Validating in-situ measurements of ULF Waves in Low Earth Orbit

Ultra-low frequency (ULF) waves in the Earth’s magnetosphere are commonly observed in space as well as on the ground stations. Located between the magnetosphere and the earth’s surface, the lower-thermosphere-ionosphere region is expected to be abundant in ULF waves. Transverse Alfven waves are of interest since they have one-dimensional nature along the magnetic field lines with radial variation in their characteristic frequencies. We have conducted three-dimensional magnetohydrodynamic wave simulations to investigate ULF observations by various types of low-Earth orbit (LEO) satellites. Virtual spacecraft are embedded in our model to measure electromagnetic wave signals as they move at different altitudes and latitudes. Our results present that the waveform and frequency of transverse waves change significantly when they are observed in low Earth orbit. Since the majority of LEO satellites lay in polar orbit, they traverse different field lines at relatively high speeds. Thus, fast movement through Alfven speed gradient along the spacecraft trajectory alters the frequency of transverse Alfven waves. It is worth noting that the observed frequency by virtual satellites in low-Earth orbit becomes ~8 times higher than the original frequency. It indicates that frequency distortion from LEO satellite observations can cause serious differences between ground-based and satellite observations. We suggest validating ULF wave observations in low-Earth orbit using a series of spacecraft such as the CubeSat constellation can improve the robustness of electromagnetic field measurements in LEO.