Local Alfven Research Articles

Mode conversion into non‐MHD waves at the Alfvén layer: The case against the field line resonance concept

According to ideal magnetohydrodynamics (MHD), a magnetic field line in an inhomogeneous plasma becomes “resonant” when the condition ω = k‖vA is satisfied on the field line; here ω is the frequency of a perturbation, k| is the component of the wavenumber parallel to the magnetic field, and υA is the local Alfven velocity on the field line. This concept of field line resonance (FLR) has long been used to explain observed magnetospheric pulsations and, in particular, recent models have proposed that magnetospheric FLRs are excited by compressional MHD waves trapped in a surrounding cavity or waveguide. We present here a reexamination of this problem using an analysis based directly on the Maxwell‐Lorentz equations, a more fundamental description of plasma behavior than MHD. Our analysis shows that the ω = k‖υA layer is actually the location where an incident compressional MHD wave mode converts into a non‐MHD inertial electron Alfvén wave. We find that all fields are finite and regular at the ω = k‖υA layer and conclude that field line resonance does not exist in reality and so is an artifact of ideal MHD.

Dynamics of MHD wave propagation in the low‐latitude magnetosphere

The propagation of MHD waves depends on a local Alfven speed and ambient geometry. The dynamical properties of MHD waves in the plasmasphere and magnetosphere are investigated by assuming a realistic Alfven speed profile in a dipole field. The WKB approximation is used to determine the cutoff boundaries and estimate the wave dispersion over a whole meridional plane. It is found that most wave energy may be transmitted effectively into the inner magnetosphere near the equatorial region, since the reflection of incoming waves at the plasmapause becomes weakest at the equator. It is also examined how the transmission from the outer magnetosphere to the inner magnetosphere depends on wave frequencies and azimuthal wavenumbers. The cutoff boundaries of wave propagation are quantitatively determined for each mode and wavenumber, which show various structures on the meridian. The results suggest that the propagation region may consist of two separable domains of the inner and outer magnetosphere for a relatively low‐frequency wave. The inner region of propagation appears to be a distorted torus around the dipole axis. Such spatial separation of the two regions becomes weak and gradually interconnected for the waves with a relatively small azimuthal wavenumber or high frequency. The wave spectra and energy distribution are also investigated for different azimuthal wavenumbers. The numerical results show that the cavity modes may exist in the plasmasphere even at the absence of the outer magnetospheric boundary, which is found to be strongly associated with the characteristics of wave parameters. In particular, it is suggested that the plasmaspheric cavity modes, in the nightside region, may play a crucial role in producing Pi 2 pulsations. In addition, theories of waveguide and cavity modes based on the dipole model are discussed in detail.

Structure and Kinetic Properties of Plasmoids and Their Boundary Regions

Based on GEOTAIL/LEP observations in the distant magnetotail, this paper reports on several new features of velocity distribution functions of electrons and ions within a plasmoid and at its boundary. Here we use the term 'plasmoid' in a wider meaning than usual in spite of the presence of significant magnetic By fields. In the lobe, as expected from MHD simulations of magnetic reconnection processes, cold plasmas are pushed away from the plasma sheet before the arrival of plasmoids, while after the plasmoid passage the convection is enhanced toward the normal direction to the plasma sheet. The cold ions flow into the plasmoid along magnetic field lines, are heated and accelerated perpendicularly at the boundary, and finally merge with hot plasmas deeper inside the plasmoids. Deep inside plasmoids, however, the ion distribution functions often show the existence of counterstreaming ion beams, while the simultaneously measured electron distribution functions show a flat-top distribution. It is noted that the presence of the counterstreaming ions is a fine structure along magnetic field lines inside the whole distribution convecting tailward with speeds of 500-900 km/s. The relative velocity of the two components along the magnetic field line reaches 1000-1500 km/s, which is much higher than the local Alfven speed. Each component has an anisotropic distribution with respect to its center in the velocity space; the perpendicular temperature is several times higher than the parallel temperature. We conclude that these counterstreaming ions are most likely of lobe origin, and they have not had time enough for thermalization. They might have entered the plasmoid from the northern and southern lobes, being heated and accelerated through slow-mode shocks at the boundaries. Hence, these field lines are open, and both ends are connected to the northern and southern lobes. This phenomenon is observed predominantly in the latter part of the plasmoid after southward turning of the magnetic field, especially after the plasma bulk speed has increased stepwise and the Ba/B, field magnitudes have attained the peak value. It is also observed even near the neutral sheet, where the magnetic Bx field is very small, but significant By and/or Bz fields exist. Since the tailward flow speed becomes faster associated with the above phenomenon, these open field lines would be draped around the leading (core) part of plasmoids. The compression due to the draping may increase the field intensity.

