Field-aligned Electron Research Articles

<i>Letter to the Editor</i> Low-frequency electric field fluctuations and field-aligned electron beams around the edge of an auroral acceleration region

Abstract. Electron beams narrowly collimated to the magnetic field line were observed continuously from a down-ward current region to an auroral acceleration region (i.e., upward current region). They were well correlated with low-frequency electric field fluctuations in the auroral acceleration region as well as in the adjacent downward current region. Magnetic field fluctuations were found only in the downward current region. The analysis suggests that static field-aligned electric fields are not fully responsible for the filed-aligned electron acceleration; the ac electric field, presumably associated with Alfvenic fluctuations, should also be involved in the acceleration of ionospheric electrons.Key words. Ionosphere (particle acceleration) – Magnetospheric physics (auroral phenomena; magnetosphere-ionosphere interactions)

Alfven wave instability of current sheets in force-free plasmas: comparison to ion acoustic instability

Current sheets necessarily form at the interface between adjacent twisted flux tubes and so are ubiquitous to magnetic configurations having non-trivial field topology. In an earlier publication (P. M. Bellan, Phys. Rev. Letters, 83,4768, 1999) the author showed that current sheets in a low β plasma will be kinetically unstable with respect to Alfvén wave emission if the current sheet becomes sufficiently thin for the field-aligned electron flow to become super-Alfvenic. At first sight it might appear that before the current sheet becomes thin enough to develop a super-Alfvenic flow, ion acoustic instabilities will develop when the sheet becomes thin enough for the electron flow to exceed the ion acoustic velocity. However, it is shown here that because of strong ion Landau damping on ion acoustic waves for plasmas with comparable electron and ion temperatures, ion acoustic waves have a much higher instability threshold than the Alfvén instability. Thus, Alfvén instability should dominate ion acoustic instability in thin current sheets.

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.

Enhancement of optical aurora triggered by the solar wind negative pressure impulse (SI−)

A geomagnetic negative sudden impulse (SI−) occurred on 3 August 1997 in association with a sudden decrease of solar wind dynamic pressure. The discrete auroras observed by an all‐sky TV imager at Zhongshan in Antarctica (magnetic latitude is ∼74.5°S) suddenly enhanced their luminosity and moved poleward at the time of the SI−. Following the primary enhancement, three cycles of quasi‐periodic luminosity pulsations were observed with periods of about 10–14 min and their average position drifted equatorward. The luminosity pulsations showed one‐to‐one correspondence with the magnetic pulsations, with the characteristic features of a field‐line resonance. The low‐altitude DMSP‐F13 satellite was almost directly over Zhongshan and observed accelerated electron precipitation with an inverted ‘V' structure and an upward field‐aligned current. These signatures suggest that the solar wind negative pressure impulse triggered a field‐line resonance of the geomagnetic field. The resonance caused the upward and downward field‐aligned current sheets, and the field‐aligned electron acceleration.

High-Density Current Injection to Magnetically Confined Plasmas with a Miniature Plasma Source

Influences of high-density current injection with a miniature plasma source on a reversed-field pinch (RFP) have been studied in the Separatrix Test Experiment-2 (STE-2) device. The current injector has successfully produced a unidirectional field-aligned electron beam. No deterioration or slight improvement of the RFP discharge was observed with injection of poloidal current of up to 1 kA. The deepening of the local edge toroidal magnetic field reversal has been observed in the vicinity of the injection port. The global change in the current profile and the stabilization of m=1 tearing modes will require current injection at multiple toroidal locations with a higher bias voltage. However, the gas throughput from the injector is too high, hindering the addition of more injectors in the STE-2 device. Global current profile control experiments with multiple miniature plasma sources are expected in larger devices.

Alfvén Waves, Density Cavities and Electron Acceleration Observed from the FAST Spacecraft

Inertial Alfvén waves propagating in regions of auroral electron acceleration are observed from the FAST (Fast Auroral Snapshot) spacecraft over its entire altitude range (350–4180 km). These electron skin depth sized field structures are dispersive and carry an electric field component parallel to the geomagnetic field leading to a variety of non-linear effects including the formation of magnetic field-aligned density cavities and electron acceleration. High resolution measurements show that the electron distribution inside the impulsive wave field envelope or density cavity consists of a cold ionospheric component and an accelerated and heated field-aligned component comprised of downgoing and reflected ionospheric and magnetosheath (solar wind like) electrons. In large amplitude examples the plasma within the wave may be dominated by the accelerated component and the depletion of plasma within the cavity approaches 100%. Comparison of the observed density depletion with the predictions of ponderomotive density cavity formation in association with electron heating shows good agreement. Furthermore, simulations show that most of the observed features of the accelerated component can be explained through Landau resonance of the cold ionospheric and magnetosheath electrons with the inertial Alfvén wave as it propagates through an altitude dependent density profile.

