Cross-tail Current Sheet Research Articles

Semi-annual, annual and Universal Time variations in the magnetosphere and in geomagnetic activity: 2. Response to solar wind power input and relationships with solar wind dynamic pressure and magnetospheric flux transport

This is the second in a series of papers that investigate the semi-annual, annual and Universal Time (UT) variations in the magnetosphere. We present a varied collection of empirical results that can be used to constrain theories and modelling of these variations. An initial study of two years’ data on transpolar voltage shows that there is a semi-annual variation in magnetospheric flux circulation; however, it is not as large in amplitude as that in geomagnetic activity, consistent with the latter showing a non-linear (quadratic) variation with transpolar voltage. We find that during the persistent minimum of the UT variation in geomagnetic activity, between about 2 and 10 UT, there is also a persistent decrease in observed transpolar voltage, which may be, in part, caused by a decrease in reconnection voltage in the nightside cross-tail current sheet. We study the response of geomagnetic activity to estimated power input into the magnetosphere using interplanetary data from 1995 onwards, an interval for which the data are relatively free of data gaps. We find no consistent variation in the response delay with time-of-year F and, using the optimum lag, we show that the patterns of variation in F-year spectrograms are very similar for geomagnetic activity and power input into the magnetosphere, both for average values and for the occurrence of large events. The Russell–McPherron (R–M) mechanism is shown to be the central driver of this behaviour. However, the (R–M) effect on power input into the magnetosphere is small and there is a non-linear amplification of the semi-annual variation in the geomagnetic response, such that a very small asymmetry in power input into the magnetosphere Pα between the “favourable” and “unfavourable” polarities of the IMF BY component generates a greatly amplified geomagnetic response. The analysis strongly indicates that this amplification is associated with solar wind dynamic pressure and its role in squeezing the near-Earth tail and so modulating the storage and release of energy extracted from the solar wind. In this paper, we show that the equinoctial pattern is found in the residuals of fits of Pα to the am index and that the amplitude of these equinoctial patterns in the am fit residuals increases linearly with solar wind dynamic pressure. Similarly, the UT variation in am is also found in these fit residuals and also increases in amplitude with solar wind dynamic pressure.

MESSENGER Observations of Giant Plasmoids in Mercury’s Magnetotail

Abstract Small-scale flux ropes, with estimated diameters of ∼500 km and that pass over the MESSENGER spacecraft on timescales of seconds or less, are a common feature in Mercury’s magnetosphere. These magnetic structures, sometimes referred to as plasmoids, are believed to form as a result of rapid transient reconnection in the cross-tail current sheet at Mercury and the other planets. Here we report the occurrence of unusually large plasmoids in Mercury’s magnetotail observed, by the MESSENGER spacecraft. These plasmoids are remarkable for several reasons. Their spatial scales in the north–south direction exceed Mercury’s radius of 2440 km, and their time durations are comparable to or longer than the average Dungey cycle time of ∼200 s. They also have a more loop-like magnetic structure than the more common helical-like flux rope topology. These new MESSENGER observations suggest that Mercury’s magnetosphere can dissipate large quantities of magnetic flux and energy not only through the formation of a large number of small flux ropes, but also occasionally by the formation and release of a single giant plasmoid.

Mercury's cross‐tail current sheet: Structure, X‐line location and stress balance

AbstractThe structure, X‐line location, and magnetohydrodynamic (MHD) stress balance of Mercury's magnetotail were examined between −2.6 < XMSM < −1.4 RM using MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) measurements from 319 central plasma sheet (CPS) crossings. The mean plasma β in the CPS calculated from MESSENGER data is ~ 6. The CPS magnetic field was southward (i.e., tailward of X‐line) ~ 2–18% of the time. Extrapolation of downtail variations in BZ indicates an average X‐line location at −3 RM. Modeling of magnetic field measurements produced a cross‐tail current sheet (CS) thickness, current density, and inner CS edge location of 0.39 RM, 92 nA/m2 and −1.22 RM, respectively. Application of MHD stress balance suggests that heavy planetary ions may be important in maintaining stress balance within Mercury's CPS. Qualitative similarities between Mercury's and Earth's magnetotail are remarkable given the differences in upstream conditions, internal plasma composition, finite gyro‐radius scaling, and Mercury's lack of ionosphere.

