Mirror Mode Research Articles

Magnetic pileup boundary and field draping at Comet Halley

The approach of the Rosetta S/C to Comet Churyumov–Gerasimenko in 2014 reactivates the interest in the plasma interaction of the solar wind with the cometary coma. In preparation for the upcoming S/C observations and the start of outgassing of the cometary nucleus, we reinvestigate the magnetic field data from the Vega-1 S/C at the flyby of Comet Halley (1986), in search of the magnetic pileup boundary and increase of field line draping. The magnetic pileup boundary has been identified as a common feature for unmagnetized bodies with an induced magnetosphere. This boundary marks the outer edge of the magnetic pileup region, also known as the magnetic barrier region, in which the magnetic field is strong and highly draped. Initially, the magnetic field draping around Comet Halley was clearly identified from the Vega-1 magnetometer data through reversal of the field component in direction to the Sun at closest approach.Here, a detailed analysis is performed in regions further upstream in the magnetosheath. The Vega-1 high resolution magnetometer data on the in- and outbound leg but inside the bow wave are reinvestigated in search for the magnetic pileup boundary as an indicator for the outer edge of the magnetic barrier. The magnetic field pileup region is studied using the correlation between the field component towards the Sun and the radial component in an aberrated cometocentric frame; this technique proved very successful for Mars and also for comets Giacobini–Zinner and Halley in the case of Giotto observations. We can clearly identify the different regimes in the magnetic field data, on the in- and outbound leg of the orbit. Waves just within the newly determined magnetic pileup region have properties different from mirror mode waves, whereas waves observed out of the magnetic pileup boundary are confirmed as mirror mode. The boundaries found at Comet Halley prove that also the detailed structure of the interaction of unmagnetized bodies with an atmosphere with the solar wind is valid for active comets, but with larger space scale.

Brief Communication: Weibel, Firehose and Mirror mode relations

Abstract. Excitation of Weibel magnetic fields in an initially non-magnetized though anisotropic plasma may trigger other low-frequency instabilities fed by pressure anisotropy. It is shown that under Weibel-like stable conditions the Weibel-like thermal fluctuation magnetic field allows for restricted Firehose-mode growth. In addition, low-frequency Whistlers can also propagate in the plasma under certain anisotropic conditions. When the Weibel-like mode becomes unstable, Firehose instability ceases but Mirror modes take over. This will cause bubble structures in the Weibel-like field in addition to filamentation.

Anatomical or mirror mode imitation? A behavioral approach

Anatomical or mirror mode imitation? A behavioral approach

Ion distributions upstream and downstream of the Earth's bow shock: first results from Vlasiator

Abstract. A novel hybrid-Vlasov code, Vlasiator, is developed for global simulations of magnetospheric plasma kinetics. The code is applied to model the collisionless bow shock on scales of the Earth's magnetosphere in two spatial dimensions and three dimensions in velocity space retrieving ion distribution functions over the entire foreshock and magnetosheath regions with unprecedented detail. The hybrid-Vlasov approach produces noise-free uniformly discretized ion distribution functions comparable to those measured in situ by spacecraft. Vlasiator can reproduce features of the ion foreshock and magnetosheath well known from spacecraft observations, such as compressional magnetosonic waves generated by backstreaming ion populations in the foreshock and mirror modes in the magnetosheath. An overview of ion distributions from various regions of the bow shock is presented, demonstrating the great opportunities for comparison with multi-spacecraft observations.

