Abstract
Abstract. We present the results of a statistical study using optical images from ALIS (Auroral Large Imaging System) to investigate the spatial and temporal variations of structures in diffuse aurora. Analysis of conjugate Reimei data shows that such fine structures are a result of modulation of high-energy precipitating electrons. Pitch angle diffusion into the loss cone due to interaction of whistler mode waves with plasma sheet electrons is the most feasible mechanism leading to high-energy electron precipitation. This suggests that the fine structure is an indication of modulations of the efficiency of the wave–particle interaction. The scale sizes and variations of these structures, mapped to the magnetosphere, can give us information about the characteristics of the modulating wave activity. We found the scale size of the auroral stripes and the spacing between them to be on average 13–14 km, which corresponds to 3–4 ion gyro radii for protons with an energy of 7 keV. The structures move southward with a speed close to zero in the plasma convection frame.
Highlights
Diffuse aurora is characterized by a broad area of fairly uniform luminosity
Recent studies have shown that the diffuse aurora is caused by precipitation of plasma sheet electrons with energies of a few to tens of keV that are scattered into the loss cone due to interaction with whistler mode waves (Ni et al, 2011a,b), while the more dynamic discrete aurora is caused by particles that are accelerated along the magnetic field lines
By fitting a Gaussian function to each peak, we were able to retrieve the full width at half maximum (FWHM), which corresponds to the stripe width
Summary
Diffuse aurora is characterized by a broad area of fairly uniform luminosity. Some might call it the least spectacular form of aurora, but it is the most energetically important one. Recent studies have shown that the diffuse aurora is caused by precipitation of plasma sheet electrons with energies of a few to tens of keV that are scattered into the loss cone due to interaction with whistler mode waves (Ni et al, 2011a,b), while the more dynamic discrete aurora is caused by particles that are accelerated along the magnetic field lines. Ebihara et al (2010) and Samara et al (2010), among others, have reported observations of smallscale structures in diffuse aurora caused by precipitation of non-accelerated high-energy electrons At first it was believed that these structures were small-scale discrete auroral structures, caused by accelerated electrons, but later studies show that they are variations of the modulation of the diffuse auroral precipitation. Ebihara et al (2010) and Samara et al (2010), among others, have reported observations of smallscale structures in diffuse aurora caused by precipitation of non-accelerated high-energy electrons
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