Earth's Distant Magnetotail Research Articles

Properties of the Equatorial Magnetotail Flanks ∼50–200 RE Downtail

AbstractIn space, thin boundaries separating plasmas with different properties serve as a free energy source for various plasma instabilities and determine the global dynamics of large‐scale systems. In planetary magnetopauses and shock waves, classical examples of such boundaries, the magnetic field makes a significant contribution to the pressure balance and plasma dynamics. The configuration and properties of such boundaries have been well investigated and modeled. However, much less is known about boundaries that form between demagnetized plasmas where the magnetic field is not important for pressure balance. The most accessible example of such a plasma boundary is the equatorial boundary layer of the Earth's distant magnetotail. Rather, limited measurements since its first encounter in the late 1970s by the International Sun‐Earth Explorer‐3 spacecraft revealed the basic properties of this boundary, but its statistical properties and structure have not been studied to date. In this study, we use Geotail and Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) missions to investigate the equatorial boundary layer from lunar orbit (∼55 Earth radii, RE, downtail) to as far downtail as ∼200 RE. Although the magnetic field has almost no effect on the structure of the boundary layer, the layer separates well the hot, rarefied plasma sheet from dense cold magnetosheath plasmas. We suggest that the most important role in plasma separation is played by polarization electric fields, which modify the efficiency of magnetosheath ion penetration into the plasma sheet. We also show that the total energies (bulk flow plus thermal) of plasma sheet ions and magnetosheath ions are very similar; that is, magnetosheath ion thermalization (e.g., via ion scattering by magnetic field fluctuations) is sufficient to produce hot plasma sheet ions without any additional acceleration.

Earth's distant magnetotail current sheet near and beyond lunar orbit

AbstractWe analyze the structure of the Earth magnetotail current sheet (CS) in middle, X∈[−50,−20] RE, and distant, X∈[−100,−80] RE, regions using data set of 573 CS crossings by Geotail in 1994–1995. For a subset of 213 CSs we determine the CS thickness L, average current density j0, and velocity vD=j0/en0 (n0 is the ion number density). We find similar dawn‐dusk distributions of CS parameters for middle and distant tail: L is about 3000 km at the dusk flank and grows up to 12,000 km toward the dawn flank; j0 grows toward the dusk flank by a factor of 2–3; and the most intense CSs (with higher vD) are observed near midnight. We show that ion‐scale CSs with the thickness of several ion thermal gyroradii (say less than seven) are observed in middle and distant tail in more than 50% of crossings. For observed CSs electrons likely provide the dominant contribution to the current density. We divide the subset into intense and weak CSs (using parameter vD). Weak CSs have thickness of about 20 ion thermal gyroradii and Bz of about 1.5 nT. Intense CSs have thickness of about 3–7 thermal gyroradii and much smaller Bz implying more stretched field line configuration. Intense CSs are accompanied by fast ion flows: vD is larger for larger amplitudes of ion bulk velocity vx that is likely due to larger contribution of Speiser ions. The properties of the CS in middle and distant tail are compared with those found for the near‐Earth tail.

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.

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.

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.

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.

Observations of a slow‐mode shock at the lobe‐plasma sheet boundary in Earth's distant magnetotail

A lobe‐plasma sheet boundary observed within Earth's distant magnetotail (XGSM ≃ −135 RE) with the Geotail spacecraft is interpreted in terms of a slow‐mode shock. Measurements of the three‐dimensional velocity distributions of protons and electrons and of the magnetic fields allow a quantitative comparison with theoretical predictions for a steady‐state, one‐dimensional shock. The observed plasma moments and magnetic field upstream from the shock are successfully used to predict these parameters in the downstream region. The measured parameters for this example of a lobe‐plasma sheet boundary are in sufficient agreement with the theoretical predictions to provide considerable confidence that this boundary is a slow‐mode shock.

