Abstract
This study presents results from magnetic field line conjunctions
between the medium-Earth orbiting Demonstration and Science Experiments
(DSX) satellite and the low-Earth orbiting VLF Propagation Mapper (VPM)
satellite. DSX transmitted at very low frequencies (VLF) towards VPM,
which was equipped with a single-axis dipole electric field antenna,
when the two spacecraft passed near the same magnetic field line. VPM
did not observe DSX signals in any of the 27 attempted conjunction
experiments; the goal of this study, therefore, is to explain why DSX
signals were not received. Explanations include i) the predicted power
at LEO from DSX transmissions was too low for VPM to observe; ii) VPM’s
trajectory missed the “spot” of highest intensity due to the focused
ray paths reaching LEO; or iii) rays mirrored before reaching VPM.
Different combinations of these explanations are found. We present
ray-tracing analysis for each conjunction event to predict the
distribution of power and wave normal angles in the vicinity of VPM at
LEO altitudes. We find that, for low-frequency (below 4kHz)
transmissions, nearly all rays mirror before reaching LEO, resulting in
low amplitudes at LEO. For mid- and high-frequency transmissions
(~8kHz and 28kHz respectively), the power at LEO is
above the noise threshold of the VPM receiver (between 0.5µV/m and
1µV/m). We conclude that the antenna efficiency and plasmasphere model
are critical in determining the predicted power at LEO, and are also the
two most significant sources of uncertainty that could explain the
apparent discrepancy between predicted amplitudes and VPM observations.