Star of Lima: Overview and Optical Diagnostics of a Barium Alfvén Critical Velocity Experiment

Four high‐velocity shaped charge Ba+ injections were conducted from two Black Brant‐10 rockets at collision‐free altitudes (770‐975 km) over northern Alaska (L = 7.4–10.6) in April 1984 under active auroral and magnetic disturbance (Kp 4+ and 5) conditions. The motions of the Ba+ “pencil” beams from these injections were accurately triangulated to altitudes ranging from 9000 to 14000 km from multistation image observations. Scanning photometer observations from five sites provided checks on the triangulations as well as auxiliary data, including observations that illustrate the lack of any anomalous ionization that might be expected from critical velocity effects. Well‐defined initial conditions and improved software for predicting the unperturbed, E∥ = 0, trajectories in the presence of convection, E⊥, fields permitted an accurate detection of changes in the motion which could be attributed to E∥ fields. Large (>1 keV) potential changes that might be anticipated from double‐layer or V‐, U‐, and S‐shaped potential structures were not encountered even though the Ba+ rays were clearly located on auroral arc flux tubes on at least several occasions and were at various times in close proximity to auroral flux tubes for many minutes. Abnormally intense E⊥ fields that might also indicate that the above potential structures were also not observed. Transient accelerations and/or decelerations involving magnetic field‐aligned energy changes ≤375 eV were, however, encountered by each of the seven principal Ba+ rays tracked to high altitudes. Acceleration events were only slightly more frequent than deceleration events. Interpretation, taking into account limits on the duration of the events and simultaneous auroral conditions, favors explanation in terms of propagating waves, soliton trains, or other pulse forms provided that the propagation is primarily field‐aligned. The similarity of energy and intermittent occurrence characteristics suggests that these E∥ perturbations are likely to be associated with suprathermal electron bursts, both upward and downward. One release, which uniquely took place in a region of intense (200 V/km) rotational shear in the E⊥ field, was followed by the appearance of a small population of suprafast Ba+ ions traveling well ahead of the main high‐velocity Ba+ rays. A second set of faint streaks, representing suprafast Ba+ ions from the low‐velocity debris cloud, simultaneously appeared above the debris rays. These suprafast ions are attributed to transverse energizations of small fractions of both the initial Ba+ jet and Ba+ debris immediately following the release. Transverse energizations for the highest velocity ions extended to 5879 eV with maximum flux near 952 eV. Tip motions for the fastest Ba+ debris ions were not obtained, but triangulation of one streak showed a maximum energization of 131 eV. Wave‐particle stochastic heating models may be relevant to explaining both the high energies and the dependence of the energy on initial velocities.

Section: High-velocity Ray Characteristicsmentioning

confidence: 99%

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

Heppner¹,

Ledley²,

Miller³

et al. 1989

“…In light of the negative or inconclusive results reported recently from space-based CIV experiments [Haerendel, 1982;Torbert, 1988;Wescott et al, 1986;Hallinan, 1988] in comparison with the success of the previously mentioned laboratory experiments, it is clear that a variety of factors besides the threshold velocity V, affect whether or not a CIV discharge will occur. Since there would be important consequences if CIV were to occur in the interaction of spacecraft with the near space environment , it is important to understand the underlying mechanisms at work in CIV for predictive purposes.…”

Section: Introductionmentioning

confidence: 98%

Interplay between collective and collisional processes in critical velocity ionization

McNeil¹,

Lai²,

Murad³

1990

Numerical simulations of critical ionization velocity (CIV) discharges have been performed, taking into account several collisional processes. The simulations are one‐dimensional with density variation in the direction of plasma‐neutral relative velocity. The heating of electrons by wave‐particle interactions results in an extended electron distribution with “hot tail” formation, which enables excitation and ionization of neutrals to occur. The tail formation is observed to persist with a maximum energy less than twice the beam energy. The collisional processes included in this simulation are ground state ionization, excitation and ionization of metastable states, ion‐neutral charge exchange, and electron‐neutral elastic collisions. It is found that near the critical velocity, metastable states participate strongly in ionization, while at high velocities the metastable effect becomes relatively unimportant. Electron‐neutral elastic collisions, within the approximations used in this model, appear to be unimportant. Charge exchange is found to be crucial in sustaining the discharge when the beam velocity is low but is unimportant when the beam velocity is high. Detailed results are shown for the case of neon, while threshold behavior is investigated for helium, xenon, and barium as well. The density dependence of ionization in neon and barium is investigated. Although we examine the effect of line excitation on thresholds for CIV in all species, xenon and barium are especially of interest because both species have been used or proposed to be used in space CIV experiments. We report that line excitation strongly drains energy for barium beams in space. As a result, despite a low ionization potential, the threshold velocity for critical ionization of barium is raised substantially. This suggests a reason why barium beam ejections, the most common method for investigating CIV in space, have so far achieved inconclusive results. We conclude that low ionization potential may not be the best criterion for choosing the optimal beams in space experiments on CIV.

“…Upper ionospheric barium release using sounding rockets to test Alfv6n's critical ionization velocity hypothesis (1954) has been conducted actively more than a half dozen times over the last decade [e.g., Haerendel, 1982;Wescott et al, 1986;Wescott et al, 1990; Stenbaek-Nielsen et al, 1990a, b; Totbert et al, 1992]. According to Alfv6n's critical ionization velocity (CIV) hypothesis, a rapid gas ionization will occur when a neutral gas cloud moves across a magnetized plasma with a relative velocity perpendicular to the magnetic field exceeding a critical value, vc, defined by V/2eqJion/M, where (•ion and M are the ionization potential and mass of neutral atom, respectively.…”

Section: Introductionmentioning

confidence: 99%

Momentum coupling in the “CRIT II” critical ionization velocity experiment

Liou¹,

Torbert²,

Haerendel³

1996

A unified theory has been developed to explain the formation of a quasi‐dc electric pulse (ω ≤ Ωi) induced by an ionizing neutral barium beam across an ionospheric plasma. We obtained a generalized form for the dc electric field in the plane perpendicular to the magnetic field within a simplified slab barium cloud moving perpendicular to the geomagnetic fields. A current system associated with the quasi‐dc electric field was also proposed to provide a way to transfer the momentum between the streaming barium cloud and the ambient plasma. The characteristic time derived by using this model for momentum coupling between the streaming barium cloud and the ambient plasma is found to be in agreement with the previous result. The quasi‐dc electric field predicted by this model is reasonably consistent with the “CRIT II” critical ionization velocity measurements. On the basis of the constrains of the conservation of energy and momentum, we found that Alfvén's critical ionization velocity (CIV) effect is a self‐limiting ionization process in a finite extent neutral cloud. It may be the reason why the CIV effect took place in CRIT II but lasted only for a very short period, and it may have resulted in low ionization yields in most of space CIV experiments.