Cosmic Ray Subsystem Research Articles

The properties of 0.11 keV–344 MeV ion spectra in the inner heliosheath using regularizedκ-distributions

Context.The shape of the ion energy spectra plays a critical role in determining the ion energetics, the acceleration mechanisms, and the possible sources of different plasma and suprathermal ion populations. The determination of the exact shape of the total particle spectrum provides the necessary means to address the inner heliosheath dynamics. Apart from various modelling efforts, a direct fit to the measured ion spectra for an extended energy range of ∼0.11–344 MeV has not been performed to date.Aims.We use an extended set of combined 0.11–55 keV remotely sensed energetic neutral atoms (ENA) measurements from the Interstellar Boundary Explorer (IBEX-Lo and IBEX-Hi) and the Cassini/Ion and Neutral Camera (INCA), converted to protons, together with ∼28 keV–344 MeV in situ ion measurements from the low-energy charged particle (LECP) and cosmic ray subsystem (CRS) experiments on Voyager 2, over the declining phase of solar cycle 23 (SC23) and the ascending phase of solar cycle 24 (SC24) to study the characteristics of the particle energy spectrum.Methods.We fitted the 0.11 keV–344 MeV composite ion spectra with a set of regularized isotropicκ-distribution functions (RKDs), which allowed us to determine the macroscopic physical properties.Results.We demonstrate that the 2009–2012 composite spectrum that corresponds to the declining phase of SC23 is well fitted by three different RKDs, while the 2013–2016 spectrum, associated with the rise of SC24, can only be approximated with six differentκ-distribution functions.Conclusions.Our results are generally consistent with shock accelerated particles that undergo additional acceleration inside the inner heliosheath. We identify a low-energy transmitted population of particles, a suprathermal reflected population and a very-high-energy component that is modulated by galactic cosmic rays. The 2013–2016 time period is most likely associated with a mixture of particles from SC23 and SC24, which is reflected by the need to employ six RDKs.

Using Magnetic Flux Conservation to Determine Heliosheath Speeds

Abstract The heliosheath (HSH) radial speeds at Voyager 2 (V2) derived from the plasma instrument (PLS) and from particle instruments using the Compton–Getting (CG) effect are different. At V2 the CG speeds are more variable than the plasma speeds and decrease about 2 yr before the heliopause. We use magnetic flux conservation to differentiate between these two speed profiles at V2, comparing the magnetic flux observed at 1 au and in the HSH. For V2 the PLS speed profile is significantly more consistent with magnetic flux conservation than the CG speeds. For Voyager 1 (V1), we present new V R derivations from the Cosmic Ray Subsystem (CRS) using the CG method that agree reasonably well with those previously obtained from the low energy charged particle (LECP) instrument. If we use the V2 PLS speed profile to calculate the magnetic flux at V1, we again find much better agreement than if we use the V1 CG speeds. These results suggest that the radial speeds derived from particle anisotropy observations in the HSH are not reliable.

No Stagnation Region before the Heliopause at Voyager 1? Inferences from New Voyager 2 Results

Abstract We present anisotropy results for anomalous cosmic-ray protons in the energy range ∼0.5–35 MeV from Cosmic Ray Subsystem (CRS) data collected during calibration roll maneuvers for the magnetometer instrument when Voyager 2 (V2) was in the inner heliosheath. We use a new technique to derive for the first time the radial component of the anisotropy vector from CRS data. We find that the CRS-derived radial solar wind speeds, when converted from the radial components of the anisotropy vectors via the Compton–Getting (C–G) effect, generally agree with those similarly derived speeds from the Low Energy Charged Particle experiment using 28–43 keV data. However, they often differ significantly from the radial solar wind speeds measured directly by the Plasma Science (PLS) instrument. There are both periods when the C–G-derived radial solar wind speeds are significantly higher than those measured by PLS and times when they are significantly lower. The differences are not expected nor explained, but it appears that after a few years in the heliosheath the V2 radial solar wind speeds derived from the C–G method underestimate the true speeds as the spacecraft approaches the heliopause. We discuss the implications of this observation for the stagnation region reported along the Voyager 1 trajectory as it approached the heliopause inferred using the C–G method.

