Multiple Flux Ropes Research Articles

Asymmetry in the current sheet and secondary magnetic flux ropes during guide field magnetic reconnection

A magnetic reconnection event with a moderate guide field encountered by Cluster in the near‐Earth tail on 28 August 2002 is reported. The guide field points dawnward during this event. The quadrupolar structure of the Hall magnetic field within the ion diffusion region is distorted toward the northern hemisphere in the earthward part while toward the southern hemisphere tailward part of X‐line. Observations of current density and electron pitch angle distribution indicate that the distorted quadrupolar structure is formed due to a deformed Hall electron current system. Cluster crossed the ion diffusion region from south to north earthward of the X‐line. An electron density cavity is confirmed in the northern separatrix layer while a thin current layer (TCL) is measured in the southern separatrix layer. The TCL is formed due to electrons injected into the X‐line along the magnetic field. These observations are different from simulation results where the cavity is produced associated with inflow electrons along the southern separatrix while the strong current sheet appears with the outflow electron beam along the northern separatrix. The energy of the inflowing electron in the separatrix layer could extend up to 10 keV. Energetic electron fluxes up to 50 keV have a clear peak in the TCL. The length of the separatrix layer is estimated to be at least 65 c/ωpi. These observations suggest that electrons could be pre‐accelerated before they are ejected into the X‐line region along the separatrix. Multiple secondary flux ropes moving earthward are observed within the diffusion region. These secondary flux ropes are all identified earthward of the observed TCL. These observations further suggest there are numerous small scale structures within the ion diffusion region.

A chain of magnetic flux ropes in the magnetotail of Mars

The interaction of Mars with the solar wind leads to the formation of a magnetotail through which significant quantities of planetary plasma are transported. Of particular interest is the extent to which this plasma transport could be affected by magnetotail dynamics, for example by magnetic reconnection and flux rope formation. Here we show observations from Mars Global Surveyor of multiple flux ropes observed in Mars' magnetotail current sheet. A chain of at least three flux ropes is observed; based on the geometry of the encounter, the flux ropes are all being ejected in the same direction from a single dominant site and modeling shows that at least two of the flux ropes are close to being in a force free condition. Given geometrical considerations, it is likely that the flux ropes are generated sequentially rather than simultaneously, suggesting periodic generation via secondary instabilities at the reconnection site.

Investigating the observational signatures of magnetic cloud substructure

[1] Magnetic clouds (MCs) represent a subset of interplanetary coronal mass ejections (ICMEs) that exhibit a magnetic flux rope structure. They are primarily identified by smooth, large-scale rotations of the magnetic field. However, both small- and large-scale fluctuations of the magnetic field are observed within some magnetic clouds. We analyzed the magnetic field in the frames of the flux ropes, approximated using a minimum variance analysis (MVA), and have identified a small number of MCs within which multiple reversals of the gradient of the azimuthal magnetic field are observed. We herein use the term “substructure” to refer to regions that exhibit this signature. We examine, in detail, one such MC observed on 13 April 2006 by the ACE and WIND spacecraft and show that substructure has distinct signatures in both the magnetic field and plasma observations. We identify two thin current sheets within the substructure and find that they bound the region in which the observations deviate most significantly from those typically expected in MCs. The majority of these clouds are followed by fast solar wind streams, and a comparison of the properties of this magnetic cloud with five similar events reveals that they have lower nondimensional expansion rates than nonovertaken magnetic clouds. We discuss and evaluate several possible explanations for this type of substructure, including the presence of multiple flux ropes and warping of the MC structure, but we conclude that none of these scenarios is able to fully explain all of the aspects of the substructure observations.

