REPORTS

but there is a statistically significant difference connected to their source (the blunt termination 16. V. Florinski, G. P. Zank, N. V. Pogorelov, J. Geophys. Res.
from the spiral field direction in the HDR, name- shock), because V1 crossed a topologic bound- 110, A07104 (2005).
17. S. N. Borovikov, N. V. Pogorelov, G. P. Zank, I. A. Kryukov,
ly lA – lP = 17° T 1° and dA – dP = 14° T 2° as ary in the magnetic field of the inner heliosheath Astrophys. J. 682, 1404–1415 (2008).
shown in Fig. 2. The magnetic polarity of the beyond the last magnetic connection point to 18. E. N. Parker, Interplanetary Dynamical Processes
magnetic field in the HDR indicates that it has the termination shock (27). Alternatively, the en- (Interscience Publishers, New York, 1963).
moved from the southern hemisphere to the po- ergetic particles could have escaped into inter- 19. M. Opher et al., Nature 462, 1036–1038 (2009).
20. N. V. Pogorelov, J. Heerikhuisen, J. J. Mitchell, I. H. Cairns,
sition of V1 in the northern hemisphere. The stellar space, if the heliosheath magnetic field G. P. Zank, Astrophys. J. 695, L31–L34 (2009).
small departure from the spiral field direction reconnected with the interstellar magnetic field 21. V. Izmodenov et al., Space Sci. Rev. 146, 329–351
might be the result of a flow that carried the beyond the position of V1. (2009).
magnetic field northward in the heliosheath to 22. J. Heerikhuisen et al., Astrophys. J. Lett. 708, L126–L130
(2010).
the location of V1. It has been suggested that such References and Notes 23. P. C. Frisch, Proc. 10th Ann. Int. Astrophys. Conf. AIP
a flow moves northward in the heliosheath be- 1. E. C. Stone et al., Science 309, 2017–2020 (2005). Conf. Proc. 1436, 239 (2012).
tween a “magnetic wall” or “magnetic barrier” and 2. L. F. Burlaga et al., Science 309, 2027–2029 (2005). 24. G. P. Zank et al., Astrophys. J. 763, 20 (2013).
the heliopause at the latitude of V1 (5, 25). 3. R. B. Decker et al., Science 309, 2020–2024 (2005). 25. H. Washimi, T. Tanaka, Space Sci. Rev. 78, 85–94
4. N. V. Pogorelov et al., Astrophys. J. Lett. 750, L4 (1996).
Increasingly strong magnetic fields from the
(2012). 26. L. F. Burlaga, N. F. Ness, Astrophys. J. 749, 13 (2012).
middle of 2010 until at least the middle of 2011 5. H. Washimi et al., Mon. Not. R. Astron. Soc. 416, 27. D. J. McComas, N. A. Schwadron, Astrophys. J. 758, 19
(possibly extending up to 150, 2012 as shown in 1475–1485 (2011). (2012).
this paper) were reported in (26), where it was 6. M. Opher et al., Astrophys. J. 734, 71 (2011). 28. www.srl.caltech.edu/ACE/ASC/coordinate_systems.html
suggested that these strong magnetic fields might 7. S. M. Krimigis, E. C. Roelof, R. B. Decker, M. E. Hill,

Downloaded from https://www.science.org at University of Texas El Paso on April 11, 2024

Nature 474, 359–361 (2011). Acknowledgments: T. McClanahan and S. Kramer provided
be related to a magnetic wall or magnetic barrier. support in the processing of the data and D. Berdichevsky
8. E. C. Stone et al., Proc. 32nd Int. Cosmic Ray Conf. 12,
Thus, it is conceivable that the HDR corresponds 29 (2011). computed correction tables for the three sensors on each of the
to this northward heliosheath flow near the helio- 9. R. B. Decker, S. M. Krimigis, E. C. Roelof, M. E. Hill, two magnetometers. N.F.N. was partially supported by NASA
pause, and the boundary of the HDR represents a Nature 489, 124–127 (2012). grant NNX12AC63G to the Catholic University of America.
10. E. C. Stone et al., Science 341, 150–153 (2013). L.F.B. was supported by NASA contract NNG11PN48P. The data
boundary of material that was moving radially are available at NASA’s Virtual Heliospheric Observatory
closer to the Sun. The strong magnetic fields ob- 11. W. R. Webber, F. B. McDonald, Geophys. Res. Lett. 40,
(http://vho.nasa.gov/), maintained within the Heliospheric Physics
1665–1668 (2013).
served from mid-2010 to 270, 2012 could be an 12. K. Behannon et al., Space Sci. Rev. 21, 235 (1997).
Laboratory at NASA’s Goddard Space Flight Center.
interaction region that extends into the HDR, 13. D. B. Berdichevsky, Voyager mission, detailed processing Supplementary Materials
produced by the collision of these two flows. The of weak magnetic fields; constraints to the uncertainties www.sciencemag.org/cgi/content/full/science.1235451/DC1
stronger magnetic field in the HDR might be of the calibrated magnetic field signal in the Voyager Supplementary Text
missions (2009); http://vgrmag.gsfc.nasa.gov/ Fig. S1
produced in response to the reduction of pres-
Berdichevsky-VOY_sensor_opu090518.pdf.
sure owing to the absence of energetic particles. 14. E. C. Stone et al., Space Sci. Rev. 21, 355 (1977). 21 January 2013; accepted 30 May 2013
The absence of energetic particles could indicate 15. L. F. Burlaga, N. F. Ness, J. Geophys. Res. 116, A05102 Published online 27 June 2013;
that magnetic lines passing V1 were no longer (2011). 10.1126/science.1235451

