Strong Core Field Research Articles

Kinetic-scale Flux Ropes: Observations and Applications of Kinetic Equilibrium Models

Abstract Magnetic flux ropes with helical field lines and a strong core field are ubiquitous structures in space plasmas. Recently, kinetic-scale flux ropes have been identified by high-resolution observations from the Magnetospheric Multiscale (MMS) spacecraft in the magnetosheath, which have drawn a lot of attention because of their nonideal behavior and internal structures. Detailed investigation of flux rope structure and dynamics requires the development of realistic kinetic models. In this paper, we generalize an equilibrium model to reconstruct a kinetic-scale flux rope previously reported via MMS observations. The key features in the magnetic field and electron pitch-angle distribution measurements of all four satellites are simultaneously reproduced in this reconstruction. Besides validating the model, our results also indicate that the anisotropic features previously attributed to asymmetric magnetic topologies in the magnetosheath can be alternatively explained by the spacecraft motion in the flux rope rest frame.

Guide field effects on the distribution of plasmoids in multiple scale reconnection

The effects of a finite guide field on the distribution of plasmoids in high-Lundquist-number current sheets undergoing magnetic reconnection in large plasmas are investigated with statistical models. Merging of plasmoids is taken into account either assuming that guide field flux is conserved resulting in nonforce-free profiles in general, or that magnetic helicity is conserved and Taylor relaxation occurs to convert part of the summed guide field flux into reconnecting field flux toward minimum energy states resulting in force-free profiles. It is found that the plasmoid distribution in terms of reconnecting field flux follows a power law with index 7/4 or 1 depending on whether merger frequencies are independent of or dependent on their relative velocity to the outflow speed, respectively. This result is approximately the same for the force-free and nonforce-free models, with nonforce-free models exhibiting indices of 2 and 1 for the same velocity dependencies. Distributions in terms of guide field flux yield indices of 3/2 for the nonforce-free model regardless of velocity dependence. This is notably distinct from the indices of 11/8 and 1 for the force-free models independent of and dependent on velocity, respectively. At low guide field fluxes, the force-free models exhibit a second power law index of 1/2 due to nonconstant flux growth rates. The velocity-dependent force-free model predicts the production of slightly more rapidly moving large guide field flux plasmoids which are supported by observational evidence of flux ropes with strong core fields. Implications are discussed on particle acceleration via Fermi processes.

Particle-in-cell Simulations of Secondary Magnetic Islands: Ion-scale Flux Ropes and Plasmoids

Abstract Ion-scale flux ropes and plasmoids are secondary magnetic islands produced during magnetic reconnection in various heliospheric plasma environments. Here we study the structure of secondary islands and the particle dynamics within them using particle-in-cell simulations. Ion-scale flux ropes (secondary islands with a strong core field) are formed in a strong guide field regime, whereas ion-scale plasmoids (secondary islands with a weak core field) are formed in a weak guide field regime. Currents in both types of secondary islands are carried primarily by electrons. Both types of secondary islands have a magnetic tension force pointing radially inward toward their center. In the flux rope type, this inward tension force is balanced by an outward magnetic pressure gradient of the strong core field; in the plasmoid type, it is countered by an outward thermal pressure gradient caused by electron and ion energizations. The transition between these two types occurs when B g /B 0 = 0.1–0.2 (B g is the guide field, and B 0 is the asymptotic magnetic field).

A new method to identify flux ropes in space plasmas

Abstract. Flux ropes are frequently observed in the space plasmas, such as solar wind, planetary magnetosphere and magnetosheath etc., and play an important role in the reconnection process and mass and flux transportation. One usually uses bipolar signature and strong core field to identify the flux ropes. We propose here one new method to identify flux ropes based on the correlations between the variables of the data from in situ spacecraft observations and the “target function to be correlated” (TFC) from the ideal flux rope model. Through comparing the correlation coefficients of different variables at different times and scales, and performing weighted-average techniques, this method can derive the scales and locations of the flux ropes. We compare it with other methods and also discuss the limitation of our method.

