Polar-cap electric field distributions related to the interplanetary magnetic field direction

TLDR: The correlation between the azimuthal direction of the interplanetary magnetic field and the most simple polar cap signatures is discussed in this article, where only the spatial distribution of the dawn-dusk polar cap field is considered.

Abstract: The correlations between the azimuthal direction of the interplanetary magnetic field and the most simple polar cap signatures are discussed. Only the spatial distribution of the dawn-dusk polar cap field is considered. For each OGO 6 traverse across the northern or southern polar cap, the simultaneous values of the interplanetary magnetic field in solar-equatorial coordinates were recorded by the Explorer 33 magnetometer. Histograms of these values are presented and are discussed. The high degree of correlation with the longitudinal angle indicates that the relative geometry of the interplanetary magnetic field and magnetospheric magnetic fields must be fundamental to explaining the distribution of polar cap electric fields. The sign of the solar-equatorial component perpendicular to the sun-earth line appears to be a more critical parameter than the sign of the component toward the sun. The Svalgaard-Mansurov correlation and the correspondence between fast convection and parallel magnetospheric and interplanetary magnetic fields are described.

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Polar
Cap
Electric
Field
Distributions
related
to
the
Interplanetary
Magnetic
Field
Direction
J.
P.
Heppner
NASA
Goddard
Space
Flight
Center
Greenbelt,
Maryland
June 1972
"Letter"
for
publication
in
the
Journal
of
Geophysical
Research

Polar
Cap
Electric
Field
Distributions
related
to the
Interplanetary
Magnetic
Field Direction
J.
P.
Heppner
NASA
Goddard
Space
Flight
Center
Greenbelt,
Maryland
In
a
previous
paper
(Heppner,
1972a)
measurements
of
the electric
field
taken
by
OGO-6
throughout
a
13
day
period
(June
10-22,
1969)
when
the orbit
plane
was
centered
on
the
1
8
h-
6
h
local
time
meridian
were
described
with
emphasis
on
the
stability
of the
basic
high
latitude
electric
field
pattern.
The
term "pattern"
implies
the
direction
of
the
electric
field
as
a
function
of
location
in
magnetic
time
(MLT)
and invariant
latitude
(A)
coordinates.
In
essence
it
was
shown
that
at
auroral
belt latitudes
in
both
hemispheres
the
electric
field
is
consistently
directed
poleward
in
the
mid-evening
sector
and
equatorward
in
the
mid-morning
sector
(i.e.,
from
dusk
toward
dawn)
whereas
the
field
at
higher,
polar
cap,
latitudes
is
consistently
directed from
dawn
toward
dusk.
By
consistently,
one
means independent
of
magnetic
disturbance
conditions,
interplanetary
field
directions,
and
other
parameters
that
might
be
expected
to
have
some
relationship
to the
electric
field
pattern.
The
continuous
existence
of
a
sequence
of
signs
as
a
function
of
latitude
does
not
however
mean
that
there
are
not
substantial
variations
in
magnitudes
and the
latitudes
where
the
sign
reverses
between
auroral
belt
and
polar
cap
directions.
Also,
multiple
sign
reversals
are
often
encountered
near
the
transition
(or
boundary)
between
auroral
belt
and
polar
cap
fields.
Examples
are
given
in
Heppner
(1972a)
and
discussed
further
relative
to
complementary
information
provided by barium
ion
cloud experiments
in
Heppner
(1972b).
Figure
1,
which
shows
data
from
successive passes
over
southern
and
northern
high
latitudes,
is
chosen
to
illustrate
how
grossly
asymmetric
the
distribution