Combined ∼10 eV to ∼344 MeV Particle Spectra and Pressures in the Heliosheath along the Voyager 2 Trajectory

Abstract We report a unique combination of ∼10 eV to ∼344 MeV in situ ion measurements from the Plasma Science (PLS), Low Energy Charged Particle (LECP), and Cosmic Ray Subsystem (CRS) experiments on the Voyager 2 (V2) spacecraft, and remotely sensed ∼110 eV to ∼55 keV energetic neutral atom (ENA) measurements from the Interstellar Boundary Explorer (IBEX) mission and Ion and Neutral Camera (INCA) on the Cassini mission. This combination is done over the time period from 2009 to the end of 2016, along the V2 trajectory, toward assessing the properties of the ion energy spectra inside the heliosheath. The combined energy spectra exhibit a series of softening and hardening breaks, providing important insights on the various ion acceleration processes inside the heliosheath. Ions in the <6 keV energy range dominate the total pressure distribution inside the heliosheath but the ion distributions at higher energies (>5.2 keV) provide a significant contribution to the total pressure. With the assumption that all ENAs (∼110 eV to 55 keV) are created by charge-exchange interactions inside the heliosheath, we estimate that the magnetic field upstream at the heliopause required to balance the pressure from the heliosheath in the direction of V2 is ∼0.67 nT. This number is consistent with the measured magnetic field at V2 from 2018 November, when the spacecraft entered interstellar space.

Galactic Cosmic-ray Anisotropies: Electrons Observed by Voyager 1 in the Very Local Interstellar Medium

Abstract Now over seven years into its journey beyond the heliopause, Voyager 1 continues to return unprecedented observations of energetic particles, magnetic fields, and plasma emissions from the very local interstellar medium. Shortly after its heliopause crossing, Voyager 1 discovered an unusual time-varying galactic cosmic-ray anisotropy, characterized by smoothly changing intensity reductions in particles propagating nearly perpendicular to the magnetic field; outside of this isolated region, cosmic rays appear mostly unvarying, without a significant radial gradient. These small (∼15%) but lasting (∼100 to ∼630 days) anisotropic events are still not fully understood. Nevertheless, they serve as clear markers, together with shorter-lived cosmic-ray intensity enhancements, electron plasma oscillations, and weak laminar shocks, that even beyond the heliopause, the Sun’s variable output significantly influences its surroundings. So far, these unusual energetic particle occurrences have mainly been studied using integrated proton intensities of ∼20 MeV and higher. Using data from the Voyager 1 Cosmic Ray Subsystem, we extend the analysis to electrons, as well as lower energy protons, and discover the surprising new result that the ∼3 to ∼105 MeV electrons remain mostly isotropic and unchanging, in sharp contrast to their proton counterparts. We search for clues to explain this underlying species dependence and rule out potential causes related to instrumental effects, velocity and energy, trapping and energy loss, drifts, and turbulence-induced scattering. We also explore some plausible mechanisms and open the door for more detailed follow-up theories and simulations.

Heliosheath Properties Measured from a Voyager 2 to Voyager 1 Transient

Abstract In mid-2012, a global merged interaction region (GMIR) observed by Voyager 2 crossed through the heliosheath and collided with the heliopause, generating a pressure pulse that propagated into the very local interstellar medium. The effects of the transmitted wave were seen by Voyager 1 just 93 days after its own heliopause crossing. The passage of the transient was accompanied by long-lasting decreases in Galactic cosmic ray intensities that occurred from ∼2012.55 to ∼2013.35 and ∼2012.91 to ∼2013.70 at Voyager 2 and Voyager 1, respectively. Omnidirectional (≳20 MeV) proton-dominated measurements from each spacecraft’s Cosmic Ray Subsystem reveal a remarkable similarity between these causally related events, with a correlation coefficient of 91.2% and a time lag of 130 days. Knowing the locations of the two spacecraft, we use the observed time delay to calculate the GMIR’s average speed through the heliosheath (inside the heliopause) as a function of temperature in the very local interstellar medium. This, combined with particle, field, and plasma observations, enables us to infer previously unmeasured properties of the heliosheath, including a range of sound speeds and total effective pressures. For a nominal temperature of ∼20,000 K just outside the heliopause, we find a sound speed of 314 ± 32 km s−1 and total effective pressure of 267 ± 55 fPa inside the heliopause. We compare these results with the Interstellar Boundary Explorer’s data-driven models of heliosheath pressures derived from energetic neutral atom fluxes (the globally distributed flux) and present them as additional evidence that the heliosheath’s dynamics are driven by suprathermal energetic processes.

Galactic Cosmic-Ray Anisotropies: Voyager 1 in the Local Interstellar Medium

Abstract Since crossing the heliopause on 2012 August 25, Voyager 1 observed reductions in galactic cosmic ray count rates caused by a time-varying depletion of particles with pitch angles near 90°, while intensities of particles with other pitch angles remain unchanged. Between late 2012 and mid-2017, three large-scale events occurred, lasting from ∼100 to ∼630 days. Omnidirectional and directional high-energy data from Voyager 1’s Cosmic Ray Subsystem are used to report cosmic ray intensity variations. Omnidirectional (≳20 MeV) proton-dominated measurements show up to a 3.8% intensity reduction. Bidirectional (≳70 MeV) proton-dominated measurements taken from various spacecraft orientations provide insight about the depletion region’s spatial properties. We characterize the anisotropy as a “notch” in an otherwise uniform pitch angle distribution of varying depth and width centered about 90° in pitch angle space. The notch averages 22° wide and 15% deep, signifying a depletion region that is broad and shallow. There are indications that the anisotropy is formed by a combination of magnetic trapping and cooling downstream of solar-induced transient disturbances in a region that is also likely influenced by the highly compressed fields near the heliopause.

