different view angle of the thin tube-like transition region which must be present around each cool thread. Title: The Prominence - Corona and the Filament - Corona Transition Region: is There any Difference? Authors: Chiuderi Drago, F.; Landi, E. Bibcode: 2002SoPh..206..315C Altcode: 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. Title: Joint EUV/Radio Observations of a Solar Filament Authors: Chiuderi Drago, F.; Alissandrakis, C. E.; Bastian, T.; Bocchialini, K.; Harrison, R. A. Bibcode: 2001SoPh..199..115C Altcode: In this paper we compare simultaneous extreme ultraviolet (EUV) line intensity and microwave observations of a filament on the disk. The EUV line intensities were observed by the CDS and SUMER instruments on board SOHO and the radio data by the Very Large Array and the Nobeyama radioheliograph. The main results of this study are the following: (1) The Lyman continuum absorption is responsible for the lower intensity observed above the filament in the EUV lines formed in the transition region (TR) at short wavelengths. In the TR lines at long wavelengths the filament is not visible. This indicates that the proper emission of the TR at the filament top is negligible. (2) The lower intensity of coronal lines and at radio wave lengths is due to the lack of coronal emission: the radio data supply the height of the prominence, while EUV coronal lines supply the missing hot matter emission measure (EM). (3) Our observations support a prominence model of cool threads embedded in the hot coronal plasma, with a sheath-like TR around them. From the missing EM we deduce the TR thickness and from the neutral hydrogen column density, derived from the Lyman continuum and He i absorption, we estimate the hydrogen density in the cool threads. Title: Radio and UV observations of a solar filament Authors: Chiuderi Drago, F. Bibcode: 2001MmSAI..72..578C Altcode: An analysis of a solar filament observed by SUMER and CDS instruments (SOHO) and by VLA at three wavelengths between 2 and 20 cm is presented. The UV observations show that the filament, which is visible only at wavelength less than 912 Å, is due to the Lyman continuum absorption in the protuberance and provide an estimate of the neutral hydrogen column density. The absence of the filament at wavelengths longer than 912 Å indicates a negligible emission from the transition region between hot and cold matter of the filament. The comparison between such observations suggests that the cold matter is concentrated in thin filaments lying inside the hot plasma. The filling factor, the physical and geometrical parameters in the transition region around the cold filaments and the hydrogen density inside them are estimated. Title: Coordinated BeppoSAX and VLA observations of UX Arietis (CD-ROM Directory: contribs/francio1) Authors: Franciosini, E.; Pallavicini, R.; Bastian, T.; Chiuderi-Drago, F.; Randich, S.; Tagliaferri, G.; Massi, M.; Neidhöfer, J. Bibcode: 2001ASPC..223..930F Altcode: 2001csss...11..930F No abstract at ADS Title: Structure and Dynamics in the Transition Region Authors: Gontikakis, C. P.; Dara, H. C.; Alissandrakis, C. E.; Zachariadis, Th. G.; Vial, J. -C.; Bastian, T.; Chiuderi Drago, F. Bibcode: 1999ESASP.448..297G Altcode: 1999mfsp.conf..297G; 1999ESPM....9..297G No abstract at ADS Title: Comparison Between Euv And Radio Observations : A Powerful Diagnostic For The Upper Solar Atmosphere Authors: Chiuderi Drago, F.; Landi, E.; Fludra, A.; Delouis, J. M.; Kerdraon, A. Bibcode: 1999ESASP.446..287C Altcode: 1999soho....8..287C In the present work we show that EUV and radio observation can provide a very powerful plasma diagnostics when combined together. The intensity of EUV lines formed in the range of temperature corresponding to the Chromosphere-Corona Transition Region, supplies the DEM function defined as DEM(T)=Ne2 dh/dT. Above the temperature where no more lines are detected, the DEM can be either truncated or arbitrarily extrapolated to a very low value at a very high temperature. The DEM derived with both assumptions reproduce of course very well the observed line intensities, being derived from a best fit of these lines. We have used the DEM, derived from a set of EUV lines observed by CDS in an equatorial Coronal Hole using both assumptions, to compute the radio brightness temperature. The comparison of the computed Tb with the values observed by the Nancay Radiheliograph in the same hole, shows that both DEM assumptions are unable to reproduce the observations and that it is necessary to add the contribution of an isothermal Corona above the region where the DEM is defined. From the fit of the radio observations at four frequencies between 164 and 410 MHz, the coronal hole Emission Measure and an upper limit of the coronal electron temperature (T < 9. 105) are derived. Using this coronal model, together with the DEM up to T < 9.105, we have computed the theoretical EUV line intensities, obtaining a very good agreement with the observations for T > 8x105 K, thus providing also a lower limit to the hole temperature. Title: EUV and radio observations of an equatorial coronal hole Authors: Chiuderi Drago, F.; Landi, E.; Fludra, A.; Kerdraon, A. Bibcode: 1999A&A...348..261C Altcode: EUV and radio data of an equatorial coronal hole, observed in October 1996 at its central meridian transit, are compared. EUV lines were observed by the CDS instrument onboard SOHO and the radio emission by the Nancay Radioheliograph (France) in the frequency range 164-410 MHz. Using the Differential Emission Measure (DEM), derived from EUV line intensities, we have computed the radio brightness temperature T_b, leaving the coronal temperature (upper limit of the DEM definition) and pressure as free parameters, to be determined from the comparison with the observations. This analysis has shown that radio data, contrarily to EUV line intensities, cannot be fitted without the presence of isothermal plasma above the region where the DEM is defined, independently of the choice of parameters. A model, nicely fitting both sets of data, is derived, in which the coronal temperature and density are T_c <= 9 x 10(5) K and N_e(0) =~ 3x 10(8 cm(-3)) respectively. The assumption of hydrostatic equilibrium with a scale height derived from the electron temperature is discussed. Title: EUV and Radio Observations of an Equatorial Coronal Hole Authors: Chiuderi-Drago, F.; Kerdraon, A.; Landi, E.; Fludra, A. Bibcode: 1999SSRv...87..141C Altcode: An equatorial coronal hole has been observed on 18 and 19 October 1996 with SOHO-CDS and with the Nancąy Radioheliograph (RH). The CDS EUV line intensities are used to determine