Electron Bursts Research Articles

Laser‐induced burst corona and streamer corona for a positive DC nonuniform field gap in N2/NO mixtures

AbstractThe characteristic of sparkover in a DC nonuniform field gap is closely related to the discharge mode at the onset time of corona. In order to investigate the transition mechanism from burst corona to glow corona or streamer corona, a discharge experiment which is controlled by a pulsed laser in N2/NO mixture has been performed.From highly sensitive observation of discharge luminescence which corresponds to the discharge current wave, the transition from burst corona to streamer corona is identified by the electron avalanche, which is formed initially at the tip of the rod electrode with the highest electric field, and the effect of secondary ionization. Moreover, it is verified that the measured electron burst drift time with laser irradiation in the discharge gap is determined by the electric field of the discharge gap. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 140(2): 10–17, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10007

Modulation of auroral field-aligned electron fluxes under two inverted-V structures at different altitudes

[1] This paper presents observations of field-aligned electron flux bursts from the auroral sounding rocket flight, Alaska 2000, launched February 25, 2000 from Poker Flat, Alaska. In one particular energetic electron burst, the field-aligned electron flux is observed to be modulated at 22–30 Hz at energies up to 30 keV. By assuming that the observed energy dispersion in the bursts is caused by time-of-flight effects, we obtain characteristic source altitudes of 87 dispersive electron bursts, and find that the average characteristic source altitude of the bursts is 4015 ± 684 km in the first inverted-V and 5411 ± 972 km in the second inverted-V. This result implies that the electron-wave or other interaction depends more strongly on which arc it occurs in, rather than another factor such as energy of the electrons, since dispersive bursts at a wide range of energies and durations were used to arrive at the result.

Formation of reflected electron bursts by the nonstationarity and nonuniformity of a collisionless shock front

The impact on electron dynamics of the nonstationarity and nonuniformity of a quasi‐perpendicular supercritical collisionless planar shock is analyzed by means of a two‐dimensional full‐particle electromagnetic simulation code. Trajectories of preselected self‐consistent electrons (in contrast with the test particles approach) have been analyzed in order to determine the reflection conditions. Four different classes of reflected electrons have been identified according to the time these spent within the shock front itself before being reinjected into the upstream region. However, the shock front is revealed to be strongly nonstationary and nonuniform (front rippling). Different sources of shock front nonstationarity are shown to take place over different time and spatial scales. The longer time scale is due to the shock front reformation associated to the dynamics of reflected ions. The smaller time scale is due to the propagation of front rippling along the shock front. One key result is that electrons hitting the shock front are not reflected uniformly in time; instead, bursts of energetic reflected electrons are formed by the shock front reformation. In addition, electrons are not reflected uniformly in space, but packs of reflected electrons are formed along the rippled shock front. Comparison of two‐dimensional with one‐dimensional simulation results evidences that nonuniformity (rippling) of the shock front contributes to diffuse electron bursty patterns both in real and velocities space. Persistent (one‐dimensional) and more diffuse (two‐dimensional) electron bursts have different impacts on upstream wave emission, which are also discussed.

A pulsating auroral X-ray hot spot on Jupiter.

Jupiter's X-ray aurora has been thought to be excited by energetic sulphur and oxygen ions precipitating from the inner magnetosphere into the planet's polar regions. Here we report high-spatial-resolution observations that demonstrate that most of Jupiter's northern auroral X-rays come from a 'hot spot' located significantly poleward of the latitudes connected to the inner magnetosphere. The hot spot seems to be fixed in magnetic latitude and longitude and occurs in a region where anomalous infrared and ultraviolet emissions have also been observed. We infer from the data that the particles that excite the aurora originate in the outer magnetosphere. The hot spot X-rays pulsate with an approximately 45-min period, a period similar to that reported for high-latitude radio and energetic electron bursts observed by near-Jupiter spacecraft. These results invalidate the idea that jovian auroral X-ray emissions are mainly excited by steady precipitation of energetic heavy ions from the inner magnetosphere. Instead, the X-rays seem to result from currently unexplained processes in the outer magnetosphere that produce highly localized and highly variable emissions over an extremely wide range of wavelengths.

