Dawn-dusk asymmetries in the auroral particle precipitation and their modulations by substorms

textabstractAuroral particle precipitation exhibits dawn-dusk asymmetries that reflect the asymmetries in the particle populations, waves, and processes in the magnetosphere. The diffuse auroral electrons can be observed mainly in 22:00 – 09:00 MLT, which coincides much with the spatial distribution of the whistler-mode chorus waves that have been shown to be the predominant mechanism for pitch-angle scatterring magnetospheric electrons into the loss cone. On the other hand, the monoenergetic auroral electrons can be observed at dusk-midnight sector. The monoenergetic electrons are magnetospheric electrons that have gone through a quasi-static parallel electric field in the upward field-aligned current regions. The broadband auroral electrons can be found mostly at 22:00 – 02:00 MLT where a peak in the Poynting flux of Alfven waves is observed. Alfven waves are known to cause broadband acceleration of electrons. There may be a connection between monoenergetic and broadband electrons in that the low frequency Alfven wave–electron interaction can result in monoenergetic electron signature. Substorms increase the power of the diffuse, monoenergetic, and broadband electron aurora by 310%, 71%, and 170%, respectively. The duration of the substorm cycle for monenergetic and broadband auroral is ~5 hr, but it is larger than 5 hr for diffuse auroral electrons.