Voyager 1 Observes Low-Energy magnetic field even though the field intensity
abruptly increased by 60%, indicating that the
magnetic field lines in this region originated at
Galactic Cosmic Rays in a Region the Sun, not from interstellar space (2). So, V1
appears to have entered a previously unknown
Depleted of Heliospheric Ions region that is depleted of energetic heliospheric
ions and accessible to low-energy cosmic rays
[see also (3, 4)].
E. C. Stone,1* A. C. Cummings,1 F. B. McDonald,2† B. C. Heikkila,3 N. Lal,3 W. R. Webber4 The first indication of a heliospheric deple-
tion region was observed on 28 July 2012, when
On 25 August 2012, Voyager 1 was at 122 astronomical units when the steady intensity of the intensity of protons from inside the helio-
low-energy ions it had observed for the previous 6 years suddenly dropped for a third time and sphere with energies 0.5 MeV ≤ E ≤ 60 MeV
soon completely disappeared as the ions streamed away into interstellar space. Although the abruptly decreased and subsequently recovered
magnetic field observations indicate that Voyager 1 remained inside the heliosphere, the intensity 5 days later (counting rates C and D in Fig. 1).
of cosmic ray nuclei from outside the heliosphere abruptly increased. We report the spectra of A second decrease on 13 August lasted 8 days
galactic cosmic rays down to ~3 × 106 electron volts per nucleon, revealing H and He energy and was followed 4 days later by the durable
spectra with broad peaks from 10 × 106 to 40 × 106 electron volts per nucleon and an increasing entry of V1 into the heliospheric depletion region
galactic cosmic-ray electron intensity down to ~10 × 106 electron volts. on 25 August. The magnetic field increased simul-
taneously with the decreases in energetic protons,
key objective of the Voyager Cosmic (GCR) nuclei and electrons in the interstellar suggesting that lower-energy plasma may also

A
1
Ray Subsystem (1) is the determination
of the intensity of galactic cosmic-ray
medium outside of the heliosphere. On 25 August
2012, Voyager 1 (V1) entered a region where
the heliospheric ions were depleted and replaced
have escaped, with the resulting decrease in plas-
ma pressure leading to a compression of the mag-
netic field (2).
California Institute of Technology, Pasadena, CA 91125, by low-energy GCR nuclei and electrons. This The intensity changes for four distinct pop-
USA. 2University of Maryland, College Park, MD 20742, would have been expected had V1 crossed the ulations of energetic particles are strongly cor-
USA. 3NASA/Goddard Space Flight Center, Greenbelt, MD
20771, USA. 4New Mexico State University, Las Cruces, NM heliopause, the boundary separating the solar related as shown in Fig. 1. Because of their small
88003, USA. wind plasma and magnetic field from the in- mass, the GCR electrons have the smallest radii
*Corresponding author. E-mail: ecs@srl.caltech.edu terstellar plasma and magnetic field. However, of gyration around the magnetic field lines,
†Deceased there was no change in the direction of the typically 0.0006 astronomical units (AU) for a

