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

Abstract

[1] 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 À1 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 À1 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.

Energy-dispersed ions in the plasma sheet boundary layer and associated phenomena: Ion heating, electron acceleration, Alfvén waves, broadband waves, perpendicular electric field spikes, and auroral emissions

“…A similar association of enhanced large pitch angle ion outflow and velocity-dispersed ions was recently reported in a Polar-spacecraft-based study at distances of 6 Re and higher (Lennartsson, 2003). In addition, here we found that the enhanced conical outflow coincided with the edges (boundaries) of energetic magnetotail ion beamlets.…”

“…Some observational evidence has been reported that ion beams could lead to broadband electrostatic noise (BEN) (e.g., Gurnet et al, 1976;Grabbe and Eastman, 1984) via beam-related instabilities. Magnetotail ion beams have been invoked for the acceleration and heating of ionospheric ion outflow to form ion beams and conics, respectively (Alfvén and Fälthammar, 1963;Schriver et al, 2003;Lennartsson, 2003;Janhunen et al, 2003). Furthermore, Elphinstone et al (1995) associated velocity-dispersed ion structures (VDIS) with the double oval, and Janhunen et al (2003) suggested that the free energy associated with ion shell distributions in the PSBL could lead in a sequence of events to the acceleration of auroral electrons causing auroral arcs.…”

Solar wind control of Earth's H+ and O+ outflow rates in the 15‐eV to 33‐keV energy range

“…Under more normal and near-steady conditions the centrifugal acceleration is not expected to generate keV energies until the ions reach the strongly curved portion of field lines at the tail midplane [see Cladis, 1986, Figure 2; see also Delcourt et al, 1988Delcourt et al, , 1989. Furthermore, this mechanism by itself generates field-aligned velocity distributions, whereas the TIMAS observations very often show broad pitch angle distributions of up-flowing ions at all altitudes [Lennartsson, 2003].…”

“…Such a mechanism may involve both transverse ion acceleration, creating ''newly born conics'' in the parlance of Miyake et al [1993], and the parallel kind. A random-type, narrow-scale (filaments) mechanism to fit that mold is outlined by Lennartsson [2003], although its specific purpose is to help explain the collocation of transversely accelerated H + , He + , and O + ions with transient bursts of hot H + ions from the near-midnight tail, rather than with cusp ion bursts. This mechanism would be powered by the kinetic energy flux associated with the H + bursts (and with accompanying electrons).…”

“…This probably has the greatest impact on the H + samplings, especially the samplings of H + energy flow density, since these bursts are 10-30 times more energetic than the typical ionosphere origin H + ions. These bursts are, in fact, regularly associated with enhanced outward flows of ionosphere origin H + , O + , and He + ions [Lennartsson, 2003], but the net field-aligned number flow densities are normally dominated by the latter ions [see also Yau et al, 1985b]. A related but different issue with ion flows in the high-altitude (northern) cusp is dealt with in section 5.2.2.…”