The document summarizes recent findings from Voyager 1, which has been traveling through the heliosheath region between the solar wind termination shock and the heliopause boundary. Key findings include: 1) The radial velocity of plasma detected by Voyager 1 has decreased nearly linearly from 70 km/s to 0 km/s over the past 3 years and has remained at 0 km/s for the past 8 months, indicating Voyager 1 has entered a transition layer with zero radial flow. 2) This transition layer was not predicted by models and contradicts expectations of an abrupt discontinuity at the heliopause. 3) Analysis of plasma velocity measurements suggests Voyager 1 may have crossed the he

1. LETTER doi:10.1038/nature10115
Zero outward flow velocity for plasma in a
heliosheath transition layer
Stamatios M. Krimigis1,2, Edmond C. Roelof1, Robert B. Decker1 & Matthew E. Hill1
Voyager 1 has been in the reservoir of energetic ions and electrons predictions7—generally assumed by conceptual models—of a sharp
that constitutes the heliosheath since it crossed the solar wind discontinuity at the heliopause.
termination shock1–3 on 16 December 2004 at a distance from Figure 1a shows that there has been little change in the intensity of
the Sun of 94 astronomical units (1 AU 5 1.5 3 108 km). It is now lower-energy ions at the position of Voyager 1 since April 2007.
22 AU past the termination shock crossing4. The bulk velocity of Indeed, a flat intensity profile and near-constant power-law energy
the plasma in the radial–transverse plane has been determined5 spectrum (j / E2c) have been persistently observed at all ion energies
using measurements of the anisotropy of the convected energetic from ,40 keV to 2 MeV, as is indicated by the constancy of the expo-
ion distribution6. Here we report that the radial component of the nent c (Fig. 1a, diamonds), which has been computed at 53–85 keV but
velocity has been decreasing almost linearly over the past three is typical of the entire energy range. We note for later reference the
years, from 70 km s21 to 0 km s21, where it has remained for small but steep intensity decrease during the last three months of 2010
the past eight months. It now seems that Voyager 1 has entered a observed for all low-energy ions (because of the nearly constant spec-
finite transition layer of zero-radial-velocity plasma flow, indi- tral shape).
cating that the spacecraft may be close to the heliopause, the border The heliosheath plasma flow velocity as estimated from ion intensity
between the heliosheath and the interstellar plasma. The existence anisotropies measured using LECP8 is shown in Fig. 1b–d. In Fig. 1b,
of a flow transition layer in the heliosheath contradicts current the radial component, VR, undergoes a long, steady, nearly linear
decline beginning at ,70 km s21 and reaching ,0 km s21 in April
2010 with an abrupt change in slope to nearly zero. This remarkable
Distance from Sun (AU) near-zero radial velocity continued until at least February 2011 (the
97.76 101.35 104.93 108.53 112.11 115.68 spacecraft velocity of 17 km s21 has been subtracted). We regard the
350
(cm–2 s–1 sr–1 MeV–1)
300 Voyager 1, heliosheath a abrupt change in ÆhVR/htæ in April 2010 (from negative to zero) as an
Ion intensity
Protons, 53–85 keV γ indication that Voyager 1 had entered a finite ‘transition layer’, ter-
250
–0.9 minology suggested in ref. 9. The more conventional conception of the
200 –1.2 heliopause has been that of a surface (separatrix) between the heated
150 –1.5
solar wind plasma of the heliosheath and the cold interstellar plasma of
–1.8
100 the very local near-interstellar medium, such that the theoretically
100 b
〈∂VR/∂t〉 = –18.8 ± 1.5 km s–1 yr–1
80 Figure 1 | Directional velocity measurements from Voyager 1 at the edge of
the heliosphere. The Low Energy Charged Particle (LECP) instrument on
VR (km s–1)
60
Voyager 1 provides angular information via a mechanically stepped platform in
