Simulation of substorm-time acceleration of oxygen ions on azimuthally directed magnetic field lines in the near-Earth plasma sheet

Abstract. In this paper, we present an interpretation of the observed field-aligned acceleration events measured by GEOS-2 near the night-side synchronous orbit at substorm onsets (Chen et al., 2000). We show that field-aligned acceleration of ions (with pitch angle asymmetry) is closely related to strong short-lived electric fields in the Ey direction. The acceleration is associated with either rapid dipolarization or further stretching of local magnetic field lines. Theoretical analysis suggests that a centrifugal mechanism is a likely candidate for the parallel energization. Equatorward or anti-equatorward energization occurs when the tail current sheet is thinner tailward or earthward of the spacecraft, respectively. The magnetic field topology leading to anti-equatorward energization corresponds to a situation where the near-Earth tail undergoes further compression and the inner edge of the plasma sheet extends inwards as close as the night-side geosynchronous altitudes.Key words. Magnetospheric physics (magnetospheric configuration and dynamics; plasma sheet; storms and sub-storms)

Download Full-text

A physical mechanism producing suprathermal populations and initiating substorms in the Earth's magnetotail

Annales Geophysicae ◽

10.5194/angeo-26-1617-2008 ◽

2008 ◽

Vol 26 (6) ◽

pp. 1617-1639 ◽

Cited By ~ 10

Author(s):

D. V. Sarafopoulos

Keyword(s):

Magnetic Field ◽

Electric Fields ◽

Plasma Sheet ◽

Physical Mechanism ◽

Energetic Particles ◽

Dimensional Structure ◽

Magnetospheric Substorms ◽

Physical Description ◽

Fundamental Building Block ◽

Field Lines

Abstract. We suggest a candidate physical mechanism, combining there dimensional structure and temporal development, which is potentially able to produce suprathermal populations and cross-tail current disruptions in the Earth's plasma sheet. At the core of the proposed process is the "akis" structure; in a thin current sheet (TCS) the stretched (tail-like) magnetic field lines locally terminate into a sharp tip around the tail midplane. At this sharp tip of the TCS, ions become non-adiabatic, while a percentage of electrons are accumulated and trapped: The strong and transient electrostatic electric fields established along the magnetic field lines produce suprathermal populations. In parallel, the tip structure is associated with field aligned and mutually attracted parallel filamentary currents which progressively become more intense and inevitably the structure collapses, and so does the local TCS. The mechanism is observationally based on elementary, almost autonomous and spatiotemporal entities that correspond each to a local thinning/dipolarization pair having duration of ~1 min. Energetic proton and electron populations do not occur simultaneously, and we infer that they are separately accelerated at local thinnings and dipolarizations, respectively. In one example energetic particles are accelerated without any dB/dt variation and before the substorm expansion phase onset. A particular effort is undertaken demonstrating that the proposed acceleration mechanism may explain the plasma sheet ratio Ti/Te≈7. All our inferences are checked by the highest resolution datasets obtained by the Geotail Energetic Particles and Ion Composition (EPIC) instrument. The energetic particles are used as the best diagnostics for the accelerating source. Near Earth (X≈10 RE) selected events support our basic concept. The proposed mechanism seems to reveal a fundamental building block of the substorm phenomenon and may be the basic process/structure, which is now missing, that might help explain the persistent, outstanding deficiencies in our physical description of magnetospheric substorms. The mechanism is tested, checked, and found consistent with substorm associated observations performed ~30 and 60 RE away from Earth.

Download Full-text

Strong Southward and Northward Currents Observed in the Inner Plasma Sheet

10.5194/angeo-2019-56 ◽

2019 ◽

Author(s):

Yanyan Yang ◽

Chao Shen ◽

Yong Ji

Keyword(s):

Magnetic Field ◽

Plasma Sheet ◽

Ring Current ◽

Generation Mechanism ◽

Storm Events ◽

Current Generation ◽

Inner Magnetosphere ◽

Latitude Region ◽

Field Lines ◽

The Magnetic Field

Abstract. It is generally believed that field aligned currents (FACs) and the ring current (RC) are two dominant parts of the inner magnetosphere. However, using the Cluster spacecraft crossing of the pre-midnight inner plasma sheet in the latitude region between 10° N and 30° N, it is found that, during large storm events, in addition to FACs and the RC, there also exist strong southward and northward currents, which cannot be FACs, because the magnetic field in these regions is mainly along the XY plane. Detailed investigation shows that both magnetic field lines (MFLs) and currents in these regions highly fluctuate. When the curvature of MFLs changes direction in the XY plane, the current also alternatively switches between southward and northward. Further analysis of the current generation mechanism indicates that the most reasonable candidate for the origin of these southward and northward currents is the curvature drift of energetic particles.

