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On the origin of polar radio brightenings at short millimeter wavelengths
Received 5 April 2000 / Accepted 10 July 2000
Polar regions of the Sun are areas where coronal holes are most profound and where the heating and acceleration of solar wind plasma takes place. The observed and yet unexplained radio emission from these regions may be related to the origin of the solar wind flow. The recent analysis of radio brightenings near the poles at 87 GHz (3.5 mm) suggested structural counterparts like polar plumes, EUV and/or soft X-ray bright points, and diffuse EUV brightenings especially near the borders of coronal holes. Coronal holes themselves were mainly seen as radio depressed, with local radio brightenings inside them.
The radio brightness temperatures for these types of features are now calculated by using the average temperature and density values from EUV and soft X-ray observations, assuming isothermal and optically thin plasma. The calculated values are in agreement with the observed ones. The calculations show some bright points and plume bases to be observable in mm-waves (calculated brightness temperature enhancements 10-390 K, observed 30-150 K), but that the observability depends strongly on density and loop geometry (line of sight source length) of individual sources. Coronal holes in general should be seen as radio depressions at 87 GHz (calculated brightness temperature drops 30-40 K, observed drops 22-130 K). Also a slight, 2.0% maximum, smooth limb brightening is now found to affect the radio observations inside 0.6-0.92 . This limb brightening is much less than reported in an earlier study using a similar telescope and same wavelength, which suggests a slightly different chromospheric model. The wide limb brightening does explain why some less intense EUV and soft X-ray features become observable in radio, as they get superposed on the smooth brightening.
Comparison to previous results obtained at several radio frequencies suggest that some of the high-latitude (70 degrees) radio brightenings at 87 GHz could be formed in the same atmospheric layer as the diffuse radio emission sources in the polar-cap regions and the diffuse radio emission sources inside equatorial coronal holes - as they all show correlation to EUV emission sources, especially in the He II 304 Å (80 000 K) line. However, the unexplained 87 GHz radio brightenings inside high-latitude coronal holes, the radio bright patches observed in polar-cap regions, and the bright compact sources seen inside equatorial coronal holes all seem to share the fact that they are not associated with any features seen in EUV. The best candidates for the formation of these radio sources are magnetic flux elements and density/temperature enhancements below the 80 000 K layer.
Key words: Sun: chromosphere Sun: corona Sun: radio radiation Sun: transition region Sun: UV radiation
Present address: Tuorla Observatory, University of Turku, Väisäläntie 20, FIN-21500 Piikkiö, Finland
Online publication: September 5, 2000