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MICRO-SCALE STRUCTURES
,
IN
THE INTERPLANETARY
MEDIUM
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I
LEONARD
F.
BURLAGA
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Microfiche
(M
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653
July
65
GODDARD
SPACE
FLIGHT
CENTER
GREENBELT,
MARYLAND
SEPTEMBER
1967
MICRO-SCALE STRUCTURES
IN
THE
INTERP LANETARY
MEDIUM
Leonard
F,
Burlaga>k
Laboratory
for
Space Sciences
NASA-Goddard Space F1 ight Center
Greenbelt, Maryland
September
1967
*NAS-NRC
Postdoctoral Resident Research Associate
!
ABSTRACT
Micro-Scale Structures in the Interplanetary Medium
This report describes and analyses the following micro-scale
((
.01
AU)
structures which
were
found in combined interplanetary magnetic
field and plasma data obtained by the deep space probe, Pioneer
6:
1)
Several types of simultaneous discontinuities in the magnetic field
and plasma parameters;
2)
at
least
one
clear
example
of
a
transitional
region (D-sheet) associated with
a
plasma
discontinuity;
3)
other D-sheets
which give evidence
of
magnetic field annihilation;
4)
inhomogeneous
isothermal regions in which the square of the magnetic field intensity
is
proportional to the density;
5)
periodic variations in the magnetic
field intensity which are associated with discontinuities in the bulk
speed.
It
is
suggested that
small
velocity discontinuities play
a
iuidamental role in reducing stresses in the interplanetary medium, and
that
large velocity discontinuities may give
rise
to waves
and turbulence.
Micro-Scale Structures in the Interplanetary Medium
A.
Introduction
Recently experimental results
were
published which suggest that
several types of structures predicted by hydromagnetic theory
(see
Landau
and Lifshitz,
1960)
are
present in the interplanetary medium. Indirect
,evidence
for the existence of
tangentia1,discontinuities
in the interplanetary
medium,
based on magnetic field data alone,
was
presented by
Ness
et
al.
(1966),
Burlaga and-Ness
(1966)
and by Siscoe
et
al.
(1966).
Indirect
evidence for
the
existence of tangential discontinuities, based only
on
plasma
data
was
given by Gosling
et
al.
(1967).
Mihalov
et
al.
(1967)
have shown that temperature peaks
are
sometimes associated with magnetic
field discontinuities and have suggested that such heating may be produced
by
magnetic field annihilation.
This report presents simultaneous, high resolution
plasma
and magnetic
field
data,
obtained by instruments on the deep
space
probe Pioneer
6,
which
demonstrate the existence of several distinct types of micro-scale
(<
-
.01
AU)
structures in the interplanetary medium. These structures
will
be
discussed
from the point of view of hydromagnetic theory and
will
be
shown to satisfy certain necessary conditions for hydromagnetic structures.
-2-
B.
Orbit and Instruments
The Pioneer 6 spacecraft
was
launched toward the sun into heliocentric
orbit on December 16, 1965.
It
penetrated to
z
.81
AU
in mid-May 1966,
when
it
was
25O ahead of the earth relative to the sun, and later
it
moved
away from the sun and
still
farther ahead of the earth. The interplanetary
medium
was
monitored essentially continuously from the date
of
launch
through April 1966. Thereafter, data
were
transmitted only intermittently.
The magnetometer which provided the magnetic field data that
will
be discussed below has been described by
Ness,
Scearce and Cantarano (1966).
Briefly,
it
is
a uniaxial fluxgate magnetometer, mounted
at
an angle of
54O
45'
to the spacecraft spin axis which
is
nearly perpendicular to the
ecliptic plane. The instrument zero-level
was
calibrated in flight by
physically reversing the direction of the sensor by
180:
Each orthogonal
component
of
the magnetic field vector
is
known
to
f.
.24
gamma.
A
complete vectar measurement, consisting of three axial measurements per
spacecraft rotation,
is
transmitted on average every 1.5 sec.
at
the highest
bit
rate.
-+
The data were reduced to give
[BI
5
B,
0
and
@
as
a
function
of
time,
where these quantities were computed from thirty-second averages
of
the measured components. Here
9
and
@
give the direction
of
B
in solar
4
ecliptic coordinates
,
i.e.
,
6
is
the latitude angle measured with respect
to the ecliptic plane
(+
above the plane), and
@
is
the azimuth angle
relative
to
the spacecraft sun line
(@
=
0
when
B
is
along the line and
+
+
toward the sun, and increases
as
B
moves toward the eastern hemisphere of
the sun),