150 12 JULY 2013 VOL 341 SCIENCE www.sciencemag.org

 REPORTS
10-MeV electron in a heliosheath magnetic field enhanced electron intensities are closely aligned seen in the counting rate of >70-MeV cosmic-
of 0.4 nT. Because V1 crosses that distance in with the five boundaries of the regions of en- ray nuclei. For example, V1 crosses the 0.025-AU
<1.5 hours, the electrons provide the sharpest hanced magnetic field that occurred on day of gyroradius of a 100-MeV proton in ~2.5 days.
indication of when V1 crossed the boundary the year (DOY) 210.6, 215.6, 225.7, 233.5, and Anomalous cosmic rays (ACRs) are also accel-
of the region where there is enhanced access of 237.7 (2). The heavier ions have larger gyroradii erated in the outer heliosphere, and at ener-
GCRs from outside. The edges of the regions of that result in broader intensity transitions, as gies below ~100 MeV per nucleon their intensity

Fig. 1. The counting rates (6-hour averages) of

four different energetic particle species in the
vicinity of the depletion region. (A) (y axis on
right) GCR nuclei (mainly protons with E > 70 MeV)
penetrating the High Energy Telescope 1 (HET 1).
(B) (y axis on left) GCR electrons with energies be-
tween 6 and ~100 MeV observed by the Electron
Telescope (TET). (C) (y axis on left) Protons with 7
to 60 MeV stopping in HET 1 (rate shown is divided
by 11.55) are mainly anomalous cosmic rays be-
fore 2012/238 (25 August) and galactic cosmic rays

Downloaded from https://www.science.org at University of Texas El Paso on April 11, 2024

after that. (D) (y axis on left) Low-energy particles
observed in the LET A (rate shown is divided by
124.5) are mainly protons with 0.5 to ~30 MeV ac-
celerated at the termination shock and in the helio-
sheath plus a scaled background rate of 0.017 s−1
because of higher-energy nuclei. Three distinct
periods in 2012 on days 210 to 215 (28 July to
2 August), 226 to 233 (13 to 20 August), and from
238 (25 August) are indicated by vertical lines corre-
sponding to the magnetic boundaries of the deple-
tion region (2). The simultaneous intensity changes
coincide with abrupt increases and decreases in the magnitude of the magnetic field, suggesting that, after two brief encounters with a depletion region
or regions, V1 durably entered a broad depletion region on 25 August (DOY 238).

Fig. 2. Intensities of H, He, and O from V1 for the

last half of 2012. (A) One-day average intensities
of H with 3.0 to 7.8 MeV. Intensities are shown for
two of the four LETs [see (1) for arrangement of the
telescopes]. The bore sight of LET D is pointed roughly
perpendicular to the magnetic field direction. The
bore sight of LET C is oriented at 90° to that of LET D.
(B) Similar to (A) except for 13-day averages of the
intensities of He with 3.0 to 7.8 MeV per nucleon. (C)
A
Similar to (B) except for O with 5.4 to 13.9 MeV per
nucleon, and the average intensity from the LET A, B,
and C is plotted instead of only LET C, in order to
improve the statistical significance of the result. The
LET A bore sight is oppositely directed to that of LET
C, and the bore sights of LET A, B, and D form an
orthogonal set. Error bars indicate statistical uncer-
tainties (T1 s).

2012.5 2012.6 2012.7 2012.8 2012.9

www.sciencemag.org SCIENCE VOL 341 12 JULY 2013 151

REPORTS
greatly exceeds that of GCRs. As seen by the Fig. 3. Differential energy spec-
intensity of 7- to 60-MeV protons in Fig. 1, the tra of H, He, C, and O from V1.
intensities of ACRs also decrease in the deple- Two spectra are shown for H: one for
tion regions as they escape out of the heliosphere. a reference period before the deple-
Their disappearance beginning on DOY 238 re- tion region was reached, 2011/274
veals the intensity of GCRs of the same energy to 2012/121, and one for a new
that have flowed into the depletion region from period within the depletion region,
outside. 2012/303 to 366. For these two H
Voyager has four Low-Energy Telescopes spectra, intensities from all four LETs
(LETs) arranged in an orthogonal array (1). As were averaged together. At higher
energies, >57 MeV, only intensities
illustrated in Fig. 2, there are substantial differ-
from HET 2 are shown. The same tel-
ences in intensity among the telescopes over ex-
escopes were used in deriving the
tended periods. LET D is oriented so that it He spectrum for the period 2012/303
observes protons with pitch angles from 50 to to 366. For C, intensities from all
100 to the spiral magnetic field, which is point- four LETs were used, and at higher
ing outward along the spiral direction (2), so LET energies, >20 MeV per nucleon, in-
D is sensitive to ions moving outward along the tensities from both HETs were aver-
field (q < 90°) and also inward (q > 90°). LET C aged together. For O with 5.4 to
observes protons with pitch angles of from 110 17.1 MeV per nucleon, only LET A,

