Abstract
Abstract. We present results of an investigation of the fine structure of the night sector diffuse auroral zone, observed simultaneously with optical instruments (ALIS) from the ground and the FAST electron spectrometer from space 16 February 1997. Both the optical and particle data show that the diffuse auroral zone consisted of two regions. The equatorward part of the diffuse aurora was occupied by a pattern of regular, parallel auroral stripes. The auroral stripes were significantly brighter than the background luminosity, had widths of approximately 5 km and moved southward with a velocity of about 100 m/s. The second region, located between the region with auroral stripes and the discrete auroral arcs to the north, was filled with weak and almost homogeneous luminosity, against which short-lived auroral rays and small patches appeared chaotically. From analysis of the electron differential fluxes corresponding to the different regions of the diffuse aurora and based on existing theories of the scattering process we conclude the following: Strong pitch angle diffusion by electron cyclotron harmonic waves (ECH) of plasma sheet electrons in the energy range from a few hundred eV to 3–4 keV was responsible for the electron precipitation, that produced the background luminosity within the whole diffuse zone. The fine structure, represented by the auroral stripes, was created by precipitation of electrons above 3–4 keV as a result of pitch angle diffusion into the loss cone by whistler mode waves. A so called "internal gravity wave" (Safargaleev and Maltsev, 1986) may explain the formation of the regular spatial pattern formed by the auroral stripes in the equatorward part of the diffuse auroral zone.
Highlights
Diffuse aurora is the most common and the most widespread type of aurora
We here only consider the images from the three stations that observe the diffuse auroral zone, though, as it is seen from Fig. 1, the Kiruna camera, that was directed northward, provided images of the discrete auroral arcs
From the calculated emission intensity it is seen that the plasma sheet electrons create homogenous luminosity in the whole diffuse zone, while the discrete auroral features are created by fluctuations of the high energy part of the downward electron spectra
Summary
Diffuse aurora is the most common and the most widespread type of aurora. The optical definition of diffuse aurora was introduced by Lui and Anger (1973). Peticolas et al (2002) studied black aurora using FAST plasma and field measurements and optical TV observations and found that black aurora occurred in narrow spatial regions where electrons had partial double loss cones, while the electrons in the ambient broad region of diffuse aurora were characterized by single loss cones. Using data from the wave and particle experiments on the CRRES satellite, Meredith et al (2000) studied the evolution of electron pitch angle distributions in the equatorial plane and their relation to ECH and whistler mode wave activity They could show that both types of waves are responsible for the electron precipitation causing the diffuse aurora. The term black aurora is used for relatively small, well defined regions with a distinct reduction of luminosity within large-scale regions of otherwise homogeneous, diffuse aurora
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