The Effects of Upper-Hybrid Waves on Energy Dissipation in the Electron Diffusion Region.

Using a two-dimensional particle-in-cell simulation, we investigate the effects and roles of upper-hybrid waves (UHW) near the electron diffusion region (EDR). The energy dissipation via the wave-particle interaction in our simulation agrees with J · E ' measured by magnetospheric multiscale (MMS) spacecraft. It means that UHW contributes to the local energy dissipation. As a result of wave-particle interactions, plasma parameters which determine the larger-scale energy dissipation in the EDR are changed. The y-directional current decreases while the pressure tensor P y z increases/decreases when the agyrotropic beam density is low/high, where (x, y, z)-coordinates correspond the (L, M, N)-boundary coordinates. Because the reconnection electric field comes from -∂ P y z /∂ z, our result implies that UHW plays an additional role in affecting larger-scale energy dissipation in the EDR by changing plasma parameters. We provide a simple diagram that shows how the UHW activities change the profiles of plasma parameters near the EDR comparing cases with and without UHW. ©2020. The Authors.

Introduction

Magnetic reconnection is a fundamental physical process in space, astrophysical, and experimental plasmas. By reconfiguring topologies of magnetic field lines, it converts energy stored in the magnetic field into the kinetic energy of particles. The magnetic null or the in‐plane magnetic null, where the breaking and reconnection of magnetic field lines occur, is encircled by the small central region of reconnection, the electron diffusion region (EDR) (Fu et al., 2015, 2019, 2016). NASA's magnetospheric multiscale (MMS) mission was designed to investigate phenomena in the EDR (Burch, Moore, et al., 2016). Providing high‐resolution data, MMS has revealed the electron kinetic physics governing the EDR. One of the most important findings of MMS is the existence of the crescent‐shaped agyrotropic electron distribution in the EDR (Burch, Torbert, et al., 2016). With the help of the existence of a nonzero dissipative electric field ( ), the agyrotropic electrons distribution has been used to identify an EDR crossing. Moreover, it is known that the agyrotropic electrons play crucial roles in the reconnection process. Not only do they generate the reconnection electric field via off‐diagonal electron pressure tensor terms (Hesse & Winske, 1994; Hesse et al., 2014; Scudder & Daughton, 2008), but they are also free energy sources of various waves (Burch et al., 2019; Cao et al., 2017; Dokgo et al., 2019, 2020; Graham et al., 2017; Jiang et al., 2019; Li et al., 2020).

Recently, high‐frequency waves generated by the agyrotropic electrons in the EDR have been studied intensively. Large amplitude upper‐hybrid waves (UHW) of several hundred mV/m amplitudes were reported near the EDR of the dayside reconnection for the first time (Graham et al., 2017), and the linear upper‐hybrid instability in the magnetotail reconnection was studied (Burch et al., 2019; Graham et al., 2017). A particle‐in‐cell (PIC) simulation study showed that nonlinear processes generate electrostatic harmonics and electromagnetic radiation (Dokgo et al., 2019). Moreover, electron Bernstein waves (EBW) were also observed near the EDR (Li et al., 2020). For explaining both UHW and EBW generation from the agyrotropic beam, a unified dispersion relation was derived (Dokgo et al., 2020). These UHW and EBW are generally conjectured to be dynamically significant so as to affect the plasma inside the EDR via wave‐particle interactions. Lapenta et al. (2020) showed Fourier spectrum of UHW and EBW in the inflow region using 3‐D kinetic simulation, and it was shown that UHW could intensify scattering and diffusion of the electrons in the EDR using MMS observation (Jiang et al., 2019). However, quantitative analysis of the influence of upper‐hybrid frequency range fluctuations excited by agyrotropic electrons has not been carried out.