Downloaded from https://www.science.org at University of Texas El Paso on April 11, 2024

to 160, so it is sensitive to ions coming inward B, and C were used in order to min-
along the spiral field. imize the contribution from helio-
These ions originate at the termination shock spheric particles. For O with 17.1 to
or in the heliosheath and diffuse mainly along 21.6 MeV per nucleon, only HET 2
the spiral magnetic field. Before 28 July, there was used, and for energies >21.6
was sufficient scattering on the field line that MeV per nucleon, intensities from
the intensity of ions in LET C was the same as in both HETs were averaged together.
LET D. However, during the first two decreases, The C and O spectra are for the pe-
the intensity of ions diffusing inward toward LET riod 2012/261 to 366. Several esti-
C was significantly lower than in LET D, indi- mates of the local interstellar galactic
cating some of the ions spiraling outward were cosmic ray H and He spectra are
1 10 100 1000
shown. The solid lines are model a
lost and not scattered back toward V1.
from Ip and Axford (13). The dotted
After the boundary crossing on 25 August lines represent the leaky-box model
(DOY 238), the intensity of H ions (protons) from Webber and Higbie (14). The dashed lines are the DC model from Moskalenko et al. (19), and the dot-
in LET C dropped much more rapidly than the dash line for H is from Fisk and Gloeckler (20). Error bars indicate statistical uncertainties (T1 s).
intensity of protons in LET D. The two rates
converged after 2012.72 (DOY 263), indicating
that low-energy protons from the heliosphere Fig. 4. Differential energy spectra
were no longer dominating the intensity near of electrons from the V1 TET. Two
5 MeV at V1. Instead, the intensity was isotropic, pairs of spectra are shown, one
as expected if the remaining protons are low- pair for a reference period before the
energy GCRs diffusing in along the magnetic new region was reached, 2011/274
field. to 2012/121, and one pair for a pe-
Figure 2 also shows similar anisotropies for riod within the new region, 2012/303
He and O. Although longer time averages are to 366. The open symbols represent
required because of the lower intensities, there is spectra derived by using response
evidence for losses in LET C during the events functions from a prelaunch acceler-
before DOY 238 and for extended periods after. ator calibration. The solid symbols use
At those times, the outward flow was observed response functions from a GEANT4
in LET D, while LET C was already observing a simulation. The intensity differences
lower intensity of isotropic GCRs diffusing inward in the solid and open symbols for a
along the magnetic field. The longer persistence given period are an indication of
of the heavier heliospheric ions is consistent the systematic uncertainly in the
with the expectation that singly ionized 5–MeV electron spectrum that is proportion-
al to E –1.45 T 0.09 in the new region.
per nucleon He+ and 9–MeV per nucleon O+
The method used in deriving the
with gyroradii of 0.022 and 0.12 AU, respec-
energy spectra is described in the
tively, will have larger scattering mean free supplementary materials. Three esti-
paths and will be scattered into the loss cone more mates of the local interstellar GCR
slowly than 5-MeV H+, which has a gyroradius electron spectrum are shown. The
of 0.0054 AU. solid line is model a from Ip and
The disappearance of most of the heliospheric Axford (13), the dot-dash line rep-
ions after 25 August 2012 provides an oppor- resents model IS7 from Webber and 1 10 100 1000
tunity to examine the energy spectra of GCRs Higbie (21), the short-dashed line
to lower energies than previously possible (4). represents the polar model from
Figure 2 shows that GCR H and He dominate Langner et al. (16), and the long-dashed and dotted lines are from Strong et al. (17) for electron source
the 3– to 7.8–MeV per nucleon energy range spectra proportional to E −1.6 and E −2.0, respectively, and include positrons.

