Generation of kinetic Alfvén waves by velocity shear instability on auroral field lines

Ion beams observed in the plasma sheet boundary layer (PSBL), cusp, and on the auroral zone field lines are expected to have spatial gradients in their drift velocity. Generation of kinetic Alfvén waves by velocity shear of the ion beams is discussed. It is shown that a hot ion beam can excite both a resonant kinetic Alfvén wave instability and a non-resonant coupled Alfvén-ion acoustic instability. For typical parameters, observed on the auroral field lines in the altitude range of 5–7 R_E (where R_E is the Earth’s radius), the frequency of the velocity shear modes, in the satellite frame of reference, lie in the ultra-low frequency (ULF) range. The noise due to velocity shear driven Alfvén modes is electromagnetic in nature, and has a finite parallel electric field component.     

Standing Alfvén waves with m ≫ 1 in a dipole magnetosphere with moving plasma and aurorae

The structure of standing Alfvén waves with large azimuthal wave numbers (m ? 1) is studied in a dipole model of the magnetosphere with rotating plasma. In the direction across magnetic shells the structure of such waves is determined by their dispersion associated with curvature of geomagnetic field lines and corresponds to the travelling wave localized between toroidal and poloidal resonant surfaces. In projection into the ionosphere (along geomagnetic field lines) this structure is similar to the structure of a discrete auroral arc. The azimuthal structure of an auroral arc is similar to azimuthal structure of Alfvén waves with m ∼ 100. Possible interaction mechanisms between the Alfvén waves and energetic electron fluxes forming auroral arcs are discussed.     

Theoretical model of steady-state magnetic reconnection in collisionless incompressible plasma based on the Grad–Shafranov equation solution

The problem of steady-state magnetic reconnection in an infinite current layer in collisionless, incompressible, nonresistive plasma, except of the electron diffusion region, is examined analytically using the electron Hall magnetohydrodynamics approach. It is found that this approach allows reducing the problem to the magnetic field potential finding, while last one has to satisfy the Grad–Shafranov equation. The obtained solution demonstrates all essential Hall reconnection features, namely proton acceleration up to Alfvén velocities, the forming of Hall current systems and the magnetic field structure expected. It turns out that the necessary condition of steady-state reconnection to exist is an electric field potential jump across the electron diffusion region and the separatrices. Besides, the powerful mechanism of electron acceleration in X-line direction is required. It must accelerate electrons up to the electron Alfvén velocity inside the diffusion region and on the separatrixes. This is a necessary condition for steady-state reconnection as well.     

Study of the solar Slow Sonic,Alfvén and Fast Magnetosonic transition surfaces

In this paper we study the shape, extend and time variations of the solar wind transition surfaces using the Lima and Priest (1993) hydrodynamic model adequately adapted for the case of the solar wind flow. The transition surfaces, namely the Slow (Sonic), the Alfvén, and the Fast Magnetosonic surface, are important boundaries around the Sun and play a crucial role in the development of the solar wind and the structure of the inner heliosphere. We determine the shape and dimension of these surfaces as a function of heliographic latitude using measurements from Ulysses spacecraft, and we also study their temporal variation using data from spacecrafts at 1 AU (OMNI database). Furthermore, we establish their dependence with the solar activity, demonstrating their shape and location for the last two solar cycles. From this we noticed that the temporal variation of all transition surfaces follows the 11-year solar cycle. Finally, from the OMNI database, we have studied the temporal variation over the past 40 years of the plasma β parameter, the kinetic to magnetic and the kinetic to thermal energy ratios, at a distance of 1 AU from the Sun.     

The Io-control of Jupiter's decametric radiation: The Alfvén wave model

Geometrical considerations based on a conical sheet model of the observed DAM sources show that DAM is generated on field lines in the active longitude sector φ ～ 200 ± 90°. We show how Io excites Alfvén waves and calculate the propagation of these waves through the inhomogenous torus and magnetosphere. The power flux at high latitudes is largest at two longitudes which are seen as the B₁, B₂, A and C sources. We also discuss the parallel electric field accompanying the Alfvén wave pulses and show that at high latitudes electrons can be accelerated to energies in excess of 1000 eV. It is suggested that these current carrying electrons excite ion-cyclotron, upper and lower-hybrid electrostatic waves which may all play crucial roles in the generation of DAM.     