Download Full-text

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.

Download Full-text

Double discontinuities at the magnetotail plasma sheet-lobe boundary

Annales Geophysicae ◽

10.5194/angeo-19-1095-2001 ◽

2001 ◽

Vol 19 (9) ◽

pp. 1095-1105 ◽

Cited By ~ 4

Author(s):

Y. C. Whang ◽

D. Fairfield ◽

R. P. Lepping ◽

T. Mukai ◽

Y. Saito ◽

...

Keyword(s):

Magnetic Field ◽

Plasma Density ◽

Field Intensity ◽

Plasma Sheet ◽

Shock Layer ◽

Magnetic Field Intensity ◽

Field Energy ◽

Slow Shock ◽

Field Lines ◽

The Magnetic Field

Abstract. A double discontinuity is a compound structure composed of a slow shock layer and an adjoining rotational discontinuity layer on the postshock side. We use high-resolution data from Geotail and Wind spacecraft to examine the interior structure within the finite thickness of the discontinuity at the plasma sheet-lobe boundary and found that recognizable MHD structures at the boundary can be stand-alone slow shocks or double discontinuities. The plasma density increases significantly and the magnetic field intensity decreases significantly across the interior of the slow shock layer. Through the rotational layer, the magnetic field rotates about the normal direction of the shock surface, as the plasma density and the magnetic field intensity remain nearly unchanged. The rotational angle can vary over a wide range. We notice that the observations of double discontinuities are no less frequent than the observations of stand-alone slow shocks. Identification of slow shocks and double discontinuities infers that plasma and magnetic field lines continuously move across the boundary surface from the lobe into the plasma sheet, and there is a conversion of magnetic field energy into plasma thermal energy through the slow shock layer. The double discontinuities also allows for a rapid rotation of the postshock magnetic field lines immediately behind the shock layer to accommodate the environment of the MHD flow in the plasma sheet region.Key words. Magnetospheric physics (plasma sheet) Space plasma physics (discontinuities; shock waves)

Download Full-text

A possible mechanism for <theta> aurora formation

Annales Geophysicae ◽

10.1007/s00585-995-0698-3 ◽

1995 ◽

Vol 13 (7) ◽

pp. 698-703 ◽

Cited By ~ 4

Author(s):

B. V. Rezhenov ◽

I. M. Vardavas

Keyword(s):

Magnetic Field ◽

Boundary Layer ◽

Plasma Sheet ◽

Energy Spectra ◽

System Of Equations ◽

The Sun ◽

High Latitudes ◽

Plasma Sheet Boundary Layer ◽

Interchange Instability ◽

Field Lines

Abstract. A mechanism for the formation of <theta> aurora connected with the development of an interchange instability on the plasma sheet boundary layer (PSBL) is suggested. The PSBL is assumed to be deep inside the region of closed magnetic field lines. A system of equations connecting currents in the ionosphere and magnetosphere is solved numerically. It is found, using realistic ionospheric and magnetospheric parameters, that in a period of 8–10 min a system of plasma bars directed to the Sun arises at high latitudes. The system of bars is about 1000 km in width and 3000 km in length and approximates the Θ aurora. The suggested mechanism allows an explanation of a number of Θ aurora features such as the appearance probability, electric field directions, energy spectra of precipitating particles, and its location.

Download Full-text

Tail reconnection in the global magnetospheric context: Vlasiator first results

Annales Geophysicae ◽

10.5194/angeo-35-1269-2017 ◽

2017 ◽

Vol 35 (6) ◽

pp. 1269-1274 ◽

Cited By ~ 13

Author(s):

Minna Palmroth ◽

Sanni Hoilijoki ◽

Liisa Juusola ◽

Tuija I. Pulkkinen ◽

Heli Hietala ◽

...

