Sun's Corona at Solar Min and Solar Max

Sun's Corona at Solar Min and Solar Max

The Sun's corona around solar min (top) and solar max (bottom).
Credit: NCAR's High Altitude Observatory and Rhodes College

The Sun's corona (upper atmosphere) looks very different at "calm" and "stormy" times during the sunspot cycle. When there are few sunspots the Sun is relatively calm; scientists call these periods "solar min". When there are lots of sunspots, the Sun is very magnetically active and there are frequent "solar storms"; scientists refer to these times as "solar max".

During solar min periods, the corona has a fairly simple structure which extends outward from the low latitude regions near the Sun's equator. From our viewpoint on Earth, the corona looks a bit like a pair of wings extending outward on either side of the Sun. Solar scientists also know that the solar wind is much different near the Sun's equator compared to the Sun's poles at solar min. The "slow solar wind" flows outward from low latitude regions at a speed of "only" 400 km/sec (nearly 900,000 miles per hour). The "fast solar wind" zooms outward from the poles at speeds around 700 km/sec (more than 1.5 million mph).

When the Sun is very magnetically active and disturbed and there are lots of sunspots, the corona looks much different. The appearance of the corona at solar max reflects the complex and chaotic state of the Sun's magnetic field. Photos of the corona at solar max show dense regions called streamers bursting forth in a haphazard fashion from all latitudes on the Sun. The Sun's scrambled magnetic field at solar max produces many more fluctuations in solar wind speed with latitude... unlike the clear equatorial vs. polar distinction found at solar min.

Under normal conditions, the extremely bright surface of the Sun prevents us from seeing the much dimmer corona. However, images taken during a solar eclipse or by a special instrument called a coronagraph can provide us with spectacular views of the Sun's atmosphere.

© 2012 UCAR with portions adapted from Windows to the Universe (© 2009 NESTA)