Space Science & Space Physics Editors' Vox

New Findings from Old Data

Recalibrated and reanalyzed data from the Voyager flybys of Jupiter 40 years ago, presented in a series of papers in JGR: Space Physics, show the value of archival data.

By

We are approaching the 40-year anniversary of the two Voyager spacecraft making their 1979 flybys of the planet Jupiter. Jupiter’s magnetosphere is big. The satellites were moving fast, and each one only spent a few days passing through the magnetic bubble around this planet so two flybys does not seem like much of an opportunity to gather data. But the plasma science instruments on the spacecraft were high quality sensors that led to numerous papers on the magnetospheric structure, dynamics, composition, and dominant physical processes.

Long after the Voyager observations the Galileo spacecraft orbited Jupiter for 8 years in the 1990s and 2000s, providing a rich set of magnetospheric measurements for nearly a solar cycle.

Artist’s impression of Voyager 1, launched on September 5, 1977, and still sending data back from interstellar space. Credit: NASA

The Voyager flybys, however, offer comparative and complementary measurements from a different solar cycle and are still proving themselves to be useful.

Several of those initial studies were authored by Fran Bagenal, now a professor at the University of Colorado in Boulder.

In the decades since, she has never lost her love of Jupiter’s magnetosphere and her publication list is full of papers on the topic.

In fact, this summer Bagenal and coworkers (primarily two undergraduate students) have a three-paper series just published in JGR Space Physics on the recalibration and reanalysis of the Voyager data.

In the first one [Bagenal et al., 2017], the ion composition data from the Plasma Science Instrument (PSI) onboard Voyager 1 and 2 were reprocessed with the help of modern physical chemistry models to “constrain the composition and reduce the number of free parameters.”

Output from the new empirical model of Jupiter’s electrically charged space environment, based on the recalibrated Voyager data set. The three plots show, from top to bottom, electron density (Ne), oxygen ion density (NO+), and proton density (NH+) for a representative meridional slice through the magnetosphere. The plot reveals that heavy ions like O+ are the dominant ions and protons play a minor role in Jupiter’s equatorial plasma sheet. Credit: Dougherty et al., 2017 and Bodisch et al., 2017

In the second article, led by student Logan Dougherty [Dougherty et al., 2017], they focus on the main constituents, specifically oxygen and sulfur ions, including a detailed examination of their charge states and flow speeds as a function of radial distance from the planet.

In the third paper, led by student Kaleb Bodisch [Bodisch et al., 2017], the focus shifts to the minor ions, including protons, which are less than 20 per cent of the particles in the Jovian magnetosphere, as well as sodium and sulfur dioxide ions, which have even smaller abundances.

Throughout the series, the researchers develop a robust 2-D model of the Jovian plasma sheet. They provide several key points of new understanding of Jupiter’s space environment that could be highly valuable for comparative planetary investigations. In addition, this work is particularly important with Juno currently orbiting Jupiter and two other missions, the European-led Jupiter Icy Moons Explorer and NASA’s Europa Clipper, in development.

In our field, emphasis is often put on the spectacular and first-look and ground-breaking observations, like Voyager’s continued outward journey into interstellar space. We often do not celebrate and recognize the fundamental yet longer-term work of producing high quality measurements with documentation and access to the wider community.

Fran and her co-workers have pulled this data from the deep reaches of the Planetary Data System (NASA’s repository for planetary mission observations), updated old codes written in the 1970s, and, perhaps, best of all, made their new data set available online. They have not only gleaned new insight from archival data but also made this old data accessible for the current generation of researchers.

—Mike Liemohn, Editor-in-Chief of JGR: Space Physics and Department of Climate and Space Sciences and Engineering, University of Michigan; email: [email protected]