Monitoring Traveling Magnetospheric Waves In Optical Aurora
The network of THEMIS all-sky imagers provides an unprecedented opportunity of studying time evolution of multiscale auroral disturbances associated with the magnetospheric substorm onset. In this talk, we report our recent results suggesting a possibility of direct optical monitoring of propagating low-frequency magnetotail disturbances associated with pre- and post-onset conditions. The results show a significant enhancement of spatial and temporal coherence of the growth phase aurora accompanied by increased ambipolar electric field fluctuations in the tail signaling a formation of a thin current sheet about ∼5 minutes prior to the main auroral breakup. During this time, growth phase arcs reveal various forms of azimuthal plasma motion including multiscale turbulence and westward propagating waves with traveling speeds of 2.0 -- 5.0 km/s. The post-breakup auroral dynamics exhibits wave-like forms which typically travel duskward at the velocity 8.0 -- 12.0 km/s, recurrence time 20 -- 30 s. These waves first appear within a local pre-midnight sector (∼1 hour MLT wide) shortly before to the global expansion onset, and they can be reliably detected during the first 10+ minutes following the breakup. The analysis of simultaneous in situ THEMIS measurements suggests that the observed pre-onset wave patterns can be an auroral footprint of flapping oscillations and/or other drift wave modes in the magnetotail which can arguably be related to the development of the initial plasma instability triggering the substorm onset.
Evolution of Wave Numbers in Auroral Structures at a Substorm Onset
Recent developments of optical instruments for ground-based auroral observations, e.g. THEMIS All-Sky Imagers array, reveal existences of fine-scale auroral structures at substorm expansion-phase onsets, which we could not identify previously due to limitation of both temporal and spatial coverage/resolution. Donovan et al.  reported that a pseudo auroral break up occurred on a pre-existing aurora consisting of eastward propagating beads with a wavelength of ~100 km. Similar auroral structures with wavelengths of 50-200 km at substorm expansion-phase onsets were reported by Friedrich et al.  and Liang et al. . These authors pointed out that the fine structures imply the dynamics of some plasma instabilities associated with substorm onsets in the magnetosphere were projected onto the structures of the onset auroras. In this presentation, we show time evolutions of auroral structures observed at a pseudo substorm onset on January 15, 2008 at Gillam, Canada (56.4N, 265.4E, dipole geomagnetic latitude 65.6N). The auroral initial brightening started at 2223 MLT just in the center of a field of view of a panchromatic (white-light) all-sky camera with a sampling rate of 30 Hz. The fast-sampling images enable us to analyze differential images every sampling interval (0.03 s). The differential images show that the initial brightening at the first 1 second had a longitudinal size of ~30 km. One of the brightening edges expanded westward with an average speed of ~20 km/s at the first 10 s, forming a longitudinal ripple-like structure. The speed gradually decreased to ~10 km/s in the following 10 s. Two-dimensional Fourier analyses of the wave number are applied for auroral images with horizontal sizes of 256 km x 256 km. The results show that the brightening auroras are subdivided into smaller scales, especially during the first 10 s of the onset. The first harmonic structures with a wave number k=3, or a wavelength ~80 km, appeared in 4 s after the onset, and the second one with a wave number k=6, or a wavelength ~40 km, appeared in 8 s after the onset. These harmonic structures are similar to those indicated by a simulation of inertial Alfven waves in the ionospheric Alfven resonator by Lysak and Song . Considering the excitation of higher modes, a decrease in the auroral expanding (phase) speed as above is consistent with the dispersion relation of inertial Alfvén waves. We suppose that auroral fine structures just after the onset can be formed by interferences of inertial Alfven waves in higher altitudes.
A Mechanism for Filamentation and Electron Acceleration Within Expansion-Phase Auroras
The M-I coupling medium prior to substorm onset is reasonably characterized by a large-scale field aligned current (FAC) sheet (or sheets) coupling the magnetosphere and ionosphere, plus earthward convection, or equivalently, a quasi-steady large-scale dawn-dusk electric field Ed-d. Imagine now an enhancement in the FAC. This has the effect of displacing the current-carrying field lines in longitude, meaning in the direction of Ed-d, causing electric potential to vary along B. Ideally, these field-aligned potential gradients would be nullified immediately by currents parallel to B. However, finite electron mass not only allows field-aligned electric fields within convecting current sheets to persist indefinitely, the resulting FAC enhancements tend to reinforce the initial perturbation. A non-linear two-fluid model of this system [ Knudsen, J. Geophys. Res., 101, 10761, 1996] shows that perturbations in field-aligned potential drop, electron energy and FAC intensity grow in the direction of the convection flow, then saturate nonlinearly and return to their undisturbed value further downstream. As these oscillations represent strong enhancements in electron energy flux, they would be visible as spatially periodic enhancements in auroral brightness. In the absence of any resistivity mechanism other than electron inertia, brightenings recur with a spatial period that can range from one to tens or even hundreds of electron inertial lengths, meaning hundreds of meters to tens of km in the direction perpendicular to B when projected to ionospheric heights. Importantly, this spatial structure is determined by intrinsic properties of the M-I coupling medium, and not by a distant source. This is one aspect that distinguishes these "stationary inertial Alfvén waves" (StIAW) from conventional, time- oscillating inertial Alfvén waves, another being that electron acceleration within StIAW can reach many times the Alfvén speed in principle. While these structures have been proposed as explanations for pre-breakup, quasi-static auroral arcs, one can expect that they would intensify and multiply rapidly within the drastically enhanced FAC and convection that characterize substorm breakup.
