This paper discusses plasma characteristics in the heliosheath region before the heliopause (HP), at the HP, and in the very local interstellar medium (VLISM). The Voyager 2 (V2) HP was a sharp boundary where the radial plasma currents went to background levels. The radial flow speeds derived from 53-85 keV (V1) and 28-43 keV (V2) ion data decreased about 2 years (8 AU) before the HP at V1 and V2. A speed decrease was not observed by the V2 plasma instrument until 160 days (1.5 AU) before the HP crossing when V2 entered the plasma boundary layer where the plasma density and 28-43 keV ion intensity increased. We determine the HP orientation based on the plasma flow and magnetic field data and show these observations are consistent with models predicting a blunt HP. Variations are observed in the currents observed in the VLISM; roll data from this region clearly show the plasma instrument observes the interstellar plasma and may be consistent with larger than expected VLISM temperatures near the HP

Belcher, John W.

Burlaga, Leonard F.

Cummings, Alan C.

Decker, Robert B.

Opher, Merav

Richardson, John D.

Stone, Edward C.

English

Caltech Authors - Main

Journal of Physics: Conference Series
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Voyager 2 Observations Near the Heliopause
To cite this article: John D. Richardson et al 2020 J. Phys.: Conf. Ser. 1620 012016
 
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19th Annual International Astrophysics Conference
Journal of Physics: Conference Series 1620 (2020) 012016
IOP Publishing
doi:10.1088/1742-6596/1620/1/012016
1
 
 
 
 
 
Voyager 2 Observations Near the Heliopause 
John D. Richardson1, John W. Belcher1, Leonard F. Burlaga2, Alan C. Cummings3, Robert B. 
Decker4, Merav Opher5, Edward C. Stone3 
 
                    1Massachusetts Institute of Technology, Cambridge, MA 02139 USA 
            2NASA Goddard Space Flight Center, Greenbelt, MD, USA 
             3California Institute of Technology, Pasadena, CA 91125 USA 
            4Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723 USA 
            5Boston University, Boston, MA, USA 
                                              jdr@space.mit.edu 
 
Abstract. This paper discusses plasma characteristics in the heliosheath region before the heliopause (HP), at the 
HP, and in the very local interstellar medium (VLISM). The Voyager 2 (V2) HP was a sharp boundary where the 
radial plasma currents went to background levels. The radial flow speeds derived from 53-85 keV (V1) and 28-
43 keV (V2) ion data decreased about 2 years (8 AU) before the HP at V1 and V2. A speed decrease was not 
observed by the V2 plasma instrument until 160 days (1.5 AU) before the HP crossing when V2 entered the  
plasma boundary layer where the plasma density and 28-43 keV ion intensity increased.   We determine the HP 
orientation based on the plasma flow and magnetic field data and show these observations are consistent with 
models predicting a blunt HP. Variations are observed in the currents observed in the VLISM; roll data from this 
region clearly show the plasma instrument observes the interstellar plasma and may be consistent with larger 
than expected VLISM temperatures near the HP. 
1.  Introduction 
Voyager 2  (V2) crossed the heliopause (HP) on Nov. 5 2018 when it was 119 AU from the Sun at a 
heliolatitude of 31° S [1,2,3,4], close to distance the predicted by Washimi et al. [5]. The Voyager 1 
(V1) plasma instrument failed in 1980, so V2 provides the first plasma data from the HP boundary 
regions, at the HP, and in the very local interstellar medium (VLISM) [6]. V1 crossed the HP in 
August 2012 at 121.7 AU [7,8,9,10] and in 2020 passed 150 AU. This paper describes the V2 
observations near the HP and compares them with those from V1.  
2.   HP Overview 
 
