On the over-collection of electrons by high voltage probes in space

It has been two decades since Parker and Murphy proposed a theory to place a rigorous upper bound on the current to an electron collecting probe in space, due to the influence of the geomagnetic field. Any greater current is then thought to involve some form of anomalous cross field transport to explain over-collection. Over-collection is commonly observed and is usually explained by invoking turbulence as a source of collisions. Although the turbulence hypothesis has some theoretical basis, no compelling kinetic theory or model has been put forth. Another possible explanation for over-collection is the ionization of the neutral gas within the charge sheath. Recently, in support of the SPEAR I rocket experiment, the three dimensional code POLAR has been used to model electron collection in a low earth orbit plasma. SPEAR I was a sounding rocket experiment that exposed to the space plasma, a pair of 10cm spherical probes biased to 44kV positive. The POLAR runs included single sphere, two sphere, and two sphere plus rocket modes. The single sphere model did not predict collection in excess of the Parker-Murphy limit. Models of two sphere collection, and of the entire experiment do however show that increased collection is possible as a result of asymetries introduced by geometry and a negative rocket body potential. These models were in close agreement with the experiment. In addition to the symmetry factor, sheath ionization is reviewed. A discussion of the role and kinetic nature of collisionless turbulence is presented