4 Summary of comparisons

We have observed polar brightenings at 87 GHz (3.5 mm wavelength), with a typical 0.5-1.5% brightness enhancement over the quiet Sun. Taking an average quiet Sun brightness temperature of 7200 K at 87 GHz, we get brightness enhancements in the scale of 35-110 K. In comparison, the active regions observed at 20-30 degrees in latitude in the same solar maps showed 1-4% brightness enhancements.

Radio brightenings were found to correlate with several different types of structures seen in EUV (see Sect. 3 for the exact numbers).

• Localized intensity enhancements (LIE - 17 locations): At 87 GHz, they often showed a brightness enhancement of about 1%. The borders of coronal holes often show diffuse emission, as the open field line areas change into a border of closed lines.
• Localized intensity decreases (LID - 12 locations): Surprisingly many of the radio enhancement peaks were located inside the small less bright regions. The corresponding radio brightenings varied in intensity, between 0.2% and 2.4%. However, these EUV reduced brightness areas usually had very small spatial size, and were often surrounded by diffuse matter or small bright areas that could contribute to the radio flux, so the connection can be ambiguous.
• Coronal holes (CH - 9 locations): In our study the radio brightenings for coronal holes were between 0.4% and 2.3%. Radio brightenings inside coronal holes were linked with the changes in B₀ angle: when more of the polar regions and coronal holes were visible, the clearer were the radio brightenings inside them. On April 12, 1996, on April 15, 1996 (see Figs. 29 and 30 for close-ups), and on May 20, 1997, radio bright regions were located inside the southern CHs, and on August 9, 1996, and on August 8, 27, and 28 the radio bright regions were detected in the northern CHs.
• Polar plumes (PP - 3 locations): The radio enhancements for the three locations that corresponded to plumes were 1.0% (location 3N on August 13, 1996), 1.4% (location 2N on August 13, 1996), and 1.5% (location 2N on August 14, 1996). The fact that plumes are inside coronal holes, which are cool and less dense, and that the plumes may not fill the whole radio beam area and may be diluted within the beam, make this radio signature even more significant.
• Active regions (AR - 3 locations): The tops of the loops in EUV were seen as radio brightenings, as expected. Active region loops showed higher brightness temperature in radio (3.6-4.0%) than, e.g., the plumes.
• Bright points (BP - no definite detections): Three radio bright polar locations could be correlated to EUV bright points, with the help of soft X-ray images (EUV-only analysis classified them as mixed features). These were location 2S on August 9, 1996 (2.1% enhancement), location 1N on August 13, 1996 (1.3% enhancement), and 1S on August 14, 1996 (2.1% enhancement). Yohkoh SXT also showed 4 bright points that were not classified as those in EUV, on April 12, 1996 (locations 3N with 0.6% and 4N with 0.4% radio enhancements), on August 9, 1996 (location 2S with 2.1% radio enhancement), and on May 20, 1997 (location 3S with 0.6% radio enhancement).

Radio depressions (23 altogether) at 87 GHz showed 0.5-2.0% (35-145 K) brightness drops, and were clearly correlated with coronal holes, localized intensity decreases, coronal hole borders seen as intensity enhancements, or a combination of these three features in EUV. The soft X-ray images confirmed the correlation: of the 23 radio depressions 17 were located within coronal holes or reduced brightness regions. And further, if we look at the classifications in both EUV and soft X-rays, only one location (7S, on August 13, 1996) was not connected to coronal holes. Therefore we can say that radio depressions in 3.5 mm are connected to the coronal hole phenomenon.

However, this is not so in the reverse situation: coronal holes and local less bright regions seen in EUV, and in soft X-rays, were often seen as brightenings at 87 GHz. This study showed 15 radio depressions vs. 22 radio brightenings for coronal hole-like EUV sources, and 17 radio depressions vs. 21 radio brightenings for coronal hole-like soft X-ray sources.

X-ray bright points have previously been detected at cm-wavelengths, using observations made with the Nobeyama Radioheliograph and the VLA (Nitta et al. [1992]; Kundu et al. [1994]). We now report detection of bright points also at 3.5 mm wavelength. A few radio brightenings correlated with bright points that were strong both in EUV and soft X-rays. Besides the 3 that were reported earlier, there were some found at lower latitudes: location S22W06 on August 9, 1996, and location N35E33 on August 13, 1996.

However, many of the bright points seen on the solar disk in the EIT and SXT images had no, or just very faint, counterparts in millimeter waves. By looking at some low latitude bright points we were able to conclude that even if the bright points were strong in EUV, they would not show up in millimeter radio if they were not strong enough (N56E30 on August 9, 1996), or spatially large enough (S42W26 and S57E24 on April 15, 1996 - see Figs. 29 and 30), in soft X-rays. Also, if the bright points lie inside coronal holes instead of bright diffuse sources, their radio flux will be diminished (N16W17 and S30E33 on April 12, 1996). There were also cases where no obvious reason could be found for why some bright points showed up in radio (S27W13 and S30W30 on August 28, 1997) and some did not (S15W13 on the same day), see Fig. 31.