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

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.