These combined images display UV light emitted by the the solar corona
on 17 August 1996.  The solar disk image is from the EIT instrument
aboard SOHO, and the extended solar corona (outside the black ring
about 0.5 solar radii from the disk) is from the UVCS instrument
aboard SOHO.  The color in the latter image represents the total UV
intensity emitted by O VI 1032 Angstroms.

The white rays are lines of magnetic force from a theoretical model
of the magnetic field of the Sun at the minimum of solar activity
(see Banaszkiewicz et al. 1998, Astron. Astrophys., 337, 940).
The polar coronal holes (dark regions on the solar disk and in the
extended corona) have primarily open magnetic field lines along
which the high-speed component of the solar wind accelerates away
from the Sun.  In the extended corona viewed by UVCS, the density of
particles is so low that individual ions very rarely collide with
other particles, and thus they execute spiraling motions around the
magnetic field lines (see green curve for an illustration of an
example particle's motion).

The transverse spiraling velocities are detected in the broad spectral
emission lines seen by UVCS.  Surprisingly, UVCS found that the gyration
speeds of the ionized oxygen atoms are significantly higher than those
of lighter particles like hydrogen; this has crucial ramifications for
theories of the extended heating of the solar corona and the acceleration
of the solar wind.  So far, the best theoretical explanation for the
faster oxygen spiraling (and for the faster oxygen outflow) is that
these particles absorb energy from high-frequency (short period)
waves in the solar wind.  These waves must have periods of about a
millisecond for the particles to be able to sap their energy.  In
contrast, most of the observed waves in the solar wind have periods
of more than a few seconds, up to minutes and hours.  The UVCS
observations are thus suggestive evidence for the production of
these shorter period waves in the solar corona.

(see, e.g., Cranmer et al. 1999, Astrophys. J., 518, 937)