Until now, there have been many hypotheses to explain what triggers a substorm in the magnetotail. Each hypothesis differs regarding the type of trigger, place of occurrence, and method of propagating the impact to the surrounding regions. Over the past decades, active research and intensive argument have been made to find the correct theory. Here I will introduce the magnetic reconnection model,Eone of the most promising theories among the hypotheses.
While energy is accumulated in the magnetotail prior to the onset of the substorm, magnetic field lines (indicating direction of magnetic force) in the magnetotail are extremely stretched as shown in the top of Fig. 2. The magnetic field lines in the south side and north side become parallel, but direction is antiparallel. At a certain time, the configuration of the magnetic field lines changes drastically as the magnetic field lines of south and north in antiparallel state are reconnected, caused by some physical process whose details are still unknown. Magnetic reconnection (1. in the bottom of Fig. 2) occurs at about 20 times the distance of the Earths radius (about 6,400 km) from the Earth on the opposite side of the Sun (hereinafter referred to as X~-20 REE. For reference, the lunar orbit is at about 60 times the distance of the Earths radius from the Earth.
When magnetic reconnection occurs, a substorm is induced by the release of energy stored in the magnetotail. On the opposite side of the magnetic reconnection region from the Earth, a cluster of plasma called plasmoid is formed and ejected away from the Earth at a high speed of several hundred to a few thousand km per second. The plasmoid carries away the southward magnetic fields produced by magnetic reconnection and a part of the released energy. On the other hand, on the Earth side of the magnetic reconnection region, a high-speed plasma flow appears moving toward the Earth, which has northward magnetic fields contrary to the plasmoid. When the flow reaches X~-10 RE,Ea process called magnetic field dipolarization occurs in which the stretched magnetic field lines return to their original state (2. in the bottom of Fig. 2). When this process occurs, particles and part of the released energy rush toward the Earth to cause intensive auroral activity. As stated above, the magnetic reconnection model posits that magnetic reconnection triggers successive substorm processes.