SMALL-SCALE PRESSURE-BALANCED STRUCTURES DRIVEN BY MIRROR-MODE WAVES IN THE SOLAR WIND

Recently, small-scale pressure-balanced structures (PBSs) have been studied with regard to their dependence on the direction of the local mean magnetic field B0. The present work continues these studies by investigating the compressive wave mode forming small PBSs, here for B0 quasi-perpendicular to the x-axis of Geocentric Solar Ecliptic coordinates (GSE-x). All the data used were measured by WIND in the quiet solar wind. From the distribution of PBSs on the plane determined by the temporal scale and angle θxB between the GSE-x and B0 ,w e notice that at θxB = 115 ◦ the PBSs appear at temporal scales ranging from 700 s to 60 s. In the corresponding temporal segment, the correlations between the plasma thermal pressure Pth and the magnetic pressure PB ,a s well as that between the proton density Np and the magnetic field strength B, are investigated. In addition, we use the proton velocity distribution functions to calculate the proton temperatures T⊥ and T� . Minimum Variance Analysis is applied to find the magnetic field minimum variance vector BN. We also study the time variation of the cross-helicity σc and the compressibility Cp and compare these with values from numerical predictions for the mirror mode. In this way, we finally identify a short segment that has T⊥ >T � ,p rotonβ � 1, both pairs of Pth‐PB and Np‐B showing anti-correlation, and σc ≈ 0 with Cp > 0. Although the examination of σc and Cp is not conclusive, it provides helpful additional information for the wave mode identification. Additionally, BN is found to be highly oblique to B0. Thus, this work suggests that a candidate mechanism for forming small-scale PBSs in the quiet solar wind is due to mirror-mode waves.

Global sound modes in mirror traps with anisotropic pressure

Global oscillations of inhomogeneous plasma with frequencies close to the bounce frequency of ions in mirror traps have been studied. It has been shown that, in some cases, the sound can be reflected from the axial plasma inhomogeneity. The ideal magnetohydrodynamic (MHD) model with Chew-Goldberger-Low approximation has been utilized to determine conditions of existence of the standing waves in the mirror-confined plasma. Linearized wave equation for the longitudinal plasma oscillations in thin anisotropic inhomogeneous plasma with finite β has been derived. The wave equation has been treated numerically. The oscillations are studied for the case of the trap with partially filled loss-cone and the trap with sloshing ions. It has been shown that in cells of the multiple-mirror trap standing waves can exist. The frequency of the wave is of the order of the mean bounce-frequency of ions. In the trap with sloshing ions, the mode supported by the pressure of fast ions could exist. The results of oscillations observation in the experiment on the Gas Dynamic Trap have been presented.

SMALL-SCALE PRESSURE-BALANCED STRUCTURES DRIVEN BY OBLIQUE SLOW MODE WAVES MEASURED IN THE SOLAR WIND

Recently, small-scale pressure-balanced structures (PBSs) were identified in the solar wind, but their formation mechanism remains unclear. This work aims to reveal the dependence of the properties of small-scale PBSs on the background magnetic field (B 0) direction and thus to corroborate the in situ mechanism that forms them. We analyze the plasma and magnetic field data obtained by WIND in the quiet solar wind at 1 AU. First, we use a developed moving-average method to obtain B 0(s, t) for every temporal scale (s) at each time moment (t). By wavelet cross-coherence analysis, we obtain the correlation coefficients between the thermal pressure P th and the magnetic pressure P B, distributing against the temporal scale and the angle θxB between B 0(s, t) and Geocentric Solar Ecliptic coordinates (GSE)-x. We note that the angle coverage of a PBS decreases with shorter temporal scale, but the occurrence of the PBSs is independent of θxB. Suspecting that the isolated small PBSs are formed by compressive waves in situ, we continue this study by testing the wave modes forming a small-scale PBS with B 0(s, t) quasi-parallel to GSE-x. As a result, we identify that the cross-helicity and the compressibility attain values for a slow mode from theoretical calculations. The wave vector is derived from minimum variance analysis. Besides, the proton temperatures obey T ⊥ < T ∥ derived from the velocity distribution functions, excluding a mirror mode, which is the other candidate for the formation of PBSs in situ. Thus, a small-scale PBS is shown to be driven by oblique, slow-mode waves in the solar wind.

