Schematics of the whistler instability driven by the sunward deficit in the anti-strahl direction. The core population is shown as a blue shaded semi-circle, with a blue semi-circle marking its thermal velocity (w_c) and v_c∥ its bulk velocity in plasma rest frame. The dashed yellow curve indicates the strahl. Pink semi-circles mark curves of constant energy in the rest frame of the wave moving with v_ph = ω/k_∥, and black dashed semi-circles the curves of constant kinetic energy in the plasma frame. Pink arrows indicate the electron diffusion paths. (a) Example without the sunward deficit; (b) The sunward deficit can trigger an instability.