222 Encyclopedia of the Solar System
FIGURE 12 (a) Results from a numerical simulation of a CME,
showing the distortion of the interplanetary plasma density
(contours) and magnetic field (white lines) as it travels toward
the Earth. The vectors indicate directions of the velocity in a
selected meridional slice. Note the flux rope ejecta that drives
the leading interplanetary shock (sharp red contour outer
boundary). (Courtesy of D. Odstrcil, University of Colorado.)
(b) Geospace response to the CME in (a), showing density
contours (log scale), in the local solar wind, the surface of
outermost closed magnetospheric field lines, and a view of the
resulting energy input into the earth’s high latitude atmosphere
at three different times. (Luhmann et al., 2004,J. Atmosph.
Solar. Terr. Phys.v. 66, p. 1243–1256, 2004.)
There are several types of precipitating particles:∼1–
20 keV auroral electrons,∼10–100 keV ring current ions
(protons and some oxygen ions),∼1–10 MeV radiation
belt electrons, and∼1–100 MeV solar energetic particles
(primarily protons). The more energetic the particles, the
deeper they penetrate; the altitude ranges to which these
various particles penetrate to deposit their energy are il-
lustrated in Fig. 13. As mentioned earlier, when these par-
ticles encounter atmospheric atoms and molecules, they
cause impact ionization and dissociate molecules into their
atomic elements. They also excite bound electrons to un-
stable states, which then radiatively decay to produce pho-
tons with specific energies and thus wavelengths that give
the aurora its colors. Chemical reactions caused by the in-
teractions of ions with the dissociated and excited atomic
products also excite particular emission features. The char-
acteristic green and red auroral emissions at 557.7 and 630.0
nm are produced by the excitation of the upper atmosphere
oxygen atoms; other auroral emission features in the blue
and near-ultraviolet spectral regions are formed from exci-
tation of molecular nitrogen and its ion.
Most auroral emissions occur in an oval-shaped band just
equatorward of the open field lines at high latitude, giving
the auroral oval (shown in Figs. 1 and 10) its name. Ring cur-
rent ion precipitation can produce high-altitude red aurora
at lower latitudes. In contrast, radiation-belt electrons and
solar energetic protons leave mainly chemical signatures.
Along with other chemical by-products, all particle precip-
itation produces nitric oxide (NO) from molecular nitro-
gen dissociation. Very energetic particles can produce NO
in the mesosphere (50–90 km) and even the stratosphere