Reconstructing the flux-rope topology using the FOTE method

With high-resolution data of the Magnetospheric Multiscale (MMS) mission, we observe a magnetic flux rope (MFR) in the Earth’s magnetosheath. This MFR, showing a clear bipolar variation of the magnetic field in the normal component to local current sheet, contains a strong core field. We use the FOTE method to reconstruct the topology of this MFR and find it is consistent with previous expectation. For the first time, the spiral field and core field of the MFR are both revealed from the FOTE method. Comparing topologies reconstructed at different times, we suggest that the axis of the MFR rotates about 88° at different spatial location. Shape and size of the normal projection to axis vary with the spatial location as well. Inside the MFR, a significant increase of plasma density from 40 to 80 cm−3, a sharp decrease of ion temperature from 200 to 50 eV, an enhancement of cold ions and a series of filamentary currents are found.

MMS observations of ion‐scale magnetic island in the magnetosheath turbulent plasma

AbstractIn this letter, first observations of ion‐scale magnetic island from the Magnetospheric Multiscale mission in the magnetosheath turbulent plasma are presented. The magnetic island is characterized by bipolar variation of magnetic fields with magnetic field compression, strong core field, density depletion, and strong currents dominated by the parallel component to the local magnetic field. The estimated size of magnetic island is about 8 di, where di is the ion inertial length. Distinct particle behaviors and wave activities inside and at the edges of the magnetic island are observed: parallel electron beam accompanied with electrostatic solitary waves and strong electromagnetic lower hybrid drift waves inside the magnetic island and bidirectional electron beams, whistler waves, weak electromagnetic lower hybrid drift waves, and strong broadband electrostatic noise at the edges of the magnetic island. Our observations demonstrate that highly dynamical, strong wave activities and electron‐scale physics occur within ion‐scale magnetic islands in the magnetosheath turbulent plasma.

ASTEROSEISMIC SIGNATURES OF EVOLVING INTERNAL STELLAR MAGNETIC FIELDS

ABSTRACT Recent asteroseismic analyses indicate the presence of strong (B ≳ 105 G) magnetic fields in the cores of many red giant stars. Here, we examine the implications of these results for the evolution of stellar magnetic fields, and we make predictions for future observations. Those stars with suppressed dipole modes indicative of strong core fields should exhibit moderate but detectable quadrupole mode suppression. The long magnetic diffusion times within stellar cores ensure that dynamo-generated fields are confined to mass coordinates within the main-sequence (MS) convective core, and the observed sharp increase in dipole mode suppression rates above 1.5 M ⊙ is likely explained by the larger convective core masses and faster rotation of these more massive stars. In clump stars, core fields of ∼105 G can suppress dipole modes, whose visibility should be equal to or less than the visibility of suppressed modes in ascending red giants. High dipole mode suppression rates in low-mass (M ≲ 2 M ⊙) clump stars would indicate that magnetic fields generated during the MS can withstand subsequent convective phases and survive into the compact remnant phase. Finally, we discuss implications for observed magnetic fields in white dwarfs and neutron stars, as well as the effects of magnetic fields in various types of pulsating stars.

A prevalence of dynamo-generated magnetic fields in the cores of intermediate-mass stars.

Magnetic fields play a part in almost all stages of stellar evolution. Most low-mass stars, including the Sun, show surface fields that are generated by dynamo processes in their convective envelopes. Intermediate-mass stars do not have deep convective envelopes, although 10 per cent exhibit strong surface fields that are presumed to be residuals from the star formation process. These stars do have convective cores that might produce internal magnetic fields, and these fields might survive into later stages of stellar evolution, but information has been limited by our inability to measure the fields below the stellar surface. Here we report the strength of dipolar oscillation modes for a sample of 3,600 red giant stars. About 20 per cent of our sample show mode suppression, by strong magnetic fields in the cores, but this fraction is a strong function of mass. Strong core fields occur only in red giants heavier than 1.1 solar masses, and the occurrence rate is at least 50 per cent for intermediate-mass stars (1.6-2.0 solar masses), indicating that powerful dynamos were very common in the previously convective cores of these stars.

