Robert Pfaff

The electric field instrument on the polar satellite

The Polar satellite carries a system of four wire booms in the spacecraft spin plane and two rigid booms along the spin axis. Each of the booms has a spherical sensor at its tip along with nearby guard and stub surfaces whose potentials relative to that of their sphere are controlled by associated electronics. The potential differences between opposite sphere pairs are measured to yield the three components of the DC to >1 MHz electric field. Spheres can also be operated in a mode in which their collected current is measured to give information on the plasma density and its fluctuations. The scientific studies to be performed by this experiment as well as the mechanical and electrical properties of the detector system are described.

Solitary potential structures associated with ion and electron beams near 1RE altitude

Small-scale solitary electric potential structures are commonly observed on auroral field lines with the Polar Electric Field Instrument (EFI). This study focuses on observations of solitary structures in the southern hemisphere auroral zone at altitudes between 5500 and 7500 km. Some of the potential structures are similar to those observed previously by the S3-3 and Viking satellites and are inferred to be negative potential pulses traveling upward along the auroral magnetic field lines, associated with upgoing ion beams and upward currents. The velocities of these “ion” solitary potential structures are estimated, using spaced EFI measurements, to be distributed within the range of ∼75 – 300 km s−1. In addition to these structures, a different type of solitary potential structure with.opposite polarity has been observed with faster propagation velocities. These faster structures (termed “electron” solitary potential structures) are distinguishable from the slower, ion solitary structures in that their distinctive bipolar electric field signature, common to both types of solitary structure, is reversed. The ultimate distinction for the electron solitary potential structures is that they are observed on auroral field lines in conjunction with magnetically field-aligned upflowing electron beams. The electron solitary potential structures propagate up the field line in the same direction as the electron beam. An example is shown of the polarity reversal from ion to electron solitary potential structures coincident with a simultaneous shift from upgoing ion beams to upgoing electron beams.

FAST observations of preferentially accelerated He+ in association with auroral electromagnetic ion cyclotron waves

The TEAMS instrument on the FAST satellite has detected events in which He+ ions are resonantly accelerated perpendicular to the magnetic field to energies of several keV. The events occur in association with electromagnetic ion cyclotron (EMIC) waves and conic distributions of up to a few hundred eV in H+ and a few keV in O+. Concentrations of He+ can be significantly elevated during the events. Our interpretation is that the He+ ions are accelerated through a cyclotron resonance with the waves. This acceleration is similar to a proposed mechanism for selective ion acceleration in impulsive solar flares.

DC polarization electric field, current density, and plasma density measurements in the daytime equatorial electrojet

Measurements of the vector DC electric field, current density, and plasma number density were gathered in the daytime equatorial electrojet on a sounding rocket launched from Alcântara, Brazil. The data set provides a self-consistent picture of the electrodynamics of the daytime electrojet, permitting a detailed comparison of the altitude profiles of the vertical component of the DC electric field, Evert, the current density, J, and the plasma number density, Ne. Good agreement in the upper electrojet region between both the magnitude and the profile of the plasma drifts, calculated independently from Evert/B and Jzonal/qNe, demonstrates that the equatorial electrojet is a Hall current consisting of electron motion driven by a vertical DC electric field. The data support the basic Cowling conductivity ideas of electron flow at the dip equator within a narrow conducting layer in which the ions are at rest.

Survey of the frequency dependent latitudinal distribution of the fast magnetosonic wave mode from Van Allen Probes Electric and Magnetic Field Instrument and Integrated Science waveform receiver plasma wave analysis

We present a statistical survey of the latitudinal structure of the fast magnetosonic wave mode detected by the Van Allen Probes spanning the time interval of 21 September 2012 to 1 August 2014. We show that statistically, the latitudinal occurrence of the wave frequency (f) normalized by the local proton cyclotron frequency (f(sub cP)) has a distinct funnel-shaped appearance in latitude about the magnetic equator similar to that found in case studies. By comparing the observed E/B ratios with the model E/B ratio, using the observed plasma density and background magnetic field magnitude as input to the model E/B ratio, we show that this mode is consistent with the extra-ordinary (whistler) mode at wave normal angles (theta(sub k)) near 90 deg. Performing polarization analysis on synthetic waveforms composed from a superposition of extra-ordinary mode plane waves with theta(sub k) randomly chosen between 87 and 90 deg, we show that the uncertainty in the derived wave normal is substantially broadened, with a tail extending down to theta(sub k) of 60 deg, suggesting that another approach is necessary to estimate the true distribution of theta(sub k). We find that the histograms of the synthetically derived ellipticities and theta(sub k) are consistent with the observations of ellipticities and theta(sub k) derived using polarization analysis.We make estimates of the median equatorial theta(sub k) by comparing observed and model ray tracing frequency-dependent probability occurrence with latitude and give preliminary frequency dependent estimates of the equatorial theta(sub k) distribution around noon and 4 R(sub E), with the median of approximately 4 to 7 deg from 90 deg at f/f(sub cP) = 2 and dropping to approximately 0.5 deg from 90 deg at f/f(sub cP) = 30. The occurrence of waves in this mode peaks around noon near the equator at all radial distances, and we find that the overall intensity of these waves increases with AE*, similar to findings of other studies.

