Proton Intensity Dropout in Isotropic Turbulence Regime during Gradual Solar Energetic Particle Events in Solar Cycle 23

Abstract When the Wind spacecraft started its only journey to Lagrange point L2 in 2003 October, it entered an isotropic turbulent environment with radially mean magnetic field ( B m ) and slow solar wind speed ( V SW ). On October 26, it detected a particle intensity dropout interval following a gradual solar energetic particle event. In addition, it recorded three magnetic field rotations corresponding to one-sided increases in V SW , indicating the occurrence of magnetic switchback in the Alfvénic solar wind. Since the last switchback interval is just before the dropout interval, we studied the difference of the pitch angle distribution (PAD) of protons between them. In the switchback interval ( B m perpendicular to V sw ), the PAD maximum is located at μ ∼ +1, where μ is the pitch angle cosine, while, in the dropout interval ( B m antiparallel to V sw ), the maximum is at μ ∼ 0. The difference indicates that protons with greater gyroradii cannot be confined in the curved magnetic field lines and can be injected into the dropout region with μ ∼ 0. Therefore, the analysis of proton pitch angle scattering in the dropout interval can be greatly simplified due to the short duration of the interval and the sharp initial conditions. Compared with the analytical solution of the Fokker–Planck equation, we deduce that the parallel mean free path of 2.1 MeV protons is 3.7 ± 0.5 au, which means that the dropout of proton intensity in the isotropic turbulent regime is also caused by insufficient spatial diffusion of protons.