Partition of released energy between ions and electrons
In plain words
When reconnection releases magnetic energy, some goes to heating ions, some to heating electrons and some to fast flows. Spacecraft see ions heated much more than electrons, and there is no first-principles theory of the split.
Precise statement
In collisionless reconnection of electron-ion plasma with inflow $\beta\ 0.01\ \text{to}\ 1$, determine the fractions of released magnetic energy that go to ion heating, electron heating and bulk outflow, and their dependence on guide field, beta and $T_i/T_e$. Spacecraft data show outflow temperature increments proportional to $m_i v_A^2$ with ion heating about an order of magnitude larger than electron heating. An answer is derived coefficients matching kinetic simulations and spacecraft statistics.
What would settle it
A theory of the heating coefficients confirmed by 3D kinetic simulations and MMS event statistics across guide-field values.
Status in the literature
Empirical scalings with $m_i v_{\mathrm{A}}^{2}$ were established from spacecraft data around 2013-2014; first-principles coefficients and their guide-field dependence remain open.