Characteristics of the Termination Shock: Insights from Voyager

We examine the energy spectra obtained from the cosmic ray instrument on the Voyager 1 spacecraft during 2002/215 through 2005/60. We find that the energy spectra of protons below ~20 MeV often resemble two power laws with a relatively hard index at low energies and a softer index at higher energies. The point of intersection of the two power laws is ~3 MeV. Beginning in 2005, the low-energy index is typically –1.5, corresponding to a shock strength (compression ratio) of 2.5. We attribute these characteristics to a restricted region of the solar wind termination shock that is sporadically connected to the Voyager 1 spacecraft by the interplanetary magnetic field. The absence of significant spectral variability in 2005 suggests that Voyager 1 entered a region with minimal spatial gradients of the lowest energy ions

The anomalous cosmic-ray component

This brief report is intended to update the anomalous component section of the summary report of the galactic cosmic-ray working group (Mewaldt et al., 1987), which was drafted at the March 1987 Workshop on the Interplanetary Charged Particle Environment at the Jet Propulsion Laboratory. The description of the spectrum of the anomalous cosmic-ray component is contained in section 3.3 of that report. That description is based on data analyzed through day 310 of 1986, and in it we proposed that the energy spectrum of the various species of the anomalous component could be derived by scaling from two generic spectra. Two generic spectra were required because the energy spectrum of the anomalous component changed shape near the time of the solar magnetic field reversal in 1980. These two generic spectra are shown in Figure 2 of the summary report

The Intensities of Cosmic Ray H and He Nuclei at ~250 MeV/nuc Measured by Voyagers 1 and 2 - Using these Intensities to Determine the Solar Modulation Parameter in the Inner Heliosphere and the Heliosheath Over a 40 Year Time Period

We have determined the solar modulation potential, phi, vs. time that is observed at Voyager 1 and 2 from measurements of the H and He nuclei intensities at a common energy of 250 MeVnuc. The H nuclei have a rigidity 0.7 GV, the He nuclei 1.4 GV. These measurements cover a 40 year time period, which includes almost 4 cycles of solar 11 year sunspot variations, throughout the inner heliosphere out to the HTS at distances of 95 AU and 85 AU, respectively at V1 and V2, and then beyond in the heliosheath. Inside the HTS the modulation potential vs. time curves at V1 and V2 show a very similar temporal structure to those observed at the Earth. During a later period of maximum solar modulation from 2000.0 to 2005.0 when V1 and V2 are in the outer heliosphere between 60-94 AU, the main temporal features of the modulation potential curves at all 3 locations match up with appropriate time delays at V1 and V2 if it is assumed that spatially coherent structures are moving outward past V1 and V2, with outward speeds of up to 700 Kms negative 1. After 2004.0 V1 and V2 are at latitudes of positive 35 and negative 30 respectively, placing lower limits on the latitude extent of these structures. Beyond the HTS in the heliosheath the modulation potential slowly decreases at both spacecraft with only a weak evidence of the unusual modulation minimum observed at the Earth in 2009, for example. A sudden decrease of the modulation potential 50 MV for both H and He nuclei occurs at V1 just before the heliopause crossing at about 122 AU. This decrease has not yet been observed at V2, which is now at 113 AU and still observing a modulation potential 60 MV.Comment: 28 pages, 9 Figure

Possible role of transients on the energy spectra of energetic particles at the solar wind termination shock

We suggest that a series of merged interaction regions interacted with the solar wind termination shock for several years prior to its crossing by Voyager 1 in December 2004 and created the observed spectral shape of energetic particles. We also find that the charge-state of He with 3 to 70 MeV/nuc and O with 1 to 14 MeV/nuc is +1 and that the rigidity dependence of the diffusion coefficient from 170 MV to 2.7 GV is given by

Onset of Solar Modulation in the Outer Heliosphere as Seen in Anomalous Cosmic Rays

We examine intensities and gradients of anomalous cosmic rays determined from observations on the Voyager 1 and 2 spacecraft to study the rapid increase in solar modulation that begins near mid-2000. We suggest that these effects are due to the arrival in the region of the spacecraft of complex magnetic field topologies associated with the reversal of the Sun's magnetic field

Location of the Termination Shock at Solar Maximum

During the recent solar maximum, Voyager 1 was beyond 80 AU. Extrapolation of the small gradients of anomalous cosmic rays at solar minimum and the larger gradients at solar maximum indicate that the solar wind termination shock is at ≲ 92 AU at the beginning of 2002

Inferring Energetic Particle Mean Free Paths from Observations of Anomalous Cosmic Rays in the Outer Heliosphere at Solar Maximum

We find that the mean free pathlength is higher by a factor of 10 or more at solar minimum than at the last solar maximum in 1900-1991, consistent with previous findings. In 2000-2001, the mean free pathlength has declined by a factor of ~10, but is still larger by a factor of ~3.6 than its value of ~0.15 AU in 1990-1991. As solar maximum conditions become durably established in the outer heliosphere over the next few months, it may be possible to deduce the radial dependence of the mean free pathlength

Rigidity Dependence of the Interplanetary Mean Free Path

We use energy spectra of anomalous cosmic rays derived from data from experiments on the Voyager and Pioneer spacecraft to inf er the rigidity dependence of the perpendicular interplanetary mean free path over the rigidity range ~0.3 to 4 GV. Data are from three intervals: 1993/157-209, 1994/157- 313, and l 996/1-52. We use two techniques to estimate that the rigidity dependence is approximately proportional to R^2 at low rigidities with a flattening above ~1 GV, roughly consistent with recent theoretical estimates (Bieber et al. 1995)

A Tilt Model for Anomalous Cosmic Rays and the Location of the Solar Wind Termination Shock

We use the observed relationship of the radial gradient of anomalous cosmic rays (ACRs), as measured in the outer heliosphere with instruments on the Voyager and Pioneer spacecraft, and the tilt of the heliospheric neutral current sheet to develop a model for the time dependence of ACRs at all radii near the helioequatorial plane. This phenomenological model reproduces the observations at 1 AU, Voyagers 1 and 2, and Pioneer 10 quite well. The model also indicates that the helioequatorial source intensity at the shock varies by a factor of 10, depending on solar magnetic polarity. The location of the solar wind termination shock during 1980-1990 is found to be 92 AU, with a best-fit of 84 AU