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Voyager 1 (V1) crossed the termination shock and entered the heli-
osheath on ≈ December 16, 2004, moving in the northern hemisphere
~34.5° above the solar equatorial plane in the general direction of the
nose in the heliosphere. V1 moved radially from 94 AU to 121 AU at
34.5°N in the heliosheath since it crossed the termination shock (1–3).
Recent estimates of the position of the heliopause (the boundary of the
heliosheath and the interstellar medium) along the V1 trajectory range
from ≈110 – 150 AU (4–6). During 2010 the radial component of the
velocity at V1 was near 0 km/s (7), and from day 2010/126 through
2011/308 the average northward component of the velocity was 28 ± 3
km/ s (8). The speed slowed during 2011 to form a quasi-stagnation
region extending from 113 to beyond 119 AU (8, 9) suggesting that V1
may be approaching the heliopause.
We present V1 magnetic field observations from 2012/150 through
2012/270 [day of year (DOY) 1 = January 1] in Fig. 1. The particles >0.5
MeV/nuc are discussed in more detail in (10, 11), and the magnetometer
and data are described in (12, 13) and in the supplementary materials).
During this interval, V1 was at 34.5°N moving from 120.7 - 121.9 AU
radially away from the Sun. From 2000/150 to 2012/210, there is no
correlation between B (which varies from 0.072 nT to 0.36 nT) and the
counting rate of particles >0.5 MeV/nuc (14) (which remains nearly
constant). In contrast, from 2012/210 to 2012/270 there is a strong anti-
correlation between B and the particle counting rates.
Figure 1 shows a series of jumps in B starting on 2012/210 and end-
ing on 2012/240, labeled B1, B2, B3, B4, and B5 (Table 1). The jumps
indicate multiple crossings of a boundary unlike anything observed pre-
viously by V1. On 2012/210 (B1), B increased abruptly from 0.17 nT to
0.43 nT (the strongest magnetic fields observed by V1 in the heliosheath
since crossing the termination shock in 2004) and the particle counting
rate dropped by a factor of ≈2 (from 23.6 to 12.2 count/s) at the same
time. The energetic particle data show that jump B3 did not correspond
to a complete entry into the region beyond the boundary, because the
counting rates did not drop to near the minimum value and B did not rise
to the level observed after B1 and B5. At the last jump on ~2012/238
(B5) B increased to ≈0.43 nT and it remained at that value until at least
2012/270, while the particle counting rate dropped from ≈25 counts/s to
≈2 counts/s (background) until at least 2012/270.
In the heliosheath, the average
magnetic B between the termination
shock crossing at the end of 2004 and
the beginning of 2011 was 0.1 nT, cor-
responding to a magnetic pressure
/8π = 0.04 × 10−12
pPa. In the heliosheath depletion region
(HDR), B is (0.44 ± 0.01) nT and the
magnetic pressure is B2
/8π = 0.8 × 10−12
= 0.08 pPa, nearly 20 times
greater than observed for five years
following the termination shock cross-
The enhancements of B between B1
and B2 and between B3 and B4 and
following B5 are possibly largely the
response of B to the decrease in pres-
sure caused by loss of the energetic
particles, in order to maintain pressure
balance normal to B and equilibrium in
the region. In this case the boundaries
are pressure balanced structures (15),
which correspond to MHD tangential
discontinuities such as stream interfac-
It is generally assumed that the helio-
pause is a pressure balanced structure
(or tangential discontinuity in the MHD approximation) which is possi-
bly rippled by waves and turbulence generated by instabilities (16, 17)
and punctuated by reconnection events (6). Because the observations
above suggest that the boundaries observed by V1 during 2012 are pres-
sure balanced structures, one must consider the hypothesis that the
boundaries represent multiple crossings of the heliopause and that V1
has entered the interstellar medium.
Due to the rotation of the Sun, the solar magnetic field forms the
Parker spiral field as it is carried radially outward by the solar wind (18)
which is observed to have an east-west orientation at V1. In contrast, the
10-AU difference in the location of the termination shock in the northern
and southern hemisphere implies that the interstellar magnetic field must
have a component in the north-south direction (19–21) and is not parallel
to the east-west direction of the solar magnetic field in the heliosheath.
Consequently, the magnetic field direction should change when V1
crosses the heliopause (fig. S1). The magnetic field direction could re-
main constant across the heliopause only if the interstellar magnetic field
were nearly parallel to the solar ecliptic plane and tangent to the helio-
spheric magnetic field. Such a configuration is highly improbable and
would have to be a remarkable coincidence, because the interstellar
magnetic field has no causal relation to the solar magnetic field (22, 23).
Higher resolution magnetic field observations from 2012/210 to
2012/270 (Fig. 2) suggests that V1 did not observe a significant change
in the direction of B at any of the five crossings of the boundary. Table 1
shows the angles on the low field (subscript L) and high field (subscript
H) sides of each boundary crossing as well as the absolute value of the
differences of these angles. The changes in the direction of B for each of
the 5 boundary crossings are indeed very small. The weighted averages
of the changes in direction angles are <Δλ> = <| λH – λL|> = (1.8 ± 1.9)°
and <Δδ> = <|δH – δL |> = (1.8 ± 1.5)°, consistent with no change in the
direction of B.
During the last boundary crossing (Fig. 3) the strength of B increased
from 0.272 nT to 0.438 nT during an interval of ≈18.4 hours centered at
day 237.7. The changes in the angles across B5 are Δδ = (0 ± 2)°, Δλ =
(1 ± 3)°. Because the uncertainties refer to differences of angles within
one day, they probably represent statistical uncertainties, relatively unaf-
Magnetic Field Observations as
Voyager 1 Entered the Heliosheath
L. F. Burlaga,1
* N. F. Ness,2
E. C. Stone3
NASA-Goddard Space Flight Center, Greenbelt, MD 20771, USA.
