Small-Scale Dynamic Aurora

Small-scale dynamic auroras have spatial scales of a few km or less, and temporal scales of a few seconds or less, which visualize the complex interplay among charged particles, Alfvén waves, and plasma instabilities working in the magnetosphere-ionosphere coupled regions. We summarize the observed properties of flickering auroras, vortex motions, and filamentary structures. We also summarize the development of fundamental theories, such as dispersive Alfvén waves (DAWs), plasma instabilities in the auroral acceleration region, ionospheric feedback instabilities (IFI), and the ionospheric Alfvén resonator (IAR)

Downward current electron beam observed by Cluster and FAST

[1] We report observations from a conjunction of FAST and Cluster during an interval of downward current at an MLT of 3-4 h on field lines mapping to the PSBL. Both spacecraft see upgoing electrons with an energy of a few hundred eV, suggesting substantial acceleration has occurred below FAST's altitude of 3200 km. At Cluster, isolated bursts of electrons are seen, and modeling indicates that the current mapped from the ionosphere exists as a collection of current filaments at Cluster (4-5 R-E). The current filaments are aligned with the background magnetic field and have a perpendicular scale at Cluster of about 100 km (which maps to 10-20 km in the ionosphere), and is similar to the mapped width observed by FAST. The electron beams are quasi-steady during a Cluster spacecraft transit time of 1 min. The field aligned current densities at FAST and Cluster are of the order of a few mu Ams(-2) and 0.05 mu Am-2, respectively, and j/B is conserved along a current filament.</p

Energy Transport and Conversion Within Earth's Supercritical Bow Shock: The Role of Intense Lower‐Hybrid Whistler Waves

International audienceDetailed analysis of a high Mach number quasiperpendicular Earth bow shock crossing by the Magnetospheric Multiscale (MMS) spacecraft fleet reveal that lower‐hybrid (LH) whistler waves generated in the shock foot region transport energy predominately along the shock surface and slightly toward the shock ramp in the shock normal incidence frame, where wave energy accumulates and is dissipated into the plasma. This suggests the LH whistlers play an integral role in energy reconfiguration at high Mach number collisionless shocks with ramifications to plasma heating. The multipoint observations are used to quantify the wave characteristic parameters (via interferometry), Poynting fluxes, and energy conversion rates D, and to assess their scale dependencies and spatial and temporal properties. The whistler associated energy transport and conversion are found to depend on scale and location within the layer. High‐frequency electrostatic waves yield largest values of D. However, the dominant net energy exchange contribution is from the LH whistlers. In the foot spatially temporally coherent net energy exchange from the plasma to whistlers is observed, whereas deeper in the ramp net wave energy dissipation to the plasma is observed exhibiting significant space‐time variability. These results are consistent with the modified two stream instability driven by the relative drift between reflected ions and electrons as the mechanism for wave growth in the foot. Owing to strong electron heating, whistler energy dissipation in the ramp is attributed to Landau damping, which out‐competes the destabilizing effect of the reflected ion and electron drift

Birth and life of auroral arcs embedded in the evening auroral oval convection: A critical comparison of observations with theory

International audienceWe present and analyze data on auroral arcs obtained during a pass of the FAST satellite over the field-of-view of the all-sky camera at Ft. Simpson (Canada), supported by ground-based magnetometer and SuperDARN radar data, and plasma data from THEMIS-A near the source region of the auroral currents. The auroral event took place at 19:00 MLT during substorm activity further east. Active auroral arcs were present over six degrees in latitude moving equatorward with significant changes in brightness and structure. New arcs were forming continuously at the polar border of the auroral oval which was marked by an Alfvénic arc. The data analysis revealed that the equatorward drift of the arcs was in part due to convective motion of the plasma frame but was rather dominated by proper motions of the arcs. Interpretation of these findings in the framework of theoretical work by one of the authors reproduces quantitatively the observed proper motion as a consequence of the progressive erosion of magnetic shear stresses. Most important was the possibility to deduce the interaction time scale between arc and source region. On average it corresponded to about six to eight transit times of an Alfvén wave between arc and source plasma or two fundamental eigenperiods of toroidal mode or azimuthally polarized Alfvén waves. However, large variations of the interaction times and corresponding proper motions were found. They are attributed to temporal and spatial variations of the energy input from the source plasma. The more remarkable is the fact that analysis on the basis of a quasi-stationary model produces consistent results. The progressive release of shear stresses during the equatorward motion of the arcs leads to the conclusion that they are dying after having reached the maximum of the poleward Pedersen current