Distant Magnetotail Research Articles

Convection in the distant magnetotail under extremely quiet and weakly disturbed conditions

Measurements of the magnetic field and low‐energy plasma by the Geotail spacecraft have been used to study magnetospheric convection in the distant tail at X = −(79 – 200)RE under extremely quiet and weakly disturbed conditions. The analysis was carried out separately for the tail lobes and the plasma sheet; these regions were identified by plasma and magnetic field parameters. It is shown that plasmas in the northern and southern tail lobes move tailward along magnetic field lines at the same time as magnetic field lines converge towards the plasma sheet. The main processes occurring in the distant tail under extremely quiet conditions (northward interplanetary magnetic field) and weakly disturbed conditions (southward IMF) are very similar, excluding the passage of high‐speed tailward plasmoids, which are obviously generated in the near‐Earth plasma sheet. Magnetic turbulence, that is, fluctuations of the magnetic field occurring against the background of a totally northward field (BZ>0), consistent with intense oscillations in the plasma flow VX component, is typical of all conditions. It is concluded that the stretched and antiparallel field lines of the northern and southern tail lobes are reconnected in the plasma sheet. Tail lobe plasma constantly enters the plasma sheet, where the lobe magnetic field energy converts into the kinetic energy of the plasma producing the magnetic turbulence. It seems likely that the direction (northward or southward) of the IMF is not necessary for tail formation, and processes in the near tail (substorm activity) does not strongly influence reconnection in the distant tail.

Foreword [to Special Section on The November 1993 Geomagnetic Storm

Foreword [to Special Section on The November 1993 Geomagnetic Storm

Plasma sheet beyond the distant neutral line

We have studied the properties of the plasma sheet in the distant magnetotail during a 12‐h interval when the GEOTAIL satellite stayed rather continually in the plasma sheet. When magnetic reconnection was operative at the distant neutral line the tailward flow speed in the distant plasma sheet is given by the sum of the Alfven speed and the tailward flow speed in the tail lobe. Appreciable twist of the tail should be incorporated in order to properly estimate the magnetic flux that traverses the distant neutral sheet to the south. The magnetic field beyond the distant neutral line is characterized by active and persistent oscillations whose power spectrum has a kink around 0.01 Hz. These oscillations roughly satisfy the Walén relation and could be due to incidence of the interplanetary Alfven waves along the IMF‐type field lines that fill the plasma sheet beyond the distant neutral line.

Magnetospheric plasma regimes identified using Geotail measurements: 1. Regime identification and distant tail variability

For many important scientific objectives, the analysis and interpretation of magnetospheric data sets require a sorting by primary plasma regimes. This sorting process is much more difficult in the distant magnetotail than for data sets sampled close to Earth. We have carried out systematic regime identifications for the entire deep‐tail portion of Geotail data at timescales down to ∼l min. Criteria were developed for selection of five basic plasma regimes: plasma sheet, lobe, magnetospheric boundary layer, magnetosheath, and solar wind. In addition to low‐energy plasma data central to the identification process, critical supplementary information derives from magnetic field and plasma wave data. We find that all basic plasma regimes remain physically distinct populations at all downtail distances sampled by the Geotail spacecraft. Preliminary automated separation of subregimes using a B versus δB method shows that plasma mantle intervals increase in probability of occurrence from 18% near Earth to about 50% near 205 RE. In contrast, plasma sheet intervals decrease overall with increasing tailward distance. Beyond about X = −100 RE, the frequency of short regime intervals steadily increases, overall variability increases, and the lobe and plasma sheet are progressively displaced by the boundary layer, here mostly plasma mantle. Our plasma regime identifications provide a critical baseline for the development of automated identification and a key resource for both Geotail data analysis and correlative studies between Geotail and other International Solar Terrestrial Physics mission data sets.

Magnetospheric plasma regimes identified using Geotail measurements: 2. Statistics, spatial distribution, and geomagnetic dependence

