Geomagnetotail dynamics: Different types of equilibriums and transitions between them

Electron firehose instabilities can be excited at dipolarization fronts and in the magnetic reconnection outflow in the terrestrial magnetotail, but their occurrence rate in the plasma sheet is unclear. Here, we investigate the characteristics of electron firehose unstable conditions in the magnetotail plasma sheet based on observations of the Magnetospheric Multiscale mission. We find an Alfvénic magnetic field fluctuation accompanied by a strong field-aligned current during a flapping motion. This fluctuation occurs where the local plasma is electron firehose unstable, indicating that the electron firehose instability in the plasma sheet can occur in the region besides dipolarization fronts and magnetic reconnection outflow. We statistically find that the local plasma near the neutral sheet has a small probability with the maximum value <1.4% to be electron firehose unstable, which mainly occurs in the central plasma sheet with BXY/BL < 0.3. The maximum probability of Tef > 0 (electron firehose unstable condition) is ∼1.36% (1.32%) at BXY/BL ≈ 0.05 (0.15) during fast (non-fast) flows. During fast flows, the plasma near the neutral sheet tends to have a higher probability of Tef > 0 when the local VT is larger. During non-fast flows, the plasma near the neutral sheet tends to have a higher probability of Tef > 0 when Te is larger. The probability of Tef > 0 shows a dawn-dusk asymmetry during fast flows and non-fast flows. In addition, the probability of Tef > 0 during fast flows tends to be larger when the ambient BZ is weak, which shows opposite characteristics during non-fast flows. These findings help to assess the importance of the role of electron firehose instabilities in the magnetotail plasma sheet.

We have used the Auburn Global Hybrid Code in 3‐D to study the generation, dynamics, and global structure of kinetic Alfvén waves (KAWs) from the magnetotail to the ionosphere. Our results show that KAWs are generated in magnetic reconnection in the plasma sheet, located around fast flows, and carrying transverse electromagnetic perturbations, parallel Poynting fluxes, parallel currents, and parallel electric field. Overall, shear Alfvénic turbulent spectrum is found in the plasma sheet. The KAWs are shear Alfvén waves possessing short perpendicular wavelength with , where is the perpendicular wave number and the ion Larmor radius. The KAWs are identified by their dispersion relation and polarizations. The structures of these KAWs embedded in the plasma sheet are also revealed by placing a virtual satellite in the tail. In order to understand whether the Poynting fluxes carried by the shear Alfvén waves/KAWs in the plasma sheet can be carried directly along field lines to the ionosphere, we have tracked the wave propagation from the plasma sheet to the ionosphere. It is found that in front of the flow‐braking region, the structure and strength of the shear Alfvén waves are significantly altered due to interaction with the dipole‐like field, mainly by the flow shear associated with the azimuthal convection. Also in front of the dipole‐like field region, ion kinetic effects (Hall effects) lead to the generation of additional pairs of KAWs. As such, the generation and transport of the shear Alfvén waves/KAWs to the ionosphere are illustrated for the first time in a comprehensive manner on the global scale.

