Observations of a Radial Density Gradient in the Very Local Interstellar Medium by Voyager 2

Beginning on 2020 June 19, at a heliocentric distance of 124.2 au, the Voyager 2 Plasma Wave Science instrument began to observe radio emissions followed by electron plasma oscillations in its 3.11 kHz spectrum analyzer channel. Plasma oscillations at this frequency imply an electron density in the range of 0.12 cm−3 ± 15%, although some response in the 1.78 kHz channel near the peak of the plasma oscillations suggest a density of 0.087 cm−3 ± 8%. Shortly after Voyager 2 crossed the heliopause, in late 2019 January, the Voyager 2 Plasma Wave Science instrument detected plasma oscillations in its 1.78 kHz channel giving an electron density of 0.039 cm−3 ± 15%. While the Voyager spectrum analyzer affords relatively poor spectral resolution, the recent observation of plasma oscillations in the 3.11 kHz channel provides definitive evidence of a radial density gradient in the very local interstellar medium (VLISM), just beyond the heliopause with a magnitude similar to that observed by Voyager 1 obtained with higher spectral resolution measurements. Plasma oscillations observed by Voyager 1 range from frequencies as low as 2.1 kHz increasing to about 3.2 kHz, giving an electron density profile that increases from about 0.055 to about 0.13 cm−3 over a distance spanning some 20 au. Given the 67° difference in heliographic latitude and 43° difference in longitude between the two Voyagers, the new Voyager 2 observations imply that the density gradient is a large-scale feature of the VLISM in the general direction of the heliospheric nose.