Initial results of a survey of equatorial noise emissions observed by the Cluster spacecraft

Initial results of a survey of equatorial noise emissions are presented. These plasma wave emissions are observed in the inner magnetosphere close to the geomagnetic equator at frequencies below the local lower hybrid frequency. We use the data recorded by the four Cluster spacecraft during the first 24 months of measurements. The data set was processed in three steps. First, we have selected the data with a nearly linear polarization corresponding to the known properties of the equatorial noise. Second, we have found parameters of a Gaussian model of the frequency-averaged power-spectral density of the selected waves as a function of the geomagnetic latitude. Third, we have analyzed the data as a function of frequency in the latitudinal interval defined by the width of the Gaussian model. Our results show that most intensity peaks of equatorial noise occur within 2∘ of the magnetic equator and the full-width at half-maximum (FWHM) of these peaks is below 3∘ in the majority of cases. The most probable frequency of the emissions is between 4 and 5 local proton cyclotron frequencies. The probability density of occurrence of the emissions then slowly decreases toward higher frequencies. Multipoint measurements indicate that the variations of the ratios of amplitudes of the equatorial noise emissions measured on different spacecraft do not increase at spatial scales up to 0.7 Earth radii in the equatorial plane. On the other hand, the variations do increase with time delay between measurements in an interval from tenths to hundreds of minutes.