Comparison of 87 GHz solar polar structures with EUV and soft X-ray emission

¹ DASOP, Observatoire de Paris, Meudon, France
² Metsähovi Radio Observatory, Helsinki Univ. of Technology, Espoo, Finland
³ Equipe SOHO/EIT, Laboratoire d'Astronomie Spatiale, LAS-CNRS, Marseille, France
⁴ DEA, Observatoire de Paris, Meudon, France

Send offprint request to: S. Pohjolainen, e-mail: pohjola@mesopy.obspm.fr

Received: 21 April 1999
Accepted: 21 January 2000

Abstract

Polar radio brightenings at 87 GHz (3.5 mm) are compared for the first time with features seen in EUV and soft X-rays. The data consist of nearly simultaneous full disk images and maps from Metsähovi Radio Observatory, SOHO/EIT, and Yohkoh/SXT on 9 selected days near the solar minimum (1996-1997).
The observed radio brightenings corresponded to various features seen in EUV, such as diffuse or localized intensity enhancements (e.g., bright points and bases of polar plumes), and intensity depressions of varying sizes (e.g., coronal holes). Some of these features were also visible in soft X-rays. The visibility of radio bright coronal holes seemed to depend on how much of the polar area was exposed, due to the variation of the B₀-angle.
The observed radio depressions near the solar poles were very well correlated with coronal holes and other EUV and/or soft X-ray intensity drops. More than half of the coronal holes, or coronal hole-like intensity drops in EUV and soft X-rays, had radio brightenings inside them. Therefore coronal holes do not have uniform radio brightness at 87 GHz. 
Many of the bright points seen at lower latitudes in the EIT and SXT images had no, or just faint, counterparts in the millimeter radio maps. It appears that for an EUV bright point to show up at 87 GHz it has to be bright and/or spatially large also in soft X-rays.