A. A. Petrukovich

Substorm dipolarization and recovery

On the basis of ∼2 years of Geotail data, we use a superposed epoch approach to study the average behavior of plasma and magnetic fields at different radial distances, between 11 and 31 RE, during 66 substorms in the premidnight sector. Magnetic field dipolarization is first seen in the innermost region (11–16 RE) around substorm onset and subsequently moves tailward at a rate of 35 km/s. Fast earthward and tailward ion bulk flows in the central plasma sheet indicate that during substorm expansion the near-Earth neutral line is located between 21 and 26 RE, with a tendency to be closer to 21 RE near substorm onset. About 45 min after onset, the tailward moving dipolarization front reaches the distance range where the near-Earth neutral line is located. Thereafter the near-Earth neutral line disappears beyond 31 RE. This is the classical signature of the start of the recovery phase. We conclude that substorm recovery sets in when the tailward moving dipolarization front reaches the near-Earth neutral line, because the near-Earth neutral line cannot operate in a dipolar field geometry.

Embedded current sheets in the Earth’s magnetotail

[1]xa0In this investigation we introduce and discuss quantitative parameters of a thin current sheet embedded in the background plasma sheet. We use Cluster statistics and empirical models, as well as self-consistent simulations, to understand the formation and development of embedded current sheets, in particular in the course of substorms. Data and theory show that the embedded sheet thickness is of the order of a proton larmor radius, a constraint equivalent to magnetic flux conservation. The embedded sheet can be essentially described by two dimensionless parameters B0/Bext and F0/Fext. B0 is the magnetic field at the embedded sheet boundary, Bext is the field at the boundary of the background plasma sheet, and F0 and Fext are magnetic flux values. During the growth phase current density in embedded sheet and B0 increase, while thickness decreases. Sheets with the most intense currents (large B0) are observed after onset. The self-consistent anisotropic sheet model, including both electron and proton currents and combined with the Harris-type background shows that when the proton-scale embedded sheet becomes sufficiently thin, an electron-scale current sheet can appear inside it due to enhanced electron curvature drift.

Origins of plasma sheet By

[1]xa0With 11 years of Geotail measurements we analyzed sources of plasma sheet By and constructed a model, depending on IMF By, coordinates X, Y, and geodipole tilt angle. By dependence on dipole tilt due to the neutral sheet warping and hinging has an odd (antisymmetric) profile with respect to Y. In addition, a new, even with respect to Y, By component was discovered, which is positively correlated with dipole tilt with the maximal amplitude ±1–2 nT. In the postmidnight sector the dipole tilt effects in By almost cancel each other, while at the premidnight sector they are summed up and are comparable with the IMF penetration. Such season-dependent net By creates a principal azimuthal asymmetry of the magnetotail and is consistent with some polar convection and aurora observations. Plasma sheet By is often substantially larger than the statistically expected value. This effect can be understood as “amplification” due to internal plasma sheet dynamics. As a result an asymmetric tail of the By distribution forms, causing certain overestimation of the regression coefficients in statistical models.

Cluster statistics of thin current sheets in the Earth magnetotail: Specifics of the dawn flank, proton temperature profiles and electrostatic effects

[1]xa0In this paper we use the statistics of 70 crossings by the Cluster mission to study and compare properties of thin current sheets observed at the dawn and dusk flanks of the Earth magnetotail. Special attention is devoted to the current sheet embedding: we define the degree of the current sheet embedding as be = Bext/B0 (B0 and Bext are magnetic field magnitudes at the thin current sheet boundary and in the lobes). We determine the current density by curlometer technique and calculate the current sheet thickness. We demonstrate that the current sheets at the dawn flank have larger be, smaller magnitude of the current density and larger relative thicknesses in Larmor radii than the current sheets at the dusk flank. Protons in thin current sheets are divided into two populations (the current-carrying particles and the background) and the temperatures of these populations have been estimated. The distribution of the proton temperature p inside typical current sheet is approximated as p ≈ Tp(1 − αT(Bx/Bext)2), where Tp is the p value in the central region of the current sheet. The average value 〈αT〉 ≈ 0.8. The proton current density (flow velocity in Y) is positive at the dusk flank and negative at the dawn flank, while the electron current density is positive at both flanks. This difference of the proton current density at two flanks is explained by the E × B drift due to the presence of the earthward electrostatic field Ex. We develop a simple model of the earthward electrostatic field to incorporate the influence of the embedding and the dawn-dusk magnetic field component.

