I. Voronkov

Relation of substorm breakup arc to other growth-phase auroral arcs

[1]xa0High-resolution CANOPUS meridian-scanning photometer and all-sky imager observations of preonset and expansion-phase auroral arcs are analyzed for expansion-phase onsets that evolve into full substorms and into pseudobreakups. One or more arcs are seen across the sky throughout the growth phase prior to onset. The onsets we have examined indicate that auroral breakup at onset does not generally occur along one of these arcs but instead often occurs along a thin breakup arc that forms equatorward of all growth phase arcs a few minutes prior to onset. The intensity of this breakup arc increases monotonically for the few minutes prior to the time normally identified as substorm onset and then typically increases dramatically. These results imply that the processes responsible for auroral breakup initiate a few minutes prior to the time normally identified as substorm expansion-phase onset. We also find that arcs poleward of the arc that breaks up appear to be unaffected by substorm onset until expansion-phase auroral activity moves poleward to the location of such arcs. Arcs poleward of the poleward-most extent of pseudobreakup auroral activity show no apparent effects of a pseudobreakup. These results imply that the process that initiates the onset of substorms does not require the occurrence of plasma sheet changes, significant enough to affect magnetosphere-ionosphere electrodynamics, along field lines that cross the equator tailward of the substorm onset region.

Ground‐based optical determination of the b2i boundary: A basis for an optical MT‐index

[1] The equatorward boundary of the proton aurora corresponds to a transition from strong pitch angle scattering to bounce trapped particles. This transition has been identified as the b2i boundary in Defense Meteorological Satellite Program (DMSP) ion data [Newell et al., 1996]. We use ion data from 29 DMSP overflights of the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) Meridian Scanning Photometer (MSP) located at Gillam, Canada, to develop a simple algorithm to identify the b2i boundary in latitude profiles of proton auroral (486 nm) brightness. Applying this algorithm to a ten year set of Gillam MSP data, we obtain ∼250,000 identifications of the optical b2i, the magnetic latitude of which we refer to as b2i Λ . We intercompare ∼1600 near-simultaneous optical and in situ b2i Λ , concluding that the optical b2i Λ is a reasonable basis for an optical equivalent to the MT-index put forward by Sergeev and Gvozdevsky [1995]. Using ∼17,000 simultaneous measurements, we demonstrate a strong correlation between the optical b2i Λ and the inclination of the magnetic field as measured at GOES 8. We develop an empirical model for predicting the GOES 8 inclination, given theuniversal time, dipole tilt, and the optical b2i Λ , as determined at Gillam. We also show that in terms of information content, the b2i boundary is an optimal boundary upon which to base such an empirical model.

Coupling of shear flow and pressure gradient instabilities

The nonlinear dynamics of a shear flow and its subsequent evolution in the equatorial plane of the inner plasma sheet is studied. A linear analysis of the ideal MHD equations reveals a hybrid vortex instability which appears because of the coupling of Kelvin-Helmholtz (KH) and Rayleigh-Taylor instabilities. The hybrid vortex mode grows faster than a KH mode, extracts ambient potential energy, and leads to vortex cells that have a larger spatial extent than a simple KH vortex. In the nonlinear stage, vortices become surge-like and may destroy the shear flow region. The relevance of this model to vortex generation and auroral arc intensifications at the inner edge of the plasma sheet is discussed.

Auroral density fluctuations on dispersive field line resonances

A model is presented that describes the excitation of density perturbations and parallel electric fields by standing shear Alfven waves on dipole fields in Earths magnetosphere. The model includes the effects of electron inertia and gyro-kinetic dispersion, accounting for field-aligned variations of the electron and ion temperatures and the ambient plasma density. In a model dipole magnetosphere, it is found that dispersion and nonlinearity determine the depth, spatial structure, and temporal evolution of large-amplitude density fluctuations near to the polar ionospheres. The characteristics of magnetospheric density cavities and their relationship to auroral luminosity, or field-aligned currents, is discussed in the context of recent satellite and ground based observations.

