Conjugate ground‐spacecraft observations of VLF chorus elements

Abstract

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>We present results of simultaneous observations of VLF chorus elements at the ground‐based station Kannuslehto in Northern Finland and on board Van Allen Probe A. Visual inspection and correlation analysis of the data reveal one‐to‐one correspondence of several (at least 12) chorus elements following each other in a sequence. Poynting flux calculated from electromagnetic fields measured by the Electric and Magnetic Field Instrument Suite and Integrated Science instrument on board Van Allen Probe A shows that the waves propagate at small angles to the geomagnetic field and oppositely to its direction, that is, from northern to southern geographic hemisphere. The spacecraft was located at L≃4.1 at a geomagnetic latitude of −12.4^∘ close to the plasmapause and inside a localized density inhomogeneity with about 30% density increase and a transverse size of about 600 km. The time delay between the waves detected on the ground and on the spacecraft is about 1.3 s, with ground‐based detection leading spacecraft detection. The measured time delay is consistent with the wave travel time of quasi‐parallel whistler‐mode waves for a realistic profile of the plasma density distribution along the field line. The results suggest that chorus discrete elements can preserve their spectral shape during a hop from the generation region to the ground followed by reflection from the ionosphere and return to the near‐equatorial region.</p></div>