Earth's Magnetotail Current Sheet Research Articles

The dynamics of electron holes in current sheets

We present 1.5D Vlasov code simulations of the dynamics of electron holes in non-uniform magnetic and electric fields typical of current sheets and, particularly, of the Earth's magnetotail current sheet. The simulations show that spatial width and amplitude of electron holes do not substantially vary in the course of propagation, but there arises a double layer localized around the electron hole and manifested as a drop of the electrostatic potential along the electron hole. We demonstrate that electron holes produced around the neutral plane of a current sheet slow down in the course of propagation toward the current sheet boundaries. The leading contribution to electron hole braking is provided by the non-uniform magnetic field although electrostatic fields typical of the current sheets do provide a noticeable contribution. The simulations also show that electron holes with larger amplitudes are slowed faster. The simulation results suggest that some of the slow electron holes recently reported in the Earth's plasma sheet boundary layer may appear due to braking of initially fast electron holes in the course of propagation in the current sheet.

Superthin current sheets supported by anisotropic electrons

Current sheets with strong transverse (cross field) currents are commonly observed in planetary magnetospheres and serve as a natural energy source for magnetic reconnection. As the most investigated current sheet, the current sheet in the Earth's magnetotail forms in a high-β plasma, with hot ions dominantly contributing to the diamagnetic currents. Spacecraft observations have shown, however, that a superthin electron dominated current sheet can be embedded in the Earth's magnetotail current sheet. In this paper, we develop a model of such superthin current sheets with strong currents produced by anisotropic electrons. We also compare the model with spacecraft observations, which shows reasonable agreement in spatial profiles and magnitudes of the current density. The spatial scale (thickness) of the superthin current sheet is controlled by the equatorial magnetic field component, whereas the current density magnitude is controlled by the electron fire-hose parameter at the equator. Although the current density peak within the superthin current sheet can significantly exceed the background (embedding) current density, the magnetic field magnitude at the superthin current sheet boundary does not exceed 10% of the total magnetic field magnitude. These superthin current sheets are sub-ion (or even electron-scale) structures, which are not sufficiently large/intense to perturb ion dynamics. We discuss applications of the proposed model for the analysis of plasma instabilities in superthin electron-dominated current sheets.

Cluster Observations of a Dispersive Flapping Event of Magnetotail Current Sheet

AbstractA kink‐like flapping event of Earth magnetotail current sheet, which consists of two frequency bands successively, is studied by the multipoint observations of Cluster. The multipoint analysis of Cluster observations demonstrates that the higher frequency band (period is about 10 min) has faster propagation velocity (about 30 km/s), shorter wavelength (about 3 RE), and smaller amplitude (1–1.5 RE). In contrast, the lower frequency band (period is about 22 min) shows slower propagation velocity (about 21 km/s), longer wavelength (about 4.4 RE), and larger amplitude (2–3 RE). Comparison with the flapping models demonstrates that the dispersion of theoretical models does not show consistency with the results of this event, which suggests that new or more advanced kink‐like flapping theories or models in the future have to consider the constraints of the dispersive properties demonstrated by this event.

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.

Current sheets with inhomogeneous plasma temperature: Effects of polarization electric field and 2D solutions

We develop current sheet models which allow to regulate the level of plasma temperature and density inhomogeneities across the sheet. These models generalize the classical Harris model via including two current-carrying plasma populations with different temperature and the background plasma not contributing to the current density. The parameters of these plasma populations allow regulating contributions of plasma density and temperature to the pressure balance. A brief comparison with spacecraft observations demonstrates the model applicability for describing the Earth magnetotail current sheet. We also develop a two dimensional (2D) generalization of the proposed model. The interesting effect found for 2D models is the nonmonotonous profile (along the current sheet) of the magnetic field component perpendicular to the current sheet. Possible applications of the model are discussed.

Thin current sheets with strong bell-shape guide field: Cluster observations and models with beams

Abstract. We study the kinetic structure of intense ion-scale current sheets with strong electron currents and the guide field having a bell-shape profile. We consider four crossings of the Earth magnetotail current sheet by the Cluster mission in 2003. The thickness of these current sheets is about the ion inertial length and significantly smaller than the characteristic ion gyroradius. We analyze the asymmetry of the electron velocity distribution functions and show that the electron current is provided by the small electron subpopulation interpreted as an electron beam or two counter-streaming electron beams. The beam (counter-streaming beams) has a bulk velocity of the order of the electron thermal velocity and a density (difference of beam densities) of about 1–5% of the plasma density. To describe the observed current sheets we develop a kinetic model with particle beams. The model predicts different thickness of the current sheet for different types of current carriers (one electron beam or two counter-streaming electron beams). The observed ion-scale current sheets can be explained assuming that the current is carried by one electron beam and a co-streaming ion beam. Although the ion beam does not carry a significant current, this beam is required to balance the electron current perpendicular to the current sheet neutral plane. The developed model explains the dominance of the electron current and the ion scales of the current sheets.

