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How do magnetospheric substorms produce bright auroral displays?
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Ionosphere and magnetosphere
The layers of Earth's atmosphere, with a yellow line showing the air temperature at various heights.
Magnetospheric substorms produce bright auroral displays through a sequence of processes involving magnetic reconnection and the energization of charged particles.12
The process typically follows these stages:
- Growth Phase: When the interplanetary magnetic field turns southward, magnetic flux is eroded from the dayside and added to the magnetotail. During this time, quiet auroral arcs drift equatorward.3
- Expansion Phase: This phase begins with a sudden brightening of the most equatorward auroral arc near midnight, often called the auroral breakup. In the "near-Earth neutral-line" model, this occurs when reconnection reaches the last-closed field lines. The substorm-wedge current system then diverts tail currents into the ionosphere, creating strong localized electric fields.34
- Particle Acceleration: These transient electric fields energize electrons and protons, propelling them earthward. As these particles collide with oxygen and nitrogen atoms in the upper atmosphere, they leave the atoms in excited states, which emit the light observed as bright auroras.425
This expansion manifests as a poleward and westward surge of bright, dynamic auroral forms.3