Distant Tail Research Articles

Escape rates and variability constraints for high‐energy sodium sources at Mercury

We present a 3‐D time‐dependent modeling of Mercury's extended sodium exosphere in the region of the distant tail. Recent simulations in support of the MESSENGER and BepiColombo missions have shown spatial distributions in Mercury's exosphere can be linked to specific source processes. Our model builds upon these efforts by concentrating on the escaping component of Mercury's surface‐bound exosphere to assess the wide‐field (7°) data described in the studies by Baumgardner et al. (2008) and Schmidt et al. (2010). Escape rates of several source processes in Mercury's exosphere are determined, and the effects of orbital motion, surface‐gas interactions, variable source rates, and spatially heterogeneous sources are explored. Our simulations demonstrate that both photon‐stimulated desorption and micrometeoroid impacts can result in a ∼20% loss of Mercury's sodium atmosphere, depending on orbital phase, and are jointly responsible for the observed comet‐like tail as driven by solar radiation pressure. For a constant supply of both these sources into the exosphere, the modeled average column density over Mercury's disc decreases by ∼1/3 at 70° true anomaly (the orbital position for peak escape), consistent with observations over many orbits.

A study of the changes of the near‐Earth plasma sheet and lobe driven by multiple substorms: Comparison with a full particle simulation of reconnection

Comparisons of multispacecraft observations and full‐particle simulations are used to understand magnetotail changes during substorms and the related cross‐tail current disruptions/reductions. We first show that the electric field accompanying current disruptions can be measured in the tail lobe from the drift velocity of oxygen beams. A stormy period is studied here with a fleet of spacecraft including the four Cluster spacecraft and the Double Star spacecraft TC‐1 in the tail, ACE and Geotail respectively in the solar wind and magnetosheath, and five LANL geostationary satellites, thus allowing the determination of the direction of propagation of the substorm disturbances. Each substorm here corresponds to an energy‐loading period followed by a dipolarization of the magnetic field seen from 11 to 18 RE. Plasma sheet thinning inside 12 RE occurs during energy loading and is enhanced at the onset of strong dissipations of magnetic energy, which precede by several minutes particle injections at 6.6 RE. Dipolarizations coincide with an increase of the lobe electric field, up to several mV/m. This study shows that the onset of the magnetic energy conversion occurs at about ∼10–11 RE and that once initiated, the perturbation propagates both toward the Earth and toward the distant tail. Comparisons of the measurements with recently published 2D full particle simulations of the reconnection process by Oka et al. (2008) indicate a good agreement between data and simulated magnetic lobe signatures. This suggests that the lobe magnetic changes are the signature of a tailward retreating neutral line, with its associated current disruption/reduction.

Energetic O+and H+ions in the plasma sheet: Implications for the transport of ionospheric ions

The present study statistically examines the characteristics of energetic ions in the plasma sheet using the Geotail/Energetic Particle and Ion Composition data. An emphasis is placed on the O+ ions, and the characteristics of the H+ ions are used as references. The following is a summary of the results. (1) The average O+ energy is lower during solar maximum and higher during solar minimum. A similar tendency is also found for the average H+ energy, but only for geomagnetically active times; (2) The O+ -to -H+ ratios of number and energy densities are several times higher during solar maximum than during solar minimum; (3) The average H+ and O+ energies and the O+ -to -H+ ratios of number and energy densities all increase with geomagnetic activity. The differences among different solar phases not only persist but also increase with increasing geomagnetic activity; (4) Whereas the average H+ energy increases toward Earth, the average O+ energy decreases toward Earth. The average energy increases toward dusk for both the H+ and O+ ions; (5) The O+ -to -H+ ratios of number and energy densities increase toward Earth during all solar phases, but most clearly during solar maximum. These results suggest that the solar illumination enhances the ionospheric outflow more effectively with increasing geomagnetic activity and that a significant portion of the O+ ions is transported directly from the ionosphere to the near ]Earth region rather than through the distant tail.