the coronal hole Differential Emission Measure (DEM); in turn this is used to compute the radio brightness temperature Tb at the observed frequencies, leaving the coronal electron temperature and density as free parameters. EUV line intensities, calculated from the derived models, show a good agreement with EUV observations. Title: Microwave, Soft and Hard X-Ray Observations of Solar Flares - a Self-Consistent Model of the Flare Site Authors: Chiuderi Drago, F.; Alissandrakis, C. E.; Bentley, R. D.; Philips, A. T. Bibcode: 1998SoPh..182..459C Altcode: High-resolution microwave observations of several flares performed with the Westerbork Synthesis Radio Telescope (WRST) on 3 and 4 July 1993 are compared with Yohkoh observations in the soft and hard X-ray domain. Only for one flare, among the six analyzed, was the hard X-ray spectrum between 20 and 200 keV available from the Wide Bragg Spectrometer, supplying the energy spectrum of non-thermal particles responsible for this radiation and for the radio emission. A complete model of this flare is derived which accounts for all available observations in the X-ray and radio wavelengths. Title: Activity cycles in UX ARIETIS Authors: Massi, M.; Neidhofer, J.; Torricelli-Ciamponi, G.; Chiuderi-Drago, F. Bibcode: 1998A&A...332..149M Altcode: By analysing long term radio observations of the binary system UX Arietis, activity cycles have been discovered which are strongly reminiscent of those present in the Sun. It is well known that the activity cycle in the Sun is 11 years, while the general magnetic field reverses every 11 years, returning to its initial configuration after 22 years, namely after two consecutive cycles of activity. Now we have discovered in UX Arietis an activity cycle of 25.5 days during which the polarization reverses and returns to its initial value after about two consecutive cycles of activity. Moreover as the cycle of 11 years of the Sun is modulated with a period of 90-110 years, poorly estimated due to its long term occurence, we found that the 25.5 days activity cycle in UX Arietis is also modulated with a period of 158 days. The solar activity cycle is related to the dynamo at work in the sun's interior. The fact of having in UX Arietis the same phenomena as in the Sun but at much shorter time scales (i.e. days instead of years) should make possible to acquire better statistics in the future and improve our understanding of the dynamo processes. Title: EUV and Microwave Observations of a Filament Authors: Chiuderi-Drago, F.; Bocchialini, K.; Lamartinie, S.; Vial, J. -C.; Bastian, T.; Alissandrakis, C. E.; Harrison, R.; Delaboudiniere, J. -P.; Lemaire, P. Bibcode: 1998ASPC..150...55C Altcode: 1998IAUCo.167...55C; 1998npsp.conf...55C No abstract at ADS Title: Quiescent filament diagnostic. Authors: Lamartinie, S.; Bocchialini, K.; Vial, J. -C.; Chiuderi-Drago, F.; Harrison, R.; Alissandrakis, C. E. Bibcode: 1998joso.proc..139L Altcode: UV observations of a quiescent filament were performed on July 28, 1996 by SUMER and CDS, two spectrometers onboard SOHO. Title: Testing the Extrapolated Coronal Magnetic Fields by Using Propagation Effects at Microwaves Authors: Chiuderi Drago, F.; Alissandrakis, C. E.; Borgioli, F. Bibcode: 1997ESASP.404..275C Altcode: 1997cswn.conf..275C No abstract at ADS Title: Non-Thermal Weak Radio Bursts: X-ray and/or EUV Counterparts? Authors: Crosby, N.; Vilmer, N.; Chiuderi Drago, F.; Pick, M.; Kerdraon, A.; Khan, J.; Schwenn, R. Bibcode: 1997ESASP.404..299C Altcode: 1997cswn.conf..299C No abstract at ADS Title: Microwave and UV observations of filaments with SOHO and the VLA Authors: Alissandrakis, C. E.; Drago, F.; Bastian, T.; Bocchialini, K.; Delaboudiniere, J. -P.; Lemaire, P.; Vial, J. -C.; Harrison, R. A.; Thompson, B. Bibcode: 1997ASPC..118..289A Altcode: 1997fasp.conf..289A Observations performed in coordination between SOHO instruments and ground-based observatories offer the unique possibility to derive information simultaneously in several wavelengths formed at different altitudes and/or temperatures in the solar atmosphere. The SUMER and CDS spectrometers, the imaging telescope EIT aboard SOHO, and the VLA provide complementary information in the UV and the radio ranges. We illustrate such a coordination with observations of filaments in the transition region, performed in July 1996. The observations in the UV between 10(4) and 10(6) K provide the differential emission measure as a function of temperature; this can be used to compute the expected brightness temperature in the microwave range and check models of the filament-corona transition region. Title: Coronal Magnetic Fields from Microwave Polarization Observations Authors: Alissandrakis, C. E.; Borgioli, F.; Chiuderi Drago, F.; Hagyard, M.; Shibasaki, K. Bibcode: 1996SoPh..167..167A Altcode: 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. Title: Rotational Modulation of the Radio Emission from Active Stars Authors: Franciosini, E.; Chiuderi Drago, F. Bibcode: 1996ASPC...93..300F Altcode: 1996ress.conf..300F No abstract at ADS Title: Radio Observations of UX Arietis: Analysis of its Variability Authors: Massi, M.; Neidhofer, J.; Torricelli-Ciamponi, G.; Chiuderi-Drago, F. Bibcode: 1996ASPC...93..330M Altcode: 1996ress.conf..330M No abstract at ADS Title: Radio and X-ray Emission in Stellar Magnetic Loops Authors: Franciosini, E.; Chiuderi Drago, F. Bibcode: 1996mpsa.conf..477F Altcode: 1996IAUCo.153..477F No abstract at ADS Title: Coronal Magnetic Fields from Polarization Observations at Microwaves Authors: Chiuderi Drago, F.; Borgioli, F.; Alissandrakis, C. E.; Hagyard, M.; Shibasaki, K. Bibcode: 1996mpsa.conf..443C Altcode: 1996IAUCo.153..443C No abstract at ADS Title: Coronal Magnetic Fields from Faraday Rotation Observations Authors: Alissandrakis, C. A.; Chiuderi Drago, F. Bibcode: 1995SoPh..160..171A Altcode: 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. Title: Radio and X-ray emission in stellar magnetic loops. Authors: Franciosini, E.; Chiuderi Drago, F. Bibcode: 1995A&A...297..535F Altcode: In this paper we present a model of a magnetic loop in an RS CVn active star, in which nonthermal particles are episodically injected in the local thermal plasma. The main parameters of the loop, starspot dimension and maximum photospheric field, are derived from optical observations, while the thermal plasma density is left as a free parameter. It is shown that the computed radio emission, at different times after the injection, reproduces very well the observed common characteristics of the radio spectra observed at different epochs. The agreement with the observations, however, can