Magnetospheric Line Radiation Observed Close to the Source: Properties and Propagation

Abstract Magnetospheric Line Radiation (MLR) is a type of whistler mode electromagnetic wave phenomenon observed in the inner magnetosphere at frequencies of a few kilohertz, that is characterized by a frequency modulation of the wave intensity. Although such events are quite regularly observed by ground‐based stations and low‐altitude spacecraft, their observations in the equatorial region at larger radial distances (i.e., close to tentative source regions) are extremely limited, likely due to the generally low frequency resolution of available measurements. A systematic search for MLR in continuous intervals of high‐resolution multicomponent wave data obtained by the Van Allen Probes spacecraft detects 15 events. They occur primarily on the dayside at frequencies between about 1 and 5 kHz, propagating with oblique wave normals away from the geomagnetic equator. For one event, simultaneous ground‐based observations are available, providing limits on the spatial extent of the event: it does not extend beyond the high‐density plasmasphere region. An electrostatic wave at a frequency corresponding to the modulation frequency of MLR is observed in three events. This is likely linked to the event formation mechanism and has not been observed before. Our results can lead to an understanding of the formation mechanism of MLR.