LAUSR

A long‐standing problem concerning dipolarization front (DF) is why energetic electrons only appear in half the DF events? By analyzing MMS measurements, here we answer this question. We find a DF structure, behind which energetic‐electron fluxes are modulated by magnetosonic waves: At wave troughs (B‐minimum) electron fluxes are high; at wave crests (B‐maximum) electron fluxes are low. This phenomenon challenges the classical theory of betatron mechanism, so we need to propose a new theory to explain it. In our theory, there exists a magnetic bottle with time‐varying belly but steady neck behind the DF. When the belly expands, a magnetic bottle is formed, and electrons are trapped; when the belly contracts, the magnetic bottle disappears, and electrons are expelled. Quantitatively, we validate the existence of this bottle and estimate its size as 2–3 RE. Our theory can explain both the presence and absence of energetic electrons behind DFs.