The Low Energy Particle Instruments on the Voyager Spacecraft

The Voyager 1 and 2 spacecrafts both have an impressive array of scientific instruments and experiments on board. During their encounters with various celestial objects, they took a variety of measurements and observations, revealing unprecedented details of our solar system.

The Voyager spacecraft has multiple experiments dedicated to charged particle detection. The Plasma Spectrometer (PLS), was covered in the previous article. This article looks at the Cosmic-Ray System (CRS), and the Low Energy Charged Particle System (LECP). These experiments worked together to reveal the location, quantity, and flux characteristics of various particles, and provided scientists with data about the boundaries of our solar system.

Low Energy Charged Particle (LECP)

According to the Voyager Backgrounder, the Low Energy Charged Particle experiment uses “two solid-state detector systems on a rotating platform mounted on the scan platform boom. One system is a low-energy magnetospheric particle analyzer,” (LEMPA) while the “second detector is a low-energy particle telescope” (LEPT). The LEPT provides measurements on interplanetary and interstellar particles, while the LEMPA is more specialized in particle species detection and near-planet measurements.

The low energy charged particle instrument. Image courtesy of NASA.

The LECP has seven science objectives:

1. Determine the atomic makeup of galactic cosmic radiation with a focus on low energy particles
2. Measure the time variation of galactic cosmic rays
3. Measure the radial gradient of galactic cosmic rays
4. Identify particles originating from solar flares and active solar regions
5. Identify particles originating from planets
6. Identify energetic particles in between planets

Of the Voyager probes’ three main particle systems (PLS, LECP, and CRS), the LECP’s energy range was the broadest.

The LEMPA operates by deflecting medium to low energy particles into beta and gamma total energy detectors where a sintered-cobalt rare earth magnet filters out smaller particles, allowing for protons and ions to pass and reach the alpha total energy detector. Energies down to 12 keV are detectable.

The LEPT is an array of solid state detectors that measure the charge and energy distribution of low and medium energy nuclei in low energy environments. This experiment is designed to identify protons and heavier nuclei (atomic number less than 32).

The platform stepper motor receives a 15.7-watt pulse every 192 seconds, provided by a bank of capacitors. The rotating platform provides the LECP with a 360-degree field of view.

During their inter-solar system journey, the Voyager 1 and 2 probes took measurements of the magnetospheres of Jupiter, Saturn, Uranus, and Neptune. The LECP has also been used to determine the environment of the Voyager 1 probe. When Voyager 1 entered interstellar space, detected galactic cosmic rays increased significantly, while protons originating from the sun virtually became undetectable.

Particle measurements that indicated Voyager 1 was crossing the boundary of the solar system. Image courtesy of NASA.

Cosmic Ray Subsystem

The CRS (Cosmic Ray Subsystem) has six science objectives:

1. Take energy spectrum measurements of electrons in the range of three to 110 MeV.
2. Measure the energy spectra and composition of cosmic ray nuclei in the energy range of 1-500MeV/nuclei. Composition ranges from hydrogen to iron.
3. Provide insight into galactic cosmic rays including origin, acceleration process, dynamics, history, and energy content to understand nucleosynthesis from cosmic ray sources.
4. Provide insight into how cosmic rays, Jovian electrons, and low energy particles from interplanetary space are transported.
5. Measure streaming patterns in three-dimensions of Hydrogen to Iron nuclei.
6. Measure the composition of particle charge in the magnetospheres of Jupiter, Saturn, Uranus, and Neptune.

The CRS is the most sensitive of the particle detecting instruments onboard the Voyager 1 and 2 probes. It has been designed to detect high energy plasma particles coming from intense radiation around planets like Jupiter, as well as from other stars, and continues to operate today.

The CRS is comprised of three solid state subsystems: the High Energy Telescope System (HETS), the Low Energy Telescope System (LETS), and The Electron Telescope (TETS).

The CRS helped determine when both the Voyager 1 and 2 probes crossed the solar system termination shock, in 2004 and 2007 respectively, and also helped determine when Voyager 1 entered interstellar space in 2012.

The CRS took measurements of the composition of solar winds during a solar minimum in 1977, detecting Helium, Carbon, Nitrogen, Oxygen, and Neon.

Measurements from the CRS indicating Voyager 1 leaving the solar system. Image courtesy of NASA.

Conclusion

The three low energy particle instruments on board the Voyager 1 and 2 probes were essential to providing insight into the particle and radiation environments of our solar system. It gave us a wide range of measurements on the magnetospheres of Jupiter, Saturn, Neptune, and Uranus, and helped us map the boundaries of our solar system.