http://lasco-www.nrl.navy.mil/index.php?p=content/handbook/hndbk_2
2.1.1 Coronal Heating and Acceleration of the Solar Wind
How is the corona heated? This is perhaps the major unsolved problem in
solar coronal physics. Current theories of coronal heating center around
either heating by waves guided by the magnetic field, or heating by
small-scale reconnection. If wave heating is taking place in the corona,
it should be possible to detect it by measuring root mean square (RMS)
velocity fluctuations in emission lines formed at coronal temperatures. UV
coronal observations show that these velocities should be in the range 20-
30 km/s. We would like to measure the nonthermal velocities of large
active region loops, which frequently extend to heights > 10E5 km, with an
accuracy of at least 10 km/s. Measurement to this accuracy will provide a
critical test of wave heating theories. In addition to being able to
measure velocities, it is necessary to image the corona with sufficient
spatial resolution to distinguish the major structural elements of the low
corona. Images from Skylab, SMM, and Yohkoh in soft X-rays and the XUV
show that a large active region loop has a cross-sectional diameter of
about 10,000 km. Thus, detailed measurements along a loop requires a
spatial resolution in the low corona of roughly 10 arc sec (7200 km). For
an isolated large loop, a spatial resolution of 20 arc sec should be
adequate for comparing line widths with the typical quiet corona.
If heating by small-scale reconnection is taking place, then theory
predicts that the heating rate should drop off rapidly with loop length.
Testing this model requires the ability to measure the temperatures,
densities, velocities and, hence, energy losses of the coronal plasma as a
function of height. To determine the lengths of the loops being observed,
it is important to be able to distinguish the large scale structure of the
inner corona. As with the wave heating observations, this requires
measurements with a spatial resolution in the low corona of 10 to 20 arc
sec.
It is not clear that waves alone account for the energy input to the solar
wind. Magnetic loops are sometimes ejected during coronal eruptions and
flares, and it has been conjectured that many small magnetic loops may be
continually emitted outward through coronal holes, heating the gas and
perhaps imparting some outward momentum as well. LASCO will measure, in
the first few solar radii above the surface of the Sun where the
acceleration moves the gas up to supersonic speeds and where most of the
thermal energy is consumed, the electron density directly and the electron
and ion "temperatures" of the observed outward flow indirectly. It will
also measure the direction of the magnetic field. Line intensity ratios
utilizing iron lines can provide a measure of the "frozen-in" temperature
of the gas, and line widths provide a measure of the unresolved
small-scale RMS fluid motions. The variation of temperature and motions
with radial distance is related to the energy budget and gas pressure for
the expansion. By combining all the observations from LASCO, the equations
for conservation of matter, momentum, and energy for steady flow along a
smooth magnetic field can be examined term by term.