Stamatios M. Krimigis

We analyze ∼40–200 keV energetic particle observations from the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) on board the New Horizons (NH) spacecraft from 2007 to 2024 and compare them with similar measurements from the Low Energy Charged Particle experiments on the Voyager 1 and Voyager 2 spacecraft when they were at comparable heliocentric distances (5–60 au). In both Voyager data sets, particle intensities generally decrease with increasing radial distance from the Sun, reaching a minimum in the outer heliosphere before rising again prior to their respective encounters with the heliospheric termination shock (TS). This radial behavior in the intensity-time profiles is described as a heliospheric valley. The NH/PEPSSI time series from 5 au (2007) to 60 au (2024) exhibits a similar decrease in particle fluxes with distance. Analysis of the radial dependence of energetic particle intensities from all three spacecraft normalized by observations at 1 au to account for the solar cycle effects reveals an approximate piece-wise composite power-law relationship, with a slope break (steeper decline) beyond ∼33 au. This break may reflect differences in the dominant transport and acceleration mechanisms operating in the two regions demarcated by this radial break. In addition, a radial scaling method is applied to Voyager observations to best match the NH data. This comparison provides an estimate for the NH TS crossing between 2027 (68 au) and 2034 (83 au).

We analyze the count rates of 40–139 keV ions that were measured in situ by the Low Energy Charged Particle instrument on Voyager 1 in order to identify the suprathermal ion anisotropies beyond the termination shock and in the very local interstellar (IS) medium (VLISM). The analysis results in a region of ∼9–10 au before the heliopause (HP) where the radial anisotropy of ions is negative, while the azimuthal ion anisotropy inside the heliosheath lies in the −T direction. In agreement to our previous analyses, we identify a positive radial anisotropy of ions up to at least ∼30 au beyond the HP, which becomes nearly zero from 2021 up to 2023 November (for ∼10 au). Notably, the anisotropy in the azimuthal direction is statistically zero throughout the upstream region, i.e., for ∼40 au past the HP, showing that the 40–139 keV ion anisotropy in the VLISM is only in the radial direction and has no azimuthal component. The presence of suprathermal ions of solar origin over such long spatial scales in the VLISM, along with the inflow of ions from IS space into the heliosheath, are important constraints for characterizing the interaction of the heliosheath with the VLISM. Our observations provide indications that V1 has entered a new regime in the VLISM since (at least) the year 2021, progressively developing characteristics akin to the pristine IS medium. Alternatively, this drop to nearly zero radial anisotropies beyond 2021 could be a manifestation of a prolonged compression/shock of solar origin.

Whitepaper #169 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Main topics: basic research. Additional topics: outer heliosphere; space-based missions/projects; emerging opportunities.

Whitepaper #096 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Main topics: basic research; infrastructure/workforce/other programmatic. Additional topics: outer heliosphere; emerging opportunities; other programmatic; space-based […]

We report on the energy dependence of Galactic cosmic rays (GCRs) in the very local interstellar medium (VLISM) as measured by the Low Energy Charged Particle (LECP) instrument on the Voyager 1 spacecraft. The LECP instrument includes a dual-ended solid-state detector particle telescope mechanically scanning through 360° across eight equally spaced angular sectors. As reported previously, LECP measurements showed a dramatic increase in GCR intensities for all sectors of the ≥211 MeV count rate (CH31) at the Voyager 1 heliopause (HP) crossing in 2012; however, since then the count rate data have demonstrated systematic episodes of intensity decrease for particles around 90° pitch angle. To shed light on the energy dependence of these GCR anisotropies over a wide range of energies, we use Voyager 1 LECP count rate and pulse height analyzer (PHA) data from ≥211 MeV channel together with lower-energy LECP channels. Our analysis shows that, while GCR anisotropies are present over a wide range of energies, there is a decreasing trend in the amplitude of second-order anisotropy with increasing energy during anisotropy episodes. A stronger pitch angle scattering at higher velocities is argued as a potential cause for this energy dependence. A possible cause for this velocity dependence arising from weak rigidity dependence of the scattering mean free path and resulting velocity-dominated scattering rate is discussed. This interpretation is consistent with a recently reported lack of corresponding GCR electron anisotropies.

