Postplasmoid Plasma Sheet Research Articles

MESSENGER observations of flux ropes in Mercury’s magnetotail

We report an investigation of magnetic reconnection in Mercury’s magnetotail conducted with MESSENGER Magnetometer and Fast Imaging Plasma Spectrometer measurements during seven “hot seasons” when the periapsis of the spacecraft orbit is on Mercury’s dayside. Flux ropes are formed in the cross-tail current sheet by reconnection. We have analyzed 49 flux ropes observed between 1.7 RM and 2.8 RM (where RM is Mercury’s radius, or 2440km) down the tail from the center of the planet, for which minimum variance analysis indicates that the spacecraft passed near the central axis of the structure. An average Alfvén speed of 465kms−1 is measured in the plasma sheet surrounding these flux ropes. Under the assumption that the flux ropes moved at the local Alfvén speed, the mean duration of 0.74±0.15s determined for these structures implies a typical diameter of ~345km, or ~0.14 RM, which is comparable to a proton gyroradius in the plasma sheet of ~380km. We successfully fit the magnetic signatures of 16 flux ropes to a force-free model. The mean radius and core field determined in this manner were ~450km, or ~0.18 RM, and ~40nT, respectively. A superposed epoch analysis of the magnetic field during these events shows variations similar to those observed at Earth, including the presence of a post-plasmoid plasma sheet, filled with disconnected magnetic flux, but the timescales are 40 times shorter at Mercury. The results of this flux rope survey indicate that intense magnetic reconnection occurs frequently in the cross-tail current layer of this small but extremely dynamic magnetosphere.

Structure and statistical properties of plasmoids in Jupiter's magnetotail

AbstractPlasmoids and other reconnection‐related signatures have been observed in Jupiter's magnetotail through analysis of magnetic field and energetic particle data. Previous studies have established the spatial distribution and recurrence period of tail reconnection events, and identified the location of a statistical X‐line separating inward and outward flow. Here we present new analysis focusing specifically on 43 plasmoid signatures observed in magnetometer data in order to establish the average properties and internal structure of Jovian plasmoids. We present statistics on the observed plasmoid length scale, duration, radial position, and local time distribution. On average, the observed plasmoids have a ~3 RJ radial extent and ~7 min duration and result in the closure of ~4–8 GWb of open flux from reconnection of open field lines in the postplasmoid plasma sheet. We also determine the amount of mass released and the magnetic flux closed in order to understand the role of tail reconnection in the transport of mass and flux in Jupiter's magnetosphere. The observed plasmoid properties are consistent with a mass loss rate of ~0.7–120 kg/s and a flux closure rate of ~7–70 GWb/d. We conclude that tail reconnection and plasmoid release is an important method of flux transport at Jupiter but likely cannot account for the mass input from Io, suggesting that additional mass loss mechanisms may be significant. Finally, we examine the plasmoid interior structure through minimum variance analysis and find that most plasmoids lack a core field and are better described by magnetic loops rather than flux ropes.

Cassini observations of plasmoid structure and dynamics: Implications for the role of magnetic reconnection in magnetospheric circulation at Saturn

We survey the Cassini magnetometer data during the deep tail orbits in 2006, and find 34 direct encounters with plasmoids. They occur as single, isolated events but also in groups of two or more plasmoids as is frequently observed at Earth . We show a case study example of three such plasmoids over three hours, where we estimate an upper limit of 5.68 GWb of flux closure, and derive a reconnection rate over this interval of 526 kV. We show the results of a superposed epoch analysis of al1 34 plasmoids indicating that, on average, plasmoids at Saturn are approix.8 min in duration and they tend toward a loop-like, as opposed to flux rope-like topology, with little or no axial core magnetic field. Our analysis shows that plasmoids at Saturn are followed by an extended interval of the post-plasmoid plasma sheet (PPPS) lasting approx.58 min. The average open magnetic flux disconnected by the continued reconnet:tion following plasmoid formation that creates the PPPS is approx.3 GWb. We calculate expected recurrence rates for plasmoids, and compare these with a derived observational recurrence rate of one plasmoid every approx.2.4 days, explaining the reasons why the spacecraft has not observed as many plasmoids as we predict will be released. We conclude that the Cassini magnetometer measurements require a combination of Vasyliunas-type closed-flux plasma sheet and Dungey-type open-flux lobe reconnection to account for the observed properties of the plasmoids and PPPS in Saturn's magnetotail.