Energetic particle bursts before the main substorm injection

We examine the relationship between the energetic electron bursts (15 – 30 s) observed before the main injection and the changes in the magnetic field during substorms on the CRRES. Pitch angle distrubution (PAD) of electron flux during these bursts and injections depends on the spacecraft position relative to the cross-tail current sheet and boundaries of energetic particle trapped region. Near the inner edge of plasma sheet (PS) the pancake PAD was observed. Inside the PS and near the outer energetic particle trapped boundary the cigar and isotropic PADs were observed. The lack of dispersion in some observed injection events are evidence of local acceleration. The burst onsets correlate with the magnetic bursts of ∂Bz/∂t with the time duration ∼ 2–4 s. It is likely that particles are accelerated by induced E field arising from these observed magnetic bursts.

Relation between optical emissions, particles, electric fields, and Alfvén waves in a multiple rayed arc

Velocities of rays in auroral arcs were used to infer the perpendicular electric fields above the acceleration region. Using rocket measurements of electron energy as a proxy for the high‐altitude potential, the high‐altitude perpendicular electric fields were calculated and found to be in good agreement with those derived from the ray motions. Additionally, a 0.6 Hz oscillating electric field at high altitude was postulated on the basis of the passing rays. Such a field was also calculated from the electron energy measurements and was found to be closely related to an Alfvén wave measured on the payload following a delay of 0.8 s. The measured electron energy flux agreed well with the auroral luminosity down to scale sizes of about 10 km. The combination of ground‐based imaging and the measured energy flux also allowed a determination of the lower border altitude of the arcs. They were found to be somewhat higher (130 km) than expected on the basis of the electron energy. A tall rayed arc with a lower border height of 170 km was associated with a burst of suprathermal electrons on the poleward edge of the aurora.

Alfvén wave generated electron time dispersion

The results from a model of kinetic Alfvén waves which includes varying magnetic field and density show that time‐dispersed bursts of auroral electrons can be accelerated by Alfvén wave pulses propagating from the magnetosphere to the ionosphere. The modeled electron signatures have similar energy range and temporal structure to those observed on sounding rockets and satellites suggesting that electron time dispersion is generated by Alfvén waves.

Magnetoinertial Oscillations of Jupiter’s Inner Radiation Belt

Quasi-periodic 40 minute (QP-40) bursts of both relativistic electrons (energy E ~ 10 MeV) and radio emissions (frequency ν 700 kHz) from the south Jovian pole were detected by the instrument onboard the Ulysses spacecraft at high jovigraphic latitudes (~30-40 S°). We show here that Jupiter's inner radiation belt (IRB) may oscillate globally with a quasi-period of ~40 minutes or so and propose that, as results of asymmetries and small-scale circumpolar magnetic irregularities/anomalies, a fraction of relativistic electrons normally trapped within the IRB may go astray from a roughly circular zone surrounding the Jovian magnetic pole during certain phases of such IRB magnetoinertial oscillations. The escaped relativistic electrons rapidly convert their transverse motions to (anti)parallel motions along the magnetic field lines by the mirror force. The observed QP-40 radio bursts are closely associated with such QP-40 leaks of relativistic electrons in near-antiparallel motions. Our prediction for QP-40 magnetoinertial oscillations of the IRB can be tested by directly observing variations of IRB decimetric brightness using ground-based radio telescopes, especially upon the arrival of high-speed solar winds at Jupiter. Several relevant aspects of this scenario are also discussed.

