Equatorial noise: Statistical study of its localization and the derived number density

Abstract

Results of a statistical study of equatorial noise emissions are presented. These electromagnetic emissions are observed in the inner magnetosphere in the vicinity of the geomagnetic equator at frequencies below the lower hybrid frequency. We use the data recorded by four Cluster spacecraft during years 2001–2003. The data set was processed in three steps. In the first one, we have selected the data with a nearly linear polarization (ellipticity less than 0.2), corresponding to the known properties of the equatorial noise. Secondly, we have found parameters of a Gaussian model of the frequency-averaged power-spectral density of those selected waves as a function of the geomagnetic latitude. Finally, we have analyzed the data in the latitudinal interval defined by the width of the Gaussian model. Our results show that most intensity peaks of equatorial noise occur exactly at the magnetic equator. Incidental deviations are most probably caused by problems in determination of the true magnetic equator, which is shown by using different magnetic field models. We have estimated the plasma number density at the observation points using the cold plasma theory. These estimates are, within experimental errors, close to the values obtained from the spacecraft potential data measured by the EFW instrument.