Comparison of E‐region electric fields observed with a sounding rocket and a Doppler radar in the Seek Campaign

This paper describes results from simultaneous measurements of mid-latitude irregularities by a sounding rocket and a Doppler radar. The experiment was conducted on August 26, 1996 as part of the SEEK (Sporadic-E Experiment over Kyushu) campaign in Japan. Electric field observed with a double-probe sensor on the rocket showed very large fluctuations of ±10 mV/m where irregularity echoes were quite intense. Doppler velocities observed with the radar agreed well with the electric fields. We estimated growth rates of the gradient-drift instability, which showed positive values around the major sporadic-E layer. It was found that, within intense irregularity region with quasi-periodic structures, there were large polarization electric fields that was likely to be associated with spatial structure of the sporadic-E layer.

Downdrafting plasma flow in equatorial bubbles

The electric field experiment carried aboard the San Marco D equatorial ionospheric satellite regularly measured updrafting in plasma depletion channels or “equatorial bubbles” which form on the bottomside of the nightside equatorial F region. We report here observations of downdrafting vertical plasma velocities inside such depletion regions in the nightside equatorial ionosphere. Both updrafting and downdrafting motions can be expected on the basis of a generalized gradient drift/collisional Rayleigh-Taylor instability process in the ionospheric F region. Although the gravitation can only drive upward plasma flow in plasma depletion regions, both background westward zonal electric fields and upward vertical neutral winds can cause an occurrence of downdrafting (i.e., a downward motion of the plasma within the bubble) if those parameters are strong enough. We show that as the background zonal electric field becomes westward (often after ∼2100 LT) in the equatorial ionosphere, the plasma interior to an existing bubble at altitudes of ∼400 km and less at the magnetic equator may assume a downdrafting motion, while at higher altitudes in the same bubble channel, the plasma flow remains upward. Such a simultaneous occurrence of the updrafting and downdrafting plasma flow in a single bubble channel may lead to the pinching off of the upper part of the depletion region from the lower altitude regions, causing the decay of a bubble or the formation of a “dead” bubble.

Electric and magnetic field measurements inside a high-velocity neutral beam undergoing ionization

Vector electric field measurements have been made inside two ionizing, high-velocity streams of barium atoms in the Earths ionosphere. A variety of electrical phenomena were observed across the frequency spectrum and are presented in this paper, which emphasizes the experimental results. Comparisons with a theoretical model for the interactions of the stream with the magnetic field and the ionosphere are presented in a companion paper (Brenning et al., this issue (a)). A most startling result is that a very large quasi-dc electric field was detected antiparallel to the beam velocity. This by itself is not unreasonable since newly ionized barium ions with their large gyroradii are expected to create such a field. But since the beam had roughly a 45° angle with the magnetic field, Bo, we find a very large (≳500 mV/m) component of E parallel to Bo. The fluctuating electric fields were also quite large, in fact, of the same order of magnitude as the quasi-dc pulse. The wave energy was found to maximize at frequencies below the barium lower hybrid frequency and included strong signatures of the oxygen cyclotron frequency. Measurements made on a subpayload separated across Bo by several hundred meters and along Bo by several kilometers do not show the large pulse, although a variety of wave emissions were seen. In addition, very large amplitude magnetic field fluctuations were detected in both bursts. Although we have no clear explanation, they appear to be a real phenomenon and worthy of future study. Finally, we note that even though the critical ionization velocity effect did not go into a discharge mode in this experiment, remarkable electromagnetic effects were seen in the neutral beam-plasma interaction.

The E-region rocket/radar instability study (ERRRIS): scientific objectives and campaign overview

Abstract The E -region Rocket/Radar Instability Study (Project ERRRIS) investigated in detail the plasma instabilities in the low altitude ( E -region) auroral ionosphere and the sources of free energy that drive these waves. Three independent sets of experiments were launched on NASA sounding rockets from Esrange, Sweden, in 1988 and 1989, attaining apogees of 124, 129 and 176km. The lower apogee rockets were flown into the unstable auroral electrojet and encountered intense two-stream waves driven by d.c. electric fields that ranged from 35 to 115 mV/m. The higher apogee rocket returned fields and particle data from an active auroral arc, yet observed a remarkably quiescent electrojet region as the weak d.c. electric fields (~ 10–15 mV/m) there were below the threshold required to excite two-stream waves. The rocket instrumentation included electric field instruments (d.c. and wave), plasma density fluctuation ( δn / n ) receivers, d.c. fluxgate magnetometers, energetic particle detectors (ions and electrons), ion drift meters, and swept Langmuir probes to determine absolute plasma density and temperature. The wave experiments included spatially separated sensors to provide wave vector and phase velocity information. All three rockets were flown in conjunction with radar backscatter measurements taken by the 50MHz CUPRI system, which was the primary tool used to determine the launch conditions. Two of the rockets were flown in conjunction with plasma drift, density, and temperature measurements taken by the EISCAT incoherent scattar radar. The STARE radar also made measurements during this campaign. This paper describes the scientific objectives of these rocket/radar experiments, provides a summary of the geophysical conditions during each launch, and gives an overview of the principal rocket and radar observations.

Wavevector observations of the two‐stream instability in the daytime equatorial electrojet

Detailed measurements of electric field wavevectors of the Farley-Buneman two-stream instability in the daytime equatorial electrojet were gathered on a sounding rocket flown from Alcântara, Brazil. Primary two-stream waves were detected in the altitude region of 104–108 km, where the current density and polarization electric field maximized and where intense 3 m vertical echoes were observed by a 50 MHz backscatter radar. The in-situ measured wave components are predominantly oriented in the horizontal, zonal direction. Spaced receivers reveal wavelengths of 11–14 m with phase velocities of about 370 m/s at the peak wave power near 106.5 km. These velocities are constant from 1–15 m, indicative of dispersionless flow. Although the neutral wind was not measured, the in-situ phase velocity appears to be below that predicted by linear theory.