Radial distribution of magnetic field in earth magnetotail current sheet

Knowing the magnetic field distribution in the magnetotail current sheet (CS) is essential for exploring magnetotail dynamics. In this study, using a joint dataset of Cluster/TC-1, the radial profile of the magnetic field in the magnetotail CS with radial distances covering 8<r<20RE under different geomagnetic activity states (i.e., AE≤100nT for quiet intervals while AE>100nT for active times) and solar wind parameters are statistically surveyed. Our new findings demonstrate that, independent of the activity state, the field strength and Bz component (GSM coordinates) start the monotonic increase prominently as r decreases down to ~11.5RE, which means the dipole field starts to make a significant contribution from there. At least in the surveyed radial range, the Bz component is found to be weaker in the midnight and dusk sectors than that in the dawn sector, displaying a dawn–dusk asymmetry. The occurrence rate of negative Bz in active times also exhibits a similar asymmetric distribution, which implies active dynamics may occur more frequently at midnight and dusk flank. In comparison with that in quiet intervals, several features can be seen in active times: (1) a local Bz minimum between 10.5<r<12.5RE is found in the dusk region, (2) the Bz component around the midnight region is generally stronger and experiences larger fluctuations, and (3) a sharp positive/negative-excursion of the By component occurs at the dawn/dusk flank regions inside r<10RE. The response to solar wind parameters revealed that the Bz component is generally stronger under higher dynamic pressure (Pdy>5nPa), which may support the dawn–dusk squeezing effect as presented by Miyashita et al. (2010). The CS By is generally correlated with the interplanetary magnetic field (IMF) By component, and the correlation quality is found to be better with higher penetration coefficient (the ratio of CS By to IMF By) when IMF Bz is positive. The implications of the present results are discussed.

On the origin of pressure and magnetic perturbations ahead of dipolarization fronts

Dipolarization fronts (DFs), earthward‐propagating structures in the Earth's magnetotail current sheet with sharp enhancements of the northward magnetic field Bz, are typically preceded by minor decreases in Bz. Other characteristic DF precursor signatures, including earthward flows and plasma density/pressure enhancements, have been explained in the context of ion acceleration and reflection at dipolarization fronts. In the same context here we simulate the spatial distribution of plasma pressure earthward of a convex DF. The resultant pressure distribution, which shows clear dawn‐dusk asymmetries with greater enhancements at the DF duskside, agrees with statistical observations. The simulation further reveals that the reflected ions can carry a secondary current earthward of the advancing DF, which explains the characteristic signature of the Bz dip immediately ahead of the DF.

Cluster observations near reconnection X lines in Earth's magnetotail current sheet

Magnetic reconnection is an efficient way to convert magnetic energy into particle energy. In this paper, we use Cluster thermal electron and ion measurements in the vicinity of a reconnection X line to delineate the structure of the reconnection current sheet. Multispacecraft observations made by Cluster on 18 August 2002 indicate that an X line drifted close to the spacecraft, about 3.4 RE earthward of the position where another X line had been observed earlier. Comparison of the Hall magnetic and electric field geometry and the observed properties of energetic electron beams streaming along the separatrix between the Cluster spacecraft indicates that the second X line formed within 20 s of the observation of the first X line. Repeated flow reversals and Hall field geometry together with the presence of a magnetic island embedded in the outflow region downstream of the first X line suggest that the initial current sheet was unstable, perhaps to the tearing mode. We identify a region with a thickness of 0.72 ion inertial lengths (29 electron inertial lengths, de) of super‐Alfvénic electron outflow (greater than the ion in‐flow Alfvén speed) during the period when the spacecraft was in the vicinity of the neutral sheet. Slightly below the neutral sheet, Cluster observed asymmetric counter‐streaming electrons with a loss of axisymmetry in the electron (V⟂1,V⟂2) distribution functions over a thin boundary with a thickness of several de. This electron‐scale transition layer was embedded in a much wider region where both the ion and electron Walén tests failed, and the electron super‐Alfvénic bulk outflow jets with high‐energy electron beams were detected. Those phenomena provide details of the substructure of the reconnection current sheet and suggest that the spacecraft traversed or skimmed the tailward edge of an elongated electron current layer. We also note that this event differs from a previously reported reconnection event in that strong electron temperature anisotropy (T∥&gt;T⟂) is observed both in the inflow region and in the exhaust, where the anisotropy appears to be associated with the elongated electron outflow jets.

MHD aspect of current sheet oscillations related to magnetic field gradients

Abstract. One-fluid ideal MHD model is applied for description of current sheet flapping disturbances appearing due to a gradient of the normal magnetic field component. The wave modes are studied which are associated to the flapping waves observed in the Earth's magnetotail current sheet. In a linear approximation, solutions are obtained for model profiles of the electric current and plasma densities across the current sheet, which are described by hyperbolic functions. The flapping eigenfrequency is found as a function of wave number. For the Earth's magnetotail conditions, the estimated wave group speed is of the order of a few tens kilometers per second. The current sheet can be stable or unstable in dependence on the direction of the gradient of the normal magnetic field component. The obtained dispersion function is used for calculation of the flapping wave disturbances, which are produced by the given initial Gaussian perturbation at the center of the current sheet and propagating towards the flanks. The propagating flapping pulse has a smooth leading front, and a small scale oscillating trailing front, because the short wave oscillations propagate much slower than the long wave ones.

