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7.
.
THE
LIGHT
ION
TROUGH,
THE
~1AIN
TROUGH,
AND
THE
PLASNAPAUSE
by
Harry
A.
Taylor,
Jr.
Laboratory
for
Planetary'
Atmospheres
Goddard Space
Flight
Center
Greenbelt,
~aryland
and
William
J.
Walsh
Aero
Geo
Astro
Beltsville,
Maryland
Goddard Space
Flight
Center
Greenbelt,
Maryland
THE
LIGHT
ION
TROUGH,
THE
MAIN
TROUGH,
AND
THE
PLASMAPAUSE
by
Harry
A.
Taylor,
Jr.
Laboratory
for
Planetary
Atmospheres
~d
William
J.
Walsh
Aero
Geo
Astro
ABSTRACT
Extensive
observations
of
mid
latitude
depletions
in
electron
and
total
ion
density
by
both
direct
and
indirect
techniques,
have prompted
numerous
studies
of
the
possible
association
between
these
troughs,
observed
both
in
the
F-region
and
in
the
topside
ionosphere,
and
the
plasmapause.
Within
this
work, one
basic
problem
arises
in
that
while
the
plasmapause
has
been
detected
as
a
global
phenomenon
both
by
vlf
and
ion
composition
measurements,
the
electron
and
ion
density
troughs
have
been
identified
primarily
as
nightside
features.
This
problem,
as
well
as
the
difficulty
in
explaining
various
inconsistencies
in
relating
the
position
of
the
plasma-
pause
and
the
ionization
trough,
is
explained
by a
close
examination
of
the
ion
composition,
generally
unavailable
in
previous
trough
studies.
In
par-
ticular,
ion
composition
results
from
the
polar
orbiting
aGO
satellites
+ +
identify
the
persistence
of
a pronounced
light
ion
trough
in
H
~d
He
,
often
identified
by
order
of
magnitude
decreases
in
the
light
ion
con-
centrations,
occurring
within
a
few
degrees
of
latitude,
reaching
resid-
ual
concentration
levels
of
10
2
to
10
3
ions/cm
3
near
60°
dipole
latitude.
i
The
light
ion
trough,
observed
both
within
the
thermosphere
and
at
high
latitudes
in
the
magnetosphere
has
been
correlated
directly
with
the
vlf
whistler
identification
of
the
plasmapause,
On
the
nights
ide
near
1000
km,
a complex
relationship
is
observed
between
the
dominant
ions
0+
and
H+.
The
rapidly
changing
mean
ion
mass
observed
at
the
plasmapause
typically
produces
a
resultant
total
ion
density
distribution
of
com-
plexity
sufficient
to
confuse
correlative
studies
with
the
plasmapause,
even though
the
light
ion
trough
is
very
sharply
defined
within
the
ion
composition.
+
On
the
dayside,
where 0
is
typically
the
predominant
ion
near
1000 km.,
light
ion
trough
structure
is
often
hidden
beneath
the
+ +
o
layer,
which
may
exhibit
little
or
no
depletion
even though
Hand
+
He
show
trough
structure
at
the
plasmapause.
This
analysis
of
the
mid
latitude
topside
ion
composition
indicates
the
need
for
caution
both
in
studies
of
the
plasmapause
per
se,
and
in
phenomenological
studies
such
as
the
relationship
between
the
sub
auroral
red
arc
and
the
plasmapause,
which
are
most
likely
dependent upon
the
detailed
information
available
within
the
ion
composition.
ii
THE
LIGHT
ION
TROUGH,
THE
MAIN
TROUGH,
AND
THE
PLASMAPAUSE
INTRODUCTION
Since
the
early
work
of
Muldrew [1965]
there
has
been
continuing
interest
in
the
phenomena
of
mid
and
high
latitude
charged
particle
depletions
and
the
possible
association
of
these
events
with
mechanisms
responsible
for
the
plasmapause.
In
early
phases
of
such
studies,
atten-
tion
was
focused
upon
F-layer
effects,
since
both
the
work
of
MUldrew
as
well
as
the
later
results
of
Sharp
[1966]
described
charged
particle
depletions
or
'troughs'
in
the
300
to
400
km.
region.
~1uldrew,
observing
a
variety
of
trough-like
features
in
the
electron
density,
identified
a
persistent
'main
trough'
on
the
nightside,
near
60°
magnetic
latitude.
Sharp
observed
abrupt
nightside
depletions
in
total
ion
density
in
the
same
latitude
range,
identifying
the
result
as
the
'mid
latitude
trough'.
Alth~ugh
these
F-layer
variations
have
not
been
substantiated
to
be
of
,
identical
origin,
their
similar
features
have
led
some
investigators
to
the
assumption
that
such
troughs
may
be
the
result
of
a
similar
or
common
mechanism.
One
important
feature
observed
in
the
lower
ionospheric
trough
is
the
tendency
for
the
depletion
to
dissipate
and
often
become
indistin-
guishable
during
dayside
hours,
including
some
evidence
that
the
trough
position
may
move
to
significantly
higher
latitudes
as day
approaches.
This
evidence
is
summarized
in
Figure
1,
taken
in
part
from
Jelly
and
Petrie
[1969],
who
illustrated
the
diurnal
variation
of
the
F-layer
trough
observed
by Muldrew.
As
shown,
the
'main
trough'
shifts
sharply
to
higher
latitudes
between
70-80°
in
the
afternoon
sector,
and
disappears
altogether
between
about
0700 and 1400
LT.
In
contrast
to
this
observed
-1-