A Magnetic Pressure Front Upstream of the Heliopause and the Heliosheath Magnetic Fields and Plasma, Observed during 2017

Abstract

Voyager 1 (V1), moving in the interstellar magnetic field, observed an increase in the daily averages of B beginning on day ≈346, 2016, rising to a local maximum on day ≈382, and declining nearly monotonically for the most part until day 720, measured from 2016.0. A pressure front was observed during a ≈35-day interval beginning on day 346, 2016. The pressure front observed by V1 was not a shock, although one might expect it to evolve into a shock. Voyager 2 (V2) observed the distant heliosheath during 2017. The average B in the heliosheath was relatively high, 0.130 nT. The distribution of azimuthal angles had two nearly equal maxima at approximately 90°and 180°. An unusual transition of the BT component from a large "away" sector to a large "toward" sector occurred during 2017 from day 101 to day 239. Abrupt but small changes in magnetic polarity occurred between day 146 and day 239, when the average BT component of B was close to zero. Changes in the >70 MeV nucleon −1 cosmic-ray intensity were qualitatively related to the B(t) profile described by the CR-B relationship. There was no net decrease in magnetic flux at V2 in the heliosheath during 2017 that might be attributed to ongoing magnetic reconnection in the heliosheath. Small-scale increments in B can be described by a q-Gaussian distribution with q=1.64±0.02 for hourly averages of B and q=1.54±0.08 for daily averages of B.

Heliosheath Properties Measured from a Voyager 2 to Voyager 1 Transient

“…IBEX regularly observes time variation in the morphology of the globallydistributed flux (see e.g., Schwadron et al 2018 for the first 9 years of LOS-integrated pressure observations), and the Voyagers have a long history of measuring MIRs, GMIRs, and other transient disturbances in situ at their various locations through the heliosphere and beyond (e.g., McDonald et al 1981;Burlaga et al 1985;Gurnett et al 1993;Whang & Burlaga 1995;Burlaga & Ness 1998;Paularena et al 2001;Burlaga et al 2003;Richardson et al 2006;Webber et al 2009;Burlaga et al 2011;Luo et al 2011). In addition to the 2012 events used in this study, Voyager 2 observed several GMIRs and MIRs in the heliosheath from 2015 to 2018 (e.g., Burlaga et al 2018aBurlaga et al , 2019, and Voyager 1 measured larger, more extended GCR anisotropy episodes starting in 2015 (e.g., Rankin et al 2019) and 2018. Likewise, IBEX has now collected 10 years of data.…”

“…The Plasma Wave (PWS) instrument detected several locally-generated electron plasma emissions, typically preceded by GCR intensity enhancements measured by the Cosmic Ray Subsystem (CRS) and Low Energy Charged Particle Instruments (LECP; Gurnett et al , 2015. Following these precursors, the magnetometer (MAG) reported several disturbances in the form of weak, quasi-perpendicular, laminar changes to the magnetic field Burlaga et al 2019). LECP and CRS additionally observed an unexpected anisotropy in the GCRs characterized by a time-varying depletion of particles with near 90° pitch angles (Krimigis et al 2013;Rankin et al 2019).…”

Magnetic Field and Plasma Density Observations of a Pressure Front by Voyager 1 during 2020 in the Very Local Interstellar Medium

“…The purpose of this paper is to present recent observations of a fourth major event in the VLISM, the 2020 event, to discuss the nature of this event, and to show that there was a density jump associated with the previous pressure front, pf1, identified by Burlaga et al (2019). The 2020 event, together with the previous three major events provide a relatively simple view of the structure in the VLISM as we shall show below.…”

“…A distinctly different feature, related to an increase in B moved past V1 in ∼35 days during 2017 (Burlaga et al 2019). The jump in B was 1.19, comparable to the jump in the second shock (1.13).…”

“…The event was not a shock wave, since the increase in B occurred during a ≈35 day interval (which is significantly larger than that predicted for collisional shocks by Mostafavi & Zank 2018a, 2018b and it was not accompanied by intense impulsive plasma oscillations of the type normally driven by upstream electron beams associated with interstellar shocks. Burlaga et al (2019) identified the event as a pressure front "pf1." Since this was associated with a compressive wave that was propagating through the VLISM, presumably as a fast magnetosonic wave, one might also regard it as a "pressure wave front.…”