A study on the microwave signature of X-ray coronal holes

qs061.sakurai04
Posted:  31-Jul-93
Updated: 12-Dec-93, 23-Sep-94, 03-Apr-95, 02-Aug-95, 15-Nov-95, 16-Dec-96
Events specified: N/A

PROPOSERS: T.Sakurai(PI,4)

           H. Woehl(1, hw@kis.uni-freiburg.de),
           R. Brajsa(1,2, rbrajsa@kis.uni-freiburg.de)
           S. Pohjolainen(3), V. Ruzdjak(2), H. Teraesranta(3),
           S. Urpo(3), B. Vrsnak(2),
           K.Ichimoto(4)

ABSTRACT: We will conduct an analysis of Low brightness Temperature microwave Regions (LTR), H-alpha filaments, He 10830 features, as well as X-ray coronal holes. The data to be used, which should be nearly simultaneous, are microwave maps from Metsahovi, H-alpha images from Hvar and Mitaka, He 10830 heliograms from KPNO (in Solar Geophysical Data) and from Norikura, and X-ray images of Yohkoh SXT. The main motivation is to investigate how X-ray coronal holes can be seen in microwaves: in absorption (as LTRs), in emission, or as a combination of both processes. From this study we look into the radiation mechanisms responsible for microwave radiation in LTRs.

Update 16-Dec-96

A study on the microwave signature of X-ray coronal holes proposers: T. Sakurai, H. Woehl, R. Brajsa, S. Pohjolainen, V. Ruzdjak, H. Teraesranta, S. Urpo, B. Vrsnak and K. Ichimoto

1) Following paper was submitted (to "Solar Physics" as a Letter): CORONAL HOLES DETECTED USING SOLAR MICROWAVE MEASUREMENTS (R. Brajsa, V. Ruzdjak, B. Vrsnak, S. Pohjolainen, S. Urpo, T. Sakurai and H. Woehl)

2) The rotation of various coronal structures, esp. coronal holes, was studied by using Yohkoh - SXT images for the period 24 - 31 May, 1993.

Update 15-Nov-95

Our study was reported in 'Radio emission from the stars and the sun', held in Barcelona, 1995 July. The abstract is enclosed, and the proceedings will appear in Astronomical Society of the Pacific Conference Series (eds. J.M.Paredes and R.Taylor).

Coronal holes detected using solar microwave measurements

R.Brajsa, S.Pohjolainen, V.Ruzdak, T.Sakurai, S.Urpo, B.Vrsnak, and H.Woehl

Microwave measurements of the sun performed at the frequency of 37 GHz (Metsahove Radio Research Station, Helsinki University of Technology, Finland) were numerically reduced using different procedures, so that various features on the full-disk solar maps are detectable. The low brightness temperature regions (LTRs) have a lower brightness temperature than the estimated quiet sun level of 7800 K, contrary to the high brightness temperature regions (HTRs), which have a higher brightness temperature than the quiet sun level. The 37 GHz data were compared with measurements in the He I 10830 Angstrom absorption line (National Solar Observatory, Kitt Peak, USA, and German Solar Telescopes, Teide Observatory, Spain) and in soft X-rays at the wavelength of about 20 Angstrom (SXT-Yohkoh). Coronal holes appear as LTRs (sometimes containing a HTR inside) in microwaves, and as regions of weaker absorption in the He I 10830 line, as well as areas with significantly weaker soft X-ray emission.

The LTRs can be divided in two types: small, compact LTRs, having brightness temperatures up to 20% lower than the quiet sun level which we denote as LTRs type I, and large, diffuse LTRs with brightness temperatures just below the quiet sun level denoted as LTRs type II. Although the distinction between the two tpes is not a sharp one, we can say that LTRs of type I are mainly spatially connected with the inversion lines of the longitudinal component of the photospheric large-scale magnetic fields, and with H-alpha filaments, whereas LTRs of type II may disclose coronal holes.

Update 02-Aug-95

1) Our article "Helium 10830 A measurements of the Sun" (R. Brajsa, S. Pohjolainen, V. Ruzdjak, T. Sakurai, S. Urpo, B. Vrsnak and H. Woehl) has been accepted by Solar Physics.

2) V.Ruzdjak presented a poster Coronal Holes Detected Using Solar Microwave Measurements (R. Brajsa, S. Pohjolainen, V. Ruzdjak, T. Sakurai, S. Urpo, B. Vrsnak and H. Woehl) at the University of Barcelona Conference "Radio Emission from the Stars and the Sun", Barcelona, 3-7 July 1995.