be achieved only if the thermal plasma density is lower than 10^7^cm^-3^. The calculations also show that the structure of the radio source evolves with time from a core-halo to a halo type in agreement with different VLBI observations. The emission measure, derived from the loop's volume and the above mentioned upper limit of thermal plasma density, is in agreement with those inferred from the X-ray observations of these type of stars, indicating a possible co-spatiality of the two emissions. Title: Radio emission from coronal streamers Authors: Chiuderi-Drago, F. Bibcode: 1994SSRv...70..279C Altcode: No abstract at ADS Title: Detection of Linear Polarization in the Microwave Emission of Solar Active Regions Authors: Alissandrakis, C. E.; Chiuderi-Drago, F. Bibcode: 1994ApJ...428L..73A Altcode: We report the detection for the first time of linear polarization in the microwave emission above a Solar Active Region. The observations were made with the Westerbork Synthesis Radio Telescope, taking advantage of the very small bandwidth of a multichannel spectral line receiver. The intensity of the Stokes parameter U, measured at several points close to the line of zero circular polarization, shows a clear sinusoidal trend as a function of lambda2, in accordance to what is expected from Faraday rotation. From the measured period, reasonable values of integral NBLdr were derived. Such observations could present in the future a unique way to measure coronal magnetic fields above Active Regions. Title: Radio emission from stellar active regions Authors: Franciosini, E.; Drago, F. Chiuderi Bibcode: 1994R&QE...37..403F Altcode: Several types of stars present a variable radio emission. In this paper we present a time-dependent model aimed to reproduce the RS CVn emission, which however can be easily extended to other types of stars. The time evolution of a population of electrons following an initial power-law distribution is studied. It is shown that the spectrum of the emitted radiation evolves, in a time scale of a few days, from the typical spectrum of the flaring emission to the flat one observed in quiescent periods. It is also shown that the intensity distribution reproduces the core-halo structure observed using VLBI for a period of about one day after the flare. Title: Periodicities in the radio emission of UX Arietis. Authors: Neidhoefer, J.; Massi, M.; Chiuderi-Drago, F. Bibcode: 1993A&A...278L..51N Altcode: Observations of UX Arietis, in the range 9 mm - 21 cm, have been performed with the Effelsberg 100-m radio-telescope from December 1992 to May 1993. The data seem to indicate that the most powerful events take place in coincidence with the orbital phase, in which the cooler K-star is in front. Also some regular variation in the circular polarization seem to be present in the data. These results however, although very interesting, deserve to be confirmed by further, more regular observations. Title: Flaring and Quiescent Radio Emission of UX Arietis: A Time-dependent Model Authors: Chiuderi Drago, F.; Franciosini, E. Bibcode: 1993ApJ...410..301C Altcode: 1993ApJ...410..301D 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 a Maxwellian distribution at the same temperature deduced from X-ray observations and 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. It is shown that the spectrum of the emitted radiation evolves, in a time scale of a few days, from the typical spectrum of the flaring emission to the flat one observed in quiescent periods. It is also shown that the intensity distribution reproduces the core-halo structure observed by Mutel et al. (1985) using VLBI for a period of about 1 day after the flare. Title: Radio and X-ray Luminosity of RS CVn Binary Systems Authors: Chiuderi Drago, F.; Franciosini, E. Bibcode: 1993ASSL..183..405C Altcode: 1993pssc.symp..405C No abstract at ADS Title: Ultra-high observations of the prominence-corona transition region. Authors: Chiuderi-Drago, F. C. Bibcode: 1992ESASP.344...49C Altcode: 1992spai.rept...49C Two models of the prominence-corona transition region (PCTR), are analyzed in view of the possibility of ultra-high resolution observations of UV lines on filaments and prominence. The first model (Chiuderi and Chiuderi-Drago, 1991) assumes an isothermal cool filament (T < 104K) with a PCTR around it, while the second one (Chiuderi-Drago, Engvold and Jensen, 1992) assumes that a PCTR is present around each one of the thin Hα threads which are usually observed in filaments. In this latter model, dissipation of Alfven waves is invoked to heat the lower portion of the PCTR where the magnetic field is perpendicular ∇T. Title: Quiscent radio emission in UX Arietis. Authors: Massi, M.; Chiuderi-Drago, F. Bibcode: 1992A&A...253..403M Altcode: Multifrequency observations of the quiescent radio emission of UX Arietis performed with the 100-m radio telescope of Effelsberg are reported. The derived spectral index implies that besides the flaring emission, also the quiescent one is due to electrons with a power-law energy distribution. A lower limit estimate of the lifetime of the quiescent emission imposes the following constraints on magnetic field and thermal electron density: B not greater than 30 G, Ne less than 4 x 10 exp 7/cu cm. The present data are insufficient to distinguish if the emitting electrons are produced during a flare or if a continuous injection of particles must be invoked. Title: Energy Balance in the Prominence / Corona Transition Region Authors: Chiuderi, C.; Chiuderi Drago, F. Bibcode: 1991SoPh..132...81C Altcode: 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. Title: Modelling a solar flare from X-ray, UV, and radio observations Authors: Drago, F. Chiuderi; Fossi, B. C. Monsignori Bibcode: 1991SoPh..132..137D Altcode: A slowly evolving, flaring loop was observed by the UVSP, XRP, and HXIS instruments onboard SMM on June 10, 1980. Simultaneous radio observations from Toyokawa (Japan) are also available. The SMM instruments have an angular resolution ranging from 3 to 30 arc sec by which the loop structure may be determined. It appears that these observations cannot be accounted for by a single loop model even assuming a variable temperature and pressure. The additional presence of a hot and tenuous isothermal plasma is necessary to explain the harder emission (HXIS). X-ray and UV data are used to fit the differential emission measure as a function of temperature and a model of the flare is deduced, which is then checked against radio data. An estimate of the heating function along the loop and of the total energy content of the loop is also given. Title: Modelling a Solar