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Numerical Experiment ◽

Plasma Sheet ◽

Three Dimensional ◽

Full Description ◽

Meridional Plane ◽

First Results ◽

Wind Input ◽

Field Lines

Abstract. The key dynamics of the magnetotail have been researched for decades and have been associated with either three-dimensional (3-D) plasma instabilities and/or magnetic reconnection. We apply a global hybrid-Vlasov code, Vlasiator, to simulate reconnection self-consistently in the ion kinetic scales in the noon–midnight meridional plane, including both dayside and nightside reconnection regions within the same simulation box. Our simulation represents a numerical experiment, which turns off the 3-D instabilities but models ion-scale reconnection physically accurately in 2-D. We demonstrate that many known tail dynamics are present in the simulation without a full description of 3-D instabilities or without the detailed description of the electrons. While multiple reconnection sites can coexist in the plasma sheet, one reconnection point can start a global reconfiguration process, in which magnetic field lines become detached and a plasmoid is released. As the simulation run features temporally steady solar wind input, this global reconfiguration is not associated with sudden changes in the solar wind. Further, we show that lobe density variations originating from dayside reconnection may play an important role in stabilising tail reconnection.

Download Full-text

Strong southward and northward currents observed in the inner plasma sheet

Annales Geophysicae ◽

10.5194/angeo-37-931-2019 ◽

2019 ◽

Vol 37 (5) ◽

pp. 931-941

Author(s):

Yan-Yan Yang ◽

Chao Shen ◽

Yong Ji

Keyword(s):

Magnetic Field ◽

Plasma Sheet ◽

Geomagnetic Storms ◽

Energetic Particles ◽

Inner Magnetosphere ◽

Function Analyzer ◽

Cluster Ion ◽

Energetic Particle Flux ◽

Field Lines ◽

The Magnetic Field

Abstract. It is generally believed that field-aligned currents (FACs) and the ring current (RC) are two dominant parts of the inner magnetosphere. However, using the Cluster spacecraft crossing the pre-midnight inner plasma sheet in the latitudinal region between 10 and 30∘ N, it is found that, during intense geomagnetic storms, in addition to FACs and the RC, strong southward and northward currents also exist which should not be FACs because the magnetic field in these regions is mainly along the x–y plane. Detailed investigation shows that both magnetic-field lines (MFLs) and currents in these regions are highly dynamic. When the curvature of MFLs changes direction in the x–y plane, the current also alternatively switches between being southward and northward. To investigate the generation mechanism of the southward and northward current, we employed the analysis of energetic particle flux up to 1 MeV. For energetic particles below 40 keV, observations from Cluster CIS/CODIF (Cluster Ion Spectrometry COmposition and DIstribution Function analyzer) are used. However, for higher-energy particles, the flux is obtained by extrapolations of low-energy particle data through Kappa distribution. The result indicates that the most reasonable cause of these southward and northward currents is the curvature drift of energetic particles.

Download Full-text

Escape of high-energy oxygen ions through magnetopause reconnection under northward IMF

Annales Geophysicae ◽

10.5194/angeo-26-3955-2008 ◽

2008 ◽

Vol 26 (12) ◽

pp. 3955-3966 ◽

Cited By ~ 10

Author(s):

S. Kasahara ◽

H. Hasegawa ◽

K. Keika ◽

Y. Miyashita ◽

M. N. Nishino ◽

...

Keyword(s):

Magnetic Field ◽

Magnetic Reconnection ◽

Recovery Phase ◽

High Energy ◽

Larmor Radius ◽

Closed Field ◽

Escape Route ◽

Finite Larmor Radius ◽

Oxygen Ions ◽

Field Lines

Abstract. During a storm recovery phase on 15 May 2005, the Geotail spacecraft repeatedly observed high-energy (>180 keV) oxygen ions in the dayside magnetosheath near the equatorial plane. We focused on the time period from 11:20 UT to 13:00 UT, when Geotail observed the oxygen ions and the interplanetary magnetic field (IMF) was constantly northward. The magnetic reconnection occurrence northward and duskward of Geotail is indicated by the Walén analysis and convective flows in the magnetopause boundary layer. Anisotropic pitch angle distributions of ions suggest that high-energy oxygen ions escaped from the northward of Geotail along the reconnected magnetic field lines. From the low-energy particle precipitation in the polar cap observed by DMSP, which is consistent with magnetic reconnection occurring between the magnetosheath field lines and the magnetospheric closed field lines, we conclude that these oxygen ions are of ring current origin. Our results thus suggest a new escape route of oxygen ions during northward IMF. In the present event, this escape mechanism is more dominant than the leakage via the finite Larmor radius effect across the dayside equatorial magnetopause.

Download Full-text