TheL= 6.6 Oosik Barium Plasma Injection Experiment and magnetic storm of March 7, 1972

Abstract

The high-altitude detonation of a high-explosive shaped charge vaporizes a hollow conical liner of barium metal and produces a fast field-aligned jet of plasma with approximately 1 mol of ions having initial velocities distributed from 8 to 20 km/s. Such a barium plasma injection experiment was performed at 540-km altitude over Alaska (L = 6.6) in dusk of March 7, 1972 (universal time), in an attempt to trace out and observe the dynamics of an auroral field line in the magnetosphere. With image orthicon TV systems and image intensified cameras the resulting streak rising into the magnetosphere was observed for 30 min and out to 3-RE altitude. A 1.5-m-aperture photometer detected the barium streak at R (release) + 35 min at nominal look angles where the ions would be near altitudes of 5 and 2 RE from the magnetic equator. The injection occurred during a quiescent phase of a magnetic storm initiated by an ssc 10 hours prior to the experiment. Quiet auroral arcs existed over Alaska at locations more than 200 km south of the injection point. The barium flux tube drifted first eastward and then southward owing to an electric field E, with E × B/B² velocities of several hundred meters per second, while also splitting into at least seven separate flux tubes distributed along 200 km parallel to L. At R + 16 min an intense auroral activation and magnetic substorm (the Oosik substorm) began in the Alaskan sector. Subsequent poleward expansion of the aurora and formation of a classic spiral resulted in the intersection and crossing of the barium flux tubes by an auroral arc. Very rapid, almost turbulent, motions of the barium flux observed during the intersection interval resulted from local transient E fields of 200 mV/m directed inward toward the auroral arc. Observations of the distortion of the barium flux tubes on opposite sides of the aurora as compared with a field model including the superdisturbed external coefficients of Mead and Fairfield (1973) offer evidence for an upward Birkeland current sheet at the poleward edge of the auroral spiral of 8 × 10−2 A/m. The Oosik substorm was spatially limited to the dusk sector; it occurred during the decaying phase of a larger substorm observed in the midnight to dawn sector. The substorm and aurora had several unusual features. Examination of the question as to whether the plasma injection may have triggered the substorm unveiled no compelling evidence that it did; however, the unusual features of the substorm leave open that possibility.