Modular model for Mercury's magnetospheric magnetic field confined within the average observed magnetopause.

Accurate knowledge of Mercury's magnetospheric magnetic field is required to understand the sources of the planet's internal field. We present the first model of Mercury's magnetospheric magnetic field confined within a magnetopause shape derived from Magnetometer observations by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft. The field of internal origin is approximated by a dipole of magnitude 190 nT RM 3, where RM is Mercury's radius, offset northward by 479 km along the spin axis. External field sources include currents flowing on the magnetopause boundary and in the cross‐tail current sheet. The cross‐tail current is described by a disk‐shaped current near the planet and a sheet current at larger (≳ 5 RM) antisunward distances. The tail currents are constrained by minimizing the root‐mean‐square (RMS) residual between the model and the magnetic field observed within the magnetosphere. The magnetopause current contributions are derived by shielding the field of each module external to the magnetopause by minimizing the RMS normal component of the magnetic field at the magnetopause. The new model yields improvements over the previously developed paraboloid model in regions that are close to the magnetopause and the nightside magnetic equatorial plane. Magnetic field residuals remain that are distributed systematically over large areas and vary monotonically with magnetic activity. Further advances in empirical descriptions of Mercury's magnetospheric external field will need to account for the dependence of the tail and magnetopause currents on magnetic activity and additional sources within the magnetosphere associated with Birkeland currents and plasma distributions near the dayside magnetopause.

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, force balance, and evolution of incompressible cross-tail current sheet thinning

[1] THEMIS five-point observations on April 8, 2009 were used to study thinning of the current sheet in the near-Earth tail that led to the onset of a small substorm. Taking advantage of a fortuitous alignment of the five spacecraft near 2300 LT and 11 RE and within 1.5 RE of the current sheet center, latitudinal gradients are analyzed. A significant latitudinal pressure gradient is present indicating the necessity of a (J × B)z force to maintain the pre-onset equilibrium state. During thinning the total pressure remained approximately constant at all spacecraft rather than increasing. Within the plasma sheet, magnetic field strength increased while plasma pressure decreased due to decreasing temperature. We present a comprehensive explanation for the relationship between the thinning, the stretched structure, and development of intense current density. Our analysis of this event suggests that (1) the thinning in this event is an MHD force-balanced self-evolving process and is not a forced process due to an increased lobe field; (2) the thinning changes flux tube structure in length and curvature but not significantly in volume; (3) the thinning evolves with a change of the radial plasma pressure profile in the near-Earth tail, which is associated with a locally intensified current sheet. The conclusion is that the increased lobe field strength is not the necessary and the primary cause for cross tail current sheet thinning but rather thinning can occur within the plasma sheet as a result of unknown internal processes.

Magnetic fields in a flap!

Elizabeth Davey and colleagues survey the behaviour of the cross-tail current sheet, a presentation that won a Rishbeth prize at the NAM/UKSP/MIST meeting in April this year.

A THEMIS multicase study of dipolarization fronts in the magnetotail plasma sheet

[1] We discuss results of a superposed epoch analysis of dipolarization fronts, rapid (δt 10 nT) increases in the northward magnetic field component, observed during six Time History of Events and Macroscale Interactions during Substorms (THEMIS) conjunction events. All six fronts propagated earthward; time delays at multiple probes were used to determine their propagation velocity. We define typical magnetic and electric field and plasma parameter variations during dipolarization front crossings and estimate their characteristic gradient scales. The study reveals (1) a rapid 50% decrease in plasma density and ion pressure, (2) a factor of 2–3 increase in high-energy (30–200 keV) electron flux and electron temperature, and (3) transient enhancements of ∼5 mV/m in duskward and earthward electric field components. Gradient scales of magnetic field, plasma density, and particle flux were found to be comparable to the ion thermal gyroradius. Current densities associated with the Bz increase are, on average, 20 nA/m2, 5–7 times larger than the current density in the cross-tail current sheet. Because j · E > 0, the dipolarization fronts are kinetic-scale dissipative regions with Joule heating rates of 10% of the total bursty bulk flow energy.

