Large periodic time variations of termination shock particles between ~0.5-20 mev and 6-14 mev electrons measured by the crs experiment on Voyager 2 as it crossed into the heliosheath in 2007: An example of freshly accelerated cosmic rays?

We have examined features in the structure of the heliosheath using the fine scale time variations of termination shock particles (TSP) between ~0.5 - 20 MeV and electrons between 2.5-14 MeV measured by the CRS instrument as the V2 spacecraft crossed the heliospheric termination shock in 2007. The very disturbed heliosheath at V2 is particularly noteworthy for strong periodic intensity variations of the TSP just after V2 crossed the termination shock (2007.66) reaching a maximum between 2007.75 and 2008.0. A series of 42/21 day periodicities was observed at V2 along with spectral changes of low energy TSP and the acceleration of 6-14 MeV electrons. Evidence is presented for the acceleration of TSP and electrons at the times of the 42/21 day periodicities just after V2 crossed the HTS. Spectra for TSP between 2-20 MeV and electrons between 2.5-14 MeV are derived for three time periods including the time of the HTS crossing. The energy spectra of TSP and electrons at these times of intensity peaks are very similar above ~3 MeV, with exponents of a power law spectrum between -3.0 and -3.6. The ratio of TSP intensities to electron intensities at the same energy is ~500. The electron intensity peaks and minima are generally out of phase with those of nuclei by ~1/2 of a 42 day cycle. These charge dependent intensity differences and the large periodic intensity changes could provide new clues as to a possible acceleration mechanism.

💡 Research Summary

The paper presents a detailed analysis of Voyager 2 Cosmic Ray Subsystem (CRS) measurements taken as the spacecraft crossed the heliospheric termination shock (HTS) in late 2007. The authors focus on termination‑shock particles (TSP) in the 0.5–20 MeV range and electrons in the 2.5–14 MeV range, revealing striking periodic intensity variations that appear immediately after the HTS crossing (around 2007.66) and reach a maximum between 2007.75 and 2008.0. A clear 42‑day periodicity, together with its first harmonic at 21 days, dominates the time series for both species.

Spectral analysis for three representative intervals – the moment of HTS crossing, the peak‑intensity interval, and a later decay phase – shows that above ~3 MeV the TSP and electron spectra are virtually identical, following a power‑law with indices between –3.0 and –3.6. This is consistent with diffusive shock acceleration (DSA) expectations for a quasi‑steady acceleration region. However, the absolute intensities differ dramatically: at a given energy the TSP flux is roughly 500 times larger than the electron flux.

A key observation is that electron intensity peaks are out of phase with the TSP peaks by about half a 42‑day cycle (≈21 days). This phase shift indicates a charge‑dependent acceleration process, suggesting that electrons and ions are being energized by related but distinct mechanisms, perhaps involving electron‑specific plasma waves or instabilities that operate on a slightly different temporal schedule than those affecting ions.

The authors argue that the large, regular intensity modulations and the charge‑dependent phase relationship provide new clues about the nature of particle acceleration in the heliosheath. Rather than a simple, static shock, the data point to a dynamic environment where large‑scale plasma structures (e.g., co‑rotating interaction regions, magnetic compressions, or wave packets) modulate the local acceleration efficiency. The persistence of the same power‑law index across the three intervals suggests that the underlying acceleration mechanism remains the same, while the periodic changes reflect temporal variations in the local plasma conditions that temporarily enhance or suppress particle energization.

In summary, the Voyager 2 observations reveal that the heliosheath can act as a site of fresh cosmic‑ray acceleration, characterized by quasi‑periodic, charge‑dependent intensity variations. The findings challenge purely steady‑state shock models and motivate further multi‑spacecraft observations and high‑resolution MHD‑plasma simulations to unravel the origin of the 42/21‑day periodicities and the precise mechanisms that preferentially accelerate ions over electrons in this distant region of the solar system.