Abstract
The observations of the solar magnetic fields is one of the most
important basics for study of all
important processes in structuring the solar atmosphere and most kinds
of the release of the
energy. The radio methods are of the special interest here because they
gain the information on
the magnetic field strength in the solar corona and upper chromosphere
where traditional optical
methods do not work.
The construction of
the Nobeyama radio heliograph
opens a new era in
usage radio methods for
solar radio magnetography due to some unique property of the instrument:
- The 2D mapping of the whole disk of the sun both in I and V Stokes
parameters with resolution
of 10 arcsec.
- Regular observations (without breaks due to weather conditions), eight
hours a day, already for
seven years.
The most effective and representative radio method of measuring the
solar magnetic fields is to
use polarization measurements of the thermal bremsstrahlung (free-free
emission). It is
applicable both to analysis of chromospheric and coronal magnetic fields
and presents information
on longitude component of the magnetic field strength in solar active
regions. Three problems are
met, however: (i) One needs to measure very low degree of polarization
(small fraction of a
percent); (ii) To get the real value of the field the spectral data are
necessary. (iii) While
observing an active region on the disk we have got the overlapping
effects on polarized signal of
the chromospheric and coronal magnetic fields.
To get higher sensitivity the averaging of the radio maps over periods
of about ten minutes were
used with the results of sensitivity on V-maps of the order 0.1%.
Observations for a number of
dates have been analysed (August 22, 1992, October 31, 1992; June 30,
1993, July 22,1994, June
15, 1995 and some more). In all cases a very good similarity was found
of the polarized regions
(V-maps) with the Ca^ + plages in form and total coincidence with the
direction of the magnetic
fields on the optical magnetograms of the same active regions. For rough
estimation of the
magnetic field strength an a priory value of spectral index (n ~1) was
used with more or less
reasonable result.
The value of the longitude component of the magnetic field is obtained
using the solution of the
equation of transfer of the radio waves in thermal plasma, which can be
written in the form: B_l =
frac{107}{lambda cdot n}cdot P%, where spectral index n and degree of
polarization P% are to be
found from observations. In case of coronal magnetic fields n = 2 for
optically thin structure, so the
spectral data can be omitted.
To make a more detailed analysis including separate estimation for both
chromospheric and
coronal magnetic field distributions we may use some additional
information from spectral (but one
dimensional) observation made with the RATAN-600 or Pulkovo radio
telescope. Also methods
of stereoscopy are in some cases effective (in this case we need
observations of a stable active
regions for several days). However, these methods will be discussed in
another paper. This work
is supported by RFBR Grant No. 96-02-16268 and GKNT 1.5.4.6. - program
''Astronomy''.