scholarly journals Overlapping ion structures in the mid-altitude cusp under northward IMF: signature of dual lobe reconnection?

2012 ◽  
Vol 30 (3) ◽  
pp. 489-501 ◽  
Author(s):  
F. Pitout ◽  
C. P. Escoubet ◽  
M. G. G. T. Taylor ◽  
J. Berchem ◽  
A. P. Walsh
Keyword(s):  
Magnetic Field ◽  
Interplanetary Magnetic Field ◽  
Low Energy ◽  
Closed Field ◽  
Opposite Hemisphere ◽  
Cluster Ion ◽  
Low Energy Tail ◽  
Field Lines ◽  
Lobe Reconnection ◽  
Cooling Model

Abstract. On some rare occasions, data from the Cluster Ion Spectrometer (CIS) in the mid-altitude cusp reveal overlapping ion populations under northward interplanetary magnetic field (IMF). While the poleward part of the cusp exhibits the expected reverse dispersion due to lobe reconnection, its equatorward part shows a second ion population at higher-energy that coexists with the low energy tail of the dispersion. This second population is either dispersionless or slightly dispersed with energies increasing with increasing latitudes, indicative of lobe reconnection as well. Our analysis of a case that occurred 3 September 2002 when the IMF stayed northward for more than two hours suggests that the second population comes from the opposite hemisphere and is very likely on newly-closed field lines. We interpret this overlap of cusp populations as a clear mid-altitude signature of re-closed magnetic field lines by double lobe reconnection (reconnection in both hemispheres) under northward IMF. This interpretation is supported by modelling performed with the Cooling model and an MHD model.

scholarly journals O<sup>+</sup> transport in the dayside magnetosheath and its dependence on the IMF direction

2015 ◽  
Vol 33 (3) ◽  
pp. 301-307 ◽  
Author(s):  
R. Slapak ◽  
H. Nilsson ◽  
L. G. Westerberg ◽  
R. Larsson
Keyword(s):  
Magnetic Field ◽  
Interplanetary Magnetic Field ◽  
Ion Flux ◽  
Opposite Hemisphere ◽  
Oxygen Ions ◽  
Upper Limit ◽  
Field Lines ◽  
Lobe Reconnection ◽  
Open Magnetic Field ◽  
The Imf

Abstract. Recent studies have shown that the escape of oxygen ions (O+) into the magnetosheath along open magnetic field lines from the terrestrial cusp and mantle is significant. We present a study of how O+ transport in the dayside magnetosheath depends on the interplanetary magnetic field (IMF) direction. There are clear asymmetries in the O+ flows for southward and northward IMF. The asymmetries can be understood in terms of the different magnetic topologies that arise due to differences in the location of the reconnection site, which depends on the IMF direction. During southward IMF, most of the observed magnetosheath O+ is transported downstream. In contrast, for northward IMF we observe O+ flowing both downstream and equatorward towards the opposite hemisphere. We observe evidence of dual-lobe reconnection occasionally taking place during strong northward IMF conditions, a mechanism that may trap O+ and bring it back into the magnetosphere. Its effect on the overall escape is however small: we estimate the upper limit of trapped O+ to be 5%, a small number considering that ion flux calculations are rough estimates. The total O+ escape flux is higher by about a factor of 2 during times of southward IMF, in agreement with earlier studies of O+ cusp outflow.

scholarly journals The auroral and ionospheric flow signatures of dual lobe reconnection

2006 ◽  
Vol 24 (11) ◽  
pp. 3115-3129 ◽  
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.

scholarly journals Interplanetary magnetic field control of Saturn's polar cusp aurora

2005 ◽  
Vol 23 (4) ◽  
pp. 1405-1431 ◽  
Author(s):  
E. J. Bunce ◽  
S. W. H. Cowley ◽  
S. E. Milan
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Interplanetary Magnetic Field ◽  
Field Line ◽  
High Latitude ◽  
Closed Field ◽  
Vortical Flows ◽  
Dayside Magnetopause ◽  
Closed Field Line ◽  
Lobe Reconnection