Energetic ion (>∼50keV) and electron (>∼40keV) bursts observed by Ulysses near Jupiter

A careful analysis of data collected by the HISCALE experiment on board Ulysses suggests that the quasi-periodic (QP) modulation of several or tens of minutes in flux and/or anisotropy/spectral observations is an almost permanent characteristic of the energetic (>∼50 keV) ion population in the outer and the high latitude middle magnetosphere of Jupiter. In most cases a periodicity of ∼5–20 min is evident, although periodicities of ∼40 min and ∼80 min were also observed. The ∼40 min and ∼80 min periodicities were often accompanied by other harmonics, for instance ∼20 min and ∼40 min periodicities respectively, and were preferentially observed in the dusk magnetosphere. Observations of QP series of energetic ion and electron bursts in the dusk high latitude magnetosphere, as for instance, strong anisotropy, forward velocity dispersion and time delay between the ion and the electron onsets, are consistent with a periodic acceleration / injection of particles, in the form of field-aligned beams, originated from the low altitude environment of the planet (∼8 R J from the planet surface); the QP intensity gradients perpendicular to the field observed in the dusk middle magnetosphere are consistent with spatial structures of energetic particle sheets crossed by the spacecraft. The QP bursts of energetic particles are often accompanied by bursts of radio and aurora emissions and are possible sources of these Jovian phenomena.

Acceleration signatures in the dayside boundary layer and the cusp

Freja data show various electron acceleration signatures in the cusp and the dayside boundary layer: (1) time dispersive super-Alfvénic electrons followed by strong wave activity which accompanies transient downward super-thermal electron burst in both the boundary layer and the cusp; (2) quasi-stationary bidirectional electron burst coinciding with localized intense field-aligned current in the boundary layer; (3) downgoing electron burst without visible time dispersion in the cusp; and (4) thermal electrons accelerated by electrostatic potential in both the boundary layer and the cusp. The first and last signatures are different between two regions for typical energies and fluxes, and these differences probably reflect the different auroral emission in the cusp proper (red) and the boundary layer (green). Contributions of these electrons to the large-scale field-aligned currents are also different between two regions. The dispersed electron burst is probably accelerated within 1 Re above the ionosphere. From this result we believe that the cusp red aurora is caused mainly by accelerated electrons, but not by the smoothly entering magnetosheath electrons without acceleration. This also requires revisions of flux transfer event models for the structured cusp red aurora.

FAST Observations of inertial alfvén waves and electron acceleration in the dayside aurora

A prevailing feature of small scale (k⊥ c/ωpe ∼≥1) Alfven waves observed from the FAST spacecraft is an association of the impulsive wavefield with bursts of field-aligned electrons with energies of the order of 12meV2A. The accelerated distributions have broad energy spectra up to a few 100 eV and are predominantly directed downwards although upward accelerated beams are sometimes observed. Observations show that the direction of acceleration is correlated with the direction of the wave Poynting flux and anti-correlated with the direction of the field-aligned current carried by the wave. The accelerated portion of the distribution in some cases carries all the field-aligned current since the thermal population is depleted. These observations are compared with the predictions of a 1-D MHD model (with an inertial correction) (Chaston, 2000) and a fluid-kinetic model (Clark and Seyler, 1999) for waveforms where k⊥ c/ωpe ∼1 and k⊥ c/ωpe>>1 respectively.

Near-Equatorial Electrons as Measured onboard the MirSpace Station

Some results of studying the electrons with energies of tens to hundreds of keV at the low and near- equatorial geomagnetic latitudes by using the instruments Sprut-V and Ryabina-2 onboard the Mirspace station in 1991 are presented. It is found that at L< 1.2 the enhanced electron fluxes are sporadically detected, being localized within three longitudinal intervals, 180° W–0°–15° E, 90°– 120° E, and 160° E–180°–135° W. The most intense electron fluxes are observed at the lower edge of the near-equatorial boundary of the inner radiation belt on longitudes of the South Atlantic Anomaly between 14 and 20 h MLT. The occurrence of electron bursts does not depend on the geomagnetic disturbance level. A hardening of the electron spectra is observed near the geomagnetic equator. At L< 1.1, the more energetic particles are located closer to the geomagnetic equator. The results are compared with the data on the low-frequency waves and fields at low and near-equatorial latitudes obtained by the Ariel-4and San Marco Dsatellites, as well as by the spacecraft and ground-based observations of the thunderstorm global distribution. The thunderstorms are considered as a possible source of electron production near the geomagnetic equator.

