Malcolm W. Dunlop

Identifying magnetic reconnection events using the FOTE method

A magnetic reconnection event detected by Cluster is analyzed using three methods: Single-spacecraft Inference based on Flow-reversal Sequence (SIFS), Multispacecraft Inference based on Timing a Structure (MITS), and the First-Order Taylor Expansion (FOTE). Using the SIFS method, we find that the reconnection structure is an X line; while using the MITS and FOTE methods, we find it is a magnetic island (O line). We compare the efficiency and accuracy of these three methods and find that the most efficient and accurate approach to identify a reconnection event is FOTE. In both the guide and nonguide field reconnection regimes, the FOTE method is equally applicable. This study for the first time demonstrates the capability of FOTE in identifying magnetic reconnection events; it would be useful to the forthcoming Magnetospheric Multiscale (MMS) mission.

Statistical characteristics of slow earthward and tailward flows in the plasma sheet

In this paper, we statistically analyzed and compared the earthward flow (EF) and the tailward flow (TF) in the plasma sheet. It is found that the properties of the EF/TF in the central plasma sheet (CPS) of >1 and the outer plasma sheet (OPS) of 0.1<<1 are distinctly different. The main conclusions include that (1) the EFs occur in both the CPS and the OPS while the TFs mainly occur in the OPS, (2) both flows are dominantly convective in the CPS and parallel in the OPS, (3) in the CPS, the EF and the TF have similar characteristics, including their bulk velocities and ion densities and E-y components. Both flows tend to have isotropic temperatures; (4) in the OPS, the EFs tend to have higher ion velocity, density, and E-y than the TF. The EFs tend to have anisotropic temperatures, while the TFs tend to have more isotropic temperatures. As a whole, combined characteristics of the EF and the TF are consistent with (1) reflection at the magnetic mirror point near the Earth for parallel flows in the OPS and (2) bouncing off/back from the dipolar field closer to the Earth for convective flows in the CPS.

X lines in the magnetotail for southward and northward IMF conditions

Utilizing associated observations of Geotail and ACE satellites from the year of 1998 to 2005, we investigated the X lines in the near-Earth tail under different interplanetary magnetic field (IMF) conditions. The X lines are recognized by the tailward fast flows with negative B-z. Statistically, the X lines in the tail can be observed for southward as well as northward IMF, but more frequently observed for southward IMF. A typical case on 26 April 2005 showed clear evidence that the X line can occur for northward IMF while the geomagnetic activity is particularly quiet. Further analysis showed that the X line-related solar wind has stronger E-y and B-z components for southward than northward IMF. In addition, the X line-related geomagnetic activities are stronger for southward than northward IMF.

In-situ observations of flux ropes formed in association with a pair of spiral nulls in magnetotail plasmas

Signatures of secondary islands are frequently observed in the magnetic reconnection regions of magnetotail plasmas. In this paper, magnetic structures with the secondary-island signatures observed by Cluster are reassembled by a fitting-reconstruction method. The results show three-dimensionally that a secondary island event can manifest the flux rope formed with an As-type null and a Bs-type null paired via their spines. We call this As-spine-Bs-like configuration the helically wrapped spine model. The reconstructed field lines wrap around the spine to form the flux rope, and an O-type topology is therefore seen on the plane perpendicular to the spine. Magnetized electrons are found to rotate on and cross the fan surface, suggesting that both the torsional-spine and the spine-fan reconnection take place in the configuration. Furthermore, detailed analysis implies that the spiral nulls and flux ropes were locally generated nearby the spacecraft in the reconnection outflow region, indicating that secondary reconnection may occur in the exhaust away from the primary reconnection site.

Parallel-dominant and perpendicular-dominant components of the fast bulk flow: Comparing with the PSBL beams†

Utilizing multipoint observations by the Cluster satellites, we investigated the ion distributions of the fast bulk flows (FBFs) in the plasma sheet. Simultaneous observation by C1 and C3 revealed that parallel-dominant and perpendicular-dominant components of the flows coexist and correspond to B-x-dominant and B-z-dominant magnetic field regions within the FBFs, respectively. In both cases, the ions distributions are characterized by a single-beam/crescent shape. In particular, no reflected ions are found within the FBFs. Statistical analysis showed that within the FBFs, the strength of the B-x component is typically less than 5 nT for B-z-dominant regions and above 10 nT for B-x-dominant regions. To distinguish between the parallel-dominant component of the FBFs and the field-aligned beams in the plasma sheet boundary layer (PSBL), we further statistically analyzed the tailward parallel flows (TPF) with positive B-z in the plasma sheet. The results indicated that the FBFs tend to have higher velocity, weaker B, and higher magnetic tilt angle (theta(MTA)) than the TPFs/PSBL beams. Statistically, in the region of B > 30 nT (theta(MTA) > 10 degrees), only PSBL beams can be observed, while in the region of B 30 degrees), the FBFs are dominant. In the intermediate region (10 degrees < theta(MTA) < 30 degrees) of the plasma sheet, the FBFs and the PSBL beams cooccur. These Cluster observations suggest that the X line can produce both perpendicular flow in central plasma sheet and parallel flow in the PSBL. In addition, the parallel-dominant component of the FBFs could be an important origin for the PSBL beams.

Earthward and Tailward Flows in the Plasma Sheet

Utilizing C3/Cluster satellite observations from the year of 2001 to 2006, we investigated the earthward flow (EF) and tailward flow (TF) at B-z> 0 in the plasma sheet. We found that the EF and the TF have similar spatial distributions. Both characteristics are independent of the distance beyond 14 RE. Both flows are deflected while closer to the Earth. Statistical results further showed that the EF/TF occur in the central plasma sheet as well as the plasma sheet boundary layer and can be observed during quiet times and periods of geomagnetic activity. A typical event reveals that the EF and the TF have different plasma population. A transition region (TR) can be formed at the interface between the EF and TF. Very significant duskward components appeared in bulk velocities for both populations. It appears that the vortical-like structure can be formed near the TR. The magnetic field within the TR is twisted and strongly fluctuates. No clear magnetic flux pileups are observed inside the TR.

Carriers and Sources of Magnetopause Current: MMS Case Study

We investigate the current carriers and current sources of an ion scale tangential magnetopause current layer using the Magnetospheric Multiscale four spacecraft data. Within this magnetopause current layer, ions and electrons equally contribute to the perpendicular current, while electrons carry nearly all the parallel current. The energy range of all these current carriers is predominantly from middle to high (>100 eV), where particles with higher energies are more efficient in producing the current. By comparing each term, two-fluid magnetohydrodynamic (MHD) theory is able to describe the current sources to a large degree because the sum of all the perpendicular currents from MHD theory could account for the currents observed. In addition, we find that the ion diamagnetic current is the main source of the total perpendicular current, while the curvature current can be neglected. Nevertheless, ions and electrons both carry comparable current due to the redistribution of the electric field and show features beyond the classic Chapman-Ferraro model, particularly on the front side of the boundary layer where the electric field reversal is most intense. We also show a second, comparative event in which ions do not satisfy MHD theory, while the electrons do. The small-scale, adiabatic parameter (square of curvature radius/gyroradius) supports our interpretation that this second event contains ion scale substructure. We suggest that comparing the predicted MHD current with plasma current can be a good method to judge whether the MHD theory is satisfied in each specific circumstance, especially for high-precision Magnetospheric Multiscale data.