L. Muschietti

FAST satellite observations of large‐amplitude solitary structures

We report observations of “fast solitary waves” that are ubiquitous in downward current regions of the mid-altitude auroral zone. The single-period structures have large amplitudes (up to 2.5 V/m), travel much faster than the ion acoustic speed, carry substantial potentials (up to ∼100 Volts), and are associated with strong modulations of energetic electron fluxes. The amplitude and speed of the structures distinguishes them from ion-acoustic solitary waves or weak double layers. The electromagnetic signature appears to be that of an positive charge (electron hole) traveling anti-earthward. We present evidence that the structures are in or near regions of magnetic-field-aligned electric fields and propose that these nonlinear structures play a key role in supporting parallel electric fields in the downward current region of the auroral zone.

Wind Spacecraft Observations of Solar Impulsive Electron Events Associated with Solar Type III Radio Bursts

We present Wind spacecraft observations of solar impulsive electron events associated with locally generated Langmuir waves during solar type III radio bursts. The solar impulsive electrons had energies from ~600 eV to greater than 300 keV. Local Langmuir emissions associated with these fluxes generally coincided with the arrival of 2-12 keV electrons. A survey of 27 events over 1 yr shows that there were few occurrences of electron distributions (~96 s averaged) that were unstable to Langmuir waves and none that had a substantial growth rate (>3 × 10-2 s-1) or endured for more than 96 s. Intense solar impulsive electron events that occurred on 1995 April 2 are studied in detail. Marginally stable (plateaued) distributions occasionally coincided with a periods of local Langmuir emissions, but the electron distributions were otherwise stable. These observations suggest that kinetic processes were modifying the electron distribution but also suggest that processes other than one-dimensional quasilinear relaxation were involved. We find that solar impulsive electron distributions were often unstable to oblique waves, such as quasi-electrostatic whistler waves or electromagnetic ion cyclotron waves, suggesting that competition between Langmuir and oblique emissions may be important. There are several other features in the Wind spacecraft solar impulsive electron observations that are noteworthy. Nondispersive flux modulations were visible in many of the events (also visible in the published ISEE 3 data) in ~1-4 keV electrons, suggesting that a local hydromagnetic instability may have accompanied the lowest energy solar impulsive electron fluxes. The Wind data differ from the ISEE 3 data in the energy spectra of the electron events. ISEE 3 recorded few events with only high-energy (>10 keV) electron fluxes, whereas a survey of the Wind events shows a substantially higher ratio of high-energy events. The high-energy events were often associated with solar flares that could not have been magnetically well connected with the satellite.

FAST observations of electron distributions within AKR source regions

Observations of high-time resolution 3-D electron distributions within the source regions of Auroral Kilometric Radiation (AKR) are reported. In general, the electron data display a broad plateau over a wide range of pitch angles indicating that these distributions have been rapidly stabilized by AKR wave growth. The source of the electron instability appears to come from several features in the distribution, including an isotropic beam feature and its mirroring components, occasional electrons in the “trapped” region, as well as steep gradients present in the atmospheric loss cone. Taken together these features may provide a nearly continuous region of ∂f/∂v⟂ which could contribute to the relativistic cyclotron maser instability. Computer simulations of the evolution of the electron distribution which assume plasma conditions similar to the parameters measured by FAST show similar results to the observed electron distributions. The FAST observations also show that relativistic corrections to the AKR dispersion relation may enable a small k|| mode with a resonance condition that is able to take maximum advantage of the initial instability in the mono-energetic electron distributions within the auroral acceleration regions.

Phase-space electron holes along magnetic field lines

Recent observations from satellites crossing active magnetic field lines have revealed solitary potential structures that move at speeds substantially greater than the ion thermal velocity. The structures appear as positive potential pulses rapidly drifting along the magnetic field. We interpret them as BGK electron holes supported by a population of trapped and passing electrons. Using Laplace transform techniques, we analyse the behavior of one phase-space electron hole. The resulting potential shapes and electron distribution functions are self-consistent and compatible with the field and particle data associated with the observed pulses. In particular, the spatial width increases with increasing amplitude. The stability of the analytic solution is tested by means of a two-dimensional particle-in-cell simulation code with open boundaries. We also use our code to briefly investigate the influence of the ions. The nonlinear structure appears to be remarkably resilient.

