Polar/Toroidal Imaging Mass‐Angle Spectrograph survey of earthward field‐aligned proton flows from the near‐midnight tail

Abstract

Data from the Toroidal Imaging Mass‐Angle Spectrograph (TIMAS) ion mass spectrometer on the Polar satellite, covering 15 eV e−1 to 33 keV e−1 in energy and essentially 4 π in view angles, are used to investigate the properties of earthward (sunward) field‐aligned flows of ions, especially protons, in the plasma sheet‐lobe transition region near local midnight. A total of 142 crossings of this region are analyzed at 12‐s time resolution, all in the Northern Hemisphere, at R(SM) ∼4–7 RE, and most (106) in the poleward (sunward) direction. Earthward proton flows are prominent in this transition region (>50% of the time), typically appearing as sudden “blasts” with the most energetic protons (∼33 keV) arriving first with weak flux, followed by protons of decreasing energy and increasing flux until either (1) a new “blast” appears, (2) the flux ends at a sharp boundary, or (3) the flux fades away within a few minutes as the mean energy drops to a few keV. Frequent step‐like changes (<12 s) of the flux suggest that perpendicular gradients on the scale of proton gyroradii are common. Peak flux is similar to central plasma sheet proton flux (105–106 (cm2 sr s keV/e)−1) and usually occurs at E ∼ 4–12 keV. Only the initial phase of each “blast” (∼1 min) displays pronounced field alignment of the proton velocity distribution, consistent with the time‐of‐flight separation of an otherwise more or less isotropic distribution (at Polar altitude) with dƒ/dv < 0. The temporal dispersive signatures are often consistent with a source at R(SM) ≤ 30 RE. No systematic latitudinal velocity dispersion is found, implying that the equatorial plasma source is itself convecting. In short, the proton “blasts” appear as sudden local expansions of central plasma sheet particles along reconfigured (“dipolarized”) magnetic field lines.