Flare from X-Ray Ultraviolet and Radio Observations Authors: Chiuderi Drago, F.; Monsignori Fossi, B. C. Bibcode: 1991SoPh..132..137C Altcode: A slowly evolving, flaring loop was observed by the UVSP, XRP, and HXIS instruments onboard SMM on June 10, 1980. Simultaneous radio observations from Toyokawa (Japan) are also available. The SMM instruments have an angular resolution ranging from 3 to 30 arc sec by which the loop structure may be determined. It appears that these observations cannot be accounted for by a single loop model even assuming a variable temperature and pressure. The additional presence of a hot and tenuous isothermal plasma is necessary to explain the harder emission (HXIS). X-ray and UV data are used to fit the differential emission measure as a function of temperature and a model of the flare is deduced, which is then checked against radio data. An estimate of the heating function along the loop and of the total energy content of the loop is also given. Title: Quiscent radio emission of RS CVn binary systems. Authors: Chiuderi Drago, F.; Klein, K. -L. Bibcode: 1990Ap&SS.170...81C Altcode: 1990Ap&SS.170...81D It is shown that the quiescent radio emission observed in some RS Vc binary systems cannot be ascribed to the same thermal electrons population responsible for the X-ray emission. An alternative explanation to account for the observed correlation between X-ray and radio luminosity is suggested. Title: Energy Balance in Prominence-Corona Transition Regions Authors: Chiuderi, C.; Chiuderi-Drago, F. Bibcode: 1990IAUS..142..325C Altcode: No abstract at ADS Title: Modelling a Solar Flare from Extreme Ultraviolet and Radio Observations Authors: Chiuderi-Drago, F.; Fossi, B. C. M. Bibcode: 1990IAUS..142..454C Altcode: No abstract at ADS Title: Radio Observations of Prominences Authors: Chiuderi Drago, F. Bibcode: 1990LNP...363...70C Altcode: 1990IAUCo.117...70C; 1990doqp.coll...70C; 1990LNP...363...70D It is shown that prominences can be analyzed through radio observations only at very short wavelengths (lambda not less than 1 cm). Longer wavelength observations can provide information on the prominence-corona transition region (PCTR). Radio observations of filaments could give an excellent determination of the angle between the temperature gradient and the magnetic field if the pressure in the transition region is known. A PCTR model that assumes a balance among the energy radiated and conducted and a heating function can reproduce the observed radio brightness temperature and the UV line intensities, at T more than 8 x 10,000 if the proper 3D geometry is correctly taken into account. The model fails to reproduce the low temperature line intensities. Title: Birth of a slowly evolving flare. Authors: Chiuderi Drago, F.; Monsignori Fossi, Brunella C. Bibcode: 1989mse..proc..215C Altcode: A gradual rise and decline flare with a duration of about one hour was observed on June 10, 1980 in the radio (Toyokawa and VLA) optical (Big Bear) and XUV (SMM satellite) ranges of wavelengths. The flare developed as a large loop connecting two regions of opposite polarity in a pre-existing active region. A model of the differential emission measure of the loop observed at the beginning of the flare is deduced from the analysis of the XUV images in C IV (1549 Å), O VIII (18.97 Å), Ne IX (13.45 Å), Mg XI (9.17 Å) and Si XIII (6.65 Å) emission lines. The differential emission measure as a function of temperature is controlled by the conductive flux via the temperature gradient. Title: Model of slowly evolving flare. Authors: Chiuderi Drago, F.; Landini, M.; Monsignori Fossi, B. C. Bibcode: 1989sasf.confP.195C Altcode: 1989IAUCo.104P.195C; 1988sasf.conf..195C A gradual rise and fall flare with a duration of about one hour was observed on June 10, 1980 in the radio (Toyokawa and VLA), optical (Bing Bear) and XUV (SMM satellite) ranges of wavelengths. The flare developed as a large loop connecting two regions of opposite polarity in a pre-existing active region. A model of the differential emission measure of the loop observed at three different stages of the flare is deduced from the analysis of the XUV images in C IV (1549 Å), O VIII (18.97 Å), Ne IX (13.45 Å), Mg XI (9.17 Å) and Si XIII (6.65 Å) emission lines. The differential emission measure as a function of temperature is controlled by the conductive flux via the temperature gradient; the evaluation of the divergence of the conductive flux is used in the energy balance to have information on the power deposition function. Title: Microwave Emission above Steady and Moving Sunspots Authors: Chiuderi Drago, F.; Alissandrakis, C.; Hagyard, M. Bibcode: 1987SoPh..112...89C Altcode: 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 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. Title: Radio outbursts in HR 1099 : quantitative analysis of flux spectrum and intensity distribution. Authors: Klein, K. -L.; Chiuderi-Drago, F. Bibcode: 1987A&A...175..179K Altcode: In this paper a detailed quantitative analysis of a well observed outburst in HR 1099 is presented. Gyrosynchrotron radiation from mildly relativistic nonthermal electrons is considered in a spatially varying magnetic field. Although the free parameters are not uniquely determined by the spectral observations, the model permits an analysis of the size and location of the radio source at different frequencies. It is shown that in the framework of such a model the low-frequency source comprises the whole binary system, while the high-frequency source has typically stellar or sub-stellar dimension. From the derived source parameters characteristic time scales for the variation of the radio flux density are estimated and discussed with respect to the observations. Title: Radio Radiation on the AR:2490 Authors: Ji, S. C.; Chiuderi-Drago, F. Bibcode: 1987PrA.....5..332J Altcode: No abstract at ADS Title: Microwave emission from steady and moving sunspots. Authors: Alissandrakis, C. E.; Chiuderi Drago, F.; Hagyard, M. J. Bibcode: 1987PAICz..66..267A Altcode: 1987eram....1..267A The authors have used force-free extrapolations of photospheric magnetic field observations from Marshall Space Flight Center to compute the total intensity and circular polarisation of sunspot associated emission from active region 2502 in the period June 13 to 15, 1980. Title: Study of radio radiation on the AR 2490. Authors: Ji, S.; Drago, F. C. Bibcode: 1987PABei...5..330J Altcode: No abstract at ADS Title: Radio spectrum of a solar active region. Authors: Chiuderi-Drago, F.; Ji, S. -C. Bibcode: 1987SSSMP..30.1199C Altcode: The radio spectrum of the A.R. (active region) 