Example proton VDF (5 April 2019 20:21:36.7407) in the SPAN-Ai instrument frame. The dotted contours are the bi-Maxwellian fit to the proton core using Eq. (3) with u indicated by the black cross. The blue square is the bulk velocity moment of the SPAN-Ai distribution and the red diamond is the average proton core bulk velocity measured by SPC during the SPAN-Ai integration time. The black arrow gives the average direction of B during this time.