Thermospheric shock waves and lensing of light in the Barium Release, Optical and Radio rocket experiment

Abstract. The Barium Release, Optical and Radio (BROR) rocket experiment, launched from Esrange near Kiruna, Sweden, involved eight releases of barium at different altitudes in the thermosphere to study electric fields near small scale auroral structures. The barium was ejected into the thermosphere by the explosion-like combustion of ignited copper oxide thermite. Shock waves could be observed optically from the ground to follow the barium ejections when the rocket speed was supersonic, but not when it was subsonic. The shock waves are attributed to copper droplets that resulted from the thermite combustion and other particulate matter in the ejecta that traveled at supersonic speed. The observed deceleration of the shock waves can be explained by frictional drag together with gravity on copper droplets having a mean radius of about 1 mm. The actual observation of the shock waves from the ground is attributed to scattering of sunlight on the particulate matter in the ejecta. Also, following the ejections at the highest speeds, optical lensing of sunlight reflected from the rocket itself was observed, in that the rocket appeared brighter just after the barium release than before the release.