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- Aikio, A. T., et al.
(författare)
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EISCAT and Cluster observations in the vicinity of the dynamical polar cap boundary
- 2008
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 26:1, s. 87-105
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics of the polar cap boundary and auroral oval in the nightside ionosphere are studied during late expansion and recovery of a substorm from the region between Tromso (66.6 degrees cgmLat) and Longyearbyen (75.2 degrees cgmLat) on 27 February 2004 by using the coordinated EISCAT incoherent scatter radar, MIRACLE magnetometer and Cluster satellite measurements. During the late substorm expansion/early recovery phase, the polar cap boundary (PCB) made zig-zag-type motion with amplitude of 2.5 degrees cgmLat and period of about 30 min near magnetic midnight. We suggest that the poleward motions of the PCB were produced by bursts of enhanced reconnection at the near-Earth neutral line (NENL). The subsequent equatorward motions of the PCB would then represent the recovery of the merging line towards the equilibrium state (Cowley and Lockwood, 1992). The observed bursts of enhanced westward electrojet just equatorward of the polar cap boundary during poleward expansions were produced plausibly by particles accelerated in the vicinity of the neutral line and thus lend evidence to the Cowley-Lockwood paradigm. During the substorm recovery phase, the footpoints of the Cluster satellites at a geocentric distance of 4.4 R-E mapped in the vicinity of EISCAT measurements. Cluster data indicate that outflow of H+ and O+ ions took place within the plasma sheet boundary layer (PSBL) as noted in some earlier studies as well. We show that in this case the PSBL corresponded to a region of enhanced electron temperature in the ionospheric F region. It is suggested that the ion outflow originates from the F region as a result of increased ambipolar diffusion. At higher altitudes, the ions could be further energized by waves, which at Cluster altitudes were observed as BBELF (broad band extra low frequency) fluctuations. The four-satellite configuration of Cluster revealed a sudden poleward expansion of the PSBL by 2 degrees during similar to 5 min. The beginning of the poleward motion of the PCB was associated with an intensification of the downward FAC at the boundary. We suggest that the downward FAC sheet at the PCB is the high-altitude counterpart of the Earthward flowing FAC produced in the vicinity of the magnetotail neutral line by the Hall effect (Sonnerup, 1979) during a short-lived reconnection pulse.
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| 4. |
- Aikio, Anita T., et al.
(författare)
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IMF effect on the polar cap contraction and expansion during a period of substorms
- 2013
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 31, s. 1021-1034
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Tidskriftsartikel (refereegranskat)abstract
- The polar cap boundary (PCB) location and motion in the nightside ionosphere has been studied by using measurements from the EISCAT radars and the MIRACLE magnetometers during a period of four substorms on 18 February 2004. The OMNI database has been used for observations of the solar wind and the Geotail satellite for magnetospheric measurements. In addition, the event was modelled by the GUMICS-4 MHD simulation. The simulation of the PCB location was in a rather good agreement with the experimental estimates at the EISCAT longitude. During the first three substorm expansion phases, neither the local observations nor the global simulation showed any poleward motions of the PCB, even though the electrojets intensified. Rapid poleward motions of the PCB took place only in the early recovery phases of the substorms. Hence, in these cases the nightside reconnection rate was locally higher in the recovery phase than in the expansion phase.In addition, we suggest that the IMF Bz component correlated with the nightside tail inclination angle and the PCB location with about a 17-min delay from the bow shock. By taking the delay into account, the IMF northward turnings were associated with dipolarizations of the magnetotail and poleward motions of the PCB in the recovery phase. The mechanism behind this effect should be studied further.
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| 5. |
- Chong, Ghai Siung, et al.
(författare)
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Dawn-Dusk Ion Flow Asymmetry in the Plasma Sheet : Interplanetary Magnetic Field B-y Versus Distance With Respect to the Neutral Sheet
- 2022
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 127:4
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Tidskriftsartikel (refereegranskat)abstract
- Previous studies have shown that the average dawn-dusk component of the perpendicular plasma flow in the plasma sheet (V-perpendicular to) can vary depending on the distance relative to the neutral sheet and the dawn-dusk component of the interplanetary magnetic field (IMF B-y). In this study, we combined 33 years of data from the Geotail, Time History of Events and Macroscale Interactions during Substorms, Cluster, and magnetospheric multiscale missions to study the slow (<200 km/s) ion flows perpendicular to the magnetic field. We find that IMF B-y has a hemispheric dependent influence on both the tail B-y and tail V-perpendicular to. Particularly, the influence is more prominent in the midnight sector (compared to both the pre- and post-midnight sectors) and at distances far from the neutral sheet (compared to the distances close to the neutral sheet). However, at distances close to the neutral sheet, there is an increased dominance of duskward flows which dominates over the systematic influence of IMF B-y on tail V-perpendicular to. Our results indicate that IMF B-y has a major influence on the magnetic flux transport in the magnetotail, mainly at distances far from the neutral sheet. The influence is weaker at distances close to the neutral sheet.