Precipitation of energetic electrons in Io's atmosphere: Production of UV emissions

Abstract Recent observations of Io by Galileo spacecraft showed the presence of a dense ionosphere, which is identified by a cold dense plasma that is at rest with respect to Io, and a tentative possibility of an intrinsic magnetic field. Also, there are measurements of intense magnetic field-aligned energetic bidirectional electron beams of tens of keV in the wake region of Io. These energetic electrons may enter into the atmosphere of Io, where they can degrade their energy via collisions with the neutral atmospheric gases. A Monte Carlo model has been generated to study the spatial degradation of electrons of energy ≤ 10 keV in the SO 2 -dominated atmosphere of Io. This model is employed to study the excitation of Io's atmosphere. The intensities of neutral atomic oxygen (OI) and sulfur (SI) ultraviolet emissions due to the energetic electron impact on SO 2 are calculated and are compared with the Hubble Space Telescope-observed intensities and the intensities produced due to photoelectron impact excitation.

Potential formation triggered by field-aligned electron acceleration due to electron cyclotron resonance along diverging magnetic-field lines

Stationary potential drop and ion acceleration along diverging magnetic-field lines are observed in a fully ionized collisionless plasma flow in the presence of electron cyclotron resonance (ECR) heating (ECRH). Time-resolved measurements for the purpose of clarifying the potential-formation dynamics show that temporal evolutions of the potential profile and electron energy distribution function correlate well with each other. The strong potential drop is found to be caused by a field-aligned electron acceleration due to ECRH, being accompanied by an effective ion acceleration so as to maintain the charge neutrality condition in the downstream region from the ECR point.

Substorms in the inner plasma sheet

Thin Current Sheets (TCS) are regularly formed prior to substorm breakup, even in the near-Earth plasma sheet, as close as the geostationary orbit. A self-consistent kinetic theory describing the response of the plasma sheet to an electromagnetic perturbation is given. This perturbation corresponds to an external forcing, for instance caused by the solar wind (not an internal instability). The equilibrium of the configuration of this TCS in the presence of a time varying perturbation is shown to produce a strong parallel thermal anisotropy ( T ‖ ≫ T ⊥) of energetic electrons and ions (E>50keV) as well as an enhanced diamagnetic current carried by low energy ions (E<50keV). Both currents tend to enhance the confinement of this current sheet near the magnetic equator. These results are compared with data gathered by GEOS-2 at the geostationary orbit, where the magnetic signatures of TCS, and parallel anisotropics are regularly observed prior to breakup. By ensuring quasi-neutrality everywhere we find, when low frequency electromagnetic perturbations are applied, that although the magnetic field line remains an equipotential to the lowest order in T e/ T i, a field-aligned potential drop exists to the next order in ( T e/ T i). Thus the development of a TCS implies the formation of a field-aligned potential drop (⋍ few hundred volts) to ensure the quasi-neutrality everywhere. For an earthward directed pressure gradient, a field-aligned electric field, directed towards the ionosphere, is obtained, on the western edge of the perturbation (i.e. western edge of the current sheet). Thus field aligned beams of electrons are expected to flow towards the equatorial region on the western edge of the current sheet. We study the stability of these electron beams and show that they are unstable to “High Frequency” (HF) waves. These “HF” waves are regularly observed at frequencies of the order of the proton gyrofrequency (f H+) just before, or at breakup. The amplitude of these HF waves is so large that they can produce a strong pitch-angle diffusion of energetic ions and a spatial diffusion that leads to a reduction of the diamagnetic current. The signature of a fast ion diffusion is indeed regularly observed during the early breakup; it coincides with the sudden development of large amplitude transient fluctuations, ballooning modes, observed at much lower frequencies (f<f H+). These results suggest that the HF waves, generated by field-aligned electron beams, provide the dissipation which is necessary to destabilize low frequency (ballooning) modes.

The second pair in the INTERBALL quartet: Some main results

The INTERBALL-2 satellite (AURORAL PROBE) was launched on August 29, 1996, about a year after the INTERBALL-1 and MAGION-4 pair already worked on a higher orbit. It performed comprehensive measurements of magnetospheric plasmas and fields mainly at altitudes 2 – 3 R E during more than two years. Some of the main findings from these measurements are briefly reviewed, in particular: 1. Discovery of long-duration high-energy electron dispersion events above the polar edge of the post-midnight auroral oval. 2. Discovery of high-energy (> 20 keV, i.e. super-auroral energy) upward moving field-aligned electron beams accompanied by bi-directional electrons ∼ 1 keV, also in these auroral regions. 3. Mass-resolved velocity-dispersed ion structures (VDIS) extended down to superthermal energies injected in the plasma flux tubes at the auroral oval polar border in a wide range of local times. 4. Mid-altitude ion distribution functions in the cusp showing a combination of downward moving, and mirror reflected, magnetosheath ions, with upward moving thermal and superthermal H +, He +, and O + ions of the “cleft ion fountain” starting from eV energy range. 5. Thermal and superthermal plasma outflows of the classic polar wind type (only H + and He +, no O +) as contrasted to conditions when additional acceleration mechanisms are operating which accelerate thermal heavy ions also (O +, etc.).

