Strong Core Field Research Articles

Anatomy of a flux transfer event seen by Cluster

Data from Cluster are used to study the structure of a flux transfer event (FTE), seen near the northern cusp. We employ Grad‐Shafranov reconstruction, using measured fields from all four spacecraft to produce a map of the FTE cross section. The FTE consists of a flux rope of approximate size 1RE and irregular shape, embedded in the magnetopause. Its axis is tangential to the magnetopause. Since no reconnection signatures are seen, the map provides a fossil record of the prior reconnection process that created the flux rope: the strong core field indicates that it was generated by component merging. An average reconnection electric field ≥0.18 mV/m must have occurred in the burst of reconnection that created the FTE. The total axial () current and magnetic flux in the FTE were −0.66 MAmp and +2.07 MWeber, respectively.

Magnetic structure of the reconnection layer and core field generation in plasmoids

Plasmoids/flux ropes have been observed both at Earth's magnetopause as well as in the magnetotail. Magnetic field measurements of such structures often reveal that rather than a minimum in field strength at their centers as expected from a simple O‐type neutral line picture, they exhibit a strong core field. To address this issue, two‐dimensional (2‐D) and 3‐D hybrid simulations are used to investigate the magnetic structure of reconnection layer in general and the formation of the core field within plasmoids in particular. The reconnection layer in the magnetotail is found to be unstable to the fire hose instability. As a result, the region between the lobe and the central plasma sheet is nearly at the marginal fire hose condition. The magnetic signatures of single and multiple X line geometries are contrasted, and it is shown that the interaction of outflowing jets from neighboring X lines leads in general to a highly complex magnetic structure within a plasmoid. The large observed core fields are explained in terms of Hall‐generated currents which can naturally lead to core field strengths that even exceed the ambient lobe field in magnitude. Ion beta and the presence of a preexisting guide field are two important factors controlling the Hall‐generated fields. In particular, it is shown that the presence of the small ubiquitous cross‐tail field component in the magnetotail can under certain conditions lead to a strong unipolar plasmoid core field. There exist significant differences between core fields associated with plasmoids at the magnetopause and those in the tail. This is due to (1) high plasma beta in the magnetosheath and (2) the asymmetry in plasma density across the magnetopause. The former leads to smaller core fields at the magnetopause, whereas the latter leads to differences in the polarity and structure of core fields within magnetopause and magnetotail plasmoids. Such differences are illustrated through examples.

MHD simulations of magnetotail dynamics

Resistive MHD simulations of large‐scale magnetotail dynamics demonstrate that the same unstable mode causes plasmoid formation and ejection into the far tail and dipolarization and the formation of the substorm current wedge in the inner tail, consistent with the neutral line model of substorms. However, they have also modified some aspects of the model and added details that could not easily be inferred without the self‐consistent approach. We review recent results that include the externally driven formation of a thin current sheet in the near tail, which eases the onset of instability and leads to a faster dynamic evolution. In contrast to earlier expectations, the field‐aligned current generation and diversion takes place in the inner tail earthward of the reconnection site, resulting from shear and diversion of the earthward flow caused by reconnection farther out. Dipolarization starts most pronounced in the tail‐dipole transition region, propagating both tailward and flankward. Strong electric fields and plasma heating also are most prominent in the inner tail. Three‐dimensional simulations without mirror symmetries have generalized the picture of plasmoid formation and ejection, demonstrating a tangled geometry of helical flux ropes with different connections that change increasingly from the Earth to the magnetosheath. The interconnection with the magnetosheath may also play a role in generating plasmoid flux ropes with strong core fields. Mass, energy, and momentum gain of plasmoids results mainly from the accumulation of already accelerated plasma rather than from a sling shot effect acting on the entire plasmoid.

