Instabilities and Structure in the High-Latitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 478

this high-latitude plasma from the solar-produced plasma in the equatorward
region.

10.1.4 Temperature Enhancements in the Trough and Stable
Auroral Red Arcs
The localized temperature enhancements in the trough shown in Fig. 10.3b are
also very interesting. Cole (1965) proposed that the ionospheric electrons are
in good thermal contact with the hot magnetosphere along the magnetic field
lines. In the trough, where the electron density is low, this input of heat raises
the electron temperature more than, say, in the region poleward of the trough,
where the heat input is also large but the electron density is high. The enhanced
ion temperatures may be explained by a combination of thermal exchange with
electrons and Joule heating due to the large electric field in the region of interest.
Care must be exercised in interpreting ion temperatures from radar data, since
the ion composition changes, shear flow on the sampled volume, and anisotropic
ion distribution functions can all lead to incorrect interpretation of the width
of the incoherent scatter spectrum and the T i estimate that results (Providakes
et al., 1989).
The high electron temperature in the trough leads to the phenomenon of stable
auroral red (SAR) arcs. The emission is O( 1 D), the same as that which occurs
in recombination as illustrated in Fig. 10.1, and is due to the impact of elec-
trons from the tail of this distribution function on oxygen atoms. Very intense
events can be visible to the naked eye (M. Mendillo, personal communication,
2005). These emissions usually reach from horizon to horizon. An example is
presented in Fig. 10.6a, using a camera located in western New York State at
approximately L 5 (Nicolls et al., 2005). The red arc stretches from horizon

to horizon but is not as unstructured as previous publications seem to imply. Pos-
sible explanations for the undulations on the poleward edge include the

= 3 .

T
instability (Hudson and Kelly, 1978), the current convective instability (Ossakow
and Chaturvedi, 1979), and possibly the thermomagnetic instability (Erukhimov
and Kagan, 1994). DMSP data in Fig. 10.6b show that high electron temper-
ature (over 6000 K), low electron density, and high velocity characterize the
red arc.

∇ n ×∇

10.1.5 Horizontal Plasma Variations Due to Localized Plasma
Production and Heating
When energetic particles precipitate into the atmosphere, energy loss occurs pri-
marily via ionizing collisions. To a crude first approximation, for every 35 eV
lost, one ion-electron pair is produced. A single kilo-electron-volt electron would
thus yield about 30 pairs. The type of ion produced depends on the altitude where
the collision occurs, since the atmospheric constituents vary with height. The ini-
tial particle energy determines how deeply it can penetrate into the atmosphere.