ar081.alissandrakis01 Posted: 03-APR-95 Updated: 26-Nov-95, 17-Jan-96, 20-Sep-96 Events specified: AR 7530
C.E. ALISSANDRAKIS (Greece), F. CHIUDERI DRAGO (Italy), K.SHIBASAKI (Japan)
Observing period: July 3 and 4, 1993
Motivation: The microwave radio observations were performed, by Alissandrakis and Chiuderi Drago, at the Westerbork Synthesis Radio Telescope (WSRT), using, for the first time for solar observations, a multi-channel receiver with very narrow bandwidths (19.4 kHz), commonly used for line observations. The central wavelength was 6 cm (4995 MHz). The main purpose of this observation was to detect the linear polarization, which is usually cancelled by the Faraday rotation in the solar corona within the bandwidths commonly used for the observations in the continuum (10 MHz). The observations (Alissandrakis and Chiuderi Drago,1994) have revealed for the first time the presence of a certain percentage of linear polarization in an A.R. and they have also shown a sinusoidal trend of the U and Q Stokes parameters from channel to channel, in agreement with the Faraday rotation. The measurement of the period of the Faraday rotation gives the value of the integral, along the line of sight, of the product N*BL where N is the electron density and BL the longitudinal component of the magnetic field. The integral is performed from the depolarization point (BL=0, B=BT), usually located at a height of (0.5-1.)10**5 Km above the phtosphere, to the observer. It is clear that an independent measurement of N, which could come from the emission measure derived from SXT observations, will provide important information on the coronal magnetic fields at such a high levels.
An interesting byproduct of the comparison between SXR and microwave emission is the separation of the bremstrahlung emission from the gyroresonance one. This can be achieved by comparing the map of the calculated brightness temperature, using Te and EM derived from the SXT, with the observed one. Also this comparison can supply information on the magnetic fields in various points of the region of formation of the radio emission (5-10)10**3 Km above the photosphere.
The magnetic fields inferred in these ways could represent a quantitative check of the extrapolated magnetic fields using the force-free or current free approximation.
Optical data available: White light and Halpha images from BBSO Vector magnetograms from MSFC
References:
Alissandrakis C.E. and Chiuderi Drago F. 1994: Ap.J. 428, L73.
Update 20-Sep-96
The work has been finished some months ago, it has been presented as a poster at the Makuhari meeting last May and it is now published on Solar Phys. 167, 167,1996. The abstract follows:
Coronal magnetic fields from polarization observationsat microwaves
by
C.E. Alissandrakis, F. Borgioli, F. Chiuderi Drago, M. Hagyard and K. Shibasaki
The Solar Active Region (A.R.) 7230 has been observed, on July 3 and 4, 1993, at lambda=6 cm, with the Westerbork Synthesis Radio Telescope, using a multi-channel receiver with very narrow bandwidths.
In the observation of July 4 an inversion and a strong suppression of the circular polarization were clearly present above different portions of the A.R., 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.
In this paper we compare the radio data with Yohkoh observations of the same A.R. and with the magnetic field extrapolated from the Marshall Vector Magnetograms in the force-free and current-free approximation.
The comparison with x-ray observations shows that, although a general agreement exists between the shape of the radio intensity map and the x-ray loops, the brightness temperatures, T_b, obtained using the parameters (T_e and EM) derived from SXT, are much lower than the observed ones.
The comparison with the extrapolated photospheric fields shows instead that, at low level, they account very well for the observed T_b above the main sunspots, if gyroresonance emission is assumed, and, at a much higher level, they satisfy to the constraints required by the wave propagation theory all over the A.R.
It appears however that a very low electron density is derived from this comparison.
Update 17-Jan-96
Here is the summary of the paper related to the proposal presented by Alissandrakis, Shibasaki and myself last March.
Authora:
F. Chiuderi Drago, C.E. Alissandrakis, F. Borgioli, M. Hagyard and K. Shibasaki
Summary:
The Solar Active Region (A.R.) 7530 has been observed, on July 3 and 4, 1993, at $ \lambda$=6 cm, with the Westerbork Synthesis Radio Telescope, using a multi-channel receiver with very narrow bandwidths.
In the observation of July 4 an inversion and a strong suppression of the circular polarization were clearly present above different portions of the A.R., 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.
In this paper we compare the radio data with Yohkoh observations of the same A.R. and with the magnetic field extrapolated from the Marshall Vector Magnetograms in the force-free and current-free approximation.
The comparison with x-ray observations shows that, although a general agreement exists between the shape of the radio intensity map and the x-ray loops, the brightness temperatures, $T_b$, obtained using the parameters ($T_e$ and $EM$) derived from SXT, are much lower than the observed ones.
The comparison with the extrapolated photospheric fields shows instead that, at low level, they account very well for the observed $T_b$ above the main sunspots, if gyroresonance emission is assumed, and, at a much higher level, they satisfy to the constraints required by the wave propagation theory all over the A.R.
It appears however that a very low electron density is derived from this comparison.
Since the paper is not yet ready for publication, same changes could be done on the above summary. I will send the final version to you before submitting the paper to Solar Physics.
Update 26-Nov-95
Preliminary results on the observed Active Region have been presented as a poster at the Makuhary meeting with the following Tiltle: "CORONAL MAGNETIC FIELDS FROM POLARIZATION OBSERVATIONS AT MICROWAVES". by : Chiuderi Drago, Borgioli, Alissandrakis, Hagyard, Shibasaki. The corresponding paper will be submitted in a short time to Solar Physics.