TESTS OF DYNAMICAL FLUX EMERGENCE AS A MECHANISM FOR CORONAL MASS EJECTION INITIATION

Current coronal mass ejection (CME) models set their lower boundary to be in the lower corona. They do not calculate accurately the transfer of free magnetic energy from the convection zone to the magnetically dominated corona because they model the effects of flux emergence using kinematic boundary conditions or simply assume the appearance of flux at these heights. We test the importance of including dynamical flux emergence in CME modeling by simulating, in 2.5D, the emergence of sub-surface flux tubes into different coronal magnetic field configurations. We investigate how much free magnetic energy, in the form of shear magnetic field, is transported from the convection zone to the corona, and whether dynamical flux emergence can drive CMEs. We find that multiple coronal flux ropes can be formed during flux emergence, and although they carry some shear field into the corona, the majority of shear field is confined to the lower atmosphere. Less than 10% of the magnetic energy in the corona is in the shear field, and this, combined with the fact that the coronal flux ropes bring up significant dense material, means that they do not erupt. Our results have significant implications for all CME models which rely on the transfer of free magnetic energy from the lower atmosphere into the corona but which do not explicitly model this transfer. Such studies of flux emergence and CMEs are timely, as we have new capabilities to observe this with Hinode and the Solar Dynamics Observatory, and therefore to test the models against observations.

The Formation of Large-Scale Current Sheets within Magnetic Clouds

Magnetic clouds are a class of interplanetary coronal mass ejections (CME) predominantly characterised by a smooth rotation in the magnetic field direction, indicative of a magnetic flux rope structure. Many magnetic clouds, however, also contain sharp discontinuities within the smoothly varying magnetic field, suggestive of narrow current sheets. In this study we present observations and modelling of magnetic clouds with strong current sheet signatures close to the centre of the apparent flux rope structure. Using an analytical magnetic flux rope model, we demonstrate how such current sheets can form as a result of a cloud’s kinematic propagation from the Sun to the Earth, without any external forces or influences. This model is shown to match observations of four particular magnetic clouds remarkably well. The model predicts that current sheet intensity increases for increasing CME angular extent and decreasing CME radial expansion speed. Assuming such current sheets facilitate magnetic reconnection, the process of current sheet formation could ultimately lead a single flux rope becoming fragmented into multiple flux ropes. This change in topology has consequences for magnetic clouds as barriers to energetic particle propagation.

Cluster observations of earthward propagating plasmoid and flux ropes in the near-tail during the course of a substorm event

We analyze the magnetic structures in the near-tail at X gsm = −17.5 R e on September 19, 2003 by Cluster. During the course of a substorm event, the earthward propagating plasmoid and flux ropes in the near-tail are observed. The earthward propagating plasmoid is associated with the bipolar Bz and By signatures. The two flux ropes are embedded within the earthward plasma flows, which might be referred to the population as ‘‘BBF-type’’ flux ropes. The first flux rope diameter is about 0.7 R e and duration based upon the Bz signature is ∼20 s, while the second one diameter is about 1.4 R e and duration is ∼30 s. The earthward propagating plasmoid and flux ropes could have influence upon the dipolarization and injection in inner magnetosphere. The Cluster observations of earthward propagating plasmoid and flux ropes can be interpreted as strong evidence for multiple X-lines. Our observations are consistent with that multiple plasmoids or flux ropes are formed repeatedly and ejected tailward in the course of geomagnetically active time.

Stream Interactions and Interplanetary Coronal Mass Ejections at 5.3 AU near the Solar Ecliptic Plane

We have performed a survey of the characteristics of two types of large spatial-scale solar-wind structures, stream interaction regions (SIRs), and interplanetary coronal mass ejections (ICMEs), near 5.3 AU, using solar-wind observations from Ulysses. Our study is confined to the three aphelion passes of Ulysses, and also within ± 10° of the solar ecliptic plane, covering a part of 1992, 1997 – 1998, and 2003 – 2005, representing three slices of different phases of the solar activity cycle. Overall, there are 54 SIRs and 60 ICMEs in the survey. Many are merged in hybrid events, suggesting that they have undergone multiple interactions prior to reaching Jovian orbit. About 91% of SIRs occur with shocks, with 47% of such shocks being forward – reverse shock pairs. The solar-wind velocity sometimes stays constant or even decreases within the interaction region near 5.3 AU, in contrast with the gradual velocity increase during SIRs at 1 AU. Shocks are driven by 58% of ICMEs, with 94% of them being forward shocks. Some ICMEs seem to have multiple small flux ropes with different scales and properties. We quantitatively compare various properties of SIRs and ICMEs at 5.3 AU, and study their statistical distributions and variations with solar activity. The width, maximum dynamic pressure, and peak perpendicular pressure of SIRs all become larger than ICMEs. Dynamic pressure (Pdyn) is expected to be important for Jovian magnetospheric activity. We have examined the distributions of Pdyn of SIRs, ICMEs, and general solar wind, but these cannot explain the observed bimodal distribution of the location of the Jovian magnetopause. By comparing the properties of SIRs and ICMEs at 0.72, 1, and 5.3 AU, we find that the ICME expansion slows down significantly between 1 and 5.3 AU. Some transient and small streams in the inner heliosphere have merged into a single interaction region.

