Fermi acceleration of auroral particles

A number of nighttime acceleration mechanisms proposed in the literature are reviewed and rejected for the role of producing the kev nighttime auroral-particle fluxes. Parallel electric fields are rejected for several reasons, but particularly because of the observed simultaneous precipitation of electrons and protons. Acceleration in the neutral sheet is inadequate for producing the particle energies, the observed field-aligned pitch-angle distribution at high latitudes, and the spectral hardening toward lower latitudes. Neutral point mechanisms, although often suggested in principle, have never been demonstrated satisfactorily in theory or in practice. Pitch-angle scattering from a trapped population produced by transverse adiabatic compression is also incapable of producing the field-aligned distribution. We therefore suggest that longitudinal or Fermi acceleration, which results from the known magnetospheric convection, is the main nighttime auroral acceleration mechanism. The argument is supported by data obtained with the soft-particle spectrometer on Isis 1. In particular, the rotational scanning provided by the spinning satellite shows the regular change from a field-aligned pitch-angle distribution at high latitudes to a trapped distribution at low latitudes, reflecting a change from Fermi acceleration on the distant taillike field lines to betatron acceleration on the more dipolar field lines closer to the earth. The operation of the Fermi mechanism requires multiple bounces between hemispheres, and therefore a loss cone must develop at both 0° and 180°; such distribution is shown by the Isis 1 data when the range of the pitch-angle scan is sufficiently wide. One other feature is evident in the data: a hardening of the spectrum at small pitch angles. This energy-dependent pitch-angle distribution has a characteristic signature on the energy-time spectrograms that is designated as a Λ structure; when both loss cones show, the characteristic signature is a topless Λ structure. Adiabatic compression thus plays a vital role in auroral and magnetospheric substorms; longitudinal or Fermi acceleration causes auroral-particle precipitation, whereas transverse or betatron acceleration is responsible for lower-latitude phenomena, such as the ring current.

[1]  L. Block.  Acceleration of Auroral Particles by Electric Double Layers , 1972 .

[2]  H. Rème,et al.  Evidence near the auroral ionosphere of a parallel electric field deduced from energy and angular distributions of low‐energy particles , 1971 .

[3]  D. Gurnett,et al.  Distributions of plasmas and electric fields over the auroral zones and polar caps , 1971 .

[4]  T. Davis.  Magnetospheric convection pattern inferred from magnetic disturbance and auroral motions , 1971 .

[5]  F. Mozer,et al.  Electric fields in the nighttime and daytime auroral zone , 1971 .

[6]  G. Paulikas.  The patterns and sources of high‐latitude particle precipitation , 1971 .

[7]  F. Mozer,et al.  Relationship between magnetospheric electric fields and the motion of auroral forms , 1971 .

[8]  J. Cornwall,et al.  UNIFIED THEORY OF SAR ARC FORMATION AT THE PLASMAPAUSE. Report for July 1970--July 1971. , 1971 .

[9]  C. Mcilwain,et al.  PLASMA CLOUDS IN THE MAGNETOSPHERE. , 1971 .

[10]  L. Frank,et al.  Observations of charged particle precipitation into the auroral zone , 1971 .

[11]  L. Frank.  Comments on a proposed magnetospheric model , 1971 .

[12]  W. Riedler,et al.  Observations of magnetic-field aligned anisotropy for 1 and 6 keV positive ions in the upper ionosphere , 1971 .

[13]  N. Brice,et al.  Influence of magnetospheric convection and polar wind on loss of electrons from the outer radiation belt , 1971 .

[14]  J. Winningham,et al.  Penetration of magnetosheath plasma to low altitudes through the dayside magnetospheric cusps , 1971 .

[15]  E. W. Hones,et al.  Energy spectra and angular distributions of particles in the plasma sheet and their comparison with rocket measurements over the auroral zone , 1971 .

[16]  P. Lindstrom,et al.  Auroral-Particle Precipitation and Trapping Caused by Electrostatic Double Layers in the Ionosphere , 1970, Science.

[17]  B. O'Brien.  CONSIDERATIONS THAT THE SOURCE OF AURORAL ENERGETIC PARTICLES IS NOT A PARALLEL ELECTROSTATIC FIELD. , 1970 .

[18]  N. Ness,et al.  Configuration of the geomagnetic tail during substorms , 1970 .

[19]  J. Winningham,et al.  The Soft Particle Spectrometer in the ISIS‐I Satellite , 1970 .

[20]  W. Heikkila.  Satellite Observations of Soft Particle Fluxes in the Auroral Zone , 1970, Nature.

[21]  J. W. Chamberlain.  Electric acceleration of auroral particles , 1969 .

[22]  G. Haskell.  Anisotropic fluxes of energetic particles in the outer magnetosphere. , 1969 .

[23]  Charles F. Kennel,et al.  Consequences of a magnetospheric plasma , 1969 .

[24]  J. Mihalov,et al.  The cislunar geomagnetic tail gradient in 1967 , 1968 .

[25]  N. Brice.  Bulk motion of the magnetosphere , 1967 .

[26]  J. H. Piddington.  A theory of auroras and the ring current , 1967 .

[27]  P. Serlemitsos.  LOW-ENERGY ELECTRONS IN THE DARK MAGNETOSPHERE , 1966 .

[28]  Charles F. Kennel,et al.  LIMIT ON STABLY TRAPPED PARTICLE FLUXES , 1966 .

[29]  J. H. Piddington.  The magnetosphere and its environs , 1965 .

[30]  G. Siscoe,et al.  Aerodynamic aspects of the magnetospheric flow , 1964 .

[31]  C. Hines.  The energization of plasma in the magnetosphere: Hydromagnetic and particle-drift approaches , 2013 .

[32]  E. N. Parker,et al.  THE SOLAR FLARE PHENOMENON AND THE THEORY OF RECONNECTION AND ANNIHILATION OF MAGNETIC FIELDS , 1963 .

[33]  Harry E. Petschek,et al.  Magnetic Field Annihilation , 1963 .

[34]  C. O. Hines,et al.  A UNIFYING THEORY OF HIGH-LATITUDE GEOPHYSICAL PHENOMENA AND GEOMAGNETIC STORMS , 1961 .

[35]  J. Dungey.  Interplanetary Magnetic Field and the Auroral Zones , 1961 .

[36]  P. Sweet.  14. The neutral point theory of solar flares , 1958 .

[37]  P. A. Sweet,et al.  The production of high energy particles in solar flares , 1958 .