Cassini observations of plasmoid structure and dynamics: Implications for the role of magnetic reconnection in magnetospheric circulation at Saturn

We survey the Cassini magnetometer data during the deep tail orbits in 2006, and find 34 direct encounters with plasmoids. They occur as single, isolated events but also in groups of two or more plasmoids as is frequently observed at Earth . We show a case study example of three such plasmoids over three hours, where we estimate an upper limit of 5.68 GWb of flux closure, and derive a reconnection rate over this interval of 526 kV. We show the results of a superposed epoch analysis of al1 34 plasmoids indicating that, on average, plasmoids at Saturn are approix.8 min in duration and they tend toward a loop-like, as opposed to flux rope-like topology, with little or no axial core magnetic field. Our analysis shows that plasmoids at Saturn are followed by an extended interval of the post-plasmoid plasma sheet (PPPS) lasting approx.58 min. The average open magnetic flux disconnected by the continued reconnet:tion following plasmoid formation that creates the PPPS is approx.3 GWb. We calculate expected recurrence rates for plasmoids, and compare these with a derived observational recurrence rate of one plasmoid every approx.2.4 days, explaining the reasons why the spacecraft has not observed as many plasmoids as we predict will be released. We conclude that the Cassini magnetometer measurements require a combination of Vasyliunas-type closed-flux plasma sheet and Dungey-type open-flux lobe reconnection to account for the observed properties of the plasmoids and PPPS in Saturn's magnetotail.

Magnetospheric energy budget and the epsilon parameter

Determination of the energy input for the magnetospheric energy budget is a nontrivial matter. As no direct means to measure the input are known, various solar wind‐derived proxies have been developed. In this article we discuss one of the most widely used energy input functions, the so‐called epsilon parameter of Akasofu. While practice has shown it to be a very useful parameter, there is no convincing evidence that it is superior to all other coupling parameters. Furthermore, its somewhat unclear definition and lack of physical foundation sometimes lead to confusing interpretation of the parameter in practical studies of magnetospheric energy cycle. For example, the parameter is sometimes understood to describe the transfer of solar wind Poynting flux into the magnetosphere, whereas the actual physical energy transfer involves conversion of solar wind kinetic energy to magnetic energy measured inside the magnetopause. Another questionable interpretation is to relate the size of the energy transfer region to the length of the reconnection line, as the scale factor in epsilon has the physical unit of area. These confusions may partly result from mixing the concepts of energy source and energy transfer. In spite of these problems the present empirical formulation of the epsilon parameter appears, from the global energy budget point of view, to give a remarkably good estimate for the total energy input into the inner magnetosphere in substorm and storm timescales. This is even more remarkable as after the parameter was first formulated we have learned that the ionosphere is a major sink of storm and substorm energy, exceeding the ring current in importance as an energy output channel. An additional issue is the energy carried away by the plasmoids and outflow of the postplasmoid plasma sheet. One can argue that the application of epsilon should be restricted to the energy consumption in the inner magnetosphere. However, as the intermittent plasmoid releases are essential parts of the same complex of processes as the ring current enhancement and ionospheric particle injections, we argue that they should be included in the energy budget, even if that might result in rejection of the epsilon as a useful input parameter. The recent analyses of energy output suggest that we can still use epsilon by scaling the parameter up by a factor of 1.5–2. It should be noted, however, that this energy budget does not account for all energy passing through the magnetosphere but only that part which is consumed in the storm and substorm processes.

Magnetotail reconnection: Simulation predictions on magnetic time series

In a recent kinetic computer simulation (Arzner and Scholer, 2001), magnetotail reconnection was studied with emphasis on the kinetic (particle) aspect. Complementary, we focus in the present paper on the magnetic fields. Examples of time series of the magnetic field at a fixed position, as predicted for spacecraft probes, are given. Different spatial regions are classified according to their signatures in these time series, and criteria for the proximity to the reconnection point, the post-plasmoid plasma sheet (PPPS) and the boundary layer are derived. The kinetic simulation reveals an instability of the PPPS, which is discussed in terms of an anisotropic 3-layer fluid model with the free energy is provided by the velocity anisotropy (T∥ > T⊥) due to the simultaneous presence of inflowing lobe ions and outflowing ions deflected in the field reversal. In its nonlinear stage, this instability creates higher harmonics and finally leads to turbulence characterized by power law spatial and temporal spectra. The latter allow a direct comparison with GEOTAIL and AMPTE observations. It is found that the simulated and observed turbulence spectra agree well.

