Meridian-scanning photometer, coherent HF radar, and magnetometer observations of the cusp: a case study

Abstract

Abstract. The dynamics of the cusp region and postnoon sector for an interval of predominantly IMF B y , B z < 0 nT are studied with the CUTLASS Finland coherent HF radar, a meridian-scanning photometer located at Ny A Ê lesund, Svalbard, and a meridional network of magnetometers. The scanning mode of the radar is such that one beam is sampled every 14 s, and a 30°azimuthal sweep is completed every 2 minutes, all at 15 km range resolution. Both the radar backscatter and red line (630 nm) optical observations are closely colocated, especially at their equatorward boundary. The optical and radar aurora reveal three dierent behaviours which can interchange on the scale of minutes, and which are believed to be related to the dynamic nature of energy and momentum transfer from the solar wind to the magnetosphere through transient dayside reconnection. Two interpretations of the observations are presented, based upon the assumed location of the open/closed ®eld line boundary (OCFLB). In the ®rst, the OCFLB is co-located with equatorward boundary of the optical and radar aurora, placing most of the observations on open ®eld lines. In the second, the observed aurora are interpreted as the ionospheric footprint of the region 1 current system, and the OCFLB is placed near the poleward edge of the radar backscatter and visible aurora; in this interpretation, most of the observations are placed on closed ®eld lines, though transient brightenings of the optical aurora occur on open ®eld lines. The observations reveal several transient features, including poleward and equatorward steps in the observed boundaries,``braiding'' of the backscatter power, and 2 minute quasi-periodic enhancements of the plasma drift and optical intensity, predominantly on closed ®eld lines.

Are dayside long-period pulsations related to the cusp?

“…Comparison of the cusp boundary identification with the Super Dual Auroral Radar Network (SuperDARN) data and with DMSP particle detector showed good correspondence between those techniques (Cai et al, 2009;Baker et al, 1990). Third, ground optical observations of the red (630.0 nm) and green (557.7 nm) line aurorae around noon can give very relevant information on precipitation boundaries and the location of the cusp (e.g., Milan et al, 1999;; however, this is possible only during polar night (in December and early January at the Svalbard location) and with clear sky conditions. For the present study we have no suitable optical data.…”

Estimating the location of the open-closed magnetic field line boundary from auroral images

“…Many other instruments have been used to provide estimates of the location of the OCB with higher spatial and temporal coverage. These include HF radars (e.g., Baker et al, 1995Baker et al, , 1997Milan et al, 1999;Milan and Lester, 2001;Chisham et al, 2001Chisham et al, , 2002, ground-based magnetometers (e.g., Iijima and Potemra, 1978;Mishin, 1990), all-sky cameras (e.g., Akasofu and Kimball, 1965;Feldstein and Galperin, 1985), meridian scanning photometers (e.g., Blanchard et al, 1995;Sandholt et al, 1998), and satellite-based imagers (e.g., Brittnacher et al, 1999;Kauristie et al, 1999;Baker et al, 2000;Carbary et al, 2003;Boakes et al, 2008). Arguably the best instrument to estimate the location of the complete OCB (in a single hemisphere) is the satellite-based imager, which can image the whole auroral oval at a time resolution of the order of minutes, for hours at a time (see e.g., Boakes et al, 2008).…”

Plasma flow during the brightening of proton aurora in the cusp

“…This interval is similar to the mean interval between FTE signatures, 8 min, and this similarity supports the possibility of PMAFs being associated with FTEs. Milan et al [1999] performed simultaneous observations of such periodic aurora and flow in the cusp using a meridianscanning photometer and HF radar, and they showed that some PMAFs are accompanied by flow enhancements.…”