Structure of the current sheets in the near-Mars magnetotail. Maven observations

During the last 15 years, the Current Sheets (CSs) have been intensively studied in the tail of the terrestrial magnetosphere, where protons are the dominated ion component. On the contrary, in the Martian magnetotail heavy ions (O⁺ and⁺ ₀) usually dominate while the abundance of protons can be negligible. Hence it is interesting to study the spatial structure and plasma characteristics of such “oxygen” CSs. MAVEN spacecraft (s/c) currently operating on the Martian orbit with a unique set of scientific instruments allows observation of the magnetic field and three-dimensional distribution functions of various ion components and electrons with a high time resolution. In this paper, we analyse nine intervals of the CSs observed by MAVEN in the near-Mars tail at the distances from the planet ~1.5–1R _M , where R _M is the radius of Mars. We analyse the spatial structure of the CSs and estimate their thickness for different magnetic configurations and relative abundance of the heavy and light ions in the sheets. It is shown that, similarly to the CSs in the Earth’s magnetotail, the thickness and complexity of the spatial structure of the Maritan CSs (i.e. the presence of embedded and / or peripheral current structures) depend on the magnetic configuration of the sheets, which, in turn, affects the fraction of the quasi-adiabatic particles in the CSs.     

Origin of Mercury’s double magnetopause: 3D hybrid simulation study with A.I.K.E.F.

During the first and second Mercury flyby the MESSENGER spacecraft detected a dawn side double-current sheet inside the Hermean magnetosphere that was labeled the “double magnetopause” (Slavin, J.A. et al. [2008]. Science 321, 85). This double current sheet confines a region of decreased magnetic field that is referred to as Mercury’s “dayside boundary layer” (Anderson, M., Slavin, J., Horth, H. [2011]. Planet. Space Sci.). Up to the present day the double current sheet, the boundary layer and the key processes leading to their formation are not well understood. In order to advance the understanding of this region we have carried out self-consistent plasma simulations of the Hermean magnetosphere by means of the hybrid simulation code A.I.K.E.F. (Müller, J., Simon, S., Motschmann, U., Schüle, J., Glassmeier, K., Pringle, G.J. [2011]. Comput. Phys. Commun. 182, 946–966). Magnetic field and plasma results are in excellent agreement with the MESSENGER observations. In contrast to former speculations our results prove this double current sheet may exist in a pure solar wind hydrogen plasma, i.e. in the absence of any exospheric ions like sodium. Both currents are similar in orientation but the outer is stronger in intensity. While the outer current sheet can be considered the “classical” magnetopause, the inner current sheet between the magnetopause and Mercury’s surface reveals to be sustained by a diamagnetic current that originates from proton pressure gradients at Mercury’s inner magnetosphere. The pressure gradients in turn exist due to protons that are trapped on closed magnetic field lines and mirrored between north and south pole. Both, the dayside and nightside diamagnetic decreases that have been observed during the MESSENGER mission show to be direct consequences of this diamagnetic current that we label Mercury’s “boundary-layer-current“.     

The Effects of the Velocity Filtration Mechanism on the Minor Ions of the Corona

Due to their different mass and varying charge states, heavy solar ions provide invaluable information on the physical mechanisms responsible for the heating of the corona and the solar wind acceleration. It is shown in the present work that some key characteristics observed in the corona are straightforward features predicted by the velocity filtration effect. This mechanism originally proposed by Scudder (1992a,b) accounts for the heating of the corona by just assuming that the velocity distribution functions have enhanced supra-thermal tails. The apparent `heating' is then a natural consequence of the increasing ratio of supra thermal over thermal particles as a function of altitude. Applied to the minor ions, it offers a natural explanation for the high temperatures of the heavy ions compared to the protons. The kinetic filtration theory predicts temperatures more than proportional to the mass of the ions, with a small correction for the charge state. With sufficiently high temperatures, the ions flow faster than the protons. These characteristics are in good agreement with the most recent spectroscopic and in-situ solar wind observations.     

Non-Maxwellian velocity distribution functions associated with steep temperature gradients in the solar transition region

Non-Maxwellian electron velocity distribution functions, computed for Dupree's (1972) model of the solar transition region in a previous paper, are used to calculate ionization rates for ions of carbon, nitrogen, and oxygen. Ionization equilibrium populations for these ions are then computed and compared with similar calculations assuming Maxwellian distribution functions for the electrons. The results show that the ion populations change (compared to the values computed with a Maxwellian) in some cases by several orders of magnitude depending on the ion and its temperature of formation.     

