NTRS - NASA Technical Reports Server
the peculiar solar minimum 23/24 revealed by the microwave butterfly diagramThe diminished polar magnetic field strength during the minimum between cycles 23 and 24 is also reflected in the thermal radio emission originating from the polar chromosphere. During solar minima, the polar corona has extended coronal holes containing intense unipolar flux. In microwave images, the coronal holes appear bright, with a brightness enhancement of 500 to 2000 K with respect to the quiet Sun. The brightness enhancement corresponds to the upper chromosphere, where the plasma temperature is approx.10000 K. We constructed a microwave butterfly diagram using the synoptic images obtained by the Nobeyama radioheliograph (NoRH) showing the evolution of the polar and low latitude brightness temperature. While the polar brightness reveals the chromospheric conditions, the low latitude brightness is attributed to active regions in the corona. When we compared the microwave butterfly diagram with the magnetic butterfly diagram, we found a good correlation between the microwave brightness enhancement and the polar field strength. The microwave butterfly diagram covers part of solar cycle 22, whole of cycle 23, and part of cycle 24, thus enabling comparison between the cycle 23/24 and cycle 22/23 minima. The microwave brightness during the cycle 23/24 minimum was found to be lower than that during the cycle 22/23 minimum by approx.250 K. The reduced brightness temperature is consistent with the reduced polar field strength during the cycle 23/24 minimum seen in the magnetic butterfly diagram. We suggest that the microwave brightness at the solar poles is a good indicator of the speed of the solar wind sampled by Ulysses at high latitudes..
Gopalswamy, Natchimuthuk (NASA Goddard Space Flight Center Greenbelt, MD, United States) Yashiro, Seiji (Catholic Univ. of America Washington, DC, United States) Makela, Pertti (Catholic Univ. of America Washington, DC, United States) Shibasaki, Kiyoto (Nobeyama Radio Observatory Nobeyama, Japan) Hathaway, David (NASA Marshall Space Flight Center Huntsville, AL, United States)
August 24, 2013
January 1, 2010