Anisotropic core ion temperatures associated with strong zonal flows and upflows

The Swarm satellites observe strongly anisotropic ion temperatures at 500 km altitude. The ion temperature anisotropy ratios going up to 5 for the strongest electric fields presented in this paper. The largest observed anisotropy ratios exceed the values predicted by theories of collisional heating in strong flows by a factor of 2, indicating that collisional cross sections should perhaps be revised for O+ ion colliding with O. Temperature anisotropy is also found not to be a simple function of electric field strength. This could be understood in terms a time delay needed to advect hot anisotropic ion velocity distributions from strongly collisional regions below 400 km to weakly collisional regions at 500 km and above. The mirror force associated with these events is insufficient to account for the observed upward flows (> 500 m/s). For gyroresonant heating to be consistent with our observations, an additional mechanism for field-aligned acceleration is required.