Signatures of distorted geomagnetic field lines in Antarctic magnetometer data
Sean Ables

Authors

Ables S
The University of Newcastle

Sciffer M
The University of Newcastle

Waters C
The University of Newcastle

Inness E
Institution not provided

Fraser B
The University of Newcastle

Abstract

Ultra-low frequency (ULF) wave signatures in the magnetometer data from Davis, Antarctica (74.49° S, 100.03° E CGM) often exhibit spectral characteristics which can be interpreted as field line resonances (FLR) of the outermost closed field lines in the dayside magnetosphere. Diurnal variations in the frequency at which maximum power occurs in the Pc5 (1-10 MHz) band, often show an arch-shaped variation with local magnetic time. The general topology of the dayside high latitude magnetosphere results in longer field lines and therefore lower resonance frequencies on the dawn and dusk flanks, and the shortest field lines and higher resonant frequency for field lines near magnetic noon, passing through the sub-solar point. Similar FLR variations are seen in magnetometer data from the Mawson, Casey and Scott Base, Antarctica. We present an eigenfunction solution for shear Alfvén waves which also accounts for the curvature and torsion of the magnetic field that threads high latitude stations. This provides an explanation for our observations of a saddle in the Pc5 arch - a decrease in frequency centred on local magnetic noon - which is seen in Davis magnetometer data and to a lesser extent in Mawson, Casey and Scott Base data. Our modelling indicates that this saddle is due to increased field torsion on the dawn and dusk flanks producing increased pre- and post noon frequencies while the noon frequencies are depressed by field curvature.