Observations of dawn–dusk aligned polar cap aurora during the substorms of January 21, 2005

A new type of polar cap aurora, dawn–dusk aligned polar cap aurora (DDAPCA), was detected during the exceptionally intense January 21, 2005 substorm (AEmax=3504nT). The DDAPCA was located at very high latitude (>85°MLAT) in the polar cap region. As the interplanetary magnetic field (IMF) GSM By component rotated from a positive to a negative value, the DDAPCA tilt angle relative to the dawn–dusk direction rotated anticlockwise and reached ∼45°. It is speculated that the DDAPCA arises from the formation of an X-line in the distant (>80RE) tail due to polar cap magnetic field reconnection under unusually high solar wind compression conditions.

Statistical study of O+ transport from the cusp to the lobes with Cluster CODIF data

Ionospheric origin O+ accelerated in the cusp/cleft region convects over the polar cap and flows along open field lines to the tail lobes. Some O+ in the lobes enters the near‐Earth plasma sheet where it is then convected to the inner magnetosphere, while some O+ ends up in the distant tail where it is lost. In order to understand the transport of ionospheric O+ to the plasma sheet so as to understand its contribution to the formation of geomagnetic storms, we have determined the occurrence frequency of cusp source O+ over the polar caps and in the lobes to determine where and when it is observed. The results show that the probability of observing O+ along the transport path is high even during nonstorm times, although, as expected, the highest probability is found during storm times. It was also found that when interplanetary magnetic field (IMF) By is positive, O+ from the northern cusp/cleft tends to stream toward the dawnside tail lobe while O+ from the south are observed on the duskside. The transport path for negative IMF By is more symmetric, but shows some evidence for a reversed asymmetry when IMF By is strongly negative. IMF Bz has little influence on the asymmetry. The asymmetry for positive By and lack of mirror symmetry between positive and negative By most likely result from the combination of convection driven by the solar wind and coupling with the ionosphere. Similar asymmetries have been observed in the convection patterns over the polar caps, which are attributed to a day‐night ionospheric conductivity gradient adding to the IMF By effect. However, there are some disagreements between the asymmetries observed in polar cap potential patterns and the asymmetries observed in the O+ spatial distribution, indicating there may be other causes for the symmetry breaking, in addition to the day‐night conductivity gradient.

Effects of nightside O+ outflow on magnetospheric dynamics: Results of multifluid MHD modeling

Through spacecraft observations and numerical modeling, it is becoming increasingly well established that ionospheric oxygen is present in the magnetosphere in amounts that can have an effect on magnetospheric dynamics, affecting pressure balance, currents, convection flows, and cross polar cap potential (CPCP). However, an understanding of the various processes through which oxygen may bring about changes in magnetospheric structure and dynamics is still lacking. In this paper, we focus on the role that ionospheric oxygen outflow from the nightside auroral zone plays in determining plasma sheet density, size, and pressure and relate the changes in plasma sheet parameters to changes in CPCP, tail geometry, and magnetospheric convection. We use the Multifluid Lyon‐Fedder‐Mobarry model as a virtual laboratory to analyze the effects of ionospheric oxygen on magnetospheric parameters. Our simulations use idealized solar wind conditions, constant and uniform ionospheric conductance, and constant nightside oxygen outflow in order to isolate the effects of a change in outflow flux. We concentrate the ionospheric outflow in the nightside auroral region, so as to avoid the possibility of directly loading the distant tail X line with oxygen and hence to avoid a direct modification of the reconnection rate in order to focus on effects within the magnetosphere. We show that the presence of O+ outflowing from an isolated patch in the nightside auroral zone increases plasma sheet density and thermal pressure, slows convection, decreases the polar cap potential, and increases the length and width of the nightside magnetosphere. We determine that ionospheric O+ mass loading can play a significant role but that strong effects on magnetospheric dynamics require a nightside flux that significantly exceeds statistical observed levels. A more realistic outflow would include both dayside and nightside sources whose total combined flux would be similar to the total fluxes used here.