Observations of plasmas and magnetic fields in Earth's distant magnetotail: Comparison with a global MHD model

We are reporting the first direct comparison of in situ observations of plasmas and magnetic fields in Earth's distant magnetotail with the results of a time‐dependent, global magnetohydrodynamic (MHD) simulation of the interaction of the solar wind with the magnetosphere. The magnetotail observations were taken with the Geotail spacecraft during the period 0300–0630 UT on October 27, 1992 at a position near the dawnside magnetopause at a downstream distance of about 81 RE. During this period a dense, cold ion stream similar in density and speed to that expected for the magnetosheath plasmas was intermittently observed. When the cold ion stream was not present, the spacecraft was located in the northern magnetotail lobe. The dense, cold ion stream differed from that expected for the magnetosheath in the Y and Z components of ion bulk flow and in the Y component of the magnetic field. These cold ion streams are associated with a magnetopause accommodation region positioned just outside the classical magnetopause, as identified by a well‐defined transition from magnetic fields typical of those found in the lobe to the lesser and more fluctuating fields in the magnetosheath. This accommodation region exhibits perturbations in plasma flows and magnetic fields that appear to be related to the complex topology of the magnetopause at these large downstream positions. Simultaneous observations of the solar wind ions and the interplanetary magnetic field (IMF) with the IMP 8 spacecraft upstream from Earth provided the driving input for a global MHD model. The solar wind ion flow was steady during this period, and the IMF exhibited a series of rotations from northward to duskward. The dynamics of the magnetotail were controlled by the Y and Z components of the IMF. When this By was strongly positive, the magnetotail lobe appeared at the downstream Geotail position. Examination of the modeled plasma parameters in the Y‐Z plane through the spacecraft position shows that this By provides a torque on the magnetotail about its central axis. The MHD model also accurately positions the spacecraft alternately in the magnetopause accommodation region and the magnetotail lobe as the IMF clock angle varied from northward to duskward, respectively. The temporal variations of modeled parameters, i.e., ion densities, temperatures, and bulk flow velocities and the magnetic field components, are directly compared with the Geotail measurements. This first comparison of the Geotail observations with the modeled plasma parameters and magnetic fields provides substantial encouragement that a global MHD model can provide a valid description of important aspects of the large‐scale topology and dynamics of the magnetotail.

Observations of plasmas associated with the magnetic signature of a plasmoid in the distant magnetotail

The three‐dimensional electron and positive ion velocity distributions were examined for four events for which the magnetic signature of plasmoids occurred at the Geotail spacecraft position in Earth's distant magnetotail. The proton bulk parameters and the electron velocity distributions for one of the events are presented in order to characterize the qualitative character of the set of events. The north‐to‐south turnings of the magnetic field that were taken for evidence of the passage of a plasmoid were often accompanied by a significant strong core of By that was centered on the transition of Bz from north to south. The magnetic signatures thus appeared to be more indicative of flux ropes than of classical plasmoids with weak core fields. The proton and electron bulk velocities in these magnetic structures are not equal, a situation that provides an earthward current. The plasma β was typically one or greater. Counterstreaming of low‐energy electrons was often found on the magnetic field lines around the central magnetic core and suggested the existence of a closed field line topology in these regions. These events appeared to be the tailward flowing debris from acceleration processes occurring in the vicinity of a separator positioned nearer to Earth.

Survey of plasma parameters in Earth's distant magnetotail with the Geotail spacecraft

The three‐dimensional velocity distributions for both electrons and positive ions as determined with plasma instrumentation on board the Geotail spacecraft are used to compute number densities, ion bulk velocities, and electron and ion temperatures. We present a statistical survey of these plasma parameters for the magnetotail at distances from Earth in the range of 10 to 210 RE for the period 19 October 1992 through 10 July 1993. The ion temperature is used to discriminate “hot” plasmas from “cold” plasmas. The hot ion plasmas are frequently found to stream either towards or away from Earth and we identify these with the plasma sheet, plasma sheet boundary layer, and regions of acceleration. The cold ion plasmas are typically streaming away from Earth. These cold plasmas appear to be associated with the plasma mantle, the magnetosheath, and boundary layers interior to the magnetosheath, although the contribution from the ionosphere has not yet been evaluated.

Narrow‐band electrostatic noise generated by an electron velocity space hole

Narrow‐band Electrostatic Noise (NEN) is a common occurrence in the Earth's distant magnetotail. NEN is observed in a frequency range (100‐316 Hz) that falls roughly between the electron and ion plasma frequencies. This mode may result from holes in the electron distribution function associated with slow shocks. An instability that is associated with this mode is studied using numerical simulations. The growth of the instability depends on the size and shape of the hole. The hole mode can also be driven unstable by either an anisotropy in the electron distribution function or an ion beam. In all these cases the instability saturates at a low level and only a fraction of the available free energy is released.

