Bryant C. Heikkila

An asymmetric solar wind termination shock

Voyager 2 crossed the solar wind termination shock at 83.7 au in the southern hemisphere, ∼10 au closer to the Sun than found by Voyager 1 in the north. This asymmetry could indicate an asymmetric pressure from an interstellar magnetic field, from transient-induced shock motion, or from the solar wind dynamic pressure. Here we report that the intensity of 4–5 MeV protons accelerated by the shock near Voyager 2 was three times that observed concurrently by Voyager 1, indicating differences in the shock at the two locations. (Companion papers report on the plasma, magnetic field, plasma-wave and lower energy particle observations at the shock.) Voyager 2 did not find the source of anomalous cosmic rays at the shock, suggesting that the source is elsewhere on the shock or in the heliosheath. The small intensity gradient of Galactic cosmic ray helium indicates that either the gradient is further out in the heliosheath or the local interstellar Galactic cosmic ray intensity is lower than expected.

Voyager 1 observes low-energy galactic cosmic rays in a region depleted of heliospheric ions.

Unexpected Magnetic Highway The heliopause is thought to separate the heliosphere (the bubble of plasma and magnetic field originating at the Sun) from interstellar plasma and magnetic field. In August last year, the Voyager 1 spacecraft, which was launched 35 years ago, was 18.5 billion kilometers away from the Sun, close to the expected location of the heliopause. Krimigis et al. (p. 144, published online 27 June) report observations of energetic ions and electrons by Voyager 1 that suggest that a sharp and distinct boundary was crossed five times over ∼30 days. Burlaga et al. (p. 147, published online 27 June) found that the magnetic field direction did not change across any of the boundary crossings, indicating that Voyager 1 had not crossed the heliopause but had entered a region in the heliosphere that serves as a magnetic highway along which low-energy ions from inside stream away and galactic cosmic rays flow in from interstellar space. Stone et al. (p. 150, published online 27 June) report the spectra of low-energy galactic cosmic rays in this unexpected region. The Voyager 1 spacecraft entered an unexpected region of the heliosphere at the boundary with interstellar space. On 25 August 2012, Voyager 1 was at 122 astronomical units when the steady intensity of low-energy ions it had observed for the previous 6 years suddenly dropped for a third time and soon completely disappeared as the ions streamed away into interstellar space. Although the magnetic field observations indicate that Voyager 1 remained inside the heliosphere, the intensity of cosmic ray nuclei from outside the heliosphere abruptly increased. We report the spectra of galactic cosmic rays down to ~3 × 106 electron volts per nucleon, revealing H and He energy spectra with broad peaks from 10 × 106 to 40 × 106 electron volts per nucleon and an increasing galactic cosmic-ray electron intensity down to ~10 × 106 electron volts.

Enhancements of energetic particles near the heliospheric termination shock

The spacecraft Voyager 1 is at a distance greater than 85 au from the Sun, in the vicinity of the termination shock that marks the abrupt slowing of the supersonic solar wind and the beginning of the extended and unexplored distant heliosphere. This shock is expected to accelerate ‘anomalous cosmic rays’, as well as to re-accelerate Galactic cosmic rays and low-energy particles from the inner Solar System. Here we report a significant increase in the numbers of energetic ions and electrons that persisted for seven months beginning in mid-2002. This increase differs from any previously observed in that there was a simultaneous increase in Galactic cosmic ray ions and electrons, anomalous cosmic rays and low-energy ions. The low-intensity level and spectral energy distribution of the anomalous cosmic rays, however, indicates that Voyager 1 still has not reached the termination shock. Rather, the observed increase is an expected precursor event. We argue that the radial anisotropy of the cosmic rays is expected to be small in the foreshock region, as is observed.

