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A substorm is a series of magnetic disturbances that occurs in the magnetosphere and ionosphere when the interplanetary magnetic field (IMF) turns southward and increased energy flows from the solar wind into the magnetosphere. The most obvious manifestation of a substorm is the aurora in the high-latitude ionosphere. During a substorm, quiet auroral arcs suddenly explode into brilliance and become intensely active. Very strong ionospheric current, termed auroral electrojet, is generated in the auroral zone. Magnetic disturbances produced by the auroral electrojet can be measured with ground magnetometers.

The entire process of a substorm can be divided into three phases: growth, expansion, and recovery. The growth phase of a substorm starts when the dayside merging between the interplanetary and magnetospheric magnetic field lines is distinctly enhanced due to a southward turning of the IMF. The magnetic flux eroded on the dayside magnetopause is transported into the tail, resulting in an increase of the tail lobe magnetic field. The period of loading of the magnetotail with magnetic flux typically lasts for about one hour. After that time period too much magnetic flux and thus magnetic energy has been accumulated in the tail. The tail becomes unstable and tries to get rid of the surplus energy. This is the time of substorm onset and the beginning of the expansion phase. A widely accepted mechanism for the substorm onset is magnetic reconnection in a near-Earth neutral line between -20 and -30 Earth radii. During the expansion phase, the magnetic energy stored in the tail is released, which is called the energy unloading process. The expansion phase lasts about 30-60 minutes. Then the reconnection in the near-Earth neutral line ceases and substorm activity settles. The magnetosphere starts to recover, and the recovery phase takes about 1-2 hours. During magnetic storms, a series of substorms may occur. The term, periodic substorms, has been used to describe such a substorm chain that lasts for many (e.g., 6-10) cycles, shows well-defined waveforms, and has nearly constant periods. A prominent feature of energetic plasma particle flux variations during periodic substorms measured by geosynchronous satellites is the sawtooth-like shape with periodic repetitions of sudden flux increases followed by gradual flux decreases, so the periodic substorms are also called sawtooth events. Multiple space-based and ground-based instrumental measurements show that each cycle of periodic substorms includes the following processes: a magnetic reconnection onset in the near tail, an injection of energetic plasma particles from the tail to the inner magnetosphere, an enhancement of energetic neutral atom emissions in the ring current, a magnetic dipolarization of the magnetotail, an intensification of the auroral emissions, a latitudinal motion of the enhanced auroral emission band, an increase of the polar cap index, an increase of the auroral electrojet index, and a positive bay in the ground magnetometer northward component at middle and low latitudes. In other words, each cycle of periodic substorms has all well known characteristics of isolated substorms. Because the magnetosphere is very dynamic with continuously southward IMF during magnetic storms, periodic substorms are generally stronger than quiet-time isolated substorms.



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