| During polar-cap absorption events, which are caused by the incidence of
energetic solar protons, the high-latitude ionospheric D region is extended
down to relatively low altitudes. While the incoming proton fluxes may be
monitored by satellite-borne detectors, and the resulting radio-wave
absorption with a ground-based riometer, the enhancement of electron density
at a given altitude is less easily determined. Direct measurements by
incoherent-scatter radar are infrequent and they tend to lack the necessary
sensitivity at the lower levels. Computations of the electron density from
the observed particle fluxes are handicapped by uncertainties in the height
profile of the effective recombination coefficient.
This paper describes a new approach based on finding the best-fit solution
to an over-determined set of equations. The D region is treated as a set of
slabs, each contributing to the total radio absorption, and the method
relies on the fact that the proton spectrum varies during the event. The
analysis produces a set of coefficients relating the absorption increment in
the slab to the square root of the production rate, as a function of height.
Values of effective recombination coefficient are also deduced over a range
of heights, and these agree with previous estimates (Gledhill, 1986) to
within a factor of 2. However, whereas the latter do not generally go below
60km altitude the new determination extends the values down to 40km.
The new method provides a measurement of the height profile of the
absorption in PCA events. It is shown that the slabs centred from
45 to 65km typically account for 80% of the total daytime absorption, and that
less than 1% of the total arises above 80km or below 30km. At night
most of the absorption comes from the slabs at 75 and 80km, with no
significant contribution from slabs below 75 or above 85km. These results
would not differ significantly from estimates based on the Gledhill profiles
if extrapolated downward.
Predictions based on the coefficients generated by the procedure are
compared with the polar-cap absorption observed during some recent events.
Typical electron-density values are derived, and the study provides an
independent confirmation that the electron density and the production rate
are related by a square-root law. |