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

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

Apr 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001jgr...106.5859l&link_type=abstract

Journal of Geophysical Research, Volume 106, Issue A4, p. 5859-5872

Physics

13

Magnetospheric Physics: Auroral Phenomena, Magnetospheric Physics: Magnetotail Boundary Layers, Magnetospheric Physics: Plasma Sheet, Magnetospheric Physics: Storms And Substorms

Scientific paper

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 4π 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)~4-7RE, 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(cm2srskeV/e)-1) and usually occurs at E~4-12keV. 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 df/dv<0. The temporal dispersive signatures are often consistent with a source at R(SM)<=30RE. 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.

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