152 12 JULY 2013 VOL 341 SCIENCE www.sciencemag.org

 REPORTS
from ~2012.77 (DOY 282) onward, because the The leaky-box model of Ip and Axford (13) this could be a disconnection region where the
intensity apparently became isotropic at that addresses the low-energy portion of the GCR spiral field has been convected far enough be-
time, consistent with the expectation for GCRs. spectra (Fig. 3). Their model appears to have yond the termination shock so that there is not
Figure 3 shows the energy spectra for the period about the right peak intensity for H, but the an effective connection to the source of anom-
2012/303 to 366 for H and He from 3 to sev- energy of the peak is lower than observed. For alous cosmic rays at the termination shock (18).
eral hundred MeV per nucleon along with an He, both the peak intensity and the energy of Further development of this and other possible
energy spectrum of H from a period before the the peak are somewhat displaced from the models will benefit our understanding of the
onset of the recent activity. This reference spec- observations. At higher energies, >70 MeV per region beyond 122 AU that Voyager 1 is now
trum shows the dominance of the spectrum by nucleon, the leaky-box model spectra from Webber exploring.
the termination shock particle and ACR helio- and Higbie (14) are in good agreement with the
spheric particle populations below ~100 MeV observed H and He spectra.
References and Notes
per nucleon during this time. These particles have Also shown in Fig. 3 are C and O spectra for 1. E. C. Stone et al., Space Sci. Rev. 21, 355 (1977).
largely streamed away in the more recent pe- 2012/261 to 366. This longer period improves 2. L. F. Burlaga, N. F. Ness, E. C. Stone, Science 341,
riod (DOY 303 to 366), replaced by the inflow the statistical significance of the observations and 147–150 (2013); 10.1126/science.1235451.
of low-energy GCRs. is justified by the intensity-versus-time profile of 3. S. M. Krimigis et al., Science 341, 144–147 (2013);
10.1126/science.1235721.
However, it is uncertain whether GCRs have O with 5.4 to 13.9 MeV per nucleon for LETs 4. W. R. Webber, F. B. McDonald, Geophys. Res. Lett. 40,
fully unimpeded access into this region. In ad- A, B, and C shown in Fig. 2. In constructing the 1665–1668 (2013).
dition, the GCR intensity immediately outside energy spectra for O in Fig. 3, the LET D tel- 5. K. Herbst, B. Heber, A. Kopp, O. Sternal, F. Steinhilber,
the heliosphere may be lower than the galactic escope was not used, because it has a more per- Astrophys. J. 761, 17 (2012).

Downloaded from https://www.science.org at University of Texas El Paso on April 11, 2024

