Dual-lobe reconnection and horse-collar auroras

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.

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Dual‐Lobe Reconnection and Horse‐Collar Auroras

Journal of Geophysical Research Space Physics ◽

10.1029/2020ja028567 ◽

2020 ◽

Vol 125 (10) ◽

Author(s):

S. E. Milan ◽

J. A. Carter ◽

G. E. Bower ◽

S. M. Imber ◽

L. J. Paxton ◽

...

Keyword(s):

Lobe Reconnection

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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

Annales Geophysicae ◽

10.5194/angeo-19-487-2001 ◽

2001 ◽

Vol 19 (5) ◽

pp. 487-493 ◽

Cited By ~ 13

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)

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

Annales Geophysicae ◽

10.5194/angeo-33-301-2015 ◽

2015 ◽

Vol 33 (3) ◽

pp. 301-307 ◽

Cited By ~ 12

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.

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Footprints of lobe reconnection observed in ionospheric electron density under steady northward IMF

Geophysical Research Letters ◽

10.1029/1998gl900242 ◽

1999 ◽

Vol 26 (1) ◽

pp. 25-28 ◽

Cited By ~ 1

Author(s):

S. E. Pryse ◽

A. M. Smith ◽

J. Moen ◽

D. A. Lorentzen

Keyword(s):

Electron Density ◽

Ionospheric Electron Density ◽

Lobe Reconnection

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Reconfiguration and closure of lobe flux by reconnection during northward IMF: possible evidence for signatures in cusp/cleft auroral emissions

Annales Geophysicae ◽

10.1007/s00585-999-0996-2 ◽

1999 ◽

Vol 17 (8) ◽

pp. 996-1011 ◽

Cited By ~ 33

Author(s):

M. Lockwood ◽

J. Moen

Keyword(s):

Elevation Angle ◽

Lower Latitude ◽

Winter Solstice ◽

Latitude Band ◽

Wind Spacecraft ◽

Summer Hemisphere ◽

Auroral Emissions ◽

Lobe Reconnection ◽

Winter Hemisphere ◽

The Imf

Abstract. Observations are presented of the response of the dayside cusp/cleft aurora to changes in both the clock and elevation angles of the interplanetary magnetic field (IMF) vector, as monitored by the WIND spacecraft. The auroral observations are made in 630 nm light at the winter solstice near magnetic noon, using an all-sky camera and a meridian-scanning photometer on the island of Spitsbergen. The dominant change was the response to a northward turning of the IMF which caused a poleward retreat of the dayside aurora. A second, higher-latitude band of aurora was seen to form following the northward turning, which is interpreted as the effect of lobe reconnection which reconfigures open flux. We suggest that this was made possible in the winter hemisphere, despite the effect of the Earth's dipole tilt, by a relatively large negative X component of the IMF. A series of five events then formed in the poleward band and these propagated in a southwestward direction and faded at the equatorward edge of the lower-latitude band as it migrated poleward. It is shown that the auroral observations are consistent with overdraped lobe flux being generated by lobe reconnection in the winter hemisphere and subsequently being re-closed by lobe reconnection in the summer hemisphere. We propose that the balance between the reconnection rates at these two sites is modulated by the IMF elevation angle, such that when the IMF points more directly northward, the summer lobe reconnection site dominates, re-closing all overdraped lobe flux and eventually becoming disconnected from the Northern Hemisphere.Key words. Magnetospheric physics (magnetopause · cusp and boundary layers; solar-wind-magnetosphere interactions) · Space plasma physics (magnetic reconnection)

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Different responses of northern and southern high latitude ionospheric convection to IMF rotations: a case study based on SuperDARN observations

Annales Geophysicae ◽

10.5194/angeo-27-2423-2009 ◽

2009 ◽

Vol 27 (6) ◽

pp. 2423-2438 ◽

Cited By ~ 4

Author(s):

D. Ambrosino ◽

E. Amata ◽

M. F. Marcucci ◽

I. Coco ◽

W. Bristow ◽

...

Keyword(s):