- 2 -
of
magnitudes
can sometimes
be,
and
also
to
provide
an
example
of
the
anti-
correlation
between
southern
and
northern polar
cap
magnitude
distributions
discussed
later.
The
anti-correlation
to
note
in
Figure
1
is
the
existence
of
maximum,
dawn-dusk
directed,
polar
cap
field
adjacent
to
the
evening
auroral
belt
in the
southern
hemisphere
but adjacent
to
the
morning
auroral
belt
in
the
northern
hemisphere.
The
two
northern
hemisphere
passes
in
Figure
1
also
illustrate
one
of
a
number
of
characteristic
field
distributions
observed
in
the
northern
polar
cap;
that
is,
in
crossing
the polar
cap
from
evening
toward
morning,
IExI
increases
almost
linearly until
near
the
morning
auroral
belt
where
a
rapid
decrease
in
JExJ
occurs
and the
sign
reverses.
Signature
Classification
The
fact
that
different
characteristic
distributions
of
the
polar
cap
field
reappeared
on
different
days
and
at
a
variety
of
UT
times
throughout
the
13
days,
led
to
the
idea
that
one
could
classify
the
distributions
in
terms
of "signatures"
which
could
then
be compared
with
other
geophysical
parameters
with
the
hope that correlations
might
appear
which
would
suggest
an
explanation
for
the
changes
in
distribution.
After
several
trial
exami-
nations
it
was
apparent that
for
roughly
two-thirds
of the
northern hemisphere
traverses
the
polar
cap
distributions could
be
identified
in
terms
of the
"signatures" shown
in
Figure
2.
For
the
other
one-third
one
could only
use
combinations
of
the
Figure
2
signatures
or
end
up
with
an
unreasonable number
of
signatures. These
cases
are
thus
omitted
from
the
statistics
given
later.
Also passes intersecting
the
noon-midnight
meridian
at
A < 75°
on the night-
side
are
omitted because
the
polar
cap
cross-section
at
these
latitudes
becomes
too
small to
regard
as
being
representative.
The
northern
hemisphere
signatures
are
thus
only
applied
to
passes crossing
the noon-midnight
meridian

-3-
between
A
=
750
on
the
nightside
and
A =
850
on
the
dayside.
For
the
southern
polar
cap,
noon-midnight
meridian
crossings
extend
from
about
A = 70
°
on
the
dayside
to
A =
850
on
the
nightside.
As
the
electric
field
in
the
dayside
region between
70
°
and
800,
and
frequently
70
°
to
850,
is
characteristically
highly
irregular
with many
sign
reversals,
classification
of
the
southern
polar
cap
field
is
generally
limited
to
traverses
crossing
between
A = 85
°
on
the
dayside
and
A =
850
on the nightside.
A
further complication
in
the
southern
polar
cap
is
that
irregularities
are
usually
more prominent
than
in
the
northern
polar
cap
even
when
the
southern
polar
cap
crossing
is
on
the
nightside
(see
Heppner,
1972a).
Thus
the
number
of
southern
polar
cap
traverses
that can
be
classified
is
considerably
less
than
in
the
northern
polar
cap and
for
meaningful
statistics
it
becomes
necessary
to
characterize
the
field
in
broader
categories
than
the
signatures
used
in
the
northern
polar
cap,
as
noted
later.
The
fact
that
correlations
between
the
azimuthal
direction
of
the
inter-
planetary
magnetic
field
and
the
most
simple
polar
cap
signatures
were
immediately
obvious
is
the
basis
of
this communication.
Because
of
their
potential
importance
the
simple
correlations
are
presented
here
to
make
the
information
available
at
an
early
date.
Thus,
here
we
will
omit
the
more complex
signatures
(F,G,H,I
and
K)
for
discussion
at
a
later date
when
more comprehensive
analyses
can
be
completed.
These
analyses
involve
use
of
different
coordinate
systems,
testing
correlations
as
a
function
of
time
differences
between
interplanetary
and
polar
cap
observations,
and
looking
for
statistical
correlations
between
interplanetary
parameters
and
more
detailed
features
such
as
the
sharpness and
location
of
polar
cap-auroral
belt
boundaries, the
existence
of
multiple
field
reversals,
large
differences
between
magnitudes
in
morning
and
evening
auroral
belt
regions,
etc.