1 Introduction

Polar-cap brightenings in solar radio maps were first discovered in the 1970's (Babin et al. [1976]; Efanov et al. [1980a], [1980b]). Subsequently several studies were done using cm- and mm-wavelength data from, e.g., the Nobeyama 45-m telescope, the Nobeyama Radioheliograph, and the Metsähovi 14-m telescope. It has been claimed that the brightenings are restricted to a certain wavelength range, from 15 to 48 GHz (Gary et al. [1997]), but the upper limit may simply be due to the lack of good quality observations at higher frequencies. Below 15 GHz the solar poles have been observed to appear darker, but at frequencies higher than 48 GHz both uniform brightness and local depressions have been observed (Gary et al. [1997]; Kosugi et al. [1986]), as well as the above mentioned brightenings (later paper by Moiseev & Nesterov [1987]).

The studies of polar brightenings and depressions have included a variety of observed features, ranging from high-latitude polar-cap brightenings to any features observed above about 40 deg latitude. Also the appearance of coronal holes is very important in polar regions. Along with the question whether polar brightenings do appear at high frequencies (shorter than 6 mm), a puzzle exists on the origin of the brightenings: are they associated with limb brightening (Shibasaki [1997]), coronal holes (Brajsa et al. [1996]), faculae (Riehokainen et al. [1998]), magnetic features (Kosugi et al. [1986]; Gopalswamy et al. [1999]), atmospheric structures (Hiei [1987]), polar plumes (Gopalswamy et al. [1992]), or something else?

The starting point of this study was the fact that for the first time at 87 GHz (3.5 mm), the solar radio maps observed with the upgraded Metsähovi Radio Observatory 14-m telescope showed polar brightenings and depressions. We present here an analysis of data from 9 separate days observed near the solar minimum (1996-1997), with good imaging capabilities at 87 GHz.