Sources of electron pitch angle anisotropy in the magnetotail plasma sheet

Walsh, A. P.; Fazakerley, A. N.; Forsyth, C.; Owen, C. J.; Taylor, M. G. G. T.; Rae, I. J.

doi:10.1002/jgra.50553

Cited by 37 publications

(64 citation statements)

References 53 publications

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“…This 'double loss cone' is extremely strong evidence that the plasma observed by Cluster was on closed magnetic field lines. The bidirectional electrons observed beforehand (between 18:15 and 18:35 UT) are also consistent with electrons observed on closed magnetic field lines -in typical magnetotail crossings, bidirectional electrons are observed through much of the outer plasma sheet, with 'double loss cone' distributions deep in the central plasma sheet (21). Ion distributions observed at this time are indicative of the occurrence of magnetotail reconnection tailward of the spacecraft (Fig S1).…”

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confidence: 80%

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Direct observation of closed magnetic flux trapped in the high-latitude magnetosphere

Fear

Milan

Maggiolo

et al. 2014

Science

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115

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Abstract:The structure of Earth's magnetosphere is poorly understood when the interplanetary magnetic field is northward. Under this condition, uncharacteristically energetic plasma is observed in the magnetotail lobes, which is not expected in the textbook model of the magnetosphere. Using satellite observations, we show that these lobe plasma signatures occur on high latitude magnetic field lines that have been closed by the fundamental plasma process of magnetic reconnection. Previous authors have suggested that closed flux can become 'trapped' in the lobe. Their hypothesis holds that this plasma trapping process explains another poorly understood phenomenon: the presence of auroras at extremely high latitudes, called transpolar arcs. Observations of the aurora at the same time as the lobe plasma signatures reveal the presence of a transpolar arc. The excellent correspondence between the transpolar arc and the 'trapped' closed flux at high altitudes provides very strong evidence of the trapping mechanism as the cause of transpolar arcs.One Sentence Summary: Plasma observed in the magnetotail lobes is due to 'trapped' closed magnetic flux, and reveals the process behind the formation of transpolar arcs. Main Text:The night side of the terrestrial magnetosphere forms a structured magnetotail, consisting of a plasma sheet at low latitudes that is sandwiched between two regions called the magnetotail lobes (Fig 1.). The lobes consist of the regions in which the terrestrial magnetic field lines are directly connected to the interplanetary magnetic field (IMF), which is referred to as being topologically 'open' (indicated by the dashed gray lines in Fig. 1). Magnetic field lines threading the plasma sheet (solid gray lines in Fig. 1) are not connected to the IMF, and are therefore 'closed' (1, 2). Topology changes are caused by the process of magnetic ‡ This is the author's version of the work. It is posted here by permission of the AAAS for personal use, not for redistribution. The definitive version was published in Science, volume 346 on 19 th December 2014, DOI:10.1126/science.1257377. reconnection, which drives magnetospheric dynamics when the IMF is southward (1). Different plasma populations are observed in these regions -plasma in the lobes is very cool, whereas the plasma sheet is more energetic. The key way to distinguish between open and closed magnetic field lines is that electron distributions on closed field lines may exhibit a 'double loss cone', in which the distribution peaks perpendicular to the magnetic field (e.g. 3). This requires the presence of magnetic mirrors on both sides of the observation site, therefore double loss cones are unambiguous indicators that the magnetic field lines observed by a spacecraft are closed.A major problem in magnetospheric physics is the adaptation of this picture to times when the IMF is northward. A recent study (4) has reported relatively hot plasma in the lobes, which is unexpected in standard magnetosphere model. The authors attributed the presence of the pla...

