Parallel Electric Field Acceleration Research Articles

Electron acceleration in the electron dissipation region of asymmetrical magnetic reconnection driven by ultra-intensity lasers

We performed 3D Particle-In-Cell simulations to study electron acceleration in the electron dissipation region of asymmetrical electron magnetic reconnection driven by ultra-intensity lasers, which is similar to the Earth’s magnetosphere reconnection process. Within the electron dissipation region, electrons exhibit a nonthermal distribution, and as the asymmetry increases, the power-law spectrum becomes steeper. Remarkably, the electron spectrum closely resembles a delta distribution, arising from the intense acceleration imparted by the reconnection electric field near the X-line. Both parallel electric field acceleration and the Betatron acceleration mechanism play pivotal roles in this reconnection process. Furthermore, as the magnetic reconnection asymmetry intensifies, the parallel electric acceleration mechanism becomes stronger near the X-point region, whereas the Betatron acceleration mechanism wanes, primarily concentrated in the outflow region.

Betatron Acceleration of Suprathermal Electrons within a Small-scale Flux Rope in the Solar Wind

A growing body of evidence from observations, theories, and simulations indicates that particles can be effectively accelerated in solar wind regions filled with dynamic small-scale flux ropes (FRs). The main acceleration mechanisms identified in simulations include parallel electric field acceleration, first-order Fermi acceleration, and generalized betatron acceleration in contracting or merging small-scale FRs. However, direct identification of these acceleration mechanisms from in situ measurements remains a challenge. Here we present a distinct event of local betatron acceleration within a contracting small-scale FR in the solar wind, due to a local compression. In this event, the lower-energy halo electrons were effectively accelerated through the betatron mechanism, whereas the higher-energy suprathermal electrons predominated by the superhalo component were almost not energized. The halo electron energization processes via the betatron mechanism are reproduced using an analytical model. Further examination of small-scale FRs in the vicinity of the heliospheric current sheet over the period 1995–2020 indicates that in situ signatures of the betatron acceleration process are essentially elusive.

Non-thermal electron acceleration from magnetically driven reconnection in a laboratory plasma

Magnetic reconnection is a ubiquitous astrophysical process that rapidly converts magnetic energy into some combination of plasma flow energy, thermal energy, and non-thermal energetic particles, including energetic electrons. Various reconnection acceleration mechanisms in different low-$\beta$ (plasma-to-magnetic pressure ratio) and collisionless environments have been proposed theoretically and studied numerically, including first- and second-order Fermi acceleration, betatron acceleration, parallel electric field acceleration along magnetic fields, and direct acceleration by the reconnection electric field. However, none of them have been heretofore confirmed experimentally, as the direct observation of non-thermal particle acceleration in laboratory experiments has been difficult due to short Debye lengths for \textit{in-situ} measurements and short mean free paths for \textit{ex-situ} measurements. Here we report the direct measurement of accelerated non-thermal electrons from low-$\beta$ magnetically driven reconnection in experiments using a laser-powered capacitor coil platform. We use kiloJoule lasers to drive parallel currents to reconnect MegaGauss-level magnetic fields in a quasi-axisymmetric geometry. The angular dependence of the measured electron energy spectrum and the resulting accelerated energies, supported by particle-in-cell simulations, indicate that the mechanism of direct electric field acceleration by the out-of-plane reconnection electric field is at work. Scaled energies using this mechanism show direct relevance to astrophysical observations. Our results therefore validate one of the proposed acceleration mechanisms by reconnection, and establish a new approach to study reconnection particle acceleration with laboratory experiments in relevant regimes.

Magnetospheric Multiscale Observations of Waves and Parallel Electric Fields in Reconnecting Current Sheets in the Turbulent Magnetosheath

AbstractDownstream of the Earth’s bow shock, the magnetosheath can exhibit strong turbulence. This turbulent cascade is associated with electron‐scale current sheets where magnetic reconnection can occur. We present data from NASA’s Magnetospheric Multiscale mission during a turbulent magnetosheath interval that includes two electron‐scale reconnecting current sheets, one with higher guide field and one that is closer to antiparallel. We examine the characteristics of energy conversion and the role of parallel electric fields and waves in that energy conversion. The high‐guide field reconnection event shows energy conversion dominated by electric fields and currents parallel to the background magnetic field, while the lower guide field event is perpendicular dominated, consistent with observations of laminar reconnecting current sheets in the magnetosheath. Both current sheets exhibit signs of a parallel electric field acceleration channel, and the higher guide field event shows whistler‐mode waves at the x‐line. Both current sheets show variation in dissipation along the n‐direction, with perpendicular dissipation on one side and parallel on the other. Between the two is a dispersion of pitch angles from 90° to 0° or 180°. This dispersion is likely caused by the acceleration channel and rapidly suppresses whistler waves, suggesting whistler waves do not contribute to the dissipation in guide field reconnection.

