A multi-wavelength study of solar coronal-hole regions showing radio enhancements

We observed 17 GHz microwave-enhanced regions in equatorial coronal holes (ECH) together with extreme-ultraviolet (EUV), far-ultraviolet (FUV) and visible emissions in a search for temperature increases which might explain the bright spots in radio wavelengths. The ultraviolet (UV) observations span a wide range of formation temperatures (8000 K to 630 000 K). Increased UV emission was observed at the approximate location of the radio enhancements, but unlike the radio brightening, the UV emission did not exceed the mean quiet sun level. However, there were two observations showing increased H alpha brightness in radio enhancements above mean quiet sun levels. No H alpha bright spots were detected in ECHs outside of radio enhancement regions. The ECH H alpha bright spots were caused by bright fibrils, bright points and a lack of dark fibrils. Since the 17 GHz and H alpha enhancements are co-spatial, have equal integrated normalized enhanced emission and brightness temperatures, the observations suggest that the radio enhancements are caused by increased fibril radio emission. In addition, increased Fe XII EUV emission was recorded at the location of some well-defined radio enhancements, which were the bases of coronal plumes. Since the radio brightness temperature is much lower than the Fe XII formation temperature, the radio and EUV enhancements are likely both related to the presence of concentrated magnetic flux, but do not arise from the same physical layer.