Magnetic Field ◽

High Latitude ◽

Ionospheric Convection ◽

Cell Pair ◽

Cell Pattern ◽

Southern High Latitude ◽

Lobe Reconnection ◽

Magnetospheric Field ◽

The Imf

Abstract. We use SuperDARN data to study high-latitude ionospheric convection over a three hour period (starting at 22:00 UT on 2 January 2003), during which the Interplanetary Magnetic Field (IMF) flipped between two states, one with By>>|Bz| and one with Bz>0, both with negative Bx. We find, as expected from previous works, that day side ionospheric convection is controlled by the IMF in both hemispheres. For strongly northward IMF, we observed signatures of two reverse cells, both in the Northern Hemisphere (NH) and in the Southern Hemisphere (SH), due to lobe reconnection. On one occasion, we also observed in the NH two viscous cells at the sides of the reverse cell pair. For duskward IMF, we observed in the NH a large dusk clockwise cell, accompanied by a smaller dawn cell, and the signature of a corresponding pattern in the SH. On two occasions, a three cell pattern, composed of a large clockwise cell and two viscous cells, was observed in the NH. As regards the timings of the NH and SH convection reconfigurations, we find that the convection reconfiguration from a positive Bz dominated to a positive By dominated pattern occurred almost simultaneously (i.e. within a few minutes) in the two hemispheres. On the contrary, the reconfiguration from a By dominated to a northward IMF pattern started in the NH 8–13 min earlier than in the SH. We suggest that part of such a delay can be due to the following mechanism: as IMF Bx<0, the northward-tailward magnetosheath magnetic field reconnects with the magnetospheric field first tailward of the northern cusp and later on tailward of the southern cusp, due to the IMF draping around the magnetopause.

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

Journal of Space Weather and Space Climate ◽

10.1051/swsc/2020053 ◽

2020 ◽

Vol 10 ◽

pp. 51

Author(s):

Maxime Grandin ◽

Lucile Turc ◽

Markus Battarbee ◽

Urs Ganse ◽

Andreas Johlander ◽

...

Keyword(s):

Magnetic Field ◽

Interplanetary Magnetic Field ◽

Space Weather ◽

Geomagnetic Latitude ◽

Distribution Functions ◽

Lower Latitude ◽

Magnetic Field Direction ◽

Loss Cone ◽

Proton Precipitation ◽

Lobe Reconnection

Particle precipitation is a central aspect of space weather, as it strongly couples the magnetosphere and the ionosphere and can be responsible for radio signal disruption at high latitudes. 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 is also found more polewards in the northward IMF simulation than in the southward IMF simulation. In addition, we find that the bursty precipitation during southward IMF driving is 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 spot associated with the lobe reconnection from the same hemisphere, we observe lower-latitude precipitating protons which originate from the opposite hemisphere’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 previously reported observations with the Cluster spacecraft. In both runs, clear electromagnetic ion cyclotron waves are generated in the cusps and might further increase the calculated precipitating fluxes by scattering protons to the loss cone in the low-altitude cusp. Global kinetic simulations can improve the understanding of space weather by providing a detailed physical description of the entire near-Earth space and its internal couplings.

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Observational Evidence of Transient Lobe Reconnection Triggered by Sudden Northern Enhancement of IMF Bz

Journal of Geophysical Research Space Physics ◽

10.1029/2021ja029410 ◽

2021 ◽

Author(s):

Zhiwei Wang ◽

Hongqiao Hu ◽

Jianyong Lu ◽

Desheng Han ◽

Jianjun Liu ◽

...

Keyword(s):

Observational Evidence ◽

Lobe Reconnection

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Convection For Northward Interplanetary Field

Convection and Substorms ◽

10.1093/oso/9780195085297.003.0013 ◽

1996 ◽

Author(s):

Charles F. Kennel

Keyword(s):

Field Line ◽

Dynamic Pressure ◽

Field Conditions ◽

Closed Field ◽

Flow State ◽

Dayside Magnetopause ◽

Statistical Studies ◽

Closed Field Line ◽

Field Lines ◽

Lobe Reconnection

Besides common sense, a number of results suggest that we can learn more about the slow “viscous” flow state by studying the magnetosphere during northward interplanetary field conditions. In particular, statistical studies have consistently identified a “residual” state of magnetospheric and ionospheric convection in northward field conditions. The integrated potential across the high latitudeionosphere does not drop below a certain resting value of about 20 kV even when the interplanetary field has been due north for several hours. There appears to be a similar residual component of geomagnetic activity that is independent of the direction of the interplanetary field (Scurry and Russell, 1991). Its correlation with the dynamic pressure of the solar wind strengthens our suspicion that it is related to viscosity. Will we be able to prove the convection in this residual state is driven by viscosity? Does the flow in northward field conditions resemble the underlying irregular flow state of the plasma sheet found at other times? Does the magnetosphere approach the teardrop configuration during prolonged intervals of northward interplanetary field? These are but a few of the questions that whet our interest in convection during northward field conditions. One does not arrive at the state of pure viscous convection immediately after the interplanetary field swings northward. Dungey (1963) was the first of many to argue that a northward magnetosheath field line will reconnect with an open tail lobe field line to create one that is connected to the ionosphere at one end and draped over the dayside magnetopause at the other. The sudden reconfiguration of stress will lead to sunward convection on the newly reconnected field lines. In the ionosphere, this superposes a “reverse” two-cell convection pattern in the central polar cap upon the two “direct” convection cells. If and when the draped reconnected field line finds a partner in the opposite tail lobe with which to reconnect, a newly closed field line will form. Dungey had imagined that the same magnetosheath field line would reconnect simultaneously with both tail lobes, in which case the rate at which open magnetic flux is closed depends upon the rate of tail-lobe reconnection.

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