Dynamics of thin current sheets associated with magnetotail reconnection

We have examined the detailed structure of thin current sheets and their evolution during a substorm interval on 24 August 2003, when Cluster experienced several rapid current sheet crossings within a couple of ion gyrotimes. These crossings took place during an interval of high‐speed ion flows with BZ reversals and signatures of accelerated electrons, suggesting crossing of the reconnection region. On the basis of four‐point observations with a tetrahedron scale of ∼200 km, we could quantify for the first time the thickness of the current sheet, which was comparable to or less than one ion inertia length, and resolve some internal structures such as multiple peaks within these thin current sheets. Different patterns in jX and in electron anisotropy were identified during the current sheet crossings: two crossings during tailward flow interval exhibited a quadrupole‐type Hall current in the ion diffusion region without a guide field, while one crossing during earthward flow showed a current system as predicted in the ion diffusion region under the presence of a guide field. Multiple flux rope type signatures or transient skewed structures are observed in the thin current sheets, particularly in regions where signatures of electron acceleration are observed. These observations suggest that three‐dimensional localized/transient structures could play an essential role in the dynamics of the thin current sheets, while a gross X‐line picture can be established only in an average sense.

Further Study of Multiple Flux Rope Events at the Magnetopause Observed by TC‐1 on 18 March, 2004

AbstractFrom 23:10 to 23:50 UT on 18 March 2004, the Double Star TC‐1 spacecraft detected nine flux ropes at the outbound crossing of the southern dawnside magnetopause. During this time period the Cluster constellation was staying in the magnetosheath. The four spacecraft observed that the interplanetary magnetic field (IMF) showed a dominant negative BY component together with a southward BZ. Notable guide field was found inside all nine magnetic flux ropes detected by TC‐1. The first seven ropes during 23:14~23:27 UT appeared quasi‐periodically with a repeated period about 1–4 minutes, which is much shorter than the averaged occurrence period (about 8~11 minutes) of the flux transfer events (FTEs) at the dayside magnetopause. The Minimum Variation Analysis (MVA) and the Grad‐Shafronov Reconstruction (GSR) technique are applied in present study of the multiple flux rope events. By and large, all the principal axes of flux ropes are along the dawn‐dusk direction. The deHoffmann‐Teller (HT) analysis shows that the HT velocities (VHT) of these flux ropes were all directed dawnward and poleward, indicating that they came from the duskward and equatorward side of the spacecraft location and were produced via component magnetic reconnection in the subsolar region at the magnetopause.

