Browsing by Author "Swenson, C. M."
Now showing 1 - 3 of 3
Results Per Page
Sort Options
Item Open Access Onset conditions for equatorial spread F determined during EQUIS II(American Geophysical Union, 2005-12-22) Hysell, D. L.; Larsen, M. F.; Swenson, C. M.; Barjatya, A.; Wheeler, T. F.; Sarango, M. F.; Woodman Pollitt, Ronald Francisco; Chau Chong Shing, Jorge LuisAn investigation into the dynamics and layer structure of the postsunset ionosphere prior to the onset of equatorial spread F (ESF) took place during the NASA EQUIS II campaign on Kwajalein Atoll on August 7 and 15, 2004. On both nights, an instrumented rocket measured plasma number density and vector electric fields to an apogee of about 450 km. Two chemical release rockets were launched both nights to measure lower thermospheric wind profiles. The Altair UHF/VHF radar was used to monitor coherent and incoherent scatter. In both experiments, strong plasma shear flow was detected. Periodic, patchy bottom-type scattering layers were observed in the westward-drifting plasma below the shear nodes. The large-scale plasma depletions that formed later during ESF reproduced the periodic structure of the original, precursor layers. The layers were therefore predictive of the ESF that followed. We surmise that collisional shear instabilities may have given rise to large-scale plasma waves that were highlighted by the bottom- type layer structure and that preconditioned the postsunset ionosphere for ESF.Item Open Access Rocket and radar investigation of background electrodynamics and bottom-type scattering layers at the onset of equatorial spread F(European Geosciences Union (EGU), 2006-07) Hysell, D. L.; Larsen, M. F.; Swenson, C. M.; Barjatya, A.; Wheeler, T. F.; Bullett, T. W.; Sarango, M. F.; Woodman Pollitt, Ronald Francisco; Chau Chong Shing, Jorge Luis; Sponseller, D.Sounding rocket experiments were conducted during the NASA EQUIS II campaign on Kwajalein Atoll designed to elucidate the electrodynamics and layer structure of the postsunset equatorial F region ionosphere prior to the onset of equatorial spread F (ESF). Experiments took place on 7 and 15 August 2004, each comprised of the launch of an instrumented and two chemical release sounding rockets. The instrumented rockets measured plasma number density, vector electric fields, and other parameters to an apogee of about 450 km. The chemical release rockets deployed trails of trimethyl aluminum (TMA) which yielded wind profile measurements. The Altair radar was used to monitor coherent and incoherent scatter in UHF and VHF bands. Electron density profiles were also measured with rocket beacons and an ionosonde. Strong plasma shear flow was evident in both experiments. Bottom-type scattering layers were observed mainly in the valley region, below the shear nodes, in westward-drifting plasma strata. The layers were likely produced by wind-driven interchange instabilities as proposed by Kudeki and Bhattacharyya (1999). In both experiments, the layers were patchy and distributed periodically in space. Their horizontal structure was similar to that of the large-scale plasma depletions that formed later at higher altitude during ESF conditions. We argue that the bottom-type layers were modulated by the same large-scale waves that seeded the ESF. A scenario where the large-scale waves were themselves produced by collisional shear instabilities is described.Item Restricted Shear flow effects at the onset of equatorial spread F(American Geophysical Union, 2006-11-23) Hysell, D. L.; Larsen, M. F.; Swenson, C. M.; Wheeler, T. F.Experiments were conducted during the NASA Equatorial Ionospheric Study (EQUIS) II sounding rocket campaign on Kwajalein Atoll to investigate the dynamics and stability of the postsunset equatorial F region ionosphere prior to the onset of equatorial spread F (ESF). Experiments took place on 7 and 15 August 2004, each composed of the launch of an instrumented and two chemical release sounding rockets. The instrumented rockets measured plasma number density, vector electric fields, and other parameters to an apogee of about 450 km. The chemical release rockets yielded lower thermospheric wind profile measurements. The Altair radar was used to monitor coherent and incoherent scatter in UHF and VHF bands. In both experiments, large‐scale waves preceded the emergence of ESF depletions and irregularities. We propose that the large‐scale waves were produced by a collisional shear instability. A three‐dimensional electrodynamic model is constructed that can reproduce the shear flow observed by the rockets. On the basis of the model, we calculate the linear growth rate for the collisional shear instability and show that it can compete with the generalized Rayleigh Taylor instability. A scenario where the former generates the large‐scale waves and initiates the latter is discussed.