Two-band spectral filtering in instruments for measuring solar magnetic fields

Astronomy and Astrophysics Supplement Series, Jul 2018

In this paper we formulate the basic concept of a bichromatic image technique. Specific examples of its application are analyzed in the context of research into solar plasma characteristics by pinpointing its merits and demerits. Specialized requirements to the spectral devices are set forth, which may be summarized as: a) the presence of two nearby identical spectral passbands; b) the mutual orthogonality for the polarization of the light that has passed through neighboring bands; and c) the possibility of controlling the relative position of the bands. Several alternative plausible implementations of the method are considered with the use of Fabry-Perot interferometers (FPI) and magnetooptical filters (MOF) for measuring the longitudinal magnetic field strength and the intensity field. A new design of a filter magnetograph is proposed, based on MOF, with the combination of two optical resonance cells into one.

A PDF file should load here. If you do not see its contents the file may be temporarily unavailable at the journal website or you do not have a PDF plug-in installed and enabled in your browser.

Alternatively, you can download the file locally and open with any standalone PDF reader:

https://aas.aanda.org/articles/aas/pdf/1997/05/dst6825.pdf

Two-band spectral filtering in instruments for measuring solar magnetic fields

Astron. Astrophys. Suppl. Ser. ltering in instruments for measuring V.M. Grigoryev 0 N.I. Kobanov 0 0 Institute of Solar-Terrestrial Physics , P.O. Box 4026, Irkutsk , Russia In this paper we formulate the basic concept of a bichromatic image technique. Speci c examples of its application are analyzed in the context of research into solar plasma characteristics by pinpointing its merits and demerits. Specialized requirements to the spectral devices are set forth, which may be summarized as: a) the presence of two nearby identical spectral passbands; b) the mutual orthogonality for the polarization of the light that has passed through neighboring bands; and c) the possibility of controlling the relative position of the bands. Several alternative plausible implementations of the method are considered with the use of Fabry-Perot interferometers (FPI) and magnetooptical lters (MOF) for measuring the longitudinal magnetic eld strength and the intensity eld. A new design of a lter magnetograph is proposed, based on MOF, with the combination of two optical resonance cells into one. lter magnetographic methods 1. Introduction This paper is primarily concerned with a technique for obtaining longitudinal magnetic eld magnetograms, based on two-passband spectral ltering. Longitudinal magnetic eld measurements usually use intensity fluctuations measured in the blue and red wings of a spectral line at the frequency of the modulator that consecutively transmits the left and right-handedly circularly polarized radiation. In this case, measurements in the two line wings are made either simultaneously with two photodetectors or sequentially with a single detector (Babcock & Babcock 1952; Beckers 1968; Cacciani 1981) , i.e. measurements in each of the spectral bands are separated either spatially or temporally. Note that the identity problem of dynamic characteristics of several photodetectors is a long-standing complicated challenge in experimental astrophysics. A single photodetector and the modulated signal are increasingly favored by the investigator because these two factors act to improve dramatically the sensitivity of measurement. With the advent of CCD and high-speed computers, the essence of the problem did not alter. As before, achieving a higher sensitivity involves the possibility of using a single photodetector for measuring a particular parameter in each pixel, as well as using the rapid modulationdemodulation. It is by no means accidental that this problem was further addressed in publications devoted to the LEST Project (Povel 1990; Keller et al. 1992; Povel et al. 1994) . The above considerations are characteristics both for most one-channel magnetographs and for multichannel instruments based on using lter systems and CCDreceivers; the latter are gaining increasing acceptance. Of course, with the advent of CCD, stokes-polarimeters have been and are being developed (Elmore et al. 1992; Mein 1991; Bendlin et al. 1992; Mickey et al. 1996) , which measure the distribution of all Stokes parameters in the line pro le at each image point. When using methods for solving inverse problems of radiation transfer in a line, they provide a wealth of information about the magnetic eld structure and the velocity through the layer thickness where the line forms. However, such observations do not enjoy reasonably high time resolution, but from 8 min to 70 min (Mickey et al. 1996). Some problems in solar physics that involve the study of shortlived and highly dynamic phenomena (flares, ejections, eruptive prominences) require a time resolution over 1 min. Conventional magnetographs with lter systems and CCD (Hagyard et al. 1982; West 1985; Sakurai et al. 1991) will remain e ective for much time to come in such observations to make measurements in xed spectral bands of the blue and red line wings. It is important and useful to further upgrade such systems with the aim to maximize their time resolution and sensitivity. Moreover, the method under consideration does not rule out the possibility of scanning a line pro le. And these issues are covered by the present paper. The central idea is that intensity fluctuations are measured simultaneously in two spectral bands 1 and 2 (in the blue and red wings) with a single photodetector, without any spatial separation into two images. As applied to lter magnetographs, this technique may be called the method of bichromatic image. The idea of the bichromatic image for measuring Hk was suggested in 1971 by Ramsey for birefringent lters and independently by a group including these authors at SibIZMIR (Lebedev et al. 1972) for the di raction spectrograph. Subsequently, the Ramsey's idea of obtaning two bands in birefringent lters for measuring all Stokes parameters was implemented by Chinese investigators (Guoxiang 1990) . This same concept was used in di erential line-ofsight velocity measurements and for measuring @I=@ the spectral line pro le (Kobanov 1983, 1993) with (...truncated)


This is a preview of a remote PDF: https://aas.aanda.org/articles/aas/pdf/1997/05/dst6825.pdf

V. M. Grigoryev, N. I. Kobanov. Two-band spectral filtering in instruments for measuring solar magnetic fields, Astronomy and Astrophysics Supplement Series, pp. 293-297, Volume 122, Issue 2, DOI: 10.1051/aas:1997135