Substorm growth phase on the magnetotail
Authors: Watanabe M. , Iijima T. 
Published: February 2008

Using the magnetic field and the plasma measurements acquired with the DMSP F7 satellite at an altitude of ∼840 km, and in the premidnight (∼2000‐0100 MLT (magnetic local time)) sector, we have determined the characteristics of field‐aligned currents and particle precipitation during the substorm growth phase. Characteristics obtained here include the following: (1) plasma sheet structure diagnosed by the ion precipitation consists of two parts, namely, the inner plasma sheet (IPS) that fills the inner part of magnetotail plasma sheet with hot particles (average energy ≈ 8.5‐12 keV), and the outer plasma sheet (OPS) that occurs tailward of and adjacent to the former and carries weaker energy flux (<2.5 × 1010 eV cm−2 sr−1 s−1 at 840 km altitudes); (2) from the latitudinal profile in the total energy flux of ion precipitation, it is statistically confirmed for the first time from the low‐altitude observation that during the growth phase, L‐dependent profile in the ion plasma pressure associates a definite earthward ▽P spreading all over the IPS and further into the OPS; (3) through the so‐called Harang discontinuity MLT sector, above mentioned plasma features are invariable, however, spatial distribution of field‐aligned current pattern changes systematically and includes the following. The classic evening‐type region 2 plus region 1 field‐aligned current system occurs in the late‐evening MLT sector, while classic morning‐type region 2 plus region 1 system in the midnight sector. In either case, the region 2 roughly coincides with the IPS and the region 1 with the OPS. In addition to region 2 plus region 1 system, a current system whose current polarity is directed opposite to region 1 system often occurs near the poleward limit of the OPS and poleward of the adjacent region 1 system for both the morning‐type and the evening‐type current system, which we tentatively refer to region 0 system here. It is strongly suggested that three‐region current pattern (evening‐type region 2 plus region 1 plus region 0) doesn't mean a westward intrusion of the morning‐type region 2 and region 1 system into the premidnight sector but is totally attributable to source mechanism that is proper to the evening domain; (4) the intensity of field‐aligned current is almost balanced latitudinally even when the currents are multiple sheets (imbalance is less than 20% of total amplitude). The value of intensity is (255 ± 100) nT in transverse magnetic disturbance (equivalent to (0.203 ± 0.080) A/m). From these facts, we propose here a stress balance model in the magnetotail plasma sheet, in which the inhomogeneity in plasma pressure and magnetic field intensity causes the azimuthal and radial current loop, respectively, in the inner and middle part of the plasma sheet. The azimuthal current loop associates the region 2 sense field‐aligned current and the radial current loop associates the region 2 plus region 1 sense field‐aligned currents in pair. In addition, two‐cell convective motion is thought to occur in the OPS and associates the region 1 sense field‐aligned current. We suggest that during the growth phase, field‐aligned current system includes multiple‐source mechanisms and the observed magnetic disturbances at low latitudes (so‐called region 2, region 1 field‐aligned current, etc.) are manifestation of superimposed effect of these different mechanisms operating in the magnetotail plasma sheet.