Evidence for the core field polarity of magnetic flux ropes against the reconnection guide field

AbstractWe present the observational evidence by Time History of Events and Macroscale Interactions during Substorms (THEMIS)‐D spacecraft to demonstrate that magnetic flux rope can have a core field polarity opposite to the guide field during asymmetric reconnection with a high magnetic shear across the magnetopause. In the presented event, a bipolar reconnection outflow was observed at the magnetopause, across which the magnetic shear was ~149° corresponding to a predicted guide field BG = −15.6 nT. The ratios for magnetic field strength and ion density between the magnetospheric and magnetosheath sides of the magnetopause were ~1.6 and ~0.15, respectively. During the event, the component of magnetic field along the M axis (the intermediate variance direction) remained negative and stable for most of the time, and its mean value agreed with the BG. Two magnetic flux ropes with a strong core field were identified, but one flux rope had a core field polarity against the guide field BG. Our finding differs from current theoretical simulations that the core field polarity of the flux ropes inherits from the guide field.

Plasma physics of magnetic island coalescence during magnetic reconnection

AbstractA large‐scale two‐dimensional electromagnetic particle‐in‐cell simulation was employed to study the magnetic island coalescence/merging process during magnetic reconnection with guide field. The merged island after coalescence is characterized by strong core field and plasma density dip at the island center. We found that the core field enhancement is caused by the out‐of‐plane magnetic field pileup, as well as the field line twisting due to Hall effect. There is an in‐plane electric current loop, which is mainly carried by electrons, circumventing the enhanced core field region. Total force points away or tangentially to the surface of density dip within the merged island, which prevents electrons from higher‐density region entering the lower density region between two merging islands. This is contrary to the force in the secondary island, inside which the total force points toward the island center and constrains plasma there. The coalescence process involves a reconnection at the merging sheet between two islands. Electron frozen‐in condition is violated locally along the merging sheet. It is contributed by both the divergence of electron pressure tensor and electron inertial. Energy dissipation is concentrated on the merging line during coalescence, while a train of dynamo (j′ · E′ < 0) and dissipation (j′ · E′ > 0) regions are distributed along the merging sheet after coalescence. Electrons and ions within the density dip at the island center are accelerated antiparallel to the ambient magnetic field, i.e., along the out‐of‐plane direction. The resultant distribution can excite the Buneman instability and two‐streaming instability, which probably account for the electrostatic solitary waves observed by satellite.

Giant flux ropes observed in the magnetized ionosphere at Venus

The Venus ionospheric response to solar and solar wind variations is most evident in its magnetic field properties. Early Pioneer Venus observations during the solar maximum revealed that the Venus ionosphere exhibits two magnetic states depending on the solar wind dynamic pressure conditions: magnetized ionosphere with large‐scale horizontal magnetic field; or unmagnetized ionosphere with numerous small‐scale thin structures, so‐called flux ropes. Here we report yet another magnetic state of Venus' ionosphere, giant flux ropes in the magnetized ionosphere, using Venus Express magnetic field measurements during solar minimum. These giant flux ropes all have strong core fields and diameters of hundreds of kilometers, which is about the vertical dimension of the ionosphere. This finding represents the first observation of these giant flux ropes at Venus. The cause of these giant flux ropes remains unknown and speculative.

FAST MAGNETIC RECONNECTION AND PARTICLE ACCELERATION IN RELATIVISTIC LOW-DENSITY ELECTRON-POSITRON PLASMAS WITHOUT GUIDE FIELD

Magnetic reconnection and particle acceleration in relativistic Harris sheets in low-density electron-positron plasmas with no guide field have been studied by means of two-dimensional particle-in-cell simulations. Reconnection rates are of the order of one when the background density in a Harris sheet is of the order of 1% of the density in the current sheet, which is consistent with previous results in the non-relativistic regime. It has been demonstrated that the increase of the Lorentz factors of accelerated particles significantly enhances the collisionless resistivity needed to sustain a large reconnection electric field. It is shown analytically and numerically that the energy spectrum of accelerated particles near the X-line is the product of a power law and an exponential function of energy, γ–1/4exp (– aγ1/2), where γ is the Lorentz factor and a is a constant. However, in the low-density regime, while the most energetic particles are produced near X-lines, many more particles are energized within magnetic islands. Particles are energized in contracting islands by multiple reflection, but the mechanism is different from Fermi acceleration in magnetic islands for magnetized particles in the presence of a guide field. In magnetic islands, strong core fields are generated and plasma beta values are reduced. As a consequence, the fire-hose instability condition is not satisfied in most of the island region, and island contraction and particle acceleration can continue. In island coalescence, reconnection between two islands can accelerate some particles, however, many particles are decelerated and cooled, which is contrary to what has been discussed in the literature on particle acceleration due to reconnection in non-relativistic hydrogen plasmas.

