• Auroral arc;
  • Energetic particle precipitation;
  • Magnetotail;
  • Substorm onset

Prebreakup arcs (PBAs), thin, faint tendrils of glowing gas stretching thousands of kilometers in either direction, are very different from the multicolored lights typically associated with auroras. PBAs develop a few minutes before the onset of the full display, brightening and expanding (“breaking up”) to form the more prominent auroras. Previous research has shown that the PBA breakup can be tied to brief pulses of energetic particles (auroral substorms) flowing into the upper atmosphere from the nightside of Earth's magnetic field, but the PBA generation mechanism has largely remained a mystery. Understanding the formation of PBAs would be an important step toward answering one of the most longstanding and pressing questions in space physics research: How do substorms form