The observation and interpretation of electric fields have been crucial to the understanding of space physics, particularly on the kinetic scale. The basic principle for electric field measurements can be considered straightforward. However, it has proven difficult to measure electric fields from current state-of-the-art voltage probes without perturbing the space plasma itself. There are numerous documented observations from electric field instruments that have no geophysical explanation for their behavior, implying that they can only be induced by the interaction between probe and plasma. The cause of these anomalies requires a full understanding of the factors that contribute to the measurements of voltage probes. We developed a procedure to isolate the sources of impedance between various components of a physical voltage probe model and a controlled plasma population via the space physics simulation chamber at the Naval Research Laboratory. This paper describes the laboratory setup, procedure, and results from the first round of tests performed on this voltage probe model and discusses improvements to future campaigns. Initial results show that some changes to the model design and the measurement of plasma parameters are required, but interactions with the plasma are observed, and the calibration method used successfully returns impedance values similar to values from direct measurements.

This abstract presents a supervised deep learning framework, centered on a Convolutional Neural Network (CNN), trained to identify and classify simulated orbital debris using electron saturation Langmuir probe data obtained from the Space Physics Simulation Chamber (SPSC) [1]. The CNN is capable of analyzing both spatial (from moving the probe) and temporal information (from leaving the probe at one place), the model achieved a detection accuracy of over 99% able to determine the presence of debris generated signatures or the absence (determined by a low probe voltage) on a training set of 1550784 samples (80%) and a test set of 387696 (20%) events.

The design and implementation of double probe electric field sensors for space plasma applications has a long and successful history spanning nearly 50 years. Existing and future designs for the E-field sensor elements and surrounding surfaces (guards, stubs, etc.) include options for DC and low-frequency current and voltage biasing of those surfaces, as well as driving of those surfaces at significant amplitudes and bandwidths. These features allow the optimization of the DC and AC coupling of the sensors to potential fluctuations in the ambient plasma and the reduction of the systematic errors in the E-field measurement due to stray and imbalanced currents to one or the other of the probes used for a given measurement.

Spacecraft data reveal a nonuniform ambipolar electric field transverse to the magnetic field in a thin current sheet in Earth's magnetotail that leads to intense E×B velocity shear and nongyrotropic particle distributions. The E×B drift far exceeds the diamagnetic drift and thus drives observed lower hybrid waves. The shear-driven waves are localized to the magnetic field reversal region and are therefore ideally suited for the anomalous dissipation necessary for reconnection. It also reveals substructures embedded in the current density, indicating a compressed current sheet.

Plasma impedance probes are often used in laboratory experiments as well as in space to make measurements of important plasma parameters such as the electron density. Conventional impedance probe methods involve sweeping the frequency applied to the probe through a range containing the plasma frequency, which can take on the order of a second to complete. This acquisition time leads to very low spatial resolution when making measurements from sounding rockets in the ionosphere. A high-time resolution impedance probe is under development at the U.S. Naval Research Laboratory with the goal of increasing the spatial resolution of measurements in space. To achieve this, a short-time Gaussian monopulse with a center frequency of 40 MHz and containing a full spectrum of frequencies is applied to an electrically short dipole antenna. Laboratory experiments were performed with the Gaussian monopulse triggered once every 10 µs and averaged over ten shots, equating to a spatial resolution of 13 cm for a typical sounding rocket speed. This paper discusses the development of the new high-time/spatial resolution self-impedance probe and illustrates that the short-time pulse method yields results that match well with data taken using conventional methods. It is shown that plasma parameters such as the electron density, sheath frequency, and electron–neutral collision frequency can also be derived from the data. In addition, data from the high-time/spatial resolution impedance probe are shown to compare well with those from theoretical impedance models.

The ability to morph electrostatic plasma turbulence into electromagnetic has promising applications, including the possibility of actively influencing the near‐Earth plasma state, aka the space weather. This dual (electrostatic/electromagnetic) nature is a fundamental property of plasma turbulence, which has not been well explored but could explain many phenomena including the formation of a resonant cavity that can amplify the turbulence energy. The upcoming Space Measurement of A Rocket‐Released Turbulence (SMART) mission is designed to understand the evolution of plasma turbulence and the nonlocal consequences of its dual nature. This includes the flow of energy into all possible wavelengths, as well as the transport of energy over a large geographical volume. The resulting energy redistribution in both waves and particles in an extended geographical volume creates a unique electromagnetic environment, which is important for space weather.

In a laboratory experiment, we demonstrate the substantial effects that collisions between charged and neutral particles have on low-frequency (Omega(i) << omega << Omega(e)) shear-driven electrostatic lower hybrid waves in a plasma. We establish a strong (up to 2.5 kV/m) highly localized electric field with a length scale shorter than the ion gyroradius, so that the ions in the plasma, unlike the electrons, do not develop the full E x B drift velocity. The resulting shear in the particle velocities initiates the electron-ion hybrid (EIH) instability, and we observe the formation of strong waves in the vicinity of the shear with variations in plasma densities of 10% or greater. Our experimental configuration allows us to vary the neutral background density by more than a factor of two while holding the charged particle density effectively constant. Not surprisingly, increasing the neutral density decreases the growth rate/saturation amplitude of the waves and increases the threshold electric field necessary for wave formation, but the presence of neutrals affects the dominant wave frequency as well. We show that a 50% increase in the neutral density decreases the wave frequency by 20% while also suppressing the electric field dependence of the frequency that is observed when fewer neutrals are present. The majority of these effects, as well as the values of the frequencies we observe, closely match the predictions of previously developed linear EIH instability theory, for which we present the results of a numerical solution.

