Opher et al. 2006 showed that an interstellar magnetic field parallel to the
plane defined by the deflection of interstellar hydrogen atoms can produce a
north/south asymmetry in the distortion of the solar wind termination shock.
This distortion is consistent with Voyager 1 and Voyager 2 observations of the
direction of field-aligned streaming of the termination shock particles
upstream the shock. The model also indicates that such a distortion will result
in a significant north/south asymmetry in the distance to the shock and the
thickness of heliosheath. The two Voyager spacecraft should reveal the nature
and degree of the asymmetry in the termination shock and heliosheath.Comment: 6 pages, 5 figures, AIP Proceedings of the 5th IGPP "The Physics of
  the Inner Heliosheath: Voyager Observations, Theory and Future Prospects

Gombosi, Tamas

Liewer, Paulett C.

Opher, Merav

Stone, Edward C.

English

arXiv

Merav Opher

Edward C. Stone

Paulett C. Liewer

Tamas Gombosi

Crossref

Global asymmetry of the heliosphere

Opher et al. showed that an interstellar magnetic field parallel to the plane defined by the deflection of interstellar hydrogen atoms can produce a north/south asymmetry in the distortion of the solar wind termination shock. This distortion is consistent with Voyager 1 and Voyager 2 observations of the direction of field-aligned streaming of the termination shock particles upstream the shock. The model also indicates that such a distortion will result in a significant north/south asymmetry in the distance to the shock and the thickness of heliosheath. The two Voyager spacecraft should reveal the nature and degree of the asymmetry in the termination shock and heliosheath

Stone, E. C.