Effects of the low‐latitude ionospheric boundary condition on the global magnetosphere

In common treatment of magnetosphere‐ionosphere coupling at high latitudes, the ionosphere is represented by a thin conducting spherical shell, which closes field‐aligned currents generated in the magnetosphere. In this approach, the current continuity yields a Poisson equation for the electrostatic potential associated with the ionospheric convection pattern. Solution of the Poisson equation then provides a means of self‐consistently describing magnetospheric and ionospheric plasma convection with a feedback of one on the other. While the high‐latitude ionospheric convection is driven by the solar wind and magnetosphere interaction, at lower latitudes atmospheric neutral winds start to dominate. The question that arises then is whether and how midlaltitude and low‐latitude ionospheric convection affects high‐latitude ionospheric and magnetospheric convection. In global magnetospheric models, ionospheric convection equatorward of the low‐latitude boundary is excluded from the simulation domain. However, the boundary condition applied at that boundary to the electrostatic potential may be used as a proxy of this convection. In this paper, we explore effects that different idealized low‐latitude boundary conditions have on the magnetospheric configuration simulated by the Lyon‐Fedder‐Mobarry global magnetohydrodynamic model. To this end, we perform a number of idealized simulations different only in the low‐latitude ionospheric boundary condition used. We find that the behavior of the system can be influenced rather significantly by the different boundary conditions, which is expressed by changes in the evolution of the polar cap potential, global magnetospheric convection, and plasma pressure distribution in the magnetotail and on the dayside. The differences in the cross‐polar cap potential can reach up to >10%, dependent on the boundary condition used. In the magnetosphere the low‐latitude ionospheric boundary condition affects the strength and location of the plasma outflow from the distant tail x‐line and the subsequent earthward convection. Changes in the plasma pressure distribution on the nightside are accompanied by noticeable differences in the shape of the magnetotail. We confirm that the changes in the magnetospheric and ionospheric configuration are not just temporal deviations of the system from the same average dynamical state by considering 1 h averages of the magnetospheric flow and pressure distribution. These results verify that the simulated system reaches similar but distinctly different dynamical states dependent on the low‐latitude boundary condition applied.

A simulation study of dynamics in the distant Jovian magnetotail

In late February 2007, the New Horizons spacecraft flew past Jupiter and repeatedly observed tailward moving plasma structures in Jupiter's distant tail (x > −2000 RJ). There were no solar wind observations in the outer solar system at this time. In this study, we have used a new magnetohydrodynamic model with a 1500 RJ long tail to investigate the propagation of these structures under different solar wind conditions. In the simulation we find that periodic plasmoid ejection from a reconnection line in Jupiter's plasma sheet can occur when the solar wind dynamic pressure is low and the IMF has a northward component along the dipole direction. In particular, periodic plasmoids which are similar to New Horizon observations are reproduced when the interplanetary magnetic field (IMF) was oscillating from northward to southward with a 10 h period similar to what would be expected for a rotating planetary magnetic field interacting with an IMF with only a y component. When the IMF had a southward component along the dipole direction, high‐latitude reconnection can form plasmoid‐like structures. As a result plasma structures moving tailward can be found over a large region of the tail, as is observed. Oscillations in the solar wind dynamic pressure lead to oscillations in the plasma sheet.

A three‐dimensional asymmetric magnetopause model

A new three‐dimensional asymmetric magnetopause model has been developed for corrected GSM coordinates and parameterized by the solar wind dynamic and magnetic pressures (Pd + Pm), the interplanetary magnetic field (IMF) Bz, and the dipole tilt angle. On the basis of the magnetopause crossings from Geotail, IMP 8, Interball, TC1, Time History of Events and Macroscale Interactions during Substorms (THEMIS), Wind, Cluster, Polar, Los Alamos National Laboratory (LANL), GOES, and Hawkeye, and the corresponding upstream solar wind parameters from ACE, Wind, or OMNI, this model is constructed by the Levenberg‐Marquardt method for nonlinear multiparameter fitting step‐by‐step over the divided regions. The asymmetries of the magnetopause and the indentations near the cusps are appropriately described in this new model. In addition, the saturation effect of IMF Bz on the subsolar distance and the extrapolation for the distant tail magnetopause are also considered. On the basis of this model, the power law index for the subsolar distance versus Pd + Pm is a bit less than −1/6, the northward IMF Bz almost does not influence the magnetopause, and the dipole tilt angle is very important to the north–south asymmetry and the location of indentations. In comparison with the previous empirical magnetopause models based on our database, the new model improves prediction capability to describe the three‐dimensional structure of the magnetopause. It is shown that this new model can be used to quantitatively study how Pd + Pm compresses the magnetopause, how the southward IMF Bz erodes the magnetopause, and how the dipole tilt angle influences the north–south asymmetry and the indentations.