2490, in the range of wavelengths between 8 mm and 20 cm, is presented. The A.R. presents a double structure at λ ≥ 2.8 cm. It is shown that gyroresonance absorption at λ = 6 cm, which is the main source of opacity in the portion of the A.R. overhanging the sunspots, plays an important role also in the radio source associated with the Hα plage. Title: Unstable Chromospheric Dark Features and Type-Iii Bursts Association Authors: Chiuderi-Drago, F.; Mein, N.; Pick, M. Bibcode: 1986SoPh..103..235C Altcode: The temporal association between the kinematic parameters of chromospheric dark features (DF) and the production of radio type-III bursts is investigated during a period of five months. The Doppler shifts inside six different DF are measured by means of the Meudon Multichannel Subtracting Double Pass Spectrograph (MSDP) during periods of some minutes around 24 type-III bursts. The position of the radio bursts has been checked to be associated with the same active region observed by MSDP, by using the Nançay Radioheliograph. Title: Radio outbursts in HR 1099: An inhomogeneous gyrosynchrotron model Authors: Klein, K. -L.; Chiuderi-Drago, F. Bibcode: 1986AdSpR...6h.113K Altcode: 1986AdSpR...6..113K Radio emission is the only available diagnostic of nonthermal phenomena in stars. Both quiescent and flaring emission have been detected in more than 30 RS CVn type binaries /1/. A set of almost simultaneous observations of the RS CVn system HR 1099 at five microwave frequencies between 1.4 and 86 GHz was performed during a long-lasting period of activity in February 1978 /2/,/3/. During most of this period the radio emission is moderately polarized and has a broad-band flux spectrum with positive slope at low frequencies and a turnover around 15 GHz. These properties of the radiation resemble those of solar microwave bursts. Because of this analogy the flux spectrum was interpreted in terms of gyrosynchrotron radiation from mildly relativistic non thermal electrons in /4/. A uniform magnetic field and a strong density decrease of energetic electrons with height above the photosphere of the primary star was assumed in their study. Although this model perfectly fits the observed spectrum, the assumption of constant field seems to be rather unrealistic, especially for the large source size implied if the observed fluxes arise from a noncoherent mechanism. The case of a heterogeneous magnetic field is considered in this contribution with emphasis on the spatial distribution of the brightness.

on leave from Instituto di Astronomia, Universita di Firenze, Italy Title: Synchrotron model for a radio burst of HR 1099. Authors: Borghi, S.; Chiuderi-Drago, F. Bibcode: 1985A&A...143..226B Altcode: The radio spectrum of a long-lasting burst observed in the flaring star HR 1099, in the frequency range 1.4 GHz ≤ ν ≤ 15.5 GHz, is interpreted in terms of gyrosynchrotron radiation by mildly relativistic electrons. The observed flux at lower frequencies puts a firm lower limit on the dimension of the source, d ≡ 4-5 R*, making unlikely the hypothesis that the burst, at least at decimetric wavelengths, involves only a small portion of the stellar atmosphere. An electron density NR ∝ R-&alpha; in a uniform field B ≡ 100 G perfectly fits the observed points if α = 5. Title: Coronal structures. Authors: Chiuderi Drago, F. Bibcode: 1984ESASP.220..109C Altcode: 1984ESPM....4..109C Outer and inner coronal structures are shortly reviewed. Title: Erratum - Non-Thermal Radio Sources in Solar Active Regions Authors: Chiuderi-Drago, F.; Melozzi, M. Bibcode: 1984A&A...134..198C Altcode: No abstract at ADS Title: Procedures for Improving the Image Stability of Kodak Spectroscopic Plates Type-Iiia Authors: Lee, W. E.; Drago, F. J. Bibcode: 1984asph.conf..107L Altcode: No abstract at ADS Title: Non-thermal radio sources in solar active regions Authors: Chiuderi Drago, F.; Melozzi, M. Bibcode: 1984A&A...131..103C Altcode: High-resolution observations of solar active regions performed at 6 cm have often shown the presence of high-temperature sources (brightness temperature greater than 2 x 10 to the 6th K) which cannot be explained simply by increasing the opacity of the coronal plasma with thermal free-free or gyroresonance absorption. These sources are explained by synchrotron emission of mildly relativistic electrons following a power-law distribution in a field of about 100 G. A type-III burst-like mechanism for the electron acceleration is proposed, and it is shown that electrons trapped in regions of low pressure can survive for about 1 h, a time longer than the average interval between two consecutive bursts in some active regions. The emissivity and the absorption coefficient in the ordinary and extraordinary waves are computed with an approximate method which gives a perfect agreement with the exact equation after the seventh harmonic. The computed brightness temperature and polarization, which perfectly agree with the observed ones at 6 cm, present a maximum at 10-15 cm. Title: Procedures for improving the image stability of KODAK spectroscopic plates, type IIIa-J. Authors: Lee, W. E.; Drago, F. J. Bibcode: 1984ORROE..14..107L Altcode: No abstract at ADS Title: New procedures for processing and storage of Kodak spectroscopic plates, type IIIa-J. Authors: Lee, W. E.; Drago, F. J.; Ram, A. T. Bibcode: 1984JImT...10...22L Altcode: No abstract at ADS Title: Coordinated radio observations during SMM Authors: Chiuderi Drago, F. Bibcode: 1984MmSAI..55..801C Altcode: Combined X-UV and high resolution cm-wavelength observations conducted during SMM are discussed. The results reveal that very high magnetic fields are present at coronal levels either above very large sunspots or above small spots of the same polarity. High brightness temperature sources are sometimes observed in active regions without any association with X-ray or magnetic structures. They can be explained only by the presence of currents generating a strong nonpotential field in the corona or by a nonthermal mechanism; the high brightness temperatures seem to indicate the latter mechanism. Some results on correlated coronograph and metric wavelength observations of type II and type IV radio bursts detected during coronal transients are also presented. These indicate that the former bursts are not excited by the shock wave generated by coronal mass ejection (CME), while the latter can be generated by plasma oscillations in a denser plasmoid travelling with the CME. Title: Microwave, ultraviolet, and soft X-Ray observations of hale region 16898 Authors: Shibasaki, K.; Chiuderi-Drago, F.; Melozzi, M.; Slottje, C.; Antonucci, E. Bibcode: 