docs: def add_mag_drifts(inst):
      """Adds ion drifts in magnetic coordinates using ion drifts in S/C coordinates
      along with pre-calculated unit vectors for magnetic coordinates.
      
      Note
      ----
          Requires ion drifts under labels 'iv_*' where * = (x,y,z) along with
          unit vectors labels 'unit_zonal_*', 'unit_fa_*', and 'unit_mer_*',
          where the unit vectors are expressed in S/C coordinates. These
          vectors are calculated by add_mag_drift_unit_vectors.
      
      Parameters
      ----------
      inst : pysat.Instrument
          Instrument object will be modified to include new ion drift magnitudes
          
      Returns
      -------
      None
          Instrument object modified in place
      
      """
      
      inst['iv_zon'] = {'data':inst['unit_zon_x'] * inst['iv_x'] + inst['unit_zon_y']*inst['iv_y'] + inst['unit_zon_z']*inst['iv_z'],
                        'units':'m/s',
                        'long_name':'Zonal ion velocity',
                        'notes':('Ion velocity relative to co-rotation along zonal '
                                 'direction, normal to meridional plane. Positive east. '
                                 'Velocity obtained using ion velocities relative '
                                 'to co-rotation in the instrument frame along '
                                 'with the corresponding unit vectors expressed in '
                                 'the instrument frame. '),
                        'label': 'Zonal Ion Velocity',
                        'axis': 'Zonal Ion Velocity',
                        'desc': 'Zonal ion velocity',
                        'scale': 'Linear',
                        'value_min':-500., 
                        'value_max':500.}
                        
      inst['iv_fa'] = {'data':inst['unit_fa_x'] * inst['iv_x'] + inst['unit_fa_y'] * inst['iv_y'] + inst['unit_fa_z'] * inst['iv_z'],
                        'units':'m/s',
                        'long_name':'Field-Aligned ion velocity',
                        'notes':('Ion velocity relative to co-rotation along magnetic field line. Positive along the field. ',
                                 'Velocity obtained using ion velocities relative '
                                 'to co-rotation in the instrument frame along '
                                 'with the corresponding unit vectors expressed in '
                                 'the instrument frame. '),
                        'label':'Field-Aligned Ion Velocity',
                        'axis':'Field-Aligned Ion Velocity',
                        'desc':'Field-Aligned Ion Velocity',
                        'scale':'Linear',
                        'value_min':-500., 
                        'value_max':500.}
  
      inst['iv_mer'] = {'data':inst['unit_mer_x'] * inst['iv_x'] + inst['unit_mer_y']*inst['iv_y'] + inst['unit_mer_z']*inst['iv_z'],
                        'units':'m/s',
                        'long_name':'Meridional ion velocity',
                        'notes':('Velocity along meridional direction, perpendicular '
                                 'to field and within meridional plane. Positive is up at magnetic equator. ',
                                 'Velocity obtained using ion velocities relative '
                                 'to co-rotation in the instrument frame along '
                                 'with the corresponding unit vectors expressed in '
                                 'the instrument frame. '),
                        'label':'Meridional Ion Velocity',
                        'axis':'Meridional Ion Velocity',
                        'desc':'Meridional Ion Velocity',
                        'scale':'Linear',
                        'value_min':-500., 
                        'value_max':500.}
      
      return