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Nightside Magnetosphere‐Ionosphere Current Circuit: Implications for Auroral Streamers and Pi2 Pulsations
Author(s) -
Ohtani S.,
Uozumi T.
Publication year - 2018
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1002/2017ja024792
Subject(s) - substorm , ionosphere , physics , resistor , magnetosphere , oscillation (cell signaling) , plasma sheet , time constant , coupling (piping) , geophysics , plasma , computational physics , voltage , electrical engineering , chemistry , materials science , quantum mechanics , biochemistry , engineering , metallurgy
We investigate the electrodynamic coupling of the nightside magnetosphere‐ionosphere system using the analogy of a current circuit. In our model circuit the generator drives a constant current, which flows through the magnetotail and ionosphere branches. The magnetotail branch has a capacitor C and resistor R T , whereas the ionospheric branch has an inductor L and resistor R I . Each element is physically described with local quantities and geometries. For R T ≪ R I the electric circuit is characterized by three time constants: τ CR (=CR T ) , τ LC =LC , and τ L/R (= L / R I ) . It is found that τ CR is of the order of the ion gyroperiod in the plasma sheet, and τ LC and τ L / R correspond to the eigenperiod and decay time of the field line oscillation, respectively. Therefore, despite the variability of each circuit element, τ CR ≪ τ LC ≪ τ L/R holds generally. It is found that under this condition the current circuit is characterized as overdamped, and its decay time constant is given by τ L/R . R I is smaller, and therefore, τ L/R is longer as the structure is more elongated in the direction of convection. This may explain why the auroral streamers, which are considered to be the ionospheric manifestation of fast flows in the plasma sheet, last significantly longer than the flows themselves. Another application is the Pi2 pulsations at the substorm onsets. If R T increases by a factor of τ LC / τ CR , the system indeed becomes underdamped, and the oscillation period is given by 2 πτ LC . It is suggested that the substorm initiation is a distinct process with a significant enhancement of tail resistivity in a localized area.