We investigate the spatial distribution, solar‐terrestrial correlation, and basic statistics of a long sequence of plasma regime identifications through the probability of observation P at downtail distances of ∼30 < xagsm < 210 RE, in an aberrated GSM coordinate system. The data are from Geotail spacecraft measurements obtained from October 1992 through October 1994. Earth's local plasma environment has been classified into five principal plasma regimes: PS, the plasma sheet, LB, the lobe (vacuum lobe, with only trace plasma content), and BL, the magnetospheric boundary layer, which together comprise the magnetosphere proper, and MS, the magnetosheath, and SW, the nearby solar wind. Eastman et al. [this issue] describe the process, rationale, and instrumentation used in the regime identifications. In the middle magnetotail (30 < xagsm < 100 RE): At geomagnetic activity index values Kp < 3o, (a) The average magnetospheric morphology appears nominal, with no large‐scale dawn‐dusk asymmetries; (b) PS is the regime most likely to be observed, and the spatial interrelationships of the PS, LB, BL, and MS appear similar to those commonly observed within −20 RE of Earth; that is, the magnetosphere is inside and the magnetosheath is outside; and (c) P(BL) increases gradually with increasing xagsm. At Kp ≥ 3o, (d) The centroid of P(PS) lies duskward of the XAGSM axis, producing a distinct dawn‐dusk asymmetry; (e) P(BL) increases sharply at xagsm > 60 RE and remains high tailward; and (f) At xagsm < 75 RE, MS is rarely observed for |YAGSM| < 20 RE. In the distant magnetotail (xagsm > 100 RE): (a) Conditions appear more chaotic. PS, BL, or MS can be observed anywhere inside the nominal magnetotail domain; (b) Compared to the middle magnetotail, PS and BL are observed more dawnward of the XAGSM axis and P(PS) is significantly lower (most distinctly at Kp > 3o); (c) P(PS) is centered dawnward of the XAGSM axis for both low and high Kp; and (d) At xagsm ≥ 170 RE, P(BL) is greater than P(PS) or P(MS), with P(MS) ≈ P(PS). P(LB) > 0, but low, out to xagsm ≥ 200 RE. In general, the downtail distributions of the magnetospheric plasma regimes depend more strongly on xagsm than on Kp, however the YAGSM distribution of P(PS) depends strongly with both xagsm and Kp. Magnetotail width varies weakly with xagsm and Kp.

Ion and electron distribution functions in the distant magnetotail: modeling of Geotail observations

Ion and electron distribution functions measured by the Geotail spacecraft in the distant magnetotail are investigated considering single‐particle dynamics in sharp field reversals. The observed H+ distribution functions exhibit two components, namely, a low‐energy core and a high‐energy population propagating tailward. In agreement with previous studies, we show that ion dynamics in a two‐dimensional (2‐D) reconnection field geometry naturally leads to such distributions and that the neutral line location can be analytically inferred from the velocity space structure. As for electron distribution functions, Geotail observations reveal characteristic “flattop” profiles with enhanced flux in the parallel direction at low energies and in the perpendicular direction at high energies. We interpret these distributions as the result of the electron chaotic behavior in the reconnection region. This chaotic regime, which occurs when the particle Larmor radius is comparable to the magnetic field line curvature radius, can lead to prominent pitch angle scattering. Near the neutral line, we show that at a given energy, this regime yields flux depletion in the parallel direction and enhancement in the perpendicular one. The flattop profile arises from this behavior affecting only the (relatively) high‐energy part of the electron distribution. Conversely, the threshold energy above which electrons exhibit such a behavior contains information on the field reversal characteristics. The electron distribution functions obtained numerically are in agreement with the observations, which support this interpretation framework and allow us to estimate the thickness of the neutral sheet.

Energetic oxygen ion bursts in the distant magnetotail as a product of intense substorms: Three case studies

On the basis of Geotail high energy particle ‐ low energy particle detector (HEP‐LD) observations, this paper reports on three energetic (144 – 4000 keV) oxygen burst sequences detected in the distant tail (Xgse = −40 to −66 RE) and their relation to substorm signatures. Those energetic oxygen ion bursts lasted only 20 to 30 min and exhibited strong beam‐like structures. Two of the events (at about 1000 and 1900 UT on February 13, 1994) occurred in tailward flowing plasma after the flow direction changed from earthward to tailward; geostationary and ground based observations detected intense substorm activity during these periods, and the local magnetic field component Bz assumed predominantly negative values. The occurrence of a magnetic field with southward polarity and oxygen bursts embedded in tailward flowing plasma is consistent with the basic signatures of reconnection (formation of a neutral line) in the magnetotail. The third energetic oxygen burst with earthward flow was observed relatively close to Earth on August 27, 1993, X= −40 RE. No reversal in the plasma flow direction was seen, and the magnetic field polarity was essentially positive throughout the event. This is consistent with a “near‐Earth” neutral line that had formed beyond X=−40 RE. We conclude that (1) a large amount of heavy ions from the ionosphere can be transferred to the distant tail and accelerated to high energies during substorm activity and that (2) these oxygen O+ ions from the polar ionosphere can be considered as “tracer ions” in the substorm dynamical process.