Statistical survey on the magnetic structure in magnetotail current sheets

On the basis of the multipoint magnetic observations of Cluster in the region 15-19 R-E downtail, the magnetic field structure in magnetotail current sheet (CS) center is statistically surveyed. It is found that the B-y component (in GSM coordinates) is distributed mainly within vertical bar B-y vertical bar < 5nT, while the B-z component is mostly positive and distributes mainly within 1 similar to 10 nT. The plane of the magnetic field lines (MFLs) is mostly vertical to the equatorial plane, with the radius of curvature (Rc) of the MFLs being directed earthward and the binormal (perpendicular to the curvature and magnetic field direction) being directed azimuthally westward. The curvature radius of MFLs reaches a minimum, R-c,R-min, at the CS center and is larger than the corresponding local half thickness of the neutral sheet, h. Statistically, it is found that the overall surface of the CS, with the normal pointing basically along the south-north direction, can be approximated to be a plane parallel to equatorial plane, although the local CS may be flapping and is frequently tilted to the equatorial plane. The tilted CS (normal inclined to the equatorial plane) is apt to be observed near both flanks and is mainly associated with the slippage of magnetic flux tubes. It is statistically verified that the minimum curvature radius, R-c,R-min, half thickness of neutral sheet, h, and the slipping angle of MFLs, delta, in the CS satisfies h = R-c,R-min cos delta. The current density, with a mean strength of 4-8 nA/m(2), basically flows azimuthally and tangentially to the surface of the CS, from dawn side to the dusk side. There is an obvious dawn-dusk asymmetry of CS, however. For magnetic local times (MLT) similar to 21:00-similar to 01:00, the CS is relatively thinner; the minimum curvature radius of MFLs, R-c,R-min (0.6-1 R-E) and the half-thickness of neutral sheet, h (0.2-0.4 R-E), are relatively smaller, and B-z (3-5 nT) and the minimum magnetic field, B-min (5-7 nT), are weaker. It is also found that negative B-z has a higher probability of occurrence and the cross-tail current density j(Y) is dominant (2-4 nA/m(2)) in comparison to those values near both flanks. This implies that magnetic activity, e. g., magnetic reconnection and current disruption, could be triggered more frequently in CS with similar to 21:00-similar to 01:00 MLT. Accordingly, if mapped to the region in the auroral ionosphere, it is expected that substorm onset would be optically observed with higher probability for similar to 21:00-similar to 01:00 MLT, which is well in agreement with statistical observations of auroral substorm onset.

Substorm‐associated pressure variations in the magnetotail plasma sheet and lobe

Simultaneous pressure measurements by Interball-Tail in the high-latitude lobe and by Geotail in the equatorial plasma sheet were analyzed for 30 substorms which exhibited significant pressure changes. At the onset of a few substorms we observed equatorial pressure peaks with magnitudes up to 50% higher than those in the lobe. These pileups are probably rather localized, and their properties are consistent with plasma sheet thickening between two active regions in the tail. During expansion and recovery phases of more than half of substorms, we observed equatorial pressure depletions relative to the high-latitude lobe pressure. These depletions can last more than 2 hours and are likely formed during the substorm expansion phase near the equatorial plane behind (tailward of) the strongly dipolar near-Earth magnetotail region. The observed pressure gradient is probably a nonstationary feature and can be compensated partially by magnetic tension on the curved field lines. Magnitude and history of the solar wind dynamic pressure appear to significantly influence substorm scenarios in the magnetotail. Possible existence of the pressure difference should be taken into account in single-spacecraft substorm studies.

