Haosheng Lin
The magnetic field in the solar corona is generally believed to be responsible for a wide range of phenomena—from being the carrier of MHD waves to heat the corona, to producing the gyro-synchrotron radiation in the radio wavelength range. Yet, there are scarcely any direct measurements of the coronal magnetic field to date. Early magnetograph observations in the "green" Fe XIIII 5303 Å coronal emission line (Harvey 1969) gave us a pretty good idea of just how difficult these measurements could be. The strongest flux he detected in the rising phase of a solar cycle (1967, 1968) was only 13 ± 20 G. More recent spectro-polarimetric measurements (Kuhn 1995) placed an upper limit of 50 G, using the near-infrared Fe XIII 10797 Å line. Linear polarization observations in the 1980's (Querfeld and Smartt 1984, Arnaud and Newkirk 1987) were more successful in mapping the direction of the coronal magnetic fields. However, because the linear polarization of coronal emission lines (CEL) is not sensitive to the strength of the magnetic field, no quantitative information of the magnetic flux can be derived from these linear polarization experiments. We are faced with the dilemma of believing that magnetic fields exist in the solar corona and play a crucial rule in almost all coronal activities; numerous new theoretical investigations and results from current space-borne experiments (SoHO, TRACE) only reinforce this conviction, while definitive quantitative measurement of the magnetic field is conspicuously missing. In order to further advance our knowledge of the solar corona, direct measurement of the coronal magnetic field strength and configuration is indispensable.
To first order approximation, the CEL linear polarization measurement only provides information on the orientation of the magnetic field vector. Information about the magnetic field strength (flux) is contained only in the circular polarization of the CELs (Cassini and Judge 1999, Lin and Cassini 1999). Measurement of the Stokes V signal due to the coronal magnetic field is one of the most difficult experiments in the field of observational solar astronomy. The magnetic field strength is expected to be very small, around 10 G; therefore, the degree of circular polarization is expected to be minute. The high coronal temperature broadens the spectral lines, further reducing the Stokes V signal. Under coronal conditions, the expected Stokes V amplitude is only 10-3 Iline or smaller. Also, the linear polarization of the CELs is typically two orders of magnitude higher than the circular polarization. Therefore, measurement of the coronal Stokes V signal is only feasible with a very sensitive polarimeter with extremely small instrumental (telescope and polarimeter) polarization cross talk. The low photon flux of the emission line solar corona (~~10-5 disk intensity) also aggravates the situation. Nevertheless, encouraged by recent successes of high-precision spectro-polarimetric observations of weak photospheric magnetic fields, Haosheng Lin (NSO), Steve Tomczyk (HAO), and Matt Penn (Cal State, Northridge) attempted to obtain a quantitative measurement of the coronal magnetic field.
The NSO/Sac Peak Evans Solar Facility's (ESF) 40-cm coronagraph was used for the experiment. Lin and colleagues designed and built a new Echelle spectrograph optimized for the Fe XIII 10747 Å and 10798 Å line pair. A liquid crystal variable retarder polarimeter and the Michigan State University NICMOS 3 IR camera were used to analyze and record the Stokes spectra. To eliminate telescope polarization cross talk, the polarimeter was mounted directly behind the O1 focus of the coronagraph before any of the polarization cross talk generating reflecting optics. An iterative retardation tuning algorithm was developed to minimize the linear to circular polarization cross talk within the polarimeter itself. With these special considerations in minimizing the instrumental cross talk, the linear to circular cross talk is typically at the low 10-3 level.
The first positive detection of a coronal Stokes V signal was achieved on 14
October 1999, observing in a region of the solar corona with strong (83
millionth of disk-center intensity) Fe XIIII 5303 Å green-line emission.
Several attempts during subsequent days in weaker green-line regions yielded
weak Stokes V signals that were barely above the noise level. Figure 1 shows
the IR Fe XIII 10747 Å and 10798 Å emission line I, Q, and V polarization
spectra of the October 14 observation, and the SoHO EIT Fe IX 171 Å image to
indicate the IR target region (the square box in the EIT image). The
integration times for the Stokes Q and V frames are both 20 minutes, while
the total observing time was 70 minutes. A total of 2,048 exposures were
taken for this observation. The Stokes Q spectrum image clearly shows the
linear polarization of both the Fe XIII 10747 Å and 10798 Å lines. The
anti-symmetric Stokes V signal is only barely discernible in the circular
polarization spectrum image. To improve the signal-to-noise ratio of the
Stokes V signal, we averaged over the full frame to generate an averaged
Stokes V profile, shown with the averaged Stokes Q profile in Figure 2.
Here, the anti-symmetric signature of the Stokes V profile is unmistakable.
There was no evidence of Stokes Q to V cross talk in the image. A magnetic
flux of 33 ± 0.7 (3) G was derived from this Stokes V spectrum using the
standard magnetograph formula. The alignment effect correction to the Stokes
V profile (Cassini and Judge 1999, Lin and Cassini 1999) was not applied
since we did not measure the full Stokes vector. A second detection was
obtained on 23 October 1999, when a large active region just rotated behind
the west limb of the Sun, which also yielded strong (79 millionths) green
line intensity. The amplitude of the Stokes V signal of the October 23
observation was smaller compared to the October 23 measurement in line
intensity units, but due to better sky conditions (13 millionths) and longer
integration time (34 minutes for each Stokes state, 150 minutes total
observing time), it was possible to detect a weak Stokes V signal. A
magnetic flux of 10 ± 0.5 (3
) G was derived from this region.
These preliminary results provide the first definitive quantitative measurements of the coronal magnetic field, which will be an important input to all the theoretical modeling of the solar corona. Future measurements with improved signal-to-noise ratio, spatial coverage, and temporal resolution will make major contributions to the advancement of the physics of the solar and stellar coronae.