Three-dimensional particle anisotropies in and near the plasma sheet of Jupiter observed by the EPAC experiment onboard the Ulysses spacecraft

During its inbound journey into Jupiter's magnetosphere, Ulysses had several encounters with the Jovian plasma sheet near the magnetic equator, which were related with intensity maxima in the energetic particles. We show for the first time anisotropies in three dimensions of three ion species (protons, helium and oxygen) in the energy range 0.24 < E < 0.77 [MeV/nucleon]. The data, obtained with the Energetic Particle Composition Experiment (EPAC) onboard Ulysses have been analysed by using spherical harmonics in three dimensions. We show that the first-order anisotropies of ions in or near the plasma sheet are strongest in a plane parallel to the ecliptic plane and more or less azimuthal with respect to the rotation of Jupiter. We show that the first-order anisotropy amplitude is larger for helium and oxygen ions than for protons in nearly the same energy per nucleon range. We find flow velocities for helium ions which are not consistent with corotation, but are larger by a factor of 2 in and near the Jovian plasma sheet on the dayside magnetosphere. In contrast for protons we observe nearly corotation. Far from the plasma sheet, at high magnetic latitudes, the flow velocities are less than corotation for protons, as well as for helium and oxygen. The azimuthal particle anisotropies are explained by intensity gradients perpendicular to the centre of the plasma sheet, by E × B particle drifts, and by nonadiabatic orbits of the particles near the Jovian plasma sheet. All of the three phenomena act in the same azimuthal direction, perpendicular to the mainly radial magnetic field direction. Each of them can be estimated, but their individual effects cannot be distinguished from each other. In addition, we find a radial component of the anisotropy which apparently is stronger for protons than for heavier ions. This radial anisotropy component is interpreted as a result of the radial outward displacement of ions in an azimuthally swept back magnetic field.     

The estimate of the Venus magnetotail length

We consider the process of flux tubes straightening in the Venus magnetotail on the basis of MHD model. We estimate the distance x _t, where flux tubes are fully straightened due to the magnetic tension and the magnetotail with the characteristic geometry of field lines (“slingshot” geometry) ends. We investigate the influence of the transversal current sheet scale on the process of flux tubes straightening. The assumption of a thin current sheet allows to obtain a lower estimate of the magnetotail length, x _t > 31R _V (R _V is the Venus radius), while the assumption of a broad current sheet allows to obtain an upper estimate, x _t < 44R _V. We show that kinetic effects associated with the losses of particles with small pitch angles from the flux tube and the influx of magnetosheath plasma into the flux tube do not significantly affect the estimate of the magnetotail length. The model predicts the existence of energetic fluxes of protons H⁺ (2–5 keV) and oxygen ions O⁺ (35–80 keV) in the distant tail. We discuss the magnetotail structure at x > x _t.     

Non-extensive statistics and slarr neutrinos

In this paper we will show that, assuming the existence of a long-range microscopic memory of the random force, acting in the solar core, mainly on the electrons and the protons rather than on the light and heavy ions (or, equally, assuming the existence of an anomalous diffusion of solar core constituents of light mass and of normal diffusion of heavy ions), the equilibrium statistical distribution that these particles must obey, is that of the Tsallis non-extensive statistics, the distribution differing very slightly from the usual Maxwellian distribution. Due to the high-energy depleted tail of the distribution, the nuclear rates are reduced and, using earlier results on the standard solar model neutrino fluxes, calculated by Clayton and collaborators, we can evaluate fluxes in good agreement with the experimental data. While the proton distribution is only very slightly different from the Maxwell one there is a slightly larger difference with the electron distribution. This revised version was published online in July 2006 with corrections to the Cover Date.     

The spatial distribution of energetic ions and electrons in the magnetotail

The spatial distributions of energetic ion and electron bursts observed on the IMP 7 and 8 satellites in the Earth's magnetotail were studied. It was found that the ion bursts were more frequently detected in the dusk than in the dawn quarter of the neutral sheet whereas the electron bursts, more frequently in the dawn than the dusk quarter. The degree of dawn-dusk asymmetry is however energy dependent; the distribution for higher energy particle bursts exhibits higher degree of asymmetry. The morphologies of the distributions manifest themselves as seasonal variations of the most probable solar ecliptic latitudes at which the ion and electron bursts were observed. The amplitudes of the variations are about 25° with the seasonal variation for ions leading that for electrons by about 2 months.     