In this letter, we investigate the effects and roles of UHW in the EDR in a quantitative manner by employing the PIC method. PIC simulations show that the wave‐particle interaction energizes electrons. The growth rate of electron energy in our simulation agrees with the local energy dissipation rate measured by the MMS spacecraft at the time of UHW (Burch et al., 2019). As a result of wave‐particle interaction, the current, the pressure tensor terms, and the factor (Swisdak, 2016) measuring the agyrotropy of distributions are changed. This means that UHW can change plasma parameter profiles in the region of UHW activities. The change rates of plasma quantities depend on the intensities of the wave activities, which are set differently using various beam densities. Considering the reconnection electric field in the EDR comes from the gradient of the off‐diagonal pressure tensor, we conclude that UHW could play an additional role in affecting the large‐scale energy dissipation process in the EDR by changing profiles of such plasma quantities. In the remainder of this letter, we detail our findings.

Simulation Conditions

We use a two‐dimensional electromagnetic PIC code (Matsumoto & Omura, 1993) to investigate the effects of UHW driven by agyrotropic electrons. We employ similar simulation conditions with the previous study by Dokgo et al. (2019). The simulation domain locates in the plane, and the boundaries are periodic. The ambient magnetic field lies on the x‐axis, and the agyrotropic electrons propagate to the y‐direction, so ( coordinates in our simulation corresponds the ( boundary coordinates, respectively (L: Reconnecting B direction, M: out‐of‐plane guide field direction, and N: current sheet normal direction).

The simulation parameters are chosen as follows: the grid size , number of grids , time step , and the number of particle per one cell is 2,160, where c is the speed of light in vacuum and is the electron plasma frequency. Plasma parameters are based on the July 3, 2017, event observed by MMS3 (2017 July 03 at 05:26:50.450‐550 UT) (Burch et al., 2019; Dokgo et al., 2019), so they are chosen as follows: densities of core electrons and the agyrotropic crescent electrons , thermal speed , the beam speed of agyrotropic crescent electron , the ratio between the electron plasma frequency and the electron gyro‐frequency , and the range of gyro‐angle of the crescent electrons . We employ a modified version of particle loading and exchanging method modeling transient properties of electrons near the EDR and carry out presimulation without E and B field calculations in order to make a stationary distribution at the beginning of the simulation (Dokgo et al., 2019). We use real proton‐to‐electron mass ratio and the temperature ratio between electrons and ions ; however, there is no significant effects and changes of ion dynamics. Thus, we consider electron quantities only hereafter.

Simulation Results

Figure 1 shows changes of energies, pressure tensor terms, and during the simulation where time (x‐axis) and energies (y‐axis) are normalized by and the total kinetic energy (KE) of electrons at the beginning of the simulation, respectively. The average speed at the beginning is c, where the angle brackets ⟨⟩ denote an average of the subindex species.

Figure 1

Time history of energies and pressure tensors. (a) Total kinetic energy, (b) electric field energy, (c) kinetic energies of core and crescent, and kinetic energies in (d) x‐ (e) y‐, and (f) z‐directions. (g) , (h) , (i) , (j) , and (k) . and terms are not shown because they are at very small values, close to the noise level. Time (x‐axis) and energy (y‐axis) are normalized with respect to the initial kinetic energy and electron plasma frequency , respectively.

Figure 1a shows that the total KE of electrons increases during , then saturates, and this energization of electrons occurs just after the enhancement of the E field energy as shown in Figure 1b. Thus, the wave‐particle interaction between UHW and electrons causes the electron energization. The maximum increase rate of total KE is as denoted by the red dashed line in Figure 1a. This corresponds to 2.88 × 10 nW/m considering plasma parameters observed in the 20170703 MMS event ( /m,  kHz). We note that this value agrees with the energy dissipation rate measured by (Burch et al., 2019). This agreement means that UHW contributes to the local energy dissipation process via the wave‐particle interaction in the MMS observation.

The wave‐particle interaction occurs in the y‐direction (the beam propagating direction) because the energies of and fields are negligible, as shown in Figure 1b. As a result of the wave‐particle interaction, the KE of crescent decreases, but the KE of core increases (Figure 1c). As shown in Figures 1d–1f, the changes of total KE come from mainly y‐ and z‐directions. However, the fact that the increase of KE is delayed from KE by approximately 1 means that the z‐directional energization comes from the 90‐degree rotation of core electrons energized in the y‐direction.