[1] McComas and Bagenal [2007] (hereinafter referred to as MB) have presented a discussion of the reconnectionmediated interaction of the Jovian magnetosphere with the interplanetary medium, which they suggest to be significantly different to that at Earth. In the latter case, it is well established that ‘open’ flux is produced at the magnetopause when the interplanetary magnetic field (IMF) is directed opposite to the equatorial planetary field, is transported to the tail by the solar wind, and returns as closed flux via plasma sheet reconnection preferentially during substorms, thus forming the Dungey cycle of flux transport [e.g., Dungey, 1961]. MB propose that the consequences of open flux production at Jupiter are different, however, due to a suggested difficulty of closed flux tube return from the tail against a substantial down-tail flow of iogenic plasma. They suggest instead that open flux is effectively removed by two-lobe reconnection when the IMF has the opposite polarity, such that the open flux in the system remains small. Two-lobe reconnection has been discussed theoretically for many years [e.g., Dungey, 1963; Cowley, 1981], though convincing evidence for its occurrence at Earth has only recently been found [e.g., Imber et al., 2006, 2007]. Here, however, we question both aspects of MB’s discussion. [2] With regard to the return of tail flux by plasma sheet reconnection, MB characterise the process as requiring closed field contraction over distances of 1500–2000 RJ at speeds of 40 km s , thus requiring 30–40 days. They suggest this to be unlikely given the surrounding fast down-tail flow of iogenic plasma. However, we regard this scenario as being unduly pessimistic, first because the estimate of the distance to the tail reconnection site is unrealistically large, and second because the closed flux tube contraction speed is unrealistically small, both contributing to unrealistically large estimates of the transport time. MB’s estimate of the distance to the tail reconnection site is essentially the length of the entire tail of open field lines, obtained by multiplying the solar wind speed by the residence time of open flux tubes in the lobe. This time is estimated to be 3–4 days on the basis that open field lines flow toward the plasma sheet at 10% of the solar wind speed ( 40 km s ) for 10% reconnection efficiency with the IMF, leading to a tail length of 1500–2000 RJ as indicated above. In fact, this significantly underestimates the length of the Jovian tail, since the lobe flow speed is slowed relative to MB’s estimate by the ratio of the lobe and IMF field strengths, i.e. factors of two to three, while an overall magnetopause reconnection efficiency of 10% seems optimistic. A more realistic residence time is 10– 20 days [Nichols et al., 2006], leading to tail lengths of 5000–10000 RJ in agreement with Lepping et al. [1983]. [3] The main point to emphasise, however, is not the inaccuracy of MB’s tail length estimate, but that such estimates provide no information about the location of the tail reconnection sites, other than an upper limit. For Earth, for example, similar estimates produce tail lengths of 1000 RE [e.g., Milan, 2004], while substorm-related reconnection is typically initiated at down-tail distances of 20–30 RE [e.g., Nagai and Machida, 1998]. While flux return from the distant tail may be unlikely as MB suggest, a reasonable conclusion is that open flux will then accumulate until reconnection occurs substorm-like sufficiently close to the planet that the closed flux is indeed able to return. The return flow speeds are then expected to be comparable to the lobe Alfven speed [e.g., Badman and Cowley, 2007], at least an order of magnitude faster than the return speeds employed in MB’s estimate. [4] Significant evidence indeed exists for sporadic reconnection in the Jovian nightside plasma sheet at distances of 100 RJ, resulting in ion jets directed both toward and away from the planet [e.g., Woch et al., 2002]. These dynamics are generally assumed to relate to pinch-off of distended closed field lines and the down-tail release of iogenic plasma occurring as part of the Vasyliunas cycle [Vasyliunas, 1983]. However, supposing that after plasmoid release the reconnection continues into the tail lobe, then closed flux is generated that will clearly flow back to the planet unencumbered by surrounding down-tail flow, whether the combined reconnection is envisaged as largescale [Cowley et al., 2003], or occurs more sporadically and multiply on smaller scales [Kivelson and Southwood, 2005]. While Dungeyand Vasyliunas-cycle tail reconnection need not be coherently related in this way, the argument is sufficient to show that open flux return from the Jovian tail by plasma sheet reconnection is not as problematic as MB suggest. [5] We now turn to MB’s second argument, that open flux can instead be effectively removed from the tail by two-lobe reconnection poleward of the cusp, such that the amount of open flux in the system remains small. This requires the open flux removal rate by two-lobe reconnection for southward-directed IMF, averaged over typical GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L10101, doi:10.1029/2007GL032645, 2008

We perform a statistical study of flux ropes and reconnection fronts based on MErcury Surface, Space ENviroment, GEochemistry, and Ranging (MESSENGER) magnetic field and plasma observations to study the implications for the spatial distribution of reconnection sites in Mercury's near magnetotail. The results show important differences of temporal and spatial distributions as compared to Earth. We have surveyed the plasma sheet crossings between −2 RM and −3 RM downtail from the planet, i.e., the location of Near‐Mercury Neutral Line (NMNL). Plasma sheets were defined to be regions with β ≥ 0.5. Using this definition, 39 flux ropes and 86 reconnection fronts were identified in the plasma sheet. At Mercury, the distributions of flux ropes and reconnection fronts show clear dawn‐dusk asymmetry with much higher occurrence rate on the dawnside plasma sheet than on the duskside. This suggests that magnetic reconnection in Mercury's magnetotail occurs more frequently in the dawnside than in the duskside plasma sheet, which is different than the observations in Earth's magnetotail showing more reconnection signatures in the duskside plasma sheet. The distribution of plasma sheet thickness shows that plasma sheet near the midnight is the thinnest part and does not show obvious asymmetry. Thus, the reasons that cause magnetic reconnection to preferentially occur on the dawnside of the magnetotail at Mercury may not be the plasma sheet thickness and require further study. The peak occurrence rates of flux ropes and reconnection fronts in Mercury's plasma sheet are ~ 60 times higher than that of Earth's values, which we interpret to be due to the highly variable magnetospheric conditions at Mercury. Such higher occurrence rate of magnetic reconnection would generate more plasma flows in the dawnside plasma sheet than in the duskside. These plasma flows would mostly brake and initiate the substorm dipolarization on the postmidnight sector at Mercury rather than the premidnight susbtorm onset location at Earth.

