Unexpected strong parallel electric field at the lower E region observed during a barium release experiment

The Barium Release Optical Rocket (BROR) mission conducted at Esrange, Sweden, on 23rd March 2023, performed barium releases into the earth’s atmosphere at eight different altitudes between 130 and 245 km to investigate small-scale electromagnetic phenomena in the auroral ionosphere. In the initial three barium releases of the BROR experiment, which were performed at 132 km, 160 km, and 193 km, the motions of both neutral and ionized barium clouds were so clearly and distinctively observed by the ground-based optical camera network that we can reconstruct a three-dimensional tomography-like reconstruction. 

In the horizontal plane, all neutral clouds resulting from the initial three releases had a strong westward component in their motion with almost constant velocity. In contrast, the ionized clouds behaved quite differently from each other, which may represent the effect of altitudinal variation of collision frequency between barium ion and neutral particles and the electric field, which may be associated with auroral activity. On the other hand, from the vertical motion of ionized clouds, we found that the first (the lowest) and second (the middle) released ion clouds show significant deviation from the theoretical estimated value, while the third released ion cloud shows a good agreement with the theoretical value. This observed fact implies that there may be a parallel electric field up to a few mV/m at the altitude below about 160 km, and the electric fields at the altitude of the first release and second release are in the opposite sense.