1983SoPh...89..307S Altcode: Hale region 16898 was observed by the Westerbork Synthesis Radio Telescope at 6 cm and by the Ultraviolet Spectrometer and Polarimeter and the X-Ray Spectrometer on the Solar Maximum Mission satellite. Optical pictures of the same active region were taken at Sacramento Peak, Big Bear, and Meudon Observatories. The radio emission mechanisms are identified by comparing radio data with ultraviolet and soft X-ray data. The height of the radio sources and the magnetic field strength at that height are deduced. A radio source above a large sunspot shows a crescent shaped depression of circular polarization and a high brightness temperature. The emission mechanism is identified as gyroresonance at the second and the third harmonic layers and it is found that the second harmonic layer, where the magnetic field strength is 900 G, must be in the corona. An extended loop-like source connecting the leading and the following part of the active region as well as the sources associated with small spots are mainly due to thermal free-free emission by hot and dense plasma which is also observed in ultraviolet and soft X-ray radiation. The calculated radio brightness temperature, using the physical parameters deduced from the ultraviolet and soft X-ray line intensities, agrees with the observed brightness temperature. The height of the low brightness temperature sources above the small spots is 6000 ± 3000 km and that above the large spot is less than 3000 km: the source above the large spot does not show any shift relative to the sunspot due to the projection effect. Very strong radio emission was found which was associated with the merging of a group of small spots into the large sunspot. In the same day, warm (≤ 106 K) and dense matter was present above the large spot. Evidence for nonthermal emission is presented. Title: Network to Cell Contrast at Microwaves Authors: Chiuderi Drago, F.; Kundu, M. R.; Schmahl, E. J. Bibcode: 1983SoPh...85..237C Altcode: 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. Title: Multiple Wavelength Observations of a Solar Active Region Authors: Chiuderi-Drago, F.; Bandiera, R.; Willson, R. F.; Slottje, C.; Falciani, R.; Antonucci, E.; Lang, K. R.; Shibasaki, K. Bibcode: 1982SoPh...80...71C Altcode: The Solar Maximum Mission Satellite, the Sacramento Peak Vacuum Tower Telescope, the Very Large Array and the Westerbork Synthesis Radio Telescope have been used to observe active region AR 2490 on two consecutive days at soft X-ray, ultraviolet, optical and radio wavelengths (2, 6, and 20 cm), with comparable angular resolution (2″ to 15″) and field of view (4' × 4'). The radio emissions at λ = 6 cm and 20 cm show a double structure in which one component is associated with bright Hα plage, C IV and soft X-ray emission, and the other component is associated only with sunspots. No radiation at λ = 2 cm is detected in this latter component. Coronal temperature and emission measure derived from X-ray lines indicate that the dominant radiation mechanism of the plage-associated component is due to thermal bremsstrahlung while the gyroresonance absorption coefficient must be invoked to account for the high brightness temperature (Tb≈2×106K) observed in the sunspot associated component. The high magnetic field strength needed (600 G at a level where T∼2×106K) is explained assuming a thin transition zone, in order to reach a high electron temperature close to the sunspot, where the magnetic fields are stronger. A higher temperature gradient above sunspots is also consistent with the absence of detectable C IV emission. Title: Space-born and Ground-based Observations of a Solar Active Region and a Flare Authors: Chiuderi Drago, F. Bibcode: 1982ASSL...96..115C Altcode: 1982spls.meet..115D Observational data of the active solar region AR 2490 are discussed with an eye to underlying physical processes. Ground- and spaceborne measurements were made by radio, optical, and XUV instrumentation. A double structure observed at 6 and 20 cm wavelengths was overlying a sunspot group which displayed north polarity. The 6 cm emission was attributed to free-free emission, while the 20 cm feature was thought to be caused by gyroresonance absorption. An analytical formulation was developed which described the thermal component for maximum X ray intensities. A flare observed on June 10, 1980 was detected on H-alpha and C IV spectrographic bands. The origin of the emissions was fixed at the two feet of the X ray loop, with a radio emission coming from the top of the loop. Title: Simultaneous Observations of AR2490 Performed in the X-Ray Ultraviolet Optical and Radio Wavelength Domain Authors: Drago, F. Chiuderi Bibcode: 1981SSRv...29..439D Altcode: No abstract at ADS Title: Simultaneous observations of AR 2490 performed in the X-ray, UV, optical, and radio wavelength domain. Authors: Chiuderi Drago, F. Bibcode: 1981SSRv...29..439C Altcode: No abstract at ADS Title: EUV and radio spectrum of coronal holes Authors: Chiuderi Drago, F. Bibcode: 1980SoPh...65..237C Altcode: From the intensity of 19 EUV lines whose formation temperature ranges from 30,000 to 1,400,000, two different models of the transition region and corona for the cell-center and the network are derived. It is shown that both these models give radio brightness temperatures systematically higher than the observed ones. An agreement with radio data can be found only with lines formed at low temperature (less than 850,000) by decreasing the coronal temperature and the emission measure. The possibility of resolving the discrepancy by using different ion abundances has been also investigated, with negative results. Title: Analysis of the Supergranulation Structure in Coronal Holes Authors: Calamai, G.; Drago, F. C.; Pettini, G. Bibcode: 1980SoPh...65..167C Altcode: Monochromatic rasters of six EUV lines taken in the centre of a coronal hole are analysed and the cell-network structure at different heights in the solar atmosphere is shown. Also a comparison with the average cell-centre and network intensities of another coronal hole is performed showing a good agreement between the two. Title: Bursts observed at 21 CM with the Westerbork interferometer Authors: Chiuderi-Drago, F.; Palagi, F. Bibcode: 1980IAUS...86..187C Altcode: The characteristics of two solar radio bursts observed at 21 cm with the Westerbork radio interferometer are discussed. The first burst was observed simultaneously with an optical flare of class I and was found to be composed of several sources. Of the two longest-lasting sources, one was observed to be located above the optical flare and to be completely