The Catholic University of America,
Washington, DC 20064, USA.
California Institute of Technology, Pasadena, CA 91125, USA.
*Corresponding author. E- mail: lburlagahsp@Verizon.net
Magnetic fields measured by Voyager 1 (V1) show that the spacecraft crossed the
boundary of an unexpected region five times between days 210 and ~238 in 2012.
The magnetic field strength B increased across this boundary from ≈ 0.2 nT to ≈0.4
nT, and B remained near 0.4 nT until at least day 270, 2012. The strong magnetic
fields were associated with unusually low counting rates of >0.5 MeV/nuc particles.
The direction of B did not change significantly across any of the 5 boundary
crossings; it was very uniform and very close to the spiral magnetic field direction,
which was observed throughout the heliosheath. The observations indicate that V1
entered a region of the heliosheath (“the heliosheath depletion region”), rather than
the interstellar medium.
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fected by drifts and other systematic errors. Because there was no
change in the direction of B with a high degree of certainty, it is very
unlikely that the boundary B5 is the heliopause.
The magnetic properties of the HDR from 2012/238 to at least
2012/270 define the region, because they differ from all previous obser-
vations within the heliosheath. The average magnetic field strength is
(0.436 ± 0.010) nT. An interstellar magnetic field strength of this magni-
tude or greater has been ruled out as being too high to explain the IBEX
ribbon (24), which adds support to our conclusion that the HDR is asso-
ciated with the heliosheath rather than the interstellar medium. The mag-
netic field components in this region are BR = (0.126 ± 0.008) nT, BT =
(-0.400 ± 0.010) nT and BN = (0.120 ± 0.013) nT. The uncertainties in
these average values are the standard deviations, and their values are
close to the digitization level and RMS noise of the instrument, 0.004 nT
and 0.003 nT, respectively. Thus, the fluctuations in the components of
B are extremely small in the HDR. The region is not turbulent.
The average direction of the magnetic field in the HDR is λA = (287°
± 1°) and δA = (14° ± 2°). The average magnetic field direction is close
to the Parker spiral magnetic field direction (Fig. 2), but there is a statis-
tically significant difference from the spiral field direction in the HDR,
namely λA – λP = (17 ± 1)° and δA - δP = (14 ± 2)° as shown in Fig. 2 .
The magnetic polarity of the magnetic field in the HDR indicates that it
has moved from the southern hemisphere to the position of V1 in the
northern hemisphere. The small departure from the spiral field direction
might be the result of a flow that carried the magnetic field northward in
the heliosheath to the location of V1. It has been suggested that such a
flow moves northward in the heliosheath between a “magnetic wall” or
“magnetic barrier” and the heliopause at the latitude of V1 (5, 25).
Increasingly strong magnetic fields from the middle of 2010 until at
least the middle of 2011 (possibly extending up to 2012/150 as shown in
this paper) were reported in (26), where it was suggested that these
strong magnetic fields might be related to a magnetic wall or magnetic
barrier. Thus, it is conceivable that the HDR corresponds to this north-
ward heliosheath flow near the heliopause, and the boundary of the HDR
represents a boundary of material that was moving radially closer to the
Sun. The strong magnetic fields observed from mid-2010 to 2012/270
could be an interaction region that extends into the HDR, produced by
the collision of these two flows. The stronger magnetic field in the HDR
might be produced in response to the reduction of pressure owing to the
absence of energetic particles. The absence of energetic particles could
indicate that magnetic lines passing V1 were no longer connected to
their source (the blunt termination shock), because V1 crossed a topolog-
ic boundary in the magnetic field of the inner heliosheath beyond the last
magnetic connection point to the termination shock (27). Alternatively,
the energetic particles could have escaped into interstellar space, if the
heliosheath magnetic field reconnected with the interstellar magnetic
field beyond the position of V1.
References and Notes
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Acknowledgments: T. McClanahan and S. Kramer provided support in
the processing of the data. D. Berdichevsky computed correction
tables for the 3 sensors on each of the two magnetometers. N. F. Ness
was partially supported by NASA Grant NNX12AC63G to the
Catholic University of America. L. F. Burlaga was supported by
NASA Contract NNG11PN48P. The data are available at NASA’s
Virtual Heliospheric Observatory http://vho.nasa.gov/ maintained
within the Heliospheric Physics Laboratory at NASA’s Goddard
Space Flight Center.
21 January 2013; accepted 30 May 2013
Published online 27 June 2013; 10.1126/science.1235451
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Fig. 2. 48-s averages of the magnetic field strength B (A), azimuthal angle λ (B),
and elevation angle δ (C), as a function of time measured from DOY 150 to DOY
270, 2012. The angles are in RTN coordinates (28). Prior to 2012/210, V1
observed magnetic fields characteristic of the heliosheath (26). The elevation and
azimuthal angles are close to the Parker spiral direction, δP ≈ 0° and λP ≈ 90° or
270°, respectively. A magnetic sector in which B was directed sunward along the
Parker spiral angle was observed between 2012/171 and 2012/208. The
magnetic field strength varied from 0.07 nT to 3.36 nT prior to the boundary
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Fig. 3. 48-s averages of the magnetic field profile during the
fifth crossing of the boundary into the heliosheath depletion
region (see Fig. 2). The solid curve is a sigmoid function, B(t)
= B2 + [B1 – B2]/[1 + exp(t – to)/(w/4.4)], which provides an
excellent fit to the data (coefficient of determination R
0.98). The parameter w gives the time required for B to
change from 10% to 90% of the way to the asymptotic values