The relative recovery of galactic and anomalous cosmic rays in the distant heliosphere: Evidence for modulation in the heliosheath

At Voyager 1 (46 AU, 33°N) the recovery of anomalous cosmic rays (ACR) is found to be very different from that of galactic cosmic rays (GCR) following the passage of the large interplanetary disturbances produced by the intensive solar activity of March/June 1991. If the modulation boundary for the GCR were at the termination shock, where anomalous cosmic rays are believed to originate, it would be expected that the intensity of the higher-energy galactic cosmic rays would recover more rapidly than the relatively low energy anomalous component. On the contrary, we find that the time constant for the recovery of 265 MeV/nucleon GCR He is approximately twice as large as that of 43 MeV/nucleon ACR He^+ and 13 MeV/nucleon O^+. A regression plot of the ACR versus GCR intensity indicates a broad plateau in the ACR intensity over a period of several years while the GCR continues to increase. These differences in the relative recovery of the ACR and GCR strongly suggest that the combined interplanetary disturbances in the form of a global merged interaction region (GMIR) produced by the March/June 1991 solar activity remain an effective modulation agent for GCR after passing beyond the termination shock and into the region of the heliosheath. Some 0.37 years after the passage of the leading portion of the GMIR by Voyager 1, there is a large anisotropy in the ACR He^+. One possible interpretation of this anisotropy is that it is produced by the initial flow of the ACR back into the heliosphere at the time that the leading portion of the interplanetary disturbance moves beyond the termination shock. If this interpretation is correct, then the inferred transit time between Voyager 1 and the termination shock of the GMIR along with an estimate of its velocity at 40 AU based on similar features in the Voyager 1 and Pioneer 11 energetic particle data give a value of the heliocentric distance to the termination shock of 88.5 ± 7 AU at ∼33°N in early 1992.

GALACTIC COSMIC RAYS IN THE LOCAL INTERSTELLAR MEDIUM: VOYAGER 1 OBSERVATIONS AND MODEL RESULTS

Since 2012 August Voyager 1 has been observing the local interstellar energy spectra of Galactic cosmic-ray nuclei down to 3 MeV nuc^(−1) and electrons down to 2.7 MeV. The H and He spectra have the same energy dependence between 3 and 346 MeV nuc^(−1), with a broad maximum in the 10–50 MeV nuc^(−1) range and a H/He ratio of 12.2 ± 0.9. The peak H intensity is ~15 times that observed at 1 AU, and the observed local interstellar gradient of 3–346 MeV H is −0.009 ± 0.055% AU^(−1), consistent with models having no local interstellar gradient. The energy spectrum of electrons (e^− + e^+) with 2.7–74 MeV is consistent with E^(−1.30±0.05) and exceeds the H intensity at energies below ~50 MeV. Propagation model fits to the observed spectra indicate that the energy density of cosmic-ray nuclei with >3 MeV nuc^(−1) and electrons with >3 MeV is 0.83–1.02 eV cm−3 and the ionization rate of atomic H is in the range of 1.51–1.64 × 10^(−17) s^(−1). This rate is a factor >10 lower than the ionization rate in diffuse interstellar clouds, suggesting significant spatial inhomogeneity in low-energy cosmic rays or the presence of a suprathermal tail on the energy spectrum at much lower energies. The propagation model fits also provide improved estimates of the elemental abundances in the source of Galactic cosmic rays.

Galactic cosmic ray H and He nuclei energy spectra measured by Voyagers 1 and 2 near the heliospheric termination shock in positive and negative solar magnetic polarity cycles

Using data from the Voyager 1 and 2 spacecraft, we have followed the intensity variations of H, He and C + O nuclei between 1998 and 2008 and determined the spectra for H and He at the time of minimum modulation in 1998, when the solar magnetic polarity was positive and again in 2008 when the solar magnetic polarity was negative. At these times these data are representative of conditions near a heliospheric termination shock assumed to be located at ∼90 AU. Above ∼400 MeV/nuc for He nuclei the 11-year solar modulation cycle observed at the Earth is not seen; instead there is a 22-year variation. The negative polarity cycle intensities above ∼150 MeV/nuc are higher than those in the positive polarity cycle by a factor of 1.4–1.7 times for both H and He nuclei. Below ∼100 MeV/nuc the C nuclei intensities are similar in the two cycles to within ±10%. These observations are compared with theoretical calculations which also show a negative to positive polarity cycle intensity difference at higher energies, most likely associated with energy changes due to drifts near the termination shock, but the comparison suggests that improved estimates of the local interstellar spectra are required.