40 eight 45u sectors. a–c, The velocity components of plasma flow (b, c) are
20 calculated, using a method published elsewhere4,6, from the directional
0 measurements of the 53–85-keV ion intensities, whose scan-average intensity
and power-law spectral index (c) are shown in a. Briefly, spacecraft-frame
–20
intensities, j(Q), are represented by a second-order Fourier series in the scan
0
〈VT′〉 = –38.7 ± 1.4 km s–1 angle, Q (0 , Q , 2p), where j(Q) 5 A0 1 A1sin(Q 2 Q1) 1 A2sin(Q 2 Q2). The
c parameters A0, (A1, Q1) and (A2, Q2), which generate the harmonic anisotropy
–20
amplitudes, j1 and j2, are determined by a least-squares fit6 to intensities in
VTʹ (km s–1)
–40 sectors 1–7. For j2 = j1, j1 < 2(c 1 1)V/v, implying that V 5 vj1/2(c 1 1),
where v is the velocity of the energetic particles. The velocity is resolved into
–60
Transition components in the Voyager 1 R–T9 instrument scan plane, which is rotated 20u
–80 layer anticlockwise about the radial (1R) direction from the R–T plane in the
flow conventional R–T–N heliographic polar coordinates in which the transverse
20 direction (1T) is that of planetary motion around the Sun. The error bars (in
d a–c; smaller than the point sizes in a) are computed from the Poisson standard
0 deviations in the directional rates. Plots of the adjacent channels (40–53 keV
VRTʹ (km s–1)
and 80–139 keV) show nearly identical results4. Such agreement independent of
–20
particle energy is an assurance that the energetic ion distribution is indeed being
–40
advected with the plasma bulk velocity. A least-squares fit to the last ten points
T′ of VR (b, heavy dashed line) gives an average velocity of ÆVRæ 5 1.0 6 2.4 km s21
30 km s–1 with an average slope of ÆhVR/htæ 5 26.1 6 12.1 km s21 yr21, such that both
–60
R mean and slope are statistically consistent with zero. In c, a least-squares fit to
–80 the transverse component, VT9, is shown by the dashed line. d, The plasma flow
2006 2007 2008 2009 2010 2011 velocity in the R–T9 plane represented as vectors, the head giving the velocity
Year (VRT9) and the tail being located at the observation time along the time axis.
1
Applied Physics Laboratory, The Johns Hopkins University, Laurel, Maryland 20723, USA. 2Academy of Athens, Athens 11527, Greece.
1 6 J U N E 2 0 1 1 | VO L 4 7 4 | N AT U R E | 3 5 9
©2011 Macmillan Publishers Limited. All rights reserved

2. RESEARCH LETTER
Figure 2 | The heliosphere and its boundaries in the general direction of heliopause estimated from energetic-neutral-atom (ENA) images made by
Voyager 1. a, Large-scale illustration depicting the solar wind radial flow inside NASA’s Cassini spacecraft. For the last, we used the simplified formula10
the termination shock and the expected deflection of the heliosheath flow jENA 5 s#dr njion < sLnjion, which relates the ENA intensity, jENA, at Cassini to
between the termination shock and the heliopause. The heliopause marks the the ion intensity, jion, along the line of sight (distance, L) between the
(theoretical) boundary between the heliosheath plasma flow and the much termination shock and the heliopause through the image pixel containing
denser, colder and slower flow of interstellar plasma being deflected around the Voyager 1. Here n 5 0.1 cm23 is the cold interstellar neutral density and s is the
heliosheath. V1, Voyager 1; V2, Voyager 2. b, Scale drawing of the unexpected (energy-dependent) charge-exchange cross-section for proton–hydrogen
transition layer that Voyager 1 has encountered within the heliosheath. For collisions. We showed11 that the ENA and ion spectra could be brought into
illustrative purposes, we have assigned a value (VN9 < 40 km s21) to the agreement at Voyager 2 with a heliosheath thickness of L2 ~54{15 AU, whereas
z30
(unmeasured) meridional velocity component throughout the layer. Voyager 1 the same normalization procedure applied to Voyager 1 results in
{11
measurements have also revealed the thickness of the transition layer, a possible L1 ~27z26 AU.