Comment on “A quantitative assessment of energy storage and release in the Earth's magnetotail”, by D. N. Baker, T. I. Pulkkinen, M. Hesse, and R. L. McPherron

Journal of Geophysical Research: Space PhysicsVolume 103, Issue A8 p. 17729-17732 CommentariesFree Access Comment on “A quantitative assessment of energy storage and release in the Earth's magnetotail”, by D. N. Baker, T. I. Pulkkinen, M. Hesse, and R. L. McPherron Walter J. Heikkila, Walter J. HeikkilaSearch for more papers by this author Walter J. Heikkila, Walter J. HeikkilaSearch for more papers by this author First published: 01 August 1998 https://doi.org/10.1029/98JA01039Citations: 1AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat References Axford, W. I., C. O. Hines, A unifying theory of high-latitude geophysical phenomena and geomagnetic storms, Can. J. Phys., 39, 1433, 1961. Baker, D. N., T. I. Pulkkinen, M. Hesse, R. L. McPherron, A quantitative assessment of energy storage and release in the Earth's magnetotail, J. Geophys. Res., 102, 7159, 1997. Cole, K. D., On solar wind generation of polar geomagnetic disturbances, Geophys. J. R.A.S., 6, 103, 1961. Dungey, J. W., Interplanetary magnetic field and the auroral zones, Phys. Rev. Lett., 6, 47, 1961. Eastman, T. E., E. W. Hones Jr., S. J. Bame, J. R. Asbridge, The magnetospheric boundary layer: Site of plasma, momentum, and energy transfer from the magnetosheath into the magnetosphere, Geophys. Res. Lett., 3, 685, 1976. Heikkila, W. J., Magnetospheric topology of fields and current, Magnetospheric Currents, Geophys. Monogr. Ser., 28 T. Potemra, 208, AGU, Washington, D. C., 1984. Heikkila, W. J., Current sheet crossings in the distant magnetotail, Geophys. Res. Lett., 15, 299, 1988. Heikkila, W. J., Interpretation of recent AMPTE data at the magnetopause, J. Geophys. Res., 102, 2115, 1997a. Heikkila, W. J., Comment on “The alternative paradigm for magnetospheric physics” by E. N. Parker, J. Geophys. Res., 102, 9651, 1997b. Heikkila, W. J., Cause and effect at the magnetopause, Space Sci. Rev., 1998. Iijima, T., T. A. Potemra, Large-scale characteristics of field-aligned currents associated with substorms, J. Geophys. Res., 83, 599, 1978. Parker, E. N., The alternative paradigm for magnetospheric physics, J. Geophys. Res., 101, 10587, 1996. Phan, T.-D., G. Paschmann, The low-latitude dayside magnetopause and boundary layer for high magnetic shear, J. Geophys. Res., 101, 7801, 1995. Pulkkinen, T. I., D. N. Baker, C. J. Owen, J. A. Slavin, A model for the distant tail field: ISEE-3 revisited, J. Geomagn. Geoelectr., 48, 455, 1996. Slavin, J. A., E. J. Smith, D. G. Sibeck, D. N. Baker, R. D. Zwickl, S.-I. Akasofu, An ISEE-3 study of average and substorm conditions in the distant magnetotail, J. Geophys. Res., 90, 10875, 1985. Wangness, R., Magnetic energy, Electromagnetic Fields, 284, John Wiley, New York, 1986. Yamamoto, T., K. Shiokawa, S. Kokobun, Magnetic field structures of the magnetotail as observed by GEOTAIL, Geophys. Res. Lett., 21, 2875, 1994a. Yamamoto, T., A. Matsuoka, K. Tsuruda, H. Hayakawa, A. Nishida, M. Nakamura, S. Kokubun, Dense plasmas in the distant magnetotail as observed by GEOTAIL, Geophys. Res. Lett., 21, 2879, 1994b. Citing Literature Volume103, IssueA81 August 1998Pages 17729-17732 ReferencesRelatedInformation