Passage of a large interplanetary shock from the inner heliosphere to the heliospheric termination shock and beyond: Its effects on cosmic rays at Voyagers 1 and 2

Using data from the charged particle telescopes on V1 and V2 we have followed the progress of a large interplanetary shock as it passes V2 at a distance of 79 AU at about 2006.16, then later crosses the heliospheric termination shock finally reaching V1 at a distance ∼100 AU. A decrease ∼15% is observed in the V2 >70 MeV rate starting at 2006.19 and three smaller decreases starting at 2006.29, 2006.50 and 2006.86 are observed at V1. From the timing of the first two decreases at V1 we are able to determine that the average shock speed slows down at the termination shock from ∼600 km s^(−1) to 210–270 km s^(−1). Decreases of ∼30–50% in anomalous He and galactic H are observed at V2 when the shock passes this location inside the termination shock. Smaller decreases are observed for both of these components when the weakened interplanetary shock passes V1 at 2006.50. These results define the extent and magnitude of solar modulation effects on cosmic rays caused by transients both inside and beyond the termination shock.

Transient intensity changes of cosmic rays beyond the heliospheric termination shock as observed at Voyager 1

This paper continues our studies of temporal variations of cosmic rays beyond the heliospheric termination shock (HTS) using Voyager 1 (V1) data when V1 was beyond 94 AU. This new study utilizes cosmic ray protons and electrons of several energies. Notable transient decreases of 5–50% are observed in galactic cosmic ray nuclei and electrons at V1 shortly after similar decreases are observed at Voyager 2 (V2) still inside the HTS. These decreases at V1 appear to be related to the large solar events in September 2005 and December 2006 and the resulting outward moving interplanetary shock. These two large interplanetary shocks were the largest observed at V2 after V1 crossed the HTS at the end of 2004. They were observed at V2 just inside the HTS at 2006.16 and 2007.43 providing timing markers for V1. From the timing of the intensity decreases observed at V1 as the shocks first reach the HTS and then later reach V1 itself, we can estimate the shock speed beyond the HTS to be between 240 and 300 km s^(−1) in both cases. From the timing of the decreases observed when the shock first reaches the HTS and then several months later encounters the heliopause, we can estimate the heliosheath thickness to be 31 ± 4 and 37 ± 6 AU, respectively, for the two sequences of three decreases seen at V1. These values, along with the distances to the HTS that are determined, give distances from the Sun to the heliopause of 121 ± 4 and 124 ± 6 AU, respectively.

Using transient decreases of cosmic rays observed at Voyagers 1 and 2 to estimate the location of the heliospheric termination shock

We have examined the intensity-time profiles of outward moving transient decreases of anomalous and galactic cosmic rays observed by Voyager 1 (Vl) and Voyager 2 (V2) in 1998 and 1999 in the outer heliosphere. The goal of this study is to compare these intensity-time profiles with those obtained by le Roux and Fichtner [1999] using numerical simulations of the time-dependent cosmic ray modulation produced by the passage of large global merged interaction regions. Their calculations show that when these interaction regions reach the heliospheric termination shock, they weaken rapidly causing the intensity of both galactic and anomalous cosmic rays to increase rapidly. They suggest using this time of rapid increase and the propagation times of these events to determine the location of the termination shock. Three outward propagating transient decreases were observed during the 1998–1999 time period. Because of local temporal variations the first two events did not exhibit a le Roux-Fichtner type of rapid recovery. However, the third event starting on day 50 of 1999 at V1 exhibited a very sharply defined intensity-time profile with a very rapid recovery starting on day 88 ± 3 of 1999 that was observed in three energy channels on V1. From this time delay we are able to determine that the termination shock was 9.5–10.7 AU beyond V1 or at a distance of 82.6–83.8 AU at this time. The intensity-time profile at V2 was less sharp, but the rapid recovery that was observed occurred at the same time (±5 days) as that at V1, thus confirming the conclusions from the V1 data. At its present outward speed of 3.6 AU yr−1, V1 would be expected to encounter the termination shock at this location as early as the last quarter of 2001.

Termination shock particle spectral features

Spectral features of energetic H ions accelerated at the termination shock may be evidence of two components. At low energies the energy spectrum is ∼−1.55, with break at ∼0.4 MeV to E−2.2. A second component appears above ∼1 MeV with a spectrum of E−1.27 with a break at ∼3.2 MeV. Even though the intensities upstream are highly variable, the same spectral break energies are observed, suggesting that these are durable features of the source spectrum. The acceleration processes for the two components may differ, with the lower energy component serving as the injection source for diffusive shock acceleration of the higher energy component. Alternatively, the spectral features may result from the energy dependence of the diffusion tensor that affects the threshold for diffusive shock acceleration.