Active phase of a substorm as a chain of two types of reconnection: In the closed plasma sheet and in the open tail lobes

The maps of the field-aligned current (FAC) density distribution in the ionosphere obtained by the TIM-2 magnetogram inversion technique are used to investigate the August 27, 2001 substorm. The open magnetic flux Ψ and intensity J of the substorm current wedge (SCW) have been determined with a step of 1–5 min. The substorm onsets are divided into two types, PSR (plasma sheet reconnection) and TLR (tail lobe reconnection). The fast expansion tailward of the PSR region is described as the transition from PSR to PSR+TLR. Assuming that the SCW FACs flow down into the ionosphere from the edges of the disruption region of the cross-tail dawn-dusk current, several parameters of the disruption region have been estimated. The disrupted magnetic field has been found to be ∼5% of the undisrupted one for PSR and ∼95% for PSR+TLR. The disturbance power Q for PSR is an order of magnitude lower than that for PSR+TLR. The abrupt growth of Q during the transition from PSR to PSR+TLR is observed over the entire SCW area from its near-Earth part to the midtail and distant tail.

Effects on the distant geomagnetic tail of a fivefold density drop in the inner sheath region of a magnetic cloud: A joint Wind–ACE study

Using a serendipitous configuration of the ACE and Wind spacecraft, we monitor the response of the distant geomagnetic tail (∼ −220 R E ) to an abrupt, approx. fivefold pressure drop (from ∼19.0 to ∼3.5 nPa) at the front boundary of a magnetic cloud (MC) on November 20, 2003. The interplanetary data are from ACE in orbit around the L1 point. The far-tail observations are from Wind, which was nominally in the magnetosheath, separated from the Sun–Earth line by ∼ 40 R E . The magnetic field in the innermost sheath region of the MC had a large B y ( ∼ 30 nT ) and substantial and variable flows lateral to the Sun–Earth line. There was also a significant northward field (∼35 nT), unique in the vicinity of this MC. These extreme values are reached in a filament forming the earliest relic of material accreted by the MC en route to Earth. The effects resulting from these on the far geomagnetic tail are: (1) expansion, (2) tail twisting, and (3) tail tilting. These extreme conditions were in part responsible for a crossing by Wind of a neutral sheet which is tilted by ∼85° to the ecliptic. Further, Wind made two successive excursions deep into the geomagnetic tail, in the first of which a tailward flow burst of ∼1200 km/s was observed. The dayside part of the interaction of the sudden and large dynamic pressure drop with the bow shock is studied with a local 3D MHD simulation. This work is a contribution to the area ICME/MC-sheaths–magnetosheath interactions.

Simultaneous entry of oxygen ions originating from the Sun and Earth into the inner magnetosphere during magnetic storms

The aim of this paper is to present, for the first time, almost simultaneous enhancements of both low‐ and high‐charge‐state oxygen ions in the inner magnetosphere during magnetic storms. Storm‐time enhancements of low‐charge‐state O ions are well recognized, but the behavior of high‐charge‐state O ions is less known. Data simultaneously collected from the ACE, Geotail, and Polar satellites indicate the following: (1) In the inner magnetosphere (at L = 3–5), the number density of high‐charge‐state O ions was increased during the early phase of magnetic storms (Polar). (2) No corresponding enhancements were identified in the number density of O ions observed in the solar wind (ACE) and the near‐Earth magnetotail (Geotail). (3) The number density of high‐charge‐state O ions present in the near‐Earth magnetotail was considerably lower than in the solar wind and the inner magnetosphere. We calculated trajectories of O6+ ions under electric and magnetic field models. The O6+ ions that became observable in the energy window of Polar were transported from the high‐latitude magnetopause to the inner magnetosphere when the convection electric field was strong. When the convection electric field was weak, the ions were reflected toward the distant tail. The O6+ ions that became observable in the energy window of Geotail were sufficiently transported from the low‐latitude magnetopause to the near‐Earth magnetotail regardless of the strength of the convection electric field. The observational facts may be adequately explained in terms of ion transport paths depending on the convection electric field with different entry points.