Effect of magnetic turbulence on the ion dynamics in the distant magnetotail

The ion dynamics in the distant Earth's magnetotail is studied in the case that a cross tail electric field E0 and reconnection‐driven magnetic turbulence are present in the neutral sheet. The magnetic turbulence observed by the Geotail spacecraft is modeled numerically by a power law magnetic fluctuation spectrum. The magnetic fluctuations have the tearing mode parity with respect to the neutral sheet and are superimposed on a modified Harris sheet. A test particle simulation is performed for the ions, and the particle density, current density, bulk velocity, temperature, pressure, and heat flux are obtained for every point in the distant tail and as a function of the magnetic fluctuation level, δB/B0. It appears that the magnetic turbulence is very effective in maintaining the stationary structure of the current sheet and in changing the ion acceleration due to the electric field to thermal motion. Also, magnetic turbulence can inflate the current carrying region up to a thick current sheet, in contrast with the often assumed thin current sheet. The values obtained for the ion temperature are consistent with those observed in the distant tail by the Geotail spacecraft. The main results are the following: (1) the thickness of the current sheet increases with the level of fluctuations. The thickness λ corresponding to the average magnetic field current profile is obtained for δB/B0 ≃ 0.3. (2) The magnetic pressure outside the current sheet is balanced by particle pressure for δB/B0 ≃ 0.3. This is obtained mostly by an increase in the temperature, while the density profile is not much peaked in the neutral sheet. (3) For low fluctuation levels, heating is anisotropic, most heating going into the y direction; increasing δB/B0 and making reference to the local magnetic field, more heating goes in the parallel rather than in the perpendicular direction, in agreement with part of the observations. (4) A possible splitting of the bulk velocity and of the current density in two sheets is obtained for δB/B0 ≥ 0.2. In general, a relevant level of magnetic turbulence, like δB/B0 ≃ 0.3, appears to be a basic ingredient of the distant magnetotail equilibrium structure.

Energetic solar particle dropouts detected by Ulysses at 1.63 AU: A possible encounter with the Earth's distant magnetotail

Fast solar particles are used to trace the topology of the interplanetary magnetic field during a solar event detected by the heliosphere instrument for spectra composition and anisotropy at low energy (HISCALE) on the Ulysses spacecraft on January 2, 1991. Two sharp‐edged dropouts, lasting for 10 and 25 min, were detected in the fluxes of solar ions from ∼130 keV to >1.8 MeV and halo (heat flux) electrons from 71–461 eV, while simultaneously the flux of high‐energy, 38–315 keV solar electrons and 56–78 keV ions remained constant. The halo electrons and 130 keV to >1.8 MeV solar ions are traveling with similar speeds, ∼4 × 106 to 3 × 107 m/s, much slower than the energetic solar electrons (>108 m/s), and faster than the 56–78 keV ions, suggesting that the dropout field lines were first disconnected from the Sun and then reconnected, with the distance to the reconnection point and time of the reconnection such that >38 keV electrons had already repopulated the dropout field lines. At the time Ulysses was 0.63 AU from the Earth on its way to Jupiter, ∼2° above the ecliptic with a Sun‐Earth‐spacecraft angle of ∼172.8°, approximately where the Earth's magnetotail would be expected to be if it extended to 15,000 Earth radii. We consider the possibility that Ulysses encountered interplanetary field lines connected to the magnetotail.

Observations of flux rope − associated particle bursts with GEOTAIL in the distant tail

Abstract. Geotail energetic particle, magnetic field data and plasma observations (EPIC, MGF and CPI experiments) have been examined for a number of energetic particle bursts in the distant tail (120Re<|XGSM|< 130 Re), associated with moving magnetic field structures, following substorm onsets. The features obtained from this data analysis are consistent with the distant magnetotail dynamics determined first by ISEE3 observations and explained in terms of the neutral line model. At the onset of the bursts, before plasma sheet entrance, energetic electrons appear as a field-aligned beam flowing in the tailward direction, followed by anisotropic ions. Within the flux rope region, suprathermal ions exhibit a convective anisotropy, which allows determination of the plasma flow velocity, assuming that the anisotropy arises from the Compton-Getting effect. The velocities thus determined in the plasma sheet are estimated to be 200–650 km/s, and compare favourably with the velocities derived from the CPI electron and proton experiment. The estimated length of magnetic field structures varies between 28 and 56 Re and depends on the strength of the westward electrojet intensification. Finally, the three structures reported here show clear magnetic field signatures of flux rope topology. The existence of a strong magnetic field aligned approximately along the Y-axis and centred on the north-to-south excursion of the field, and the bipolar signature in both By and/or Bz components, is consistent with the existence of closed field lines extending from Earth and wrapping around the core of the flux rope structure.Key words. Magnetospheric configuration and dynamics · Magnetotail

Observations of bidirectional electrons in the distant tail lobes: GEOTAIL results