Phase-space dynamics of runaway electrons in magnetic fields

Dynamics of runaway electrons in magnetic fields are governed by the competition of three dominant physics: parallel electric field acceleration, Coulomb collision, and synchrotron radiation. Examination of the energy and pitch-angle flows reveals that the presence of local vortex structure and global circulation is crucial to the saturation of primary runaway electrons. Models for the vortex structure, which has an O-point to X-point connection, and the bump of runaway electron distribution in energy space have been developed and compared against the simulation data. Identification of these velocity-space structures opens a new venue to re-examine the conventional understanding of runaway electron dynamics in magnetic fields.

Solar cycle effects on parallel electric field acceleration of auroral electron beams

AbstractWe present the occurrence frequency of downgoing auroral electron beams in magnetic local time and invariant latitude, and the dependence on solar cycle, as indicated by F10.7, on whether the ionospheric foot point of the satellite is illuminated or dark, and on the energy flux carried by the electrons. As previously reported, we find that the occurrence of electron beams peaks in the premidnight local time sector and that solar illumination at the foot point (solar zenith angle) reduces both the occurrence and energy of the electron beams. The effect of solar maximum conditions (indicated by F10.7) is almost as large as the effect of the solar zenith angle. The characteristic energy of the electron beams is dependent on the energy flux carried, in addition to both solar zenith angle and F10.7. The beam energy (and therefore the parallel potential drop) is ~1.6 times higher for during solar minimum than during solar maximum for both dark and illuminated foot points. The beam energy during dark solar minimum conditions is a factor of ~3 more than during sunlit minimum conditions. The “area” covered by intense aurora is also reduced during solar maximum, for both sunlit and dark conditions. There is no evidence that the statistical results are due to the fact that acceleration via parallel electric fields moves to lower latitudes during solar maximum.

Electromagnetic ion cyclotron resonance heating in the VASIMR

Plasma physics has found an increasing range of practical industrial applications, including the development of electric spacecraft propulsion systems. One of these systems, the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) engine, both applies several important physical processes occurring in the magnetosphere. These processes include the mechanisms involved in the ion acceleration and heating that occur in the Birkeland currents of an auroral arc system. Auroral current region processes that are simulated in VASIMR include lower hybrid heating, parallel electric field acceleration and ion cyclotron acceleration. This paper will focus on using a physics demonstration model VASIMR to study ion cyclotron resonance heating (ICRH). The major purpose is to provide a VASIMR status report to the COSPAR community. The VASIMR uses a helicon antenna with up to 20 kW of power to generate plasma. This plasma is energized by an RF booster stage that uses left hand polarized slow mode waves launched from the high field side of the ion cyclotron resonance. The present setup for the booster uses 2–4 MHz waves with up to 20 kW of power. This process is similar to the ion cyclotron heating in tokamaks, but in the VASIMR the ions only pass through the resonance region once. The rapid absorption of ion cyclotron waves has been predicted in recent theoretical studies. These theoretical predictions have been supported with several independent measurements in this paper. The ICRH produced a substantial increase in ion velocity. Pitch angle distribution studies show that this increase takes place in the resonance region where the ion cyclotron frequency is equal to the frequency on the injected RF waves. Downstream of the resonance region the perpendicular velocity boost should be converted to axial flow velocity through the conservation of the first adiabatic invariant as the magnetic field decreases in the exhaust region of the VASIMR. In deuterium plasma, 80% efficient absorption of 20 kW of ICRH input power has been achieved. No evidence for power limiting instabilities in the exhaust beam has been observed.

Accelerated electrons as the source of Auroral Kilometric Radiation

Data from the Fast Auroral Snapshot Explorer (FAST) have confirmed that Auroral Kilometric Radiation (AKR) is generated by the accelerated electrons in the auroral density cavity. FAST observations of the electron distribution within the AKR source region show a characteristic horse-shoe distribution expected from the parallel electric field acceleration and magnetic mirroring of the electrons, the precipitating portion of which constitute the auroral electrons. Within the density cavity it is likely that the hot electrons carry most, if not all, of the plasma density. The AKR wave characteristics within the source region show that the lower frequency cut-off is below the cold electron gyro-frequency, and near the cut-off, the wave electric and magnetic field polarization is consistent with perpendicularly propagating X-mode waves. Taken together these observations are consistent with the cyclotron maser instability, but with the inclusion of relativistic effects in the wave dispersion, as well as the resonance condition. This allows the accelerated electrons to generate AKR, and further the replenishment of the distribution by the parallel electric field ensures that AKR can be generated over a large altitude range on auroral zone field lines

Velocity filter mechanism for ion bowl distributions (bimodal conics)

Klumpar et al. (1984) have recently observed ion distributions in the high‐altitude auroral region which are conical at high energies and field‐aligned at lower energies. In velocity space these distributions appear “bowl‐shaped.” These observed distributions were interpreted as being due to two‐stage acceleration involving transverse ion heating and upward field‐aligned electric field acceleration. Here we show that qualitatively similar bowl‐shaped distributions can also be formed by transverse heating in a region of finite horizontal extent, followed by essentially adiabatic convective flow to the observation location. The latter stage contributes a velocity‐filtering effect which produces some characteristics of the distributions observed. For the published distributions of Klumpar et al. the evidence favors the parallel electric field acceleration as the dominant mechanism for the second stage. However, in general, it may be possible to use multiple ion species observations to distinguish these two mechanisms for ion bowl distributions.

