Figure 1: ICRF2 radio sources and minimum elongation angles.
From: Probing the solar corona with very long baseline interferometry
Plot a shows the defining radio sources (green) and all other radio sources (purple) of the ICRF2 in an equatorial system. The defining radio sources are of high astrometric quality and their positions define the axes directions of the frame. They are preferred for scheduling VLBI experiments. The orange line represents the apparent movement of the Sun as seen from Earth. Plot b only includes the 962 radio sources within 15° elongation to the ecliptic, out of which 79 are defining sources. Here, an ecliptic coordinate system is used. The minimum angular distance between the Sun and the ICRF2 radio sources for each day of the year (doy), exemplarily for 2012, is displayed in plot c.
