Rippled Electron‐Scale Structure of a Dipolarization Front

We carried out a statistical study of equatorial dipolarization fronts (DFs) detected by the Magnetospheric Multiscale mission during the full 2017 Earth's magnetotail season. We found that two DF classes are distinguished: class I (74.4%) corresponds to the standard DF properties and energy dissipation and a new class II (25.6%). This new class includes the six DF discussed in Alqeeq et al. (2022, https://doi.org/10.1063/5.0069432) and corresponds to a bump of the magnetic field associated with a minimum in the ion and electron pressures and a reversal of the energy conversion process. The possible origin of this second class is discussed. Both DF classes show that the energy conversion process in the spacecraft frame is driven by the diamagnetic current dominated by the ion pressure gradient. In the fluid frame, it is driven by the electron pressure gradient. In addition, we have shown that the energy conversion processes are not homogeneous at the electron scale mostly due to the variations of the electric fields for both DF classes.

Dipolarization fronts (DFs), ion‐scale magnetic‐dipolar transients in the magnetotail, are favorable regions for the generation of various types of plasma waves, which are important for energy transport and dissipation in the magnetosphere. The plasma waves hitherto reported near the DFs are typically low‐frequency (lower than electron cyclotron frequency) modes. Here, we report MMS observations of high‐frequency waves, including Langmuir and upper hybrid waves near the electron plasma frequency, inside flux pileup regions behind the DFs. Using MMS high‐cadence data, we revealed that the Langmuir waves were possibly generated by local electron beams and rapidly thermalized the beams, and the upper hybrid waves, which were associated with perpendicularly anisotropic electrons, may propagate from other regions. These waves can drive localized energy transfer and accelerate local electrons at a rate of ∼1.5 eV/s, indicating that the high‐frequency waves can play a role in wave‐particle energy transfer near the DFs.

<p>The mass and energy transfer across Earth’s magnetopause is caused by a variety of different plasma processes. One of these processes is the Kelvin-Helmholtz instability (KHI), excited by the velocity shear between the fast-flowing magnetosheath plasma and the relatively stagnant magnetosphere. It has been frequently observed during periods of northward interplanetary magnetic field (IMF), however much less is known about its behaviour during southward IMF conditions.</p><p>We present the first Magnetospheric Multiscale (MMS) observations of KH waves and vortices at the dusk-flank magnetopause during southward IMF conditions on September 23, 2017. The instability criterion for the KHI was fulfilled during this event. The boundary normal vectors, obtained by using multi-point methods, are consistent with the predicted structures of the KH waves. We further performed a series of realistic 2D and 3D fully kinetic PIC simulations based on the plasma parameters observed during this MMS event. A comparison to results from these simulations demonstrated quantitative consistencies with the MMS data in many aspects such as the flow and total pressure variations in the KH waves, and the signatures of the non-linearly rolled up KH vortices including the Low Density Faster Than Sheath (LDFTS) plasma.</p><p>The simulations further showed that secondary instabilities are excited at the edges of the primary KHI. The Rayleigh-Taylor instability (RTI) can lead to the penetration of high-density arms into the magnetospheric side and disturb the structures of the vortex layer, leading to irregular variations of the surface waves. This can be an important factor in explaining the lower observational probability of KH waves during southward IMF than northward IMF. In the non-linear growth stage of the primary KHI, the lower-hybrid drift instability (LHDI) is excited at the vortex edges leading to efficient plasma mixing across the magnetopause.</p><p>The high-time resolution of MMS measurements demonstrated the occurrence of kinetic-scale plasma waves mainly on the low-density side of the edges of the KH waves. Given quantitative consistencies with the simulations, these waves can be interpreted as being generated by the LHDI. These observed waves form due to the strong density gradient between the two sides of the boundary layer and can lead to a flattening of the edge layers.</p><p><br>In this presentation, we will show the consistencies between MMS observations and 2D and 3D simulation runs focusing on the large-scale surface waves (KHI, RTI) and the small-scale fluctuations (LHDI) and outline the multi-scale properties of the observed KH waves during southward IMF.</p>

Recent DE‐2 data from the Langmuir probe, vector electric field, and ion drift meter instruments are employed to study equatorial spread F in the frequency regime of the low frequency drift and the lower hybrid drift instabilities. Strong electron density gradients observed along the orbital path through the topside equatorial F region correspond to regions of high electric field waves and large ion drift velocities. The electric field waves are seen in two distinct wavelength ranges which correspond to the parameter regimes of the low frequency drift and the lower hybrid drift instabilities. In the smaller of the two wavelength ranges the lower hybrid drift instability is found to be unstable, based on the ion drift velocity and the other plasma parameters measured on DE‐2, and using published theory. Thus there is experimental evidence that the lower hybrid drift instability may produce the observed short wavelength waves without invoking a cascading mechanism.

