Voyager Special Events - MIT Space Plasma Group

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Voyager Special Events Page

Voyager 2 Plasma Research Described in Astrobites Article

Astrobites is a consortium targeting an undergraduate audience. Briley Lewis, a graduate student at UCLA, has written a summary of the Nature article "Voyager 2 plasma observations of the heliopause and interstellar medium" referenced below. The Astrobites article is available at their site ("https://astrobites.org/2020/02/06/where-the-solar-system-ends/").

Voyager 2 Enters the Interstellar Medium

Science Overview:

The goal of the Voyager Interstellar Mission was to explore the region outside our heliosphere. In November 2018 Voyager 2 crossed the outer boundary of the heliosphere, the heliopause (HP), and entered the interstellar medium, the vast region between the stars. These observations were documented in a collection of papers published in Nature Astronomy in November 2019 describing the magnetic field, plasma wave, energetic particle, galactic cosmic ray, and (for the first time) plasma observations. The heliopause was crossed at 119.0 AU, very similar to the Voyager 1 crossing distance of 121.7 AU. Several new features were discovered ahead of the HP crossing. A plasma boundary layer was observed starting 1.5 AU before the HP in which the plasma density increased and speed decreased. A magnetic barrier was observed starting 0.95 AU before the HP where the magnetic field increased by a factor of 3 and the galactic cosmic ray intensity increased. The HP was a sharp boundary where the outward plasma flow ceased, magnetic field strength and galactic cosmic ray intensities increased, and heliospheric energetic particles decreased. The magnetic field direction did not change at the HP at V2 (nor at V1), a surprise we are trying to understand. The first measurements of ions in the interstellar medium show that the plasma is variable and suggest it is hotter near the HP than expected.

Figure 1. The Voyagers, having left the heliosphere and entered the interstellar medium

[cartoon crossing the heliopause]

Figure 2. Overview of the crossing of the heliopause by Voyager 2

[overview plot of crossing the heliopause by Voyager 2]

The radial plasma flow decreases and the plasma density increases in the plasma boundary region then the radial flow stops at the HP. The magnetic field and cosmic rays increase in the magnetic barrier then jump up at the HP. The 28-43 keV ions increase in the plasma boundary layer then drop out at the HP

Significance:

The Voyagers are the first spacecraft to leave our heliosphere, a milestone in our exploration of our galaxy. All stars are surrounded by astrospheres; these are the first in situ observations of the interaction of a star with the interstellar medium.

References:

Burlaga, L.F., Ness, N.F., Berdichevsky, D.B. et al. Magnetic field and particle measurements made by Voyager 2 at and near the heliopause. Nat. Astron. 3, 1007-1012 (2019).

Gurnett, D.A., Kurth, W.S. Plasma densities near and beyond the heliopause from the Voyager 1 and 2 plasma wave instruments. Nat. Astron. 3, 1024-1028 (2019).

Krimigis, S.M., Decker, R.B., Roelof, E.C. et al. Energetic charged particle measurements from Voyager 2 at the heliopause and beyond. Nat. Astron. 3, 997-1006 (2019).

Richardson, J.D., Belcher, J.W., Garcia-Galindo, P. et al. Voyager 2 plasma observations of the heliopause and interstellar medium. Nat. Astron. 3, 1019-1023 (2019).

Stone, E.C., Cummings, A.C., Heikkila, B.C. et al. Cosmic ray measurements from Voyager 2 as it crossed into interstellar space. Nat. Astron. 3, 1013-1018 (2019).

Interesting Events Observed by the Voyager 2 Plasma Experiment in the Solar Wind

Energy-time spectrograms showing velocity jumps or plasma structure plots observed on the dates listed below are available.

[ map image]

For more information contact John Richardson at MIT (e-mail: jdr@space.mit.edu).

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