Observations of the phases of the substorm

[1]xa0Following the database of large-scale vortices during pseudo-breakup and breakup registered by the Gillam All-Sky Imager, we selected one event (19 February 1996) for a detailed consideration. This event is a sequence of pseudo-breakup and local substorm, and breakup followed by the large substorm, which is isolated from the previous pseudo-breakup by the second growth phase. Commencement of these elements of auroral activity was clearly seen above the Churchill line of the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS; pseudo-breakup was completely covered by the field of view of the Gillam All-Sky Imager). Geotail was located at ∼19 RE in the equatorial plane of midnight sector, which, along with supporting observations from two geostationary satellites (GOES 8 and 9), allowed for a comparison of ground-based, geostationary orbit and midtail signatures. The pseudo-breakup consisted of two distinct stages: a near-exponential arc intensity growth and a poleward vortex expansion that started simultaneously with dipolarization in the inner magnetosphere. The latter corresponded to explosive onset of short-period (tens of millihertz) pulsations observed at geostationary orbit and on the ground in the vicinity of the arc. No significant disturbances poleward of the vortex were observed. Pseudo-breakup was followed by the second growth phase, which involved a significant thinning of the plasma sheet. Breakup was of a similar two-stage character as the pseudo-breakup. Full onset of the expansive phase that followed breakup was seen simultaneously by all instruments including Geotail, which detected strong perturbations in the midtail. The expansive phase onset launched the second postbreakup package of Pi2 pulsations that were of larger amplitude. Finally, during the substorm recovery phase, the poleward boundary intensifications (PBIs) were observed as long-period, on the order of 10 min, pulses of electron precipitation. PBI commencement coincided with bursty flows and pulses of plasma energization in the midtail. Observed features support recent ideas claiming that we are dealing with processes (breakup, full onset of the expansive phase, and PBIs) of a distinct physical nature that require different commencement thresholds, namely, the inner plasma sheet instability (pseudo-breakup and breakup), midtail reconnection (expansive phase onset), and further magnetotail dynamics during the recovery phase (PBIs).

Shear flow instability in the dipolar magnetosphere

The three-dimensional, nonlinear evolution of a shear flow (or Kelvin-Helmholtz (KH)) instability driven by a large-amplitude shear Alfven wave (SAW) in the Earths magnetosphere is studied by using numerical solutions to the complete set of ideal magnetohydrodynamic equations. An initial setup is chosen to simulate a standing SAW associated with field line resonances (FLRs) in a dipolar magnetosphere. It is shown that KH vortices grow most rapidly in the equatorial plane. In this region, the growth rate is reduced by the ratio of the KH and SAW frequencies when compared to the growth rate predicted by a two-dimensional theory for transient magnetic field lines. For typical parameters of FLRs, this ratio is small. Field-aligned gradients of the KH mode vorticity and azimuthal phase velocity initiate Alfven waves, which carry energy toward the ionosphere. This results in partial restructuring of field-aligned currents with scale size of ∼10 km above the ionosphere. After one period of the SAW, energy in the KH mode returns to the SAW flow. This suggests that vortex formation might be largely periodic in evolution, reconfiguring after each period of the FLR. Finally, we show that this restructuring of field-aligned currents does not depend on the initial phase of the SAW. For example, the model predicts that a ground-based observer in the Northern Hemisphere (looking antiparallel to the Earths magnetic field) will see that the downward current wraps clockwise and the upward current wraps counterclockwise, though the positions of the currents change latitudes for different phases.

Nonlinear shear Alfvén resonances in a dipolar magnetic field

The effect of the ponderomotive force (PF) on the temporal evolution of shear Alfven field line resonances (FLRs) is considered for a magnetic dipole geometry appropriate to the Earths magnetosphere. We derive a set of equations which describes the coupling of shear Alfven and slow mode waves and show that in a dipole field, the PF initiates a spectrum of standing slow mode waves, rather than just the fundamental mode that arises in a Cartesian box model magnetosphere. Magnetic field aligned slow mode density perturbations lead to a nonlinear temporal phase shift between the compressional driver and shear Alfven wave. This results in nonlinear saturation of the wave fields of FLRs and may occur well in advance of linear saturation as a result of ionospheric dissipation. We derive expressions for the nonlinear frequency shifts caused by the slow mode spectrum and determine the timescale for nonlinear saturation of the shear Alfven wave fields. Finally, we compare our results with previous estimates made in a box model magnetosphere and show that our main conclusions remain valid.