[1] We use magnetometer chains collaborating with SuperMAG to derive SME, a generalization of the auroral electrojet indices calculated from 100 or more sites instead of the 12 used in the official auroral electrojet indices, AE(12) = AU(12) − AL(12). We investigate how these various indices relate to nightside auroral power by using both particle (DMSP) and image (Polar Ultraviolet Imager (UVI)) data. The best correlation is between SME and total nightside auroral power, namely, r = 0.86. Hence, nearly 3/4 of the minute-by-minute variance in nightside power can be determined by SME alone. Interestingly, although the geophysical meaning of AE(12) has sometimes been challenged, we show that even that index correlates at the r = 0.81 level, or 2/3 of the variance, in nightside power. Most auroral power stems from the diffuse aurora, with a linear relationship between the auroral electrojet indices and nightside diffuse power. Thus, SME has a clear geophysical meaning: It samples the thermal portion of the magnetotail plasma sheet. We study how well SML (the generalization of AL) identifies substorms in two types of tests. The first is a comparison with a set of 1081 substorms determined from Polar UVI in 1997–1998, and the second is the use of DMSP particle precipitation data for superposed epoch analysis. The same algorithm applied to SML is much more likely (about 50% more likely) to identify an onset seen by UVI than is AL(12). Even when both indices can be used to identify onsets, the median delay after imaging onset until the AL indicator is less than half using SML (about 4 min versus 8 min). There are 10,719 onsets in the SML data between 1 January 1997 and 31 December 2002, of which 5084 are isolated. Isolated SML onsets behave almost identically to the onsets determined by global imagers. Specifically, they represent the same sharp spike in auroral power, which is most pronounced in broadband (wave) precipitation, with the same duration and subsequent recovery. However, recurrent substorms (those following less than 2 h after a previous onset) rise from a higher baseline by a smaller percentage but with the same absolute change in auroral power, thus reaching a higher peak power.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2011JA016779

Evaluation of SuperMAG auroral electrojet indices as indicators of substorms and auroral power

The impulsive phase of a solar flare marks the epoch of rapid conversion of energy stored in the preflare coronal magnetic field. Hard X-ray observations imply that a substantial fraction of flare energy released during the impulsive phase is converted to the kinetic energy of mildly relativistic electrons (10-100 keV). The liberation of the magnetic free energy can occur as the coronal magnetic field reconfigures and relaxes following reconnection. We investigate a scenario in which products of the reconfiguration?large-scale Alfv?n wave pulses?transport the energy and the magnetic field changes rapidly through the corona to the lower atmosphere. This offers two possibilities for electron acceleration. First, in a coronal plasma with -->? < me/mp, the waves propagate as inertial Alfv?n waves. In the presence of strong spatial gradients, these generate field-aligned electric fields that can accelerate electrons to energies on the order of 10 keV and above, including by repeated interactions between electrons and wave fronts. Second, when they reflect and mode-convert in the chromosphere, a cascade to high wavenumbers may develop. This will also accelerate electrons by turbulence, in a medium with a locally high electron number density. This concept, which bridges MHD-based and particle-based views of a flare, provides an interpretation of the recently observed rapid variations of the line-of-sight component of the photospheric magnetic field across the flare impulsive phase, and offers solutions to some perplexing flare problems, such as the flare number problem of finding and resupplying sufficient electrons to explain the impulsive-phase hard X-ray emission.

/pdf/impulsive-phase-flare-energy-transport-by-large-scale-alfven-1usqe2ljpv.pdf

Impulsive phase flare energy transport by large-scale Alfven waves and the electron acceleration problem

The nature of small-scale turbulent fluctuations in the solar wind is investigated using a comparison of Cluster magnetic and electric field measurements to predictions arising from models consisting of either kinetic Alfven waves or whistler waves. The electric and magnetic field properties of these waves from linear theory are used to construct spacecraft-frame frequency spectra of (|δE|/|δB|) s/c and (|δB ∥|/|δB|) s/c , allowing for a direct comparison to spacecraft data. The measured properties of the small-scale turbulent fluctuations, found to be inconsistent with the whistler wave model, agree well with the prediction of a spectrum of kinetic Alfven waves with nearly perpendicular wavevectors.

https://iopscience.iop.org/article/10.1088/2041-8205/745/1/L9/pdf

Identification of kinetic alfven wave turbulence in the solar wind

We present a statistical study of the temporal- and spatial-scale characteristics of different field-aligned current (FAC) types derived with the Swarm satellite formation. We divide FACs into two classes: small-scale, up to some 10 km, which are carried predominantly by kinetic Alfven waves, and large-scale FACs with sizes of more than 150 km. For determining temporal variability we consider measurements at the same point, the orbital crossovers near the poles, but at different times. From correlation analysis we obtain a persistent period of small-scale FACs of order 10 s, while large-scale FACs can be regarded stationary for more than 60 s. For the first time we investigate the longitudinal scales. Large-scale FACs are different on dayside and nightside. On the nightside the longitudinal extension is on average 4 times the latitudinal width, while on the dayside, particularly in the cusp region, latitudinal and longitudinal scales are comparable.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014GL062453

Field‐aligned currents' scale analysis performed with the Swarm constellation

We review basic theoretical concepts in particle acceleration, with particular emphasis on processes likely to occur in regions of magnetic reconnection. Several new developments are discussed, including detailed studies of reconnection in three-dimensional magnetic field configurations (e.g., current sheets, collapsing traps, separatrix regions) and stochastic acceleration in a turbulent environment. Fluid, test-particle, and particle-in-cell approaches are used and results compared. While these studies show considerable promise in accounting for the various observational manifestations of solar flares, they are limited by a number of factors, mostly relating to available computational power. Not the least of these issues is the need to explicitly incorporate the electrodynamic feedback of the accelerated particles themselves on the environment in which they are accelerated. A brief prognosis for future advancement is offered.

Recent Advances in Understanding Particle Acceleration Processes in Solar Flares

This paper presents a comprehensive review of dispersive Alfven waves in space and laboratory plasmas. We start with linear properties of Alfven waves and show how the inclusion of ion gyroradius, parallel electron inertia, and finite frequency effects modify the Alfven wave properties. Detailed discussions of inertial and kinetic Alfven waves and their polarizations as well as their relations to drift Alfven waves are presented. Up to date observations of waves and field parameters deduced from the measurements by Freja, Fast, and other spacecraft are summarized. We also present laboratory measurements of dispersive Alfven waves, that are of most interest to auroral physics. Electron acceleration by Alfven waves and possible connections of dispersive Alfven waves with ionospheric-magnetospheric resonator and global field-line resonances are also reviewed. Theoretical efforts are directed on studies of Alfven resonance cones, generation of dispersive Alfven waves, as well their nonlinear interactions with the background plasma and self-interaction. Such topics as the dispersive Alfven wave ponderomotive force, density cavitation, wave modulation/filamentation, and Alfven wave self-focusing are reviewed. The nonlinear dispersive Alfven wave studies also include the formation of vortices and their dynamics as well as chaos in Alfven wave turbulence. Finally, we present a rigorous evaluation of theoretical and experimental investigations and point out applications and future perspectives of auroral Alfven wave physics.

/pdf/small-scale-alfvenic-structure-in-the-aurora-3hopdgfj09.pdf

J. Maggs

Papers

Small scale alfvénic structure in the aurora