The role of ULF waves in energy transport in the magnetosphere

We examine the role of ULF waves in energy transport in the Earth's magnetosphere for four distinct types of events. Firstly we present ground-based observations of an N=0 Pc5 drift resonance in the afternoon sector that is extremely well-defined and narrow-band in both frequency and azimuthal wavenumber. We also present the first characterisation of the 2-D ionospheric velocity flows of such a wave, and we explain the wave growth via in-situ observations of an unstable particle energy distribution in the Earth's ring current.

Secondly we show how field line resonance (FLR) on closed auroral field lines in the dusk sector can modulate auroral particle precipitation, and that field line stretching can overcome the typical poleward FLR phase. We argue that the flank waveguide may be excited via fast mode waves propagating sunward from the magnetotail, producing equatorward and sunward propagating east-west aligned auroral arcs via mode-conversion to Alfvén waves on stretched auroral field lines.

Thirdly we present the first EISCAT measurements of ionospheric heating rates due to auroral poleward boundary intensifications (PBIs), and show that PBIs can deposit a similar amount of energy per unit area into the ionosphere as substorms.

Finally we characterise the ground signature of PBIs in ground-based optical and magnetometer data, and show that the PBI repetition rate matches the frequency of Alfvén waves observed in-situ in the PSBL at altitudes of 5-6 R_E. The observation of identical periodicities provides definitive evidence that high altitude ULF wave activity may be a major mechanism for auroral particle precipitation, and can therefore play an important role in energy coupling between the magnetotail and the ionosphere. We also find that the observed Alfvén waves carry an Earthward directed field-aligned Poynting flux that contains enough energy to explain the heating rates due to PBIs as measured by the EISCAT Svalbard radar. Satellite measurements show that high frequency oscillations are superimposed on top of the ULF waves in the PSBL and these high frequency waves have kinetic scale sizes and may therefore produce appreciable parallel electric fields. We show that these high frequency waves are characteristic of Alfvénic turbulence, probably driven by a current-convective interchange instability.