Ion cyclotron emission due to collective instability of fusion products and beam ions in TFTR and JET

Ion cyclotron emission (ICE) has been observed from neutral beam heated TFTR, and JET tritium experiments at sequential cyclotron harmonics of both fusion products and beam ions. The emission originates from the outer midplane plasma, where fusion products and beam ions are likely to have a drifting ring-type velocity-space distribution that is anisotropic and sharply peaked. Fusion product driven ICE in both TFTR and JET can be attributed to the magnetoacoustic cyclotron instability, which involves the excitation of obliquely propagating waves on the fast Alfven/ion Bernstein branch at cyclotron harmonics of the fusion products. Differences between ICE observations in JET and TFTR appear to reflect the sensitivity of the instability growth rate to the ratio vbirth/cA where vbirth is the fusion product birth speed and cA is the local Alfven speed for fusion products in the outer midplane edge of TFTR supershots, vbirth < cA for alpha particles in the outer midplane edge of JET, the opposite inequality applies. If sub-Alfvenic fusion products are isotropic or have undergone even a moderate degree of thermalization, the magnetoacoustic instability cannot occur. In contrast, the super-Alfvenic alpha particles that are present in the outer midplane of JET can drive the magnetoacoustic cyclotron instability even if they are isotropic or have a relatively broad distribution of speeds. These conclusions may account for the observation that fusion product driven ICE in JET persists for longer than fusion product driven ICE in TFTR. Moreover, the time evolution of the maximum growth rate, obtained using the Sigmar model for the alpha particle distribution and TFTR data for the fusion product source rate, closely follows the observed time evolution of the ICE amplitude in TFTR supershot discharges. Other observed features of fusion product driven ICE that match the linear instability include the scaling with fusion product density, doublet splitting of spectral peaks, the relative strength of certain harmonics and source localization. A separate mechanism is proposed forthe excitation of beam driven ICE in TFTR: electrostatic ion cyclotron harmonic waves, supported by stronglysub-Alfvénic beam ions, can be destabilized by a low concentration of such ions with a very narrow spread ofvelocities in the parallel direction. Sufficiently narrow distributions are likely to exist in the edge plasma, close tothe point of beam injection.

Anomalous magnetic diffusion in coronal current layers

The current-driven kinetic Alfven instability is proposed as an anomalous transport mechanism for regions of concentrated, field-aligned currents in the solar corona. Anomalous magnetic diffusivity (ηe f f ≤ 109cm2s−1), produced by kinetic Alfven turbulence in the vicinity of the saturation level, provides fast magnetic energy release with a local inflow Alfven Mach numberMin ≤ 0.1.

Kinetic properties of heavy ions in the solar wind from SWICS/Ulysses

The kinetic properties of heavy ions in the solar wind are known to behave in a well organized way under most solar wind flow conditions: Their speeds are all equal and faster than that of hydrogen by about the local Alfven speed, and their kinetic temperatures are proportional to their mass. The simplicity of these properties points to a straightforward physical interpretation; wave-particle interactions with Alfven waves are the probable cause. With the SWICS sensor on board Ulysses, it is now possible to investigate the kinetic properties of many more ion species than before. Furthermore, the transition of Ulysses into the fast stream emanating from the south polar coronal hole since 1992 allows us to study these properties both in the slow, interstream solar wind, as well as in an unambiguously identified fast stream. We present data from SWICS/Ulysses on the dominant ions of He, C, O, Ne, and Mg. As a result we find that, both in the slow wind and in fast streams, the isotachic property is obeyed even better than it could be determined by the ICI instrument on ISEE-3. The mass proportionality ofTkin is also shown to hold for these ions, including the newly identified C and Mg.