Using high‐resolution MMS data, we report the observations of broadband electrostatic waves associated with parallel electron temperature anisotropy () behind a dipolarization front (DF). These electrostatic waves include electrostatic solitary waves and electron cyclotron waves. To quantify electron anisotropy, we define the parallel flux anisotropy parameter , where is phase space densities at each energy. By performing correlation analyses between parallel flux anisotropies () and power spectral densities (PSDs) of these waves, we find that: (a) in 0.8–20 keV are positively correlated with the parallel electric field fluctuations in 50–3,000 Hz; (b) in 0.8–20 keV are positively correlated with perpendicular electric field fluctuations in all frequency ranges; (c) the correlation between parallel flux anisotropies and perpendicular electric field fluctuations is more significant in this case. We also discuss the possibility of electron firehose instability and wave–particle interactions. Our study promotes the understanding of the properties and dynamics of DFs.

The current sheet structure modification by lower‐hybrid drift instability (LHDI) has been shown to lead to quick magnetic reconnection triggering (QMRT) even in a thick current sheet (Tanaka et al., 2005). The three‐dimensional simulations, however, have been limited to certain sets of parameters. Having learned which effects of LHDI are crucial for QMRT in these three‐dimensional results, we perform a series of two‐dimensional simulations of LHDI (reconnection itself not included) with various current sheet thickness D, guide field intensity B 0y , and the frequency ratio τ ( τ = ω pe /Ω e , where ω pe is the electron plasma frequency and Ω e is the electron gyrofrequency) and inspect how these parameters affect the QMRT process. While QMRT looked promising in the three‐dimensional studies with B 0y = 0 and τ = 1, the two realistic factors, nonzero guide field and τ larger than 1, requires rather small current sheet thickness for QMRT to be operative in the magnetospheric situations.

Using the high‐resolution measurements from magnetospheric multiscale mission, here we present the observational evidence of kinetic interchange instabilities (ICIs) at dipolarization fronts (DFs). Specifically, we present the observation of two DFs caused by ICI, and discover the sub‐ion scale magnetic depression caused by kinetic‐scale ICI at the second DF. By using multi‐spacecraft observation, the typical process inside the magnetic hole (MH)—by electron trapping process—is clearly shown in the magnetic depression, evidencing that the depression is a sub‐ion MH generated at DF by kinetic ICI. Based on observations and Minimum Variance Analysis (MVA) analysis, we present a two‐dimensional image of the ICI in the nonlinear stage. This observational study demonstrates the kinetic ICI at DFs, confirming the existence of MH generated by ICI at DF, and improves the understanding of dynamics at DFs.

<p>At the leading edges of reconnection jets in the magnetotail, commonly referred to as Dipolarization Fronts (DF), strong fluctuations in the electric field δE and the magnetic field δB are observed. Recent results from a fully kinetic PIC simulation (Nakamura et al., 2019) demonstrate that a Lower Hybrid Drift Instability-driven (LHDI) disturbance at the DF front region can be responsible for these electric and magnetic field fluctuations. These findings are well in line with an observed event (Liu at al., 2018), comparable to the simulated plasma conditions. However, a general experimental validation under a wider range of conditions yet remains absent. The present work experimentally investigates δE and δB fluctuations for a selection of DF events between July 2017 and September 2018 using Magnetospheric Multiscale (MMS) mission data. Aiming for a statistical approach, the analysis consists of a quantitative evaluation of dynamic wave power spectra of both δE and δB in the lower hybrid frequency range. Furthermore, propagation properties of associated wave structures are analyzed and related to present plasma conditions. Findings include the identification of peak wave power occurrence times relative to the magnetic DF structure and the associated density gradient.</p>

The behavior of the lower hybrid drift instability is investigated in the low drift velocity regime by including the effects of the magnetic field on the ions. It is found that the mode is transformed from the nonresonant lower hybrid drift instability to the resonant ion cyclotron drift instability as the drift velocity Vd decreases and is finally stabilized when Vd/Vti∼ (me/mi)1/2.