Assessing access of galactic cosmic rays at Moon's orbit

Characterizing the lunar radiation environment is essential for preparing future robotic and human explorations on lunar bases. Galactic cosmic rays (GCR) represent one source of ionizing radiation at the Moon that poses a biological risk. Because GCR are charged particles, their paths are affected by the magnetic fields along their trajectories. Unlike the Earth, the Moon has no strong, shielding magnetic field of its own. However, as it orbits Earth, the Moon traverses not only the weak interplanetary magnetic field but also the distant magnetic tail of Earth's magnetosphere. We combine an empirical magnetic field model of Earth's magnetosphere with a fully‐relativistic charged particle trajectory code to model and assess the access of GCR at the Moon's orbit. We follow protons with energies of 1, 10 and 100 MeV starting from an isotropic distribution at large distances outside a volume of space including Earth's magnetosphere and the lunar orbit. The simulation result shows that Earth's magnetosphere does not measurably modify protons of energy greater than 1 MeV at distances outside the geomagnetic cutoff imposed by Earth's strong dipole field very near to the planet. Therefore, in contrast to Winglee and Harnett (2007), we conclude that Earth's magnetosphere does not provide any substantial magnetic shielding at the Moon's orbit. These simulation results will be compared to LRO/CRaTER data after its planned launch in June 2009.

Transport Regimes and Concentration Tails for Classical Diffusion in Heterogeneous Media with Sharply Contrasting Properties

In this study, we solved a problem of tracer transport in heterogeneous media with a sharp contrast in properties. Two kinds of media were considered. The first was a heterogeneous medium with a highly permeable domain in the shape of a plain‐parallel layer or a straight cylinder. A highly permeable domain in the second medium corresponded to a percolation system. Classical diffusion was considered for both kinds of media as the basic physical mechanism for tracer transport. Due to long residence times of tracer particles in the low‐permeability domain, three transport regimes (fast classical diffusion, subdiffusion, and slow classical diffusion) occurred in the first heterogeneous medium. Similarly, in the second medium the presence of sinks (dead ends of percolation clusters) led to subdiffusion at early times, changing to slow classical diffusion at large times when the system was above the percolation threshold. For both kinds of media, the change of transport regimes with time resulted in a complex multistage structure of breakthrough curves. The structure of the more distant tail segments was found to be more strongly affected by the earlier time transport regimes.

Universal and nonuniversal tails of distribution functions in the directed polymer and Kardar-Parisi-Zhang problems

The optimal-fluctuation approach is applied to study the most distant (nonuniversal) tails of the free-energy distribution function ${P}_{L}(F)$ for an elastic string (of a large but finite length $L$) interacting with a quenched random potential. A further modification of this approach is proposed which takes into account the renormalization effects and allows one to study the closest (universal) parts of the tails. The problem is analyzed for different dimensions of a space in which the polymer is imbedded. In terms of the stochastic growth problem, the same distribution function describes the distribution of heights in the regime of a nonstationary growth in the situation when an interface starts to grow from a flat configuration.

Comment on “Jupiter: A fundamentally different magnetospheric interaction with the solar wind” by D. J. McComas and F. Bagenal

Comment on “Jupiter: A fundamentally different magnetospheric interaction with the solar wind” by D. J. McComas and F. Bagenal

Cold ionospheric plasma in Titan's magnetotail

The interaction between Titan and the corotating Saturnian plasma forms an induced magnetosphere with an elongated Alfven‐wing‐style magnetotail. On 26 December 2005, the Cassini spacecraft flew through Titan's magnetotail, providing the first distant tail observation, over 5 Titan radii downstream. We examine measurements observed by the magnetometer and Langmuir probe during this pass. We use the direction of the magnetic field along the trajectory to identify the source regions of plasma reaching the spacecraft. Cold plasma, with a density of about 10 cm−3, is found magnetically connected to the ionosphere. Titan's ionosphere appears to be escaping along field lines down the tail, leading to particle loss from the atmosphere.