The proton temperature anisotropy associated with bursty bulk flows in the magnetotail

[1] We study the development of the proton temperature anisotropy T⊥/T|| in bursty bulk flows (BBFs), as observed by THEMIS Mission. For a set of 10 selected events, during which at least three spacecraft are aligned in the same flow, we can sample the plasma parameters along the Earth's magnetotail. The temperature anisotropy in the quiescent tail is negligible. However, as soon as the BBF passes over the spacecraft a strong anisotropy is measured. We analyze T⊥/T|| as a function of parallel plasma beta-β||(=nkT||/(B2/2μ0)) for the different THEMIS satellites and compare the spread of the data points with various instability thresholds over ion scales that can reduce the temperature anisotropy: for T⊥/T||<1 the parallel and oblique firehose; for T⊥/T||>1 the proton cyclotron and mirror mode. It is shown that the anisotropy reduces whilst the BBF is moving Earthward, and the strongest fluctuations are enhanced along the instability thresholds, indicating that these instabilities reduce the proton temperature anisotropy.

A picosecond ytterbium-doped double-clad fiber amplifier with a SESAM mode locking PCF oscillator as the seed source

We report a picosecond ytterbium-doped double-clad fiber amplifier with a semiconductor saturable absorber mirror (SESAM) mode locking large mode area photonic crystal fiber (PCF) oscillator as the seed source. In the seed source, without any dispersion compensator or tunable elements in the cavity, we obtain self-starting mode locking of a tunable ytterbium-doped fiber oscillator. The 1037–1054 nm tuning range is achieved by proper adjustment of the half-wave plate and SESAM in the laser cavity. More than 1 W output power at the different wavelengths of operation has been achieved in experiments. For the fiber amplifier seeded with an SESAM mode locking PCF fiber oscillator, an average output power of 21 W has been demonstrated at a repetition rate of 45.9 MHz with a pulse duration of 8.8 ps, corresponding to a pulse energy of 458 nJ and a peak power of 52 kW. The nonlinear effect of self-phase-modulation (SPM), which results in spectral broadening, has been observed during the amplified experiments.

Physical mechanisms for electron mirror and field swelling modes

Ion mirror instability is dominant in planetary and cometary magnetosheaths and other high-beta plasmas where the ions are hotter than the electrons. It is associated with a zero-frequency non-propagating mode with the wave vector nearly perpendicular to the ambient magnetic field. The counterparts of this instability in hot electron plasmas are the field swelling and electron mirror instabilities. A theory for these instabilities was developed more than two decades ago (Basu B and Coppi B 1982 Phys. Rev. Lett.48 799, 1984 Phys. Fluids27 1187) within the framework of a fluid model. The connection between the two types of instabilities has been analyzed in (Migliuolo S 1986 J. Geophys. Res.91 7981). In contrast to these papers, we shall here adopt the standard quasi-hydrodynamic approach that is usually used for the study of mirror instabilities. To analyze the electron mirror and field swelling instabilities, we will only use the perpendicular balance condition and the Liouville theorem. We have found that such a description is easier to understand and gives us increased physical insight into the basic physical features of both these instabilities.

Ion temperature anisotropy instabilities in planetary magnetosheaths

There has been a lack of understanding why mirror modes are present in planetary magnetosheaths, at comets, and in the heliosheath. Linear theory indicates that the ion cyclotron instability should dominate over the mirror mode instability in electron‐proton plasma. In this paper, we take a new approach. We examine the role of plasma electron temperature anisotropy on the ion cyclotron and mirror mode instabilities. It will be shown that an inclusion of anisotropic electrons with T⊥ e/T∥ e ≥ 1.2 reduces the ion cyclotron growth rate substantially and increases the mirror mode growth rate. The minimum plasma beta for mirror instability dominance (over the ion cyclotron instability) is βp = 0.5.