We present observations of ≳10–100 keV nucleon⁻¹ suprathermal (ST) H, He, O, and Fe ions associated with crossings of the heliospheric current sheet (HCS) at radial distances of E ⁻⁴–E ⁶; and (5) maximum energies E _X increase with the ion’s charge-to-mass (Q/M) ratio as E X / E H ∝ ( Q X / M X ) δ , where δ ∼ 0.65–0.76, assuming that the average Q states are similar to those measured in gradual and impulsive solar energetic particle events at 1 au. The absence of velocity dispersion in combination with strong field-aligned anisotropies closer to the HCS appears to rule out solar flares and near-Sun coronal-mass-ejection-driven shocks. These new observations present challenges not only for mechanisms that employ direct parallel electric fields and organize maximum energies according to E/Q but also for local diffusive and magnetic-reconnection-driven acceleration models. Reevaluation of our current understanding of the production and transport of energetic ions is necessary to understand this near-solar, current-sheet-associated population of ST ions.

We report 40–139 keV in situ ion measurements from the Voyager 1/Low Energy Charged Particle (LECP) instrument, over the time period from 2000 to the first 3 months of 2020, toward assessing energetic ion flow properties inside the heliosheath (HS) and upstream of the heliopause (HP). We identify an average radial outflow of ions at ∼46, 67, and 109 keV of ∼2.33 × 10⁻³, ∼1.37 × 10⁻³, and ∼7.27 × 10⁻⁴ ( cm 2 sr s keV ) ^–1, respectively, from the beginning of 2013 up to 2020, i.e., over a spatial range of ∼28 au past the HP. The raw 40–139 keV rates, uncorrected for background, show distinct intensity dropouts for particles with 90° pitch angles, in concert with the >211 MeV Galactic cosmic-ray anisotropy events, reported after Voyager 1 exited the HS in 2012 August. The intensity spectrum is consistent with a power-law form in energy with a spectral index of ∼−1.4, similar to energy spectra measured by both Voyager 1 and 2/LECP inside the HS, providing strong indication that these particles correspond to an ion population leaking from the HS into interstellar space, most likely due to the flux tube interchange instability at the boundary. Future modeling efforts should incorporate this ion population, which provides the framework for establishing a communication between the HS and the very local interstellar medium.

The Voyager 1 (V1) and Voyager 2 (V2) spacecraft were launched in 1977 on a mission to explore the outer planets and reach the heliopause, the boundary between the hot solar plasma and the relatively cool interstellar plasma. V1 reached the heliopause on 2012 August 25, at 121.6 au, and V2 reached the heliopause on 2018 November 5, at 119.0 au. One of their remarkable discoveries was the detection of shocks propagating into the interstellar plasma from energetic solar events. These shocks are typically preceded by electron plasma oscillations excited by electron beams streaming along interstellar magnetic field lines ahead of the shocks. The frequencies of the plasma oscillations have now provided radial electron density profiles in the outer heliosphere and in the interstellar medium to radial distances of more than 145 au. The oscillations are typically preceded by bursts of high-energy ∼5–100 MeV electrons. These electron bursts are interpreted as being due to the reflection (and acceleration) of cosmic-ray electrons by the shock at the time the shock first contacts the magnetic field line that passes through the spacecraft. Relative timing between the cosmic rays reflected by the shock and the onset of the plasma oscillations allow us, for the first time, to estimate the energy, ∼20–100 eV, of the electron beams responsible for the plasma oscillations. These observations are combined into a self-consistent model called the foreshock model that describes the interaction of shocks of solar origin with the interstellar plasma.

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 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.

We augment the heliospheric network of galactic cosmic ray (GCR) monitors using 2012–2017 penetrating radiation measurements from the New Horizons (NH) Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI), obtaining intensities of ≳75 MeV particles. The new, predominantly GCR observations provide critical links between the Sun and Voyager 2 and Voyager 1 (V2 and V1), in the heliosheath and local interstellar medium (LISM), respectively. We provide NH, Advanced Composition Explorer (ACE), V2, and V1 GCR observations, using them to track solar cycle variations and short-term Forbush decreases from the Sun to the LISM, and to examine the interaction that results in the surprising, previously reported V1 LISM anisotropy episodes. To investigate these episodes and the hitherto unexplained lagging of associated in situ shock features at V1, propagating disturbances seen at ACE, NH, and V2 were compared to V1. We conclude that the region where LISM magnetic field lines drape around the heliopause is likely critical for communicating solar disturbance signals upstream of the heliosheath to V1. We propose that the anisotropy-causing physical process that suppresses intensities at ∼90° pitch angles relies on GCRs escaping from a single compression in the draping region, not on GCRs trapped between two compressions. We also show that NH suprathermal and energetic particle data from PEPSSI are consistent with the interpretation that traveling shocks and corotating interaction region (CIR) remnants can be distinguished by the existence or lack of Forbush decreases, respectively, because turbulent magnetic fields at local shocks inhibit GCR transport while older CIR structures reaching the outer heliosphere do not.