Alfvén waves as a driving mechanism in stellar winds

Alfvén waves have been invoked as an important mechanism of particle acceleration in stellar winds of cool stars. After their identification in the solar wind they started to be studied in winds of stars located in different regions of the HR diagram. We discuss here some characteristics of these waves and we present a direct application in the acceleration of late-type stellar winds.     

Heating heavy ions in the polar corona by collisionless shocks: A one-dimensional simulation

Recently a new model for explaining the observations of preferential heating of heavy ions in the polar solar corona was proposed ( and ). In that model the ion energization mechanism is the ion reflection off supercritical quasi-perpendicular collisionless shocks in the corona and the subsequent acceleration by the motional electric field E = −V × B/c. The mechanism of heavy ion reflection is based on ion gyration in the magnetic overshoot of the shock. The acceleration due to the motional electric field is perpendicular to the magnetic field, giving rise to large temperature anisotropy with T_⊥ ? T_∥, in agreement with SoHO observations. Such a model is tested here by means of a one dimensional test particle simulation where ions are launched toward electric and magnetic profiles representing the shock transition. We study the dynamics of O⁵⁺, as representative of coronal heavy ions for Alfvénic Mach numbers of 2–4, as appropriate to solar corona. It is found that O⁵⁺ ions are easily reflected and gain more than mass proportional energy with respect to protons.     

Investigation of natural and artificial stimulation of the ionospheric Alfvén resonator at high latitude

A brief review is provided of recent progress in understanding the ionospheric Alfvén resonator (IAR) at high latitude. Firstly, naturally occurring resonances of the IAR as detected by pulsation magnetometers in the auroral zone at Sodankylä and in the polar cap at Barentsburg are considered. The characteristics of the IAR in the two regions are broadly similar, although the effects of solar illumination are less clear at the higher latitudes. Secondly we review recent attempts to stimulate the IAR through high-power radio frequency experiments both in the auroral zone at Tromsø with the European Incoherent SCATter (EISCAT) heater, and within the polar cap at Longyearbyen with the Space Plasma Exploration by Active Radar (SPEAR) facility. In the auroral zone at, Tromsø the stimulated IAR has been observed by ground-based magnetometers, and through electron acceleration observed on the FAST spacecraft. At SPEAR in the polar cap, the stimulated IAR has been investigated, with ground magnetometers, with the first results indicative of a positive detection.     

Ion dynamics associated with Alfven wave in the near-Earth magnetotail: Two-dimensional global hybrid simulation

Ion dynamics in the near-Earth magnetotail region is examined during periods of fast Earthward flow with a two-dimensional (2-D) global-scale hybrid simulation. The simulation shows that shear Alfven waves are generated at x ∼ −10R_E, where the strong earthward flow is arrested by the dipole field, and propagate along field lines from the equator to both southern and northern polar ionosphere. Non-gyrotropic ion velocity distributions occur where the large-amplitude Alfven waves are dominant. The simulation indicates that the Alfven waves are generated by interaction of the fast earthward flow with the stationary near-Earth plasma. Beam ions are found to be pitch-angle scattered and trapped in the wave field, leading to the non-gyrotropic ion distributions in the high-latitude plasma sheet boundary. In addition, significant particle heating and acceleration are found to occur behind the dipolarization front due to the effect of wave turbulence.     

Generation mechanism of the whistler-mode waves in the plasma sheet prior to magnetic reconnection

The whistler-mode waves and electron temperature anisotropy play a key role prior to and during magnetic reconnection. On August 21, 2002, the Cluster spacecrafts encountered a quasi-collisionless magnetic reconnection event when they crossed the plasma sheet. Prior to the southward turning of magnetospheric magnetic field and high speed ion flow, the whistler-mode waves and positive electron temperature anisotropy are simultaneously observed. Theoretic analysis shows that the electrons with positive temperature anisotropy can excite the whistler-mode waves via cyclotron resonances. Using the data of particles and magnetic field, we estimated the whistler-mode wave growth rate and the ratio of whistler-mode growth rate to wave frequency. They are 0.0016f_ce (Electron cyclotron frequency) and 0.0086f_ce, respectively. Therefore the whistler-mode waves can grow quickly in the current sheet. The combined observations of energetic electron beams and waves show that after the southward turning of magnetic field, energetic electrons in the reconnection process are accelerated by the whistler-mode waves.     

