Characterization of Joule heating in structured electric field environments

Abstract

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 structuring. Since Joule heating is often regarded as an important mechanism contributing to energization of outflowing heavy ions observed by higher‐altitude auroral satellites, this work has particular relevance to space physics issues, and, to our knowledge, has not been investigated systematically in a controlled environment. Since transverse (to B) ionospheric electric fields are often spatially and temporally structured, with scale lengths often as small as an ion gyroradius, the ability to systematically vary the spatial extent and magnitude of an electric field region and to observe the effect on ion energy is important. The experiment makes use of a concentric set of separately biasable ring anodes which generate a radial electric field with controllable scale length perpendicular to an ambient axial magnetic field. Joule heating results from ion‐neutral collisions occurring within this transverse, dc electric field. Until there is sufficient neutral pressure to raise the ion‐neutral collision frequency (vin) to an observable Joule heating threshold, ion cyclotron wave heating, which is induced by shear in E × B rotation, can be the primary channel for ion energization. We have discussed in earlier papers the conditions under which this occurs, and we have treated the transition between the two forms of ion heating. We concentrate primarily in this work on constructing the fields themselves and on the relationship between the subsequent collisional heating and the Pedersen conductivity as an initial indication of the validity of the measurement results. We are able to demonstrate that measurable heating is produced by even relatively small scale structures of the order of the ion gyroradius. In addition, we show that measured heating is consistent with predictions 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.