Electrodynamic coupling of Jupiter's thermosphere and stratosphere: A new source of thermospheric heating?

We investigate the vertical penetration of non-uniform electric fields in a planetary ionosphere. We develop a simple theoretical description of the vertical variation of electric fields with altitude in an ionosphere permeated with a vertical magnetic field. This framework is applied to the suggestion that winds in Jupiter's stratosphere could drive currents in the thermosphere. In particular, we propose that wind structures in the upper region of Jupiter's stratosphere (200-350 km above the 1 bar level) couple with the ionospheric plasma in this region to generate electric currents, some of which close by flowing vertically along magnetic field lines and then horizontally in the Pedersen conducting region of the thermosphere. These currents extract kinetic energy from the stratospheric winds and dissipate it, via Joule heating, as thermal energy in the thermosphere, thus providing a possible contribution to the observed high temperatures in this region. While the existence of significant wind structures in the upper stratosphere is speculative, the wind speeds that are required to generate significant heating (∼100 m s−1) are not unreasonable in comparison to the observed tropospheric, lower stratospheric and thermospheric wind speeds. The scale size of the wind structures is critical to the degree of penetration of the induced electric fields. Wind structures with scale sizes less than ∼10 km do not generate electric fields that penetrate significantly into the thermosphere, while those with scale sizes of greater than ∼100 km (which includes very large, planetary-scale wind structures) generate electric fields that penetrate almost unmodified across the whole of the thermosphere. Sharp ionospheric layers or holes can prevent penetration of the electric fields to the thermosphere, doing so more effectively if they are of greater magnitude, of greater vertical width, or located at lower altitude. The timescale for damping of the stratospheric winds by ion drag is found to be strongly altitude-dependent, ranging from ∼1 planetary rotation at 350 km altitude to >100 planetary rotations at 200 km altitude. The timescales also vary strongly with the nature and scale size of the wind structures. If the timescales of the processes driving stratospheric winds are longer than these timescales, then our mechanism will damp the winds almost to zero, and supply negligible energy to the thermosphere. We also discuss the possible relevance of our results to magnetosphere-ionosphere coupling in the auroral regions of Jupiter and other planets.