chiuderi-drago

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Author name code: chiuderi-drago
ADS astronomy entries on 2022-09-14
author:"Chiuderi Drago, Franca"

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Title: The Quiet-Sun Differential Emission Measure from Radio and
UV Measurements
Authors: Landi, E.; Chiuderi Drago, F.
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 &lt; 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.

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Title: The HeI abundance in Solar filaments
Authors: Chiuderi Drago, F.
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 &gt; 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.

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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.
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.

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Title: Effect of suprathermal particles on the quiet Sun radio
emission
Authors: Chiuderi, C.; Chiuderi Drago, F.
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

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Title: SOHO CDS and SUMER observations of quiescent filaments and
their interpretation
Authors: Del Zanna, G.; Chiuderi Drago, F.; Parenti, S.
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.

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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.
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.

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Title: Effect of suprathermal particles on the quiet sun radio
emission
Authors: Chiuderi, C.; Chiuderi Drago, F.
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.

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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.
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.

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Title: Solving the Discrepancy between the Extreme-Ultraviolet and
Microwave Observations of the Quiet Sun
Authors: Landi, E.; Chiuderi Drago, F.
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&lt;=4.3) with values based on
the Vernazza, Avrett, &amp; 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.

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Title: The Solar Transition Region from UV and microwave observations
Authors: Chiuderi Drago, F.; Landi, E.
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 &lt;
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.

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Title: Effect of Superthermal Particles in the Quiet Sun Radio
Emission
Authors: Chiuderi, C.; Chiuderi Drago, F.
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.

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Title: EUV observations of the same prominence at the limb and on
the disk
Authors: Chiuderi Drago, F.
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.

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Title: The Prominence - Corona and the Filament - Corona Transition
Region: is There any Difference?
Authors: Chiuderi Drago, F.; Landi, E.
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.

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Title: Joint EUV/Radio Observations of a Solar Filament
Authors: Chiuderi Drago, F.; Alissandrakis, C. E.; Bastian, T.;
Bocchialini, K.; Harrison, R. A.
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.

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Title: Radio and UV observations of a solar filament
Authors: Chiuderi Drago, F.
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.

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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.
2001ASPC..223..930F Altcode: 2001csss...11..930F
No abstract at ADS

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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.
1999ESASP.448..297G Altcode: 1999mfsp.conf..297G; 1999ESPM....9..297G
No abstract at ADS

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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.
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 &lt; 9. 105) are derived. Using this coronal
model, together with the DEM up to T &lt; 9.105, we have computed the
theoretical EUV line intensities, obtaining a very good agreement with
the observations for T &gt; 8x105 K, thus providing also a lower limit
to the hole temperature.

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Title: EUV and radio observations of an equatorial coronal hole
Authors: Chiuderi Drago, F.; Landi, E.; Fludra, A.; Kerdraon, A.
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 &lt;= 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.

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Title: EUV and Radio Observations of an Equatorial Coronal Hole
Authors: Chiuderi-Drago, F.; Kerdraon, A.; Landi, E.; Fludra, A.
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.

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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.
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.

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Title: Activity cycles in UX ARIETIS
Authors: Massi, M.; Neidhofer, J.; Torricelli-Ciamponi, G.;
Chiuderi-Drago, F.
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.

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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.
1998ASPC..150...55C Altcode: 1998IAUCo.167...55C; 1998npsp.conf...55C
No abstract at ADS

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Title: Quiescent filament diagnostic.
Authors: Lamartinie, S.; Bocchialini, K.; Vial, J. -C.; Chiuderi-Drago,
F.; Harrison, R.; Alissandrakis, C. E.
1998joso.proc..139L Altcode:
UV observations of a quiescent filament were performed on July 28,
1996 by SUMER and CDS, two spectrometers onboard SOHO.

