Statistical Analysis of Wave Propagation Properties of Equatorial Noise Observed at Low Altitudes

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 degrees 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. Plain Language Summary We study the electromagnetic emission called equatorial noise, which occurs frequently in the magnetosphere of Earth and is known to have an impact on the radiation belt dynamics. Here we present statistics of the rare events when the emissions reached the altitudes of 700 km and were detected by the low orbiting satellite DEMETER. Our analysis reveals an unusually high spread of recorded events in the latitude, and we also notice large deviations of the wave vector from the perpendicular direction. The observed wave properties and indices of geomagnetic activity are used to set up a ray tracing simulation. We confirm that the observations agree with the theoretical propagation properties of rays in the whistler mode, which is the wave mode associated with equatorial noise. The correspondence between simulation and observation is further improved by inferring the initial wave properties in the source with the help of nonlinear least squares fitting. Additional simulations of ray propagation with nonzero initial deviation from the plane of local meridian confirm that such deviations must be minimal; otherwise, the rays become deflected before reaching the altitude of the DEMETER satellite.