Observations of energetic electrons up to 200 keV associated with a secondary island near the center of an ion diffusion region: A Cluster case study

An antiparallel reconnection event is recognized in the magnetotail, and a secondary island with a strong core magnetic field is identified near the center of the ion diffusion region. In the island, the electron density peaks in the outer region while the dip is in the core region with a strong core magnetic field. A strong electron beam parallel to the magnetic field, as well as an obvious current antiparallel to the magnetic field with density up to ∼40 nA/m2, is observed in the outer region of the island. This suggests that the strong core magnetic field inside the island is generated by the electron beam and then the antiparallel current in the outer region of the island. The electron density dip in the core region is formed due to the existing strong core field, which expels electrons out of the core region. The electron flat‐top distributions are detected in the ion diffusion region except the core region of the island, and the shoulder energy range of the flat‐top distributions is from 100 eV to 4 keV. An enhancement of the energetic electron flux up to 200 keV is found in the ion diffusion region, and a further increase of energetic electron fluxes is observed inside the island. Waves at the lower hybrid frequency are intensified in the ion diffusion region, while the intensification is strong in the outer region of the island and becomes very weak in the core region.

Evidence that crater flux transfer events are initial stages of typical flux transfer events

Bipolar magnetic perturbations along the normal to the local magnetopause associated with field magnitude enhancements are signatures of typical flux transfer events (T‐FTEs) and are interpreted as evidence of encounters with magnetic flux ropes with strong core fields. If the field magnitude dips at the center of the signature, we identify the event as a crater FTE (C‐FTE). In the multiple‐spacecraft data of the Time History of Events and Macroscale Interactions During Substorms (THEMIS) between 1 May and 31 October 2007, we have identified 622 FTEs of which only 23 manifested C‐FTE signatures. We analyze a C‐FTE (30 July 2007) that evolved into a T‐FTE and compare its properties with those of a T‐FTE (May 20, 2007). For all 23 C‐FTEs and 35 confirmed T‐FTEs, we compare solar wind conditions and internal plasma and field properties. The similarity of solar wind properties for events in the two classes suggests that differences in their structures are not related to the solar wind conditions. Systematic differences in internal peak fields (BC‐FTE < BMagnetosphere < BT‐FTE) and averaged number densities (NT‐FTE < 0.5 × NMagnetosheath < NC‐FTE) between the two groups are consistent with the evolution of C‐FTEs into T‐FTEs. We propose that parallel flows inside C‐FTEs deplete the internal ion densities and reduce the thermal pressures as the central field magnitude increases to maintain pressure balance.

Plasmoid‐like structures in multiple X line Hall MHD reconnection

Driven by a waveform inflow, multiple X line reconnection is initiated in a long current layer, and the plasmoids are bound between two neighboring reconnection sites. We investigate the behaviors of the plasmoid‐like structures in the absence and presence of an initial guide field By0 normalized by B0 (B0 is the initial intensity of Bx field at the top and bottom boundaries of the simulation domain) using a Hall magnetohydrodynamic (MHD) code. For the case with By0 = 0 the profiles of the out‐of‐plane By component are the bipolar signature or the bipolar wavelike signature which is caused by Hall effect and independent of the external mechanism. Such By features are in line with the observed signature of a closed‐loop‐like plasmoid in the magnetotail. The bipolar and fluctuation signatures of By have an asymmetric feature in the presence of a small By0(= 0.1), and the By profile becomes a positive signature as By0 reaches or exceeds 0.3. In the case of By0 = 0.5, a By bulge appears in the By signature when the enhanced By regions caused by Hall effect take place in the plasmoid. The By bulge evolves into a peaking signature, whose maximum (By∣max) is quickly raised and approaches the lobe magnetic field strength. Such a significant enhancement of the By component in the central region of the plasmoid might be representative of the observed strong core field in the magnetic flux rope. The present results indicate the following implications: (1) Hall effect and a preexisting cross‐tail component By are two important factors controlling the occurrence of various plasmoid‐like structures in the magnetotail. (2) In the later phase the nonlinear interaction between Hall effect and the By flux added by the plasma inflow makes a most important contribution to the growth of the core By field.