Particle motion and currents in the neutral sheet of the magnetospheric tail

The trajectories of plasma-sheet protons are computed numerically in magnetic-field models which simulate the neutral-sheet-type configuration observed in experiments. No electric field is included, in contrast with the reconnection theory. Entering the neutral sheet and then exiting from it, the particle performs an ordered displacement across the tail. A continuous interchange between the neutral and plasma sheets will give rise to an electric current which may be responsible for the observed magnetic-field configuration. An estimate of this current is made from the tension balance requirement, showing that a substantial anisotropy of the plasma-sheet pressure is necessary to maintain the steady state. It is shown that the neutral sheet itself can be a source of such an anisotropy, due to the non-adiabatic behaviour of protons. Other anisotropy origins are discussed briefly.     

Observations of solar wind stream with high abundance of heavy ions and relation with coronal conditions

Long intervals, during which heavy ions were detected in the high energy tail of the energy spectra of solar wind ions, were recorded by the plasma spectrometer SCS onboard the Prognoz-7 satellite. In particular, such a region with unusual features—low velocity, high density, low temperature of protons and, especially, low temperature of α-particles—was observed during 10–13 December 1978. The time dependence of these parameters makes it possible to recognize this event as “noncompressive density enhancement”. In this region heavy ions such as O⁺⁶, O⁺⁷, Si⁺⁷, Si⁺⁸, Si⁺⁹ and a group of iron from Fe⁺⁶ to Fe⁺¹³ were identified by the electrostatic analyzer.The abundance of these ions relative to protons was about ten times higher than had previously been observed. The coronal temperature, estimated from the ratios of the ion fluxes with different ionization states, is higher than that estimated earlier for the oxygen ions.     

Consequences of an isotropic static plasma sheet in models of the geomagnetic tail

The equation of momentum balance and magnetic flux conservation are given for a static tail model with an isotropic plasma sheet. The possibility of magnetic field leakage into the solar wind and across the neutral sheet is allowed. Numerical integrations for a wide variety of adjustable model parameters are presented that give the dependence on distance from Earth of all tail parameters (field strength inside and outside of the plasma sheet, plasma pressure, plasma sheet area, tail radius, and normal field component to the neutral sheet). The model gives good agreement with the observed distance dependence of the tail field strength, and accounts for the scatter in the data in terms of a mixture of the fields inside and outside the plasma sheet in the data averages. However, compared with the present interpretations of the observations the model gives a too large plasma pressure at large distances and a too small normal component to the neutral sheet. The discrepancies imply that plasma flow and/or pressure anisotropy are required for an adequate model.     

The magnetic field in the Earth's tail

Detailed magnetic fields in the Earth's tail are calculated from a proposed model containing Beard's tail surface and a current sheet inferred from satellite observations. The component inside and perpendicular to the neutral sheet permits us to construct the drift pattern in the magnetic equatorial plane for charged particles. The computed results are in reasonable agreement with the experimental results, although some deviations are noted.     

Energy and pitch angle distributions for auroral ions using the current sheet acceleration model

Using a dipole plus tail magnetic field model, H⁺, He⁺⁺ and O ₁₆ ⁺⁶ ions are followed numerically, backward in time, from an output plane perpendicular to the axis of the geomagnetic tail, to their point of entrace to the magnetosphere as solar wind particles in the magnetosheath. An adiabatic or guiding center approximation is used in regions where the particles do not interact directly with the current sheet. A Maxwellian distribution with bulk flow is assumed for solar wind particles in the magnetosheath. Bulk velocity, density, and temperature along the magnetopause are taken from the fluid calculations of Spreiter. Using Liouville's theorem, and varying initial conditions at the output plane, the distribution function is found as a function of energy and pitch angle at the output plane. These results are then mapped to the auroral ionosphere using guiding center theory. Results show that the total precipitation rate is sufficient only for particles which enter the magnetosphere near the edges of the current sheet. Small pitch angles are favored at the output plane, but mappings to the auroral ionosphere indicate isotropic pitch angle distributions are favored with some peaking of the fluxes parallel or at other angles to the field lines. Perpendicular auroral pitch angle anisotropies are at times produced by the current sheet acceleration mechanism. Therefore, caution must be used in interpreting all such observations as ‘loss cone-trapping’ distributions. Energy spectra appear to be quite narrow for small cross-tail electric fields, and a little broader as the electric field increases. Comparisons of these results with experimental observations are presented.     

