On the Impact of Thermal Gradients Across Fluxgate Sensors on In Situ Magnetic Field Measurements

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 degrees 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. Plain Language Summary Fluxgate magnetometers are important tools for studying magnetic fields in space. However, when a fluxgate sensor is exposed to changing temperatures on-orbit, the baseline measurement can be unstable, making it harder to accurately measure small magnetic fields. Some fluxgates experience on-orbit instabilities that are related, not just to the average temperature of the sensor, but also to differences in temperature across the sensor. We use a laboratory testing method to explore the relationship of temperature differences across a sensor to changes in instability of the measurement. While we observe a strong relationship between changes in instability and the average temperature of the sensor, we do not observe a meaningful relationship between instability and differences in temperature across the sensor.