CORONAL HOLE: NASA's Solar Dynamics Observatory is monitoring a hole in the sun's atmosphere--a "coronal hole." It is the dark region circled in this extreme ultraviolet image taken during the early hours of April 28th:
Coronal Holes are places where the sun's magnetic field open up and allows the solar wind to escapee.
A stream of wind flowing from this coronal hole is expected to reach Earth on April 30th-May 1st. NOAA forecasters estimate a 35% chance of geomagnetic activitty at that time.