Small substorms: Solar wind input and magnetotail dynamics

We investigated properties of 43 small magnetospheric substorms. Their general signatures were found to be consistent with the so-called contracted oval or northern Bz substorms. Small but clear pressure changes in the tail corresponding to growth and expansion phases detected in about a half of cases testify that these substorms follow the same loading-unloading scheme as the larger ones. However, rate of the solar wind energy accumulation in the magnetosphere was low due to azimuthal IMF orientation with dominating IMF By and small fluctuating IMF Bz. Plasma sheet signatures could be very strong and likely were localized in their cross-tail size. Negative bays in auroral X magnetograms were of order of 100–300 nT, with maxima at Bear Island station (71°geomagnetic latitude) and in few cases were delayed after magnetotail onsets by tens of minutes. Small substorms probably differ from their larger counterparts in a way that coherency of the magnetotail reconfiguration in the inner and middle-tail regions and across the tail is lost in smaller substorms.

Profile of strong magnetic field By component in magnetotail current sheets

The strong magnetic field B-y component (in GSM coordinates) has been increasingly noticed to play an important role in the dynamics of tail current sheet (CS). The distribution profile of strong B-y components in the tail CS (i.e., those with guide field), however, is not well known. In the present work, by using the simultaneous multipoint observations of Cluster satellites, the profile of a strong B-y component in tail current sheets is explored, through detailed case studies, as well as in a statistical study. It is discovered that around the midnight meridian, the strength of the strong B-y component, i.e., |B-y|, is basically enhanced at the center of the CS relative to that in the CS boundaries and lobes and forms a north-south symmetric distribution about the center of CS. Generally, however, for strong guide field cases in the non-midnight meridian, the profile of B-y strength basically becomes north-south asymmetric, the strength of the B-y component in the northern side of the CS is found to be either stronger or weaker than that in the counterpart southern side. By considering the modulation of the tail flaring magnetic field with magnetic local time, we propose an interpretation to account for the variation of the B-y-profile, which is supported by the statistical survey. These results offer an observation basis for further studies.

Adiabatic electron heating in the magnetotail current sheet: Cluster observations and analytical models

[1]xa0We consider the electron distribution in current sheets observed by Cluster mission in the Earth magnetotail. We use the statistics of 70 fast (less than 20 minutes) and 12 slow (more than one hour) crossings of horizontal current sheets. We demonstrate that for both types electron temperature decreases with increase of magnetic field ∣Bx∣ away from the current sheet center. We use the approximations Te⊥/Te⊥xa0maxxa0≈xa01xa0−xa0αT⊥(Bx/Bext)2 and Te∥/Te∥xa0maxxa0≈xa01xa0−xa0αT∥(Bx/Bext)2, where Bext is value of Bx in the lobes. For statistics of thin current sheets (fast crossings) we obtain mean values 〈αT⊥〉xa0≈xa0〈αT∥〉xa0≈xa01. For thick current sheets (slow crossings) we also obtain 〈αT⊥〉xa0≈xa0〈αT∥〉, but 〈αT⊥〉,xa0〈αT∥〉xa0>xa01. The electron temperature anisotropy is about Te∥/Te⊥xa0=xa01.1xa0−xa01.2 and vertical profiles Te∥/Te⊥xa0≈xa0const. Observed vertical distributions of Te∥ and Te⊥ are described by the analytical model of electron heating in the course of the earthward convection in thin current sheets with (Bx(z),xa0Bz(x)) and in thick current sheets with (Bx(x,xa0z),xa0Bz(x,xa0z)). We also show that the observed electron temperature anisotropy is provided by the electron population in the energy range between 50 ev and 3xa0keV. The cold core of electron distribution ( 5 keV) has Te∥/Te⊥xa0∼xa01 or even Te∥/Te⊥xa0<xa01. We consider electron pressure tensor in observed thin current sheets and demonstrate that electron velocity distribution is gyrotropic with high accuracy.

Are earthward bursty bulk flows convective or field‐aligned?

We use five years of Geotail and AMPTE/IRM measurements in the near-Earth magnetotail to compute angles between vectors of fast earthward plasma flow and the local magnetic field. In the low-β parts of the magnetotail, fast flows were found to be nearly field-aligned with an average angle of ∼20°. In the high-β plasma sheet the average angle was larger than 45°. The existence of a substantial convective component in the plasma sheet confirms the importance of the bursty bulk flow phenomenon for the magnetotail convection process.