Two classes of earthward fast flows in the plasma sheet

In this study we first identify earthward plasma sheet fast flows from Geotail plasma and magnetic fields with a criterion of V_⊥x > 300 km/s, where V_⊥x is the X component of the plasma flow perpendicular to the ambient magnetic field. We then estimate rates of change of the nightside auroral power over the courses of the fast flows using Polar Ultraviolet Imager auroral images. It is found that 68 earthward fast flows observed at |Y| < 6 R_E during 1997-1998 can be classified into two classes. One class of the earthward fast flows (Class I) was often observed near X = ∼-10 R_E and the other class (Class II) was found at X < - 15 R_E. The auroral power rates of change of the fast flows in Class I in terms of time are found to be high, indicating that the total auroral power on the nightside was significantly increasing during these fast flows. The corresponding auroral images show an apparent substorm bulge developed on the nightside, i.e., a significant global auroral development. The auroral power rates for most of the earthward fast flows in Class II are low. The auroral features, such as poleward boundary intensifications and pseudobreakups, are found to be associated with these fast flows. Some of the earthward fast flows in Class II can propagate earthward and provide a favorable condition for a substorm onset, leading to an auroral bulge developed on the nightside. We have also tested another criterion of V_xB_z > 2 mV/m for an identification of fast flows, where V_x is the X component of the plasma flow and B₂ is the Z component of the geomagnetic field. It is found that V_{_x} is a better parameter than V_xB_z to differentiate the two classes of earthward fast flows in the plasma sheet.