Observations of magnetic flux ropes and associated currents in Earth's magnetotail with the Galileo spacecraft

On December 8, 1990, during its first flyby of the Earth, Galileo recorded signatures of several flux rope type structures in the Earth's magnetotail. One of the prominent structures had a negative/positive polarity in the Bz component and was moving earthward, whereas another of the prominent structures had a positive/negative polarity in Bz and was observed to be traveling away from the Earth. Both of these flux ropes had strong core fields (≥10 nT) aligned in the positive YGSM direction. We have performed an analysis of combined field and particle data obtained within these flux ropes. We show that the length scale of a flux rope can be determined quite accurately from magnetic data if an independent measure of either the current density or the convection velocity is available. A surprising result that emerges from this analysis is that even though temporal durations (5–10 min) and magnetic field perturbations of the two flux ropes are similar, the scale sizes of the two structures are very different. The earthward moving flux rope with a radius ≤1000 km was traveling very slowly (υx < 10 km/s). The tailward moving flux rope with a radius of ∼8 RE had a speed of ∼150 km/s. Even though the earthward moving flux rope was small in size, the current density in its core was well above that of the tail current sheet and was detected directly by the plasma instrument. We believe that this is the first time that measurements of field aligned current in a flux rope core have been made directly by a plasma instrument. We discuss the details of the distribution functions of the current‐carrying particles. We also speculate on the conditions that led to the vast differences in the scale sizes of the two flux ropes.

CDAW 8 observations of plasmoid signatures in the geomagnetic tail: An assessment

A primary objective of the Coordinated Data Analysis Workshop 8 (CDAW 8) was the critical assessment of the plasmoid hypothesis. In this study, various types of magnetic phenomena, including closed loops and flux ropes, were considered as candidates for producing the north‐then‐south magnetic field perturbations characteristic of the “plasmoids” observed in the magnetotail by ISEE 3 during the CDAW 8 A and G events. For these two events the principal axis analyses of the magnetic field data and supporting energetic particle and plasma measurements are found to be consistent with the closed‐loop model of plasmoids for which the plane of rotation for B lies near the GSE X‐Z plane and only small field components lie in the minimum variance direction oriented largely along the Y direction. While small pitch angle spiraling of the field lines within these structures such as suggested by Hughes and Sibeck (1987) and Birn et al. (1989) cannot be definitively measured using single‐spacecraft magnetometer observations, the ISEE 3 measurements are not consistent with moderate strength or Venus‐type, strong core field flux ropes. The event A and G plasmoids were preceded by smoothly draped lobe field lines and followed by strong southward fields in the postplasmoid plasma sheet as predicted by the reconnection model of substorms. This latter feature of the observations is consistent with plasmoids moving down the tail, at least in part, due to the Maxwell stress exerted by lobe field lines which were disconnected at the near‐tail neutral line and have draped about the earthward side of the plasmoid. Calculations, based upon typical plasmoid and tail parameters, are presented which indicate that the J×B force associated with these disconnected lobe field lines may be sufficient to accelerate plasmoids up to the speeds observed by ISEE 3. A new characteristic of the plasmoid signature identified in this study is the presence of enhanced internal magnetic fields which can exceed the magnitude of the adjacent lobe fields by as much as 10–20%. While the internal structure of plasmoids remains poorly understood, an extension of the traveling compression region model (Slavin et al., 1984) is proposed to explain these high field regions. Finally, the relationship between the nature of individual substorms and the characteristics of the plasmoids observed in the tail by ISEE 3 is considered. The event A observations suggest that during isolated substorms the magnetotail may undergo an interval of intense reconnection which results in the creation of a single large plasmoid. Extended substorm intervals, exemplified by event G, may produce numerous smaller plasmoids as X lines form in the cislunar tail and retreat tailward while the magnetosphere continues to draw energy from the solar wind. Overall, the energy expended in accelerating the plasmoid down the tail appears comparable to that dissipated in the inner magnetosphere and ionosphere. Additional studies of plasmoid structure and dynamics as a function of substorm activity and distance down the tail should be conducted to determine the general validity of the conclusions reached on the basis of the CDAW 8 events.

Strong core magnetic fields in magnetopause flux transfer events

Magnetic field measurements in flux transfer events (FTEs) at the Earth's magnetopause often reveal the existence of a strong core magnetic field. We describe a possible mechanism for the origin of such strong core fields in the framework of a recently proposed model for FTEs. In this model bursty reconnection at a single X line, which extends over a large longitude segment, leads to loop like magnetic fields. As these loops move due to the Maxwell stress along the magnetopause, the magnetopause magnetic field directed perpendicular to the loops is swept up and correspondingly enhanced. When the downward pulling force due to the stress of the swept up magnetopause field equals the upward pulling force due to the loops an electromagnetic force equilibrium is reached. The core field strength determined by the force balance is proportional to the longitude extent of the reconnection line. Events with a large core field are therefore expected to have a higher detection probability than events with a small core field.