Kinetic structure of the post plasmoid plasma sheet during magnetotail reconnection

Ion dynamics during magnetotail reconnection is studied by means of a two‐dimensional, large‐scale hybrid simulation (macroparticle ions, inertialess electron fluid). The initial setup is a realistic two‐dimensional equilibrium with a normal magnetic field component through the current sheet and a flaring lobe field. Reconnection is initiated by a localized resistivity in the near‐Earth region. As in MHD, a plasmoid develops and is ejected downtail. The ion kinetic structure in the post plasmoid plasma sheet is studied in detail. In a region of about 10 RE from the neutral line the ions are demagnetized and are picked up by the electron fluid ejected from the X line. Although the magnetic field in this region is reminiscent of slow mode shocks, no shocks occur, and the structure can be described by a standing large‐amplitude whistler. The Hall current leads to a cross‐tail magnetic field up to 40% of the lobe field. This may have important consequences for mapping of low‐altitude features into the tail. Further away from the neutral line over a distance up to 30 RE a thin current sheet develops in which the lobe ions perform quasi‐adiabatic orbits. This current sheet becomes instable and disrupts; the instability is driven by the free energy contained in the non‐Maxwellian velocity distributions in the current sheet. In the region of current sheet breakup the post plasmoid plasma sheet is hot and moves with a bulk speed close to the local Alfvén speed. In order to delineate the acceleration and heating process, individual ions are followed during the simulation. There is no indication for slow mode shocks in the simulation system within the simulation time. This suggests that slow mode shocks are not to be expected in the geomagnetic tail within several tens of RE from the X line.

Small scale plasmoids in the post-plasmoid plasma sheet: Origin of MHD turbulence?

A variety of MHD turbulence can be observed in the Earth's magnetotail, but the origin of the turbulence is still a long-standing problem. We study the small-scale fluctuations excited in collsionless magnetic reconnection by using a two-dimensional, particle-in-cell numerical simulation, and propose that the magnetic diffusion region of reconnection may be one of the possible regions to excite a strong MHD turbulence. The magnetic field turbulence observed in the post-plasmoid plasma sheet is compared with the fluctuations obtained in the computer simulation.

Plasma distribution functions in plasmoids: GEOTAIL observations

GEOTAIL observations have revealed that ion velocity distribution functions in plasmoids are in general non-Maxwellian, and often contain beams, which represent the memory of their source and route associated with magnetic reconnection processes. Cold ions of the lobe origin flow into the plasmoid along magnetic field lines, are accelerated and heated through slow shocks at the boundary, and finally merge with hot components inside the plasmoid. Deep inside the plasmoid, however, their remnants are still discernible and the ion distribution functions often show the existence of counterstreaming ion beams. In this region both ends of the field lines are most likely connected to IMF through the northern and southern lobes with short distances, so that ions of the lobe origin have not had time enough for thermalization. The presence of a long-duration post-plasmoid plasma sheet (PPPS) with high tailward speeds shows the continuing occurrence of reconnections on the earthward side of the spacecraft location. The ion velocity distributions contain beams almost everywhere in the PPPS region, but their appearance is highly variable and at times non-gyrotropic. In the near-Earth region, plasma flow properties are also highly variable and bursty after the plasmoid traversal. The high variability of plasma distribution functions and flow properties may represent a consequence of multiple and bursty reconnections, and coalescence of a number of vortices may constitute the structure of the post-plasmoid plasma sheet.