location for the heliopause and the consistency of the range of locations of the
expected gradual reduction of VR (due to the rotation of the flow so as could produce the sharp change in ÆhVR/htæ to zero observed in April
to parallel the heliopause) would end with VR asymptotically (not 2010.
abruptly) going to zero, and then only at the heliopause itself. The spatial relationships in the transition layer of the plasma flow
Figure 1c shows that the transverse velocity, VT9, fluctuated around a measurements along the radial trajectory of Voyager 1 (heliographic
mean value of ÆVT9æ < 240 km s21 for ,5 yr until near the end of latitude, 36u N) are depicted in Fig. 2. If VR remains zero, and the outer
2010, when a trend towards zero began (Fig. 1c, last four points). end of the flow transition layer is really the heliopause, then the
The swing of the velocity vector (Fig. 1d) from radial to transverse Voyager 1 observations demand that the orientation of the heliopause
was completed by June 2010. Because VR has remained near zero and is normal to the radial trajectory of Voyager 1 (regardless of the values
VT’ seems to be tending to zero at the end of 2010, any remaining flow of VT9 and VN9). The measured transition region (VR < 0) extends
will have to be in the unobserved meridional component, VN9. For an from 113.5 AU to at least 115.7 AU. To relate this location to estimates
axisymmetric heliopause, it is expected that VN’ < 30 km s21 for the of the thickness, L, of the heliosheath, we have estimated L from the
meridional flow of the deflected distant interstellar plasma ENA all-sky images from the Cassini Ion and Neutrals Camera10,11,
(V 5 26 km s21). The observed flow pattern therefore could be con- whose energy range overlaps that of LECP. Using the ENA data for
sistent with Voyager 1 now being in the deflected interstellar plasma Voyager 1 and the same method of analysis11, we compute a
flow; that is, Voyager 1 may have crossed the heliopause, having passed heliosheath thickness of L1 ~27{11 AU. Assuming that the termination
z26
through the transition layer (VR 5 0). shock is still where Voyager 1 crossed it, at 94 AU, the estimated radius
We have interpreted the trends in VR after July 2007 as a spatial of the heliopause along the trajectory of Voyager 1 should be 121 AU,
phenomenon, as it is unlikely to be all or in part temporal. We cannot which is not inconsistent with our suggestion that Voyager 1 is actually
construct a reasonable scenario dominated by temporal variations for crossing the heliopause if VR and VT9 remain near zero beyond 116 AU.
the monotonic time dependence of VR, for example by ascribing it to a This is why we called attention to the small (,27%) decrease in the
continually accelerating inward motion of the termination shock for 0.04–2-MeV ions (mentioned above, in the discussion of Fig. 1a) that
almost 3 yr that brings the heliosheath at 115 AU to a dead stop for at commenced just when VT9 began increasing to zero. If the ion intensity
least eight months throughout a region ,2.5 AU thick (the distance decrease continues, we would interpret it as a draining away of the
travelled by Voyager 1 in eight months). (All absolute distances are heliosheath’s energetic ion population into the downstream interstel-
measured relative to the Sun.) The effects of any inward motion of the lar plasma flow. However, recent activity12 at Voyager 2 suggests that it
termination shock (such as to reduce VR) would take at least 1 yr to may be a global heliospheric response to a changing magnetic config-
propagate the ,20 AU to Voyager 1, and it is hard to imagine how it uration at the Sun.
3 6 0 | N AT U R E | VO L 4 7 4 | 1 6 J U N E 2 0 1 1
©2011 Macmillan Publishers Limited. All rights reserved

3. LETTER RESEARCH
We must remember that Voyager 1, because it has no operational 9. Suess, S. T. The heliopause. Rev. Geophys. 28, 97–115 (1990).