Low‐charge‐state heavy ions upstream of Earth's bow shock and sunward flux of ionospheric O+1, N+1, and O+2 ions: Geotail observations

Energetic ∼10–210 keV/e low‐charge‐state heavy ions (LCSHI) were measured sunward of Earth's bow shock (to X ∼ 30 RE) during numerous intervals lasting from minutes to hours using Geotail/STICS in 1995–1998. LCSHI fluxes are strong and continuous in a few tens of intervals, during which a strong component of ionospheric origin O+1, N+1, and O+2 streams sunward on nearly radial IMF. Most often though only ‘trace’ levels are present. LCSHI flux is typically, but not exclusively, observed during diffuse upstream ion events. LCSHI are accompanied by sunward energetic electron bursts in three of the four cases shown and twice by enhanced IMF fluctuations. As O+1 streams sunward in the spacecraft frame during the strongest case, H+, He+2, He+1, and O+6 fluxes have weak anisotropies and the H+ energy spectrum is kappa‐like. The strongest LCSHI fluxes tend to occur duskward of local noon during disturbed geomagnetic conditions. On average, LCSHI flux is more uniformly distributed across the dayside.

SAMPEX observations of precipitation bursts in the outer radiation belt

The occurrence frequency of precipitation bursts of &gt; 1 MeV electrons in the outer radiation belt is examined using data from the SAMPEX satellite. Electron burst characteristics shown in this paper include the dependence of the precipitation on magnetic local time, radial distance and geomagnetic activity. Precipitation bursts with timescales &lt;1 s, i.e., microbursts, are studied in detail, including their dependence on the phases of geomagnetic storms. It is found that precipitation bursts occur typically in the region between L = 4 and L = 6. Microbursts tend to occur at L lower than the bursts with timescales of several tens of seconds. The number of observed microbursts significantly increases during storms, appearing mainly in the morning sector early in the recovery phase of storms. These findings suggest that the microbursts may be due to interactions with electron whistler waves, which take place near the dawnside plasmapause in the density irregularities that are perhaps created in the “recovering” plasmasphere. The prevalence of bursty precipitation indicates that this enhanced loss component of the relativistic electron flux should be taken into account in any quantitative model of relativistic electron acceleration processes.

Time‐dependent transport of field‐aligned bursts of electrons in flickering aurora

We have developed a time‐dependent auroral electron transport model to study emission rates caused by field‐aligned bursts of electrons (FABs) seen in flickering aurora. We simulate flickering FABs by turning on and off a downward electron intensity distribution at a given frequency. We assume this electron beam originates and is modulated at an altitude of 4000 km. We apply collisionless transport from 4000 to 600 km and solve a time‐dependent Boltzmann equation below 600 km. Because FABs have significant flux over a large energy range, dispersion has the most important effect on the resulting emission rates. We find that for a 5 Hz flickering FAB, the column emission rate varies 93% from peak to valley, whereas for 100 Hz flickering, the variation in column emission rate is only 12% from peak to valley. This variation is dependent on the frequency and source altitude. We show that with a time‐dependent transport calculation and a filtered fast photometer or imager looking in the zenith, it is possible to obtain an upper limit on the altitude from which the optical flickering originates. We also study what electron detectors on a rocket or satellite might measure in the lower ionosphere when there exists field‐aligned bursts of electrons. Velocity dispersion calculations will give source altitudes much lower than is correct if they are derived from low energy electrons (&lt;2 keV) measured at altitudes below 150 km. Our results agree with the interpretation that field‐aligned bursts are a temporal rather than spatial feature, and from this knowledge it should be possible to reconstruct the initial electron distribution function at the source altitude.