Ion kinetic effects in magnetic reconnection: Hybrid simulations

Magnetic reconnection in a low beta plasma is studied by using a large‐scale two‐dimensional hybrid simulation code that treats the ions fully kinetically and the electrons as a massless fluid. Reconnection is realized by localizing an anomalous resistivity in the the center of a Harris‐type current sheet. The results are applicable to reconnection in the geomagnetic tail. Within a distance of about 200 ion inertial lengths from the neutral line the fast reconnection jet is characterized over most of the region by partial shell type ion distributions; i.e., the distribution in the central current sheet is not thermalized. Inward drifting cold lobe ions perform in the center of the reconnection wedge due to the small magnetic field curvature Speiser‐type orbits and are subsequently ejected again onto lobe field lines. This stadium is similar to the collisionless reconnection scenario described by Hill [1975]. The cross‐tail current of the thin current sheet is supported by the drifting ions. This current sheet warps and an instability develops, which leads further away from the neutral line to a filament ation of the cross tail current. These simulations suggested that after near‐Earth reconnection proceeds to lobe magnetic field lines a postplasmoid plasma sheet with a thin current sheet builds up, which is of a boundary layer type. The thin current sheet is instable. At about 200 ion inertial lengths away from the neutral line the instability has reached a nonlinear state and the cross‐tail current becomes patchy; the incoming and ejected cold lobe ions are isotropized in the ensuing magnetic field and thus constitute a hot plasma sheet distribution. This can then support the occurrence of slow mode shocks at the boundary of the reconnection layer with backstreaming ions upstream of the shocks.

Observations of earthward streaming electrons at the trailing boundary of a plasmoid

In this paper, we report on highly asymmetric spectrum of electrons observed at the boundary of postplasmoid plasma sheet (PPPS). The data were obtained when GEOTAIL made repeated crossings of the boundary after encountering with a tailward flowing plasmoid at XGSM ∼ −46RE. In the spectrum, electrons in 0.1–1 keV energy range are seen to flow earthward along the lobe‐like field lines into the PPPS counter to more energetic components (ions and electrons) leaking from the PPPS. In the boundary region, earthward streaming electrons are observed even when the energetic leaking components almost disappear, which makes us characterize this region more generally by tailward flowing field aligned currents (FAC). By referring to hybrid code (ion particles, massless electron fluid) results, we propose that the earthward flowing electrons sustain the FAC away from the X‐line (tailward if a spacecraft is located tailward of the X‐line), which is a part of a Hall current loop built‐up in the course of magnetic reconnection.

CDAW 8 observations of plasmoid signatures in the geomagnetic tail: An assessment

A primary objective of the Coordinated Data Analysis Workshop 8 (CDAW 8) was the critical assessment of the plasmoid hypothesis. In this study, various types of magnetic phenomena, including closed loops and flux ropes, were considered as candidates for producing the north‐then‐south magnetic field perturbations characteristic of the “plasmoids” observed in the magnetotail by ISEE 3 during the CDAW 8 A and G events. For these two events the principal axis analyses of the magnetic field data and supporting energetic particle and plasma measurements are found to be consistent with the closed‐loop model of plasmoids for which the plane of rotation for B lies near the GSE X‐Z plane and only small field components lie in the minimum variance direction oriented largely along the Y direction. While small pitch angle spiraling of the field lines within these structures such as suggested by Hughes and Sibeck (1987) and Birn et al. (1989) cannot be definitively measured using single‐spacecraft magnetometer observations, the ISEE 3 measurements are not consistent with moderate strength or Venus‐type, strong core field flux ropes. The event A and G plasmoids were preceded by smoothly draped lobe field lines and followed by strong southward fields in the postplasmoid plasma sheet as predicted by the reconnection model of substorms. This latter feature of the observations is consistent with plasmoids moving down the tail, at least in part, due to the Maxwell stress exerted by lobe field lines which were disconnected at the near‐tail neutral line and have draped about the earthward side of the plasmoid. Calculations, based upon typical plasmoid and tail parameters, are presented which indicate that the J×B force associated with these disconnected lobe field lines may be sufficient to accelerate plasmoids up to the speeds observed by ISEE 3. A new characteristic of the plasmoid signature identified in this study is the presence of enhanced internal magnetic fields which can exceed the magnitude of the adjacent lobe fields by as much as 10–20%. While the internal structure of plasmoids remains poorly understood, an extension of the traveling compression region model (Slavin et al., 1984) is proposed to explain these high field regions. Finally, the relationship between the nature of individual substorms and the characteristics of the plasmoids observed in the tail by ISEE 3 is considered. The event A observations suggest that during isolated substorms the magnetotail may undergo an interval of intense reconnection which results in the creation of a single large plasmoid. Extended substorm intervals, exemplified by event G, may produce numerous smaller plasmoids as X lines form in the cislunar tail and retreat tailward while the magnetosphere continues to draw energy from the solar wind. Overall, the energy expended in accelerating the plasmoid down the tail appears comparable to that dissipated in the inner magnetosphere and ionosphere. Additional studies of plasmoid structure and dynamics as a function of substorm activity and distance down the tail should be conducted to determine the general validity of the conclusions reached on the basis of the CDAW 8 events.