10. Krimigis, S. M., Mitchell, D. G., Roelof, E. C., Hsieh, K. C. & McComas, D. J. Imaging the
instruments that can measure them, is ‘blind’ to the particles that interaction of the heliosphere with the interstellar medium from Saturn with
produce the suprathermal (,1–20-keV) pressure in the heliosheath Cassini. Science 326, 971–973 (2009).
that dominates the dynamics within the heliosheath and hence also the 11. Krimigis, S. M., Mitchell, D. G., Roelof, E. C. & Decker, R. B. ENA (E . 5 keV) images
from Cassini and Voyager ‘‘ground truth’’: suprathermal particle pressure in the
cross-heliopause force balance with the stress applied by the interstellar heliosheath. AIP Conf. Proc. 1302, 79–85 (2010).
magnetic field13. It is this ‘unseen’ population that must be producing 12. Decker, R. B., Roelof, E. C., Krimigis, S. M. & Hill, M. E. in Physics of the Heliosphere: A
the structure we measure as the transition layer, so there remains the 10-Year Retrospective (eds Heerikhuisen, J., Li, G. & Zank, G.) (10th Annu. Internat.
possibility that the heliopause may be completely different from any- Astrophys. Conf., American Institute of Physics, 2011).
13. Roelof, E. C. et al. Implications of generalized Rankine-Hugoniot conditions for the
thing that has been suggested by contemporary theory7,14,15. It would PUI population at the Voyager 2 termination shock. AIP Conf. Proc. 1302, 133–141
not be the first time that the Voyager observations have surprised us. (2010).
14. Izmodenov, V. V. et al. Kinetic-gasdynamic modeling of the heliospheric interface.
Received 21 January; accepted 11 April 2011. Space Sci. Rev. 146, 329–351 (2009).
15. Zank, G. P. Physics of the solar wind-local interstellar medium interaction: role of
1. Decker, R. B. et al. Voyager 1 in the foreshock, termination shock, and heliosheath. magnetic fields. Space Sci. Rev. 146, 295–327 (2009).
Science 309, 2020–2024 (2005).
Acknowledgements This work was supported at The Johns Hopkins University Applied
2. Burlaga, L. F. et al. Crossing the termination shock into the heliosheath: magnetic
Physics Laboratory by NASA. We are grateful to J. Aiello (for assistance with our
fields. Science 309, 2027–2029 (2005).
graphical presentation) and R. McNutt (for a historical summary of heliosheath
3. Stone, E. C. et al. Voyager 1 explores the termination shock region and the
terminology).
heliosheath beyond. Science 309, 2012–2020 (2005).
4. Decker, R. B., Krimigis, S. M., Roelof, E. C. & Hill, M. E. Variations of low-energy ion Author Contributions S.M.K. contributed most of the text; E.C.R. contributed to the text
distributions measured in the heliosheath. AIP Conf. Proc. 1302, 51–57 (2010). and provided theory interpretation; and R.B.D. performed the data analysis with the
5. Krimigis, S. M. et al. The Low Energy Charged Particle (LECP) experiment on the assistance of M.E.H.
Voyager spacecraft. Space Sci. Rev. 21, 329–354 (1977).
6. Krimigis, S. M. et al. Voyager 1 exited the solar wind at a distance of 85 AU from the Author Information Reprints and permissions information is available at
Sun. Nature 426, 45–48 (2003). www.nature.com/reprints. The authors declare no competing financial interests.
7. Florinski, V. et al. The dynamic heliosphere: outstanding issues. Space Sci. Rev. Readers are welcome to comment on the online version of this article at
143, 57–83 (2009). www.nature.com/nature. Correspondence and requests for materials should be
8. Scientific exploration: What a long, strange trip it’s been. Nature 454, 24–25 (2008). addressed to S.M.K. (tom.krimigis@jhuapl.edu).
1 6 J U N E 2 0 1 1 | VO L 4 7 4 | N AT U R E | 3 6 1
©2011 Macmillan Publishers Limited. All rights reserved