Coronal Mass Ejections
Helmet streamers in the corona are relatively long-lived, but they can disappear rapidly in
one of the most spectacular manifestations of solar activity, a coronal mass ejection or
CME.
In such an event the Sun ejects an average of about a trillion
kilograms of coronal matter at average
speeds of about 400 km per second, though in extreme events ten times that amount of mass can be ejected at speeds of
1000 km/s.
Examples
A
coronal mass ejection is illustrated in the top right image.
They produce bursts in the solar wind that influence
much of the rest of the Solar System, including the Earth.
Thus, the observation
of a large coronal mass ejection
by the Sun is usually a signal for increased
auroras
and related activity several days hence when the ejected burst
reaches Earth.
The following figure
shows the development of a solar prominence under a large streamer into a coronal mass ejection.
However, not all coronal mass ejections are associated with underlying eruptive prominences or flares as in this
example. The largest energy events tend to involve both flares and coronal mass ejections, but
smaller ones may not. Furthermore, even when flares and CMEs occur together, the region
for the CME is often broader than that of the flare, and may not be centered on the flare.
The relationship among these kinds of events is
murky because we have only a partial understanding
of how the magnetic field produces activity on the surface and of how
coupling to the magnetic field excites the corona.
Relationship of Flares and Coronal Mass Ejections
The largest flares and CMEs can release 1027
Joules of energy, both occur only
over
regions with closed magnetic field lines (not over coronal holes where the field lines are
open), and both involve explosive release of magnetic field energy.
Beyond these points of agreement, there is controversy over the relationship between
flares and CMEs.
A flare is a burst of radiation
from the corona and chromosphere resulting from intense heating and often involves bursts
of particle acceleration. A coronal mass ejection is an eruptive motion that ejects
mass from the inner corona (the part within about half a solar radius of the surface) into the
solar wind. The definitions are similar, and the issue is further clouded
because the largest flares and CMEs
often occur in overlapping regions. The unresolved issue is whether flares cause coronal mass
ejections, or CMEs cause flares, or whether both are the result of a deeper unifying mechanism.
Heating of the Corona
As we have noted previously, the corona maintains a temperature of millions of K even though
it is expanding into space as the solar wind. Therefore, the corona requires a continuous
source of energy to maintain its high temperature. It is thought generally that the heating of
the corona comes through coupling to the solar magnetic field, but details are not well known.
A plausible picture is that interaction with
the magnetic field over the entire surface contributes a
continuous heating of the corona, but that superposed on this are large bursts of heating
associated with flares and coronal mass ejections. This burst heating occurs over
localized regions
of closed magnetic field lines (not over coronal holes) and thus
is not uniformly distributed.