Magnetic Dipole Modeling for DC and Low Frequency AC Magnetic Fields in Space Missions

Introduction

In most space missions, dedicated sensitive instruments are mounted on the spacecraft in order to perform in situ measurements of interplanetary magnetic fields (Acuna, 2002). Fluxgate and Search Coil Magnetometers are commonly placed on a long boom that extends beyond the main structure of the spacecraft. The sensitivity (resolution) of these magnetometers is very high for purposes of measuring very weak variations of magnetic flux density (Musmann, 2010). Due to their high sensitivity, however, these sensors are susceptible to magnetic field emissions generated by the spacecraft. Units and equipment on-board, as well as components of the spacecraft itself produce a combined magnetic field at the location of the magnetometers, thus interfering with the output of the instruments. Unquestionably, one of the critical requirements for these instruments is to operate in a magnetically “clean” environment; typical requirements on the total DC magnetic field generated by the spacecraft at the location of the magnetometers are in the order of 0.1-1 nT (Primdahl, 1989; Mehlem, 1987). Moreover, current and upcoming space missions (Solar Orbiter, PLATO, JUICE), have even more stringent requirements on magnetic cleanliness of slowly time varying AC magnetic fields.

The “magnetic cleanliness” of the spacecraft, which is targeted during the development phase, is realized by application of “best engineering practices”, i.e. careful design and construction of the spacecraft by using non - magnetic materials, magnetic shielding, etc. Therefore, the spacecraft residual magnetism is usually kept below the requirements determined by the scientific application and commensurate with the sensitivity of the on-board magnetometers. In most cases, however, magnetic fields emitted by subsystems and components of the spacecraft are still present at the location of the magnetometers. To overcome the magnetic fields’ measurements contamination, it has been a standard practice to perform on-ground electromagnetic compatibility (EMC) test campaigns at unit and system level (Junge & Marliani, 2011). At early stages of the space missions, the main target is to capture the magnetic signature of individual units and/or subsystems that will actually be mounted on the spacecraft and characterize them. The magnetic cleanliness test campaign is finalized at system level, with magnetic measurements and characterization of the entire spacecraft.

The unit level measurements are performed on-ground in dedicated facilities, namely Magnetic Coil Facilities (MCF). These facilities usually employ a pair of Helmholtz coils in order to compensate Earth’s magnetic field (ECSS, 2012). The Equipment under Test (EUT), is placed on a turntable that makes a full rotation of 360° while several magnetometers measure its (remanent) magnetic field at various angles (most commonly at 0°, 10°, …), ensuring that the unit’s magnetic signature is well covered. Although the location of the sensors is in close proximity to the EUT in order to obtain an adequate Signal to Noise Ratio (SNR), practical experience has shown that measurement distances below 3-5 times the maximum dimension of the unit should in principle be avoided, since the magnetic material inside the unit in most cases is not uniformly distributed and composite field behavior that deviates from the dipole approximation may emerge at smaller observation distances.

Various approaches have been proposed for identification of on-board “magnetically clean” specification points, where the magnetometers can be placed. The predominant method indicates that each EUT can be represented by a small number of equivalent magnetic dipole sources and involves the generation of models that can be used to accurately reproduce the magnetic field of the original equipment. The captured magnetic signature of each individual EUT is therefore modeled with a multiple Magnetic Dipole Model (MDM); the parameters of the model (position and magnetic moment vectors of the dipole sources) can be estimated applying either deterministic or stochastic techniques (Weikert, Mehlem, & Wiegand, 2012; Carrubba, Junge, Marliani, & Monorchio, 2012; Dumond, & Bergé, 2012; Boschetti, Gervasio, & Marziali, 2012). The magnetic cleanliness of past and recent space missions for spacecrafts with instruments susceptible to magnetic fields has been largely dependent on MDM modeling at unit level, by capturing near field magnetic measurements of real spacecraft equipment.