Catastrophic Behavior of Multiple Coronal Flux Rope System

A major solar active event called Bastille Day Event occurred in AR 9077 on July 14, 2000. Simultaneous occurrence of a filament eruption, a flare and a coronal mass ejection was observed in this event. Previous analyses of this event show that before the event, there existed an activation and eruption of a huge trans-equatorial filament, which might play a crucial role in triggering the Bastille Day event. This implies that independent flux systems are closely related to and affect each other, which has encouraged us to investigate the catastrophic behavior of a multiple coronal flux rope system with the use of a 2.5-D time-dependent MHD model. A force-free field that contains three separate coronal flux ropes is taken to be the initial state. Starting from this state, we increase either the annular or the axial flux of a certain flux rope to examine the catastrophic behavior of the system in two regimes, the ideal MHD regime and the resistive MHD regime. It is found that a catastrophe occurs if the flux exceeds a certain critical value, or the magnetic energy of the system exceeds a certain threshold: the rope of interest breaks away from the base and escapes to infinity, leaving a current sheet below. Moreover, the destiny of the remainder flux ropes relies on whether reconnection takes place across the current sheet. In the ideal MHD regime, i.e., in the absence of reconnection, these ropes remain to be attached to the base in equilibrium, whereas in the resistive MHD regime they abruptly erupt upward during reconnection and escape to infinity. Reconnection causes the field lines to close back to the base and thus changes the background field outside the attached flux ropes in such a way that the constraint on these ropes is substantially relaxed and the corresponding catastrophic energy threshold is reduced accordingly, leading to a catastrophic eruption of these ropes. Since magnetic reconnection is generally inevitable when a current sheet forms and develops through an eruption of one flux rope, the eruption of this flux rope must lead to an eruption of the others. This provides an example to demonstrate the interaction between several independent magnetic flux systems in different regions, as implied by the Bastille Day event, and may serve as a possible mechanism for sympathetic events occurring on the Sun.

Multiple flux rope events at the magnetopause observations by TC-1 on 18 March 2004

Abstract. From 23:10 to 23:50 UT on 18 March 2004, the Double Star TC-1 spacecraft detected eight flux ropes at the outbound crossing of the southern dawnside magnetopause. A notable guide field existed inside all ropes. In the mean time the Cluster spacecraft were staying in the magnetosheath and found that the events occurred under the condition of southward IMF Bz and dominant negative IMF By. There are six ropes that appeared quasi-periodically, with a repeated period being approximately 1-4 min. The last flux rope lasts for a longer time interval with a larger peak in the BN variations; it can thus be referred to as a typical FTE. The 18 March 2004 event is quite similar to the multiple flux rope event observed by Cluster on 26 January 2001 at the northern duskside high-latitude magnetopause. A detailed comparison of these two events is made in the paper. Preliminary studies imply that both of these multiple flux ropes events seem to be produced by component reconnection at the dayside low-latitude magnetopause.

Multiple Flux Rope Events at the High-Latitude Magnetopause: Cluster/Rapid Observation on 26 January, 2001

Cluster measurements of the cusp and high latitude magnetopause boundary on 26 January, 2001 confirm that the cusp is a dynamic region full of energetic charged particles and turbulence. An energetic ion layer at high-latitudes beyond and adjacent to the duskside magnetopause exists when the Interplanetary Magnetic Field (IMF) has a southward orientation. Multiple energetic ion flux bursts were observed in the energetic ion layer. Each energetic ion flux burst was closely related to a magnetic flux rope. The axes of the flux ropes lie in the direction pointing duskward/tailward and somewhat upward. An intense axis-aligned current flows inside the ropes, with the current density reaching ∼10−8 A/m2. The main components of the energetic ions are protons, helium and CNO ions, which originate from the magnetosphere, flowing out into the magnetosheath along the axis of the flux ropes. The velocity of the magnetosheath thermal plasma relative to the deHoffman-Teller (DHT) frame is found to be basically along the axis of the flux ropes also, but towards the magnetosphere. These flux ropes seem to be produced somewhere away via magnetic reconnection and move at similar DHT velocities passing over the spacecraft. These observations further confirm that the high-latitude magnetopause boundary region plays an important role in the solar wind-magnetopause coupling.

Multiple Flux Rope Events at the High‐Latitude Magnetopause on January 26, 2001: Current Density Calculation

AbstractA systematic comparison is carried out for three methods of current density calculation based on simultaneous magnetic field measurements of four‐spacecraft Cluster mission. It is demonstrated analytically and numerically that within the linear approximation, all these methods lead to exactly the same results. As a case study, the current density of multiple flux rope events at the high‐latitude magnetopause on January 26, 2001 is investigated. The result shows that an intense current flows inside the ropes with the current density reaching as high as about 10−8A/m2. The current inside rope is valuable with small relative errors. It is also found that the current direction is almost parallel to the axis of flux rope obtained through the magnetic minimum variance analysis (BMVA). Finally, it is proposed that the current MVA method (CMVA) has great effectness for study of flux rope geometry, particularly when the BMVA is insufficient to determine the flux rope orientation.

