Observations of striation formation in a barium ion cloud

The growth of a barium ion cloud released at 194 km has been documented. Within 930 sec after release, the cloud center had descended (at 27 m/sec) to 168 km with traces (∼1010/cm2 column) visible at heights of 160 and 190 km along the earth's magnetic field. During the same period, the cloud moved across (perpendicular to) field lines at a velocity of 35 m/sec relative to the neutral wind. For about 500 sec ambipolar diffusion perpendicular to the field lines led to a radial expansion with an effective diffusion rate of 6.103 m2/sec (about 10% of the neutral diffusion at that height). After 200 sec, the side facing the neutral cloud (the ‘downwind’ side of the ion cloud) steepened rather than decreased in brightness gradient; at 500 sec it had steepened to a Gaussian radius of 0.8 km, whereas the other side had grown to a 3.7-km Gaussian radius. At about 500 sec, an instability developed on the steep-gradient surface and grew into three vertical sheets, the edges of which moved much less rapidly (10 m/sec) with respect to the neutral wind than the 35-m/sec ion cloud from which they were being pulled. These sheets had dimensions about 10-km long, 30-km high, and 0.6-km wide at 930 sec after release.