Electromagnetic Ion Cyclotron Waves Observed in the Near Earth Plasma Sheet Boundary Layer.

Enhanced magnetic field fluctuations observed during an ISEE-2 crossing of the plasma sheet boundary layer at X GSE =18.2 Re have the characteristics of electromagnetic ion cyclotron waves. The observed fluctuations propagate essentially along the magnetic field and are left-hand circularly polarized in the spacecraft frame, with frequencies below the proton cyclotron frequency (Ω p ) and peak power at about 0.2Ω p . Plasma data indicate that these waves are associated with decreasing plasma β and occur in the presence of an anisotropic earthward directed proton beam with velocity less than the local Alfven speed. Linear Vlasov theory indicates that these waves are most likely excited by the electromagnetic ion cyclotron instability driven by the temperature anisotropy of the proton beam

The phase mixing of Alfven waves, coordinated modes, and coronal heating

view Abstract Citations (87) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Phase Mixing of Alfven Waves, Coordinated Modes, and Coronal Heating Parker, E. N. Abstract Phase mixing has been a popular theoretical mechanism invoked to dissipate the photospheric plane shear Alfven waves hypothesized to penetrate up into the coronal hole and solar wind, thereby maintaining the expanding coronal gas at 1.5 million K and serving as the principal energy source for the solar wind. However, phase mixing requires an ignorable coordinate, which is not available in the presumably filamentary coronal hole. The filamentary density and temperature of the coronal hole couple the waves into a coordinated mode with a unique phase velocity omega/k, which provides the other popular theoretical mechanism, viz, resonant absorption where the local Alfven speed C is equal to omega/k. This provides coronal heating at radial distances O(10 solar radii), where it serves to accelerate the solar wind to high velocity. But it does not provide the principal heat input in the first 1-2 solar radii, required by Withbroe's (1988) analysis of the observed structure of the coronal hole. Publication: The Astrophysical Journal Pub Date: July 1991 DOI: 10.1086/170285 Bibcode: 1991ApJ...376..355P Keywords: Magnetohydrodynamic Waves; Radiative Transfer; Solar Corona; Solar Heating; Solar Wind; Heat Sources; Transverse Waves; Wave Equations; Wave Propagation; Solar Physics; RADIATIVE TRANSFER; SUN: CORONA; SUN: SOLAR WIND; WAVE MOTIONS full text sources ADS |

Ion Reflection and transmission during reconnection at the Earth's subsolar magnetopause

Composition measurements in an accelerated flow event at the Earth's dayside magnetopause show evidence for reflection and transmission of magnetospheric and magnetosheath ion species. Furthermore, a single velocity transformation approximately tangent to the magnetopause surface orders the individual transmitted and reflected ion distributions on both sides of the magnetopause into field‐aligned flow at VA, the local Alfven speed. These observations provide strong evidence for a kinetic description of magnetic reconnection at the magnetopause.

A wavelike origin for clumpy structure and broad line wings in molecular clouds

The strong magnetosonic waves that are likely to be generated by virialized motions in molecular clouds should steepen into shocks and density irregularities that may be identified with some of the fine-scale or clumpy structure in these clouds. This process is illustrated here with a numerical solution to the magnetosonic equations with diffusion, in the case of two colliding, linearly polarized wavetrains. The parallel and perpendicular velocities that are associated with such clumps are comparable to the local Alfven speed. This implies that the broad wings in non-Gaussian line profiles (unassociated with embedded stars) could be the result of wave-related motions in the low-density regions of molecular clouds. Theoretical composite line profiles derived for a line of sight containing density substructures of the form n proportional to r exp -alpha and Alfven velocity variations of the form vA proportional to n exp -beta fit the observed core-wing molecular line profiles for reasonable alpha and beta. 14 refs.

