Juno will be the first spacecraft to fly over the poles of the planet Jupiter at low altitude. Juno science objectives for the polar magnetosphere include: Investigate the primary auroral processes responsible for particle acceleration, characterize the field-aligned currents that transfer angular momentum from Jupiter to its magnetosphere, identify and characterize auroral radio and plasma wave emissions associated with particle acceleration, and characterize the nature and spatial scale of auroral features.

Juno is ideally suited to determine the auroral distributions of charged particles due to its low altitude passes through the polar region of the planet. For the auroral science, The Jupiter Energetic Particle Detector Instrument (JEDI) will directly measure precipitating fluxes including particles that generate the planetary aurora and particles that heat and ionize the planetary atmosphere. Detailed analysis of the particle spectral and angular characteristics will be used to study acceleration mechanisms. JEDI has the capacity through its electron measurements to determine the magnetic topology of the polar cap region.


Jedi will measure the energetic plasma. The JEDI system covers the energy range of 25 keV to ? 500 keV for electrons, and 10 keV/nucleon to ~20 MeV total energy for ions. JEDI determines the distributions of the high-energy magnetospheric ions and electrons, including the composition of ions. It does this by measuring the energy and velocity of the particles and then using a look-up table to determine the mass and therefore the species of particle. The measured species for JEDI include electrons and ions (H, He, O, S).

Rapid spacecraft motions and slow spacecraft rotation require that JEDI simultaneously and continuously resolve both magnetic loss cones at every position inside of ~ 3 RJ. JEDI uses 2 individual instruments (JEDI-90 and JEDI-270), each with multiple views that continuously sample within a 360 degree plane roughly normal to the spacecraft spin axis. All sky coverage is additionally achieved every spin with an additional sensor (JEDI-A180) that views coplanar to the spin axis.


Each JEDI sensor consists of a 60 mm diameter, hockey-puck-like cylinder, in which a start foil and stop foil, wrapped around opposite curved sides of the cylinder, constitute the time-of-flight chamber.
An incoming energetic ion will pass through the collimator and a passive thin foil designed to keep the cold plasma out of the instrument. Then the ion passes through the start foil generating forward-scattered electrons that are then focused towards a microchannel plate. The ion will continue through the chamber to the stop foil, generating backscattered electrons, also accumulated on the MCP. The ion will then pass into a solid-state detector, providing the third component of the measurement. Since the time-of-flight (TOF) can be computed from the start and stop signals and the chamber size is known, the particle speed can be obtained. The velocity coupled with the energy yields the ion species. For ions that fall below the discrimination level of the solid-state detectors, a heavy vs. light determination can be made with the TOF and the anode pulse height.

The detectors are arranged so that each detector senses the events within a given range of incidence angles. One of the JEDI sensors also contains witness detectors. Each of the six detector modules is composed of four pixels: large and small ion and large and small electron. The electron detectors differ from the ion detectors in that they add a layer of aluminum, which excludes low-energy ions.
Each electron and ion detector is split into a small pixel and a large pixel; the large pixel has 20 times the area of the small pixel. This provides 24 detector elements.