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“…However, between 17:00 and 19:00 UT, a much more energetic plasma population was observed (~1 keV electrons and ~10 keV ions -Figs. 2c-d), which is comparable with the mean plasma sheet energy when the IMF is northward (21). The electron and ion energies and temperatures (Figs.…”

mentioning

confidence: 53%

Direct observation of closed magnetic flux trapped in the high-latitude magnetosphere

Fear

Milan

Maggiolo

et al. 2014

Science

Self Cite

115

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“…The k values lower than in earlier studies were recently obtained based on the analysis of Cluster ( k = 2.89 for the single component fits of the observed electron distribution [ Walsh et al , ]) and The Time History of Events and Macroscale Interactions during Substorms (THEMIS) ( k = 2.5–3 for 40 keV electrons during injection events [ Gabrielse et al , ]) data. It may seem that k values lower than 2 can cause a divergence of the integration when we obtain, for example, the omni directional energy flux following Vasyliunas [] (equation (11c)).…”

Section: Modeling Of Low‐energy Electrons With Imptammentioning

confidence: 66%

Nowcast model for low‐energy electrons in the inner magnetosphere

et al. 2015

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“…On the other hand, the field‐aligned and isotropic distributions are dominant at the energy range of 0.3–1 keV. The field‐aligned distribution may be originated from ambient electron distribution in plasma sheet at sub‐keV energies [ Walsh et al ., ]. The isotropic distribution at lower energies may be caused by the electron pitch angle scattering by wave‐particle interactions.…”

Section: Statistical Resultsmentioning

confidence: 99%

A statistical study on the whistler waves behind dipolarization fronts

Zhou

Deng

et al. 2015

JGR Space Physics

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We present a statistical study of whistler waves behind dipolarization fronts (DFs) based on the Cluster satellites measurements during the years [2001][2002][2003][2004][2005][2006][2007]. We find 732 DFs during the 7 year tail periods (X GSM ≤ À8 R E and |Y GSM | ≤ 10 R E ) in the plasma sheet. By constraining the whistler waves in a 1 min interval behind the DFs (the maximum B z ), we find that 381 DFs (about 50%) are followed by whistler waves. We study the occurrence rate of whistler waves, the wave characteristic parameters, and the corresponding electron distribution, not only in a global view but also in the local DF coordinate. In a global view, behind the DFs, the whistler waves mostly occur in the radial distance between 17 and 18 R E . They have a higher occurrence rate on the dawnside than the duskside. On the other hand, in the local DF coordinate, whistler waves have a higher occurrence rate around the meridian of DF. In addition, the average wave amplitudes increase toward the dawnside of DF. Associated with the whistler waves, electron distributions have a dominant perpendicular anisotropy for electrons with energy higher than 5 keV. Lower energy electron distributions do not have such perpendicular anisotropy dominance. Moreover, the perpendicular anisotropy for electrons >5 keV increases toward the dawnside of DF, which may be caused by the drift-betatron acceleration. We suggest that the free energy source for whistler waves behind the DFs is probably the perpendicular anisotropy of >5 keV electrons caused by the betatron acceleration. IntroductionDipolarization front (DF), characterized by a sharp increase of magnetic field component normal to the neutral sheet B z , has been intensively studied in recent years [Nakamura et al., 2002;Runov et al., 2009;Zhou et al., 2009]. Observations by Geotail, Cluster, and THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft have shown that the sharp B z enhancements of the DFs are typically preceded by a minor B z dip, and followed by a decrease of plasma density and plasma pressure [Ohtani, 2004;Runov et al., 2009;Schmid et al., 2011]. These DFs are thin current sheets embedded within the earthward fast plasma flows [Zhou et al., 2009;Sergeev et al., 2009]. As a boundary layer with a typical scale around the ion inertial length or gyroradius, it separates the relatively cold dense ambient plasma from the hot tenuous earthward fast flows [Runov et al., 2011]. Behind the DF, energetic electron flux increases were usually observed [e.g., Zhou et al., 2009;Deng et al., 2010;Khotyaintsev et al., 2011;Ashour-Abdalla et al., 2011;Fu et al., 2011]. The electron acceleration may be attributed to both the adiabatic and nonadiabatic processes [Birn et al., 2012]. The adiabatic process, including the Fermi and betatron acceleration [Apatenkov et al., 2007;Fu et al., 2011], elevates the energy of electrons uniformly, so that the power law index of the electron energy spectrum does not change during this process.The nonadia...

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Sources of electron pitch angle anisotropy in the magnetotail plasma sheet

Cited by 37 publications

References 53 publications

Direct observation of closed magnetic flux trapped in the high-latitude magnetosphere

Direct observation of closed magnetic flux trapped in the high-latitude magnetosphere

Nowcast model for low‐energy electrons in the inner magnetosphere

A statistical study on the whistler waves behind dipolarization fronts

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