Abstract. Dayside UV emissions in Saturn's polar ionosphere have been suggested to be the first observational evidence of the kronian "cusp" (Gérard et al., 2004). The emission has two distinct states. The first is a bright arc-like feature located in the pre-noon sector, and the second is a more diffuse "spot" of aurora which lies poleward of the general location of the main auroral oval, which may be related to different upstream interplanetary magnetic field (IMF) orientations. Here we take up the suggestion that these emissions correspond to the cusp. However, direct precipitation of electrons in the cusp regions is not capable of producing significant UV aurora. We have therefore investigated the possibility that the observed UV emissions are associated with reconnection occurring at the dayside magnetopause, possibly pulsed, akin to flux transfer events seen at the Earth. We devise a conceptual model of pulsed reconnection at the low-latitude dayside magnetopause for the case of northwards IMF which will give rise to pulsed twin-vortical flows in the magnetosphere and ionosphere in the vicinity of the open-closed field-line boundary, and hence to bi-polar field-aligned currents centred in the vortical flows. During intervals of high-latitude lobe reconnection for southward IMF, we also expect to have pulsed twin-vortical flows and corresponding bi-polar field-aligned currents. The vortical flows in this case, however, are displaced poleward of the open-closed field line boundary, and are reversed in sense, such that the field-aligned currents are also reversed. For both cases of northward and southward IMF we have also for the first time included the effects associated with the IMF By effect. We also include the modulation introduced by the structured nature of the solar wind and IMF at Saturn's orbit by developing "slow" and "fast" flow models corresponding to intermediate and high strength IMF respectively. We then consider the conditions under which the plasma populations appropriate to either sub-solar reconnection or high-latitude lobe reconnection can carry the currents indicated. We have estimated the field-aligned voltages required, the resulting precipitating particle energy fluxes, and the consequent auroral output. Overall our model of pulsed reconnection under conditions of northwards and southwards IMF, and for varying orientations of IMF By, is found to produce a range of UV emission intensities and geometries which is in good agreement with the data presented by Gérard et al. (2004). The recent HST-Cassini solar wind campaign provides a unique opportunity to test the theoretical ideas presented here.

scholarly journals Hybrid-Vlasov simulation of auroral proton precipitation in the cusps: Comparison of northward and southward interplanetary magnetic field driving

2020 ◽  
Author(s):  
Maxime Grandin ◽  
Lucile Turc ◽  
Markus Battarbee ◽  
Urs Ganse ◽  
Andreas Johlander ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Interplanetary Magnetic Field ◽  
Geomagnetic Latitude ◽  
Distribution Functions ◽  
Lower Latitude ◽  
Magnetic Field Direction ◽  
Opposite Hemisphere ◽  
Proton Precipitation ◽  
Reconnection Site ◽  
Lobe Reconnection

&lt;p&gt;We present the first hybrid-Vlasov simulations of proton precipitation in the polar cusps. We use two runs from the Vlasiator model to compare cusp proton precipitation fluxes during southward and northward interplanetary magnetic field (IMF) driving. The simulations reproduce well-known features of cusp precipitation, such as a reverse dispersion of precipitating proton energies, with proton energies increasing with increasing geomagnetic latitude under northward IMF driving, and a nonreversed dispersion under southward IMF driving. The cusp location is also found more poleward in the northward IMF simulation than in the southward IMF simulation. In addition, we find that the precipitation takes place in the form of successive bursts during southward IMF driving, those bursts being associated with the transit of flux transfer events in the vicinity of the cusp. In the northward IMF simulation, dual lobe reconnection takes place. As a consequence, in addition to the high-latitude precipitation footprint associated with the lobe reconnection from the same hemisphere, we observe lower-latitude precipitating protons which originate from the opposite hemisphere&amp;#8217;s lobe reconnection site. The proton velocity distribution functions along the newly closed dayside magnetic field lines exhibit multiple proton beams travelling parallel and antiparallel to the magnetic field direction, which is consistent with observations with the Cluster spacecraft. We suggest that precipitating protons originating from the opposite hemisphere&amp;#8217;s lobe reconnection site, albeit infrequent, could be observed in a situation of dual lobe reconnection.&lt;/p&gt;

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

2016 ◽  
Vol 34 (10) ◽  
pp. 901-915 ◽  
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.

scholarly journals Instabilities of Magnetic Fields in Stars

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.

scholarly journals Plasma convection in the magnetotail lobes: statistical results from Cluster EDI measurements

2008 ◽  
Vol 26 (8) ◽  
pp. 2371-2382 ◽  
Author(s):  
S. Haaland ◽  
G. Paschmann ◽  
M. Förster ◽  
J. Quinn ◽  
R. Torbert ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Interplanetary Magnetic Field ◽  
Geomagnetic Disturbance ◽  
Plasma Convection ◽  
Data Set ◽  
Central Plasma ◽  
Strong Convection ◽  
Central Plasma Sheet ◽  
Field Lines ◽  
Convection Patterns

Abstract. A major part of the plasma in the Earth's magnetotail is populated through transport of plasma from the solar wind via the magnetotail lobes. In this paper, we present a statistical study of plasma convection in the lobes for different directions of the interplanetary magnetic field and for different geomagnetic disturbance levels. The data set used in this study consists of roughly 340 000 one-minute vector measurements of the plasma convection from the Cluster Electron Drift Instrument (EDI) obtained during the period February 2001 to June 2007. The results show that both convection magnitude and direction are largely controlled by the interplanetary magnetic field (IMF). For a southward IMF, there is a strong convection towards the central plasma sheet with convection velocities around 10 km s−1. During periods of northward IMF, the lobe convection is almost stagnant. A By dominated IMF causes a rotation of the convection patterns in the tail with an oppositely directed dawn-dusk component of the convection for the northern and southern lobe. Our results also show that there is an overall persistent duskward component, which is most likely a result of conductivity gradients in the footpoints of the magnetic field lines in the ionosphere.