The magnetic field near Mars: A comparison between a hybrid model, Mars Global Surveyor and Mars Express observations

Mars Express (MEX) does not carry its own magnetometer which complicates interpretation of ASPERA-3/MEX ion measurements. The direction of the interplanetary magnetic field (IMF) is especially important because it, among other things, determines the direction of the convective electric field and orientation of the cross tail current sheet and tail lobes. In this paper we present a case study to show the properties of the magnetic field near Mars in a quasi-neutral hybrid (QNH) model at the orbits where the Mars Global Surveyor (MGS) has made measurements, present a method to derive the IMF clock angle by comparing fields in a hybrid model and the direction of the magnetic field measured by MGS by deriving the IMF clock angle. We also use H + ring velocity distribution observations upstream of the bow shock measured by the IMA/ASPERA-3 instrument on board MEX spacecraft. These observations are used to indirectly provide the orientation of the IMF. We use a QNH model (HYB-Mars) where ions are modeled as particles while electrons form a mass-less charge neutralizing fluid. We found that the direct MGS and non-direct IMA observations of the orientation magnetic field vectors in non-crustal magnetic field regions are consistent with the global magnetic field draping pattern predicted by the global model.

A numerical model of the ionospheric signatures of time-varying magnetic reconnection: III. Quasi-instantaneous convection responses in the Cowley-Lockwood paradigm

Abstract. Using a numerical implementation of the cowlock92 model of flow excitation in the magnetosphere-ionosphere (MI) system, we show that both an expanding (on a ~12-min timescale) and a quasi-instantaneous response in ionospheric convection to the onset of magnetopause reconnection can be accommodated by the Cowley-Lockwood conceptual framework. This model has a key feature of time dependence, necessarily considering the history of the coupled MI system. We show that a residual flow, driven by prior magnetopause reconnection, can produce a quasi-instantaneous global ionospheric convection response; perturbations from an equilibrium state may also be present from tail reconnection, which will superpose constructively to give a similar effect. On the other hand, when the MI system is relatively free of pre-existing flow, we can most clearly see the expanding nature of the response. As the open-closed field line boundary will frequently be in motion from such prior reconnection (both at the dayside magnetopause and in the cross-tail current sheet), it is expected that there will usually be some level of combined response to dayside reconnection.

Detailed analysis of low-energy electron streaming in the near-Earth neutral line region during a substorm

A synaptic view of Cluster FGM, CIS, EDI, and PEACE observations of the near-Earth neutral line region during 1800–1830 UT on 13 September 2002 is discussed. Associated with the flow reversal and a thin cross-tail current sheet, a clear signature of a quadrupolar magnetic field is observed, which indicates the formation of the Hall current system around the X line in the large-scale picture. Associated with this magnetic field structure, low energy field-aligned electron beams mainly directed toward the X line are observed. High resolution data of FGM and EDI further show the fine structure of the low energy electron beams near the plasma sheet–lobe boundary. The beam becomes much more intense in the narrower region associated with the small magnetic field disturbances. In some cases, complicated bi-directional or oppositely directed beams are observed.

Unexpected vertical current sheets in the magnetotail associated with northward IMF

Fast magnetic field polarity changes detected in the quiet magnetotail by a spacecraft are usually attributed to thin cross-tail current sheet crossings. On September 14, 2001, Cluster registered several such events. However the IMF was northward for the previous 24 h and the formation of a thin current sheet was very unlikely. The multi-point analysis revealed that these sheets were almost vertically oriented, had strong shear and vanishing normal magnetic components and therefore should be interpreted as boundaries between two independent magnetic field domains (flux tubes), rather than crossings of the main cross-tail current sheet. We suggest that such anomalous configuration might form in the course of high-latitude reconnection of geomagnetic lobe and interplanetary field lines when IMF is northward.

