Magnetospheric source region of discrete auroras inferred from their relationship with isotropy boundaries of energetic particles

Abstract. According to observations, the discrete auroral arcs can sometimes be found, either deep inside the auroral oval or at the poleward border of the wide (so-called double) auroral oval, which map to very different regions of the magnetotail. To find common physical conditions for the auroral-arc generation in these magnetotail regions, we study the spatial relationship between the diffuse and discrete auroras and the isotropic boundaries (IBs) of the precipitating energetic particles which can be used to characterise locally the equatorial magnetic field in the tail. From comparison of ground observation of auroral forms with meridional profiles of particle flux measured simultaneously by the low-altitude NOAA satellites above the ground observation region, we found that (1) discrete auroral arcs are always situated polewards from (or very close to) the IB of >30-keV electrons, whereas (2) the IB of the >30-keV protons is often seen inside the diffuse aurora. These relationships hold true for both quiet and active (substorm) conditions in the premidnight-nightside (18-01-h) MLT sector considered. In some events the auroral arcs occupy a wide latitudinal range. The most equatorial of these arcs was found at the poleward edge of the diffuse auroras (but anyway in the vicinity of the electron IB), the most poleward arcs were simultaneously observed on the closed field lines near the polar-cap boundary. These observations disagree with the notion that the discrete aurora originate exclusively in the near-Earth portion of plasma sheet or exclusively on the PSBL field lines. Result (1) may imply a fundamental feature of auroral-arc formation: they originate in the current-sheet regions having very curved and tailward-stretched magnetic field lines.

Download Full-text

Filamentary field-aligned currents at the polar cap region during northward interplanetary magnetic field derived with the Swarm constellation

Annales Geophysicae ◽

10.5194/angeo-34-901-2016 ◽

2016 ◽

Vol 34 (10) ◽

pp. 901-915 ◽

Cited By ~ 12

Author(s):

Hermann Lühr ◽

Tao Huang ◽

Simon Wing ◽

Guram Kervalishvili ◽

Jan Rauberg ◽

...

Keyword(s):

Magnetic Field ◽

Interplanetary Magnetic Field ◽

Initial Study ◽

Auroral Oval ◽

Closed Field ◽

Defense Meteorological Satellite Program ◽

Field Lines ◽

Auroral Arcs ◽

First Time ◽

Meteorological Satellite

Abstract. ESA's Swarm constellation mission makes it possible for the first time to determine field-aligned currents (FACs) in the ionosphere uniquely. In particular at high latitudes, the dual-satellite approach can reliably detect some FAC structures which are missed by the traditional single-satellite technique. These FAC events occur preferentially poleward of the auroral oval and during times of northward interplanetary magnetic field (IMF) orientation. Most events appear on the nightside. They are not related to the typical FAC structures poleward of the cusp, commonly termed NBZ. Simultaneously observed precipitating particle spectrograms and auroral images from Defense Meteorological Satellite Program (DMSP) satellites are consistent with the detected FACs and indicate that they occur on closed field lines mostly adjacent to the auroral oval. We suggest that the FACs are associated with Sun-aligned filamentary auroral arcs. Here we introduce in an initial study features of the high-latitude FAC structures which have been observed during the early phase of the Swarm mission. A more systematic survey over longer times is required to fully characterize the so far undetected field aligned currents.

Download Full-text

Random Walk and Trapping of Interplanetary Magnetic Field Lines: Global Simulation, Magnetic Connectivity, and Implications for Solar Energetic Particles

10.5194/egusphere-egu21-3719 ◽

2021 ◽

Author(s):

David Ruffolo ◽

Rohit Chhiber ◽

William H. Matthaeus ◽

Arcadi V. Usmanov ◽

Paisan Tooprakai ◽

...

Keyword(s):