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Title: Testing the Extrapolated Coronal Magnetic Fields by Using
Propagation Effects at Microwaves
Authors: Chiuderi Drago, F.; Alissandrakis, C. E.; Borgioli, F.
1997ESASP.404..275C Altcode: 1997cswn.conf..275C
No abstract at ADS

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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.
1997ESASP.404..299C Altcode: 1997cswn.conf..299C
No abstract at ADS

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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.
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.

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Title: Coronal Magnetic Fields from Microwave Polarization
Observations
Authors: Alissandrakis, C. E.; Borgioli, F.; Chiuderi Drago, F.;
Hagyard, M.; Shibasaki, K.
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.

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Title: Rotational Modulation of the Radio Emission from Active Stars
Authors: Franciosini, E.; Chiuderi Drago, F.
1996ASPC...93..300F Altcode: 1996ress.conf..300F
No abstract at ADS

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Title: Radio Observations of UX Arietis: Analysis of its Variability
Authors: Massi, M.; Neidhofer, J.; Torricelli-Ciamponi, G.;
Chiuderi-Drago, F.
1996ASPC...93..330M Altcode: 1996ress.conf..330M
No abstract at ADS

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Title: Radio and X-ray Emission in Stellar Magnetic Loops
Authors: Franciosini, E.; Chiuderi Drago, F.
1996mpsa.conf..477F Altcode: 1996IAUCo.153..477F
No abstract at ADS

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Title: Coronal Magnetic Fields from Polarization Observations at
Microwaves
Authors: Chiuderi Drago, F.; Borgioli, F.; Alissandrakis, C. E.;
Hagyard, M.; Shibasaki, K.
1996mpsa.conf..443C Altcode: 1996IAUCo.153..443C
No abstract at ADS

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Title: Coronal Magnetic Fields from Faraday Rotation Observations
Authors: Alissandrakis, C. A.; Chiuderi Drago, F.
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.
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.
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.
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
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.
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.
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.
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.
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 &lt; 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.
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.
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
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.
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.
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.
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.
1990IAUS..142..454C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Radio Observations of Prominences
Authors: Chiuderi Drago, F.
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.
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.
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.
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.
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.
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.
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.
1987PABei...5..330J Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Radio spectrum of a solar active region.
Authors: Chiuderi-Drago, F.; Ji, S. -C.
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.
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.
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.
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-&amp;alpha; in a
uniform field B ≡ 100 G perfectly fits the observed points if α = 5.

---------------------------------------------------------
Title: Coronal structures.
Authors: Chiuderi Drago, F.
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.
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.
1984asph.conf..107L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Non-thermal radio sources in solar active regions
Authors: Chiuderi Drago, F.; Melozzi, M.
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.
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.
1984JImT...10...22L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Coordinated radio observations during SMM
Authors: Chiuderi Drago, F.
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.
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.
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.
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.
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
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.
1981SSRv...29..439C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: EUV and radio spectrum of coronal holes
Authors: Chiuderi Drago, F.
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.
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.
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.
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.
1979phsp.coll...77A Altcode: 1979IAUCo..44...77A
No abstract at ADS

---------------------------------------------------------
Title: Osservazioni radio di buchi coronali.
Authors: Chiuderi-Drago, F.; Poletto, G.
1977MmSAI..48..762C Altcode:
No abstract at ADS

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Title: The filament-corona transient region from OSO-VI EUV
observations.
Authors: Chiuderi Drago, Franca; Silvi, Maurizio
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.
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.
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.
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.
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.
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.
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.
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.
1975MmArc.104..120D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A coronal hole observed at X, UV and radio wavelengths
Authors: Drago, F.
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.
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.
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.
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
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
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.
1972BAAS....4T.379C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Radio brightness of the sun at centimetric wavelength
Authors: Chiuderi-Drago, F.
1972cesra...3..114C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Radio model of the quiet sun
Authors: Chiuderi-Drago, F.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
1961MmSAI..31..437D Altcode:
No abstract at ADS