The Juno ASC as an Energetic Particle Counter

The Juno spacecraft completed 35 successful orbits around Jupiter from orbit insertion in 2016 through the end of its prime mission phase in 2021. The Advanced Stellar Compass (ASC) and associated Camera Head Units (CHUs) comprise a dedicated attitude sensing system of the Magnetic Field Investigation (MAG), one of Juno’s scientific payloads. The CHU is a CCD-based camera and is inherently susceptible to ionizing radiation. An energetic charged particle penetrating the camera head electronics shielding will deposit its energy and liberate charges in the charge wells. These events will register as isolated bright pixels in the integrated star tracker source images, and are distinguishable from optical sources that illuminate a number of collocated pixels spanning the imager’s point spread function. By simply counting the number of such isolated bright pixels, an estimate of the number of charged particles penetrating the CCD can be established, eventually constraining the local external omnidirectional radiation flux (omniflux). The ASC performs the bright pixel count onboard and includes this count rate with the attitude telemetry, providing an energetic particle omniflux measurement at high time resolution. We describe here this additional functional capability of the ASC, including the filtering required to isolate the unbiased attitude cycles, the calibration required to circumvent count statistics effects, and the calibration of the sensor sensitivity, as well as the attenuation efficacy of the sensor shielding mass. Finally, we discuss the potential of the omniflux product for radiation field mapping as well as a proxy for investigating physical phenomena otherwise unattainable.