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Journal ArticleDOI

Energetic electrons of terrestrial origin behind the bow shock and upstream in the solar wind

01 Jan 1969-Journal of Geophysical Research (John Wiley & Sons, Ltd)-Vol. 74, Iss: 1, pp 95-106

Abstract: Energetic electron fluxes associated with the earth's bow shock are found to be present about as often on the dawn side of the sun-earth line as on the dusk side. The peak fluxes attained by these spikes also show no dawn-dusk asymmetry. Upstream electron events, on the other hand, are predominantly found to the dawn side of the sun-earth line. Both phenomena have the same temporal character with characteristic times of 30 to 150 seconds. Both have characteristic energies of bout 15 kev, but the upstream electron fluxes are much weaker. The upstream events are interpreted to be of secondary origin with the bow shock spikes representing the primary acceleration event. This local acceleration process evidently is of no consequence to the problem of the Van Allen belts and auroral processes.
Topics: Bow shock (aerodynamics) (63%), Bow wave (59%), Van Allen radiation belt (56%), Magnetopause (53%), Shock wave (52%)

Summary (1 min read)

Jump to: [9.][DISCUSSION] and [10.]


  • Out to be isotropic, the hypothesis (An('erson, 1968) that fast-ii ► odc waver, propagating upstream from the shock cotiple to the solar wind to accelerate particles would be favored.


  • A remarkable feature of the bow shock spikes is that they have large intensity just behind the bow shock and rapidly decreasing intensity moving either toward or away from the magnetosphere .
  • This close confinement means that a relatively small total number of energetic particles can produce an appreciable flux without excessive demands on the source strength which presumably acts only in a sinall region behind the shock.
  • This idea was used by Jokipii and Davis (1964) in their theoretical analysis of the bow shock spikes.
  • Just what the magnetic field configuration is that produces a mirror geometry is less clear.
  • Ileppner, et al. (1967) have shown that at the shock the field strength rises by several gammas above the average magnetosheath field.


  • Evieleritly this only way to avoid the conclusion that the p; ► i•ticIvs are confined is to postulate a reversible effect by which particles ary accelerated behind the shock but when they leave this region they become deceleratcd.
  • They have a strong tendency to appear oil dawn :Hide as expected from the average; spiral character of the inLerplanetary magnetic field .
  • This kind of measurement could in principle, be made with two spacecraft.
  • They ranged from 2 to 6 minutes but a great deal of spectral power was also spread over longer periods.
  • The situation concerning bow shock spikes now appears to be that they represent an interesting case of particle acceleration by solar wind interaction with the earth's magnetosphere.

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July 30, 1965
Revised, September 11, 1968
K. A. Anderson
Physics Department and Space Sciences Laboratory
University of California
Berkeley, California
IS 6
*This work was supported in part by the National Aeronautics and
Space Administration under Contract NAS 5-9077 and Grant
NGL 05-003-017.

Energetic electron fluxes asaociated with the earth's bow shock
are found to be present about as often on the dawn side of the sun-
earth line as on the dusk side. The peak fluxes in these spikes
also show no dawn-dusk asymmetry. Upstream electron events on
the other hand are predominantly found to the d^wn side of the sun-
earth line. Both phenomena have the same temporal character
characteristic times of 30 to 150 seconds. Both have characteristic
energies of about 15 KeV but the upstream electron fluxes are much
weaker. The upstream events are interpreted to be of secondary ori-
gin with the bow shock spikes representing the primary acceleration
event. This local acceleratir
n process evidently is
no consequence
to the problem of the Van Allen belts and auroral processes.
N .

J .
Fan et al.
first attributed the existence of the energetic
electron spikes detached from the trapped radiation on the sunward
side of the earth to an effect of thr bow shock. A few months
later Frank and Van Allen (1964) described additional examples of
this phenomenon and noted their existence throughout much of the
magnetosheath region. They hypothesized that these energetic parti-
cles were the spectral tail of the hot electron component of the mag-
netosheath plasma.
Further observational studies of this phenomenon were reported
by Anderson, Harris and Paoli (1965) and by Anderson (1965).
This work confirmed the earlier studies but a still different hypothe-
sis for the origin of these particles was giver.. They thought that
trapped radiation could be sloughed off and then travel along field
lines through
the magnetosheath.
All these early
left un-
answered the
question of spatial
distribution of the
and their
precise relationship to the bow shock. In ] 966 both Anderson and
et al.
presented evidence showing that the spikes could not ori-
ginate as sloughed-off Van Allen radiation. In these 1966 studies a
relationship to the bow shock was demonstrated but the precise
location of the spikes with respect to the shock was not determined.
Also in
Lin and Anderson found that in a few cases the bow
shock spikes had the same quasi-periodic character as trapped
electrons especially in the skirt region. The observational picture
was greatly clarified following Binsack's (1966) observation that
bow shock spikes appeared in greatest number and in Largest
intensity when the magnetopause was in ra
id, large amplitude motion.
Anderson, Binsack and Fairfield (1968) then showed the magnetopause

motion was present most of the time, the spatial amplitude varied
from 0. 2 to 2 R
and that the motion was periodic or quasi-
periodic with peak spacings of 3 to 15 minutes. Furthermore, it
was shown that the bow shock moved with about the same period
and spatial amplitude as did the magnetopause and they concluded
that the magnetopause motion must be coherent over distance
10 R
. The nature of the large, coherent oscillations of the mag-
netopause is not known but they may be surface waves generated by
an instability of the Kelvin-Helmholtz type. An important consequence
of coherent magnetopause motion is that it will drive the shock in
and out with the same period and amplitude.
the magnetosphere
as a whole is a good hydromagnetic medium the surface oscillations
result in the propagation of h-m waves throughout much of the mag-
netosphere. Such waves result in a periodic modulation of trapped
energetic electron fluxes as
by Judge and Coleman (1962) and
by Lin and Anderson (1966). The energetic electron spikes were
shown to be .located in the magnetosheath just behind the shock. The
origin of the energetic alectron spikes was ascribed to dissipation of
energy in the hydromagnetic waves which populate the magnetosheath.
One more phenomenon may be fitted into this picture, Anderson,
Harris and Paoli (1965), Jokipii (1968) and Anderson (1968) have
shown that energetic electron spikes often appear far outside even
the most distant excursion of the bow shock. These particles most
likely originate as spikes just behind the bow shock, then are guided
to their upstream point of observation by field lines passing through
the bow shock into interplanetary space. Some no doubt rriove in
the other direction and account for some of the transient fluxes observed
in the magnetosheath (see Figure 1).
!+ , . '

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