J. R. Benbrook

Particle and field signatures during pseudobreakup and major expansion onset

The temporal and spatial scales of the onset of two types of substorm events are investigated. These substorms were cases where the expansion onset had precursor localized auroral activation without significant negative bay enhancement, that is, “pseudobreakup”. High-resolution energetic particle and magnetic field data at synchronous orbit are used for the analysis together with auroral and magnetic field data simultaneously taken from ground-based instrumentation. The auroral structure following the pseudobreakup significantly resembled the major expansion aurora, except in its spatial scale. Typical magnetospheric onset signatures such as tail current diversion, dipolarization, and injection were observed associated with some of the pseudobreakups. The major expansion, on the other hand, consisted of a number of rather localized injections and expansions, each of which had timescales of 2-8 min, a comparable timescale to that of pseudobreakups. This study shows that there does not appear to be any phenomenological differences between pseudobreakups and major expansion onsets. The major difference between pseudobreakups and major expansion onsets would be the number of occurrences, as well as the intensity and the scale size of the magnetospheric source.

Observation of stratospheric ozone depletion in rocket exhaust plumes

Although modelling studies have predicted that particulate and reactive gas-phase species in the exhaust plume of large rockets might cause significant local ozone depletion, the actual response of the stratosphere after rocket launches has never been directly determined. Here we report comprehensive measurements that follow the evolution of stratospheric ozone in the wake of two Titan IV rockets launched on 12 May and 20 December 1996. In both cases, ozone concentrations dropped to near-zero values in the plume wake, across regions four to eight kilometres wide, within 30 minutes after launch; intense ozone loss persisted for 30 minutes after which time concentrations recovered to ambient levels. Our data indicate that the number of ozone molecules lost in the plume regions significantly exceed the number of chlorine molecules deposited by the two rockets. This suggests that a catalytic cycle based on Cl2O2, other than Cl2, and unique to solid rocket motor (SRM) plumes might be responsible for our observations. However, the limited spatial and temporal extent of the observed ozone losses implies that neither the catalytic Cl2O2 cycle nor other reactions involving exhaust compounds from large solid-fuelled rockets have a globally significant impact on stratospheric chemistry.

Ground‐based instrumentation for measurements of atmospheric conduction current and electric field at the South Pole

We have constructed instruments to measure the atmospheric conduction current and the atmospheric electric field: two fundamental parameters of the global-electric circuit. The instruments were deployed at the Amundsen-Scott South Pole Station in January 1991 and are designed to operate continuously for up to one year without operator intervention. The atmospheric current is measured by a sensor that uses a split-hemispheric conducting shell of 17.8-cm radius, separated by a thin Teflon insulating disk. The detection electronics are inside the sphere. In principle, the atmospheric current flows into one hemisphere, through the electronics where it is measured, and out the other hemisphere. The electric field is measured by a field mill of the rotating dipole type. The electric field sensing elements are two 30-cm-long antennas, driven to rotate in the vertical plane at 1800 rotations per minute. Two arrays of identical instruments have been deployed, separated by 600 m, in order to distinguish between atmospheric electrical signals of local and global origin. The separation distance of the arrays was determined by the climatology of the Antarctic plateau. Sample data from the first days of operation at the South Pole indicate variations in the global circuit over time scales from minutes, to hours, to days.

Observation of stratospheric ozone depletion associated with Delta II rocket emissions

Ozone, chlorine monoxide, methane, and submicron particulate concentrations were measured in the stratospheric plume wake of a Delta II rocket powered by a combination of solid (NH4ClO4/Al) and liquid (LOX/kerosene) propulsion systems. We apply a simple kinetics model describing the main features of gas-phase chlorine reactions in solid propellant exhaust plumes to derive the abundance of total reactive chlorine in the plume and estimate the associated cumulative ozone loss. Measured ozone loss during two plume encounters (12 and 39 minutes after launch) exceeded the estimate by about a factor of about two. Insofar as only the most significant gas-phase chlorine reactions are included in the calculation, these results suggest that additional plume wake chemical processes or emissions other than reactive chlorine from the Delta II propulsion system affect ozone levels in the plume.