Mirror mode structures in the asymmetric Hermean magnetosheath: Hybrid simulations

Results of two global three‐dimensional hybrid simulations of the solar wind interaction with the Hermean magnetosphere are presented for southward and northward interplanetary magnetic field (IMF) orientations. Important dawn‐dusk asymmetries of the Hermean bow shock and magnetosheath are observed depending on the IMF orientation. For the southward IMF, the dawnside has a thicker magnetosheath with higher β values and slower bulk velocities compared to the duskside, whereas for the northward IMF, the duskside has a thicker and higher β magnetosheath with slower bulk velocities. Mirror mode activity consequently appears at the dawnside for the southward IMF and at the duskside for the northward IMF. A mechanism for the bow shock and magnetosheath asymmetries is proposed and discussed in the context of the Hermean and terrestrial magnetosheaths.

Nonlinear mirror modes in the presence of hot electrons

A non-perturbative calculation of the gyrotropic pressures associated with large-scale mirror modes is performed, taking into account a finite, possibly anisotropic electron temperature. In the small-amplitude limit, this leads to an extension of an asymptotic model previously derived for cold electrons. A model equation for the profile of subcritical finite-amplitude large-scale structures is also presented.

Self through the Mirror (Neurons) and Default Mode Network: What Neuroscientists Found and What Can Still be Found There

OPINION article Front. Hum. Neurosci., 24 July 2013Sec. Brain Health and Clinical Neuroscience https://doi.org/10.3389/fnhum.2013.00383

Mirror‐mode storms inside stream interaction regions and in the ambient solar wind: A kinetic study

Mirror‐mode structures have been found in the solar wind at various heliocentric distances with different missions. Recently, STEREO has observed mirror‐mode waves present as trains of holes and also as humps in the magnetic field magnitude. In some cases, mirror‐mode trains last for very long periods of time and have been called “mirror‐mode storms”. We present case studies of mirror‐mode storms observed in the solar wind using STEREO data in three different locations: in the downstream region of the forward shock of a stream interaction region, inside a stream interaction region far from the forward shock, and also in the ambient solar wind. To make a formal identification of the mirror mode, we determine wave characteristics using minimum variance analysis. Finally, we perform a kinetic dispersion analysis and discuss the possible origin of mirror‐mode structures evaluating curves of growth for different regimes of proton temperature anisotropies in a plasma with a He component.

Opto-Mechanical Effects in Superradiant Light Scattering by Bose—Einstein Condensate in an Optical Cavity

We investigate the effects of a movable mirror (cantilever) of an optical cavity on the superradiant light scattering from a Bose—Einstein condensate (BEC) in an optical lattice. We show that the mirror motion has a dynamic dispersive effect on the cavity-pump detuning. Varying the intensity of the pump beam, one can switch between the pure superradiant regime and the Bragg scattering regime. The mechanical frequency of the mirror strongly influences the time interval between two Bragg peaks. We find that when the system is in the resolved side band regime for mirror cooling, the superradiant scattering is enhanced due to coherent energy transfer from the mechanical mirror mode to the cavity field mode.

Comparing resonant photon tunneling via cavity modes and Tamm plasmon polariton modes in metal-coated Bragg mirrors

Resonant photon tunneling was investigated experimentally in multilayer structures containing a high-contrast (TiO(2)/SiO(2)) Bragg mirror capped with a semitransparent gold film. Transmission via a fundamental cavity resonance was compared with transmission via the Tamm plasmon polariton resonance that appears at the interface between a metal film and a one-dimensional photonic bandgap structure. The Tamm-plasmon-mediated transmission exhibits a smaller dependence on the angle and polarization of the incident light for similar values of peak transmission, resonance wavelength, and finesse. Implications for transparent electrical contacts based on resonant tunneling structures are discussed.

