  Particle-in-Cell simulations of collisionless magnetic reconnection with a
guide field reveal for the first time the three dimensional features of the low
density regions along the magnetic reconnection separatrices, the so-called
"cavities". It is found that structures with further lower density develop
within the cavities. Because their appearance is similar to the rib shape,
these formations are here called "low density ribs". Their location remains
approximately fixed in time and their density progressively decreases, as
electron currents along the cavities evacuate them. They develop along the
magnetic field lines and are supported by a strong perpendicular electric field
that oscillates in space. In addition, bipolar parallel electric field
structures form as isolated spheres between the cavities and the outflow
plasma, along the direction of the low density ribs and of magnetic field
lines.