3) R.Brajsa and H.Woehl performed measurements of He 10830 A line intensity using German VTT at Tenerife in the period 27 June - 2 July 1995. Simultaneously a video recording of Ca K, H-alpha, and continuum images was made. The main objective of this observation is to study the large scale strtuctures in the solar atmosphere, and to compare the VTT data with Yohkoh X-ray images and with microwaves from Metsahovi.

Update 03-Apr-95

1) Our article "Helium 10830 A measurements of the Sun" (R. Brajsa, S. Pohjolainen, V. Ruzdjak, T. Sakurai, S. Urpo, B. Vrsnak and H. Woehl) was submitted to Solar Physics. After receiving the referee's report, we improved the paper along the referee's suggestions, and sent it back to the editors.

2) R. Brajsa presented a poster SOFT X-RAY, He I 10830 A LINE AND MICROWAVE SIGNATURES OF CORONAL HOLES (R. BRAJSA, S. POHJOLAINEN, V. RUZDJAK, T. SAKURAI, S. URPO, B. VRSNAK and H. WOEHL) at the VI-th Canary Islands Winter School of Astropysics "The Structure of the Sun", Tenerife, Spain, 5 - 16 December 1994.

Update 23-Sep-94

One of the members of this collaboration (R.Brajsa) obtained PhD on June 20, 1994 at the Dept. of Physics, University of Zagreb. The thesis is partly based on this research.

A full paper coming out from this collaboration; Helium 10830 Angstrom measurement of the sun R.Brajsa, S.Pohjolainen, V.Ruzdjak, T.Sakurai, S.Urpo, B.Vrsnak, and H.Woehl will be submitted to Solar Physics soon. The abstract of this paper is given below.

Abstract Measurements of the sun in the near infrared absorption He I 10830 Angstrom line were performed using the echelle spectrograph at the Vacuum Tower Telescope (German Solar Telescope, Teide Observatory, Tenerife, Spain) with a spectral resolution of 6.71 milli-Angstrom per pixel in the period 24-30 May 1993. These measurements were compared with the full-disk soft X-ray pictures of the sun (Japanese solar satellite Yohkoh), the full-disk solar images in H-alpha (Big Bear Solar Observatory), the full-disk solar images in He I 10830 Angstrom (National Solar Observatory, Kitt Peak) and with the full-disk microwave solar maps at 37 GHz (Metsahovi Radio Research Station, Finland). In the He 10830 Angstrom line, the sun exposes a limb darkening similar as in the visible part of the spectrum. Active regions and H-alpha filaments expose a strong absorption in the He 10830 Angstrom line. On the other hand,

Update 12-Dec-93

We presented the following report at Finnish COSPAR on Januray 10 - 11, 1994, as a poster paper.

SOFT X-RAYS, H-ALPHA, MAGNETOGRAPHIC, HE 10830 AND 37 GHZ ANALYSIS OF THE WHOLE SUN

T. Sakurai, H. Woehl, R. Brajsa, S. Pohjolainen, V. Ruzdjak, S. Urpo, B. Vrsnak, and K. Ichimoto

Microwave measurements of the Sun at 37 GHz from the Metsaehovi Radio Research Station were numerically reduced so that the Low brightness Temperature Regions (LTRs) can be visible. These structures have lower brightness temperature than the quiet Sun, as opposed to the High Temperature Regions (HTRs), which have higher brightness temperature than the quiet Sun. The full disc microwave maps with the LTRs of January 1992 are compared to the soft X-rays (solar satellite YOHKOH), H-alpha (Big Bear Solar Observatory), He 10830 and magnetic (Kitt Peak National Observatory) full disc solar maps as a part of a YOHKOH project under the title: "A study on the microwave signature of X-ray coronal holes". Diffuse, larger LTRs with relatively higher brightness temperatures are spatially correlated to coronal holes, while sharp, smaller LTRs with lower brightness temperatures are connected to H-alpha filaments and to inversion lines of the longitudinal component of the photospheric magnetic field underlying the filaments. In spite of strong temporal and spatial variations of analysed data, one could roughly summarize the observed phenomenology as follows:

                    37 GHz          He 10830          SXR
H-alpha filament    LTR          strong absorption     -
active region       HTR          strong absorption    emission
coronal hole        LTR(*)       reduced absorption   no emission

These phenomenological results are compared to various models of the solar atmosphere.