How the Voyager Missions’ Plasma Science Investigations Teach Us About Solar Winds

July 30, 2017 by Mark Hughes

AAC continues to celebrate the 40th anniversary of the Voyager spacecraft by looking at the Plasma experiments. In this article, we take a look at the Voyager missions' plasma experiments.

This article looks at the Voyager plasma-science experiments. We also got in a few words with MIT's Dr. John D. Richardson, the principal investigator who is currently studying the data from Voyager spacecraft.

As the available power for the Voyager spacecraft continues to dwindle, many of the experiments have been turned off. But the two spacecraft are still returning useful data about the solar system and interstellar medium. 

AAC continues celebrating the Voyager spacecraft engineering that has allowed 40 years of continuous discovery with this article on the Plasma Science Experiments. In this article, series coordinator Mark Hughes tells us about the plasma experiments and brings us an interview with MIT's Dr. John D. Richardson, the Primary Investigator of the Plasma Science Experiments.

Our other articles on the Voyager missions are available here:

Plasma Experiment

Clouds of ionized gases called plasma travel outwards from the sun in all directions throughout the solar system with a pattern determined by the rotation speed of the sun, the radial velocity of the gases, and several other factors. (Advanced readers can find a complete accounting of influences by reading Extraterrestrial Physics by Robert F. Wimmer-Schweingruber.)  

Occasionally, turbulent processes in the sun create an additional coronal mass ejection that sends a pulse of ionized gases off in a particular direction. As the gases travel further from the sun, they spread and slow. These outward-moving gases are often called solar wind.


Heliospheric model of an interplanetary coronal mass ejection from CCMC


The Voyager spacecraft have instruments that have allowed them to detect the speed, density, temperature, flux, and pressure of solar winds as they traveled to the far edges of the solar system over the last four decades. They are now in a region of space called the "termination shock" where the gases from the sun are no longer always moving outwards, but ebb and flow as they mix with other interstellar gases.

The ionized gases initially spread out and slowed down, decreasing density and temperature, but they now mix with turbulently with the gases from other stars and undergo compression and heating. This boundary in space is marked by a sudden increase in temperature and change in phase of the plasma. Voyager 1 crossed the termination shock at a distance of 94 AU in December of 2004 and Voyager 2 crossed the termination shock at a distance of 84 AU on August 2007.


Click to enlarge this image (via NASA) that shows the location of the Voyager spacecraft in the heliosphere


Faraday Cup Plasma Detector

Clouds of ionized gases from the sun and other stars float throughout the solar system. The Voyager spacecraft has two Faraday cup plasma detectors used to detect those charged particles.  

One detector is pointed towards Earth and the other is rotated at a right angle to the first. As charged particles enter the detector, the change in the net charge is proportional to the number of ions that strike the detector. Measurement of the electric current that flows as charges move between the inner and the outer body of the detector provides data that scientists can use to determine charges in the range of 10 eV to 6000 eV.


Image of a Faraday cup plasma detector.


The stated goals of the plasma experiment are:

  1. Determination of the properties of the solar wind, including changes in the properties with increasing distance from the Sun
  2. Study of the magnetospheres that are intrinsic to the planets, themselves, and that corotate with the planets independent of solar wind activity
  3. Study of the satellites
  4. Detection and measurement of interstellar ions

From the Voyager Backgrounder:


"The Earth-pointing detector uses a novel geometrical arrangement that makes it equivalent to three Faraday cups and determines microscopic properties of the plasma ions. With this detector, accurate values of the velocity, density, and pressure can be determined for plasma from the Earth (1 A.U.) to beyond Saturn (10 A.U.). Two sequential energy scans are employed to allow the instrument to cover a broad range of energies -- from 10 electron volts (eV) to 6,000 electron volts (6 KeV)."

"The other Faraday cup, a side-looking or lateral detector, measures electrons in the range of 10 electron volts to 6 Kev and should improve spatial coverage for any drifting or rotating positive ions during planetary encounters.

The instrument was designed primarily for exploring planets' magnetospheres. It is capable of detecting hot subsonic plasma such as has been observed in the Earth's magnetosphere and is expected from ions originating in the McDonough-Brice ring of Io. The instrument's large angular acceptance allows detection of plasma flows well away from the direction of the Sun, such as plasma flows that corotate with the planet."



The plasma experiment weighs 9.9 kg (21.8 lb.) and draws 8.3 watts of power. 

Plasma Wave System

The plasma wave experiment uses two antennas to determine the wave-particle interactions of charged particles near planets and in the interplanetary medium. As the antennas move through clouds of charged particles, they detect the changing electric fields.  

The antennas are connected to a selectable pre-amplifier (x0 or x0.01 gain) and then fed to a differential amplifier. The composite signal is fed through a 2.4 kHz and 7.2 kHz notch filter to reduce the spacecraft's 1st and 3rd harmonic from the power supply. 

The PWS experiment was a late addition to the spacecraft and force the plasma scientists to use the antennas that were a part of the Planetary Radio Science experiment.


Voyager plasma wave diagram from the University of Iowa


From the Voyager Backgrounder:

"Scientific objectives of the plasma wave experiment are measurements of thermal plasma density profiles at Jupiter and Saturn; studies of wave-particle interaction and study of the interaction of the Jovian and Saturnian satellites with their planet's magnetospheres."

A Few Words with Dr. John Richardson

Massachuttes Institute of Technology Professor John Richardson is the Principal Investigator of the Plasma Field Experiments and analyzes the data that is actively returned from Voyager. He provided AAC with several comments about the still running experiment.

When asked about the most exciting part of the Voyager research, Dr. Richardson responded: "Voyager has been a fount of excitement over the past 40 years. Highlights are the discovery of volcanos on Io, the only flybys of Uranus and Neptune, which have highly tilted magnetic fields, the first crossing of the termination shock and heliopause showing the size of our heliosphere, and the first measurements of the interstellar medium which surrounds or heliosphere."


Image of the Voyager spacecraft at the interstellar boundary. Image courtesy of NASA


"We are learning that the heliosphere is not symmetric. Outside the termination shock, the flow patterns observed by Voyager 1 north of the equator and Voyager 2 south of the equator are very different. The solar wind still has a major influence even in the interstellar medium. Pressure pulses observed at Voyager 2 in the heliosheath produce shocks when they collide with the heliopause, which then propagate into the interstellar medium where they are observed by Voyager 1."