Generation of ELF electromagnetic waves in the ionosphere by localized transverse dc electric fields: Subcyclotron frequency regime

It is commonly believed that Alfvénic waves observed in the ionosphere originated at trans ionospheric altitudes. However, recent observations of waves localized over skin depth scales, which are prone to Landau damping, and upward going Poynting flux suggest an ionospheric source. This theoretical study establishes the generation of electromagnetic waves at subcyclotron frequencies by localized static electric fields of the type commonly observed in the auroral ionosphere. The problem is formulated in terms of an eigenvalue system of equations which can describe all cold plasma normal modes, ion acoustic waves, and the various mode couplings induced by nonuniform electric fields. It is found that the velocity shear associated with the background electric field can significantly affect the observable properties of Alfvén waves. In particular, Alfvén waves can be destabilized when the magnitude of the velocity shear frequency exceeds the ion cyclotron frequency. Velocity shear can also significantly modify the ratio of E/B for these waves. The analysis further reveals electromagnetic Kelvin‐Helmholtz instabilities, which are non‐Alfvénic, with lower velocity shear thresholds. We model conditions encountered by several rockets and satellites, for example, FAST, Preja, AMICIST, AT2, to analyze wave properties and find that for typical ionospheric parameters the Kelvin‐Helmholtz instabilities can generate electromagnetic waves with broadbanded spectra and other physical characteristics similar to observations. It is also shown that these waves are capable of resonant interaction with electrons over localized regions and hence may be important to understanding the generation of suprathermal electron bursts.

Effective recombination coefficient in the lower ionosphere during bursts of auroral electrons

Sudden ionospheric electron density increases, associated with impulsive electron precipitation bursts and followed by decays, are observed by the EISCAT incoherent scatter radar. The effective recombination coefficient and the ratio of the NO + and O 2 + ion concentrations are estimated from the variations by use of the Sodankylä Ion Chemistry model. Recombination coefficients obtained previously from the same data by a simpler approach are compared with the present results.

Escape into vacuum of fast electrons generated by oblique incidence of an ultrashort, high-power laser pulse on a solid target

The possibility that fast electrons can escape in a direction close to the trajectory of a reflected ultrashort laser pulse at extremely high laser radiation fluxes is examined analytically and numerically. Analytic estimates are made of the feasibility of forming electron bursts in the plasma and of their subsequent motion. The self-consistent, collisionless motion of a plasma acted on by specified incident and reflected ultrashort laser pulses is modeled in two dimensions by the particle-in-cell method. It is shown that a substantial number of electrons located in the subcritical region are gathered into bunches by the resultant forces and escape to the vacuum in a direction different from the normal to the target surface within a narrow range of solid angles. This demonstrates the feasibility of laser acceleration of an electron burst during reflection of an ultrashort laser pulse from a solid target.

Electron beam formation by small‐scale oblique inertial Alfvén waves

Particle‐in‐cell simulations reveal characteristics of small‐scale nearly perpendicular nonlinear inertial Alfvén waves in the electrostatic limit, during wave steepening and electron beam generation. These electrostatic inertial Alfvén waves (ESIAW) propagate with only weak electron Landau damping for Ti/Te ≫ 1. Nonlinear amplitude initial conditions convectively steepen and evolve higher wavenumber structures similar in morphology and magnitude to previous results from a fluid‐kinetic model. Shorter wavelength acoustic waves emergent during steepened ESIAW wave evolution are found to be instrumental in exciting cold electron beams possessing kinetic energies a few hundred times that of the thermal electron population. This mechanism is one possible explanation for suprathermal electron bursts observed in and near auroral inverted‐V events.

Particle physics with petawatt class lasers

With a Petawatt class CPA laser of the LLNL Livermore or the proposed GSI (Darmstadt) type laser, interactions with matter can be studied in the upper 1020 W/cm2 regime. With such a laser focused into an underdense plasma, strong electron bursts with energies up to several 100 MeV are ejected in a forward direction. This leads to a comparable burst of bremsstrahlung radiation in the presence of high-Z material. Here, we discuss the corresponding γ-induced nuclear reactions including secondary particle production, including pions. Due to the threshold behavior in the production, and the advantage of delayed detection, we propose to employ these reactions for probing the initial plasma conditions.