Abstract
Abstract Voyager 1 has been moving through the very local interstellar medium (VLISM) from the time that it crossed the heliopause on 2012/DOY 238 to 2020/DOY 292. Three notable objects in the magnetic field of the VLISM have been observed: two shocks and one pressure front. This paper reports the observation of a fourth object observed near 2020/DOY 147. There were no upstream electron plasma oscillations of the type often observed ahead of shocks, abrupt increases in energetic particles, or fluctuations in the 48 s increments of the magnetic field associated with this feature, suggesting that it was probably not a shock. This feature was associated with a relatively large increase in the magnetic field strength (B2/B1 = 1.35) and in the electron density determined by the Plasma Wave Science experiment (N2/N1 = 1.36) using a new method described in this paper. This feature appears to be a pressure front associated with a compressive wave in the VLISM. The two shocks and the two pressure fronts were associated with the four largest maxima observed in B(t) between 2012/DOY 238 and 2020/DOY 292. Each feature was associated with a jump–ramp structure. The jump–ramp structures were separated by long relatively undisturbed quiet intervals.
Highlights
The structure of the heliosphere, heliosheath, and heliopause and the interaction of the heliosphere with the interstellar medium was discussed by Holzer (1989), and reviewed by Zank (1999, 2015) and Zank (2015)
Both shocks were preceded by an interval containing electron plasma oscillations that were thought to be driven by low-energy, upstream electrons, which ended abruptly when the shock arrived (Gurnett et al 2013, 2015, respectively). The presence of these electron plasma oscillations supports the interpretation of the abrupt jumps in B as shocks despite the large passage times, since it is known that electron plasma oscillations can be produced by a beam of electrons accelerated by a shock. These in situ observations of shocks in the very local interstellar medium (VLISM) beyond the heliopause confirm the hypothesis of Gurnett et al (1993), based on the observations of a strong heliospheric 2–3 kHz radio emission event detected by Voyagers 1 and 2 that started in 1992 July, was generated at or near the heliopause by an interplanetary shock that originated during a period of intense solar activity in late May and early 1991 June
We have confirmed the existence of pressure fronts in the VLISM by identifying a second pressure front, observed by Voyager 1 (V1) on 2020/147, at 149 au, 34°.8 latitude, and 175°.3 longitude in heliographic inertial coordinates
Summary
The structure of the heliosphere, heliosheath, and heliopause and the interaction of the heliosphere with the interstellar medium was discussed by Holzer (1989), and reviewed by Zank (1999, 2015) and Zank (2015). After crossing the heliopause, the magnetometer (MAG) on V1 recorded an abrupt increase in the magnetic field strength on 2012.92/330 to 340 (DOY 335), with a jump in the magnetic field B2/B1 = 1.43 This event was identified as a shock (sh1) (Burlaga et al 2021), based on the observation that it was preceded by electron plasma oscillation events (Gurnett et al 2013). The presence of these electron plasma oscillations supports the interpretation of the abrupt jumps in B as shocks despite the large passage times, since it is known that electron plasma oscillations can be produced by a beam of electrons accelerated by a shock These in situ observations of shocks in the VLISM beyond the heliopause confirm the hypothesis of Gurnett et al (1993), based on the observations of a strong heliospheric 2–3 kHz radio emission event detected by Voyagers 1 and 2 that started in 1992 July, was generated at or near the heliopause by an interplanetary shock that originated during a period of intense solar activity in late May and early 1991 June. The uncertainty in B is nominally ±0.035 nT, but the uncertainty in an increment of B during a short interval is less than this because drifts of the magnetometers are generally not important on small scales
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