Three‐dimensional simulation of diamagnetic cavity formation by a finite‐sized plasma beam

The problem of collisionless coupling between a plasma beam and a background plasma is examined using a three‐dimensional hybrid code. The beam is assumed to be moving parallel to an ambient magnetic field at a speed greater than the local Alfvén speed. In addition, the beam has a finite spatial extent in the directions perpendicular to the magnetic field and is uniform and infinite in the direction parallel to the ambient magnetic field. Such a system is susceptible to coupling of the beam ions with the background ions via an electromagnetic ion beam instability. This instability isotropizes the beam and energizes the background plasma. A large‐amplitude Alfvén wave traveling radially away from the interaction region is associated with the energized background plasma. The process described here is one which may be responsible for the formation of diamagnetic cavities observed in the solar wind.

Excitation of MHD surface waves propagating with shear Alfven waves in an inhomogeneous cylindrical plasma

Experimental results are presented which show that the MHD surface waves or fast waves of m=+or-1 have been excited in the range of frequency less than ion cyclotron frequency using a helical antenna in a finite beta and cylindrical plasma. Their dispersion relation and spatial structure were successfully confirmed after the wave signals were separated from those of the local shear Alfven waves that propagated together with the surface waves. In order to obtain the dispersion relations from the signals, the authors adopted three kinds of methods: (1) conventional cross-power spectrum density using fast Fourier transform, (2) maximum entropy method, and (3) direct estimation of the phase velocity of wave packets. The observed properties of both the surface waves and the SAWs are in general agreement with theory.

Nonlinear Parker instability of isolated magnetic flux in a plasma

The nonlinear evolution of the Parker instability in an isolated horizontal magnetic-flux sheet embedded in a two-temperature layer atmosphere is studied by using a two-dimensional MHD code. In the solar case, this two-layer model is regarded as a simplified abstraction of the sun's photosphere/chromosphere and its overlying much hotter (coronal) envelope. The horizontal flux sheet is initially located in the lower temperature atmosphere so as to satisfy magnetostatic equilibrium under a constant gravitational acceleration. Ideal MHD is assumed, and only perturbations with k parallel to the magnetic-field lines are investigated. As the instability develops, the gas slides down the expanding loop, and the evacuated loop rises as a result of enhanced magnetic buoyancy. In the nonlinear regime of the instability, both the rise velocity of a magnetic loop and the local Alfven velocity at the top of the loop increase linearly with height and show self-similar behavior with height as long as the wavelength of the initial perturbation is much smaller than the horizontal size of the computing domain.

Evolution of diamagnetic cavities in the solar wind

Two‐dimensional hybrid (particle ions and fluid electrons) simulations of a finite plasma beam moving through a collisionless plasma parallel to an ambient magnetic field are considered. When the relative drift speed exceeds twice the local Alfven velocity, an electromagnetic ion beam instability is excited which produces an energetic plasma in the interaction region with β larger than unity. This energetic plasma pushes out the magnetic field, creating a diamagnetic cavity and launching outgoing Alfven waves. Application of the results to recent observations of hot diamagnetic cavities in the solar wind is discussed.

Heating of the solar corona by the resonant absorption of Alfven waves

An improved method for calculating the resonance absorption heating rate is discussed and the results are compared with observations in the solar corona. To accomplish this, the wave equation for a dissipative, compressible plasma is derived from the linearized magnetohydrodynamic equations for a plasma with transverse Alfven speed gradients. For parameters representative of the solar corona, it is found that a two-scale description of the wave motion is appropriate. The large-scale motion, which can be approximated as nearly ideal, has a scale which is on the order of the width of the loop. The small-scale wave, however, has a transverse scale much smaller than the width of the loop, with a width of about 0.3-250 km, and is highly dissipative. These two wave motions are coupled in a narrow resonance region in the loop where the global wave frequency equals the local Alfven wave frequency. Formally, this coupling comes about from using the method of matched asymptotic expansions to match the inner and outer (small and large scale) solutions. The resultant heating rate can be calculated from either of these solutions. A formula derived using the outer (ideal) solution is presented, and shown to be consistent with observations of heatingmore » and line broadening in the solar corona. 34 references.« less

