Outer Zone Relativistic Electron Response to Mesoscale Periodic Density Structures in the Solar Wind: Van Allen Probes Measurements

We investigate the relativistic and the ultra-relativistic outer radiation belt electron response to the Periodic Density Structures (PDS) in the solar wind. We have studied four intervals between 0000 UT and 1500 UT during 17 January 2013 interplanetary coronal mass ejection (ICME) sheath event following the passage of an interplanetary shock (IP) on 16 January, at 2240 UT. Earlier studies limited to geosynchronous orbit have shown that PDS induce Ultra-Low-Frequency (ULF) oscillations in the magnetosphere and modulate energetic electron intensities. Our study shows for the first time, using pitch angle resolved electron intensity measurements, that PDS modulate relativistic and ultra-relativistic electron intensities in the heart of the radiation belts (L ≈ 4). We find that the modulation of electron intensities occur at frequencies close to those of interplanetary PDS. Furthermore, electron intensity modulations occur over a wide range of energy (≈200 keV to 4 MeV) and pitch angles (≈20–120°). This event study suggests that relativistic and ultra-relativistic electron intensity modulations driven by PDS may be largely energy and pitch angle independent.