Characterization of Joule heating in structured electric field environments

Abstract

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 x 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. IntroductionJoule heating in the ionosphere can be induced by transverse electric fields in a collisional plasma. Collisions between ions and neutrals are integral to the Joule heating process since they lead to a cross-magnetic field (Pedersen) current density (Jp) and therefore a Jp ß E dissipation of electric field energy. The mechanism by which ion heating occurs (i.e., wave and/or Joule heating) has important geophysical consequences since heavy ion (oxygen) distribution functions measured at higher altitudes often display heating which is argued to have 1807

Ion Joule heating as a function of electric field scale size

“…The standard deviations of the data sets are plotted for each point. Consistent with earlier work [ Walker et al , 2001], the ion temperature tends to reach a maximum at the peak of the Joule heating rate curve which is strongly dependent on the Pedersen conductivity and therefore ν in /Ω i . In the present case, the results shown are for a plasma density of 5 × 10 7 cm −3 and a magnetic field of 40 gauss.…”

“…In this regard, for example, combined incoherent scatter radar in addition to satellite in situ measurements have unequivocally established that irregular field structures encountered in regions of small convective velocity are associated with ion heating. Earlier laboratory investigations [ Amatucci et al , 1999; Walker et al , 2001] have shown that there are two dominant processes responsible for ion temperature enhancement in the presence of transverse electric fields: (1) Joule, or collisional, heating in regions of high convective velocity and sufficient neutral density and, (2) wave heating via plasma instabilities in the presence of velocity shear. These works demonstrate conclusively the transition between these two forms of heating as a function of plasma neutral density.…”

“…Ion thermal speed is v ti ≈5 × 10 4 cm s −1 and neutral argon density varies from N n ≈ 10 11 − 10 14 cm −3 . Langmuir probes are used as diagnostics of electron temperature and density, emissive probes are used for determination of plasma potential from which electric fields are derived, and the ion temperature measurements are taken with the ion energy analyzer described in Walker et al [2001]. Table 1 provides a comparison of important plasma parameters in the ionosphere and in the laboratory.…”

“…More detailed explanations of the experimental facility, including the plasma characterization, diagnostic instrumentation and the arrangement of the electric field‐producing rings, is described in much of our earlier work, [e.g., Amatucci et al , 1996]. For further introduction to the experiments described here and their relevance to space physics issues we refer the interested reader to Walker et al [2001]. Our intention with the current work is to examine the effects of structure of the electric field on Joule heating by varying both the spatial and directional characteristics of the electric fields and measuring the resulting heating.…”

The Harris magnetic field: A laboratory realization of the topology based on energy resonance

“…The initial production of the field and experimentation however is done in vacuum at pressures near 5 × 10 −5 Torr. Further descriptions of the chamber and diagnostic measurement capability in general may be found elsewhere [ Walker et al , 1997, 2001]. To provide the x‐component of the modified Harris field of , the grid is pulsed and the field component produced by the pulsed grids adds to the constant z‐component produced by the external Helmholtz coils.…”