New aspects of whistler waves driven by an electron beam studied by a 3-D electromagnetic code

We have restudied electron beam driven whistler waves with a 3-D electromagnetic particle code. In the initialisation of the beam-plasma system, “quiet start” conditions were approached by including the poloidal magnetic field due to the current carried by beam electrons streaming along a background magnetic field. The simulation results show electromagnetic whistler wave emissions and electrostatic beam modes like those observed in the Spacelab 2 electron beam experiment. It has been suggested in the past that the spatial bunching of beam electrons associated with the beam mode may directly generate whistler waves. However, the simulation results indicate several inconsistencies with this picture: (1) the parallel (to the background magnetic field) wavelength of the whistler wave is longer than that of the beam instability, (2) the parallel phase velocity of the whistler wave is smaller than that of the beam mode, and (3) whistler waves continue to be generated even after the beam mode space charge modulation looses its coherence. The complex structure of the whistler waves in the vicinity of the beam suggest that the transverse motion (gyration) of the beam and background electrons is involved in the generation of the whistler waves.     

Ballooning perturbations in the inner magnetosphere of the Earth: Spectrum,stability and eigenmode analysis

We investigate the generation of ballooning perturbations in the inner magnetosphere of the Earth in the dipole model of the geomagnetic field taking into account ionospheric boundary conditions. The ionosphere is considered as a thin layer with finite conductivity. The eigenmode spectrum is discrete and consists of Alfvén, slow magnetosonic, flute and incompressible modes. Their interaction depends on ionospheric conductivity. The decay rate is small in noon and night sectors and large in dawn and dusk sectors. The lowest stability threshold α/γ ≈ 4.25 is determined by flute modes.     

Cluster observations showing the indication of the formation of a modified-two-stream instability in the geomagnetic tail

This study presents several observations of the Cluster spacecraft on September 24, 2003 around 15:10 UT, which show necessary prerequisites and consequences for the formation of the so-called modified-two-stream instability (MTSI). Theoretical studies suggest that the plasma is MTSI unstable if (1) a relative drift of electrons and ions is present, which exceeds the Alfvèn speed, and (2) this relative drift or current is in the cross-field direction. As consequences of the formation of a MTSI one expects to observe (1) a field-aligned electron beam, (2) heating of the plasma, and (3) an enhancement in the B-wave spectrum at frequencies in the range of the lower-hybrid-frequency (LHF). In this study we use prime parameter data of the CIS and PEACE instruments onboard the Cluster spacecraft to verify the drift velocities of ions and electrons, FGM data to calculate the expected LHF and Alfvèn velocity, and the direction of the current. The B-wave spectrum is recorded by the STAFF instrument of Cluster. Finally, a field aligned beam of electrons is observed by 3D measurements of the IES instrument of the RAPID unit. Observations are verified using a theoretical model showing the build-up of a MTSI under the given circumstances.     

Electron beam experiments in Japanese satellites and rockets

Electron beam experiments in space that have been done and planned in Japan are reviewed. 200eV, 1mA electron beam is emitted from a satellite and several types of wave excitation such as UHF and ω_ce have been observed. The satellite potential and the energy spectrum of returning electrons are measured by Langmuir probes and electrostatic energy analyser. In rocket experiments of K-10-11, K-10-12, K-9M-57, K-9M-58, K-9M-61 and K-9M-66, several types of electron guns were used whose power ranges from 1mW to 1KW. The rocket potential was measured by Langmuir probes and floating probes and optical line emission measurement and wave measurements were also done. The rocket potential was not so high as expected from the balance with ionospheric plasma but strongly affected by the plasma production by the emitted electron beam and return electrons.     

Theoretical properties of offset bipolar electric field solitary structures in space plasmas

The bipolar electric field solitary (EFS) structures have been frequently observed in the near Earth plasma regions, such as auroral zone, magnetopause, cusp regions, and magneto-tail. Sometimes these structures are observed as offset bipolar structures. In this paper, the properties of the offset bipolar EFS structures parallel to the magnetic field are studied with an ion fluid model in a cylindrical symmetry by considering electrostatic condition. The model results show that the offset bipolar EFS structures can develop from both ion-acoustic waves and ion cyclotron waves, and propagate along the magnetic field line in the space plasmas if plasma satisfies some conditions. The offset bipolar EFS structures can have both polarities. It will be first negative pulse and then positive pulse if the initial electric field E₀ < 0 or reverse in order if E₀ > 0. The amplitude of the offset bipolar EFS structures first decreases and then increases with the wave propagation velocity. Some results from our model are consistent with the observations.     

