Fig. 5

Magnetic field frequency spectra (left) and proton energy spectra (right) before (red) and during (blue) the observation of the magnetic cavity, providing additional perspective of the proton spike event. The frequency spectra are obtained from the GSE-z component of the magnetic field measured by the Wind spacecraft during 20:27:30–22:54:00 UT (outside the cavity; pink), and 23:20:30–24:33:00 (inside the cavity; light blue) of 4 June 2011. The ion cyclotron frequencies are indicated with the arrows. The energy spectra are obtained by the Low and High Energy Detectors of SOHO/ERNE during 22:00–23:00 UT (outside the cavity; red) and 22:00–23:00 UT (inside the cavity; blue). The background spectrum is obtained from a pre-SEP time period, i.e., 19:00–22:00 UT of 3 June 2011 (dashed curve) and is subtracted from the shown spectra. In both panels, both the power-law fit and spectral index are shown for each spectrum. The two proton energy spectra are almost identical below 10 MeV and have the power-law index of − 4.2 (black line)