Multiple‐point electron measurements in a nightside auroral arc: Auroral turbulence II particle observations

We report here three‐point measurements of bursty, velocity‐dispersed, field‐aligned electron precipitation at the poleward edge of a northward‐moving, post‐breakup, nightside auroral arc. The three‐point measurement allows detection of the proper motion of the inverted‐V arc, which is shown to be 550 m/sec northward. The velocity dispersion patterns are fitted to find the source altitude of the precipitation bursts as a function of distance from the poleward edge of the arc. These source points are interpreted to trace out the low‐altitude boundary of the inverted‐V potential drop, which is seen to rise both in time, and in the northward direction. The precipitation bursts under the inverted‐V are seen to have an arc‐aligned velocity which varies with time, and which is consistent with the measured E × B local drift speed.

Auroral‐plasma sheet electron anisotropy

Observations from 90 passes through the high‐latitude extension of the Earth's plasma sheet made by the Hydra particle spectrometer on Polar show clear, repeatable structure in the electron pitch angle anisotropy. Two distinct regions are seen. The first region begins at the poleward edge of the plasma sheet and consists of alternating regions of field‐aligned electron beams moving away from the ionosphere and loss‐cone distributions indicative of electron acceleration and precipitation below the spacecraft. The second region begins equatorward of the first region and is characterized by a higher temperature loss‐cone distribution superimposed on lower temperature, field aligned population. With decreasing latitude, the boundary in energy between these two populations in the second region decreases until only the trapped, loss‐cone population remains. The two regions are identified as connecting to the discrete and diffuse auroral regions, respectively, and it is shown that the observed pitch angle anisotropies can be explained as arising from auroral processes below the spacecraft.

Comparison between the Landau and Cyclotron Resonances in the Electron Beam-Plasma Interactions

It is well-known that a cool field-aligned electron beam streaming in a plasma can destabilize electrostatic waves, in the frequency ranges of 0<ω< m i n (Ω e ,ω p ) and m a x (Ω e ,ω p )<ω<ω uh , where ω p , Ω e , and ω uh are the plasma, the electron cyclotron, and the upper-hybrid frequencies, respectively. Although both the Landau and cyclotron resonances lead to the generation of the waves, not much attention has been paid to the wave generation via the cyclotron resonance. In this paper we compare the two types of interactions for both non-relativistic and relativistic electron beams by considering a simple plasma model which consists of main and beam electrons and background ions. The plasma is charge neutral and current carrying. The linear theory based on this model predicts that, when the electron beams are non-relativistic, the waves are generated predominantly via the Landau interaction for a wide range of plasma parameters, except for a limited parameter regime with cool main electrons, Ω e /...

Exact Solution with Shock-Like Structure for Nonlinear Kinetic Alfvén Waves and Electron Acceleration

An analytical solution with shock-like structure for nonlinear kinetic Alfvén waves in a low-β (β << me/mi << 1) plasma is presented. It is showed that the oblique propagating nonlinear kinetic Alfvén wave can rarefy the plasma and lead to a local density jump over a characteristic width of 10 λe at the wave front, in particular, these new shock-like structure of nonlinear kinetic Alfvén waves possess a net field-aligned electric potential drop with a typical order of (2me/miβ)Te/e >> Te/e, as a consequence, they can effectively accelerate field-aligned electrons to velocities of order of the Alfvén velocity.

Plasma Potential Formation and Particle Acceleration Due to ECRH in Diverging Magnetic-Field Lines

A plasma potential formation is investigated in a fully ionized collisionless plasma flow in the presence of electron cyclotron resonance (ECR) heating (ECRH) under simple diverging magnetic-field configurations. When ECR takes place in a diverging region, there appears a strong potential drop along the field lines, which results from a field-aligned electron acceleration, being accompanied by an effective ion acceleration so as to maintain the charge neutrality condition in the downstream region. Time-resolved measurements are performed in order to clarify dynamics of the potential formation.