Magnetic double gradient mechanism for flapping oscillations of a current sheet

A new kind of magnetohydrodynamic waves are analyzed for a current sheet in a presence of a small normal magnetic field component varying along the sheet. As a background, two simplified models of a current sheet are considered with a uniform and nonuniform current distributions in the current sheet. On a basis of these two models, the flapping‐type waves are obtained which are related to a coexistence of two gradients of the tangential and normal magnetic field components along the normal and tangential directions with respect to the current sheet. A stable situation for the current sheet is associated with a positive result of the multiplication of the two magnetic gradients, and unstable (wave growth) condition corresponds to a negative result of the product. In the stable region, the “kink”‐like wave mode is interpreted as so called flapping waves observed in the Earth's magnetotail current sheet.

Magnetic Double-Gradient Instability and Flapping Waves in a Current Sheet

A new kind of magnetohydrodynamic instability and waves are analyzed for a current sheet in the presence of a small normal magnetic field component varying along the sheet. These waves and instability are related to the existence of two gradients of the tangential (B_{tau}) and normal (B_{n}) magnetic field components along the normal (nabla_{n}B_{tau}) and tangential (nabla_{tau}B_{n}) directions with respect to the current sheet. The current sheet can be stable or unstable if the multiplication of two magnetic gradients is positive or negative. In the stable region, the kinklike wave mode is interpreted as so-called flapping waves observed in Earth's magnetotail current sheet. The kink wave group velocity estimated for the Earth's current sheet is of the order of a few tens of kilometers per second. This is in good agreement with the observations of the flapping motions of the magnetotail current sheet.

Cluster observations of broadband electromagnetic waves in and around a reconnection region in the Earth's magnetotail current sheet

We present an analysis of the electric and magnetic wave spectra on kinetic scales during several crossings of a reconnecting current sheet. The spectra were measured from 1 Hz or less up to 4096 Hz by the EFW, FGM and STAFF instruments onboard the Cluster spacecraft between 3 and 4 UT on 11 October 2001. During the event plasma flows of order of the local Alfvén speed reversed from tailward to earthward, suggesting that a reconnection site moved over the spacecraft. We ordered the observed electric and magnetic field wave spectrum by the position within the current sheet using the magnitude of the magnetic field B. We found that the electric and magnetic wave power decreased considerably at all frequencies towards the center of the current sheet (B ≈ 0 nT). The electric energy density decreases 5 orders of magnitude from the edge of the current sheet (B = 19 nT) to the center and the magnetic energy density peaks within the current sheet (B = 13 nT) and is decreased by 2.5 orders of magnitude at the center. Within the current sheet, the electric and magnetic wave spectra were dominantly broadband electromagnetic noise (i.e., power law spectra with exponents ≈−1.4 and ≈−2.4, respectively) throughout the frequency range ∼0.1–1000 Hz, spanning from MHD (i.e., ion cyclotron frequency ≈0.2 Hz) to almost the electron plasma frequency (≈4000 Hz). We argue that the wave activity is likely to be whistler wave turbulence and discuss the implications of these results for reconnection from wave‐particle interactions.

Observations of multiple X‐line structure in the Earth's magnetotail current sheet: A Cluster case study

Observations of the Earth's magnetotail made by the four Cluster spacecraft on October 2 2003 are presented. Multi‐spacecraft analysis is used to show that the variations in field and flow observed in the vicinity of the magnetotail current sheet are most consistent with a series of two active reconnection sites bounding an Earthward moving flux rope. We demonstrate that a single spacecraft analysis of the same data leads to the incorrect conclusion that a single X‐line is moving tailward. The implications of this in relation to the interpretation of single spacecraft observations are outlined. These results show that reconnection can occur simultaneously at different points in the near‐Earth magnetotail current sheet, providing (further) important experimental validation of multiple X – line reconnection theories on the mesoscale (tens of ion inertial length) level.

On the non-adiabatic particle scattering in the earth's magnetotail current sheet

The empirical model of disturbed magnetosphere of Tsyganenko and Usmanov (1982) and the semi-empirical model of the storm-time magnetospheric configuration of Tsyganenko (1981) are used to find the critical energy for non-adiabatic particle scattering in the midnight sector. Computed values of E crit vs L are compared with the appropriate experimental data of Imhof et al. (1977). It is found that none of the considered models is able to reproduce the observed steep decrease of E crit with L. The steepest slope is given by the Tsyganenko model which includes a current sheet with the finite thickness. The current sheet thickness is a crucial parameter in the non-adiabatic scattering problem. In discussion we point to natural limitations of an empirical model as far as the current sheet thickness is to be determined. Imhof et al.'s data as well as some magnetic field data sets seem to indicate that magnetosphere models incorporating a thin current sheet and allowing for the thickness dependence on the geocentric distance would probably be closer to reality than the considered models, at least during higher levels of magnetic activity.