An investigation of the field-aligned currents associated with a large-scale ULF wave using data from CUTLASS and FAST

Abstract

Abstract. On 14 December 1999, a large-scale ULF wave event was observed by the Hankasalmi radar of the Super-DARN chain. Simultaneously, the FAST satellite passed through the Hankasalmi field-of-view, measuring the magnetic field oscillations of the wave at around 2000 km altitude, along with the precipitating ion and electron populations associated with these fields. A simple field line resonance model of the wave has been created and scaled using the wave's spatial and temporal characteristics inferred from SuperDARN and IMAGE magnetometer data. Here the model calculated field-aligned current is compared with field-aligned currents derived from the FAST energetic particle spectra and magnetic field measurements. This comparison reveals the small-scale structuring and energies of the current carriers in a large-scale Alfvén wave, a topic, which at present, is of considerable theoretical interest. When FAST traverses a region of the wave involving low upward field-aligned current densities, the current appears to be carried by unstructured downgoing electrons of energies less than 30 eV. A downward current region appears to be carried partially by upgoing electrons below the FAST energy detection threshold, but also consists of a mixture of hotter downgoing magnetospheric electrons and upgoing ionospheric electrons of energies <30 eV, with the hotter upgoing electrons presumably representing those upgoing electrons which have been accelerated by the wave field above the low energy detection threshold of FAST. A stronger interval of upward current shows that small-scale structuring of scale ∼50 km has been imposed on the current carriers, which are downgoing magnetospheric electrons of energy 0-500 eV.

Equivalent currents associated with morning-sector geomagnetic Pc5 pulsations during auroral substorms

“…This vortex was at high latitudes (MLATs 74-75 • ), which gives us reason to believe that the vortex is a combined signature of morning-sector large-scale R1 and FLR FACs. Meridional sequences of two or three vortices with opposing rotation directions (upward and downward currents) which are more consistent with previously reported FLR FAC systems (Scoffield, et al, 2005;McPherron, 2005) appear in our equivalent current patterns too, but also in these structures the most poleward FAC was typically downward, i.e., parallel to morning-sector R1.…”

“…The latitude of resonant field lines can be located with data from a meridional ground-based magnetometer chain: the amplitude of oscillations is largest and the phase has a change of 180 • at the FLR footpoint. In this region also the variations in the ionospheric electric field are strongest (Scoffield et al, 2005).…”

“…The magnetic field variations, Pc5 pulsations and substorm breakups, observed at the ground are associated with the current system which the magnetohydrodynamic (MHD) waves of FLRs generate (Scoffield et al, 2005). This system consists of a set of field-aligned currents (FACs) with varying directions according to latitude and the associated Hall and Pedersen currents in the ionosphere (cf.…”

Downward current electron beam observed by Cluster and FAST

“…For downward currents, it is not clear how two‐dimensional cross‐sectional structure (perpendicular to B ) maps from the ionosphere to the magnetosphere. FAST‐SuperDARN conjunctions have shown how large latitudinal scales in the ionosphere (several 100s km) contain fragmented structures of ∼50 km at 4000 km altitude [ Scoffield et al , 2005, 2007]. Whether these fragmented structures are translationally invariant in longitude, or isolated flux tubes, is not clear.…”

Global MHD eigenmodes of the outer magnetosphere

“…Recent observations have shown that our view of a ULF wave depends upon how we choose to observe it. For example, Scoffield et al [2005] show how the latitudinal scale in radar data can be several 100 km, while in FAST data the same event has structure at 50 km. The latter scale is roughly 10λ e , where λ e is the electron inertial length (λ 2 e = m e /µ 0 ne 2 ) and is 5 km for n = 1 cm 3 (n is the magnetospheric number density).…”