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

Abstract

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...

Multi‐instrument observation of simultaneous polar cap auroras on open and closed magnetic field lines

“…As more and more flux is closed in this magnetic local time (MLT) sector, the trapped field lines will protrude deeper into the magnetotail lobes. This mechanism has been validated by several different case studies [e.g., Goudarzi et al , ; Fear et al , ]. Furthermore, the main predictions from the mechanism, i.e., a delay in the MLT dependence on the IMF B y and an association with TRINNI flows, have been statistically verified by Fear and Milan [, ].…”

“…These features are more consistent with the DMSP precipitation associated with the dawnside arcs in this study. Fear et al [] also presented observation from Cluster that showed that the plasma source for this arc was on closed field lines. Similarly, a study by Mailyan et al [] presented observations of a transpolar arc (observed by IMAGE and the TIMED/GUVI instrument, which is very similar to SSUSI) and a conjugate in situ plasma signature detected by Cluster.…”

“…In this study the Lyman‐Birge Hopfield long (LBHL) band, between 165 and 180 nm, is used. For comparison with observations of polar cap aurora by the IMAGE satellite [e.g., Milan et al , ; Fear and Milan , , ; Fear et al , , ], these wavelengths are within the range of the IMAGE wideband imaging (WIC) camera.…”

Dayside reconnection under interplanetary magnetic field By‐dominated conditions: The formation and movement of bending arcs

“…As lobe reconnection is distinct in each hemisphere, the motion of the TPA in one hemisphere is anticipated to be independent from the motion of the TPA in the opposite hemisphere. A recent case study of a TPA [ Fear et al ., ], observed during a conjunction with the Cluster spacecraft, has demonstrated that closed magnetic flux is observed at high altitudes in the magnetotail, which moves to and fro as predicted by the Milan et al [] mechanism.…”

The statistical difference between bending arcs and regular polar arcs

“…It was first suggested by Frank et al [] that the plasma sheet would be bifurcated, where the bifurcated tongue maps high into the lobes and to a theta aurora in the middle of the polar cap. Observation of filamentary plasma sheet extensions into the lobes during a transpolar arc event [ Huang et al, ] and recent observations of closed field lines filled with atypically hot plasma within the lobes [ Fear et al, ] during a transpolar arc event shows that such a plasma sheet topology may exist.…”