Substorm intensifications and field line resonances in the nightside magnetosphere

Abstract

Magnetometer and HF radar data often indicate the presence of magnetohydrodynamic, field line resonances in the nightside magnetosphere. These resonances have frequencies of about 1.3, 1.9, 2.6, and 3.4 mHz and are due to cavity modes or waveguide modes which form between the magnetopause and turning points on dipolelike magnetic shells. Energy from these cavity modes tunnels to the field line resonances which are seen in the F region by the HF radar and on the ground by the magnetometers. The presence of these field line resonances gives us an excellent diagnostic tool for determining the position of the mechanism leading to the energetic electrons and field-aligned currents associated with substorm intensifications and auroral brightening. Using data from the Canadian CANOPUS array of magnetometers, meridian scanning photometers, riometers, and bistatic auroral radars and data from the Johns Hopkins University/Applied Physics Laboratory HF radar at Goose Bay in Canada, we have identified a number of intervals in which substorm intensifications occurred during times when field line resonances existed in the region of the magnetosphere where the intensification occurred. In the events that we have analyzed in detail, the ionospheric signatures of the substorm intensification began equatorward (earthward) of existing field line resonances. These observations give very strong evidence indicating that at least one component of the substorm mechanism must be active very close to the Earth, probably on dipolelike field lines in regions with trapped and quasi-trapped energetic particles. Furthermore, the auroral intensifications started near the position of one of the equatorward resonances, indicating that the field line resonances may play a role in triggering or producing the substorm intensifications. One possible scenario is mode conversion to kinetic Alfv6n waves in the resonance. Table 1), meridian scanning photometers, a bistatic auroral radar, and a charged couple device (CCD) imager and became fully operational in December of 1989. The CANOPUS meridian scanning photometer array (MPA) uses meridian scanning, eight-channel, filter wheel photometers at Rankin Inlet, Gillam, Pinawa, and Fort Smith. Only the data from the Gillam (GILL) and Rankin Inlet (RANK) instruments are used in this study. Five of the channels measure auroral emissions (4709, 4861 (twice), 5577, and 6300-/•), and three channels measure the background near 4800, 4935, and 6250/•. The photometer scans the meridian at two revolutions per minute with a sampling rate of 510 samples per scan per channel. Data from the scans are averaged into 17 latitudinal bins, centered on the latitude of the station. The data bins are 0.5 ø wide and 0.5 ø apart for the low-altitude (110 kin) emissions (4861, 5577, and 4709 /•) and approximately 1.0 ø wide for the high-altitude (230 kin) emission (6300/•). The magnetometer array uses three-component, ring core, flux gate instruments. Each channel is sampled at 5-s intervals. The magnetometers are aligned in geogra...

Combined CUTLASS, EISCAT and ESR observations of ionospheric plasma flows at the onset of an isolated substorm

“…8a) and thus would map to a location in the near-Earth magnetotail as suggested by e.g. Samson et al (1992), Samson (1994), andYeoman et al (1994). The poleward expanding auroral structure moves initially at 0.9 km s )1 poleward, slowing to 0.6 km s )1 by 71.5°latitude.…”

Multiple current sheets in a double auroral oval observed from the MAGION-2 and MAGION-3 satellites

“…As shown by J. Samson and his group, both from observational and theoretical grounds (see Samson et al, 1992;Samson and Rankin, 1994;Rankin et al, 1994;Liu et al, 1995), non-linear standing lowfrequency waves in the mHz range can be intimately related to the arcs' formation and their multiplicity. In this theory the resonances are fed by broad-band noise in the system, and it was shown that auroral electrons accelerated by FA electric field due to large k ⊥ in the Alfven wave can reach typical auroral energies in a steady state.…”

Influence of the finite ionospheric conductivity on dispersive, nonradiative field line resonances

“…By themselves monochromatic oscillations at the very-low-frequency range (about 1 ± 5 mHz) connected with ®eld line resonances (FLRs) on the auroral magnetic ®eld lines, are frequently measured by ground based magnetometers and HF radars (e.g., Samson et al, 1992). They may be produced by surface waves excited by plasma pressure pulses in solar wind on the dayside magnetosphere and traveling along inner plasmasheet boundary layers, or by some magnetospheric cavity modes.…”

“…Because FLRs and associated auroral arcs are very narrow in latitude and practically homogeneous in local time (Greenwald and Walker, 1980;Samson et al, 1992), the azimuthal variation of the solutions can be ignored (d y 0) in the lowest approximation. The twodimensionality of the problem then eliminates the strong vector (E Â B) nonlinearity in the model, and only weak nonlinear terms associated with the density disturbance survive.…”