    The V1 HP crossing was initially controversial because, although the magnetic field magnitude B 
and galactic cosmic ray (GCR) intensities increased and the anomalous cosmic ray (ACR) intensities 
decreased as predicted, the magnetic field direction did not change as was expected [1,2,3]. Plasma 
oscillations observed beyond the boundary showed that the electron densities were much higher than 
those in the heliosheath (HSH) indicating that V1 had crossed the HP [4]. V1 has remained in the 
VLISM since 2012.  
    The V2 HP crossing was similar enough to the V1 crossing that it was immediately recognized that 
V2 had entered the VLISM. The plasma science (PLS) instrument showed that the ion flow changed at 
the same time as changes were observed in B and in the GCR and ACR intensities. Figure 1 shows the 
currents observed in the four PLS Faraday cups [11]. The A, B, and C cup look directions lie about a 
19th Annual International Astrophysics Conference
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doi:10.1088/1742-6596/1620/1/012016
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cone whose central axis points towards earth; they are well-positioned to observe the outward flowing 
solar wind and HSH plasma. The D cup looks at a right angle to this direction and was oriented as 
close as possible to the expected VLISM flow direction. Figure 1 shows currents from the lowest 
energy channel (10-30 eV) for the A and C cups and  from the lowest two (10-57 eV) for the B cup, 
which looks most directly into the HSH flow and thus has significant current in two channels. The A 
cup looks furthest from the HSH flow and has the lowest currents. Before the HP the currents in all 
four cups increased by a factor of 2 from day 140 to 190. This region of enhanced currents is the 
plasma boundary layer. The average currents decrease slowly until day 300, when they fall rapidly to 
background levels at the HP. The D-cup is often compromised by noise; we show only the most 
reliable data so the sampling rate is less but the profile shape is the same. The currents in the three 
sunward-looking cups go to background levels on day 309 of 2018. At this time outward flow ceases 
and few ions enter these detectors. The D-cup currents decrease at the HP, but not to background 
levels. These currents beyond the HP are the first direct measurements of the VLISM plasma. 
 
Figure 1. Currents in the sunward-looking A, B, and C cups (top) and in the sideways looking D-cup 
(bottom). The A, B, and C cup currents fall to background levels at the HP crossing on day 309. 
 
We compare the two Voyager HP crossings to see which features are intrinsic to the HP boundary 
and which may be time and/or location dependent. Figure 2 shows that both HP crossings have a broad 
HP boundary region with complex structure. The V1 2012 HP crossing has an abrupt increase in the 
magnetic field strength B, a sharp decrease of HSH energetic particles, and an increase in the GCR 
counting rate. Two HP precursors were observed at V1 near days 210 and 230, with decreases in B and 
the energetic particle intensities and increases in the GCR counts centered on days 212 and 230 of 
2012 [1,2,3]. After the HP, B remained high and steady, the HSH energetic particles disappeared, and 
GCR counts plateaued. Plasma wave (PWS) data confirmed the densities were high as expected in the 
VLISM [4]. The precursors may be flux tubes moving from the VLISM into the HSH [2]. 
The V2 crossing did not have precursors like those at V1. On day 309 B sharply increased, the HSH 
energetic particle intensity decreased, the GCR counting rate increased, and the plasma dynamic 
pressure and the radially outward plasma currents dropped to background levels [7,8,10]. PWS 
observations from days 35–55 of 2019 confirmed the density outside the HP was 0.04 cm-3 [9]. 
The biggest surprise of the V1 HP crossing was that the direction of B did not change [1]. Models 
can explain this observation [12], but disagree on whether this lack of B rotation at V1 was a 
19th Annual International Astrophysics Conference
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coincidence of geometry or if the rotation of the VLISM B toward the Parker spiral direction were an 
intrinsic HP feature. At the V2 HP crossing the direction of B again did not change [6], showing that 
the lack of rotation of B may be a normal HP feature.  
 
 
Figure 2. The V1 (left) and V2 (right) HP crossings.  Left: The V1 magnetic field magnitude, the 
GCR count rate, and the 53-85 keV ion intensity. The dotted lines show, from left to right, the first 
GCR increase, two close approaches to the HP, and the HP crossing. Right: The V2 plasma dynamic 
pressure, the magnetic field magnitude, the GCR count rate, and the 28-43 keV ion intensity. The 
dotted lines show, from left to right, the beginning of the plasma boundary layer, the beginning of the 
magnetic barrier, and the HP. 
3.   Plasma flows and the HP orientation 
 
