Physics
Scientific paper
May 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agusmsa44a..03k&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #SA44A-03
Physics
2411 Electric Fields (2712), 2427 Ionosphere/Atmosphere Interactions (0335), 2439 Ionospheric Irregularities, 2443 Midlatitude Ionosphere, 2772 Plasma Waves And Instabilities (2471)
Scientific paper
The SEEK II project yielded a rich data set that has provided much insight into the processes at work at mid- latitudes. Investigation of these data suggests that the electric fields associated with plasma instabilities act to destroy the gradient that feeds them and, in turn, drives the medium to near marginal stability with a small density gradient. The evidence points to a wind/wind dynamo-generated instability since externally generated electric fields cannot vary as rapidly in space as the observed fields do when the rocket moves across the structures. Estimates of the wave-driven diffusion coefficient based on the observed fluctuating electric fields are the order of DA = (δ E / B)2τ where δ E is the wave amplitude and τ is its period. Taking δ E = 1 mV/m, B = 4 × 10-5 T, and τ = λ / vph = 1 s, DA = 625 m2/s, which is two orders of magnitude above classical values. Kagan and Kelley [1998] first discussed the role of anomalous diffusion in sporadic E layers when they noticed that classical ambipolar diffusion led to gradient scale lengths the order of a meter. With the anomalous diffusion calculated here we find that L = 0.7 km, which is reasonable. For consistency we can see if this value of L corresponds to marginal stability. Following Kagan and Kelley [1998], we find that waves are unstable for wavelengths greater than λm where λm2 = Ψ (Ψ + 1)(4π2Cs2LΩi) / uvin2. If we take vin = 3000 s-1, u = 20 m/s, and L = 0.7 km, we find that waves are linearly stable for wavelengths smaller than 40 m. As the waves continue to erode the gradient, the stable boundary will correspondingly move to longer wavelengths.
Kagan Ludmila M.
Kelley Michael C.
Pfaff Robert F.
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