34 research outputs found
Magnetic Evolution and Temperature Variation in a Coronal Hole
We have explored the magnetic flux evolution and temperature variation in a
coronal-hole region, using Big Bear Solar Observatory (BBSO) deep magnetograms
and {\it SOHO}/EIT images observed from 2005 October 10 to 14. For comparison,
we also investigated a neighboring quiet region of the Sun. The coronal hole
evolved from its mature stage to its disappearance during the observing period.
We have obtained the following results: (1) When the coronal hole was well
developed on October 10, about 60 % of the magnetic flux was positive. The EUV
brightness was 420 counts pixel, and the coronal temperature, estimated
from the line ratio of the EIT 195 {\AA} and 171 {\AA} images, was 1.07 MK. (2)
On October 14, when the coronal hole had almost disappeared, 51 % of the
magnetic flux was positive, the EUV radiance was 530 counts pixel, and
the temperature was 1.10 MK. (3) In the neighboring quiet region, the fraction
of positive flux varied between 0.49 and 0.47. The EUV brightness displayed an
irregular variation, with a mean value of 870 counts pixel. The
temperature was almost constant at 1.11 MK during the five-day observation. Our
results demonstrate that in a coronal hole less imbalance of the magnetic flux
in opposite polarities leads to stronger EUV brightness and higher coronal
temperatures
Comparison of Magnetic Flux Distribution between a Coronal Hole and a Quiet Region
Employing Big Bear Solar Observatory (BBSO) deep magnetograms and H
images in a quiet region and a coronal hole, observed on September 14 and 16,
2004, respectively, we have explored the magnetic flux emergence, disappearance
and distribution in the two regions. The following results are obtained: (1)
The evolution of magnetic flux in the quiet region is much faster than that in
the coronal hole, as the flux appeared in the form of ephemeral regions in the
quiet region is 4.3 times as large as that in the coronal hole, and the flux
disappeared in the form of flux cancellation, 2.9 times as fast as in the
coronal hole. (2) More magnetic elements with opposite polarities in the quiet
region are connected by arch filaments, estimating from magnetograms and
H images. (3) We measured the magnetic flux of about 1000 magnetic
elements in each observing region. The flux distribution of network and
intranetwork (IN) elements is similar in both polarities in the quiet region.
For network fields in the coronal hole, the number of negative elements is much
more than that of positive elements. However for the IN fields, the number of
positive elements is much more than that of negative elements. (4) In the
coronal hole, the fraction of negative flux change obviously with different
threshold flux density. 73% of the magnetic fields with flux density larger
than 2 Gauss is negative polarity, and 95% of the magnetic fields is negative,
if we only measure the fields with their flux density larger than 20 Gauss. Our
results display that in a coronal hole, stronger fields is occupied by one
predominant polarity; however the majority of weaker fields, occupied by the
other polarity
Physics of Solar Prominences: I - Spectral Diagnostics and Non-LTE Modelling
This review paper outlines background information and covers recent advances
made via the analysis of spectra and images of prominence plasma and the
increased sophistication of non-LTE (ie when there is a departure from Local
Thermodynamic Equilibrium) radiative transfer models. We first describe the
spectral inversion techniques that have been used to infer the plasma
parameters important for the general properties of the prominence plasma in
both its cool core and the hotter prominence-corona transition region. We also
review studies devoted to the observation of bulk motions of the prominence
plasma and to the determination of prominence mass. However, a simple inversion
of spectroscopic data usually fails when the lines become optically thick at
certain wavelengths. Therefore, complex non-LTE models become necessary. We
thus present the basics of non-LTE radiative transfer theory and the associated
multi-level radiative transfer problems. The main results of one- and
two-dimensional models of the prominences and their fine-structures are
presented. We then discuss the energy balance in various prominence models.
Finally, we outline the outstanding observational and theoretical questions,
and the directions for future progress in our understanding of solar
prominences.Comment: 96 pages, 37 figures, Space Science Reviews. Some figures may have a
better resolution in the published version. New version reflects minor
changes brought after proof editin
Effect of suprathermal particles on the quiet Sun radio emission
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 , 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
The HeI abundance in Solar filaments
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 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, , with previous observations and models
The Solar Transition Region from UV and microwave observations
Abstract. 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
Coronal magnetic fields from faraday rotation observations
In the first part of this communication we briefly summarize the results of the first observation of linear polarization in the microwave emission above a solar active region obtained with the Westerbork Synthesis Radio Telescope, taking advantage of the very narrow bandwidths of a multi-channel spectral line receiver. The intensity of the Stokes parameter U, 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. © 1995 Kluwer Academic Publishers