A numerical model of the ionospheric signatures of time-varying magneticreconnection: I. ionospheric convection

Abstract. This paper presents a numerical model for predicting the evolution of the pattern of ionospheric convection in response to general time-dependent magnetic reconnection at the dayside magnetopause and in the cross-tail current sheet of the geomagnetic tail. The model quantifies the concepts of ionospheric flow excitation by Cowley and Lockwood (1992), assuming a uniform spatial distribution of ionospheric conductivity. The model is demonstrated using an example in which travelling reconnection pulses commence near noon and then move across the dayside magnetopause towards both dawn and dusk. Two such pulses, 8min apart, are used and each causes the reconnection to be active for 1min at every MLT that they pass over. This example demonstrates how the convection response to a given change in the interplanetary magnetic field (via the reconnection rate) depends on the previous reconnection history. The causes of this effect are explained. The inherent assumptions and the potential applications of the model are discussed. Key words. Ionosphere (ionosphere-magnetosphere interactions; plasma convection) – Magnetospheric physics (magnetosphere-ionosphere interactions; solar wind-magnetosphere interactions)

Physics of Substorm Growth Phase, Onset, and Dipolarization

A new scenario of substorm growth phase, onset and dipolarization during expansion phase and the corresponding physical processes are presented. During the growth phase, as a result of enhanced plasma convection, the plasma pressure and its gradient continue to be enhanced over the quiet-time values in the plasma sheet. Toward the late growth phase, a strong cross-tail current sheet is formed in the near-Earth plasma sheet region, where a local magnetic well is formed. The equatorial plasma beta (β eq ) can reach a local maximum with value larger than 50 and the cross-tail current density can be enhanced to over 10nA/m2 as obtained from 3D quasi-static magnetospheric equilibrium solutions for the growth phase. The most unstable kinetic ballooning instabilities (KBI) are expected to be located in the tailward side of the strong cross-tail current sheet region. The field lines in the most unstable KBI region map to the transition region between the region-l and region-2 currents in the ionosphere, which is consistent with the observed initial brightening location of the breakup arc in the intense proton precipitation region. The KBI explains the AMPTE/CCE observations that a low frequency instability with a wave period of 50–75 seconds is excited about 2–3 min prior to substorm onset and grows exponentially to a large amplitude at the onset of current disruption (or current reduction). At the onset of current disruption higher frequency instabilities are excited so that the plasma and electromagnetic field fluctuations form a strong turbulent state. Plasma transport takes place due to the strong turbulence to relax the ambient plasma pressure profile so that the plasma pressure and current density are reduced and the ambient magnetic field intensity increases by more than a factor of 2–3 in the high-β eq region and the field line geometry recovers from tail-like to dipole-like – dipolarization.

Magnetic turbulence and particle dynamics in the Earth’s magnetotail

Abstract. The influence of magnetic turbulence in the near-Earth magnetotail on ion motion is investigated by numerical simulation. The magnetotail current sheet is modelled as a magnetic field reversal with a normal magnetic field com-ponent Bn , plus a three-dimensional spectrum of magnetic fluctuations dB which represents the observed magnetic turbulence. The dawn-dusk electric field Ey is also considered. A test particle simulation is performed using different values of Bn and of the fluctuation level dB/B0. We show that when the magnetic fluctuations are taken into account, the particle dynamics is deeply affected, giving rise to an increase in the cross tail transport, ion heating, and current sheet thickness. For strong enough turbulence, the current splits in two layers, in agreement with recent Cluster observations.Key words. Magnetospheric physics (magnetospheric configuration and dynamics) – Interplanetary physics (MHD waves and turbulence) – Electromagnetics (numerical methods)

Energetic particle bursts before the main substorm injection

We examine the relationship between the energetic electron bursts (15 – 30 s) observed before the main injection and the changes in the magnetic field during substorms on the CRRES. Pitch angle distrubution (PAD) of electron flux during these bursts and injections depends on the spacecraft position relative to the cross-tail current sheet and boundaries of energetic particle trapped region. Near the inner edge of plasma sheet (PS) the pancake PAD was observed. Inside the PS and near the outer energetic particle trapped boundary the cigar and isotropic PADs were observed. The lack of dispersion in some observed injection events are evidence of local acceleration. The burst onsets correlate with the magnetic bursts of ∂Bz/∂t with the time duration ∼ 2–4 s. It is likely that particles are accelerated by induced E field arising from these observed magnetic bursts.