Voyager data starting about two years ( ~8 AU) before both HP crossings may contain precursors of 
the HP.  Figure 3 compares the flow speeds in the RTN coordinate system derived from LECP and 
CRS particle data at V1 and V2 using the Compton-Getting effect [13] and the V2 speeds reported by 
the PLS instrument. The V1 VR speeds are lower than those at V2 throughout the HSH and go to zero 
about 2 years (8 AU) before the HP. At V2, the PLS VR slowly decreases across the HSH, then drops 
sharply just before the HP. Starting about two years (~8 AU) before the HP, the VR derived from V2 
LECP data vary between near zero and the PLS VR values, with the average LECP VR about 40 km/s 
below the PLS VR values. These differences are not understood, but the low VR derived from LECP 
data at both V1 and V2 two years and 8 AU before the HP suggests they could be related to the HP 
structure. 
The magnitude of VT is also much larger at V2 than V1, perhaps because V1 is nearer the longitude 
of the HP nose. The LECP V2 VT speeds are larger than those from PLS; this difference is probably a 
19th Annual International Astrophysics Conference
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sampling bias since the highest angle flows are outside the viewing angle of PLS [13]. The magnitude 
of VN decreases at both spacecraft before the HP. 
The orientation of the HP at the V1 and V2 crossings may provide information on the global shape 
of the HP. The plasma velocity and magnetic field at the HP should be parallel to the HP boundary. 
We use median speed data from within 0.1 AU of the HP to calculate the angles of the HP plane for 
V2. We assume the LECP VT values are correct. The LECP values of VT = 139 km/s and VR = 23 
km/sec give an RT angle (atan(VR/VT)) of 261° for the HP. The PLS VR in this region is 49 km/s, 
giving an RT angle of  250°±7°; the angles in both cases signify that the HP is blunt. VN is not 
measured by LECP, but using the PLS VN of -23 ± 17 km/s gives a (RT)N plane angle of 9°± 6°, also 
in the direction indicating the HP is blunt. The (RT)N angle is defined as atan (VN /sqrt(VR2 + VT2)). 
The observed median magnetic field angles for the same time period are 266° ± 8° in the RT plane and 
19° ± 4° in the (RT)N plane; the uncertainty in the BR component is 0.06 nT and in the BT and BN 
components are 0.03 nT [7]. These results are in agreement given the uncertainties and are consistent 
with expectations of a blunt HP. The predicted magnetic field angles from a recent model (model b of 
[14]) gives a magnetic field angle in the RT plane of 260° ± ~3° and an (RT)N angle of 11.3° (+11°, -
17°), consistent with the observations reported here. 
 
 
 
 
Figure 3. The HSH plasma speeds in the RTN coordinate system from PLS and LECP data for V2 and 
LECP and CRS data for V1. The left axis is at the termination shock and the right axis at the HP. 
19th Annual International Astrophysics Conference
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4.  Plasma Boundary Layer and Magnetic Barrier 
 
V2 provided the first plasma data in the HP region. Figures 1 and 2 show that a plasma boundary 
region, not observed at V1, was entered 160 days (1.5 AU) ahead of the HP. This region is identified 
by the enhanced PLS currents and dynamic pressure, which correspond to a plasma speed decrease of 
30%, a density increase of a factor of 2, and a temperature increase of 30%, giving the 70% increase in 
the dynamic pressure in Fig. 2 [6]. The LECP 28-43 keV energy ion intensity in the plasma boundary 
region correlates well with the plasma pressure (and density). A small increase in the GCR slope was 
observed at the beginning of this region. No change in the magnetic field occurred at the start of the 
plasma boundary layer. The origin of this region is not known; compression of the plasma as it 
approaches the HP would increase the density, temperature, and LECP ion flux as observed, but would 
also cause an increase in B which was not observed. 
Figure 2 shows the V1 HP crossing is preceded by a 88 day (~1.3 AU) wide region that started at 
day 150 of 2012 with a step increase in the GCR rates and ended at the HP crossing on day 238 with a 
second step increase of GCRs, an increase in B, and a dropout of low-energy ions to background levels 
[1,2,3]. This region probably contains open field lines that connect the solar wind and VLISM 
magnetic fields, allowing GCRs to enter the HSH and HSH electrons to exit.  
V2 observed an increase in the slope of the GCR intensity 80 days (0.95 AU) before the HP, similar 
in location to the first V1 GCR increase. At V2, the GCR increase corresponded to the start of the 
magnetic barrier, a region of increased B not observed at V1 [1]. The average B in the magnetic barrier 
was ~0.40 ± 0.06 nT, significantly greater than the average B ~0.13 nT from days 1–229 of 2018 [7]. 
These strong fields were comparable in strength to the magnetic field in the VLISM observed by 
Voyager 1. Little change in the plasma or LECP ions was observed at the start of the magnetic barrier. 
The V2 magnetic field increased to 0.7 nT at the HP, significantly larger than the 0.4 nT field observed 
at V1. The larger B in the VLISM at V2 than V1 was expected, since a larger magnetic pressure in the 
south would move the TS position in the V2 direction inward compared to V1.  
 
