Characterization of Joule heating in structured electric field environments

We have recently performed a detailed characterization of ion Joule heating perpendicular to an axial magnetic field in the laboratory in a simulated ionospheric plasma environment which contains localized electric field stru cturing. Since Joule heating is often regarded as an important mechanism co ntributing to energization of outflowing heavy ions observed by higher-alti tude auroral satellites, this work has particular relevance to space physic s issues, and, to our knowledge, has not been investigated systematically i n a controlled environment. Since transverse (to B) ionospheric electric fi elds are often spatially and temporally structured, with scale lengths ofte n as small as an ion gyroradius, the ability to systematically vary the spa tial extent and magnitude of an electric field region and to observe the ef fect on ion energy is important. The experiment makes use of a concentric s et of separately biasable ring anodes which generate a radial electric fiel d with controllable scale length perpendicular to an ambient axial magnetic field. Joule heating results from ion-neutral collisions occurring within this transverse, de electric field. Until there is sufficient neutral press ure to raise the ion-neutral collision frequency (nu (in)) to an observable Joule heating threshold, ion cyclotron wave heating, which is induced by s hear in E x B rotation, can be the primary channel for ion energization. We have discussed in earlier papers the conditions under which this occurs, a nd we have treated the transition between the two forms of ion heating. We concentrate primarily in this work on constructing the fields themselves an d on the relationship between the subsequent collisional heating and the Pe dersen conductivity as an initial indication of the validity of the measure ment results. We are able to demonstrate that measurable heating is produce d by even relatively small scale structures of the order of the ion gyrorad ius. In addition, we show that measured heating is consistent with predicti ons of joule heating as a function of ion-neutral collisions. Finally, this work can have major implications for ionospheric studies where large-scale electric fields are often assumed in the calculation of Joule heating.