Multiple Flux Rope Events at the High‐Latitude Magnetopause: Cluster/Rapid Observation on January 26, 2001

AbstractFrom 11:10 to 11:40UT on January 26, 2001 the four Cluster II spacecraft were located in the duskside high latitude regions of the magnetosheath and magnetosheath boundary layer (MSBL). During this time Interval the interplanetary magnetic field (IMF) had a negative Bz component. A detailed study on the multiple flux ropes (MFRs) observed in this period is conducted in this paper. It is found that: (1) The multiple flux ropes in the high latitude MSBL appeared quasi‐periodically with a repeated time period of about 78s, which is much shorter than the averaged occurring period (about 8–11min) of the flux transfer events (FTEs) at the dayside magnetopause (MP). (2) All the flux ropes observed in this event had a strong core magnetic field. The axial orientation of the most flux ropes is found to lie in the direction of the minimum magnetic field variance; a few flux ropes had their axes lying in the direction of the middle magnetic field variance; while for the remainders their principle axes could not be determined by the method of Principal Axis Analysis (PAA). The reason that causes this complexity relys on the different trajectories of the spacecraft passing through the flux ropes. (3) Each flux rope had a good corresponding HT frame of reference in which it was in a quasi‐steady state. All flux ropes moved along the surface of the MP in a similar direction indicating that these flux ropes all came from the dawnside low latitude. Their radial scale is 1– 2RE, comparable to the normal diameter of FTEs observed atthe dayside MP. (4) The energetic ions originated from the magnetosphere flowed out to the magnetosheath on the whole, while the solar wind plasma flowed into the magnetosphere along the axis of the flux ropes. The flux ropes offered channels for the transport of the solar wind plasma into the magnetosphere and the escaping of the magnetospheric plasma into the interplanetary space. (5) Each event was accompanied by an enhanced reversal of the dusk‐dawn electric field, which could be identified to be the convective electric field in nature.

Multiple spacecraft flux rope modeling of the Bastille Day magnetic cloud

The Bastille Day magnetic cloud in July 2000 occurred with NEAR in conjunction with the Earth at a radial distance of 1.76 AU and 1.9° from the Earth‐Sun line. Propagation time from ACE at 0.99 AU to NEAR indicates the cloud did not decelerate significantly between the Earth and 1.76 AU. Using a non‐force‐free, kinematic flux rope model we find the rope contained 130 TWb of magnetic flux, was oriented with clock and cone angles of 50° and 83°, and had a radius of 0.25 AU at ACE. At NEAR its radius had expanded to 0.43 AU. Simultaneous modeling of ACE and NEAR data indicate the axial and poloidal magnetic fields vary as R−1.4 and R−1.2 where R is heliocentric distance. Magnetosheath thicknesses of 0.14 AU and 0.23 AU indicate the rope cross section is elongated normal to the cloud axis and the radial direction.

Tailward flowing energetic oxygen ion bursts associated with multiple flux ropes in the distant magnetotail: GEOTAil observations

An event of tailward flowing energetic (144–7959 keV) oxygen ion bursts was observed in the distant magnetotail (X=−63, Y=+7, Z=−3.8 RE) on February 13, 1994. The observations were made with the HEP‐LD spectrometer on board the GEOTAIL spacecraft. The event was associated with magnetic field signatures characteristic of multiple flux ropes. During the event, which lasted from 1847 to 1907 UT, strong impulsive increases in the oxygen flux were observed. From 1846 to 1900 UT the proton counting rate also exhibited an increase, followed by a decrease until the end of the oxygen event. The oxygen flux was confined to a rather narrow range in polar and azimuthal angle (only 7–10% of 4π was occupied). This implies a streaming distribution or beam‐like structure. Comparison of the particle flow angles with the polar and azimuthal angles of the magnetic field indicates that the ion beam may have been embedded in flux ropes, which may be connecting the polar ionosphere and the distant magnetotail. During the observed oxygen event the ratio is significantly higher than the ratios usually found in the center of the distant magnetotail. There is some evidence that the observed oxygen ions were more efficiently accelerated in this event than hydrogen and helium ions.