Electromagnetic disturbances observed near the dip region ahead of dipolarization front

Abstract

Dipolarization front (DF) is a thin magnetic structure embedded in fast flows in the magnetotail, which plays an important role in particle acceleration, flow braking, wave excitation, and other related processes. Electromagnetic disturbances near the magnetic dip region in front of DFs are investigated using Time History of Events and Macroscale Interactions during Substorms probe observations in this paper. Strong magnetic field and electric field fluctuations, with several wave bands below and around the lower hybrid frequency, are found in an event on 21 March 2008. The properties of the wave are similar to that of magnetosonic wave. Detailed analyses show that the phase space density for ions in the perpendicular direction has a positive slope near the local Alfvén speed, which is a possible free‐energy source for the generation of the wave. This type of ion distribution could result from the earthward reflected ions ahead of DF, though other forming mechanism could not be fully ruled out.

Observation of High‐Frequency Electrostatic Waves in the Dip Region Ahead of Dipolarization Front

“…Around 13:04:54.00 UT, B z decreases from 0.37 to −0.92 nT within 0.9 s (from 13:04:53.5 to 13:04:54.4 UT, the time period when the magnetic field B z prior the DF decreases from the maximum value to the minimum value, as marked by the magenta dash lines), and the feature of B z depression is consistent with the character of the dip region ahead of the DF described in Figure 3 in Fu, Zhao, et al (2020). This implies that MMS observed a magnetic dip region ahead of the DF (Zhao et al, 2016;Schmid et al, 2019). By adopting the timing analysis method, the spatial scale of this dip region is estimated as 48.5 km, ∼0.1 λ i (λ i ∼489 km is the ion inertial length) and 0.02 ρ i (ρ i ∼ 1,980 km is the thermal proton gyroradius).…”

“…Waves in the range of lower hybrid frequency ( f LH ; Divin et al., 2015; Huang, Fu, Yuan, 2015; Le Contel et al., 2017) are frequently observed at the DFs also. Below and around the f LH , the low frequency electromagnetic waves have been detected ahead of the DF (Zhao et al., 2016; Zhang et al., 2017). Whistler waves with frequency between f LH and electron cyclotron frequency ( f ce ) are often observed at and behind DFs (Chen et al., 2021; Deng et al., 2010; Fu et al., 2014; Grigorenko et al., 2020; Huang et al., 2012, 2016, 2019; Li et al., 2015; Liu et al., 2021).…”

“…Such study would help us to more fully understand the microphysics process (especially electron dynamics) around the DF. In the dip region, few low frequency fluctuations have been studied until now (Chen et al, 2021;Zhao et al, 2016;Zhang et al, 2017); however, high-frequency waves are not observed due to the limited time resolution of the instruments.…”

Oxygen Ion Reflection at Earthward Propagating Dipolarization Fronts in the Magnetotail

“…In the earthward propagation of DFs, precursor signatures that include gradual enhancements in plasma density, velocity, and pressure are commonly detected ~20 to ~60 s prior to the sharp enhancement of B z (e.g., Ohtani et al, ; Zhao et al, ; Zhou et al, , ). These signatures have been interpreted as preexisting plasma sheet particles being accelerated and reflected by the approaching DF (Eastwood et al, ; P. Wu & Shay, ; Zhou et al, , ).…”

Modulation of Whistler Mode Waves by Ion‐Scale Waves Observed in the Distant Magnetotail

“…The plasma sheet (Bame et al, ) in the Earth's magnetotail is a region full of plasma activities characterized by high‐speed plasma flows (Angelopoulos et al, ; Baumjohann et al, ), magnetic structures (Nakamura et al, ; Slavin et al, ), wave activities (Khotyaintsev et al, ; Zhao et al, ), and particle accelerations (Fu et al, ; Zhou et al, ). During the active times in the plasma sheet, both earthward and tailward flows are generated (Nagai et al, ).…”