Statistical studies of impulsive events at high latitudes

A statistical study has been made of the high-latitude impulsive events that were observed during the 1985–1986 South Pole Balloon Campaign. The events were selected by searching for unipolar pulses ≥ 10 nT above background in the vertical component of the magnetic field on the ground and/or pedestal or “W” shaped horizontal electric field perturbations ≥ 10 mV/m in amplitude and accompanied by perturbations in the vertical electric field at balloon altitude. A main event list comprising 112 events was compiled from the 468 hours of data available. Three aspects of the events were examined: the solar wind conditions prior to the event, local time of observation, and intrinsic properties of the events. The local time distribution was obtained from the 112 entry main event list and was found to be nearly uniform across the dayside, with no midday gap. The event rate found using our low-amplitude selection criteria was 0.7 event/hr, comparable to expectations based on in situ studies of the magnetopause. A total of 42 events were found for which data were available from Interplanetary Monitoring Platform (IMP) 8. Of these events, 12 occurred when the ZGSM component (BZ) of the interplanetary magnetic field (IMF) was northward and 30 occurred when BZ was southward or fluctuating. Only three of the BZ northward cases and only five of the BZ southward cases were preceded by pressure pulses greater than 0.4 nPa in amplitude. Ten of the events were studied in detail by means of a model-fitting method discussed elsewhere. This method infers values of several parameters, including the total current flowing in a coaxial or monopole system and a two-dimensional dipole system. The intrinsic properties of the events showed that only ∼ 10% of the total current contributed to momentum transfer to the high-latitude ionosphere, that the direction of the motion depended more on local time of observation than IMF By, and that events were usually several hundred kilometers in size. The observed Bz control found in the 42 event list and the prevalence of coaxial current dominated events are inconsistent with the predictions of the pressure pulse model.

In-situ measurement of Cl2 and O3 in a stratospheric solid rocket motor exhaust plume

The concentration of Cl2 in the stratospheric exhaust plume of a Titan IV launch vehicle was measured with a neutral mass spectrometer carried on a WB-57F aircraft at 18.9 km altitude. Twenty nine minutes after a twilight Titan IV launch, the mean Cl2 concentration across an 8 km wide plume was 126 ± 44 ppbv, consistent with model predictions that a large fraction of the HCl in solid rocket motor exhaust is converted into Cl2 by afterburning reactions in the hot plume. Co-incident measurements with ultraviolet absorption photometers also carried on the aircraft show that ozone concentration in the plume was not different from ambient levels. This is consistent with model predictions that nighttime SRM launches will not cause transient ozone loss in the lower stratosphere. The measured Cl2 concentration equals 15% of the ambient ozone concentration suggesting that transient ozone reduction in SRM plume wakes can be expected after daytime launches when solar ultraviolet radiation will photolyze the exhaust plume Cl2.

Balloon observations of stratospheric electricity above the South Pole: vertical electric field, conductivity, and conduction current

