I have output from an SPH code including magnetic fields for each particle.  

I see there are no specific fields for MHD data related to particles. For example, I have 

data = {
       'particle_position_x': pos_x,
       'particle_position_y': pos_y,
       'particle_position_z': pos_z,
       'particle_mass': pmass,
       'particle_velocity_x': v_x,
       'particle_velocity_y': v_y,
       'particle_velocity_z': v_z,
       'magnetic_field_x':b_x,
       'magnetic_field_y':b_y,
       'magnetic_field_z':b_z,
       'magnetic_field_divergence': eb,
       'smoothing_length': h,
       'density': rho,
       'temperature': Tgas,
       'magnetic_field_magnitude': bmag    
    }

and then want to plot 

mag = yt.SlicePlot(ds, 'z', ('gas', 'magnetic_field_divergence'),width=(6.0,'pc'))

But  'magnetic_field_x' is not known for particle data, so yt gives an error message.   Is there a way to circumvent the problem ?

Siegfried