Properties of suprathermal ion anisotropies from LECP on the Voyager inside the heliosheath and beyond the heliopause

The Low Energy Charged Particle (LECP; Krimigis et al. 1977) detector on Voyager is capable of measuring both the intensities and directions of energetic ions inside the heliosphere, providing direct sampling of their anisotropies at seven positions separated by 45 deg within its scan plane (one sector is blocked and is used for calibration purposes). When expressed in terms of roughly parallel (azimuthal; T) and perpendicular (Radial; R) components to the magnetic field direction, these anisotropies can provide important insights on the ion flow properties in and beyond the heliosheath. The analysis of 40-139 keV ions obtained by the LECP on Voyager 1 (V1) have shown that the azimuthal ion anisotropy turns to -T inside the heliosheath and revealed (a) the existence of a region of ~9-10 au before the HP, consistent with an inward radial flow of suprathermal ions into the heliosheath, and (b) a region of ~30 au beyond the heliopause (HP), consistent with a radial outflow of suprathermal ions leaking from the HS into interstellar space (Dialynas et al. 2021; 2024). Beyond this point, both the azimuthal and radial anisotropies are nearly zero, which roughly coincides with an abrupt and prolonged increase in both the magnetic field intensity (Burlaga et al. 2023) and electron densities (Kurth, 2024), known as “second pressure front” (pf2). The confluence of these observations indicate that V1 may have entered a new region in the VLISM beyond ~152 au, progressively developing characteristics akin to the pristine IS medium. Recent magnetic field observations (Burlaga et al. 2024a) have shown that the magnetic field parameters are consistent with a clear separation of two regions in space beyond ~155 au, exhibiting different magnetic field properties. Burlaga et al. (2024b), argued that this is a solar cycle effect in the VLISM, most likely a manifestation of a prolonged compression/shock of solar origin (e.g. Gurnett et al. 2021), whereas Fisk & Gloeckler (2022) argue that the Voyager measurements before the “pf2” are indicative of the flow of ions through the so-called “heliocliff”. We will present an update on the Voyager 1 measurements and preliminary results from the analysis of the >28 keV ion anisotropies from Voyager 2.

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