Particle injections with auroral expansions

Abstract

We compare the onset of dispersionless energetic particle injections, observed as a sudden increase of energetic (tens to hundreds of keV) electron and ion fluxes on a timescale of ∼1 min, with the start of auroral breakups. A total of 34 dispersionless injections observed by Los Alamos National Laboratory (LANL) satellites are analyzed, and their corresponding auroral breakups are determined with global auroral images acquired from the Polar ultraviolet imager. An important finding is that dispersionless injections can actually be associated with substorm intensification. The injection time at LANL relative to the start of auroral breakups varies from −2 to 8 min and can sometimes be more than 10 min. The average lag time for the injections compared to the auroral breakups is 1.8 min with a standard deviation of 2.5 min. It is suggested that particle energization must take place in the magnetotail ∼l min earlier than the start of the explosive auroral substorm onset, while the delay of the injections at LANL is due to a propagation effect. An implied average earthward injection boundary is estimated to be ∼ 6.9 – 9.2 RE. Further analysis of the delay time indicates that the transport of substorm injections is associated with the enhancement of convection electric field by a factor of ∼5, corresponding to an earthward convection flow speed of 5 – 120 km s−1. Dispersionless injections can take place in a fairly wide magnetic local time (MLT) region from 2000 to 0100 MLT with a peak at 2200 MLT, where auroral breakups occur most frequently. More importantly, dispersionless injections have ionospheric footprints clustered around the location of auroral breakup within ±1 hour of MLT, further supporting the concept of the close relationship between the substorm injections and the auroral breakups.