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| 6. |
- Chong, Ghai Siung, et al.
(författare)
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Ion Convection as a Function of Distance to the Neutral Sheet in Earth's Magnetotail
- 2021
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Ingår i: Journal of Geophysical Research - Space Physics. - : John Wiley & Sons. - 2169-9380 .- 2169-9402. ; 126:12
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Tidskriftsartikel (refereegranskat)abstract
- We utilized 33 years of data obtained by the Geotail, THEMIS, Cluster and MMS missions to investigate the slow (<200 km/s) ion flows perpendicular to the magnetic field in Earth's magnetotail plasma sheet. By using plasma β as a proxy of distance to the neutral sheet, we find that the ion flow patterns vary systematically within the plasma sheet. Particularly, in regions farther from the neutral sheet, earthward (tailward) flows exhibit a strong tendency to diverge (converge) quasi-symmetrically, with respect to the midnight meridional plane. As the distance becomes closer toward the neutral sheet, this tendency to diverge and converge gradually weakens. Moreover, duskward flows become the dominant components in both the earthward and tailward flows. These variations in ion flow patterns with distance to neutral sheet are hemispherically independent. We suggest that the spatial profiles of the electric and diamagnetic drift vary with distance to the neutral sheet and are therefore responsible for the varying ion flow patterns.
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| 7. |
- Chong, Ghai Siung, et al.
(författare)
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Tailward Flows in the Vicinity of Fast Earthward Flows
- 2021
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Ingår i: Journal of Geophysical Research - Space Physics. - : Blackwell Publishing. - 2169-9380 .- 2169-9402. ; 126:4
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Tidskriftsartikel (refereegranskat)abstract
- The occurrence of tailward flows in the magnetotail plasma sheet is closely linked to the dynamics of earthward bursty bulk flows (BBFs). Tailward flows that are observed in the vicinity of these BBFs (or TWABs – Tailward flows around BBFs) may hold unique information on its origin. In this study, we conduct a statistical survey on TWABs by using data from the Cluster mission. We find that TWABs are observed in the vicinity of ∼75% of the BBFs and their occurrence does not depend on BBF velocity magnitude. TWABs have a flow convection pattern consistent with the general tailward flows (GTWs) in the plasma sheet and they do not resemble vortical-like flows. However, TWABs have a flow velocity magnitude twice larger than the GTWs. The plasma density and temperature of TWABs are comparable with BBFs. It is more common to observe a TWAB succeeding than preceding a BBF. However, there is no distinctive difference (in flow pattern, plasma density and temperature) between preceding and succeeding TWABs. We suggest that TWABs are likely the “freshly” rebounded BBFs from the near-Earth region where the magnetic field is stronger. TWABs may represent the early stage of the evolution of tailward flows in the plasma sheet. We also discuss and argue that other mechanisms such as shear-induced vortical flows and tailward slipping of depleted flux tubes cannot be the principal causes of TWABs.
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| 10. |
- De Spiegeleer, Alexandre, et al.
(författare)
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In Which Magnetotail Hemisphere is a Satellite? Problems Using in Situ Magnetic Field Data
- 2021
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Ingår i: Journal of Geophysical Research - Space Physics. - : Blackwell Publishing. - 2169-9380 .- 2169-9402. ; 126:2
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Tidskriftsartikel (refereegranskat)abstract
- In Earth's magnetotail plasma sheet, the sunward-tailward Bx component of the magnetic field is often used to separate the region above and below the cross-tail current sheet. Using a three-dimensional magneto-hydrodynamic simulation, we show that high-speed flows do not only affect the north-south magnetic field component (causing dipolarization fronts), but also the sunward-tailward component via the formation of a magnetic dent. This dent is such that, in the Northern Hemisphere, the magnetic field is tailward while in the Southern Hemisphere, it is earthward. This is opposite to the expected signatures where Bx > 0 (Bx < 0) above (below) the neutral sheet. Therefore, the direction of the magnetic field cannot always be used to identify in which hemisphere an in situ spacecraft is located. In addition, the cross-tail currents associated with the dent is different from the currents in a tail without a dent. From the simulation, we suggest that the observation of a dawnward current and a tailward magnetic tension force, possibly together with an increase in the plasma beta, may indicate the presence of a magnetic dent. To exemplify, we also present data of a high-speed flow observed by the Cluster mission, and we show that the changing sign of Bx is likely due to such a dent, and not to the spacecraft moving across the neutral sheet.
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