Anomalous cosmic rays in the distant heliosphere and the reversal of the Sun's magnetic polarity in Cycle 23

Beginning in early June 2001 and extending over a ∼3 month period the Cosmic Ray Subsystem (CRS) experiment on Voyager 1 (80.3 AU, 34°N) observed a rapid transition in the energy of the peak intensity of anomalous cosmic ray (ACR) He from ∼11 MeV/n to ∼25 MeV/n. One month later there began a similar spectral shift at Voyager 2 (64.7 AU, 24°S). When these ACR transition times are extrapolated back from the estimated location of the heliospheric termination shock (TS) to the Sun (∼1 year) there is reasonable agreement with the range of times obtained for the polarity reversal of the interplanetary magnetic field (IPB) in the northern hemisphere near the Sun and the later reversal in the south. We interpret these changes in the ACR spectra as giving a global view of the interplanetary field reversal at the TS and confirmation of the important role of the TS in controlling the ACRs.

Io encounters past and present: A heavy ion comparison

The heavy ion counter (HIC) on the Galileo spacecraft consists of two low‐energy telescopes that were proof‐test models of the telescopes used for the cosmic ray subsystem (CRS) on Voyager. The telescopes on HIC were improved to better withstand the intense radiation environment of the Jovian system and to measure higher‐energy particles with greater sensitivity. The similarity of these two instruments makes it natural to compare the data in an effort to glean insight on the behavior of heavy ion fluxes at higher energies and how the inner Jovian magnetosphere has changed in the last 18 years. Inside L = 6, the oxygen spectrum suggests a spectral break at ∼ 7500 MeV/Gauss, corresponding to ions with gyroradii of ∼ 1.5 RIo at L = 6. A comparison of the phase space densities show remarkable agreement between the two time periods outside of the Io orbit, with differential spectra consistent with γ ∼ −8 above ∼ 6 MeV/nucleon. Near Io Galileo observed a large density gradient not apparent in the Voyager data that may be due to local time asymmetries or a decrease in the radial diffusion rate.

VoyagerMeasurements of the Mass Composition of Cosmic‐Ray Ca through Fe Nuclei

We present measurements of the charge and isotopic composition of cosmic-ray Ca through Fe nuclei made on the Voyager spacecraft. We have analyzed 18 years of data in the energy range 100-300 MeV nucleon-1 collected by the High Energy Telescope of the cosmic-ray subsystem experiment on the Voyager 1 and 2 spacecraft. The average solar modulation level for this measurement is 480 MV. These data have several times the statistical accuracy and have mass resolution comparable to the best previously published measurements covering this charge range from the ISEE cosmic-ray experiment. Many of the isotopes of these charges are dominated by secondary production from cosmic rays traversing the interstellar medium, and for these isotopes our measured isotopic fractions are generally consistent to within 10%, with the exception of certain K-capture isotopes, with those calculated for secondary production using the latest propagation codes and cross sections. For five isotopes,40Ca,52Cr,55Mn,54Fe, and 58Fe, we can determine the cosmic-ray source composition to an improved accuracy over previous values. None of these isotopes show any significant differences with respect to the solar composition. This result, taken with other recent Voyager measurements of the isotopic composition of charge Z = 6-16 nuclei as well as Co and Ni nuclei, now provide the source composition of ~20 individual cosmic-ray isotopes with Z between 6 and 28 measured by the same instrument under the same conditions. This source composition is remarkably solar-like with only 13C,14N, and 22Ne isotopes showing significant abundance differences. This degree of similarity is not well explained by present specific models for the cosmic-ray origin and is discussed further in this paper.

Cosmic-ray isotopic composition of C, N, O, Ne, Mg, SI nuclei in the energy range 50-200 MeV per nucleon measured by the Voyager spacecraft during the solar minimum period

The isotopic composition of C, N, O, Ne, Mg, Si cosmic ray nuclei has been measured in the energy range 50-200 MeV per nucleon using data collected by the High-Energy Telescope of the cosmic-ray subsystem experiment on the Voyager 1 and 2 spacecraft. These data were collected during the period of minimum solar activity in 1986-1988 at an average distance of 27 AU with an effective solar modulation that was much less than at the Earth. The isotope analysis, based on the energy loss - total energy method, has a mass resolution of 0.2 amu for carbon and 0.4 amu at silicon. We find a (C-13)/(C-12) ratio slightly lower and a (O-18)/(O-16) ratio slightly enhanced over their solar system value. We also observe the previously reported enhancement of the (Ne-22)/(Ne-20) ratio relative to solar at the cosmic-ray source but only a weak, if any, enhancement of the (Mg-25)/(Mg-24), (Mg-26)/(Mg 24), and (Si-30)/(Si-28) ratios.