Multifractal and intermittent nature of substorm-associated magnetic turbulence in the magnetotail

Analysis is performed on the magnetic field fluctuations for three events in the near-Earth tail to examine the multifractal and intermittent nature of current disruption. The three events encompass observations being made from very near the cross-tail current sheet center (neutral sheet) to some distance away from the neutral sheet. It is found that near the neutral sheet, the scaling properties of the components related to the intensity of the cross-tail current ( B v) and the field-aligned current ( B d) are similar. However, at some distance from the neutral sheet, the scaling properties of the components related to the global magnetic field reconfiguration ( B h) and the field-aligned current are similar. The intermittency coefficient, which is different for components with different scaling properties, ranges from 0.18 to 0.42. This result implies that the intermittency correction needed to apply to the power spectral index for comparison with turbulence theory prediction is ∼0.06–0.14, which is significant in comparison with the difference required to distinguish between fluid and MHD turbulence.

Magnetospheric sash dependence on IMF direction

The magnetospheric sash is a ribbon of weak field shaped like a horseshoe with its open ends adjacent to the north and south dayside, magnetopause cusps and its closed end forming the cross‐tail current sheet. The clock angle of the sash in the dawn‐dusk meridian plane (as seen from the sun) rotates from 0° to 90° as the clock angle of the interplanetary magnetic field (IMF) rotates from 0° to 180°. We use a global MHD simulation to obtain the sash clock angles for IMF clock angles of 45°, 90°, and 135°. Remarkably, the results are very close to the clock angle of the magnetic null points obtained by superposing a uniform field representing the IMF on a dipole field representing the earth. Contours of magnetic field strength on cross sections perpendicular to the solar wind flow direction show how the sash evolves tailward from the dayside cusps.

On the use of photometer data to map dynamics of the magnetotail current sheet during substorm growth phase

The Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) is a large‐scale ground‐based instrument array of remote sensing equipment monitoring the high‐latitude ionosphere from the north central to the northwest portion of North America. It provides realtime coverage of auroral events such as magnetospheric substorms. In this paper the variation of magnetotail thickness is estimated during substorm growth phase by using a simple magnetic field model and data from the meridian scanning photometers in the CANOPUS array. An important parameter in the modeling process is the κ parameter, defined as the square root of the minimum curvature radius to maximum Larmor radius ratio. Through use of this parameter we are able to grossly determine the expected proton precipitation regions in the ionosphere. Two examples presented (February 9, 1995, and March 9, 1995) show that crosstail current sheet thinning from 2–0.1 RE can occur during the course of a growth phase. Within the framework of our model it is demonstrated that the magnetotail field line stretching, associated with thinning of the current sheet, is correlated with the equatorward motion of Hβ auroral emissions. Also demonstrated is the Earthward movement of the inner edge of the plasma sheet to within 6 RE.

On the possibility of identifying heavy ion acceleration processes in the magnetotail of Mars

The dependence of the energy of ions on their mass could be a useful tool for the identification of possible acceleration processes in the Martian plasma sheet. The study is based on a few cases when heavier ions and protons were registered simultaneously in the plasma sheet by the TAUS energy spectrometer, and on published data of the ASPERA instrument. The analysis of mass dependence of the ion energy in the plasma sheet suggests that frictional preacceleration of ions probably in the pole regions of the Martian magnetosphere can be responsible for not more than 40% of the energy gain of heavy ions in the plasma sheet, the cross-tail current sheet acceleration yields up to 70%, and from 30 to 70% of the heavy ion energy is provided by the acceleration due to magnetic field line stresses (and/or electric field aligned acceleration).