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 vis ible emissions in a search for temperature increases which might explain th e 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 enhancement s, but unlike the radio brightening, the UV emission did not exceed the mea n quiet sun level. However, there were two observations showing increased H alpha brightness in radio enhancements above mean quiet sun levels. No H a lpha bright spots were detected in ECHs outside of radio enhancement region s. The ECH H alpha bright spots were caused by bright fibrils, bright point s and a lack of dark fibrils. Since the 17 GHz and H alpha enhancements are co-spatial, have equal integrated normalized enhanced emission and brightn ess temperatures, the observations suggest that the radio enhancements are caused by increased fibril radio emission. In addition, increased Fe XII EU V emission was recorded at the location of some well-defined radio enhancem ents, which were the bases of coronal plumes. Since the radio brightness te mperature is much lower than the Fe XII formation temperature, the radio an d EUV enhancements are likely both related to the presence of concentrated magnetic flux, but do not arise from the same physical layer.