Voyager 1 Studies of the HMF to 81 AU During the
Ascending Phase of Solar Cycle 23
L. F. Burlagaa, N. F. Nessb, Y.-M. Wang and N. R. Sheeley, Jr.c
aLaboratory for Extraterrestrial Physics, NASA-Goddard Space Flight Center, Greenbelt, MD 20771, USA
bBartol Research Institute, University of Delaware, Newark, DE 19716, USA
cE. O. Hulburt Center for Space Research, Naval Research Laboratory, Washington, DC 20375, USA
Abstract. The paper analyzes the magnetic field strength B and polarity observed in the distant heliosphere from 1996 to
early 2001 and will be discussed in relation to the variation of B from 1978 through 1996. The observations extend the
results of Burlaga et al. [1]. The polarity of the heliospheric magnetic field (HMF) from 1997 to early 2001 is studied in
relation to the extrapolated position of the heliospheric current sheet (HCS). These observations of polarity extend the earlier
results of Burlaga et al. [2] and Burlaga and Ness [3].
The V1 observations of the heliospheric magnetic field strength B agree with Parker’s model of the global heliospheric
magnetic field from 1 to 81 AU and from 1978 to 2001, when one considers the solar cycle variations in the source magnetic
field strength and the latitude/time variation in the solar wind speed. Parker’s model, without adjustable parameters, describe
the general tendency for B to decrease with increasing distance R from the Sun, and the solar cycle time variations causing
the three broad increases of B around 1980, 1990, and 2000, and the minima of B in 1987 and 1997. The variation of
magnetic polarity observed by V1 and V2 was caused by the increasing latitudinal width of the sector zone with increasing
solar activity, which in turn was related to the increasing maximum latitudinal extent and the decreasing minimum
latitudinal extent of the footpoints of the HCS.
increasing distance from the Sun [8]. Nevertheless, the
latitudinal extent of the HCS generally changes little
(O(10°)) with increasing distance from the Sun out to
40 AU [9]. The extent of the HCS defines the
latitudinal boundaries of a zone in which both positive
and negative polarities of the magnetic field are
observed. This zone can be identified even at large
distances, and it can be related to solar magnetic fields.
Burlaga and Ness [9] call this region the “sector zone.”
During 1989, V1 and V2 observed both positive and
negative polarities; both spacecraft were in the sector
zone, because the HCS extended to moderate latitudes
approaching solar maximum [10, 11]. Ulysses
observed a single magnetic polarity at northern
latitudes above the HCS during 1996 and opposite
polarities when it was within the heliolatitude range
covered by the excursion of the neutral line from 1997
through 2000 [12].
INTRODUCTION
The radial variation of the magnetic field strength to
66 AU was determined by Burlaga et al. [1], using
Voyager and Pioneer 11 data, and were found to agree
with Parker’s model [4, 5] when considering the
latitudinal and temporal variations of the source
magnetic field strength and the solar wind speed at 1
AU .
The heliospheric magnetic field has regions of
positive polarity (field vectors pointing away from the
Sun) and negative polarity (field vectors pointing
toward the Sun) [6]. During much of the solar cycle,
the polarity of the magnetic field is predominantly
positive (negative) above the heliospheric current
sheet (HCS) and negative (positive) below the HCS.
Schulz [7] attributed a 2-sector pattern to a tilt of the
nearly planar HCS near the Sun relative to the solar
equatorial plane, and a 4-sector pattern to warps in the
HCS when it lies close to the equatorial plane.
The HCS tends to extend from the solar corona to
the distant heliosphere. Its internal structure can be
complex and its shape is severely distorted with
CP679, Solar Wind Ten: Proceedings of the Tenth International Solar Wind Conference,
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39
variations in the solar wind speed. The top (bottom) dashed
curve is the prediction of Parker’s model for a solar wind
speed of 400 km/s (800 km/s). Standard deviations of the
observed averages are indicated also.
OBSERVATIONS OF B
This section presents new observations of the yearly
averages of the Voyager 1 data for the magnetic field
strength, BV1(t), from 1997 to 2001.34. In addition,
we compare the observations BV1(t) with Parker’s
model. At 80 AU, the distance of V1 in 2001, the
radial and azimuthal components of the heliospheric
magnetic field are expected to be ≈ 0.0007 nT and ≈
0.05 nT, respectively. The observations of BV1(t) are
shown as dots in Figure 1. As expected, B decreases
with increasing distance from the Sun (which
corresponds to increasing time as V1 moves away
from the Sun). There are also three relative maxima in
B near 1980, 1990, and 2000 and two relative minima
in B during 1987 and 1997. The relative maxima and
minima in BV1(t) in Figure 1 are related to solar cycle
variations in the source field strength (measured by the
magnetic field at 1 AU, B1(t)) and the solar wind speed
at V1, V(t, θ(t)). Figure 2 shows B1(t) and V(t, θ(t))
from 1978 through 2000.34 in the bottom and top
panels, respectively. The variations of the sunspot
number during the same interval are shown in the
middle panel. The magnetic field strength at 1 AU,
B1(t), was obtained from the NSSDC Omni data set
and some recent data from ACE. The speed V(t, θ(t))
on V1 was not measured directly beyond 10 AU, so
we determined it using the Wang and Sheeley [13]
method.
A solar cycle variation of B1(t) is evident in Figure
2; B1(t), had a minimum strength equal to ≈5 nT and a
maximum strength of ≈9 nT. The three intervals with
relatively strong fields observed by V1 were
associated with relatively strong magnetic fields at 1
AU shortly after the sunspot maxima in ≈ 1980, 1990,
and 2000. The intervals with relatively weak fields
observed by V1 were associated with relatively weak
magnetic fields at 1 AU in 1987 and 1997 when the
solar activity was low.
