The asymmetric geospace as displayed during the geomagnetic
storm on August 17, 2001

Østgaard, Nikolai; Reistad, Jone Peter; Tenfjord, P.; Laundal, K. M.; Rexer, Therese; Haaland, Stein; Snekvik, Krisian; Hesse, M.; Milan, S. E.; Ohma, Anders

doi:10.5194/angeo-2018-65

Cited by 16 publications

(29 citation statements)

References 28 publications

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“…The same could also be seen in Østgaard et al (), where the displacement of the centroid is reduced during expansion phase in the presented events. More recently, Østgaard et al () studied the longitudinal displacement of the conjugate aurora during a geomagnetic storm. Two substorms occurred in the time interval considered, and Østgaard et al () found that the relative displacement between the hemispheres was smallest after each substorm, and build up in the interval between the substorms.…”

Section: Discussionmentioning

confidence: 99%

“…More recently, Østgaard et al () studied the longitudinal displacement of the conjugate aurora during a geomagnetic storm. Two substorms occurred in the time interval considered, and Østgaard et al () found that the relative displacement between the hemispheres was smallest after each substorm, and build up in the interval between the substorms. Statistical support of the reduction is seen in Milan et al (), who did a superposed epoch analysis of substorms using auroral images from the northern hemisphere obtained by IMAGE.…”

Section: Discussionmentioning

confidence: 99%

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Evolution of Asymmetrically Displaced Footpoints During Substorms

et al. 2018

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It is well established that a transverse (y) component in the interplanetary magnetic field (IMF) induces a By component in the closed magnetosphere through asymmetric loading and/or redistribution of magnetic flux. Simultaneous images of the aurora in the two hemispheres have revealed that conjugate auroral features are displaced longitudinally during such conditions. Although the direction and magnitude of this displacement show correlations with IMF clock angle and dipole tilt, single events show large temporal and spatial variability of this displacement. For instance, we know little about how the displacement changes during a substorm. A previous case study demonstrated that displaced auroral forms, associated with the prevailing IMF orientation, returned to a more symmetric configuration during the expansion phase of two substorms. Using the far ultraviolet cameras on board the Imager for Magnetopause‐to‐Aurora Global Exploration and Polar satellites, we have identified multiple events where conjugate auroral images are available during periods with substorm activity and IMF By≠0. We identify conjugate auroral features and investigate how the asymmetry evolves during the expansion phase. We find that the system returns to a more symmetric state in the events with a clear increase in the nightside reconnection rate and that the displacement remains unchanged in the events with little or no net closure of open magnetic flux. The return to a more symmetric state can therefore be interpreted as the result of increased reconnection rate in the magnetotail during the expansion phase, which reduces the asymmetric lobe pressure.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Evolution of Asymmetrically Displaced Footpoints During Substorms

et al. 2018

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“…In an event study, Østgaard, Humberset, et al () showed that the longitudinal displacement of conjugate auroral features were reduced during the expansion phase in two subsequent substorms. In a more recent study, Østgaard et al () found that the asymmetry of conjugate auroral features reached a minimum after substorms occurring during a geomagnetic storm. Ohma et al () presented a multicase study of conjugate images from substorms, showing a reduced displacement in several events and an apparent relation between the reduction of asymmetry and increase in tail reconnection rate.…”

Section: Introductionmentioning

confidence: 96%

Observations of Asymmetric Lobe Convection for Weak and Strong Tail Activity

et al. 2019

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In this study we use high-quality convection data from the Electron Drift Instrument on board Cluster to investigate how near-Earth tail activity affects the average convection pattern in the magnetotail lobes when the interplanetary magnetic field has a dominating east-west (B y ) component. Two different proxies have been used to represent different levels of reconnection in the near-Earth tail: The value of the AL index and the substorm phases identified by the Substorm Onsets and Phases from Indices of the Electrojet algorithm. We find that the convection changes from a dominantly Y GSM direction, but opposite in the two hemispheres, to a flow oriented more toward the plasma sheet, as the north-south component of the convection increases when reconnection enhances in the near Earth tail. This result is consistent with recent observations of the convection in the ionosphere, which suggest that the nightside convection pattern becomes more north-south symmetric when tail reconnection increases. This is also supported by simultaneous auroral observations from the two hemispheres, which shows that conjugate auroral features become more symmetric during substorm expansion phase. The reduced asymmetry implies that the asymmetric pressure balance in the lobes is altered during periods with strong reconnection in the near-Earth tail.

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“…Earlier studies have found strong evidence that lobe reconnection is more efficient in the hemisphere inclined toward the Sun, that is, the local summer hemisphere (Crooker & Rich, ; Frey et al, ; Koustov et al, ; Østgaard et al, ; Wilder et al, ; Yakymenko et al, ), from observing the sunward ionospheric convection velocity in the dayside polar cap during northward IMF. However, most studies have focused on the dependence on the solar wind electric field, E sw = v sw B T , where B T is the transverse component of the IMF,

B_{T} = \sqrt{B_{y}^{2} + B_{z}^{2}}

(Koustov et al, ; Sundberg et al, ; Wilder et al, ; ; Yakymenko et al, ), likely to be the most important controlling factor on the lobe reconnection rate during purely northward IMF.…”

Section: Introductionmentioning

confidence: 99%

Separation and Quantification of Ionospheric Convection Sources: 2. The Dipole Tilt Angle Influence on Reverse Convection Cells During Northward IMF

et al. 2019

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This paper investigates the influence of Earth's dipole tilt angle on the reverse convection cells (sometimes referred to as lobe cells) in the Northern Hemisphere ionosphere during northward IMF, which we relate to high‐latitude reconnection. Super Dual Auroral Radar Network plasma drift observations in 2010–2016 are used to quantify the ionospheric convection. A novel technique based on Spherical Elementary Convection Systems (SECS) that was presented in our companion paper (Reistad et al., 2019, https://doi.org/10.1029/2019JA026634) is used to isolate and quantify the reverse convection cells. We find that the dipole tilt angle has a linear influence on the reverse cell potential. In the Northern Hemisphere the reverse cell potential is typically two times higher in summer than in winter. This change is interpreted as the change in interplanetary magnetic field‐lobe reconnection rate due to the orientation of the dipole tilt. Hence, the dipole tilt influence on reverse ionospheric convection can be a significant modification of the more known influence from vswBz. These results could be adopted by the scientific community as key input parameters for lobe reconnection coupling functions.

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The asymmetric geospace as displayed during the geomagnetic storm on August 17, 2001

Abstract: Abstract.Previous studies have shown that conjugate auroral features are displaced in the two hemispheres when the interplanetary Increased tail reconnection during substorm expansion phase reduces the asymmetry.15 Copyright statement.

Cited by 16 publications

References 28 publications

Evolution of Asymmetrically Displaced Footpoints During Substorms

Evolution of Asymmetrically Displaced Footpoints During Substorms

Observations of Asymmetric Lobe Convection for Weak and Strong Tail Activity

Separation and Quantification of Ionospheric Convection Sources: 2. The Dipole Tilt Angle Influence on Reverse Convection Cells During Northward IMF

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