Plasma irregularities adjacent to auroral patches in the postmidnight sector

Abstract

[1] We demonstrate a close association between decameter-scale plasma irregularities in the E region ionosphere and auroral patches in the postmidnight sector. In September 2009, campaign-based measurements of the aurora were conducted in Iceland with a white light all-sky camera (ASC) at Tjörnes (66.20°N, 17.12°W) and the SuperDARN radar at þykkvibaer (63.77°N, 20.54°W). On one night during the campaign period, the ASC observed the successive passage of auroral patches in the postmidnight sector after a small substorm-like activity. The patches were drifting predominantly eastward across the field-of-view of the ASC with a speed of approximately 360-450 m s, which is consistent with the sunward convection in the postmidnight westward electrojet. The simultaneous radar measurements recorded strong radar backscatter echoes (>15 dB) within the gaps between adjacent auroral patches, while such echoes were not observed or were very weak in the region of the aurora. The Doppler velocity estimation showed that the electric field was clearly reduced within the patches, which was probably the result of the enhanced conductance associated with auroral precipitation. Thus, this reduction in the electric field suppressed the generation of irregularities (i.e., radar echoes) in the regions of auroral patches. This suggests that the conductance enhancement associated with precipitating electrons not only modified the electric field within the aurora but also affected the generation of small-scale plasma structures in the vicinity of the patch-type optical auroral forms.

Models for electrostatic drift waves with density variations along magnetic field lines

“…The analysis outlined in this study has several verifiable features that distinguishes it from, for instance, the current convective instability [ Ossakow and Chaturvedi , ] that assumes a fully collisional plasma. These instabilities are often invoked for explaining observations of irregularities in the ionospheric E and F regions [ Greenwald et al , ; Hosokawa et al , ]. While most studies of convective drift instabilities [ Simon , ; Fejer et al , ] rely on steady state electric fields having a component along ∇ ⟂ n 0 so that E 0 ·∇ n 0 >0, our generator model is based on steady state electric fields E 0 ⟂∇ ⟂ n 0 ( r ⟂ ) and operates self consistently with the density enhancement in a magnetic flux tube that forms the basis for a universal drift wave instability.…”

“…[8] To generate a substantial perturbation consistent with observations [Greenwald et al, 2002;Hosokawa et al, 2010], free energy is needed to drive an instability. Several sources of energy can be recognized as relevant for the present problem.…”

“…In nature as well as many laboratory experiments, we have conditions where the equilibrium plasma density has a gradient along as well as perpendicular to an externally imposed stationary magnetic field B . In the polar ionosphere, in particular, magnetic flux tubes with enhanced plasma density are often associated with auroral precipitation and auroral patches [ Hosokawa et al , ] where the plasma density has a gradient ⟂ B as well as a decreasing density along magnetic field lines, i.e., increasing altitudes. The case with plasma density gradients perpendicular to B has previously been studied, and conditions for electrostatic drift wave instabilities have been established [ Kadomtsev , ].…”

Possible connection of polar cap flows to pre- and post-substorm onset PBIs and streamers

“…When the poleward boundary of the auroral oval (the open/closed field line boundary) was within the MSP FOV, all patches were observed to drift into that boundary with subsequent brightenings of PBIs [ Lorentzen et al , 2004]. Since these patches are known to drift with the F region plasma drift speed [e.g., Doolittle et al , 1990; Hosokawa et al , 2010], these brightenings were taken as a unique signature of tail reconnection bursts, which carry plasma from open polar cap field lines into the plasma sheet as illustrated by the flow channel from the polar cap in Figure 1. The authors did not address whether the drifting patches were associated with localized flow channels within the polar cap, however the meridional drift speeds, which are only a component of the total drift speed, exceed those seen in our observations during periods between flow channels, suggesting that the patches move across the polar cap within the flow channels.…”

Simultaneous ground‐satellite observations of meso‐scale auroral arc undulations

“…This radar has a field‐of‐view (FOV) that extends northeastward, which covers almost entire FOV of the ASC at TJO. During the interval of interest, this radar was operating with a special camping mode providing higher temporal and spatial resolution measurements of the ionospheric electric field within the FOV of the ASC at TJO [see Hosokawa et al , 2010, and references therein] for details of this mode). In this mode the radar operated with 15 km range separations giving higher spatial resolution.…”