We supplement the results of the 27 August 2001 substorm studied earlier in the series of papers. Described is the plasma flow in the nightside ionosphere from the near‐polar region from the polar cap to the auroral oval during the substorm preonset phase and two expansion onsets, EO1 and EO2, produced by reconnection in the closed tail (magnetic reconnection (MR1) and in the open tail lobes (MR2), respectively. We discuss the location of the MR2 region (is it near, middle, and/or distant tail?) and the EO2 trigger mechanism. The upward substorm current wedge field‐aligned current (FAC) and the downward FAC in the polar cap dusk sector that were both produced by different types of magnetosphere‐ionosphere feedback instability are found to provide the main contribution to the system of FACs during EO1 and EO2. Also, we obtain the estimates for the EO1 and EO2 power and energy. Addressed are the variations in the tail lobe magnetic flux and their (variations) association with EO2. In addition, we describe a 3‐D system of mesoscale cells, each of which involves a plasma vortex and a local FAC maximum. The cells of this system in the inner magnetosphere and in the tail lobes intensify one after other within 2 min interval. At last, we substantiate the assumption that the fast plasma flow recorded by the Cluster satellites 7 min prior to EO1 was a bursty bulk flow from the most distant tail.

Magnetic reconnection in Earth's magnetotail produces fast earthward flows in the plasma sheet. Tailward flows are often observed associated with the earthward flows. Both return flow vortices at the flanks of an earthward flow channel and rebound of the earthward flow from the intense dipolar magnetic field of the inner magnetosphere have been shown to explain tailward flows observed near Earth. We combine plasma sheet measurements from Cluster with conjugate ground‐based magnetic and auroral data to examine the development of earthward and tailward flow signatures during a substorm onset. We show for the first time observations of ionospheric signatures that appear to be associated with rebound flows. Because of the highly dynamic magnetotail configuration, special care is taken with the satellite footprint mapping. The ionospheric footprints produced by the event oriented AM02 model drift equatorward and poleward in response to tail magnetic field stretching and dipolarization, respectively. The footprint motion matches that of the ambient ionospheric structures, and the plasma flow measured by Cluster agrees with that inferred from the conjugate ionospheric observations, confirming the validity of the AM02 mapping. The ionospheric signatures of fast earthward flows during a substorm onset are shown to resemble the known signatures of quiet‐time flows, including equatorward propagating auroral streamers inside a channel of enhanced poleward equivalent current. However, the large‐scale dipolarization results in additional poleward expansion of the signatures, as has been predicted by simulations.

We have studied the three‐dimensional structure of substorm‐associated variations in the magnetotail with GEOTAIL data. For this study we selected 342 substorm events from the Pi2 pulsation and applied the method of superposed epoch analysis. We divided the data into those in the plasma sheet, the plasma sheet boundary layer, and the lobe by the ion β. It was found that the fast tailward flows start to develop in the premidnight plasma sheet around X ∼ −28 RE (GSM) about 0 ∼ 2 min before onset, associated with the plasmoid formation. Immediately after onset, the fast tailward flows develop further, and the magnetic field substantially increases southward. Simultaneously, the northward magnetic field increases around X ∼ −10 RE, corresponding to the dipolarization. In the lobe, the perpendicular plasma flow toward the plasma sheet, as well as the dawn‐dusk electric field, first starts to be enhanced around (X, Y) ∼ (−20,7) RE about O ∼ 2 min before onset and then in the surrounding regions successively. The total pressure decrease first occurs around (X, Y) ∼ (−18,7) RE about 0 ∼ 2 min before onset, and then propagates to the surrounding regions successively. The dawn‐dusk electric fields both calculated with E = −V × B and measured directly by the double probe, simultaneously develop around X ∼ −10 RE and X ∼ −28 RE immediately after onset, while those in the plasma sheet around.(X, Y) ∼ (−20,5) RE do not develop much even after onset. These observational results strongly suggest that an efficient magnetic reconnection takes place at least about 0 ∼ 2 min earlier than the Pi2 onset, that the substantial plasmoid evolution and the dipolarization occur simultaneously immediately after onset, and that, on average, the center of the energy release, where the near‐Earth neutral line (i.e., the diffusion region) is possibly created, is initially located around (X, Y) ∼ (−19,6) RE. These features are consistent with a thin‐current reconnection model.

