Dual-lobe reconnection and horse-collar auroras

<p>We propose a mechanism for the formation of the horse-collar auroral configuration common during periods of strongly northwards interplanetary magnetic field, invoking the action of dual-lobe reconnection (DLR). &#160;Auroral observations are provided by the Imager for Magnetopause-to-Auroras Global Exploration (IMAGE) satellite and spacecraft of the Defense Meteorological Satellite Program (DMSP). &#160;We also use ionospheric flow measurements from DMSP and polar maps of field-aligned currents (FACs) derived from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). &#160;Sunward convection is observed within the dark polar cap, with antisunwards flows within the horse-collar auroral region, together with the NBZ FAC distribution expected to be associated with DLR. &#160;We suggest that newly-closed flux is transported antisunwards and to dawn and dusk within the reverse lobe cell convection pattern associated with DLR, causing the polar cap to acquire a teardrop shape and weak auroras to form at high latitudes. &#160;Horse-collar auroras are a common feature of the quiet magnetosphere, and this model provides a first understanding of their formation, resolving several outstanding questions regarding the nature of DLR and the magnetospheric structure and dynamics during northwards IMF. &#160;The model can also provide insights into the trapping of solar wind plasma by the magnetosphere and the formation of a low-latitude boundary layer and cold, dense plasma sheet. &#160;We speculate that prolonged DLR could lead to a fully closed magnetosphere, with the formation of horse-collar auroras being an intermediate step.</p>

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Ionospheric signatures of the low-latitude boundary layer under conditions of northward IMF and small clock angle

Annales Geophysicae ◽

10.5194/angeo-24-2169-2006 ◽

2006 ◽

Vol 24 (8) ◽

pp. 2169-2178 ◽

Cited By ~ 2

Author(s):

S. E. Pryse ◽

R. W. Sims ◽

J. Moen ◽

K. Oksavik

Keyword(s):

Boundary Layer ◽

Particle Energy ◽

Polar Cap ◽

Low Latitude ◽

Central Plasma ◽

High Latitude Ionosphere ◽

Field Lines ◽

Lobe Reconnection ◽

Low Latitude Boundary Layer

Abstract. A case study is presented that concerns the footprints of the low-latitude boundary layer in the high-latitude ionosphere. The measurements were made near local magnetic noon in summertime under conditions of Bz>0 and small clock angle. Of particular interest are particle fluxes measured in the region by the NOAA-12 satellite that revealed energetic (>30 keV) electrons, characteristic of trapped particles, together with a population of softer precipitating magnetosheath particles. The particle energy-distribution was distinct from those identifying the central plasma sheet at lower latitudes. On its poleward side the layer extended to at least the latitude of the polar cap boundary as identified in ion flows and electron densities measured by the EISCAT Svalbard radar. It is proposed that the particles of the low-latitude boundary layer occurred on newly-closed magnetic field lines, which were formed by the closure of open polar cap field by lobe reconnection in both Northern and Southern Hemispheres.

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AMPERE polar cap boundaries

Annales Geophysicae ◽

10.5194/angeo-38-481-2020 ◽

2020 ◽

Vol 38 (2) ◽

pp. 481-490 ◽

Cited By ~ 1

Author(s):

Angeline G. Burrell ◽

Gareth Chisham ◽

Stephen E. Milan ◽

Liam Kilcommons ◽

Yun-Ju Chen ◽

...

Keyword(s):

Auroral Oval ◽

Neutral Atmosphere ◽

Polar Cap ◽

Data Set ◽

Defense Meteorological Satellite Program ◽

Perturbation Data ◽

Statistical Studies ◽

Coupling Processes ◽

Meteorological Satellite

Abstract. The high-latitude atmosphere is a dynamic region with processes that respond to forcing from the Sun, magnetosphere, neutral atmosphere, and ionosphere. Historically, the dominance of magnetosphere–ionosphere interactions has motivated upper atmospheric studies to use magnetic coordinates when examining magnetosphere–ionosphere–thermosphere coupling processes. However, there are significant differences between the dominant interactions within the polar cap, auroral oval, and equatorward of the auroral oval. Organising data relative to these boundaries has been shown to improve climatological and statistical studies, but the process of doing so is complicated by the shifting nature of the auroral oval and the difficulty in measuring its poleward and equatorward boundaries. This study presents a new set of open–closed magnetic field line boundaries (OCBs) obtained from Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) magnetic perturbation data. AMPERE observations of field-aligned currents (FACs) are used to determine the location of the boundary between the Region 1 (R1) and Region 2 (R2) FAC systems. This current boundary is thought to typically lie a few degrees equatorward of the OCB, making it a good candidate for obtaining OCB locations. The AMPERE R1–R2 boundaries are compared to the Defense Meteorological Satellite Program Special Sensor J (DMSP SSJ) electron energy flux boundaries to test this hypothesis and determine the best estimate of the systematic offset between the R1–R2 boundary and the OCB as a function of magnetic local time. These calibrated boundaries, as well as OCBs obtained from the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) observations, are validated using simultaneous observations of the convection reversal boundary measured by DMSP. The validation shows that the OCBs from IMAGE and AMPERE may be used together in statistical studies, providing the basis of a long-term data set that can be used to separate observations originating inside and outside of the polar cap.

