The Cameo Barium Releases: E_∥ fields over the polar cap

Abstract

Four successive thermite barium releases at an altitude of 965 km over polar cap invariant latitudes 84° to 76° near magnetic midnight were conducted from the orbiting second stage of the vehicle that launched Nimbus 7. This was the first opportunity to observe the behavior of a conventional barium release when conducted at orbital velocity in the near earth magnetic field and to demonstrate that the μ▽B force at orbital velocity would accelerate the Ba+ ions to much greater altitudes for measuring E∥ fields by observing perturbations in the motion of the visible ions. The principal unexpected characteristic in the release dynamics was the high, 1.4 to 2.6 km/s, initial Ba+ expansion velocity relative to an expected velocity of 0.9 km/s. Characteristics of the neutral gas expansion are also discussed. Under the combined influence of μ▽B and E∥ forces the finely striated Ba+ streamers from the 8 kg releases elongated along the field to dimensions >10,000 km and were photographed from California and Hawaii as well as higher latitude sites. Although observed up to one hour after release and to elevation angles >55° from Southern California, triangulations of upper tips were limited to the initial 25 min and altitudes <23,000 km. The average E⊥ convection of the Ba+ flux tubes was very normal for Kp=3 conditions and closely followed model patterns for convection between the pole and the midnight auroral zone. However, over shorter space‐time dimensions the E⊥ field is observed to be irregular; this also is a normal polar cap condition. From the analysis of field aligned motions in terms of the potential changes required in the equation of motion to match the altitude versus time observations: (1) E∥ fields do not appear below 1400 km, (2) between 1500 and 4200 km the changes in energy are typically 20–40 eV with a 71 eV maximum, (3) between 4200 and 15,000 km maximum values are in the range 80–720 eV with the highest values coming from the highest latitude releases, and (4) in the case of one release a 6 keV acceleration appears above 15,800 km. These changes appear as both accelerating (upward E∥) and decelerating (downward E∥) fields with an apparent periodicity of 3 to 4 min and are in phase at all four locations for 3, and possibly 4 or 5, cycles within the limitations imposed by a 1 min resolution and data gaps. This apparent wave characteristic could, under selective error assumptions, result from several impulsive decelerations superimposed on a more general, but variable, accelerating field. This possibility is considered to be less likely than a hydromagnetic wave association. The unique capabilities for measuring very weak fields and for separating temporal and spatial variations may explain the lack of precedent from previous E∥ measurements.

Simultaneous polar cap and magnetotail observations of intense polar rain

“…(See Meng [1978] for a review.) Parallel electric fields that might accelerate such electrons are also suggested by occasional observations of downcoming as well as upgoing ionoshperic photoelectrons [Winningham and Heikkila, 1974;Winningham and Gurgiolo, 1982] and by accelerated barium ions [Heppner et at., 1981].…”

“Electron conic” signatures observed in the nightside auroral zone and over the polar cap

“…These waves are then thought to be in Landau resonance with the ions I-Dusenbery and Lyons, 1981' Ashour-Abdalla and Okuda, 1984]. Polar cap acceleration regions have been reported before [Heppner et al, 1981;Winningham and Heikkila, 1974], but such acceleration has not been known to produce electron conic distributions. Likewise, no electron conic distributions have previously been reported in the auroral zone, even though our results suggest these distributions may be present Copyright 1985 by the American Geophysical Union.…”

Search for auroral belt E∥ fields with high‐velocity barium ion injections

“…The observed initial velocities from fitting the upper and lower tip triangulations, as discussed in section 6.2 and listed in [1977] reported initial peak ion velocities parallel to the magnetic field in excess of neutral velocities for shaped charge injections that were nearly perpendicular to the magnetic field. The Cameo thermite releases [Heppner et al, 1981] with orbital velocities nearly perpendicular to the magnetic field also produced initial ion velocities that were clearly high relative to the neutral velocities. Thus, from the available information it appears that the ions acquire additional energy only when injected at large angles to the magnetic field.…”