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Electric Fields in the Vicinity of Auroral Forms
From Motions of Barium Vapor Releases
By
Eugene M. Wescott*
John D. Stolarik
and
James P. Heppner
Laboratory for Space Sciences
NASA-Goddard Space Flight Center
Greenbelt, Maryland
* National Academy of Sciences, Resident Research Associate
ABSTRACT
In August - September, 1967, eleven barium vapor clouds were released
during evening twilight between invariant magnetic latitudes of 67.3
0
to
68.1
0
from And4ya, Norway. Two flights (8 releases) occurred during moderate
negative bays in H, while the third flight (3 releases) took place during
a positive bay in-H. Visual auroral displays were observed in the vicinity
during all flights. In the negative bay situation, barium ion cloud motions
3
were eastward, and closely parallel to auroral arc alignments. Electric
fields transverse to the magnetic field with intensities of 10 to 130 m
y
/m
3
directed southward were observed. During the positive bay event the barium
clouds spanned the breakup transition region, with the two equatorwards
clouds moving westward while the poleward cloud went east. Observed re-
versals in direction were closely correlated with magnetic variations.
North directed electric fields of up to 50 m
y
/m were found in the positive
bay sector. In all events the ion cloud motions revealed that E was per-
pendicular to the ionospheric current, hence we conclude that the auroral
electrojets, both eastward and westward, are essentially Hall currents. The
results illustrate that the magnitude of E driving ionospheric currents
cannot be deduced solely from ground magnetic observations because of the
variable ionospheric electrical conductivity. There is evidence that
while E is large near an auroral arc, the field within is very low. Large
gradients and/or irregularities in the E field are found to exist most of
the time. These are revealed on three different time-space scales: from
differences in velocity for parallel moving clouds, from velocity changes
along the path of a given cloud, and in the form of rayed structure within
a cloud.
1
t
s
3
INTRODUCTION
The development of a method for producing visible ion clouds in and
above the ionosphere has given geophysicists a powerful tool for investi-
gating ion drift motions, the fine structure of the ionosphere, electric
fields, ionospheric conductivity and magnetospheric convection. Investi-
gators at the
Max
Planck Institut, [Foppl et al., 1967] discovered that
barium vapor produced in a thermite reaction and released in the ionosphere
under solar ultraviolet radiation will form a long lasting ion cloud with
strong resonant lines in the visible spectrum. Such ion clou3c can be
formed with uncomplicated payloads from sounding rockets above about 130 km
altitude, and they can be observed and photographed under twilight conditions
as long as they are illuminated by solar radiation in the visible band.
The growth of the ion cloud after release along the magnetic field
line is controlled by gravity and collisions with the ambient atmosphere,
while growth perpendicular to the field line is controlled by ambipolar
diffusion, collisions, and inhomogeneities in the electric field. The bulk
motion of the cloud is a function of gravitational fall, interaction with
the ambient neutral wind, and of the magnetic and electric fields. If an
ion cloud is formed at an altitude where the collision frequency of the
Ba
+
ions with the ambient atmosphere is low and where the ion density in
the cloud is not large enough to create a large anomaly in electron density,
then the motion of the cloud is essentially due to the E x B/B
2
drift and
gravity. As B is known to a fraction of one percent and the expected
vertical motion attributable to gravitational fall is usually checked by
comparison of the neutral and ion fall rates, the electric field E, per-
pendicular to B, is deduced from E - -v x B by measuring the horizontal
velocity vi of the ion cloud. The high conductivity along B usually permits