aux_fields Subroutine

  subroutine aux_fields(pp,spp,gradB,curlb,Bmag)
    TYPE(SPECIES), INTENT(IN)    :: spp
    !! An instance of the derived type SPECIES containing all the parameters
    !! and simulation variables of the different species in the simulation.
    REAL(rp),DIMENSION(3),INTENT(INOUT) :: gradB
    REAL(rp),DIMENSION(3),INTENT(INOUT) :: curlb
    REAL(rp),INTENT(IN) :: Bmag
    REAL(rp) :: dRB
    REAL(rp) :: dPHIB
    REAL(rp) :: dZB
    INTEGER  :: pp

    dRB=(spp%vars%B(pp,1)*spp%vars%BR(pp,1)+ &
         spp%vars%B(pp,2)*spp%vars%BPHI(pp,1)+ &
         spp%vars%B(pp,3)*spp%vars%BZ(pp,1))/Bmag
    dPHIB=(spp%vars%B(pp,1)*spp%vars%BR(pp,2)+ &
         spp%vars%B(pp,2)*spp%vars%BPHI(pp,2)+ &
         spp%vars%B(pp,3)*spp%vars%BZ(pp,2))/Bmag
    dZB=(spp%vars%B(pp,1)*spp%vars%BR(pp,3)+ &
         spp%vars%B(pp,2)*spp%vars%BPHI(pp,3)+ &
         spp%vars%B(pp,3)*spp%vars%BZ(pp,3))/Bmag

    gradB(1)=dRB
    gradB(2)=dPHIB/spp%vars%Y(pp,1)
    gradB(3)=dZB

    curlb(1)=((Bmag*spp%vars%BZ(pp,2)-spp%vars%B(pp,3)*dPHIB)/spp%vars%Y(pp,1)- &
         (Bmag*spp%vars%BPHI(pp,3)-spp%vars%B(pp,2)*dZB))/Bmag**2
    curlb(2)=((Bmag*spp%vars%BR(pp,3)-spp%vars%B(pp,1)*dZB)- &
         (Bmag*spp%vars%BZ(pp,1)-spp%vars%B(pp,3)*dRB))/Bmag**2
    curlb(3)=((Bmag*spp%vars%BPHI(pp,1)-spp%vars%B(pp,2)*dRB) - &
         (Bmag*spp%vars%BPHI(pp,1)-spp%vars%B(pp,1)*dPHIB)/ &
         spp%vars%Y(pp,1))/Bmag**2+ &
         spp%vars%B(pp,2)/(Bmag*spp%vars%Y(pp,1))  

  end subroutine aux_fields