Simultaneous DMSP and CRRES observation of broadband electrons during a storm-time substorm on March 25, 1991

Broadband electrons observed by the DMSP satellites during storm-time substorms are characterized by an unusually intense, flux of precipitating electrons over the broadband energy range from 30 eV to 30 keV near the equatorward edge of the auroral oval (Shiokawa et al., 1996; 1997). During the broadband electron event observed by the DMSP-F9 satellite at 21 MLT on March 25, 1991, the CRRES spacecraft was at a geocentric distance of 6.3 RE and in the same local time as that of DMSP. The CRRES satellite observed 1) a large enhancement of field-aligned electron flux, 2) highly turbulent electric fields up to 10 mV/m, 3) intense west-ward excursion (∼100 nT) of magnetic field (suggesting intense field-aligned current generation) in a highly tail-like field configuration, 4) large O+ energy density, and 5) intense low-frequency (below 300 Hz) electrostatic waves. We discuss possible production mechanisms of broadband electrons on the basis of these observations.

Upward high-energy field-aligned electron beams above the polar edge of auroral oval: observations from the SKA-3 instruments onboard the Auroral Probe (Interball-2)

Abstract. A new phenomenon was found at the polar edge of the auroral oval in the postmidnight-morning sectors: field-aligned (FA) high-energy upward electron beams in the energy range 20–40 keV at altitudes about 3RE, accompanied by bidirectional electron FA beams of keV energy. The beam intensity often reaches more than 0.5·103 electrons/s·sr·keV·cm2, and the beams are observed for a relatively long time (~3·102–103s), when the satellite at the apogee moves slowly in the ILAT-MLT frame. A qualitative scenario of the acceleration mechanism is proposed, according to which the satellite is within a region of bidirectional acceleration where a stochastic FA acceleration is accomplished by waves with fluctuating FA electric field components in both directions.Key words. Ionosphere (particle acceleration; wave-particle interactions) · Magnetospheric physics (magnetosphere-ionosphere interactions)

Ion heating, electron acceleration, and the self‐consistent parallel E‐field in downward auroral current regions

Downward auroral current regions are characterized by: upflowing field‐aligned electrons; transverse ion heating; depleted plasma densities; downward‐pointing parallel E‐fields and current densities; intense ELF and VLF plasma wave activity; and the absence of hot magnetospheric particles. In this paper, we present a kinetic model that explains all of these features. We imagine a geomagnetic flux tube of finite length imbedded in a downward current region and impose appropriate kinetic boundary conditions. We assume that broadband ELF waves exist on the flux tube and heat the ions by cyclotron resonance near the ion gyrofrequency. We carry out an iterative, steady‐state solution for the one‐particle distribution functions, fα(s, ν⟂, ν∥), and the self‐consistent parallel E‐field, E∥(s). When this theory is applied to the Gorney et al. [1985] data set, we find good agreement between theory and experiment for the ion conic, the electron beam, and the downward‐pointing E∥(s).

A statistical study of field-aligned electron beams associated with ion conics events

Abstract. A statistical study of field-aligned electron beams associated with upflowing ion conics is presented from Exos-D (Akebono) observations below 10 000 km. The electron beams are narrowly collimated along the field line and generally have energies of several tens of eV. They are divided in the analysis into three types: upflowing, downflowing, and counterstreaming. All the types of electron beams are almost equivalent in their energy and pitch angle characteristics and their association rate with upflowing ion events. About 50% of ion conics are found to be coincident with field-aligned electron beams. There is also a correlation in energy between the field-aligned electrons and ion conics. These show that the association is not a mere coincidence but rather that the field-aligned acceleration of electrons is related to the perpendicular energization of ions. The association rate of upflowing electrons is high on the nightside, while that of downflowing electrons is high on the dayside. The association rate of downflowing electrons is high at low altitudes, and the rates of the three types of electron beams become equivalent at high altitudes. Data indicate that the height of the electron acceleration region is lower on the nightside. It is suggested that the average height of the electron acceleration region is around the satellite apogee (–10000 km), and the average thickness of the region is about thousands km.Key words. Ionosphere-magnetosphere interactions · Particle acceleration · Auroral phenomena

Electron modulation and ion cyclotron waves observed by FAST

New observations from the FAST satellite demonstrate strong wave‐particle interactions between energetic electrons and H+ EMIC waves in inverted‐V arcs. The intense waves are shown to occur in strong upward current regions which contain intense downgoing field‐aligned electron fluxes. Electrons near the inverted‐V spectral peak have large, factor of 2 to 10, coherent flux modulations at or near the wave frequency. The electron modulations are typically centered at about fCH+/2 where fCH+ is the local H+ cyclotron frequency. The EMIC waves are broadbanded, extending from about 0.3fCH+ to 0.7fCH+. These waves also accelerate cold secondary electrons, forming counterstreaming field‐aligned electrons at energies up to about 300 eV. In addition, electron modulations at fCH+ are observed in the density cavities associated with upgoing ion beams. Intense waves at fCH+ are simultaneously detected and shown to have a magnetic component similar to the EMIC waves.