Caltech Authors - Main

Global asymmetry of the heliosphere
Merav Opher∗, Edward C. Stone†, Paulett C. Liewer∗∗ and Tamas
Gombosi‡
∗George Mason University, 4400 University Drive, Fairfax, VA 22030
†California Institute of Technology, Pasadena, CA 91125
∗∗Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109
‡Center for Space Environment Modeling, University of Michigan, Ann Arbor, MI
Abstract. Opher et al. [1] showed that an interstellar magnetic field parallel to the plane defined by
the deflection of interstellar hydrogen atoms can produce a north/south asymmetry in the distortion
of the solar wind termination shock. This distortion is consistent with Voyager 1 and Voyager 2
observations of the direction of field-aligned streaming of the termination shock particles upstream
the shock. The model also indicates that such a distortion will result in a significant north/south
asymmetry in the distance to the shock and the thickness of heliosheath. The two Voyager spacecraft
should reveal the nature and degree of the asymmetry in the termination shock and heliosheath.
PACS: 96.50.Xy,96.60.-j,96.60.Iv
INTRODUCTION
After 27 years of anticipation, Voyager 1 crossed the inward moving termination shock
(TS) at 94 AU and is now exploring the heliosheath [2, 3, 4]. The twin Voyager spacecraft
are probing the northern and southern hemispheres of the heliosphere. As Voyager 1
crossed the TS and began exploring the heliosheath it has become increasingly clear
that this previously unexplored region is full of surprises. Using in situ spacecraft data
to constrain the shape of the heliosphere is challenging because they are single point
observations. For a quantitative global understanding of the three dimensional structure
of the heliosphere, it is necessary to use modeling in conjunction with observations to
probe this region.
In mid 2002, Voyager 1 began observing strong beams of energetic termination shock
particles (TSPs) streaming outward along the spiral magnetic field. The strong upstream
TSP beams were observed much of the time until Voyager 1 crossed the shock at 94 AU
in December 2004. Jokipii et al. [5] and Stone et al. [4] suggested that the upstream
beaming resulted from a non-spherical shock. For a spherical shock, Voyager 1 would
observe upstream TSPs streaming inward along the magnetic field. With a non-spherical
shock, Voyager 1 could be connected to the TS along magnetic field lines that crossed
the TS (the source of TSPs) and then crossed back into the supersonic solar wind.
In order to know Voyager 1 measurements in 2002 it is important to understand the
shape of the heliosphere. The interaction of the solar system with the local interstellar
medium requires an intensive modeling effort because this problem is inherently three
dimensional and involves solar and interstellar magnetic fields, ionized and neutral
atoms, and cosmic rays[6, 7]. The size and shape of the heliosphere will depend on
the properties of both the solar wind and the local interstellar medium.
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Based on the polarization of light from nearby stars, Frisch [8, 9] suggested that the
magnetic field direction is parallel to the galactic plane (and directed toward l ∼ 70◦).
Voyager 3kHz radio emission data also show preferred source locations in a plane
parallel to the galactic plane [10]. On the other hand, Lallement et al. [11], mapping the
solar Lyman-α radiation that is resonantly backscattered by interstellar hydrogen atoms,
found that the neutral hydrogen flow direction differs from the helium flow direction by
4◦. The plane of the H deflection (HDP) is tilted from the ecliptic plane by ∼ 60◦ and
is consistent with an interstellar magnetic field parallel to the HDP plane[12]. However,
Pogorolev et al.[13] show that the H deflection does not uniquely determine the plane of
the magnetic field.
If the interstellar magnetic field is inclined to the interstellar velocity, it can produce
a lateral or north-south asymmetry in the heliospheric shape [14, 15]. Opher et al. [1]
showed that that an interstellar magnetic field parallel to the H-deflection plane (HDP)
can produce a north/south asymmetry in solar wind termination shock that is consistent
with Voyager 1 and Voyager 2 observations of the direction of streaming of the particles.
We review these results and comment on the Heliosphere global asymmetry.
RESULTS
The model that we used is based on the BATS-R-US code, a three-dimensional mag-
netohydrodynamic (MHD) parallel, adaptive grid code developed by University of
Michigan[16] and adapted for the outer heliosphere[17, 18]. The boundary conditions
are described in [1]. We considered fields parallel to the HDP with different inclination
angles α , where α is the angle between the interstellar magnetic field and the interstellar
wind velocity (Figure 1).
The coordinate system in Figure 1 is the conventional one for MHD heliospheric
models, with the z-axis as the solar rotation axis of the sun, the interstellar velocity
direction in the x direction, with y completing the right handed coordinate system.
This coordinate system is essentially the heliographic inertial coordinate system (HGI),
except that in the heliographic inertial coordinates, the direction of the interstellar wind
is 5.07◦ (HGI latitude) and 178.23◦ (HGI longitude). The HGI latitude for Voyager 1 and
Voyager 2 become 29.1◦ and −31.2◦ relative to the interstellar wind in the x− y plane,
a latitude difference of 60.3◦. Although this conventional coordinate system ignores the
5.07◦ tilt of the solar rotation axis in respect to the interstellar wind, more significantly
it ignores the tilt of the heliospheric current sheet.
Figure 2 shows the heliopause (HP) surface for BISM in the HDP plane with an
inclination angle α = 45◦, BISM,y < 0, and BISM = 1.8µG (the same values are used
in Figures 2-4). The heliopause is asymmetric, north/south and east/west. It is distorted
in the direction of the plane of the interstellar magnetic field and bulges outward in the
northern hemisphere where the heliospheric current sheet is deflected (see also Figure
3). The current sheet deflection also results in an indentation at the solar equatorial plane
