Regular readers of the IPS
Monthly Solar and Geophysical Summary,
or those people who receive IPS Warning services, have probably
seen the term "coronal hole on the sun" used quite frequently.
Not surprising because at this time of the solar cycle, coronal
holes are the sources of many of the disturbances to
the ionosphere (and HF communications) and to the geomagnetic
field. But what is a coronal hole and what does one look
like?
Firstly, the solar corona is the outer atmosphere of the sun, extending from
the solar "surface" out into space. It is a
region which is difficult to observe,
being seen only during solar eclipses or with special equipment.
A coronal hole is a large region in the corona
which is less dense and is cooler than its surrounds. Such
holes may appear at any time of the solar
cycle but they are most common during the declining
phase of the cycle.
In the Yohkoh image below the bright regions indicate hotter
areas of the solar corona, mainly above the sunspot regions. A large
dark region extends from one pole right across the solar equator
and well into the other hemisphere. It is from coronal holes like this one
that IPS makes forecasts of disturbances to HF communications
and other earth and space-based technology.
Educational