Figure 2. Solar cycle variation of the solar wind speed
computed at V1 (top panel) and the magnetic field strength
at 1 AU (bottom panel). The sunspot number is shown in the
middle panel for reference.
A solar cycle variation of V(t, θ(t)) is evident in
Figure 2, but it is complicated by a latitudinal variation
that is also present. At 1 AU there are relative minima
in V near the solar maxima in 1980, 1990 and 2000,
and there are maxima in V near the solar minima in
1986 and 1996. The travel time of the solar wind from
1 AU to 81 AU is approximately 300 days, so that the
effects of solar minimum and solar maximum are
observed significantly later in the distant heliosphere
than at 1 AU.
Figure 1. The dots show the yearly averages of the magnetic
field strength B measured by Voyager 1 from 1978 to 2001.
The solid curve is Parker’s model, computed as described in
the text. The three local maxima and two local minima in B
are associated with solar cycle variations in the source field
magnetic field strength and with both solar cycle and latitude
40
panels of Figure 3. When V1 observed ≈100% positive
magnetic polarity in the northern hemisphere in 1997,
the maximum northern extent of the HCS was very
low, ≈10° N. In that year, V2 observed essentially no
positive magnetic polarity in the southern hemisphere,
and the minimum southern extent of the HCS was ≈5°
S. The percentage of positive polarity measured by V1
decreased from 1997 to 2000 as the maximum latitude
of the HCS moved northward above the latitude of V1.
The percentage of positive polarity measured by V2
from 1997 to 2000 increased (the percentage of
negative polarity decreased) as the minimum
latitudinal extent of the HCS moved southward below
the latitude of V2.
The latitudinal extent of the HCS was propagated
radially from the source surface to the distant
heliosphere in constructing Figure 3 [11].
COMPARISON OF THE
OBSERVATIONS OF B WITH
PARKER’S MODEL
We now compare the V1 observations of the 1-year
averages of the magnetic field strength B with the
predictions of Parker’s model. Using the observations
B1(t) and V1(t) at 1 AU and the speed profile V(t,θ(t))
discussed above, the magnetic field strength at V1 that
is predicted from Parker’s model was calculated. The
result is shown by the solid curve in Figure 1. The
effects of the speed on the theoretical magnetic field
strength are shown by the dotted curves in Figure 1,
which show Parker's model for the cases V(t, θ(t)) =
400 km/s (top curve) and 800 km/s (bottom curve).
Parker's model, without adjustable parameters,
reproduces the basic features of the observed magnetic
field strength profile, including: 1) the general
tendency to decrease with increasing distance R from
the Sun; 2) the three broad maxima around 1980,
1990, and 2000; and 3) the two minima in 1987 and
1997.
The values of B at V1 in ≈1987 and 1997 are lower
than the model of Parker predicts for the values of V(t,
θ(t)) that we used. It is possible that the departures of
the observations from Parker’s model in 1987 and
1997 in Figure 1 are associated with the presence of a
vortex street at those times.
VARIATIONS OF THE POLARITY OF
THE HELIOSPHERIC MAGNETIC
FIELD AND THE EXTENT OF THE
SECTOR ZONE
Figure 3. Top left. The percentage of positive polarities
observed by Voyager 1 in the years 1997 to 2001. Top right.
The percentage of positive polarities observed by Voyager 2
in the years 1997 to 2001. Bottom left. The maximum
latitudinal extent of the HCS as a function of time computed
from solar observations. Bottom right. The minimum
latitudinal extent of the HCS as a function of time computed
from solar observations.
We now consider the variation of the magnetic field
polarity in the distant heliosphere during the ascending
phase of solar cycle 23.
The percentage of positive magnetic polarity
measured at V1 and V2 from 1997 (near solar
minimum in the distant heliosphere) through 2001.5
(near solar maximum in the distant heliosphere) is
shown as a function of time in the top panels of Figure
3, respectively. The uncertainty in these measurements
is of the order of 5% - 10%. In the northern
hemisphere, V1 observed a decrease in the percentage
of positive polarities from ≈100% during 1997 to
≈50%. In the southern hemisphere, V2 observed an
increase in the percentage of positive polarities from ≈
0% during 1997 to ≈ 50% during the same period.
The percentage of positive polarities observed by V1
(V2) is related to the maximum (minimum) latitudinal
extent of the HCS, shown in the bottom left (right)
We conclude that the variation of magnetic polarity
observed by V1 and V2 from 1997 to 2001 was caused
by the increasing width of the sector zone with
increasing solar activity, which was related to the
changing northern and southern latitudinal extent of
the footpoints of the HCS.
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ACKNOWLEDGMENTS
It was largely through discussions with Dr. M.
Acuna and the outstanding efforts of T. McClanahan
and S. Kramer that we were able to utilize the
measurements of very weak magnetic fields discussed
in this paper. Valuable assistance was also provided by
Diana Taggart. N.F. Ness acknowledges partial
support by Jet Propulsion Laboratory contract 959167.
We thank T. Hoeksema (WSO/Stanford) for helpful
comments on the manuscript and for providing his
results on the positions of the heliospheric current
sheet, which are an essential part of this paper, on the
World Wide Web. N.R. Sheeley, Jr. and Y.-M. Wang
received financial support from NASA and the
NRL/ONR 6.1 Accelerated Research Initiative, Solar
Magnetism and the Earth's Environment.
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Note: The full discussion of these results is given in:
Heliospheric Magnetic Field Strength and Polarity
from 1 to 81 AU During the Ascending Phase of Solar
Cycle 23, L.F. Burlaga, N.F. Ness, Y.-M. Wang and
N.R. Sheeley, Jr., J. Geophys. Res., Submitted March,
2002.
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