Comparison of small‐scale flux rope magnetic properties to large‐scale magnetic clouds: Evidence for reconnection across the HCS?

A newly developed semi‐automated routine to identify small‐scale symmetric force‐free flux ropes in the solar wind was used to search 11 years (1995–2005) of WIND magnetic field data. We identified 68 flux ropes whose durations range from 39 minutes up to 12.2 hours, with the distribution strongly peaked at an hour or less. These small‐scale flux ropes have strong core field signatures in total magnetic field coincident with bipolar field inflection consistent with a cylindrically symmetric flux rope model. They are predominately observed in the slow and moderate solar wind (<500 km s−1). They are observed to have a solar cycle occurrence variation with most small‐scale flux rope events found during solar minimum. This database of small‐scale flux ropes is compared to previously published small‐scale flux rope surveys and interplanetary coronal mass ejection (ICME)/magnetic cloud databases. The size distribution of flux ropes in the solar wind appears to be bimodal, with the majority of events having durations 4 hours or less where the magnetic cloud distribution peaks between 12 and 16 hours. The results suggest different source mechanisms for small and large‐scale flux ropes.

Crater FTEs: Simulation results and THEMIS observations

The BATS‐R‐US magnetohydrodynamic (MHD) model predicts flux transfer events (FTEs) with strong core magnetic fields embedded within a broadened current layer of weak magnetic field strengths on the equatorial dayside magnetopause during intervals of southward and duskward interplanetary magnetic field (IMF) orientation. Multipoint THEMIS observations at 2202 UT on June 20, 2007 of a southward‐moving FTE on the post‐noon magnetopause confirm the predictions of the model. THEMIS‐E and ‐A with large impact parameters simply observe enhanced magnetic field strengths in draped magnetosheath magnetic field lines, THEMIS‐B and ‐C with moderate impact parameters pass through the broadened current layer to observe crater FTEs with deep troughs bounding a strong core field, while THEMIS‐D with a very low impact parameter observes the core magnetic field strength enhancement embedded in the current layer.

Flux transfer events simultaneously observed by Polar and Cluster: Flux rope in the subsolar region and flux tube addition to the polar cusp

In this paper we present observational evidence of a flux transfer event observed simultaneously at low‐latitude by Polar and at high‐latitude by Cluster. This event occurs on 21 March 2002, when both Cluster and Polar are located near local noon but with a large latitudinal separation. During the event, Cluster is moving outbound from the polar cusp to the magnetosheath and Polar is in the magnetosheath near the equatorial magnetopause. The observations show that a flux transfer event occurs between the equator and the northern cusp. Polar and Cluster observe the FTE's two open flux tubes: Polar encounters the southward moving flux tube near the equator and Cluster encounters the northward moving flux tube at high latitude. The low‐latitude FTE appears to be a flux rope with helical magnetic field lines as it has a strong core field and the magnetic field component in the boundary normal direction exhibits a strong bipolar variation. Unlike the low‐latitude FTE, the high‐latitude FTE observed by Cluster does not exhibit the characteristic bipolar perturbation in the magnetic field. However, the plasma data clearly reveal its open flux tube configuration. It shows that the magnetic field lines have straightened inside the FTE and become more aligned to the neighboring flux tubes as it moves to the cusp. Enhanced electrostatic fluctuations have been observed within the FTE core, both at low and high latitudes. This event provides a unique opportunity to understand high‐latitude FTE signatures and the nature of time‐varying reconnection. It shows that existing FTE models cannot accommodate all the features in global observations, and coordinated measurements from largely spaced multiple spacecraft place important constraints which are crucial to the development and refinement of FTE models.