Abstract This paper summarizes the results of measurements of the electrical conductivity σ and vertical component of the vector electric field Ez acquired from eight stratospheric balloon flights launched from Amundsen-Scott Station, South Pole, in the austral summer of 1985–1986. The major findings of this research are as follows 1. (1) The data contribute to the set of global atmospheric electricity measurements and extend the work of COBB [(1977), Atmospheric electric measurements at the South Pole. In Electrical Processes in Atmospheres, Dolezalek H. and Reiter R. (eds), pp. 161–167. Steinkopf, Darmstadt, F.R.G.] to determine the electrical environment of the south polar region 2. (2) The average vertical profile of the conductivity at the South Pole, when compared with profiles obtained at other Antarctic locations, suggests that the conductivity scale height may increase with increasing geomagnetic latitude across the polar cap. 3. (3) The conductivity profiles measured at the South Pole and other Antarctic locations differ significantly from polar cap model profiles. On the basis of these measurements, the model profiles appear to require modification 4. (4) The magnitudes of the Ez profiles were observed to vary from day-to-day by a factor of > 2 5. (5) In all of the flights the air-Earth conduction current Jz, calculated as the product of Ez and σ, decreased with altitude in agreement with previous direct measurements of the air-Earth current by Cobb [( 1977), Atmospheric electric measurements at the South Pole. In Electrical Processes in Atmospheres, Dolezalek H. and Reiter R. (eds), pp. 161–167. Steinkopf, Darmstadt, F.R.G.] 6. (6) The magnitude of Jz was 2–3 times larger than the global average, which can be attributed to the lower columnar resistance of the atmosphere above the high-elevation Antarctic plateau. The magnitude of Jz agrees with that observed by Cobb, if the Cobb measurements are multiplied by the Few and Weinheimer [(1986), Factor of 2 error in balloon-borne atmospheric conduction current measurements. J. geophys. Res.91, 10937] correction factor of 2 7. (7) Ez from all of the flights during times of balloon float demonstrates characteristics of the classical ‘Carnegie’ diurnal variation, which is indicative of global influences on the ionospheric potential 8. (8) The influence of geomagnetic activity was observed as a decrease in the amplitude of the diurnal variation of Ez with increasing geomagnetic activity index Kp, which is the predicted effect at the South Pole of the magnetospheric polar-cap potential superimposed on the ‘Carnegie’ potential variation.

Balloon measurements above the South Pole: Study of ionospheric transmission of ULF waves

We report here an experimental study of the Hughes and Southwood model of the transmission of electromagnetic signals from the ionosphere/magnetosphere to the lower atmosphere. The electric field data were obtained from one of eight high-latitude balloon payloads launched above the south geographic pole during the South Pole Balloon Campaign in the 1985–1986 austral summer. The magnetic field data are from the South Pole magnetometer. The balloon payloads were instrumented with double-probe electric field detectors and bremsstrahlung X ray detectors. One of the events from flight 7 (January 8, 1986, at 1500–1800 UT (1130–1430 MLT)) had an amplitude in the range of 10–20 mV/m and a period of several minutes. In the magnetosphere, the wave was probably an Alfven mode. The wave event was superimposed on background fluctuations that can be attributed to turbulence. The results agree with the predictions of the model in that the best coherence is observed between parallel components of E versus B. The results disagree with the predictions of the model in two respects. First, we find a frequency structure not predicted by the model, and second, we find intervals where the electric and magnetic field polarizations are of opposite handedness. The first discrepancy was modeled by taking account of the measured electrostatic turbulence. The second problem remains a puzzle.

Sprite and elve electrodynamics

Abstract Flight 3 of the Sprites99 balloon campaign flew from 00:39:32 to 11:12:00 UTC on 21 August 1999, launched from Ottumwa, Iowa. Three axis electric and magnetic field and ground-based low light level TV observations from three sites were made of more than 68 transient luminous events (TLEs) in the middle atmosphere above thunderstorms in South Dakota and Kansas. At least four TLEs were recorded by two or more stations, including sprites at 0955:36, 0541:58 and 0546:10 UTC and a sprite halo at 0746:35. An event triggered on-board memory, sampling 10 quantities at a rate of 50 kHz per channel for 160 ms per event, recorded bursts for some of the sprites and elves. At the range of the balloon from the lightning and TLE (∼400 km), the vertical electric field perturbation is roughly simulataneous with the light emission. The presence of a radial component of some magnetic pulses suggests that signal propagation was not entirely in the TM mode.

Electron precipitation near L=4: Longitudinal variation

Abstract Observations of penetrating electron precipitation at two L ∼ 4 stations in the southern hemisphere, Kerguelen and Siple Station, Antarctica, are summarized. The nearly complete absence of precipitation at Kerguelen and its frequent occurrence at Siple Station is explained in terms of their location with respect to the center of the South Atlantic geomagnetic anomaly at L ∼ 4. Conclusions which result from this explanation are discussed along with questions which remain concerning penetrating electron precipitation.