Four years ago (2019), an Israeli group published a balloon launch similar to mine (max altitude 28 km), with a magnetometer on board. Their magnetometer was much more sensitive, however. Instead of using the Hall effect, they used a fluxgate, consisting of a ring of ferromagnetic material wrapped in a coil of copper wire; changes in the magnetic flux through the core induced current in the coil for detection.
But I have tested a cheap Hall effect sensor similar to the one on my payload, and it is sufficiently sensitive to measure microtesla-scale changes in ambient magnetic field. I believe that my results will be similar to the Israeli group’s results, except I may have a lot more noise.
The way I tested the Hall effect sensor before launching was this: my teammates at my workplace took the Hall effect sensor to a nearby high-voltage pylon, and measured the ambient magnetic field at different distances from the base of the pylon, traveling on foot. The results showed a distance-dependent decline in magnetic field strength just within a few meters of the base of the pylon, until the magnetic field leveled out at around 40 microtesla, which is reasonable for the geomagnetic field. These measurements are shown below. Each measurement was taken while connected to several GPS satellites, so we imported the GPS coordinates to Google Maps for an overlay of each measurement’s location. There are eleven measurements, and the distance between each measurement is shown qualitatively by a different color on the Google Map Satellite View.
The Israeli group’s high-altitude magnetometry yielded the following confusing figure:
The figure is confusing because between 10 km and 21 km of altitude, there are two magnetic field measurements without explanation in the text of the article. The blue curve is the measured magnetic field by the balloon payload; the other lines are different mathematical models developed from ground-based and orbital measurements. I considered the possibility that between 10 km and 21 km there was divergent magnetometry on the descent compared to the ascent, but this possibility can be eliminated because the mathematical modeling also shows the strange dual-magnetometry between 10 km and 21 km. I expect that my results will be:
- noisier, since I measuring the magnetic field via the Hall effect, which is a less direct technique than the induction method used by the Israeli group
- less confusing, since I expect my measurements to be a one-to-one correspondance between magnetic field strength and altitude for the entire range of altitude.
ASHI, BOOMS, BARREL, BBC are the four launches.
ASHI:
BOOMS:
BARREL:
BBC: