Quiettime observation of a coherent compressional Pc-4 micropulsation at synchronous altitude

During a magnetically quiet interval (1740–1850 UT), on February 14, 1967, the magnetic-field intensity and energetic electron fluxes (E≈0.3–2.0 Mev) at ATS 1 exhibited coherent modulations having a frequency of 33.8 cph (period ≈106.5 sec) and a duration of approximately 40 oscillations. The electron fluxes and the magnetic field oscillated in phase. The field perturbation reached 8 γ (peak to peak) in the direction of the unperturbed geomagnetic field. The transverse component of the field perturbation was practically zero. The characteristics of the observed oscillations appear to be compatible with those of a compressional (magnetosonic) excitation of the outer magnetosphere. The substantially radial normal mode is perhaps driven by a bounce-resonant interaction with the 15-kev protons that populate the quiettime ring current.