Sweeping-magnetic-twist mechanism for the acceleration of jets in the solar atmosphere

A magnetodynamic mechanism for the acceleration of jets in the solar atmosphere (surges, Brueckner's EUV jets, and so on) is proposed, and a 2.5-dimensional MHD simulation is performed to show how this mechanism operates in the situation of the chromosphere-corona region of the solar atmosphere. It is seen from the result of simulation that together with the release of the magnetic twist, e.g., into a reconnected open flux tube, the mass in the high density twisted loop is driven out into the open flux tube due both to the pinch effect progressing with the packet of the magnetic twist into the open flux tube, and to the j × B force at the front of the packet of the unwinding twist in the off-axis part of the tube. The former, the progressing pinch, is accompanied by an accelerated hot blob, while the latter, the unwinding front of the magnetic twist, drives a cool cylindrical flow, both with velocities of the order of the local Alfven velocity. One of the characteristic properties of the jet in our model is that the jet, consisting of hot core and cool sheath, has a helical velocity field in it, explaining the thus-far unexplained observed feature. The sudden release of the magnetic twist into an open flux tube is most likely to be due to the reconnection between a twisted loop and the open flux tube. The mass is driven out in the relaxation process of the magnetic twist from the twisted loop to the open flux tube.

Local Characteristic of Axisymmetric Alfvén Wave

The local characteristic of the Alfven wave excited by a Stix coil is investigated in a radially inhomogeneous plasma. It is observed that the wave field of the standing axisymmetric Alfven wave is enhanced and the polarization sense changes at the resonance layer. It is also observed that the phase velocity of the propagating axisymmetric Alfven wave varies radially and corresponds to the local Alfven velocity.

Electric field measurements at the magnetopause: 1. Observation of large convective velocities at rotational magnetopause discontinuities

Large convective electric fields of the order of 10 mV/m (sometimes as high as 22 mV/m) are observed at rotational magnetopause discontinuities. These observations were made with the long cylindrical (179‐m base line) probes carried on the ISEE 1 satellite. These electric field observations yield convective velocity magnitudes, V* = |E×B/B²|, of the order of 150 km/s. In this V* format some of these observations are similar to the high speed plasma velocity observations that were made at the magnetopause with the plasma experiment carried on the ISEE 1 satellite. It is shown that, for many of these magnetopause crossings, there exists a special moving coordinate system where the observed electric fields vanish. Such a unique reference system is often used in theoretical studies of magnetic discontinuities. This special coordinate system does not move at the local plasma velocity but moves instead at a velocity intermediate between the convective velocity and the local Alfvén velocity. It is used here as a diagnostic tool for the experimental investigation of rotational discontinuities at the magnetopause.

A threshold effect for spacecraft charging

The existence of a threshold energy dependence is shown for the charging of geostationary spacecraft in eclipse. Applied Technology Satellite 6 and P78‐2 (SCATHA) data show that plasma sheet fluxes must extend above 10 keV for these satellites to charge in eclipse. The existence of a threshold energy is significant because magnetospheric convection boundaries produce relatively sharp phase space boundaries, with electron fluxes falling rapidly above the energy associated with the local Alfven layer. The threshold effect is due to the shape of the normal secondary yield curve, in particular the high energy crossover, where the secondary yield drops below 1. A large portion of the ambient electron flux must exceed this energy for a negative current to exist.

Electron acceleration and radiation signatures in loop coronal transients

It is proposed that in loop coronal transients an erupting loop moves away from the solar surface, with a velocity exceeding the local Alfven speed, pushing against the overlying magnetic fields and driving a shock in the front of the moving part of the loop. Lower hybrid waves are excited at the shock front and propagate radially toward the center of the loop with phase velocity along the magnetic field that exceeds the thermal velocity. The lower hybrid waves stochastically accelerate the tail of the electron distribution inside the loop. The manner in which the accelerated electrons are trapped in the moving loop are discussed, and their radiation signature is estimated. It is suggested that plasma radiation can explain the power observed in stationary and moving type IV bursts.