unpolarized with intermittent appearance, and the other was observed to be completely left-hand circularly polarized, suggesting a position above the region of the positive magnetic field. The second burst was recorded simultaneously with a subflare at the position of the first burst, and was found to exhibit a strong left-hand circular polarization which split into two components with opposite polarization which then lost their polarization. The observed polarization behavior can be explained in terms of emission from a thermal source in the presence of a magnetic field. Title: Discussion Authors: Chiuderi-Drago, F.; Kundu, M. R.; Lantos, P.; Zirin, H. Bibcode: 1979phsp.coll..134C Altcode: 1979IAUCo..44..134C No abstract at ADS Title: Discussion Authors: Anzer, U.; Chiuderi-Drago, F.; Kundu, M. R.; Leroy, J. L.; Malville, J.; Rompolt, B.; Sheeley, N.; Stenflo, J.; Tandberg-Hanssen, E.; Öhman, Y. Bibcode: 1979phsp.coll...77A Altcode: 1979IAUCo..44...77A No abstract at ADS Title: Osservazioni radio di buchi coronali. Authors: Chiuderi-Drago, F.; Poletto, G. Bibcode: 1977MmSAI..48..762C Altcode: No abstract at ADS Title: The filament-corona transient region from OSO-VI EUV observations. Authors: Chiuderi Drago, Franca; Silvi, Maurizio Bibcode: 1977SoPh...55..177C Altcode: The transition region between filament and corona is investigated measuring the intensity of six EUV lines above two filaments on the disk observed on September 1 and 2, 1969 by OSO-VI. The comparison between these intensities and those observed on quiet regions shows that there is no difference between the two transition regions. Title: The filament-corona transition region from OSO-VI EUV observations Authors: Drago, F. C.; Silvi, M. Bibcode: 1977SoPh...55..177D Altcode: The transition region between filament and corona is investigated measuring the intensity of six EUV lines above two filaments on the disk observed on September 1 and 2, 1969by OSO-VI. The comparison between these intensities and those observed on quiet regions shows that there is no difference between the two transition regions Title: High resolution intensity and polarization structure of the sun at 21 cm. Authors: Chiuderi Drago, F.; Felli, M.; Tofani, G. Bibcode: 1977A&A....61...79C Altcode: The sun has been observed at 21 cm with the Westerbork Synthesis Radio Telescope with a resolution of 21 by 61 arcsec. The maps of total intensity and circular polarization are presented. The most prominent structures are three active centers. The radio parameters derived are compared with optical and magnetic data. For one region it is also possible to combine the 21-cm data with observations at 2.8 cm obtained with a resolution of 17 by 60 arcsec. At 21 cm the active centers are very extended with no prominent peaks. The region observed also at 2.8 cm presents at this frequency a bright core of smaller size surrounded by a lower-intensity halo. This difference is interpreted in terms of an optical-thickness effect. The brightness temperatures at 21 cm are of the order of 1 to 2 million K, and the circularly polarized emission, well-correlated with the magnetic structure, has an intensity never greater than 5% of the total intensity. Title: A dynamical model of coronal holes based on radio observations Authors: Chiuderi Drago, F.; Poletto, G. Bibcode: 1977A&A....60..227C Altcode: A dynamical homogeneous model of coronal holes, taking into account a more than radial divergence of magnetic-field lines, is derived under the assumptions of a constant temperature at coronal levels and of a constant ratio (p-squared/Fc) of the square of the pressure to the conductive flux in the transition region. Available observations of brightness temperatures at several radio frequencies allow the determination of p-squared/Fe as well as the temperature and density at the base of the corona compatible with radio observations. The model so obtained gives the correct values of the EUV line intensities arising from the transition region, since they depend only on p-squared/Fc, while the Mg X coronal-line intensity at 625 A turns out to be lower than observed by about one order of magnitude. This indicates that the discrepancy previously found in the interpretation of EUV and radio data assuming a hydrostatic atmosphere still persists. Title: Structure of coronal holes from UV and radio observations Authors: Chiuderi Drago, F.; Avignon, Y.; Thomas, R. J. Bibcode: 1977SoPh...51..143C Altcode: The coronal hole observed on May 31, 1973 is studied using extreme ultraviolet and radio observations. The EUV line is the Fe XV at λ = 284 Å and the radio frequencies are 169 and 408 MHz. An unsuccessful attempt to deduce an homogeneous model of the hole from these observations, shows that EUV and radio observations are inconsistent if interpreted in such a frame and if the EUV line intensity measurements in the hole are reliable. Title: Dynamical Effects in the EUV Chromospheric Network. Authors: Chiuderi-Drago, F. G.; Poletto, G. Bibcode: 1977uxsa.coll...10C Altcode: 1977IAUCo..43...10C No abstract at ADS Title: A model of the solar transition region and corona for the minimum of activity Authors: Chiuderi Drago, F.; Noci, G. Bibcode: 1976A&A....48..367C Altcode: Constant pressure and conductive flux in the transition region and constant temperature and hydrostatic equilibrium in the corona are assumed to allow the use of simple formulas to derive the brightness temperature. Contributions to the brightness temperature from the transition region and corona are compared. Three of the very few available observations in the relevant frequency range are used to deduce a model of these two regions for solar activity minima, which turns out to be quite similar to UV and X-ray models of coronal holes. Title: Limb Brightening and Dark Features Observed at 6cm Wavelength Authors: Chiuderi-Drago, F.; Furst, E.; Hirth, W.; Lantos, P. Bibcode: 1975A&A....39..429C Altcode: Summary. Radio maps of the sun at 6 cm wavelength were obtained using the 100 m-telescope near Bonn (HPBW =2.6') during 7 days. The most interesting features present on these maps are: a) limb brightening of about 15 % without deconvolution, and b) depression zones correlated with filaments. The combination of both observations allows to determine the electron density of the low corona and leads to the conclusion that there is a transition region between filament and the surrounding corona. Key words: solar limb brightening filaments Title: A coronal hole observed at X, UV and radio wavelengths. Authors: Drago, F. Bibcode: 1975MmArc.104..120D Altcode: No abstract at ADS Title: A coronal hole observed at