In situ Polar observation of transverse cold‐ion acceleration: Evidence that electric field generation is a hot‐ion finite gyroradii effect

Recent results with the Polar TIMAS instrument (ion mass spectrometer) have revealed that earthward magnetic field‐aligned and velocity‐dispersed flows of 1‐ to 33‐keV protons (and other ions) from the outer magnetosphere are inherently “blast‐like” and exhibit a complex filamentary structure where transverse scale sizes may often be a few proton gyroradii. These velocity‐dispersed protons (and accompanying hot electrons) have been found to have a close association with enhanced large pitch angle outflow of accelerated (sometimes to more than 10 keV) ionospheric ions (H + , O + , and He + ) at Polar. Theoretical considerations suggest a mechanism by which the difference in gyroradii between hot ions and electrons in earthward plasma bursts generates charge imbalance and strong transverse electric fields at density gradients, as the bursts move into an increasingly strong magnetic field, which in turn accelerate the ambient cold ions. Model electric fields greater than 1 V m −1 at Polar are readily generated with density gradient scale lengths of a few earth radii in an equatorial source region of the bursts, assuming that the magnetic moments of the burst protons are preserved. The nonadiabatic acceleration of cold ions may act to reduce these fields to the ca 0.1‐V m −1 amplitude and few‐second period fluctuations actually observed. Sample TIMAS data are shown to be consistent with the transverse acceleration of cold O + ions to keV energy on a timescale of less than a single gyroperiod.