Magnetosphere-Ionosphere Coupling due to shear Alfven waves: Implications for Substorm Onset
Shear Alfven waves are a natural plasma mode which propagates along the magnetic field, transmitting energy and information. In the Earth's magnetosphere, shear Alfven waves have been shown to be related to substorm onset, perhaps being the medium by which information regarding the initiation region may propagate to the ionosphere. There may also be a direct relationship between shear Alfven waves and the auroral signatures which signify substorm onset. Numerical simulations and laboratory experiments have demonstrated that shear Alfven waves of short perpendicular extent can support parallel electric fields which accelerate electrons in collisionless plasma typical of magnetospheric conditions. Recent observations using data from the Canadian Geospace Monitoring (CGSM) array and THEMIS ground-based observatories have shown that the onset of ULF wave activity and structuring of the breakup arc can occur to within ∼ 10s of ULF wave activity at geosynchronous orbits, posing the question as to how this rapid magnetosphere- ionosphere coupling can occur. Shear Alfven waves may provide the key: either through direct propagation, or by accelerating electrons in the magnetosphere which communicate the initiation process to the ionosphere. In this talk, we will present simulation results from a self-consistent drift-kinetic numerical model in a night- side magnetic field topology. The simulation code (DK-1D) has been successfully modified to included inhomogeneities in the magnetic field and ambient plasma. The simulation follows the interaction between shear Alfven waves and electrons as they travel through the warm plasma of the central plasma sheet/plasma sheet boundary layer towards the ionosphere. We will also present case studies using a combination of CGSM and THEMIS observations which, together with the simulation results, will address this fundamental question.
Coordinated Observations of Auroral Arcs with ALIS and EISCAT
In March 2008, we carried out a coordinated observation campaign of auroral arcs between the European Incoherent Scatter Radar (EISCAT) located in Tromsö, Norway, and the Auroral Large Imaging System (ALIS) located near Kiruna, Sweden. The ALIS network consists of 5 ground-based stations equipped with optical cameras observing simultaneously the same volume of the sky located at altitudes around 90-100 km. From optical observations, we reconstruct the three-dimensional (3D) volume rate emissions of the aurora with tomographic-like inversion techniques and we retrieve a 2D map (in longitude and latitude) of the energy spectra of precipitating electrons at the top of the ionosphere. From radar observations, we can also infer the energy spectrum of electrons but only along the magnetic field line (1D). These results are compared to test the assumptions used in the models as well as the reconstruction techniques. We use the energy spectrum of electrons deduced from ALIS data as input to TRANS4 (a proton-electron kinetic/fluid transport code) to simulate the density and temperature profiles observed by EISCAT. The electron energy fluxes are then used to obtain the 2D field-aligned potential drops between the upper ionosphere and the magnetosphere by using a Knight-like relationship.
Coexistence of distinct power-law regimes in Self Organized Model for the Magnetosphere
It is now argued that the Central Pasma Sheet (CPS) may behave like a Self-Organized Critical (SOC) system,
driven by the the solar wind. The power law distributions for the sizes, energy and durations of substorms that
are reflected in observations can be reproduced using such SOC models. However, recent observations made
with the POLAR-UVI instrument showed that there is in fact two distinct regimes in substorms energies : small
and big events scales as different power laws, the smaller events having a steeper slope. We used a 2D-SOC
model subject to a deterministic driving, with conservative redistributions laws. We where able, with a slow
driving together with a small dissipation in energy redistribution, to reproduce the coexistence of these two
scaling regimes. The computation of the waiting-times, under the imposition of a threshold, showed truncated
exponentials distributions, which is consistent with observations. Finally, we computed statistics of substorms
depending on their onset position, and found that the southward mapping events tends to exhibit the dual
power-law scaling, while a single slope statistic was found for northward mapping substorms, which is again
consistent with recent observations.