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Investigation of the outer and inner low-latitude boundary layers

Annales Geophysicae ◽

10.5194/angeo-19-1065-2001 ◽

2001 ◽

Vol 19 (9) ◽

pp. 1065-1088 ◽

Cited By ~ 18

Author(s):

T. M. Bauer ◽

R. A. Treumann ◽

W. Baumjohann

Keyword(s):

Boundary Layer ◽

Solar Wind ◽

Tangential Stress ◽

Solar Wind Plasma ◽

Alfven Wave ◽

Bulk Velocity ◽

Closed Field ◽

Low Latitude ◽

Field Lines ◽

Low Latitude Boundary Layer

Abstract. We analyze 22 AMPTE/IRM crossings of the day-side low-latitude boundary layer for which a dense outer part can be distinguished from a dilute inner part. Whereas the plasma in the outer boundary layer (OBL) is dominated by solar wind particles, the partial densities of solar wind and magnetospheric particles are comparable in the inner boundary layer (IBL). For 11 events we find a reasonable agreement between observed plasma flows and those predicted by the tangential stress balance of an open magnetopause. Thus, we conclude that, at least in these cases, the OBL is formed by a local magnetic reconnection. The disagreement with the tangential stress balance in the other 11 cases might be due to reconnection being time-dependent and patchy. The north-south component of the proton bulk velocity in the boundary layer is, on average, directed toward high latitudes for both low and high magnetic shear across the magnetopause. This argues clearly against the possibility that the dayside low-latitude boundary layer is populated with solar wind plasma primarily from the cusps. "Warm", counterstreaming electrons that originate primarily from the magnetosheath and have a field-aligned temperature that is higher than the electron temperature in the magnetosheath by a factor of 1–5, are a characteristic feature of the IBL. Profiles of the proton bulk velocity and the density of hot ring current electrons provide evidence that the IBL is on closed field lines. Part of the IBL may be on newly opened field lines. Using the average spectra of electric and magnetic fluctuations in the boundary layer, we estimate the diffusion caused by lower hybrid drift instability, gyroresonant pitch angle scattering, or kinetic Alfvén wave turbulence. We find that cross-field diffusion cannot transport solar wind plasma into the OBL or IBL at a rate that would account for the thickness ( ~ 1000 km) of these sublayers. On the duskside, the dawn-dusk component of the proton bulk velocity in the IBL and magnetosphere is, on average, directed from the nightside toward local noon. Formation of the IBL may also be due to mechanisms operating in the magnetotail.Key words. Magnetospheric physics (magnetopause, cusp and boundary layer; magnetospheath)

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The dependence of cusp ion signatures on the reconnection rate

Annales Geophysicae ◽

10.5194/angeo-21-947-2003 ◽

2003 ◽

Vol 21 (4) ◽

pp. 947-953 ◽

Cited By ~ 2

Author(s):

S. K. Morley ◽

M. Lockwood

Keyword(s):

