Statistical analysis of wave propagation properties of equatorial noise
observed at low altitudes
Abstract
Equatorial noise is an electromagnetic emission with line spectral
structure, predominantly located in the vicinity of the geomagnetic
equatorial plane at radial distances ranging from 2 to 8 Earth’s radii.
Here we focus on the rare events of equatorial noise occurring at
ionospheric altitudes during periods of strongly increased geomagnetic
activity. We use multicomponent electromagnetic measurements from the
entire 2004–2010 DEMETER spacecraft mission and present a statistical
analysis of wave propagation properties. We show that, close to the
Earth, these emissions experience a larger spread in latitudes than they
would at large radial distances and that their wave normals can
significantly deviate from the direction perpendicular to local magnetic
field lines. These results are compared to ray tracing simulations, in
which whistler mode rays with initially nearly perpendicular wave
vectors propagate down to the low altitudes with wave properties
corresponding to the observations. We perform nonlinear fitting of the
simulated latitudinal distribution of incident rays to the observed
occurrence and estimate the distribution of wave normal angles in the
source. The assumed Gaussian distribution provides the best fit with a
standard deviation of $2^{\circ}$ from the
perpendicular direction. Ray tracing analysis further shows that small
initial deviations from the meridional plane can rapidly increase during
the propagation and result in deflection of the emissions before they
can reach the altitudes of DEMETER.