Two kinds of mirror modes in a nonzero electron-temperature plasma

This communication describes the study of mirror modes in a plasma with a small or large fraction of cold electrons within a gyrokinetic framework. It was found that if the number of cold electrons is me/mi times smaller than the number of hot electrons, the dispersion relation of the compressional perturbation represents a modification of the magnetohydrodynamics slow mode dispersion relation; if the plasma anisotropy is small, the perturbation is an oscillation, but if the anisotropy is sufficient, the square of the oscillation frequency becomes negative and one of the solutions represents the ‘fluid’ mirror instability. In the opposite case, when the number of cold electrons is me/mi times larger than the number of hot electrons, the perturbation is damping in the non-oscillatory regime if the anisotropy is small, and growing in the case of large anisotropy. This is the ‘kinetic’ mirror instability. For both kinetic instabilities, the threshold is the same, but the growth rates are different.

LOCAL TWO-DIMENSIONAL PARTICLE-IN-CELL SIMULATIONS OF THE COLLISIONLESS MAGNETOROTATIONAL INSTABILITY

The magnetorotational instability (MRI) is a crucial mechanism of angular momentum transport in a variety of astrophysical accretion disks. In systems accreting at well below the Eddington rate, such as the central black hole in the Milky Way (Sgr A*), the rate of Coulomb collisions between particles is very small, making the disk evolve essentially as a collisionless plasma. We present a nonlinear study of the collisionless MRI using first-principles particle-in-cell (PIC) plasma simulations. In this initial study we focus on local two-dimensional (axisymmetric) simulations, deferring more realistic three-dimensional simulations to future work. For simulations with net vertical magnetic flux, the MRI continuously amplifies the magnetic field until the Alfv\'en velocity, v_A, is comparable to the speed of light, c (independent of the initial value of v_A/c). This is consistent with the lack of saturation of MRI channel modes in analogous axisymmetric MHD simulations. The amplification of the magnetic field by the MRI generates a significant pressure anisotropy in the plasma (with the perpendicular pressure being larger than the parallel pressure). We find that this pressure anisotropy in turn excites mirror modes and that the volume averaged pressure anisotropy remains near the threshold for mirror mode excitation. Particle energization is due to both reconnection and viscous heating associated with the pressure anisotropy. Reconnection produces a distinctive power-law component in the energy distribution function of the particles, indicating the likelihood of non-thermal ion and electron acceleration in collisionless accretion disks. This has important implications for interpreting the observed emission -- from the radio to the gamma-rays -- of systems such as Sgr A*.

Mirror Mode Waves Downstream of a Stream Interaction Region Forward Shock near 1 AU

Mirror mode waves in the solar wind are typically observed not as quasi-periodic sinusoidal signatures, but as trains of nonperiodic structures of two types: magnetic "peaks" and magnetic "dips." Some trains of long durations have been called mirror mode storms. In this work we report mirror mode waves downstream of a stream interaction region (SIR) forward shock observed near 1 AU on 7 May 2007 with Solar Terrestrial Relations Observatory (STEREO) and Time History of Events and Macroscale Interactions during Substorms (THEMIS) data. The high-resolution magnetic-field data (0.125-second resolution) from STEREO are scanned to search for magnetic dips and peaks (or upgoing magnetic "mesas") in the solar wind. STEREO-A observes a mirror mode storm: the appearance of mirror mode waves (mainly magnetic peaks and upgoing mesas) is simultaneous with the entry into a high-density, high-temperature, and high plasma beta accompanied by a depressed field region; the magnetic dips survive in the lower plasma-beta region. STEREO-B observes mirror mode waves (mainly magnetic peaks) with different amplitudes and asymmetric forms, which can survive in a low plasma beta region. THEMIS-D, which was located in the solar wind, also observes mirror mode waves (mainly magnetic peaks and upgoing mesas) as well as an enhanced ion temperature anisotropy (T (aSyen)a parts per thousand 3T (ayen)). The enhanced ion temperature anisotropy and high plasma beta satisfy the mirror-instability criterion. These observations of STEREO and THEMIS-D show that mirror mode waves can be excited downstream of a SIR forward shock near 1 AU.