Electrostatic Turbulence and Debye-Scale Structures Associated with Electron Thermalization at Collisionless Shocks

We analyze measurements of bipolar, Debye-scale electrostatic structures and turbulence measured in the transition region of the Earths collisionless bow shock. In this region, the solar wind electron population is slowed and heated, and we show that this turbulence correlates well in amplitude with the measured electron temperature change. The observed bipolar structures are highly oblate and longitudinally polarized and may instantaneously carry up to 10% of the plasma energy ψ ≡ e/kbTe ≈ 0.1 before dissipating. The relationship between ψ and the field-aligned scale size Δ∥ of the Gaussian potential suggests that the bipolar structures are BGK trapped particle equilibria or electron hole modes. We suggest a generation scenario and a potential role in dissipation.

Kinetic localization of beam‐driven Langmuir waves

The bursts of Langmuir waves observed in the auroral ionosphere are interpreted as localized wave packets of tens to a hundred wavelengths along the geomagnetic field, which interact with streaming, energetic electrons. We investigate the interaction at a kinetic level and show by means of self-consistent, particle-in-cell simulations with open boundary conditions that the interaction itself can lead to wave localization. Ballistic perturbations of the distribution function and nonlinear orbit corrections combine to modulate the perturbed density that drives the waves. The resulting kinetic localization takes place within shorter timescales and for smaller wave amplitudes than the focusing via ponderomotive force.

On the formation of wave packets in planetary foreshocks

Kinetic localization of beam-driven Langmuir waves is studied for parameters relevant to the electron foreshock environment. Particle-in-cell simulations of the interaction occurring between energetic electrons and Langmuir waves are performed. It is shown that the nonlinearities in the resonant electrons lead to the formation of packets growing out of the noise. The characteristic spatial scale depends upon the beam velocity and has a weak dependence on the wave amplitude. The mechanism takes place within shorter timescales and for smaller amplitudes than the classic nonlinearities due to bulk electrons and ions. As an example, we apply the results to observations made in the Earth and Jovian foreshocks.

Oblique turbulence driven by field-aligned electron fluxes in the auroral ionosphere

This paper investigates how the various waves of the whistler resonance cone compete for the free energy available in field-aligned fluxes of precipitating electrons in the auroral ionosphere. A two-dimensional numerical simulation code, based on the coupled pair of quasi-linear equations, is used for the investigation. The description provided by these equations allows us to follow the nonlinear development of the instability and analyze the interplay between the modes as mediated by the changing distribution function. We present results for a parameter regime corresponding to altitudes probed by sounding rockets, around 103 km and Ωe ≳ ωp. The presence of a halo of energetic electrons significantly affects the evolution of the turbulence. The spectrum is shown to shift with time toward increasingly oblique propagation angles including a substantial share of quasi perpendicular, short-wavelength modes close to the lower hybrid frequency.

Stability of fast solitary structures on auroral field lines

Abstract The stability of the fast solitary structures which were observed onboard several auroral crossing satellites is analyzed as a dynamical system and investigated numerically. These large-amplitude potential spikes are supported by trapped electron populations. For parameters of low and mid-altitude auroral passes with gyro-to-bounce frequency ratios significantly larger than unity, the potential spikes are very resilient, while for lower magnetic fields, at ratios below unity, they develop unstable undulations in the transverse direction. The evolution of the solitary structures is related to changes in the trajectories of the trapped electrons. It is shown here that the coupling of the parallel and perpendicular dynamics is stronger when the above ratio decreases, resulting in a bifurcation of trajectories. The addition of a small perturbation to the large amplitude structure leads to a very different response of the trapped electrons in the two configurations. The electron behavior reflects the lack of spike stability at small gyro-to-bounce frequency ratios.

Unusual heliospheric energetic ion abundances due to resonant wave-particle processes

Abstract The resonant wave-particle effects which enhance spectacularly the abundance of the 3 He isotope and heavy elements in impulsive solar flares are analyzed. Primordial nucleosynthesis and galactic evolution confines the coronal ratio of the He isotopes, 3 He 4 He , to several times 10 −4 ; this ratio is enhanced during impulsive flares by up to four orders of magnitude due to interaction with electromagnetic ion cyclotron (emic) waves. The heavy elements, which are produced during galactic evolution and which are not fully ionized at coronal temperatures, are also accelerated by emic waves through higher gyroharmonic resonances. The oblique emic waves, which propagate undamped over large distances, are observed on auroral field lines in conjunction with dynamically evolving electron fluxes, and, analogously, are postulated on coronal field lines in conjunction with the X-ray emitting electrons. Quasilinear simulations show that the dynamically evolving electron velocity distribution consisting of a core, halo and beam can enhance the low-frequency modes, while quenching the high-frequency Langmuir waves and preserving some positive slope. This positive slope may drive the low-frequency electromagnetic waves. The acceleration of 3 He by the emic waves and the enrichment of specific elements and charge states, at different coronal conditions, is described.