Earthward flowing plasmoid: Structure and its related ionospheric signature

An earthward moving plasmoid has been observed on 28 October 2002 by the Cluster spacecraft with simultaneous auroral viewing by the IMAGE satellite. This offers the opportunity to ascertain the optical and the evolutional signatures in the ionosphere of the earthward moving plasmoid. The ionospheric signatures observed in this paper are not substorm‐related. Both the ground‐based measurements and IMAGE satellite auroral observations show the ionospheric signatures moving to lower latitudes, when the earthward moving plasmoid is observed by the Cluster spacecraft. The intensity of the current in the center of the plasmoid is found to be weaker than that in the adjacent region. Also, the directions of the current in the central part of the plasmoid, different from the background cross‐tail current, are more field‐aligned. Those facts are consistent with the tail current closes through the substorm‐like current wedge, since the cross‐tail current is blocked by the plasmoid. On the other hand, the current in the earthward plasmoid may close through the interhemisphere. In this paper we demonstrate that the magnetic structures, plasmoids and flux ropes, will transport flux and energy from the distant tail to the Earth.

Dynamics of equilibrium upset and electromagnetic energy transformation in the geomagnetotail: A theory and simulation using particles. 1. Evolution of configurations in an MHD approximation

The process of equilibrium disruption in the system with a current sheet (CS) under the conditions of small magnetic field component normal to CS, which is induced by an external disturbance, has been theoretically studied within the scope of MHD. In the geomagnetotail, this disturbance can be caused by a tearing instability developing in the more distant tail section, or by a ballooning instability in the tail nearest section, or by a rapid reconfiguration at the magnetopause during the disturbance passage in the solar wind. Locally, in a limited CS section, a longitudinal momentum balance is rapidly (on the Alfven time scale) upset when a fast MHD disturbance, the form of which depends on the presence of CS, passes along the tail. The nonequilibrium temperature, which subsequently evolves through splitting of CS into several current structures, originates on a substantially larger (due to the smallness of the normal field component) time scale. Such a reconfiguration SPONTANEOUSLY develops after the initial equilibrium upset under the action of an external (weak) disturbance. During an analysis within the scope of MHD, this reconfiguration can be described as the well-known process with two pairs of nonlinear waves propagating in both directions from the central sheet plane at constant velocities: these are fast rarefaction waves and the following slow “switching-off” shocks. However, the kinetic theory reveals substantially different relaxation channels. These channels are studied in the second and third work sections, where the kinetic numerical simulation of the problem is presented and the results of this simulation are analyzed.

Fate of outflowing suprathermal oxygen ions that originate in the polar ionosphere

We investigate the transport and ultimate fate of outflowing suprathermal O+ ions by kinetic means using a full‐particle tracing scheme. We launched O+ ions in order to determine their final destination in terms of the magnetopause, the distant tail, the ring current (L‐shell of 5), and the atmosphere. The number of O+ ions that reached each of the destinations was calculated quantitatively by employing an empirical model of the outflowing ion distribution. This empirical model is based on long‐term data from the Suprathermal Ion Mass Spectrometer aboard the Akebono satellite and depends on Kp and the sunspot number. Major results are as follows: (1) Under an active‐time magnetic field and a midstrength convection electric field, the majority of outflowing ions with initial speed greater than the escaping speed reached the ring current, with lesser amounts reaching the magnetopause, the distant tail, and the atmosphere. (2) The presence of an active‐time magnetic field results in an increase in the number of ions that reach the ring current by a factor of ∼3, the magnetopause by a factor of ∼0.5, the distant tail by a factor of ∼200, and the atmosphere by a factor of ∼0.01 in comparison with those under a quiet time magnetic field. (3) An energy distribution of ions at L = 5 shows that in addition to a thermal/suprathermal component, an energetic component peaking at ∼20 keV appears for the active condition, which resembles an averaged spectrum of ring current O+ ions.