Kivelson [1997,1998] has actually discussed about the idea of reconnection efficiency. It's so amazing for me. In the meantime, they had ideas of K-H instability happening at G8 magnetopause crossing based on magnetic perturbation phase analysis. Again, it is amazing.
We have some more discussions in [Zhou+, 2020].
[Hall+, 1998] derived the column density in the range from the observed UV emissions assuming a hypothetical distribution of electrons. They made use of the electron number density recorded during the Galileo G1 and G2 flybys, those which occurred closest to the moon and contained primarily electrons from Ganymede's ionosphere rather than the Jovian magnetosphere. Their estimate assumed that all electrons (~100 at the surface) are able to excite auroral emissions on O2, i.e., that all electrons are energetic enough to do so. The dissociative excitation of O2 has an energy threshold around 14–15 eV and the associated cross-section peaks near 100 eV [Kanik+, 2003]. Hence, ionospheric electrons, which dominate inside the magnetosphere [Carnielli+, 2019], would need to undergo significant acceleration, and the way this would occur is not clear, as argued also by [Eviatar+, 2001a]. The MHD-EPIC model [Zhou+, 2019] suggests that energization through magnetic reconnection at the upstream magnetopause can contribute as much as 40% of the energy flux required to produce the observed auroral emissions. The authors then argue that a way to reproduce the observations in a more realistic way would be to reduce the density of exciting electrons, which implies to increase that of .
Using radio occultation measurements, [Kliore, 1998] derived an upper limit of 4000 for the electron number density at the surface. Along the G1 and G2 flybys, the PWS instrument on board Galileo measured the electron number density (primarily of ionospheric origin near closest approach), while the PLS instrument recorded the ion bulk flow velocities and ion energy spectra [Frank+, 1997]. For the G2 flyby, the flow was originally interpreted by [Barth+, 1997] as escaping hydrogen ions. However, this was later reinterpreted as a flow of oxygen ions by [Vasyliūnas and Eviatar, 2000], which is supported by the MHD simulation results of [Jia+, 2009], and a combination of both species by [Paty+, 2008]. [Collinson+, 2018] re-analyzed the PLS data from the G1 and G2 flybys, showing different fits for different assumed ion masses. Remarkably, for the G2 flyby none of the fits can match the electron density profile – which, assuming quasi-neutrality, should equal to the total ion density – derived from PWS and reported in [Eviatar+, 2001b]. In summary, the ionospheric composition is a subject of debate.
[Kivelson+, 2004][Kivelson+2004]