SR90, Strontium Shaped‐Charge Critical Ionization Velocity Experiment

Abstract

In May 1986 we carried out an experiment to test Alfvén's critical ionization velocity (CIV) effect in free space, using the first high‐explosive shaped charge with a conical liner of strontium metal. The release, made at 540 km altitude at dawn twilight, was aimed at 48° to B. The background electron density was 1.5×104 cm−3. A faint field‐aligned Sr+ ion streak with tip velocity of 2.6 km s−1 was observed from two optical sites. Using two calibration methods, we calculate that between 4.5×1020 and 2×1021 ions were visible. We have calculated an ionization time constant of 1920 s for Sr from the solar UV spectrum and ionization cross section, which combined with a computer simulation of the injection predicts 1.7×1021 solar UV ions in the low‐velocity part of the ion streak. Thus all the observed ions are from solar UV ionization of the slow (less than critical) velocity portion of the neutral jet. The observed neutral Sr velocity distribution and computer simulations indicate that 2×1021 solar UV ions would have been created from the fast (greater than critical) part of the jet. They would have been more diffuse and were not observed. Using this fact, we estimate that any CIV ions created were less than 1021. We conclude that future Sr CIV free space experiments should be conducted below the UV shadow height and in much larger background plasma density.