The spontaneous fast reconnection model is applied to the earthward fast flow events observed in the near‐Earth plasma sheet. Here, the earthward fast flow events include both of bursty bulk flow events and flow burst events. In order to apply it directly to actual observations, virtual probes are located in the plasma sheet region in the three‐dimensional simulation domain so that we can directly observe the temporal variations of plasma quantities in accordance with the growth and proceeding of the fast reconnection. In this model, magnetic reconnection drastically evolves and Alfvénic fast plasma jet flows in the very restricted narrow channel, and a large‐scale plasmoid is formed ahead of the fast plasma jet. The results of virtual observation of these evolutions are found to be in good agreement with actual satellite observations. At the same time, in the lobe region, travelling compression regions (TCRs) are observed in connection with the fast flow events. The temporal profiles of magnetic fields detected by the virtual probes are also in good agreement with actual satellite observations. It is concluded that the earthward fast flow events and earthward TCR events result from the fast reconnection mechanism.

Intensive correlative studies of magnetic fields and plasmas within the earth's magnetotail at geocentric radial distances of ∼23–46 RE during March-October 1974 have revealed striking new features. The instruments employed in this survey were the University of Iowa Lepedea and the Goddard Space Flight Center magnetometer. The hot tenuous plasmas within the plasma sheet were found to be in a state of almost continual flow and were threaded with northward, or closed, geomagnetic field lines. The magnetic field is referred to as ‘northward’ or ‘southward’ if the component Bz is positive or negative, respectively, regardless of the magnitudes of the other components. Proton bulk speeds were in the range 50–500 km s−1. The magnetic fields are directed northward, irrespective of whether the plasma flows are tailward or earthward. These observations show the demand for a strong persistent source of magnetic flux and hot plasmas for the plasma sheet. No characteristic proton bulk flows, e.g., strong dawn-to-dusk motions, were evident during crossings of the magnetic neutral sheet. Occasionally, the satellite encountered the region of acceleration in the magnetotail, the ‘fireball.’ This spectacular phenomenon exhibits strong jetting of plasmas in excess of 1000 km s−1, proton temperatures of ∼107 °K (kT ∼ 1 keV), disordered magnetic fields, southward magnetic fields during tailward jetting of plasmas, and northward magnetic fields for fast plasma flows toward earth. Observations of electrons are used to demonstrate that earthward plasma flows within the fireball are threaded with closed geomagnetic field lines, and open magnetic field lines are embedded in the tailward jetting plasmas. The magnetosheathlike plasmas within the boundary layers which are positioned contiguously to the plasma sheet display striking evidences of plasma heating, great changes in bulk flow velocities and acceleration of energetic electrons with E > 45 keV. Typical temperatures and bulk flow speeds within the boundary layer plasmas are ∼106 °K and ∼400 km s−1, respectively. Persistent zones of southward magnetic fields are detected, which are often positioned adjacent to the plasma sheet and within the boundary layer plasmas. Rotations of the magnetic fields from southward to northward, or vice versa, in these boundary layers are accompanied by large enhancements of energetic electron intensities, substantial heating of the low-energy electron distributions, and strong perturbations of the proton velocity distribution functions. During periods for northward directed fields the proton bulk flows decelerate to speeds of ≲200 km s−1, densities decrease, and the proton velocity distributions can be fitted with two Maxwellians with temperatures of ∼2 × 105 °K and ∼ 107 to 5 × 107 °K for specific examples. The boundary layer plasmas appear to be the primal source of hot plasmas and closed geomagnetic field lines for the plasma sheet. Qualitatively, all of the major results of the present study are consistent with macroscopic features which are expected from merging of magnetic field lines in the earth's magnetotail or, more specifically, in the magnetosheathlike boundary layer plasmas.

Multi-channel opportunistic spectrum access: A mixed-scale decision perspective

A two-satellite study of nightside flux transfer events in the plasma sheet

In the emerging field of multiscale systems, many practical problems in the physical and biological sciences require the simulation of signals at the molecular level or the merging of signals generated over large and different spatial and time scales. Computationally, the former is intractable for large-sized systems (hundreds of millions of atoms) over large time scales (micro to miliseconds) while the compounding of information over different scales presents unique challenges and opportunities for wavelet-based as also other techniques. While the multiresolution aspect of wavelets has been explored in this context, the generation of a formal, rigorous methodology that would govern their use in the understanding of interscale phenomena still exists as an open problem. The purpose here is to provide a brief elementary introduction to the multiscale problem, to introduce the role that wavelets and in particular the Compound Wavelet Matrix have played in the bridging of information across scales, and to suggest related areas for exploration. A few simple illustrative multiscale simulation examples where wavelets are used to bridge scales are shown.