Effect of plasma mantle injection on the dynamics of the distant magnetotail

We investigate the role of the plasma mantle in the dynamics of the Earth's distant magnetotail. The plasma mantle can exert a substantial influence on the current‐sheet force equilibrium by transporting both momentum and mass from the dayside magnetopause to the tail current sheet. Such influences are particularly strong when the interplanetary magnetic field contains a significant southward component for a prolonged period. We find that a number of processes characterizing a substorm growth phase can be attributed, at least in part, to the plasma mantle. Among these processes are the thinning of the plasma sheet, storage of magnetic energy in the tail, and formation of plasmoids. The rates of these processes are found to increase nonlinearly with the momentum density of the mantle plasma. The present model applies only to the distant magnetotail (x ≲ −30 RE) because of various approximations that we make to allow an analytical treatment of the governing equations.

An open magnetopause model of the Earth's distant tail based on ISEE 3 evidence

It is shown that an “open” hydromagnetic equilibrium model of the Earth's distant magnetotail can describe the gross features of the cross‐tail dimensions, the magnetic field intensity, and the average plasma density in the tail lobes, as measured by ISEE 3. The tail flaring rate changes (decreases) downstream at around X ∼ −(100–120) RE. Beyond that distance our model predicts a noncircular flattened tail whose average diameter is 2 RT = 2|yz|1/2 = 60–70 RE at down‐tail distances of X ∼ −200 RE and 2RT ∼ 70–110 RE at X ∼ −1000 RE. These values agree with measurements by ISEE 3 and Pioneer 7, respectively. The degree of flattening y/z (east‐west/north‐south dimension ratio) is related to the degree of anisotropy of the plasma pressure (p∥/p⊥ ratio) and to the degree of anisotropy of the field pressure in the solar wind. The most probable flattened tail, elongated in the east‐west direction, is obtained in our model (for p∥/p⊥ ∼ 1) with an y/z ratio of 1.4–1.7 at X ∼ −200 RE and 7–12 at X ∼ −;1000 RE. A tail cross section elongated in the north‐south direction (y/z < 1) would require a large p∥/p⊥ ratio, exceeding the threshold for the excitation of the fire hose instability in the solar wind. The magnetopause in our model is assumed to be partially open as implied by the observed electron distribution functions, by magnetic field data, and by the presence of mantle‐type plasma in the distant tail lobes. The degree of openness α, defined as the fraction of the ambient interplanetary magnetic field lines that become connected to the lobe field lines, is evaluated to be α ∼ 0.1. If the initial magnetic flux in each lobe is taken to be 0.65×109 Wb and if α = 0.1, we obtain a decrease of the average (X aberrated) component of the tail lobe field from |BT| = 7.5–8.5 nT at X ∼ −200 RE to |BT| ≲ 5 nT at X ∼ −1000 RE. The related lobe plasma density increase between these distances is from nT = 0.3–0.4 to 0.8–1.5 cm−3. The field and plasma values obtained are in reasonable agreement with the observations. The total length of the tail depends on α, and the value α = 0.1 allows the tail to reach X ∼ −(2–3) × 10³ RE as supported by the evidence of magnetospheric tail‐associated phenomena seen in Pioneer 7 data. Higher values of up to α = 0.2 cannot be ruled out on the basis of plasma data, but in that case the magnetic flux loss from the lobes becomes too fast to permit the tail to extend beyond X ∼ −(700–1000) RE.

Velocity filter effect and dynamics of distant magnetotail

The velocity filter effect produces a persistent time variation of the self-consistent magnetic-field configuration in the Earth's distant magnetotail. In the presence of a mantle source of plasma, a steady-state configuration cannot be attained, even if the source supply is constant in time. The velocity filter effect produces an evolution of the magnetotail such that the plasma sheet thins throughout the region accessible to the incoming plasma. This thinning process, widely observed and theoretically predicted, leads to current-sheet disruption when the thickness of the plasma sheet shrinks to a critical value. The whole process lasts 1–2 h, consistent with the observed duration of a typical substorm growth phase.