For a wide variety of laboratory and space plasma environments, theoretical predictions state that plasmas are unstable to inhomogeneous flows over a very broad frequency range. Such sheared flows are generated in the Earth's magnetosphere and intensify during active periods. Specifically, for a velocity shear oriented perpendicular to a uniform background magnetic field, the shear scale length (L-E) compared to the ion gyroradius (rho(i)) determines the character of the shear-driven instability that may prevail. An interpenetrating plasma configuration is used to create a transverse velocity shear profile in a magnetized plasma column, a condition similar to that found in the natural boundary layers. The continuous variation of rho(i)/L-E and the associated transition of the instability regimes driven by the shear flow mechanism are demonstrated in a single laboratory experiment. Broadband wave emission correlated to increasing/decreasing stress (i.e., rho(i)/L-E), a characteristic signature of a boundary layer crossing, is found under controlled and repeatable conditions. This result holds out the promise for understanding the cause and effect of the in situ observation of broadband electrostatic noise.

: In earlier works we used spheres of various sizes as impedance probes in demonstrating a method of determining plasma potential, p, when the probe radius is much larger than the Debye length, D. The basis of the method in those works 1-4 relies on applying a small amplitude signal of fixed frequency to a probe in a plasma and measuring the complex reflection coefficient, , for varying probe bias, Vb. For a given frequency, Re(Zac) (the real part of the complex plasma impedance determined from ) is plotted versus Vb , and a minimum predicted by theory occurs at p for a large range of electron density, ne 3. However, the frequency range of the applied signal is restricted as we briefly review in this paper. As ne decreases, or the sheath grows to the order of the probe radius, the frequency range becomes even more restrictive and, in addition, the minimum in Re(Zac) in experimental data at Vb = p becomes difficult to discern.

Shear driven instabilities are commonly observed in the near-Earth space, particularly in boundary layer plasmas. When the shear scale length (LE) is much less than the ion gyro-radius (ρi) but greater than the electron gyro-radius (ρe), the electrons are magnetized in the shear layer, but the ions are effectively un-magnetized. The resulting shear driven instability, the electron-ion hybrid (EIH) instability, is investigated in a new interpenetrating plasma configuration in the Auburn Linear EXperiment for Instability Studies. In order to understand the dynamics of magnetospheric boundary layers, the EIH instability is studied in the presence of a density gradient located at the boundary layer between two plasmas. This paper reports on a recent experiment in which electrostatic lower hybrid waves are identified as the EIH instability, and the effect of a density gradient on the instability properties are investigated.

It is shown that a magnetized plasma layer with a velocity gradient in the flow perpendicular to the ambient magnetic field is unstable to waves in the Very Low Frequency band that spans the ion and electron gyrofrequencies. The waves are formally electromagnetic. However, depending on wave vector (k) over bar = kc/omega(pe) (normalized by the electron skin depth) and the obliqueness, k(perpendicular to)/k(parallel to), where k(perpendicular to,parallel to) are wave vectors perpendicular and parallel to the magnetic field, the waves are closer to electrostatic in nature when (k) over bar >> 1 and k(perpendicular to) >> k(parallel to) and electromagnetic otherwise. Inhomogeneous transverse flows are generated in plasma that contains a static electric field perpendicular to the magnetic field, a configuration that may naturally arise in the boundary layer between plasmas of different characteristics. (C) 2014 AIP Publishing LLC.

This chapter contains sections titled: Introduction Electrode Operation Wave Observations Summary

We report experimental evidence of a seamless transition between three distinct modes in a magnetized plasma with a transverse sheared flow as the ratio of the ion gyroradius to the shear scale length (a measure of shear magnitude) is varied. This was achieved using a dual plasma configuration in a laboratory experiment, where a sheared flow oriented perpendicular to a background magnetic field is localized at the boundary of the plasmas. This confirms the basic theory that plasma is unstable to transverse velocity shear in a broad frequency and wavelength range. The experiment characterizes the compression or relaxation of boundary layers often generated in a variety of laboratory and space plasma processes.

Introduction: The NRL Space Physics Simulation Chamber Laboratory conducts a broad-based research program addressing near-Earth space plasma physics. The unique Space Chamber device (shown in operation in Fig. 1) produces large-volume, steady-state, spacelike plasmas with conditions scaled to match various ionospheric and magnetospheric regions of interest. The program includes basic research illuminating the underlying physics driving key space plasma processes and applied research for understanding plasma effects on spacecraft systems, testing spacecraft hardware, and development of innovative plasma sensors.

: Presence of plasma turbulence can strongly influence propagation properties of electromagnetic signals used for surveillance and communication. In particular, we are interested in the generation of low frequency plasma turbulence in the form of coherent vortex structures coexisting with short scale density irregularities and of lower hybrid turbulence. Lower-hybrid type density irregularities are excited by plasma flows with velocity shear, whereas interchange or flute type oscillations in magnetized plasma are associated with Rayleigh-Taylor type instability. These types of density irregularities play important role in refraction and scattering of high frequency electromagnetic signals propagating in the earth ionosphere, inside a plasma sheath of reentry and hypersonic vehicles and in many other applications. We will discuss generation of low frequency density irregularities due to the presence of plasma flows with velocity shear and interchange instability.