  We report the properties of a novel type of sub-proton scale magnetic hole
found in two dimensional PIC simulations of decaying turbulence with a guide
field. The simulations were performed with a realistic value for ion to
electron mass ratio. These structures, electron vortex magnetic holes (EVMHs),
have circular cross-section. The magnetic field depression is associated with a
diamagnetic azimuthal current provided by a population of trapped electrons in
petal-like orbits. The trapped electron population provides a mean azimuthal
velocity and since trapping preferentially selects high pitch angles, a
perpendicular temperature anisotropy. The structures arise out of initial
perturbations in the course of the turbulent evolution of the plasma, and are
stable over at least 100 electron gyroperiods. We have verified the model for
the EVMH by carrying out test particle and PIC simulations of isolated
structures in a uniform plasma. It is found that (quasi-)stable structures can
be formed provided that there is some initial perpendicular temperature
anisotropy at the structure location. The properties of these structures (scale
size, trapped population etc.) are able to explain the observed properties of
magnetic holes in the terrestrial plasma sheet. EVMHs may also contribute to
turbulence properties, such as intermittency, at short scale lengths in other
astrophysical plasmas.