Long-term changes in indices of geomagnetic activity at the auroral station Sodankylä

Here we compare the traditional analog measure of geomagnetic activity, Ak, with the more recent digital indices of IHV and Ah based on hourly mean data, and their derivatives at the auroral station Sodankylä. By this selection of indices we study the effects of (i) analog vs. digital technique, and (ii) full local-time vs. local night-time coverage on quantifying local geomagnetic activity. We find that all other indices are stronger than Ak during the low-activity cycles 15–16 suggesting an excess of very low scalings in Ak at this time. The full-day indices consistently depict stronger correlation with the interplanetary magnetic field strength, while the night-time indices have higher correlation with solar wind speed. The Ak index correlates better with the digital indices of full-day coverage than with any night-time index. However, Ak depicts somewhat higher activity levels than the digital full-day indices in the declining phase of the solar cycle, indicating that, due to their different sampling rates, the latter indices are less sensitive to high-frequency variations driven by the Alfvén waves in high-speed streams. On the other hand, the night-time indices have an even stronger response to solar wind speed than Ak. The results strongly indicate that at auroral latitudes, geomagnetic indices with different local time coverage reflect different current systems, which, by an appropriate choice of indices, allows studying the century-scale dynamics of these currents separately.     

Thermonuclear flashes on magnetized neutron stars as a model for gamma-ray bursts

We propose, as a model for gamma ray bursts, that the energy released by a thermonuclear runaway at the surface of a slowly accreting, magnetized neutron star, is converted into gamma ray photons by small scale magnetic field reconnection. Most of the energy flux is transported up to the atmosphere by Alfvén waves, driven by oscillatory magnetoconvection. The electric field created by reconnection accelerates electrons to very high energies. The excitation of Landau levels by collisions is followed by gamma ray synchrotron emission. The time scales, energies, and luminosities involved agree with the observations.     

Acceleration of charged particles by fluctuating and steady electric fields in a X-type magnetic field

Release of stored magnetic energy via particle acceleration is a characteristic feature of astrophysical plasmas. Magnetic reconnection is one of the mechanisms for releasing energy from magnetized plasmas. Collisionless magnetic reconnection could provide both the energy release mechanism and the particle accelerator in space plasmas. Here we studied particle acceleration when fluctuating (in-time) electric fields are superposed on an static X-type magnetic field in collisionless hot solar plasma. This system is chosen to mimic the reconnective dissipation of a linear MHD disturbance. Our results are compared to particle acceleration from constant electric field superposed on an X-type magnetic field. The constant electric field configuration represents the effects of steady state magnetic reconnection. Time evolution of ion and electron distributions are obtained by numerically integrating particle trajectories. The frequencies of the electric field represent a turbulent range of waves. Depending on the frequency and amplitude of the electric field, electrons and ions are accelerated to different degrees and have energy distributions of bimodal form consisting of a lower energy part and a high energy tail. For frequencies (ω in dimensioless units) in the range 0.5 ? ω ? 1.0 a substantial fraction (20%–30%) of the proton distribution is accelerated to gamma-ray producing energies. For frequencies in the range 1 ? ω ? 100.0 the bulk of the electron distribution is accelerated to hard X-ray producing energies. The acceleration mechanism is important for solar flares and solar noise storms but it could be applicable to all collisionless astrophysical plasmas.     

CRIT II electric, magnetic, and density observations in an ionizing neutral jet

Measurements inside a high velocity neutral barium beam show a factor of six increase in plasma density in a moving ionizing front. This region was co-located with intense electric fields (δE ≈ 300 mV/m²) perpendicular to the local geomagnetic field and field aligned currents all fluctuating at frequencies well under the lower hybrid frequency for barium but above the oxygen cycloton frequency. It was determined that these structures were moving with the barium stream near the neutral barium velocity. Large quasi-dc electric and magnetic field fluctuations were also detected. The heart of the ionizing front, a cross beam current of nearly 10 mA/m², was estimated from the magnetic field variation. This is three orders of magnitude higher than typical auroral zone currents associated with auroral arcs. This current sheet was co-located with fluxes of soft electrons which saturated the particle detector. An Alfvén wave with a finite electric field component parallel to the geomagnetic field was observed to propagate along B_o where it was detected by an instrumented sub-payload.