Temporal Variations ◽

Closed Field ◽

Polar Cap ◽

Reconnection Rate ◽

Physical Conditions ◽

Defense Meteorological Satellite Program ◽

Closed Field Line ◽

Temporal And Spatial ◽

Meteorological Satellite ◽

F 14

Abstract. The interpretation of structure in cusp ion dispersions is important for helping to understand the temporal and spatial structure of magnetopause reconnection. "Stepped" and "sawtooth" signatures have been shown to be caused by temporal variations in the reconnection rate under the same physical conditions for different satellite trajectories. The present paper shows that even for a single satellite path, a change in the amplitude of any reconnection pulses can alter the observed signature and even turn sawtooth into stepped forms and vice versa. On 20 August 1998, the Defense Meteorological Satellite Program (DMSP) craft F-14 crossed the cusp just to the south of Longyearbyen, returning on the following orbit. The two passes by the DMSP F-14 satellites have very similar trajectories and the open-closed field line boundary (OCB) crossings, as estimated from the SSJ/4 precipitating particle data and Polar UVI images, imply a similarly-shaped polar cap, yet the cusp ion dispersion signatures differ substantially. The cusp crossing at 08:54 UT displays a stepped ion dispersion previously considered to be typical of a meridional pass, whereas the crossing at 10:38 UT is a sawtooth form ion dispersion, previously considered typical of a satellite travelling longitudinally with respect to the OCB. It is shown that this change in dispersed ion signature is likely to be due to a change in the amplitude of the pulses in the reconnection rate, causing the stepped signature. Modelling of the low-energy ion cutoff under different conditions has reproduced the forms of signature observed.Key words. Ionosphere (particle precipitation) Magnetospheric physics (energetic particles, precipitating, magnetopause, cusp and boundary layers)

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Ion and neutral temperature distributions in the E-region observed by the EISCAT Tromsø and Svalbard radars

Annales Geophysicae ◽

10.5194/angeo-20-1415-2002 ◽

2002 ◽

Vol 20 (9) ◽

pp. 1415-1427 ◽

Cited By ~ 7

Author(s):

S. Maeda ◽

S. Nozawa ◽

M. Sugino ◽

H. Fujiwara ◽

M. Suzuki

Keyword(s):

Heat Transport ◽

Frictional Heating ◽

Soft Particle ◽

Ion Temperature ◽

Polar Cap ◽

Low Latitude ◽

Neutral Temperature ◽

Defense Meteorological Satellite Program ◽

E Region ◽

Uhf Radar

Abstract. Simultaneous Common Program Two experiments by the EISCAT UHF radar at Tromsø and the EISCAT Svalbard radar at Longyearbyen from 00:00 to 15:00 UT on 22 September 1998 and 9 March 1999 have been utilized to investigate distributions of the ion and neutral temperatures in the E-region between 105 and 115 km. During the experiments, soft particle precipitations in the dayside cusp were observed over the Svalbard radar site by the Defense Meteorological Satellite Program (DMSP) F11 satellite. It is found that the dayside electric field in the regions of the low-latitude boundary of the polar cap and the cusp was greater and more variable than that in the auroral region. The ion temperature, parallel to the geomagnetic field at Longyearbyen, was higher than that at Tromsø during the daytime from 06:00 to 12:00 UT. The steady-state ion energy equation has been applied to derive neutral temperature under the assumption of no significant heat transport and viscous heating. The estimated neutral temperature at Longyearbyen was also higher than that at Tromsø. The ion and neutral energy budget was discussed in terms of the ion frictional heating and the Joule heating. The results indicate two possibilities: either the neutral temperature was high in the low latitude boundary of the polar cap and the cusp, or the heat transport by the polar cap neutral winds toward the dayside sector was significant.Key words. Ionosphere (auroral ionosphere; ionosphere–atmosphere interactions; polar ionosphere)

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High spatial and temporal resolution observations of the ionospheric cusp

Annales Geophysicae ◽

10.1007/s00585-995-0919-9 ◽

1995 ◽

Vol 13 (9) ◽

pp. 919-925 ◽

Cited By ~ 65

Author(s):

M. Pinnock ◽

A. S. Rodger ◽

J. R. Dudeney ◽

F. Rich ◽

K. B. Baker

Keyword(s):

Boundary Layer ◽

Short Duration ◽

Conclusive Evidence ◽

Hf Radar ◽

Polar Cap ◽

Low Latitude ◽

Flux Transfer Events ◽

Flux Transfer ◽

Very High ◽

Low Latitude Boundary Layer

Abstract. The Halley PACE HF radar has been operated in a new mode to provide very high time (10 s) and space (15 km) resolution measurements of the iono-spheric signatures of the cusp and the low-latitude boundary layer. The first data show that the iono-spheric signature of flux transfer events occur up to 300 km equatorward of regions showing the HF characteristics of the ionospheric cusp. Whilst larger flux transfer events are seen, on average, every 7 min, many much smaller and short-duration events have been identified. On one occasion DMSP data have been used to show that at least four flux transfer events are occurring simultaneously at the edge of the cusp over 2 h of MLT. There is strong, but not conclusive evidence, that reconnection at the magnetopause is both intermittent and patchy. These data also suggest that flux transfer events can be a significant contributor to the cross-polar cap potential.

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