Bursty bulk flow intrusion to the inner plasma sheet as inferred from auroral observations

Abstract

The physics of the interaction of bursty bulk flows (BBFs) with the near‐Earth plasma is not yet known in detail. We address this issue with comprehensive observations made with the MIRACLE network during a well‐documented streamer event. In this case an equatorward intruding auroral streamer inclined from northwest (NW) to southeast (SE) reached the region of the proton oval of 486 nm (Hβ) luminosity. This Hβ band is assumed to map poleward of the ≥30 keV proton isotropic boundary, which is at geocentric distances ∼7–8 RE according to the appropriate magnetic field models. A significant enhancement (30%) in the Hβ luminosity was recorded just at the time and in the location where the equatorward end of the streamer reached the Hβ band. This enhancement implies a corresponding pressure enhancement in the equatorial magnetosphere. The simultaneous poleward shift of the Hβ emission suggests that also a magnetic field dipolarization took place in the same region. An NW‐aligned electrojet (300–400 km in width) accompanied the equatorward development of the streamer (with streamer locating at the dusk flank of this electrojet), and it produced a sharp negative magnetic bay (∼300 nT) resembling a substorm onset. Both auroral and equivalent current observations suggest that the plasma jet was not completely dissolved or stopped during this interaction, but rather diverted in the azimuthal direction. Equivalent currents reconstructed from ground magnetic field observations indicate that a R1‐type field‐aligned current system (upward current ∼0.5 MA) accompanied the streamer development. However, our observations do not show any signatures of a R2‐type current system expected to build up during the flow braking and diversion according to previously published simulation studies.