Carrying out a large scale 3D full kinetic simulation, we have shown that the effects of lower-hybrid waves at the edges of the current sheet provide a quick triggering of magnetic reconnection even in ion-scale current sheets. A thin embedded electron current layer is formed as a result of the non-linear evolution of the lower hybrid drift instability, which is sustained by accelerated meandering electrons around the neutral sheet, and the emergence of a thin electron current layer is subject to the quick reconnection triggering. However. there may be an upper-limit to the current sheet thickness for this type of quick triggering mediated by the lower-hybrid-drift instability alone because the lower-hybrid-drift wave activity becomes weaker in a thicker current sheet. To evaluate whether the quick triggering mechanism mediated by the lower-hybrid-drift instability is truly realistic, we performed a parametric study of magnetic reconnection with the lower hybrid drift instability. Consequently, we confirmed that the proposed scenario of the quick reconnection triggering is available with a real ion-electron mass ratio m/sub i//m/sub e/=1836. We also found that the upper-limit of the current sheet thickness depends sensitively on the strength of the guide magnetic field.

The lower-hybrid drift instability (LHDI) in a thin current sheet in the intermediate-wavelength (kyρiρe∼1, where ky, ρe, and ρi are the wave vector and the electron and ion gyroradii, respectively) regime for particles with κ velocity distribution is studied. The latter is more suitable for describing nonthermal distributions with an enhanced high-energy tail and includes the Maxwellian as a limiting case. It is shown that linear electromagnetic LHDI can be excited near the center of the current sheet. The growth rate decreases, but the electromagnetic component of the LHD mode increases with increase in hot particles.

In this paper we examine the drift instability around the lower hybrid resonance (LHR) frequency in the presence of temperature and density gradients. The unstable mode which propagates with the electron drift velocity is strongly affected by the electron temperature and density gradients present in a direction perpendicular to the magnetic field. A generalised expression for the growth/damping rate for the lower hybrid drift mode in the presence of temperature and density gradients have been derived and discussed for suitable ionospheric plasma parameters. Expressions for marginal stability and maximum growth rate have also been calculated. Applications of these results to explain some of the observed features of satellite observations of ionospheric plasma have been indicated.

We perform a case study on the evolution of the current sheet in different regions around the dipolarization front (DF), including magnetic‐dip preceding the DF, front at the DF, and magnetic pileup region (MPR) behind the DF based on magnetospheric multiscale (MMS) observation on July 31, 2017. In this event, MMS1 stays inside the current sheet during the whole bursty bulk flow (BBF) interval. Our analysis reveals that the cross‐tail current sheet at the DF is rolled up, signified by the depression (−Vz/−Bz) at the dip and elevation (+Vz/+Bz) at the front. The minimum variance analysis on the magnetic field method is applied to obtain the normal direction of the current sheet. The result confirms the roll‐up, that is, downward at the depressed current sheet and upward at the elevated current sheet. The current sheet roll‐up at the DF is asymmetric, with steeper elevation than depression. The elevation angle of the elevated current sheet is evaluated to be ∼30°. Strong duskward and predominantly perpendicular J spike (∼90 nA/m2) concentrate at the interface between the dip and the front. The strength of the current of the J‐spike is about nine/three times the current at the dip/front. The front is characterized by positive E·J. In the dip/MPR, no such preference is seen. Ion/Electron pitch angle distributions exhibit significant and different evolutions in the roll‐up current sheet from dip to front, including their energy‐dependence and distributions. Finally, the roll‐up current sheet could decelerate BBF and change the flow structure. The potential significance of the roll‐up current sheet on BBF evolution is emphasized.

The dipolarization front (DF), usually observed near the leading edge of a bursty bulk flow (BBF), is thought to carry an intense current sufficient to modify the large‐scale near‐Earth magnetotail current system. However, the physical mechanism of the current generation associated with DFs is poorly understood. This is primarily due to the limitations of conventional plasma instruments which are unable to provide a sufficient number of unaliased 3‐D distribution functions on the timescale of the DF, which usually travels past a spacecraft in only a few seconds. It is thus almost impossible to unambiguously determine the detailed plasma structure of the DF at the usual temporal resolution of such instruments. Here we present detailed plasma measurements using the Cluster Plasma Electron and Current Experiment and Cluster Ion Spectrometry‐Composition and Distribution Function ion data for an event during which it was possible to observe the full pitch angle distribution at a cadence of 1/4 s. The observations clearly show details of plasma substructure within the DF, including the presence of field‐aligned electron beams. In this event, the current density carried by the electron beam is much larger than the current obtained from the curlometer method. We also suggest that the field‐aligned current around the DF obtained from the curlometer method may have been misinterpreted in previous studies. Our results imply that the nature of the DF current system needs to be revisited using high‐resolution particle measurements, such as those observations shortly to be available from the Magnetospheric Multiscale mission.

Magnetic curvature is shown to have a relatively weak effect on lower hybrid drift instabilities, but ion temperature gradients can significantly alter stability in the broad sheath limit, ai≪Ln.