Richards P.G.,George Mason University |
Nicolls M.J.,SRI International |
St.-Maurice J.-P.,Institute of Space and Atmospheric Studies |
Goodwin L.,Institute of Space and Atmospheric Studies |
Ruohoniemi J.M.,Virginia Polytechnic Institute and State University
Journal of Geophysical Research A: Space Physics | Year: 2014
This paper investigates unusually deep and sudden electron density depletions (troughs) observed in the Poker Flat (Alaska) Incoherent Scatter Radar data in middle summer of 2007 and 2008. The troughs were observed in the premidnight sector during periods of weak magnetic and solar activity. The density recovered to normal levels around midnight. At the time when the electron density was undergoing its steep decrease, there was usually a surge of the order of 100 to 400 K in the ion temperature that lasted less than 1 h. The Ti surges were usually related to similar surges in the AE index, indicating that the high-latitude convection pattern was expanding and intensifying at the time of the steep electron density drop. The convection patterns from the Super Dual Auroral Radar Network also indicate that the density troughs were associated with the expansion of the convection pattern to Poker Flat. The sudden decreases in the electron density are difficult to explain in summer because the high-latitude region remains sunlit for most of the day. This paper suggests that the summer density troughs result from lower latitude plasma that had initially been corotating in darkness for several hours post sunset and brought back toward the sunlit side as the convection pattern expanded. The magnetic declination of ∼22° east at 300 km at Poker Flat greatly facilitates the contrast between the plasma convecting from lower latitudes and the plasma that follows the high-latitude convection pattern. Key Points Modeling premidnight plasma density troughs observed in Alaska in summer Density troughs occur during weak solar and magnetic activity Model indicates cause is convection of low-density plasma from lower latitudes ©2014. American Geophysical Union. All Rights Reserved.
Zawada D.J.,Institute of Space and Atmospheric Studies |
Dueck S.R.,Institute of Space and Atmospheric Studies |
Rieger L.A.,Institute of Space and Atmospheric Studies |
Bourassa A.E.,Institute of Space and Atmospheric Studies |
And 2 more authors.
Atmospheric Measurement Techniques | Year: 2015
The Optical Spectrograph and InfraRed Imaging System (OSIRIS) instrument on board the Odin spacecraft has been measuring limb-scattered radiance since 2001. The vertical radiance profiles measured as the instrument nods are inverted, with the aid of the SASKTRAN radiative transfer model, to obtain vertical profiles of trace atmospheric constituents. Here we describe two newly developed modes of the SASKTRAN radiative transfer model: a high-spatial-resolution mode and a Monte Carlo mode. The high-spatial-resolution mode is a successive-orders model capable of modelling the multiply scattered radiance when the atmosphere is not spherically symmetric; the Monte Carlo mode is intended for use as a highly accurate reference model. It is shown that the two models agree in a wide variety of solar conditions to within 0.2 %. As an example case for both models, Odin-OSIRIS scans were simulated with the Monte Carlo model and retrieved using the high-resolution model. A systematic bias of up to 4 % in retrieved ozone number density between scans where the instrument is scanning up or scanning down was identified. The bias is largest when the sun is near the horizon and the solar scattering angle is far from 90°. It was found that calculating the multiply scattered diffuse field at five discrete solar zenith angles is sufficient to eliminate the bias for typical Odin-OSIRIS geometries. © 2015 Author(s).