Quick Spontaneous Triggering of Reconnection in Ion-Scale Current Sheets

M. Fujimoto (TITech) and I. Shinohara

Three-dimensional full particle simulations have been performed 
to investigate if spontaneous triggering of reconnection in 
current sheets of ion-scale thickness is possible. 
In contarst to two-dimensional results where quick spontaneous triggering is
impossible unless the current sheet thickness is of electron scale, 
coupling to the Lower Hybrid Drift Instability (LHDI) that develops 
at the current-sheet edges enables quick triggering 
in thicker current sheets in three-dimensional situations. 
For anti-parallel cases, we show for the ion-to-electron mass ratio of 400 
that reconnection proceeds to a significant level 
by 10 ion gyro-time for the initial current sheet thickness 
of 1.3 times the ion inertial length. 
Presence of the guide field changes the properties of LHDI
and thus the quick triggering process. For the guide field intensity of 
0.75 times the reconnecting component, we have confirmed that 
quick triggering is available in the current sheet 
whose thickness is 0.7 times the ion-inertial length. 
These results imply that, for the guide field strength expected 
in the Earth's magnetotail environment, quick triggering of reconnection 
sets in as soon as the current sheet thickness becomes comparable 
to the ion inertial length.