Abstract. Energetic electrons with 90deg pitch angle have been observed in the magnetotail at ~19 RE near local midnight during the recovery phase of a substorm event on 27 August 2001 (Baker et al., 2002). Based on auroral images Baker et al. (2002) placed the substorm expansion phase between ~04:06:16 and ~04:08:19 UT. The electron enhancements perpendicular to the ambient magnetic field occurred while the Cluster spacecraft were on closed field lines in the central plasma sheet approaching the neutral sheet. Magnetic field and energetic particle measurements have been employed from a number of satellites, in order to determine the source and the subsequent appearance of these electrons at the Cluster location. It is found that ~7.5 min after an X-line formation observed by Cluster (Baker et al., 2002) a current disruption event took place inside geosynchronous orbit and subsequently expanded both in local time and tailward, giving rise to field-aligned currents and the formation of a current wedge. A synthesis of tail reconnection and the cross-tail current disruption scenario is proposed for the substorm global initiation process: When a fast flow with northward magnetic field, produced by magnetic reconnection in the midtail, abruptly decelerates at the inner edge of the plasma sheet, it compresses the plasma populations earthward of the front, altering dynamically the Bz magnetic component in the current sheet. This provides the necessary and sufficient conditions for the kinetic cross-field streaming/current (KCSI/CFCI) instability (Lui et al., 1990, 1991) to initiate. As soon as the ionospheric conductance increases over a threshold level, the auroral electrojet is greatly intensified (see Fig. 2 in Baker et al., 2002), which leads to the formation of the substorm current wedge and dipolarization of the magnetic field. This substorm scenario combines the near-Earth neutral line and the current disruption for the initiation of substorms, at least during steady southward IMF. One can conclude the following: The observations suggest that the anisotropic electron increases observed by Cluster are not related to an acceleration mechanism associated with the X-line formation in the midtail, but rather these particles are generated in the dusk magnetospheric sector due to the longitudinal and tailward expansion of a current disruption region and subsequently observed at the Cluster location with no apparent energy dispersion. Keywords. Magnetospheric physics (Magnetotail; Plasma convection; Storms and substorms)

Despite frequent disturbances from flow, stream meiofauna form diverse and abundant assemblages suggesting that they are resistant and/or resilient to flow disturbances. Stream flow profoundly influences benthic invertebrate communities but these effects remain poorly understood. We examined the influence of flow on meiofauna colonization at small spatial scales (2–3 m) using artificial streams in conjunction with similar sites (flow, depth, substrates) in the reference stream (Illinois River, Arkansas). Colonization of meiofauna was found to be rapid and generally increased with flow rates examined (1–2, 6–7, and 11–12 cm s−1). Six of the 10 most abundant taxa successfully completed colonization in artificial channels (equaled or exceeded reference benthic densities) within 5 days. Benthic meiofauna were more abundant in fast flows in artificial channels and in fast and slow flows in reference stream sites. A diverse assemblage of meiofauna was collected from the plankton which was dominated by rotifers, copepods (mostly nauplii), dipterans, and cladocerans. Densities of drifting meiofauna (potential colonists of the benthos) were low (5 no. l−1) and similar among artificial channels and reference sites regardless of flow rates (F1,18 = 2.19, p = 0.1407). Although densities were low, the numbers of drifting meiofauna were more than sufficient to colonize the benthos. Less than 0.65% of the drifting meiofauna were needed to colonize the substrates of artificial streams. The benthic assemblage paralleled that of the plankton, consisting mainly of rotifers, copepods (mainly nauplii), and dipterans. Evidence for active control over dispersal was observed as meiofauna densities varied between the plankton and benthos over the diel cycle (F1,18 = 6.02, p = 0.0001 and F1,18 = 9.88, p = 0.006, respectively). Rotifers, copepods, and nematodes were more abundant in the plankton during the day and in the substrates at night. These results suggest that meiofauna assemblages can change rapidly in response to alterations of habitat patches by disturbance.

In a substorm event of August 14, 1996, the Geotail satellite was at XGSM ∼ −10 RE aligned with the GOES 8 satellite in the X direction in the premidnight sector. Two Pi2 onsets were identified. Whereas the first onset was associated with a pseudobreakup, an auroral bulge developed following the second onset. At the pseudobreakup Geotail observed a fast (>900 km/s) plasma flow along with dipolarization, but the geosynchronous magnetic field became stretched subsequently. In contrast, for the main onset, the magnetic field dipolarized at GOES 8, although the associated total (time‐integrated) transport of magnetic flux at Geotail was no stronger than that for the pseudobreakup. It is suggested that the braking of the fast flow does not necessarily cause tail current disruption in a large scale, and there is an additional (or alternative) condition to suffice for an initial brightening to develop to a global substorm.