Magnetic Field ◽

Random Walk ◽

Field Line ◽

Large Scale ◽

Transport Model ◽

Source Region ◽

Science Research ◽

Energetic Particles ◽

Solar Energetic Particles ◽

Field Lines

The random walk of magnetic field lines is an important ingredient in understanding how the connectivity of the magnetic field affects the spatial transport and diffusion of charged particles. As solar energetic particles (SEPs) propagate away from near-solar sources, they interact with the fluctuating magnetic field, which modifies their distributions. We develop a formalism in which the differential equation describing the field line random walk contains both effects due to localized magnetic displacements and a non-stochastic contribution from the large-scale expansion. We use this formalism together with a global magnetohydrodynamic simulation of the inner-heliospheric solar wind, which includes a turbulence transport model, to estimate the diffusive spreading of magnetic field lines that originate in different regions of the solar atmosphere. We first use this model to quantify field line spreading at 1 au, starting from a localized solar source region, and find rms angular spreads of about 20 &#8211; 60 degrees. In the second instance, we use the model to estimate the size of the source regions from which field lines observed at 1 au may have originated, thus quantifying the uncertainty in calculations of magnetic connectivity; the angular uncertainty is estimated to be about 20 degrees. Finally, we estimate the filamentation distance, i.e., the heliocentric distance up to which field lines originating in magnetic islands can remain strongly trapped in filamentary structures. We emphasize the key role of slab-like fluctuations in the transition from filamentary to more diffusive transport at greater heliocentric distances. This research has been supported in part by grant RTA6280002 from Thailand Science Research and Innovation and the Parker Solar Probe mission under the ISOIS project (contract NNN06AA01C) and a subcontract to University of Delaware from Princeton University (SUB0000165). &#160;MLG acknowledges support from the Parker Solar Probe FIELDS MAG team. &#160;Additional support is acknowledged from the&#160; NASA LWS program&#160; (NNX17AB79G) and the HSR program (80NSSC18K1210 & 80NSSC18K1648).

Download Full-text

Reconnection and energetic particles at the edge of the exterior cusp

Annales Geophysicae ◽

10.5194/angeo-24-1949-2006 ◽

2006 ◽

Vol 24 (7) ◽

pp. 1949-1956 ◽

Cited By ~ 10

Author(s):

T. Asikainen ◽

K. Mursula

Keyword(s):

Magnetic Field ◽

Pitch Angle ◽

Energetic Particles ◽

Maximum Energy ◽

Closed Field ◽

Angle Distribution ◽

Pitch Angle Distribution ◽

Flux Transfer Events ◽

Cusp Region ◽

Field Lines

Abstract. In this paper we study flux transfer events (FTE) observed at the post-noon edge of the exterior cusp region by Cluster satellites. During the outbound dayside orbit on 2 February 2003, intense bursts of energetic particles were observed in close conjuction with magnetic field FTE signatures by the RAPID instrument onboard the Cluster 4. The pitch-angle distribution of the particles showed that the enhancements consist of particles flowing antiparallel to the magnetosheath field lines away from the expected reconnection site to the exterior cusp. At the same time Cluster 3 observed enhancements of energetic particles deeper in the exterior cusp with a delay of about 40 s to the Cluster 4 enhancements. The estimated maximum energy gain per particle by reconnection remains below 1 keV, thus clearly below the tens to hundreds of keV energy range observed by the RAPID instrument. These observations support the earlier statistical result of the magnetospheric origin of energetic particles in the exterior cusp. Reconnection near the exterior cusp partly releases the particles in the closed field lines of the adjacent HLPS region into the exterior cusp.

Download Full-text

The auroral and ionospheric flow signatures of dual lobe reconnection

Annales Geophysicae ◽

10.5194/angeo-24-3115-2006 ◽

2006 ◽

Vol 24 (11) ◽

pp. 3115-3129 ◽

Cited By ~ 37

Author(s):

S. M. Imber ◽

S. E. Milan ◽

B. Hubert

Keyword(s):

Magnetic Field ◽

Interplanetary Magnetic Field ◽

Field Line ◽

Radar Data ◽

Auroral Oval ◽

Closed Field ◽

Closed Field Line ◽

Field Lines ◽

Lobe Reconnection ◽

Northern And Southern Hemispheres

Abstract. We present the first substantial evidence for the occurrence of dual lobe reconnection from ionospheric flows and auroral signatures. The process of dual lobe reconnection refers to an interplanetary magnetic field line reconnecting with lobe field lines in both the northern and southern hemispheres. Two bursts of sunward plasma flow across the noon portion of the open/closed field line boundary (OCB), indicating magnetic flux closure at the dayside, were observed in SuperDARN radar data during a period of strongly northward IMF. The OCB is identified from spacecraft, radar backscatter, and auroral observations. In order for dual lobe reconnection to take place, we estimate that the interplanetary magnetic field clock angle must be within ±10° of zero (North). The total flux crossing the OCB during each burst is small (1.8% and 0.6% of the flux contained within the polar cap for the two flows). A brightening of the noon portion of the northern auroral oval was observed as the clock angle passed through zero, and is thought to be due to enhanced precipitating particle fluxes due to the occurrence of reconnection at two locations along the field line. The number of solar wind protons captured by the flux closure process was estimated to be ~2.5×1030 (4 tonnes by mass), sufficient to populate the cold, dense plasma sheet observed following this interval.