scholarly journals Cluster observations of continuous reconnection at the magnetopause under steady interplanetary magnetic field conditions

2004 ◽  
Vol 22 (7) ◽  
pp. 2355-2367 ◽  
Author(s):  
T. D. Phan ◽  
M. W. Dunlop ◽  
G. Paschmann ◽  
B. Klecker ◽  
J. M. Bosqued ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Interplanetary Magnetic Field ◽  
High Speed ◽  
Flow Speed ◽  
Plasma Jets ◽  
Current Layer ◽  
Magnetic Shear ◽  
Opposite Hemisphere ◽  
Entire Interval ◽  
Transfer Event

Abstract. On 26 January 2001, the Cluster spacecraft detected high-speed plasma jets at multiple crossings of the high-latitude duskside magnetopause (MP) and boundary layer (BL) over a period of more than 2h. The 4 spacecraft combined spent more than half of this time in the MP/BL and jets were observed whenever a spacecraft was in the MP. These observations were made under steady southward and dawnward interplanetary magnetic field (IMF) conditions. The magnetic shear across the local MP was ~100° and β~1 in the adjacent magnetosheath. The jet velocity is in remarkable agreement with reconnection prediction throughout the entire interval, except for one crossing that had no ion measurements inside the current layer. The flow speed measured in the deHoffmann Teller frame is 90% of the Alfvén speed on average for the 10 complete MP current layer crossings that are resolved by the ion measurements. These findings strongly suggest that reconnection was continuously active for more than two hours. The jets were directed persistently in the same northward and anti-sunward direction, implying that the X-line was always below the spacecraft. This feature is inconsistent with patchy and random reconnection or convecting multiple X-lines. The majority of MP/BL crossings in this two-hour interval were partial crossings, implying that they are caused by bulges sliding along the MP, not by inward-outward motion of a uniformly thin MP/BL. The presence of the bulges suggests that, although reconnection is continuously active under steady IMF conditions, its rate may be modulated. The present investigation also reveals that (1) the predicted ion D-shaped distributions are absent in all reconnection jets on this day, (2) the electric field fluctuations are larger in the reconnecting MP than in the magnetosheath proper, but their amplitudes never exceed 20mV/m, (3) the ion-electron differential motion is ~20km/s for the observed MP current density of ~50nA/m2 (∇× B), thus inconsequential for the deHoffmann-Teller and Walén analyses, (4) flows in an isolated flux transfer event (FTE) are directed in the same direction as the MP jets and satisfy the Walén relation, suggesting that this FTE is also generated by reconnection. Finally, the present event cannot be used to evaluate the validity of component or anti-parallel merging models because, although the magnetic shear at the local MP was ~100°(≪180°), the X-line may be located more than 9RE away (in the opposite hemisphere), where the shear could be substantially different.

scholarly journals Excitation of transient lobe cell convection and auroral arc at the cusp poleward boundary during a transition of the interplanetary magnetic field from south to north

2001 ◽  
Vol 19 (5) ◽  
pp. 487-493 ◽  
Author(s):  
P. E. Sandholt ◽  
C. J. Farrugia ◽  
S. W. H. Cowley ◽  
M. Lester ◽  
J.-C. Cerisier
Keyword(s):  
Magnetic Field ◽  
Interplanetary Magnetic Field ◽  
Large Scale ◽  
Intermediate Phase ◽  
Bright Spot ◽  
Convection Cell ◽  
Plasma Convection ◽  
Cell Pattern ◽  
Auroral Phenomena ◽  
Lobe Reconnection

Abstract. We document the activation of transient polar arcs emanating from the cusp within a 15 min long intermediate phase during the transition from a standard two-cell convection pattern, representative of a strongly southward interplanetary magnetic field (IMF), to a "reverse" two-cell pattern, representative of strongly northward IMF conditions. During the 2–3 min lifetime of the arc, its base in the cusp, appearing as a bright spot, moved eastward toward noon by ~ 300 km. As the arc moved, it left in its "wake" enhanced cusp precipitation. The polar arc is a tracer of the activation of a lobe convection cell with clockwise vorticity, intruding into the previously established large-scale distorted two-cell pattern, due to an episode of localized lobe reconnection. The lobe cell gives rise to strong flow shear (converging electric field) and an associated sheet of outflowing field-aligned current, which is manifested by the polar arc. The enhanced cusp precipitation represents, in our view, the ionospheric footprint of the lobe reconnection process.Key words. Magnetospheric physics (auroral phenomena; magnetopause, cusp, and boundary layers; plasma convection)

scholarly journals Differential rotation in neutron stars with open and closed magnetic topologies

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.

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