F layer ionization patches in the polar cap

Abstract

Ground‐based optical and digital ionosonde measurements were conducted at Thule, Greenland to measure ionospheric structure and dynamics in the nighttime polar cap F layer. These observations showed the existence of large‐scale (800–1000 km) plasma patches drifting in the antisunward direction during a moderately disturbed (Kp ≥ 4) period. Simultaneous Dynamics Explorer (DE‐B) low‐altitude plasma instrument (LAPI) measurements show that these patches with peak densities of ∼106 el cm−3 are not locally produced by structured particle precipitation. The LAPI measurements show a uniform precipitation of polar rain electrons over the polar cap. The combined measurements provide a comprehensive description of patch structure and dynamics. They are produced near or equatorward of the dayside auroral zone and convect across the polar cap in the antisunward direction. Gradients within the large scale, drifting patches are subject to structuring by convective instabilities. UHF scintillation and spaced receiver measurements are used to map the resulting irregularity distribution within the patches.

Improvement of HF coherent radar line-of-sight velocities by estimating the refractive index in the scattering volume using radar frequency shifting

“…These structures may form from processes such as particle precipitation on the nightside [e.g., Noël et al, 2000Noël et al, , 2005Sofko et al, 2007] and photoionization creating polar cap patches on the dayside [Weber et al, 1984]. Other structures which may provide locations for HF coherent radar scatter are the midlatitude trough [Moffett and Quegan, 1983] and traveling ionospheric disturbances (TIDs) [e.g., Pryse et al, 1995].…”

Three‐dimensional structuring characteristics of high‐latitude plasma patches

“…Observations [Weber et al, 1986;Basu et al, 1994] as well as (large-scale) modeling [Sojka et al, 1993] show that the patches convect to distances of the order of 3000 km [Weber et al, 1986] and for periods of a few hours while retaining their integrity. The mesoscale irregularities, observed within these patches throughout the polar cap region [Weber et al, 1984[Weber et al, , 1986Tsunoda, 1988;Basu et al, 1990Basu et al, , 1995, occasionally are more intense on the trailing edge than on the leading edge [Weber et al, 1984].…”

“…The largescale structures ranging from hundreds to thousands of kilometers have been categorized as "patches" (in the polar cap) or "blobs" (at auroral latitudes) [Weber et al, 1984[Weber et al, , 1986Basu et al, 1990Basu et al, , 1994. The mesoscale features in the range of 0.1 km to tens of kilometers are believed to arise from naturally occurring instabilities in a two-component plasma.…”

Polar patches observed by ESR and their possible origin in the cusp region

“…Particle precipitation has also been cited as a possible source of patch plasma. Weber et al (1984) suggested that, if the¯ux-tube convected Sunward along the cusp for tens of minutes before turning anti-Sunward into the polar cap, then there might be su cient time for build-up of ionisation. Modelling studies by Sojka and Schunk (1986) indicated that densities might increase by a factor of two above background within a¯ux tube that convected through cusp-precipitation for some 10±15 min.…”