vpa_advection

calculate the advection speed in the vpa-direction at each grid point

update the advection speed in the parallel velocity coordinate for the case where density, flow and pressure are evolved independently from the pdf; in this case, the parallel velocity coordinate is the normalized peculiar velocity wpa = (vpa - upar)/vth

update the advection speed in the parallel velocity coordinate for the case where density and flow are evolved independently from the pdf; in this case, the parallel velocity coordinate is the peculiar velocity wpa = vpa-upar

update the advection speed in the parallel velocity coordinate for the case where no moments are evolved independently from the pdf. vpa is unchanged.

update the advection speed in the parallel velocity coordinate for the case where density is evolved independently from the pdf; in this case, the parallel velocity coordinate is unchanged.