Modeling of Proton Acceleration in a Magnetic Island Inside the Ripple of the Heliospheric Current Sheet

Crossings of the heliospheric current sheet (HCS) at the Earth’s orbit are often associated with observations of anisotropic beams of energetic protons accelerated to energies from hundreds of keV to several MeV and above. A connection between this phenomenon and the occurrence of small-scale magnetic islands (SMIs) near reconnecting current sheets has recently been found. This study shows how pre-accelerated protons can be energized additionally due to oscillations of multiple SMIs inside the ripple of the reconnecting HCS. A model of the electromagnetic field of an oscillating 3D SMI with a characteristic size of ~0.001 AU is developed. A SMI is supposed to be bombarded by protons accelerated by magnetic reconnection at the HCS to energies from ~1keV to tens of keV. Numerical simulations have demonstrated that the resulting longitudinal inductive electric fields can additionally reaccelerate protons injected into a SMI. It is shown that there is a local “acceleration” region within the island in which particles gain energy most effectively. As a result, their average escape energies range from hundreds of keV to 2 MeV and above. There is almost no particle acceleration outside the region. It is shown that energies gained by protons significantly depend on the initial phase and the place of their entry into a SMI but weakly depend on the initial energy. Therefore, low-energy particles can be accelerated more efficiently than high-energy particles, and all particles can reach the total energy limit upon their escape from a SMI. It is also found that the escape velocity possesses a strong directional anisotropy. The results are consistent with observations in the solar wind plasma.

     

Sources of sodium in the lunar exosphere: Modeling using ground-based observations of sodium emission and spacecraft data of the plasma

Observations of the equatorial lunar sodium emission are examined to quantify the effect of precipitating ions on source rates for the Moon’s exospheric volatile species. Using a model of exospheric sodium transport under lunar gravity forces, the measured emission intensity is normalized to a constant lunar phase angle to minimize the effect of different viewing geometries. Daily averages of the solar Lyman α flux and ion flux are used as the input variables for photon-stimulated desorption (PSD) and ion sputtering, respectively, while impact vaporization due to the micrometeoritic influx is assumed constant. Additionally, a proxy term proportional to both the Lyman α and to the ion flux is introduced to assess the importance of ion-enhanced diffusion and/or chemical sputtering. The combination of particle transport and constrained regression models demonstrates that, assuming sputtering yields that are typical of protons incident on lunar soils, the primary effect of ion impact on the surface of the Moon is not direct sputtering but rather an enhancement of the PSD efficiency. It is inferred that the ion-induced effects must double the PSD efficiency for flux typical of the solar wind at 1 AU. The enhancement in relative efficiency of PSD due to the bombardment of the lunar surface by the plasma sheet ions during passages through the Earth’s magnetotail is shown to be approximately two times higher than when it is due to solar wind ions. This leads to the conclusion that the priming of the surface is more efficiently carried out by the energetic plasma sheet ions.     

The warm current sheet model, and its implications on the temporal behaviour of the geomagnetic tail

A simple model current sheet is studied numerically. Consistent fields and particle trajectories, and their dependence on electron and proton temperature, convection velocity and normal field, B_z, linking through the current sheet, are presented and discussed. It is shown that the protons, which are the major current carriers, largely retain the decoupling of the motion in the x-y plane from the normal oscillations as in the ‘cold’ current sheet. The positive potential of the current sheet is shown to be sufficient to trap some energetic electrons, the motion of which enables the predominance of energetic electrons towards the dawn side of the tail to be understood. Semi-empirical relationships for the thickness and the potential of the current sheet are obtained.

The consequences of such a current sheet on the behaviour of the geomagnetic tail are investigated. Using Faraday's law and the consistent cross tail electric field it is shown that the effect of a southward turning of the interplanetary field is to lead to a decrease in B_z,an increase of the current sheet conductivity, and a growth of stored field energy, i.e. the current sheet blocks merging. The decrease of the resistance of the current sheet is limited by the finite width of the tail. Finally, it is pointed out that if the conditions which bring about the growth of field energy persist, then the collapse of the field lines characteristic of substorms may occur.     