that is apparent in both Figures 2 and 3. The heliosheath is thicker in the north where
the HCS is deflected. There is a smaller north/south asymmetry in the termination shock
that will be discussed further below.
Figure 4 shows the intersection of the cones at 29.1◦ and −31.2◦ that contain the
46
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FIGURE 1. The coordinate system showing α , the inclination angle between the interstellar wind V ISM
and magnetic field BISM , and the angle β between the plane of the field and the x-y plane; β = 60 ◦ for
HDP. The z-axis is the solar rotational axis. Note that this differs from the Solar Ecliptic Coordinate (SEC)
system in which the z-axis is perpendicular to the ecliptic plane.
FIGURE 2. A model with BISM in the HDP plane and α = 45◦ and BISM,y < 0. The 3D iso-surface of
the HP is denoted (yellow). The black lines are the interstellar magnetic field lines in the HDP plane. The
contours denote magnetic field in the HDP plane
spiral solar magnetic field lines crossing Voyager 1 and Voyager 2 respectively. In both
the northern and southern hemisphere, the cones intersect the surface of the termination
shock nearer the equator in the nose region where the shock is closer to the Sun. The
termination shock also bulges out in the northern hemisphere. As discussed in Opher et
al.[1], 2 AU upstream of the shock Voyager 1 is connected to the shock along the field
line in the direction toward the Sun, allowing TSPs to stream outward along the field as
observed.
The distortion is larger in the southern hemisphere, so that 5 AU upwind of the shock
Voyager 2 is connected to the shock and the TSP streaming should be inward along the
spiral magnetic field line, opposite to that for Voyager 1, as has been observed [19]. The
47
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FIGURE 3. Contours of magnetic field strength B(nT ) in the meridional plane with α = 45 ◦ and
BISM,y < 0. The black lines are the interstellar magnetic field and the white arrows denote the trajectories
of Voyager 1 and Voyager 2. The heliospheric current sheet (deep blue) is deflected northward in the
heliosheath.
FIGURE 4. The intersection of the termination shock surface (yellow) with the cone of interplanetary
field line connecting to Voyager 1 and Voyager 2 (black lines). The trajectories of Voyager 1 and Voyager
2 are denoted (white arrows) as well as the solar equatorial plane (red line). B ISM is in the HDP plane withα = 45◦ and BISM,y < 0.
48
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FIGURE 5. The red and blue squares are respectively the termination shock locations for V1 and V2
and the red and blue circles are the respective heliopause locations. The Izmodenov et al. and Opher et al.
hydro models do not include an interplanetary magnetic field (IMF) and do not have a large north/south
asymmetry.
radial distance to the shock at the latitudes of Voyager 1 and 2 decreases significantly as
the inclination angle α increases from α = 45◦ to 60◦, with the shock 7 to 10 AU closer
in the south at Voyager 2 than in the north at Voyager 1[1].
Although there was no direct indication of how far Voyager 1 was from the shock
when the upstream episodes of TSPs were observed, MHD models based on Voyager
2 solar wind pressure measurements [20] indicate that the distance was less than 3 to
4 AU . This suggests that α is likely in the range of 30◦ to 60◦ for which the distortion is
maximum[1].
Figure 5 summarizes the locations of the TS and the HP scaled from various recent
models. We normalized the model results to 90 AU (red squares), the estimated average
location of the shock at the latitude of Voyager 1 as the shock moves inward and
outward over the solar cycle. The conclusions that can be drawn are: a) For the models
including an interplanetary magnetic field, the TS ranged between 76 to 82 AU for
Voyager 2; b) There is somewhat less North-South asymmetry with a weaker BISM;
c) The heliosheath thickness at Voyager 1 is about 55 to 59 AU . For the model that
has an interplanetary magnetic field and strong BISM, the thickness at Voyager 2 is 33
to 45 AU . Although this is a preliminary calculation and does not incorporate a tilted
current sheet, it does indicate the possibility of a significant north-south asymmetry in
the heliosheath thickness; d) These models do not include neutrals that may affect the
degree of asymmetry. However, models such as Linde 1998 [21] (same model as in
Linde et al. 1998 [22]) that include neutrals as a fluid show a north-south asymmetry
with a ratio of ∼ 1.13 for the north/south distance to the termination shock and 1.44 for
the ratio for the HP distance, similar to models without neutrals. Izmodenov et al. [12],
using a kinetic treatment with the BISM in the HDP plane, found a smaller north-south
asymmetry. Their treatment, however, did not include the solar magnetic field.
49
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CONCLUSIONS
The recent results by Opher et al.[1] indicate that an interstellar magnetic field (B ∼
2µG) in the H-deflection plane can distort the heliosphere in a manner consistent with
the streaming observed by Voyager 1 and Voyager 2. The model also indicates that
such a distortion will result in a significant north/south asymmetry in the distance to
the shock and the thickness of heliosheath, and it is reasonable to expect that Voyager
2 will encounter the shock in the next two years. When combined with a more realistic
heliospheric model with a tilted current sheet, this should provide additional information
about the strength and inclination of the local interstellar magnetic field.
ACKNOWLEDGMENTS
The authors would like to thanks the use of Columbia cluster NASA Ames. Part of
this work is the result of research performed at the Jet Propulsion Laboratory of the
California Institute of Technology under a contract with the National Aeronautics and
Space Administration.
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A Three-Dimensional Adaptive Multifluid MHD Model of the Heliosphere,

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Global Asymmetry of the Heliosphere

Global Asymmetry of the Heliosphere

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