Turbulence plays a key role in the conversion of the energy of large-scale fields and flows to plasma heat, impacting the macroscopic evolution of the heliosphere and other astrophysical plasma systems. Although we have long been able to make direct spacecraft measurements of all aspects of the electromagnetic field and plasma fluctuations in near-Earth space, our understanding of the physical mechanisms responsible for the damping of the turbulent fluctuations in heliospheric plasmas remains incomplete. Here we propose an innovative field–particle correlation technique that can be used to measure directly the secular energy transfer from fields to particles associated with collisionless damping of the turbulent fluctuations. Furthermore, this novel procedure yields information about the collisionless energy transfer as a function of particle velocity, providing vital new information that can help to identify the dominant collisionless mechanism governing the damping of the turbulent fluctuations. Kinetic plasma theory is used to devise the appropriate correlation to diagnose Landau damping, and the field–particle correlation technique is thoroughly illustrated using the simplified case of the Landau damping of Langmuir waves in a 1D-1V (one dimension in physical space and one dimension in velocity space) Vlasov–Poisson plasma. Generalizations necessary to apply the field–particle correlation technique to diagnose the collisionless damping of turbulent fluctuations in the solar wind are discussed, highlighting several caveats. This novel field–particle correlation technique is intended to be used as a primary analysis tool for measurements from current, upcoming and proposed spacecraft missions that are focused on the kinetic microphysics of weakly collisional heliospheric plasmas, including the Magnetospheric Multiscale (MMS), Solar Probe Plus, Solar Orbiter and Turbulence Heating ObserveR (THOR) missions.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/12921EF6C25C03AC133C892D5A2E4B89/S0022377816001197a.pdf/div-class-title-diagnosing-collisionless-energy-transfer-using-field-particle-correlations-vlasov-poisson-plasmas-div.pdf

Diagnosing collisionless energy transfer using field-particle correlations: Vlasov-Poisson plasmas

Recent measurements of auroral electrons and intense Langmuir waves on a high-altitude sounding rocket show that these waves were produced by dispersive bursts of low-energy electrons. The sounding rocket was launched northward into the evening auroral zone during a substorm expansion and crossed several discrete auroral arcs having electron energy peaks greater than 25 keV. The large-amplitude Langmuir waves, which appeared in {approximately}100-ms bursts and had amplitudes greater than 200 mV/m, were seen only during periods of enhanced low-energy (0.3-3.0 keV), field-aligned electron precipitation. The enhanced electron flux displayed a dispersive signature in which the higher-energy electrons arrived before the lower-energy electrons. A wave-particle correlator, a new instrument that performed a direct correlation of the arrival times of electrons with the phase of the high-frequency wave field, detected electron bunching at the wave frequency during the bursts of intense Langmuir waves. The electron bunching events, which had amplitudes of a few percent, provided a direct identification of the energy (velocity) of the resonant electrons and therefore established the parallel wavelength ({approximately}15 m). The electron bunching events were detected at or near the energy at which a positive slope in the electron distribution function was seen. During a dispersive burst, themore » phase velocity (or wavelength) of the Langmuir waves changed in response to the changing velocity at which the positive slope occured. The authors conclude that the velocity dispersion in the low-energy, field-aligned electrons created the unstable electron distribution that was responsible for Langmuir wave growth. The proposed growth mechanism is similar to that suggested for Langmuir wave growth during solar type III radio bursts.« less

Langmuir wave growth and electron bunching: Results from a wave‐particle correlator

Auroral beam/plasma interaction observed directly

The Shuttle Potential and Return Electron Experiment (SPREE) was designed and fabricated for flight as part of the joint NASA/Agenzia Spaziale Italiana Tethered Satellite System (TSS-1) mission. The SPREE is a complex instrument package designed to measure ion and electron particle flux and wave-particle interactions. The SPREE flight hardware consists of two multiangular electrostatic analyzer units, two rotary tables, a data processing unit, a particle correlator experiment, and two data recording units. The electrostatic analyzers measure both electrons and ions, in an energy range from 10 eV to 10 keV and simultaneously over an angular fan of (100×10) degrees. These units are mounted on the rotary tables to provide a 2π steradian field of view out of the Orbiter's payload bay. To assess negative charging of the Orbiter with respect to the ambient plasma, ion data from the analyzers are processed real time by an on-board algorithm operating within the data processing unit. The particle correlator experiment determines wave-particle interactions in the frequency range 0–10 MHz for electrons and from 0–10 kHz for ions. SPREE operated successfully throughout the TSS-1 mission. Examples of the data returned by the SPREE are shown.

The Shuttle Potential and Return Electron Experiment (SPREE)

We report on the analysis of megahertz modulation of electrons as measured by the Shuttle Potential and Return Electron Experiment (SPREE) during dc firing of the shuttle electrodynamic tether system (SETS) fast pulsed electron generator (FPEG). The SPREE and FPEG were flown aboard the space shuttle Atlantis flight STS 46 as part of the Tethered Satellite System (TSS 1) mission. The principal data reported here are from the SPREE multiangular electrostatic analyzers (ESAs) and Space Particle Correlator Experiment (SPACE). The ESAs, mounted on rotary tables, measured electrons and ions in the energy range from 10 eV to 10 keV over a solid angle of 2π sr. The SPACE is a signal processing system that analyzes the pulse stream from the SPREE ESAs to identify bunching of the electrons and ions produced by coherent wave-particle interactions (WPIs). The SPACE detects modulations in the electron fluxes in frequency range 0- to 10-MHz. This paper concerns 2- to 4-MHz modulations of the electron flux detected by the SPACE when the FPEG was firing in a dc mode at pitch angles close to 90°. During such operations, FPEG emitted a current of 100 mA at an energy of 1 keV. For these times, electrons with energies from 10 to 1850 eV were measured by the SPREE. For energies between ∼10 and 100 eV the electron flux is basically isotropic. At higher energies the flux increases for pitch angles near 90°. The electron distribution functions generally decrease monotonically with increasing energy up to 100 eV. At energies >100 eV the distributions either monotonically decrease or exhibit a peak or plateau at energies near the beam emission energy. Megahertz modulations were observed for electrons with energies from 10 to 1180 eV, on both positive and negative slopes in the distribution function and throughout the 2π sr sampled by the ESAs. The occurrence and strength of the modulations exhibit no clear dependence on the pitch angle at which the electrons are measured. However, they appear to be limited to low parallel velocities (<3×10 6 m s −1 ) where beam-generated waves are in resonance with suprathermal electrons.

M.P. Gough

Papers

A technique for rocket-borne detection of electron bunching at megahertz frequencies