Plasmoid‐associated energetic ion bursts in the deep geomagnetic tail: Properties of plasmoids and the postplasmoid plasma sheet

The properties of energetic ions (>35 keV) observed in plasmoids and in the postplasmoid plasma sheet in the distant geomagnetic tail by the energetic particle anisotropy spectrometer instrument on ISEE 3 are investigated and compared with simultaneous thermal electron and magnetic field data. Two representative plasmoid‐associated bursts are discussed in detail, followed by the results of a survey of 37 such events. It is found that the bulk speeds deduced from the angular anisotropies of the energetic ions, assumed to be protons, agree with those deduced from the thermal electrons to within ∼200 km s−1 in the hot plasma regions, showing that the ions are convected with the plasma. Average speeds are 660 km s−1 in plasmoids and 840 km s−1 in the postplasmoid plasma sheet. In the energetic particle boundary layers surrounding the hot plasma regions, however, the ion anisotropies are consistent with energetic proton bulk speeds (∼1000 km s−1) which considerably exceed local plasma speeds (∼200 km s−1) such that the ions form a beam, originating in the plasma sheet, which flows along newly reconnected magnetic field lines through plasma which has tail lobe characteristics. In the plasma sheet regions a correlation is found between the energetic proton intensity and the thermal electron temperature Te, such that the intensity increases by 2 orders of magnitude as kTe increases from ∼50 to ∼200 eV. To investigate this further, the thermal ion temperature Ti, not measured on ISEE 3, is estimated from pressure balance arguments. It is found that Ti and Te are correlated, with Ti ≈ 8Te in the plasmoids and with Ti ≈ 11Te in the postplasmoid plasma sheet, such that the energetic ion fluxes are also correlated with (and presumably controlled by) Ti. Characterizing the energetic protons as a Maxwellian distribution with density np* and temperature Tp* then yields typical energies kTp* ≃ 10 keV, compared with kTi ≃ 1 keV and kTe ≃ 100 eV, and densities np* ≃ 6 × 10−3 cm−3, compared with ni ≃ ne ≃ 0.16 cm−3. Thus the energetic protons form a tail to the thermal population which is “non‐Maxwellian” in the sense that their characteristic temperature is much larger than thermal values. The energetic proton bulk speeds are also integrated to investigate plasmoid lengths. It is found that one‐loop plasmoids (as inferred from field data) have an average length ∼60 RE, while multiple‐loop plasmoids have an average length ∼110 RE. For single‐loop plasmoids the observed time delay in their appearance in the distant tail following substorm onset is consistent with pinching off at a near‐Earth neutral line (given some acceleration as the plasmoid moves downtail), while for multiple‐loop plasmoids, agreement is found only if the first loop alone is considered. This result suggests that subsequent loops are formed after substorm onset, either through tailward retreat of the substorm neutral line and further pinching off or through loop formation within the postplasmoid plasma sheet.

Correlated observations of substorm effects in the near‐Earth region and the deep magnetotail

We present simultaneous observations of energetic particle measurements from the geosynchronous satellite 1982–019 and magnetic field, electron plasma, and energetic proton and electron measurements obtained with ISEE 3 in the deep tail. The data are supplemented by ground magnetograms. A substorm occurred on March 22, 1983, close to 0300 UT as identified in the ground magnetograms and by a particle injection at geosynchronous orbit. About 10 min later, ISEE 3 observed (at a distance of ∼130 RE in the deep tail) magnetic field, plasma, and energetic particle signatures consistent with the passage of a plasmoid. After the passage of the plasmoid the satellite enters shortly into a lobelike environment, in which an energetic proton beam is observed. High‐resolution magnetic field data are indicative of small‐scale structures in the postplasmoid plasma sheet. From the plasma sheet flow speed during the plasmoid's passage it is concluded that the 0300 UT substorm is responsible for its origin. This allows an approximate timing of the plasmoid release at a near‐earth neutral line and of the plasma sheet recovery after substorm onset, and it indicates a close relationship between processes in the near‐earth plasma sheet and the deep tail during substorms.