6. K. Scherer et al., Astrophys. J. 735, 128 (2011).
intensity because of modulation in the local sistent heliospheric contribution resulting from
7. R. D. Strauss, M. S. Potgieter, S. E. S. Ferreira, H. Fichtner,
interstellar medium (5–7). Recent models indi- its bore sight looking nearly perpendicular to the K. Scherer, Astrophys. J. 765, L18 (2013).
cate a reduction of ~25 to ~40% in the intensity magnetic field direction. Even ignoring LET D, 8. J. R. Jokipii, in The Outer Heliosphere: The Next Frontiers,
of 100-MeV protons, with corresponding posi- there may be some residual heliospheric contri- K.Scherer, H. Fichtner, H. J. Fahr, E. Marsch, Eds.
tive radial gradients of ~0.5 and ~0.9%/AU near bution present below ~10 MeV per nucleon. The (Pergamon, New York, 2001), pp. 513–519.
9. K. Lodders, Astrophys. J. 591, 1220–1247 (2003).
the heliopause (7), although other models suggest C and O energy spectra above ~10 MeV per nu- 10. J. A. Simpson, Annu. Rev. Nucl. Part. Sci. 33, 323–382
that there should be no interstellar gradient (8). cleon are very similar, with the peak C intensity (1983).
The time dependence of the observed intensities occurring at ~70 MeV per nucleon. The C/O ratio 11. A. C. Cummings, E. C. Stone, C. D. Steenberg, Astrophys. J.
after the disappearance of heliospheric protons for the energy range 21.6 to 106 MeV per nucleon 578, 194–210 (2002).
12. K. Scherer, H. Fichtner, S. E. S. Ferreira, I. Büsching,
(DOY 270 to 366 in Fig. 1) corresponds to is 0.95 T 0.06, consistent with GCR observations M. S. Potgieter, Astrophys. J. 680, L105–L108
gradients of –1.4 T 0.9%/AU for 7- to 60-MeV at higher energies at 1 AU (10, 15) and not with (2008).
protons and –1.0 T 0.4%/AU for >70-MeV GCR the ACR ratio of 0.005 (11). All previous mea- 13. W.-H. Ip, W. I. Axford, Astron. Astrophys. 149, 7
nuclei, mainly protons. The gradient of 6- to surements below ~100 MeV per nucleon represent (1985).
14. W. R. Webber, P. R. Higbie, J. Geophys. Res. 114,
100-MeV electrons is also small, only –0.6 T particles decelerated from much higher energies
A02103 (2009).
0.6%/AU from DOY 239 to 366. Thus, there is by the solar modulation process. 15. J. S. George et al., Astrophys. J. 698, 1666–1681
no evidence for a positive radial gradient in the The intensity of electrons with ~6 to 100 MeV (2009).
current region. had jumps in concert with the GCR nuclei (Fig. 16. U. W. Langner, O. C. de Jager, M. S. Potgieter, in
Proceedings of the 27th International Cosmic Ray
The GCR H and He spectra have the same 1), indicating that V1 is observing GCR elec-
Conference, 7 to 15 August 2001, Hamburg, Germany
shape from ~3 to 346 MeV per nucleon. The H/He trons in this energy range as opposed to elec- (International Union of Pure and Applied Physics, Berlin,
ratio has been determined in three energy ranges trons accelerated in the heliosphere. The ratio of 2001), vol. 10, pp. 3992–3995.
corresponding to different telescope and/or op- intensities of the two periods shown in Fig. 4 is 17. A. W. Strong, E. Orlando, T. R. Jaffe, Astron. Astrophys.
eration modes of the instrument. In the lowest roughly a factor of two over the energy range 534, A54 (2011).
18. D. J. McComas, N. A. Schwadron, Astrophys. J. 758, 19
energy band, 3 to 7.8 MeV per nucleon, we find shown, indicating an energy-independent diffu- (2012).
the H/He ratio to be 11.9 T 0.4. In the 7.8–to– sive mean free path for 6- to 60-MeV electrons. 19. I. V. Moskalenko, A. W. Strong, J. F. Ormes, M. S. Potgieter,
57 MeV per nucleon band, the ratio is 12.9 T The energy spectrum for the new region is Astrophys. J. 565, 280–296 (2002).
0.6, and in the highest energy interval, 134 to proportional to E –1.45 T 0.09 and has a spectral 20. L. A. Fisk, G. Gloeckler, Astrophys. J. 744, 127
(2012).
346 MeV per nucleon, the ratio is 12.6 T 0.3. shape that is very similar to the theoretical esti- 21. W. R. Webber, P. R. Higbie, J. Geophys. Res. 113,
These uncertainties are purely statistical, and there mates of the interstellar electron spectrum of Ip A11106 (2008).
may be systematic uncertainties as well. How- and Axford (13) and Langner et al. (16), but the
ever, the reasonably good agreement of the overall intensity is about a factor of two below Acknowledgments: This work was supported by NASA
(NNN12A012). This paper is dedicated to the memory of
ratios across the range from 3 to 346 MeV per those estimates. The observed intensity exceeds Frank McDonald, whose leadership in the cosmic-ray
nucleon suggests that systematic uncertainties an extrapolation of the spectrum from a diffu- investigation on Voyager began in 1972. His contributions
are likely small. The peak intensities of the GCR sion model by Strong et al. (17) that assumes an continued until the day of his passing, just after Voyager
spectra of H and He are in the ~10– to 40–MeV electron source spectrum proportional to E−1.6 1 durably entered the depletion region and fulfilled his vision
of observing low-energy galactic cosmic rays from the local
per nucleon energy range. below a few GeV, as implied by fits to radio
interstellar medium. This paper benefited substantially
The H/He ratio of 12.9 T 0.6 in the energy synchrotron emission. A source spectrum of E−2 from discussions during meetings of the International Team
region of the peak in the spectra is consistent would better match the electron spectrum (Fig. on the Physics of the Heliopause at the International
with the recommended abundance in the solar 4) but would not be consistent with the radio Space Science Institute in Bern, Switzerland.
photosphere, 12.6 (9). It differs from previous observations without other adjustments to the
cosmic-ray observations of 4.7 T 0.5 at 100 MeV model. Supplementary Materials
www.sciencemag.org/cgi/content/full/science.1236408/DC1
per nucleon observed at 1 AU, where the spectra The presence of a region having a spiral mag- Supplementary Text
and abundance ratios are modified by the effects netic field, but depleted of energetic heliospheric Fig. S1
of solar modulation (10). It also differs from the particles and accessible by low-energy GCR nu- References
ACR ratio of 4.1 (11), indicating that ACRs do clei and electrons, is an important feature of the 11 February 2013; accepted 24 May 2013
not dominate GCRs outside the heliosphere as interaction between the heliosphere and the local Published online 27 June 2013;
has been suggested (12). interstellar medium. It has been suggested that 10.1126/science.1236408

www.sciencemag.org SCIENCE VOL 341 12 JULY 2013 153