Download Full-text

Instabilities of Magnetic Fields in Stars

Symposium - International Astronomical Union ◽

10.1017/s0074180900237467 ◽

1974 ◽

Vol 59 ◽

pp. 177-177

Author(s):

R. J. Tayler

Keyword(s):

Magnetic Field ◽

Gravitational Potential ◽

Rotation Period ◽

Region Of Interest ◽

Alfven Wave ◽

Closed Field ◽

Toroidal Plasma ◽

Wide Range ◽

Field Lines ◽

Rotating Star

It has been shown (Markey and Tayler, 1973; Tayler, 1973; Wright, 1973) that a wide range of simple magnetic field configurations in stars are unstable. Although the ultimate effect of the instabilities is unclear, it seems likely that they would lead to enhanced destruction of magnetic flux, so that magnetic field decay would be much more rapid than previously supposed. Instability is almost certain in a non-rotating star containing either a purely toroidal field or a purely poloidal field, which has closed field lines inside the star. In both cases the instability resembles the well known instabilities of cylindrical and toroidal current channels, modified by the constraint that motion must be almost entirely along surfaces of constant gravitational potential.If both toroidal and poloidal fields are present, the problem is more complicated. In a toroidal plasma with a helical field, the worst instabilities are also helical but it is impossible for a helical disturbance to be parallel to a surface of constant gravitational potential everywhere. As a result, the admixture of toroidal and poloidal fields has a stabilizing influence, but it is not at present clear whether the majority of such configurations are completely stable.The effect of rotation has not yet been studied but it will certainly be important if the rotation period is less than the time taken for an Alfvén wave to cross the region of interest. This is true in most stars unless the internal magnetic field is very much stronger than any observed field.

Download Full-text

A physical mechanism producing suprathermal populations and initiating substorms in the Earth's magnetotail

Annales Geophysicae ◽

10.5194/angeo-26-1617-2008 ◽

2008 ◽

Vol 26 (6) ◽

pp. 1617-1639 ◽

Cited By ~ 10

Author(s):

D. V. Sarafopoulos

Keyword(s):

Magnetic Field ◽

Electric Fields ◽

Plasma Sheet ◽

Physical Mechanism ◽

Energetic Particles ◽

Dimensional Structure ◽

Magnetospheric Substorms ◽

Physical Description ◽

Fundamental Building Block ◽

Field Lines

Abstract. We suggest a candidate physical mechanism, combining there dimensional structure and temporal development, which is potentially able to produce suprathermal populations and cross-tail current disruptions in the Earth's plasma sheet. At the core of the proposed process is the "akis" structure; in a thin current sheet (TCS) the stretched (tail-like) magnetic field lines locally terminate into a sharp tip around the tail midplane. At this sharp tip of the TCS, ions become non-adiabatic, while a percentage of electrons are accumulated and trapped: The strong and transient electrostatic electric fields established along the magnetic field lines produce suprathermal populations. In parallel, the tip structure is associated with field aligned and mutually attracted parallel filamentary currents which progressively become more intense and inevitably the structure collapses, and so does the local TCS. The mechanism is observationally based on elementary, almost autonomous and spatiotemporal entities that correspond each to a local thinning/dipolarization pair having duration of ~1 min. Energetic proton and electron populations do not occur simultaneously, and we infer that they are separately accelerated at local thinnings and dipolarizations, respectively. In one example energetic particles are accelerated without any dB/dt variation and before the substorm expansion phase onset. A particular effort is undertaken demonstrating that the proposed acceleration mechanism may explain the plasma sheet ratio Ti/Te≈7. All our inferences are checked by the highest resolution datasets obtained by the Geotail Energetic Particles and Ion Composition (EPIC) instrument. The energetic particles are used as the best diagnostics for the accelerating source. Near Earth (X≈10 RE) selected events support our basic concept. The proposed mechanism seems to reveal a fundamental building block of the substorm phenomenon and may be the basic process/structure, which is now missing, that might help explain the persistent, outstanding deficiencies in our physical description of magnetospheric substorms. The mechanism is tested, checked, and found consistent with substorm associated observations performed ~30 and 60 RE away from Earth.

