pitchangle

Pitch angle is a concept in plasma and space physics describing the orientation of a charged particle’s velocity relative to the local magnetic field. It is defined as the angle α between the particle’s velocity vector and the magnetic field direction. Mathematically, cos α = v̂ · B̂, where v̂ and B̂ are the unit vectors in the particle’s velocity and the magnetic field. The angle ranges from 0° to 180°, with α = 0° corresponding to motion parallel to the field, α = 90° to perpendicular motion, and α = 180° to antiparallel motion.

In a magnetized plasma, particles gyrate around field lines with a characteristic gyrofrequency. The pitch angle determines how much of the particle’s energy is in the parallel versus perpendicular component, influencing confinement and mirror reflection in magnetic bottles. The loss cone is the range of pitch angles that leads to precipitation into denser media, such as the atmosphere, when field lines connect to the planet or star. Pitch-angle distributions can be anisotropic and change due to scattering by waves, turbulence, and collisions.

Measurement and use: Pitch-angle distributions are inferred from directional particle detectors together with simultaneous magnetic-field measurements. From these, scientists compute α and study processes such as pitch-angle diffusion, radiation-belt dynamics, and solar-energetic particle transport. The concept also appears in astrophysical contexts where charged particles move in ordered magnetic fields, including cosmic-ray transport and jets.