On the Impact of Thermal Gradients across Fluxgate Sensors on In-situ
Magnetic Field Measurements
Abstract
Fluxgate magnetometers are an important tool for measuring space
plasmas. In-situ magnetic field investigations often involve measuring
small perturbations of a large background field, so robust instrument
calibration is critical to accurately resolving geophysical signals.
Fluxgate instruments aboard recent space science missions have observed
calibration anomalies that have been attributed to thermal gradients
across the sensor. Here we present data from a laboratory experimental
investigation of effects of thermal gradients on fluxgate calibration
and performance. A purpose-built laboratory apparatus fixed two thermal
reservoirs at either end of a racetrack fluxgate sensor. Varying the
reservoir temperatures allowed us to vary the sensor temperature and
impose thermal gradients as large as 50 °C across a racetrack fluxgate
sensor. We find that changes in instrumental sensitivity, offset, and
noise can be explained purely by changes in the average temperature of
the sensor without a dependence on the difference in temperature across
the sensor. We suggest that invoking concept of a static thermal
gradient inducing thermoelectric currents within the fluxgate core or
sensor may not be appropriate to explain changes in instrumental
sensitivity, offset, and noise that have been observed on orbit.