ISEE‐3 demonstrated the relatively frequent occurrence of low‐energy (50 ∼ 500 eV) electron flux enhancements within the distant magnetotail lobes. These electrons were found to be “bidirectional” in the sense that intensities were highest parallel (and antiparallel) to the lobe magnetic field lines. It was concluded that these electrons must enter the tail lobes along open field lines at the distant magnetopause and that this population would constitute the source for polar rain electrons at low altitudes in the polar cap regions. Similar plasma electron features have been found using the Low Energy Particle (LEP) sensor system onboard the GEOTAIL spacecraft. Remarkably close correspondences of low‐energy (≲ 2 keV) ion tailward flow enhancements are found to occur when bidirectional electron fluxes suddenly intensify. Such ion plasma measurements were not available with ISEE‐3 and, thus, a new aspect of solar wind entry and overall plasma dynamics is revealed by these GEOTAIL measurements in the distant tail. Other ISTP data (e.g., IMP‐8 and CANOPUS) are used to analyze selected periods.

Correction to “Observations of ion and electron velocity distributions associated with slow‐mode shocks in Earth's distant magnetotail” by J. Seon et al.

Correction to “Observations of ion and electron velocity distributions associated with slow‐mode shocks in Earth's distant magnetotail” by J. Seon et al.

Cold ion streams consisting of double proton populations and singly charged oxygen observed at the distant magnetopause by Geotail: A case study

The Geotail plasma and magnetic field observations reveal that multiple ion streams with different energies coexist at the magnetopause separating the magnetosheath and the lobe/mantle in the distant magnetotail at (−187, −23, −2) RE in the GSM coordinates. The ion streams sometimes consist of three components flowing tailward. On the basis of E × B drift analysis, we have identified that two of the three streams are protons, while the residual one of the highest energy (>7 keV/q) consists of O+. The direction of the perpendicular velocities of these ion components is toward the magnetosheath from the lobe/mantle. The higher‐energy (∼2 keV/q) proton component is detected only at the magnetopause, while the other two components (the lower‐energy (<1 keV/q) protons and O+) can also be observed in the lobe/mantle. In the lobe/mantle, the bulk velocity of the O+ flow is always nearly equal to that of the lower‐energy proton stream. In the magnetosheath, the two proton streams completely merge with each other, and the O+ component cannot be recognized clearly. In the magnetopause current layer, the lower‐energy proton component is significantly accelerated in the direction perpendicular to the magnetic field due to electric field enhancement, whereas the parallel speeds of the higher‐energy protons change according to the energization rate in the current layer. The temperature of the higher‐energy proton stream is generally 50–100 eV, approximately twice as large as that of the lower‐energy proton stream in the lobe/mantle and slightly higher than that of protons in the magnetosheath. These signatures of multiple ion streams are discussed in terms of magnetic field reconnection at the lobe/mantle magnetopause.

Structure of the distant magnetotail and its dependence on the IMF B y component: GEOTAIL observations

Structure of the distant magnetotail (x<−150 Re) in the y-z plane is studied statistically on the basis of the plasma and magnetic field data obtained by the GEOTAIL spacecraft. In order to reduce the scatter due to time variations in the solar wind direction, a coordinate system is introduced whose x-axis is taken parallel to the hourly direction of the solar wind measured in the upstream region. Our analysis shows that the distant tail has almost the same dimension in y and z directions for average IMF conditions. In other words, there is no indication of the tail flattening along the y axis for average magnitude of IMF By. The distant tail current sheet is found to be tilted by 20 degrees on average either clockwise or anticlockwise depending on the sign of the IMF By. However, the location of the core lobe, which is defined to be the region of maximum probability of observing the lobe, is not tilted from the z axis. This means that the IMF By effect is not an overall rotation of the tail structure but involves a skewing of the internal structure. The tilt angle of the current sheet is larger when the IMF is directed northward. The large tilt angle observed for the northward IMF implies that the northward magnetic field component in the tail does not necessarily mean the closure of field lines across the current sheet; this finding thus alleviates the difficulty regarding the net tailward transport of positive Bz during quiet times raised earlier.

Flux ropes in the magnetotail: Consequences for ion populations

We have investigated the origin of ions in magnetotail flux ropes by using a large scale trajectory calculation in which we launched solar wind ions into the electric and magnetic fields from a global magnetohydrodynamic simulation of the magnetosphere. We analyzed many ions entering the flux rope and found that three types of reconnection — on the dayside, in the magnetotail, and on the distant magnetotail flanks — as well as single particle drifts were important in forming this population.

