Correlated optical and ULF magnetic observations of the winter cusp – Boundary layer system

Abstract

We have found a consistent correlation between ground based ultra low frequency (ULF) magnetic and optical measurements of the dayside cusp/boundary layer system which, we believe, may allow ground based identification of the boundary layers and central cusp. We have based our analysis on data taken at Longyearbyen Svalbard, Norway. Under normal magnetic conditions, as the station rotates beneath the auroral zone each day it passes under the ionospheric projection of the cusp‐boundary layer system, near magnetic noon at approximately 0830 UT. During the passage of the cusp‐boundary layer system a distinctive sequence of magnetic spectral features is observed. The boundary layer passage is characterized by a sharp increase in the power spectral density of broad band, unpolarized noise, combined with a polarized low frequency (1–3 mHz) signal. This is followed by a period of greatly reduced power spectral density of the unpolarized noise. A narrow band polarized signal in the 3–5 mHz band persists throughout this quiet period which is observed to be predominantly left hand polarized before local magnetic noon. This narrow band feature also appears to rise in frequency as the cusp passes overhead, and then falls again after noon. Optical observations are strongly correlated with the magnetic data. We believe the observed features have their source in the ionospheric footprint of the boundary layers and central cusp. The polarized signals may arise from Alfven waves spawned at the magnetopause which propagate down the magnetic field lines that map to the boundary layers. As the cusp may move with changing geomagnetic activity, observations of the features noted here may not be seen on every day. The statistical question of how often, and under what conditions, these features appear is an ongoing research topic. We plan to address this statistical question in a future manuscript.

Coupling of magnetopause‐boundary layer to the polar ionosphere

“…Actually, we found that the total spectral density in the prenoon sector is nearly twice as the total spectral density in the postnoon sector in either the frequency domain or the wavenumber domain. Therefore the geomagnetic pulsations produced by the FACs in our simulations have similar frequency and characteristics as those observed by McHarg and Olson [1992].…”

Observations of polar patches generated by solar wind Alfvén wave coupling to the dayside magnetosphere

“…Such PMAF recurring rates, similar to Pc5 periodicities, were also found by Lockwood et al (1989) and Sandholt et al (1990) who called the events dayside auroral break up events or simply cusp/cleft auroral activity (Sandholt et al, 1994). McHarg and Olson (1992) correlated such events with the ULF wave activity and Leontyev et al (1992) showed magnetometer data for one of their`P MAF'' events but did not discuss the associated Pc5 pulsations that can clearly be identi®ed in their Fig. 5.…”

Two‐dimensional structure of long‐period pulsations at polar latitudes in Antarctica

“… Rostoker et al [1972] found that, averaged over quiet days, the position of the dayside maximum of the total Pc5 power approximately lies on the separatrix between open and closed field lines. McHarg and Olson [1992] found that the ionospheric projection of the dayside boundary layers was characterized by the combination of broad‐band unpolarized noise with the narrow band polarized signal. The typical polar cusp signal was polarized and narrowband (3–5 mHz), and had arc‐type dynamic spectra with maximal frequency at noon.…”