Properties and origin of subproton‐scale magnetic holes in the terrestrial plasma sheet

Abstract

Electron-scale magnetic depressions in the terrestrial plasma sheet are studied using Cluster multispacecraft data. The structures, which have an observed duration of~5-10 s, are approximately 200-300 km wide in the direction of propagation, and they show an average reduction in the background magnetic field of 10-20%. A majority of the events are also associated with an increase in the high-energy high pitch angle electron flux, which indicates that the depressions are presumably generated by electrons with relatively high velocity perpendicular to the background magnetic field. Differences in the recorded electron spectra in the four spacecraft indicates a possible nongyrotropic structure. Multispacecraft measurements show that a subset of events are cylindrical, elongated along the magnetic field, and with a field-parallel scale size of at a minimum 500 km. Other events seem to be better described as electron-scale sheets, about 200-300 km thick. We find that no single formation mechanism can explain this variety of events observed. Instead, several processes may be operating in the plasma sheet, giving rise to similar magnetic field structures in the single-spacecraft data, but with different 3-D structuring. The cylindrical structures have several traits that are in agreement with the electron vortex magnetic holes observed in 2-D particle-in-cell simulations of turbulent relaxation, whereas the sheets, which show nearly identical signatures in the multispacecraft data, are better explained by propagating electron solitary waves.

Kinetics of sub‐ion scale magnetic holes in the near‐Earth plasma sheet

“…In the equatorial plane, magnetic holes are stretched in the earthward direction. This configuration is not unique (both quasi‐1‐D and cylindrical magnetic holes were reported by Balikhin et al [], Sundberg et al [], and Li et al []), but it implies significant spatial anisotropy of the initial plasma configuration where MHs were formed. Within the holes, electron scattering to the loss cone is significantly reduced because of the suppression of ECH waves. The holes, on the other hand, may generate field‐aligned currents and thus couple the magnetosphere to the ionosphere on a kinetic (sub‐ion gyroradius) scale.…”

“…Balikhin et al [] noted that magnetic field polarization within magnetic holes in the magnetotail is far from linear (see also multispacecraft analysis by Sundberg et al []). We use MVA analysis of 23 MHs to define the ratios of main ( λ 1 ) to intermediate ( λ 2 ) eigenvalues and intermediate ( λ 2 ) to minimum ( λ 3 ) eigenvalues.…”

A statistical study of kinetic‐size magnetic holes in turbulent magnetosheath: MMS observations

“…Recently, small‐scale magnetic holes (less than ion gyroradius) have been detected in the plasma sheet [ Sun et al ., ; Sundberg et al ., ; Gershman et al ., ; Goodrich et al ., ]. These small‐scale magnetic holes cannot be explained by the aforementioned mechanisms.…”

Propagation of small size magnetic holes in the magnetospheric plasma sheet

“…Small size magnetic holes (SSMHs), also in the name of “subproton‐scale magnetic holes” [ Sundberg et al ., ] are structures with a scale less than, or close to, the proton gyroradius. In recent years, this kind of structures has been frequently observed by the Time History of Events and Macroscale Interactions during Substorms and the Cluster satellites in the magnetospheric plasma sheet [ Ge et al ., ; Sun et al ., ].…”

“…Sundberg et al . [] further discussed various possible generation mechanisms of SSMHs in the plasma sheet, including ion or electron mirror modes and field‐swelling instabilities [e.g., Basu and Coppi , ; Gary and Karimabadi , ; Pokhotelov et al ., ], tearing modes [ Balikhin et al ., ], electron vortices [ Haynes et al ., ], and solitary waves [e.g., Baumgärtel , ; Stasiewicz et al ., ; Stasiewicz , ; Ji et al ., ], and suggested the tearing mode, the electron vortex, and the solitary waves were candidates for the SSMHs formation in the plasma sheet.…”