Observations of slow‐mode shocks in Earth's distant magnetotail with the Geotail spacecraft

Three crossings of the lobe‐plasma sheet boundaries encountered within Earth's distant magnetotail with the Geotail spacecraft are analyzed in terms of slow‐mode shocks. Quantitative comparisons with theoretical predictions for a steady state, one‐dimensional shock are made based upon measurements of the three‐dimensional velocity distributions of protons and electrons and of the magnetic fields. The observed plasma moments and magnetic field upstream from the shock are successfully used to predict these parameters in the downstream region. Slow‐mode Mach numbers in the upstream and downstream regions are also shown to be above and below unity, respectively, as required for the slow‐mode shock transition. However, a consideration of the relative uncertainties for the upstream and downstream slow‐mode Mach numbers allows convincing identifications for only two of the boundaries as slow‐mode shocks. Low number densities of plasmas for the other boundary, ∼0.1/cm3, are responsible for the large relative uncertainties for the Mach numbers. This analysis of slow‐mode shocks, together with a comprehensive accounting of statistical uncertainties and variances, allows us to provide firm evidence for the existence of slow‐mode shocks in Earth's distant magnetotail.

Observations of ion and electron velocity distributions associated with slow‐mode shocks in Earth's distant magnetotail

Three‐dimensional observations of ion and electron velocity distributions with the Geotail spacecraft for three crossings of slow‐mode shocks encountered in Earth's distant magnetotail are presented. Important findings from the observations are (1) the ion velocity distributions in the shock layer and the downstream region of the shocks are highly anisotropic because of the presence of multiple ion components in velocity space and (2) the pressure tensors in the downstream regions as determined from the measured velocity distributions can be diagonalized in magnetic field coordinates. A modification to the procedure for the identification of the slow‐mode shocks is developed with an assumption of anisotropic, gyrotropic plasmas in order to take these observations into account. The predictions for the downstream plasma moments and magnetic fields from the modified procedure are in sufficient agreement with the measured parameters to provide considerably greater confidence in the identifications of the slow‐mode shocks. The possibility that the boundaries are tangential discontinuities is eliminated with observations of upstream plasma mantle ions that penetrate the shock layer and enter the downstream hot plasmas. Also observed in the upstream ion velocity distributions is the presence of accelerated ions escaping from the downstream region which may play an important role in the dynamics of the slow‐mode shock. Observations of the electron velocity distributions allow identification of the separatrix in Earth's magnetotail. The separatrix in the upstream region is identified with the first appearance of hot electrons streaming away from the shock. Flat‐topped electron velocity distributions are found in the downstream region.

Structure of plasma sheet in magnetotail: Double‐peaked electric current sheet

The structure of the plasma sheet in the distant magnetotail observed by the Geotail satellite is examined. We found that the observed structure of the plasma sheet is often different from the standard Harris‐type plasma sheet [Harris, 1962]. The observed structure can be expressed as a double‐peaked current sheet which has a pair of localized electric currents away from the neutral sheet, i.e., a geometrically thick zero magnetic field region attached to the thin boundary layer with a large magnetic field gradient. This type of the plasma sheet is often observed in the distant magnetotail at radial distances of −50 &gt; XGSM/RE &gt; −125. We discuss a possible model to explain the formation of the double‐peaked current sheet in terms of large‐scale magnetic reconnection associated with slow shocks.

Fractal structures and power law spectra in the distant Earth's magnetotail

This paper is aimed at the theoretical understanding of the results obtained from the survey of plasmas and magnetic fields in the Earth's distant magnetotail with the Geotail spacecraft. Our attention is concentrated on the “kink” power law spectrum of magnetic field fluctuations proposed by Hoshino et al. [1994] from an analysis of the Geotail data. In the framework of the present study, we discuss a self‐consistent theoretical model describing the magnetic field structures in the distant tail with the use of the basic concepts of fractal geometry. It is shown that the statistically significant number of the excited tearing modes in the Earth's distant magnetotail could be responsible for the development of self‐similar, fractal magnetic structures in the range of spatial scales between ∼ 4 × 102 km and ∼ 8 × 103 km. This actually leads to the existence of two kinks in the power spectrum, one around the turnover frequency ƒ* ≈ 0.03 Hz, in good agreement with the results by Hoshino et al. [1994], the other close to the turnover frequency ƒ** ≈ 0.5 Hz, beyond the Nyquist frequency in the relevant Geotail magnetic field data. The possible behavior of the power spectra of the magnetic fluctuations in a wide range of frequencies is discussed.