Evidence that the anomalous cosmic-ray component is singly ionized

Data from the Cosmic Ray Subsystem on the Voyager spacecraft were used to measure the spectra of He, C, N, O, and Ne with 4-124 MeV per nucleon (for O) during solar minimum conditions near the end of 1977. By subtracting both a low-energy interplanetary component and the high-energy galactic component, the energy spectra of the 'anomalous' cosmic-ray species He, N, O, and Ne are determined. The energy dependences of such spectra are found to depend upon the charge state of the particles, their source spectra, and the rigidity dependence of the diffusion coefficient. If the source spectra at the boundary of the modulation region have similar power-law energy dependences, then the observed energy spectra indicate that the anomalous particles are singly ionized.

Energetic oxygen and sulfur ions in the Jovian magnetosphere and their contribution to the auroral excitation

Observations of 1 to 20 MeV/nuc oxygen, sodium, and sulfur ions in the Jovian magnetosphere are reported. Measurements made by the cosmic ray subsystem on Voyager 1 and 2 are used to calculate abundances and energy spectra in the region from 5 to 20 Jovian radii (RJ). The phase space density of the oxygen ions calculated from the spectra has a positive radial gradient between 6 and 17 RJ, indicating an inward diffusive flow. The upper limit for the diffusion coefficient D at 9 RJ is ∼10−5 s−1. This limit, combined with the analysis of Voyager plasma observations by Siscoe et al. (1981), implies an upper limit to the production rate of oxygen and sulfur ions from Io of ∼1028 ions/s. If D(9 RJ) is ∼4×10−6 s−1, then ∼2×1024 oxygen and sulfur ions with > 70 MeV/nuc‐G are lost per second as they diffuse inward from 12 to 8 RJ. Assuming these ions are scattered into the loss cone, they deliver ∼4×1012 W to the Jovian atmosphere. Extrapolations to lower magnetic moments suggest that the 1013–1014 W required to produce the observed ultraviolet auroral emissions could result from the precipitation of ∼1026 oxygen and sulfur ions/s with magnetic moments ≳10 to 30 MeV/nuc‐G (≳35 to 100 keV/nuc at 10 RJ). The ions with ≳70 MeV/nuc‐G deposit their energy between ∼67° and ∼72° magnetic latitude at an average depth of ∼1019 cm−2 of H2 (∼500‐km altitude), which is above the homopause. If the extrapolated spectrum extends down to ∼10 MeV/nuc‐G, then 10 times more energy (∼1013 W) is carried inward across 10 RJ by the energetic oxygen and sulfur ions than flows outward with the plasma, indicating the presence of an energy source in the middle or outer magnetosphere.

Energetic oxygen and sulfur in the Jovian magnetosphere

This paper reports measurements made by the cosmic ray subsystem onboard Voyager 1 and 2 in the Jovian magnetosphere. Energy spectra of oxygen ions in the energy range 1–20 MeV/nuc between 5 and 20 RJ are presented and used to calculate phase space densities. There is a steep positive radial gradient in the phase space density of the energetic oxygen ions in this region, indicating an inward diffusive flow. Solutions of the diffusion equation assuming a diffusion coefficient D and loss lifetime τ of the forms D=D0Ln and τ=τ0Lm, where D0, τ0, n, and m are constants, and L is the McIlwain parameter, are fit to the radial phase space density profile of oxygen ions with magnetic moments of 680 MeV/nuc‐G. The best fits are found to have n + m ≈ 6 and 3 <n <6. On the basis of the diffusion coefficient upper limit obtained from these fits, the upper limit on the rate at which oxygen ions with >400 MeV/nuc‐G diffuse inward across 10 RJ is 5 × 1021±1 ions s−1. The observations suggest that oxygen and sulfur ions in the Io plasma torus diffuse radially outward, are nonadiabatically accelerated in some region outside 17 RJ and then diffuse inward and outward from the acceleration region.

Voyager 1: Energetic Ions and Electrons in the Jovian Magnetosphere

The observations of the cosmic-ray subsystem have added significantly to our knowledge of Jupiter's magnetosphere. The most surprising result is the existence of energetic sulfur, sodium, and oxygen nuclei with energies above 7 megaelectron volts per nucleon which were found inside of Io's orbit. Also, significant fluxes of similarly energetic ions reflecting solar cosmic-ray composition were observed throughout the magnetosphere beyond 11 times the radius of Jupiter. It was also found that energetic protons are enhanced by 30 to 70 percent in the active hemisphere. Finally, the first observations were made of the magnetospheric tail in the dawn direction out to 160 Jupiter radii.