Download Full-text

Focused transport of energetic particles along magnetic field lines draped around a coronal mass ejection

Journal of Geophysical Research Atmospheres ◽

10.1029/91ja02699 ◽

1992 ◽

Vol 97 (A2) ◽

pp. 1597 ◽

Cited By ~ 9

Author(s):

L. C. Tan ◽

G. M. Mason ◽

M. A. Lee ◽

B. Klecker ◽

F. M. Ipavich

Keyword(s):

Magnetic Field ◽

Coronal Mass Ejection ◽

Energetic Particles ◽

Field Lines ◽

Mass Ejection

Download Full-text

Differential rotation in neutron stars with open and closed magnetic topologies

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa915 ◽

2020 ◽

Vol 494 (3) ◽

pp. 3095-3109

Author(s):

F Anzuini ◽

A Melatos

Keyword(s):

Magnetic Field ◽

Field Line ◽

Open Field ◽

Flux Tube ◽

Differential Rotation ◽

Time Scale ◽

Outer Sphere ◽

Closed Field ◽

Magnetic Tension ◽

Field Lines

ABSTRACT Analytic arguments have been advanced that the degree of differential rotation in a neutron star depends on whether the topology of the internal magnetic field is open or closed. To test this assertion, the ideal-magnetohydrodynamics solver pluto is employed to investigate numerically the flow of an incompressible, viscous fluid threaded by a magnetic field with open and closed topologies in a conducting, differentially rotating, spherical shell. Rigid body corotation with the outer sphere is enforced on the Alfvén time-scale, along magnetic field lines that connect the northern and southern hemispheres of the outer sphere. Along other field lines, however, the behaviour is more complicated. For example, an initial point dipole field evolves to produce an approximately closed equatorial flux tube containing at least one predominantly toroidal and approximately closed field line surrounded by a bundle of predominantly toroidal but open field lines. Inside the equatorial flux tube, the field-line-averaged magnetic tension approaches zero, and the fluid rotates differentially, adjusting its angular velocity on the viscous time-scale to match the boundary conditions on the flux tube’s toroidal surface. Outside the equatorial flux tube, the differential rotation increases, as the magnetic tension averaged along open field lines decreases.

Download Full-text

Examining the optical intensity and magnetic field expansion factor in the open magnetic field regions associated with coronal holes

Proceedings of the International Astronomical Union ◽

10.1017/s1743921320000393 ◽

2019 ◽

Vol 15 (S354) ◽

pp. 228-231

Author(s):

Chia-Hsien Lin ◽

Guan-Han Huang ◽

Lou-Chuang Lee

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Coronal Holes ◽

Closed Field ◽

Logarithmic Scale ◽

Expansion Factor ◽

Astrophysical Journal ◽

Field Lines ◽

The Relationship ◽

Open Magnetic Field

AbstractCoronal holes can be identified as the darkest regions in EUV or soft X-ray images with predominantly unipolar magnetic fields (LIRs) or as the regions with open magnetic fields (OMF). Our study reveals that only 12% of OMF regions are coincident with LIRs. The aim of this study is to investigate the conditions that affect the EUV intensity of OMF regions. Our results indicate that the EUV intensity and the magnetic field expansion factor of the OMF regions are weakly positively correlated when plotted in logarithmic scale, and that the bright OMF regions are likely to locate inside or next to the regions with closed field lines. We empirically determined a linear relationship between the expansion factor and the EUV intensity. The relationship is demonstrated to improve the consistency from 12% to 23%. The results have been published in Astrophysical Journal (Huang et al. 2019).

Download Full-text

Energy Conversion in the Electron Stagnation Region of Magnetopause Reconnection

10.5194/egusphere-egu2020-3757 ◽

2020 ◽

Author(s):

James Burch ◽

James Webster ◽

Kristina Pritchard ◽

Kevin Genestreti ◽

Michael Hesse ◽

...

Keyword(s):

Magnetic Field ◽

Energy Conversion ◽

Closed Field ◽

Stagnation Region ◽

Strong Electron ◽

The Earth ◽

Background Magnetic Field ◽

Out Of Plane ◽

Uniform Background ◽

Field Lines

For reconnection at the Earth&#8217;s day side, which is asymmetric, the main energy conversion occurs on closed field lines in the electron stagnation region. Energy conversion, as measured by J&#10625;E, occurs where out-of-plane electric field components are embedded within larger regions of out-of-plane current, which is carried by strong electron flows in the M direction of the LMN coordinate system. Bracketing these energy conversion sites are electron jet reversals (along L and -L) and converging &#160;electron flows (along N and -N). These electron flows are like those that surround reconnection X lines, however, in these cases they occur completely within closed field lines. The question then is what, if anything, this energy conversion has to do with local reconnection of magnetic field lines. This paper reports on a study of two events observed by MMS on December 29, 2016 and April 15, 2018. The electron inflows have velocities between 0.05 VeA and 0.1 VeA, (VeA = electron Alfv&#233;n speed), which are consistent with predicted reconnection rates. Laboratory measurements and 3D simulation results offer some clues about how reconnecting current sheets can evolve in a uniform background magnetic field.

Download Full-text