Solar wind access to the plasma sheet along the flanks of the magnetotail

Low-energy particle trajectories in an idealized magnetotail magnetic field are investigated to determine the accessibility of magnetosheath protons and electrons to the plasma sheet along the flanks of the tail magnetopause. The drift motion of the positively (negatively) charged particles incident on the dawn (dusk) magnetotail flank causes such particles to penetrate deeper into the magnetotail. For certain combinations of particle energy, incident velocity vector and initial penetration point on the tail magnetopause, the incident particles can become trapped in the plasma sheet, after which their net drift motion then provides a current capable of supporting the entire observed magnetotail field. The results further indicate that the bulk of the solar wind plasma just outside the distant tail boundary, which streams preferentially in a direction along the magnetopause away from the Earth at velocities around 400 km s^?1, can be caught up in the tail if the initial penetration point is within about 2R_E, of the quasi-neutral sheet. It is suggested that a large fraction of the magnetotail plasma is composed of former solar wind particles which have penetrated the magnetospheric boundary at the tail flanks.     

The magnetic field near Mars: A comparison between a hybrid model, Mars Global Surveyor and Mars Express observations

Mars Express (MEX) does not carry its own magnetometer which complicates interpretation of ASPERA-3/MEX ion measurements. The direction of the interplanetary magnetic field (IMF) is especially important because it, among other things, determines the direction of the convective electric field and orientation of the cross tail current sheet and tail lobes. In this paper we present a case study to show the properties of the magnetic field near Mars in a quasi-neutral hybrid (QNH) model at the orbits where the Mars Global Surveyor (MGS) has made measurements, present a method to derive the IMF clock angle by comparing fields in a hybrid model and the direction of the magnetic field measured by MGS by deriving the IMF clock angle. We also use H⁺ ring velocity distribution observations upstream of the bow shock measured by the IMA/ASPERA-3 instrument on board MEX spacecraft. These observations are used to indirectly provide the orientation of the IMF. We use a QNH model (HYB-Mars) where ions are modeled as particles while electrons form a mass-less charge neutralizing fluid. We found that the direct MGS and non-direct IMA observations of the orientation magnetic field vectors in non-crustal magnetic field regions are consistent with the global magnetic field draping pattern predicted by the global model.     

Flushing effect in reconnection: Effects of minority species of oxygen ions

Effects of O⁺ ions on magnetic reconnection in the magnetotail are examined in a Harris equilibrium using a combination of linear Vlasov theory and large-scale driven and non-driven two-dimensional fully kinetic particle simulations. Linear theory of multiple species plasma indicates that the growth rate is rather insensitive to the composition of the background (lobe) or its temperature but more sensitive to the properties of the current carriers. Thus O⁺ can affect significant changes to the linear growth rate of tearing mode only as a current carrier. However, it is demonstrated that in the nonlinear stage reconnection can effectively move trace material from the lobes into the current sheet proper. If the supply of lobe markers (like O⁺) is sufficiently permanent in the lobe, an initially proton-dominated current sheet can be virtually replaced by the marker ions through this “flushing effect”. The dominance of marker ions introduces finite Larmor radius signatures with marker gyroradii scales rather than that of the protons. In this way, the presence of heavier marker species in the lobe can lead to (i) reduced efficiency of energy conversion, (ii) reduction in the number and repetition frequency of secondary islands, (iii) broadening of the quadrupole magnetic structure, (iv) slowing down of the coalescence process and (v) modification of the composition of the ion current carriers. Using Cluster observations, we show the evidence for the “flushing effect” in the data. Detailed comparison with observations is planned for future work.     

A 2-D model for the support of a polar-crown solar prominence

We present a 2-D potential-field model for the magnetic structure in the environment of a typical quiescent polar-crown prominence. The field is computed using the general method of Titov (1992) in which a curved current sheet, representing the prominence, is supported in equilibrium by upwardly directed Lorentz forces to balance the prominence weight. The mass density of the prominence sheet is computed in this solution using a simple force balance and observed values of the photospheric and prominence magnetic field. This calculation gives a mass density of the correct order of magnitude. The prominence sheet is surrounded by an inverse-polarity field configuration adjacent to a region of vertical, open polar field in agreement with observations.A perturbation analysis provides a method for studying the evolution of the current sheet as the parameters of the system are varied together with an examination of the splitting of an X-type neutral point into a current sheet.Program Systems Institute of the Russian Academy of Sciences, Pereslavl-Zalessky 152140, Russia.