Centrifugal Acceleration of the Polar Wind

Many, many years ago (17, to be exact), I spent 10 weeks at the University of Alabama in Huntsville participating in their Research Experiences for Undergrads program at the Center for Space Plasma and Aeronomic Research.

Anyhoo, the REU program at UAH was the perfect opportunity to get paid for grad-level research and to decide if said grad-level research was something I wanted to pursue after receiving my undergrad degree. Let me say this. I wouldn't trade those ten weeks for anything, but they helped me decide I didn't want to follow-through with the lifetime study of ionized particles in the Earth's magnetosphere.

Yeah.

So, for your reading pleasure, I present to you the abstract of a paper that I co-authored, published in August, 1994, in the Journal of Geophysical Research. Sixteen years ago this was big stuff. In layman's terms, we were studying the aurora borealis. Satellite data and computer modeling data regarding the behavior of the aurora borealis weren't matching back then and I spent ten weeks calculating one line of computer code to make the data match.

And it did. Go us!

Brain melt in 3... 2... 1:

The effect of parallel ion acceleration associated with convection was first applied to energization of test particle polar ions by Cladis (1986). However, this effect is typically neglected in "self-consistent" models of polar plasma outflow, apart from the fluid simulation by Swift [1990]. Here we include approximations for this acceleration, which we broadly characterize as centrifugal in nature, in our time-dependent, semikinetic model of polar plasma outflow and describe the effects on the bulk parameter profiles and distribution functions of H⁺ and O⁺. For meridional convection across the pole the approximate parallel force along a polar magnetic field line may be written as F_cent,pole=1.5m(E_i/B_i)²(r²/r_i³) where m is ion mass, r is geocentric distance, and E_i, B_i and r_i refer to the electric and magnetic field magnitudes and geocentric distance at the ionosphere, respectively. For purely longitudinal convection along a constant L shell the parallel force is F_cent,long=F_cent,pole[1-(r/(r_iL)]^3/2/[1-3r/4r_iL)]^5/2. For high latitudes the difference between these two cases is relatively unimportant below ~5 R_E. We find that the steady state O⁺ bulk velocities and parallel temperatures strongly increase and decrease, respectively, with convection strength. In particular, the bulk velocities increase from near 0 km s^-1 at 4000 km altitude to ~10 km s^-1 at 5 R_E geocentric distance for a 50-mV/m ionospheric convection electric field. However, the centrifugal effect on the steady O⁺ density profiles depends on the exobase ion and electron temperatures: for low-base temperatures (T_i = T_e = 3000 K) the O⁺ density at high altitudes increases greatly with convection, while for higher base temperatures (T_i = 5000 K, T_e = 9000 K), the high-altitude O⁺ density decreases somewhat as convection is enhanced. The centrifugal force further has a pronounced effect on the escaping O⁺ flux, especially for cool exobase conditions; as referenced to the 4000 km altitude, the steady state O⁺ flux increases from 10⁵ ions cm^-2s^-1 when the ionospheric convection field E_i=0mV/m to ~10⁷ ions cm^-2s^-1 when E_i = 100mV/m. The centrifugal effect also decreases the time scale for approach to steady-state. For example, in the plasma expansion for T_i=T_e=3000 K, the O⁺ density at 7 R_E reaches only 10^-7 of its final value ~1.5 hours after expansion onset for E_i = 0. For meridional convection driven by E_i=50mV/m, the density at the same time after initial injection is 30-50% of its asymptotic level. The centrifugal acceleration described here is a possible explanation for the large (up to ~10km s^-1 or more) O⁺ outflow velocities observed in the midaltitude polar magnetosphere with the Dynamics Explorer 1 and Akebono spacecraft.

Did your head explode? My pleasure. :)

Author's note: I have read and re-read and re-re-read this abstract, comparing it to the paper sitting next to me. As far as I can tell, I haven't made any typos. If I have, oh well.