  In space and astrophysical plasmas, turbulence is responsible for
transferring energy from large scales driven by violent events or
instabilities, to smaller scales where turbulent energy is ultimately converted
into plasma heat by dissipative mechanisms. The nonlinear interaction between
counterpropagating Alfven waves, denoted Alfven wave collisions, drives this
turbulent energy cascade, as recognized by early work with incompressible
magnetohydrodynamic (MHD) equations. Recent work employing analytical
calculations and nonlinear gyrokinetic simulations of Alfven wave collisions in
an idealized periodic initial state have demonstrated the key properties that
strong Alfven wave collisions mediate effectively the transfer of energy to
smaller perpendicular scales and self-consistently generate current sheets. For
the more realistic case of the collision between two initially separated Alfven
wavepackets, we use a nonlinear gyrokinetic simulation to show here that these
key properties persist: strong Alfven wavepacket collisions indeed facilitate
the perpendicular cascade of energy and give rise to current sheets.
Furthermore, the evolution shows that nonlinear interactions occur only while
the wavepackets overlap, followed by a clean separation of the wavepackets with
straight uniform magnetic fields and the cessation of nonlinear evolution in
between collisions, even in the gyrokinetic simulation presented here which
resolves dispersive and kinetic effects beyond the reach of the MHD theory.