Venus nightside ionospheric holes: The signatures of parallel electric field acceleration regions?

Recent observations by the instruments onboard the Pioneer Venus spacecraft have revealed the existence of "holes" or regions of density depletion in the nightside Venus ionosphere associated with regions of radial magnetic fields. The properties of the electrons within the core of these holes are suggestive of an acceleration process along the magnetic field lines as are the Venera 9 and 10 observations of energetic ions in the Venus tail. Given the observational information, we attribute these Venusian plasma depletions to the presence of parallel electric fields similar to those observed in the terrestrial auroral ionosphere. The resulting electric field accelerates electrons down the field lines producing heating of the depleted thermal electron population within the hole and production of ionization below the hole. Simultaneously ionospheric ions are accelerated outward toward the plasmasheet.

The small‐scale structure of electrostatic shocks

Small‐scale regions of large electric fields have been observed above the auroral zone by the S3‐3 satellite. The data from five such electrostatic shocks have been examined in as great a detail as is available. The three higher altitude shocks (all above 5700‐km altitude) were associated with upward‐going ion beams, indicating that the potential associated with the shock closed below the satellite to give rise to the parallel electric field required for the acceleration of the ion beam. Electrostatic ion cyclotron waves were found in all these cases adjacent to the shock and extending throughout the upward‐going ion beam region. The lack of noticeable Doppler shift in the electrostatic ion cyclotron waves in association with large convective drift velocities indicates that the wavelength of the electrostatic ion cyclotron wave can be several kilometers and the potential difference within the wave can be the order of 100 V. Regions having anisotropic electron distributions with the signature of parallel electric field acceleration close to but above the satellite were found on the other side of the shock from the ion beam region in two of these examples. Both these regions were associated with V shaped VLF hiss. In contrast, lower altitude shocks were associated with VLF saucers and conical ion distributions. The association of the lower altitude shocks with VLF saucers indicates that the low‐altitude shock region is a permanent feature on the scale of tens of seconds and that the low‐altitude shock region extends along the magnetic field. Since they occur in different current regions, the low‐altitude shocks described here are not merely the low altitude extension of the kind of higher altitude shocks associated with ion beams. Electrostatic shocks are always embedded in broader regions of low frequency turbulence. The shock itself is a complex low‐frequency structure that, if interpreted as a Doppler shift effect, implies a complicated small scale spatial structure. The important theoretical question of whether the parallel electric field inside the electrostatic shock exists over a large region of space parallel to the magnetic field or is confined to a narrow double layer or similar structure is not answered. The data are consistent with both possibilities.

Signatures of electric fields from high and low altitude farticles distributions

Measurements of high altitude (<1.3 R e) ions and electrons at auroral energies are used to provide evidence of parallel electric field acceleration over the dusk to midnight auroral regions for both the north and south hemispheres. The data, taken on August 12, 1976 by charged particle spectrometers on the S3‐3 satellite, show evidence of potential differences of ∼2 kV below and ∼1 kV above a satellite altitude of 7300 km.

Rocket-borne measurements of the dayside cleft plasma: The Tordo Experiments

Results are presented from low-energy plasma analyzers (12 eV to 12 keV) carried on two rockets launched into the dayside cleft during January 1975. It is concluded that (1) atmospheric interaction becomes important for less than 1-keV electrons at approximately 250 km, (2) characteristics of particles in 'inverted V's' observed in the afternoon cleft are consistent with their interpretation as being due to parallel electric field acceleration from a constant source population, and (3) magnetospheric energetic (greater than 2 keV) electrons intermingle with magnetosheathlike plasma in the cleft.

Geomagnetic storm particles in the high-latitude magnetotail

Nearly monoenergetic positive ions flowing outward along magnetic-field lines in the high-latitude magneto tail at ∼18 RE, outside the plasma sheet, have been observed with Vela satellites. These ions, probably mainly protons, are detected only during geomagnetic storms. The ‘storm particles’ have average energies per charge ranging from about 0.3 to 3 kv, but at any instant the energy distribution is quite narrow, sometimes <10%. Their angular distribution is usually narrow, sometimes ∼6°. Particles with storm-particle characteristics are not observed in the plasma sheet. Storm-particle fluxes cannot be determined with high accuracy because of their narrow energy distribution, but they are estimated to reach 105 particles cm−2 sec−1 or more. Possible sources of the storm particles are considered, including the solar wind, magnetosheath, polar cusps, polar wind or ionosphere, plasma sheet, solar neutral hydrogen streams, reconnection transfer of plasma into the magneto tail from the polar cusps, polar wind, or polar ionosphere, and parallel electric-field acceleration of the polar wind or ionosphere ions along polar-cap magnetic-field lines. Of these possibilities, the electric-field acceleration is favored. No matter what their source, storm particles appear to be a new feature of geomagnetic storms.