A new model to explain energetic electrons behind dipolarization front

Dipolarization front—a sharp boundary leading reconnection jets and producing colorful auroras—plays a crucial role in the magnetotail energy conversion. Behind this front, sometimes energetic electrons appear, whereas sometimes they vanish. The reason causing such uncertainty is still a mystery, owing to the lack of high-resolution measurements. Here we propose a novel model to uncover this mystery: we find that behind the front there exists a magnetic bottle with time-varying belly but steady neck. When the belly is expanding—like a man getting fat—the magnetic bottle is formed and energetic electrons are trapped; when the belly is contracting—like a man getting slim—the magnetic bottle disappears and energetic electrons are expelled. This model clearly explains how energetic electrons are trapped in the Earth’s magnetotail and in principle it can be applied to other planetary magnetotails.