M. Loranc et Jp. Stmaurice, A TIME-DEPENDENT GYRO-KINETIC MODEL OF THERMAL ION UPFLOWS IN THE HIGH-LATITUDE F-REGION, J GEO R-S P, 99(A9), 1994, pp. 17429-17451
Ample evidence supports the significance of the high-latitude ionosphe
ric contribution to magnetospheric plasma. Assuming flux conservation
along a flux tube, the upward field-aligned ion flows observed in the
magnetosphere require high-latitude ionospheric field-aligned ion upfl
ows of the order of 10(8) to 10(9) cm(-2) s(-1). Since radar and satel
lite observations of high-latitude F region flows at times exceed this
flux requirement by an order of magnitude, the thermal ionospheric up
flows are not simply the ionospheric response to a magnetospheric flux
requirement. Several ionospheric ion upflow mechanisms have been prop
osed, but simulations based on fluid theory do not reproduce all the o
bserved features of ionospheric ion upflows. Certain asymmetries in th
e statistical morphology of high-latitude F region ion upflows suggest
that the ion upflows may be generated by ion-neutral frictional heati
ng. We developed a single-component (O+), time-dependent gyro-kinetic
model of the high-latitude F region response to frictional heating in
which the neutral exobase is a discontinuous boundary between fully co
llisional and collisionless plasmas. The concept of a discontinuous ne
utral exobase and the assumption of a constant and uniform polarizatio
n electric field reduce the ion guiding center motion in the frame of
a convecting flux tube to simple one-dimensional ballistic trajectorie
s. We thus are able to analytically calculate a time and height-depend
ent ion velocity distribution function, from which we can compute the
ion density, parallel velocity, parallel and perpendicular temperature
, and parallel flux. Using our model, we simulated the response of a c
onvecting flux tube between 500 km and 2500 km to various frictional h
eating inputs; the results were both qualitatively and quantitatively
different from fluid model results, which may indicate an inadequacy o
f the fluid theory approach. The gyro-kinetic frictional heating model
responses to the various simulations were qualitatively similar: (1)
initial perturbations of all the modeled parameters propagated rapidly
up the flux tube, (2) transient values of the ion parallel velocity,
temperature, and flux exceeded 3 km s(-1), 2 x 10(4) K, and 10(9) cm(-
2) s(-1), respectively, (3) a second transient regime developed wherei
n the parallel temperature drops to very low values (a few hundred Kel
vins), and (4) well after heating ceased, large parallel temperatures
and large downward parallel velocities and fluxes developed as the flu
x tube slowly returned to diffusive equilibrium. The ion velocity dist
ributions during the simulation are often non-Maxwellian and are somet
imes composed of two distinct ion populations.