FAST FLOW EVENTS


 
Events Identification
149 Events in Total

 

 

 

 

 

Identifying Fast Flows

 

    Geotail 12-s average low energy particle (LEP) data and magnetic field (MGF) data is used to identify plasma sheet fast flows

 

    We identify periods of fast flows by using a criterion of |Vxperp| > 300 km/s, where |Vxperp| is the X component of the plasma flow perpendicular to the ambient magnetic field. If two fast flows occur within 15 min, they are considered one event. We have identified 921 events from the 2-year period of the data. The starting time of the event is designated as the time of the first data point with | Vxperp| > 300 km/s.

 

    We also impose a constraint of ß > 0.5 for covering at least 85% duration of the event on the selection of the fast flow events, where ß is the ratio of the ion pressure to the magnetic pressure. This constraint ensures that Geotail is mainly in the central plasma sheet, not in the plasma sheet boundary layer. Moreover, we select events with Geotail locations around the noon-midnight meridian (-20 < Y < 20 RE) to avoid some magnetosheath events. We only use fast flows events that occurred between 2100 and 0300 MLT.

 

Calculating the Integrated Auroral Power Rate of Change

 

    We examine the availability of Polar UVI LBH-long images taken every 3 min for a period of 30 min centered at the starting time of the fast flow events, then we have 11 corresponding Polar UVI images for each event. In real situations, some images are missing or of bad quality. We discard events with < 9 corresponding images. Also we discard events with a change of field of view of the Polar UVI camera which occurred during the events.

 

    We integrate auroral power over a region of 60-80 degree MLAT and 2000-0400 MLT for each of the UVI images for the 30 min period of each fast flow event. We do not integrate the auroral power for events with < 30% field of view over the integration region. The estimation of the auroral power can be used to evaluate the large-scale impact of the localized fast flow in terms of auroral energy transport.

 

    To quantitatively investigate the auroral energy change during which fast flows occur, we fit a straight line to all the auroral power calculated for each event by using the least-squares method. The slope of the straight line is interpreted as the auroral power rate of change.

 

A total of 149 events are left through a series of event selection criteria.

 

    For the scientific results derived from these events, readers are referred to Shue et al. [2003] (J. Geophys. Res., 108(A6), 1231, doi:10.10292002JA009794) and Shue et al. [2008] (J. Geophys. Res., 113, A02205, doi:10.1029/2007JA012456).

 

Figure 1.  Observations of Polar Ultraviolet Imager (UVI) auroral images and Geotail plasma and magnetic fields for the 27 May 1997, 2249 UT event. (a) A sequence of Polar UVI Lyman-Birge-Hopfield long band images have been shifted to the starting time of fast flows. The auroral images with negative (positive) timings marked to the left show these images were observed before (after) the first fast flow occurred. A diamond on each image indicates the foot point of Geotail, mapped by using the Tsyganenko [1989] model. (b) Auroral power for each image is integrated and plotted against timing. A vertical bar indicates the standard error of the mean of the auroral power. Data points for the auroral power are fitted to a red straight line by a linear least-squares method. The slope of the line is marked on the top of the plot. (c) Geotail data have been shifted to the starting time of the fast flows marked by the vertical dashed line. The horizontal blue line on | Vxperp| and ß indicates a threshold value of 300 km/s and 0.5, respectively.