5.  Plasma in the VLISM 
 
PLS observed VLISM plasma directly for the first time. Although the energy and flow direction of 
the VLISM are at the limits of what PLS can observe, data from roll maneuvers clearly show that PLS 
observes the VLISM [6]. Figure 1 shows VLISM currents from the 10–30 eV energy channel of the 
sideways-looking PLS D-cup.  PWS observations show plasma oscillations before the current increase 
at day 420, suggesting that this current increase is related to the shock that generates these oscillations 
[8]. The PLS data in the region where plasma oscillations are observed, combined with the density of 
0.04 cm-3 derived from the PWS data, show that the VLISM temperatures are larger than expected, 
30,000°–50,000° K, compared to most predicted values of 15,000°–30,000° K [6]. This higher 
temperature is near the upper end of the range predicted by Zank et al. [15], who incorporated VLISM 
heating via secondary charge exchange.  
Figure 4 shows the data during spacecraft rolls that clearly indicate PLS observes VLISM plasma, 
with the current vs. roll angle profile behaving qualitatively as expected. The data are from rolls on 
days 164 and 255 of 2019 and show data from two different energy channels for each roll. These data 
are affected by background noise; the currents shown are corrected for noise assuming a constant noise 
level across the roll. The dashed lines show simulations of currents during the rolls. The upper left plot 
shows that temperature of 54,000° K fits better than lower temperatures (37,000° and 24,000°), 
consistent with the higher than expected temperatures mentioned above, and the other three plots also 
show that 54,000° K gives a reasonable fit to the data. However, more roll data are needed to provide 
an estimate of the uncertainties in the noise correction procedure. After the roll on day 255 of 2019 the 
instrument playback was changed to give us double the time resolution during these rolls. The first two 
19th Annual International Astrophysics Conference
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rolls with this increased time resolution were aborted due to spacecraft problems and V2 is currently in 
a command moratorium due to DSN scheduled downtime, but these rolls are planned to resume in late 
2020 and should provide the best data to date on the plasma in the VLISM.  
 
 
 
 
Figure 4.  Observed currents in different energy ranges shown in the title of each plot [m2: 10-12 
eV; m2: 12-14 eV; m5 19-22 eV) from spacecraft rolls on days 164 and 255 of 2019. The dashed lines 
show simulations of the expected currents during these rolls for VR, VT, VN = -10, 20,-10 km/s, N =0.1 
cm-3, and T = 54,000° (red), 37,000° (green), 24,000° K (blue) and in the upper left panel and 54,000° 
K in the other panels. As V2 rolls the PLS D cup first looks away from the VLISM flow, then nearest 
to the flow before the end of the roll. 
6. Summary 
 
    This paper presents and discusses recent Voyager 2 observations near the HP. The first signs of the 
HP may have been changes in the speed derived from LECP data starting ~2 years (8 AU) before the 
HP. The PLS instrument did not see a decrease in speed until 160 days (1.5 AU) before the HP, in the 
19th Annual International Astrophysics Conference
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doi:10.1088/1742-6596/1620/1/012016
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newly discovered plasma boundary layer characterized by lower speeds and higher densities. The V2 
HP was a sharp boundary where the radial plasma currents went to background levels. Observations of 
the flows and magnetic field at the HP at V2 are consistent with models predicting a blunt HP. 
Variations are observed in the currents observed in the interstellar medium; roll data from this region 
clearly show the plasma instrument observes the interstellar plasma and may be consistent with large 
than expect temperatures near the HP. 
  
Acknowledgments. JDR was supported under NASA contract 959203 from the Jet Propulsion 
Laboratory to the Massachusetts Institute of Technology and NASA grant 80NSSC19K0260. RBD 
was supported by NASA contract NNN06AA01C. ECS and ACC were supported by NASA grant 
NNN12AA01C.  ACC, RBD, MO, and JDR acknowledge support from NASA grant 18-DRIVE18_2-
0029, Our Heliospheric Shield. 
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Voyager 2 Observations Near the Heliopause

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