Cluster and DMSP observations of SAID electric fields

We report on magnetically conjugate Cluster and the Defense Meteorological Satellite Program (DMSP) satellite observations of subauroral ion drifts (SAID) during moderate geomagnetic activity levels on 8 April 2004. To our knowledge, the field‐aligned separation of DMSP and Cluster (≈28,000 km) is the largest separation ever analyzed with respect to the SAID phenomenon. Nonetheless, we show coherent, subauroral magnetosphere‐ionosphere (MI) coupling along an entire field line in the post‐dusk sector. The four Cluster satellites crossed SAID electric field channels with meridional magnitude E M of 25 mV/m in situ and latitudinal extent ΔΛ ≈ 0.5° in the southern and northern hemispheres near 07:00 and 07:30 UT, respectively. Cluster was near perigee ( R ≈ 4 R E ) and within 5° (15°) of the magnetic equator for the southern (northern) crossing. The SAID were located near the plasmapause—within the ring current‐plasmasphere overlap region. Downward field‐aligned current signatures were observed across both SAID crossings. The most magnetically and temporally conjugate SAID field from DMSP F16A at 07:12 UT was practically identical in latitudinal size to that mapped from Cluster. Since the DMSP ion drift meter saturated at 3000 m/s (or ∼114 mV/m) and the electrostatically mapped value for E M from Cluster exceeded 300 mV/m, a magnitude comparison of E M was not possible. Although the conjugate measurements show similar large‐scale SAID features, the differences in substructure highlight the physical and chemical diversity of the conjugate regions.