Collisionless onset of reconnection in a thinning magnetotail current sheet
In plain words
In Earth's magnetic tail the solar wind slowly stretches a sheet of electric current for about an hour before reconnection suddenly starts and releases a substorm (a burst of auroral activity). A small magnetic field threading the sheet is predicted to stabilize it, and why and when the sheet nevertheless tears is not known.
Precise statement
For a current sheet with lobe field $B_0$, small normal field $B_n$ ($B_n/B_0$ about 0.01 to 0.1) and half-thickness $L$ thinned by external driving toward the ion inertial length $d_i$ or below, determine the condition on $L/d_i$, $B_n/B_0$, magnetic flux accumulation at the sheet end and driving rate under which the sheet becomes unstable to collisionless tearing or a related mode (electron tearing, which magnetized electrons are predicted to stabilize; ion tearing; ballooning-interchange modes), and the resulting growth time. An answer is an onset criterion and onset time that agree with fully kinetic simulations at realistic $m_i/m_e$ and with substorm growth-phase observations.
What would settle it
A criterion confirmed by kinetic simulations of driven sheets and by multi-spacecraft observations (e.g. THEMIS, MMS) of the tail current sheet just before substorm onset.