The J-MAG Magnetometer: Instrument design, performance, and initial in-flight results.

JUICE is an ESA L-Class interplanetary mission to the Jupiter system that was launched on the 14th April 2023 from Kourou, French Guiana. It will make in-situ and remote sensing measurements of Jupiter and the Galilean moons during a three-year science operation phase starting in July 2031. The tour will include a high-latitude phase, fly-bys of Callisto and Europa culminating in elliptical and circular orbits of Ganymede down to an altitude of 200 km. Detection and characterization of potential sub-surface oceans on Europa and Ganymede are a key science goal of the mission as is the interaction between Ganymedes internal magnetic field with the Jovian field. Constraining the depth and conductivity of any subsurface ocean on Ganymede will be achieved through measurement of response to two magnetic inducing signals, one at Ganymedes orbital period (171.7 hrs) and the other at Jupiters synodic period (10.5 hrs) which together drive a requirement for in-flight accuracy of 0.2nT. The combination of such low frequency and low amplitude scientific signals places a unique set of EMC cleanliness requirements on the spacecraft system and on the performance of the instrument configuration. J-MAG is the DC magnetometer instrument on JUICE measuring the low frequency magnetic field vector in the range 0-64 Hz. It is composed of a conventional dual fluxgate design together with a Coupled Dark State Magnetometer (CDSM) absolute scalar sensor and an electronics box accommodated within a vault on the main platform. All three sensors are mounted on the outer segment of a 10.6 m boom. The scalar sensor is included to enable calibration of the fluxgate sensors during the Ganymede phase where the magnetic field is highly variable and traditional techniques for fluxgate offset calibration (such as spacecraft rolls or analysis of incompressible waves in the solar wind) are not viable. We report on the instrument design and challenges presented by the mission environment and trajectory. We will present the performance of the magnetometer on ground and a summary of the initial results from the near-Earth commissioning phase that took place during May 2023.