The structure of flux transfer events recovered from Cluster data

Abstract. The structure and formation mechanism of a total of five Flux Transfer Events (FTEs), encountered on the equatorward side of the northern cusp by the Cluster spacecraft, with separation of ~5000 km, are studied by applying the Grad-Shafranov (GS) reconstruction technique to the events. The technique generates a magnetic field/plasma map of the FTE cross section, using combined magnetic field and plasma data from all four spacecraft, under the assumption that the structure is two-dimensional (2-D) and time-independent. The reconstructed FTEs consist of one or more magnetic flux ropes embedded in the magnetopause, suggesting that multiple X-line reconnection was involved in generating the observed FTEs. The dimension of the flux ropes in the direction normal to the magnetopause ranges from about 2000 km to more than 1 RE. The orientation of the flux rope axis can be determined through optimization of the GS map, the result being consistent with those from various single-spacecraft methods. Thanks to this, the unambiguous presence of a strong core field is confirmed, providing evidence for component merging. The amount of magnetic flux contained within each flux rope is calculated from the map and, by dividing it by the time interval between the preceding FTE and the one reconstructed, a lower limit of the reconnection electric field during the creation of the flux rope can be estimated; the estimated value ranges from ~0.11 to ~0.26 mV m-1, with an average of 0.19 mV m-1. This can be translated to the reconnection rate of 0.038 to 0.074, with an average of 0.056. Based on the success of the 2-D model in recovering the observed FTEs, the length of the X-lines is estimated to be at least a few RE.

Cluster observations of flux rope structures in the near-tail

Abstract. An investigation of the 2003 Cluster tail season has revealed small flux ropes in the near-tail plasma sheet of Earth. These flux ropes manifest themselves as a bipolar magnetic field signature (usually predominantly in the Z-component) associated with a strong transient peak in one or more of the other components (usually the Y-component). These signatures are interpreted as the passage of a cylindrical magnetic structure with a strong axial magnetic field over the spacecraft position. On the 2 October 2003 all four Cluster spacecraft observed a flux rope in the plasma sheet at X (GSM) ~-17 RE. The flux rope was travelling Earthward and duskward at ~160 kms-1, as determined from multi-spacecraft timing. This is consistent with the observed south-then-north bipolar BZ signature and corresponds to a size of ~0.3 RE (a lower estimate, measuring between the inflection points of the bipolar signature). The axis direction, determined from multi-spacecraft timing and the direction of the strong core field, was close to the intermediate variance direction of the magnetic field. The current inside the flux rope, determined from the curlometer technique, was predominantly parallel to the magnetic field. However, throughout the flux rope, but more significant in the outer sections, a non-zero component of current perpendicular to the magnetic field existed. This shows that the flux rope was not in a "constant α" force-free configuration, i.e. the magnetic force, J×B was also non-zero. In the variance frame of the magnetic field, the components of J×B suggest that the magnetic pressure force was acting to expand the flux rope, i.e. directed away from the centre of the flux rope, whereas the smaller magnetic tension force was acting to compress the flux rope. The plasma pressure is reduced inside the flux rope. A simple estimate of the total force acting on the flux rope from the magnetic forces and surrounding plasma suggests that the flux rope was experiencing an expansive total force. On 13 August 2003 all four Cluster spacecraft observed a flux rope at X (GSM) ~-18 RE. This flux rope was travelling tailward at 200 kms-1, consistent with the observed north-then-south bipolar BZ signature. The bipolar signature corresponds to a size of ~0.3 RE (lower estimate). In this case, the axis, determined from multi-spacecraft timing and the direction of the strong core field, was directed close to the maximum variance direction of the magnetic field. The current had components both parallel and perpendicular to the magnetic field, and J×B was again larger in the outer sections of the flux rope than in the centre. This flux rope was also under expansive magnetic pressure forces from J×B, i.e. directed away from the centre of the flux rope, and had a reduced plasma pressure inside the flux rope. A simple total force calculation suggests that this flux rope was experiencing a large expansive total force. The observations of a larger J×B signature in the outer sections of the flux ropes when compared to the centre may be explained if the flux ropes are observed at an intermediate stage of their evolution after creation by reconnection at multiple X lines near the Cluster apogee. It is suggested that these flux ropes are in the process of relaxing towards the force-free like configuration often observed further down the tail. The centre of the flux ropes may contain older reconnected flux at a later evolutionary stage and may therefore be more force-free.