X, UV and radio wavelengths Authors: Drago, F. Bibcode: 1975xtcg.work..120D Altcode: A large coronal hole, whose central meridian passage took place on May 31, 1973, was observed by the X-ray telescope on Skylab, by the OSO 7 satellite (at 284 A), and by the radio interferometer at 408 and 169 MHz. An attempt to determine the parameters of this hole is made by comparing the UV and radio data, but the lack of north-south resolution in the radio data does not allow a satisfactory determination. Title: Brightness Distribution at λ = 3 and 21 cm near the Solar North Pole Authors: Chiuderi Drago, F.; Patriarchi, P. Bibcode: 1974SoPh...37..403C Altcode: 1974SoPh...37..403D The brightness distribution of the Sun near the north pole at λ = 3 and 21 cm is obtained using the results of the partial eclipse of February 25, 1971. The solution obtained by expanding Bλ(r) in orthogonal polynomials does not show any brightening at either frequency, but a sensible darkening. If this result is explained by the presence of spicules, the coronal emission must be negligible with respect to the emission coming from the transition region. This is possible, at λ = 21 cm, only if we assume the presence of a coronal hole near the pole. Title: Solar radio brightness distribution from OSO IV UV spectroheliograms Authors: Chiuderi Drago, F.; Poletto, G. Bibcode: 1973MmSAI..44..171C Altcode: No abstract at ADS Title: Centre to limb brightness distribution at radiofrequencies (Results of the total solar eclipse of November 12, 1966) Authors: Chiuderi Drago, F.; Patriarchi, P. Bibcode: 1973MmSAI..44..321C Altcode: No abstract at ADS Title: On the Radio Optical Depth of the Layer Where the Temperature Equals the Brightness Temperature Authors: Chiuderi Drago, Franca Bibcode: 1972SoPh...27..132C Altcode: It is usually believed that the radio optical depth of the layer h* where the temperature equals the brightness temperature is independent of the frequency. This assumption is criticized from the theoretical point of view and the behaviour of τ(h*) as a function of the frequency v is computed for two different solar models. Title: Coronal Abundance of Elements and a Model of the Quiet Sun from Radio Observations Authors: Chiuderi, Claudio; Chiuderi Drago, Franca; Noci, Giancarlo Bibcode: 1972SoPh...26..343C Altcode: It is shown that the combined use of radio observations of the quiet Sun and UV line intensities allows to compute the absolute coronal abundance of the elements. The abundances found by this method agree very well with the most recent determinations. A model of the transition region and corona in hydrostatic equilibrium is also presented. Similarities and differences with models based on UV observations are discussed. Title: Coronal Abundances and a Model of the Quiet Sun from Radio Observations Authors: Chiuderi, C.; Chiuderi Drago, F.; Noci, G. Bibcode: 1972BAAS....4T.379C Altcode: No abstract at ADS Title: Radio brightness of the sun at centimetric wavelength Authors: Chiuderi-Drago, F. Bibcode: 1972cesra...3..114C Altcode: No abstract at ADS Title: Radio model of the quiet sun Authors: Chiuderi-Drago, F. Bibcode: 1971cesra...1...52C Altcode: No abstract at ADS Title: Le riunioni del CESRA ad Utrecht e Trieste (Congressi e Simposi) Authors: Chiuderi Drago, F.; Tofani, G. Bibcode: 1971MmSAI..42..647C Altcode: No abstract at ADS Title: Model of the quiet sun and coronal abundance of elements from radio observations Authors: Chiuderi, C.; Chiuderi-Drago, F.; Noci, G. Bibcode: 1971cesra...2...55C Altcode: No abstract at ADS Title: Radio Observation of the Solar Eclipse of May 20, 1966 Authors: Drago, F. G.; Noci, G. G. Bibcode: 1969SoPh....7..276D Altcode: The eclipse of May 20, 1966 was observed at the wavelengths of 3.2 and 9.1 cm by three Arcetri expeditions. The curves obtained by deriving the occultation curves have been filtered by digital techniques to cut off high frequency noise; by them, many characteristics of three sources of the S-component present on the disk have been studied: temperature, dimensions, emitted flux and brightness distribution. Isophotes of the latter are compared with isophotes of the corresponding Hα plages for two sources: a close similarity results for one of them. Moreover it is shown that: (a) the height above the photosphere of the sources at λ = 9.1 cm is greater than that of the sources at λ = 3.2 cm; (b) the maximum of the radio emission is not always placed exactly above a sunspot or above the sunspot group barycentre. Title: Observations of solar radio bursts from September 1965 to December 1967. Authors: Drago, F. Bibcode: 1969MmArc..90.....D Altcode: No abstract at ADS Title: Le sorgenti DI radiazione nell'intervallo spettrale 1 Ap0s-2000 Ap0s Authors: Canto, A. M.; Landini, M.; Noci, Giancarlo; Drago, F. G. Bibcode: 1969MmArc..89....1C Altcode: 1969PUFir..89.....C No abstract at ADS Title: The radio event of August 3, 1967 (Lettera alla Direzione) Authors: Drago, F. G. Bibcode: 1968MmSAI..39..377D Altcode: No abstract at ADS Title: Models of solar chromosphere and corona and the 21 cm hydrogen line in the solar spectrum. Authors: Cantù, A. M.; Drago, F. G.; Noci, G. Bibcode: 1968MmArc..89...41C Altcode: No abstract at ADS Title: Osservazione radio - (λ=3.1 cm, λ=9.1 cm) di una eclisse solare da tre stazioni distanziate Authors: Drago, F.; Noci, G. Bibcode: 1968sai....11...92D Altcode: No abstract at ADS Title: A statistical study on the association between flares and type II Bursts Authors: Drago, F. G.; Tagliaferri, G. L. Bibcode: 1967sai....10...45D Altcode: No abstract at ADS Title: Osservazioni di burst ad onde metriche e fenomeni ottici associati Authors: Drago, F.; Godoli, G.; Porta, M.; Piattelli, M. Bibcode: 1967MmSAI..38..643D Altcode: No abstract at ADS Title: The importance 2 flare of October 4, 1965. Optical and radio observations Authors: Ballario, M. C.; Drago, F. G. Bibcode: 1966MmSAI..37..317B Altcode: No abstract at ADS Title: Variazione della componente di base col ciclo solare a 600 e a 200 MHz Authors: Drago, F. G. Bibcode: 1966MmSAI..37..197D Altcode: No abstract at ADS Title: On the Association Rate between Flares and Type II Bursts Authors: Drago, F. G.; Tagliaferri, G. L. Bibcode: 1966ZA.....63..202D Altcode: No abstract at ADS Title: Inversion of the Polarization between Metric and Centimetric Wavelengths. With 1 Figure in the Text Authors: Drago, F. G. Bibcode: 1965ZA.....61..173D Altcode: No abstract at ADS Title: Ricerche di radioastronomia solare in Arcetri - Nota II: Il flusso radioelettrico del Sole quieto a 186 M Hz Authors: Drago, F.; Righini, G. Bibcode: 1961MmSAI..31..437D Altcode: No abstract at ADS