The pressure terms in Figures 1g–1j are calculated from the particle data at each time step via definition where i and j are coordinate axes (x, y, and z), s is the species (total, core, and crescent beam), and denotes the particle index of the species s. As shown in Figures 1g–1i, changes in the diagonal terms of pressure tensor in accordance with each directional kinetic energy. We omit and terms because they are very small, and not significantly different from the noise level.

We note that the dynamical changes associated with and are anti‐correlated, as shown in Figures 1j and 1k. The increases by 5.03% from 3.380 × 10 at the beginning of the simulation to 3.548 × 10 at the end, and the core population causes this increase of as shown in Figure 1j. Meanwhile, the decreases by 16.0% from 1.749 × 10 to 1.475 × 10. This anticorrelation behavior in the changes associated with and comes from that all pressure tensor terms in are changed by the wave‐particle interaction. In the definition of term, it has diagonal pressure terms in its denominator and off‐diagonal terms in its numerator (Swisdak, 2016). During the wave‐particle interaction, increases in and are much more pronounced than the increase of .

We perform simulations using various beam densities to investigate how the change of energy and pressure tensor depends on the intensity of wave or instability. Figure 2 presents time history of (a) total KE, (b) , (c) , and (d) on the left column. Each color in subpanels corresponds to a single run using = 0.03 (green), 0.05 (blue), 0.085 (black), 0.10 (yellow), and 0.12 (red). Every parameter is normalized by their initial values to compare the rates of changes. The right column of Figures 2e–2h presents differences of the same parameters where is defined by a difference between the initial value at the beginning and the saturation state at the end

Figure 2

Time history of (a) total KE, (b) , (c) , and (d) using various beam densities. Net differences between at the initial and final states of (e) total KE, (f) , (g) , and (h) . Each parameter is normalized by its own initial value to compare the rates of changes.

As shown in Figures 2a–2d, very large fluctuations occur during the wave growth phase. It is apparent that the higher the beam density, the larger the fluctuation. In particular, for , it is seen that varies from ∼0.8 to ∼1.4 during the wave growth phase, before settles in the saturation stage to a value a few % above the starting value. That is, large temporal fluctuations might be associated with the wave growth phase.

Figures 2e–2h show quantitative changes at the saturation stage more clearly. We note that increases when as the beam density increases. However, decreases when , and become negative when . These changes of in opposite directions originate from the initial ⟨ depending on the beam density. When the electron distribution becomes flatten as a result of thermalization via wave‐particle interaction, the number of electrons near the center of the flattening (≡) increases while elsewhere decreases. If ⟨ located near ⟨ due to the low density of the beam, then the change of the electron distribution looks that more electrons move away from near ⟨ to . Thus, increases. On the other hand, if ⟨ located near due to the high density of the beam, more electrons gather to near ⟨. Hence, decreases as a result of wave‐particle interaction. Therefore, the beam density and would be important to determine the increase or decrease of . Figure 2g shows that can decrease by 50% of initial values when the beam density is very high. decreases but the change rate seems to be saturated when .

Those results show that UHW activities change plasma parameters, and the amounts of the changes depend on the intensity of UHW and the local value of plasma parameters. The gradient of is closely related to the reconnection electric field, and contributes to the large‐scale (and slowly‐varying) energy dissipation through in the entire EDR. Thus, our simulation implies that UHW can also affect the large‐scale energy dissipation process in the EDR by changing the profiles of and , in addition to locally energizing particles via the wave‐particle interaction. In Figure 3, we plot a schematic diagram of plasma profiles with and without UHW to show the effect of UHW on the plasma parameters near the EDR, where the vertical axis is N direction in the coordinate, and the horizontal axis is the magnitude of parameters.

Figure 3

Schematic diagram of plasma parameter profiles in the EDR. x‐axis is magnitudes of parameters, and y‐axis is N‐direction in coordinate. (a) The growth rate of UHW and parameters determining . (b) , and the energy dissipation with/without UHW for low beam density case, (c) for high beam density case. Dashed lines represent the case without UHW, and solid lines represent the case with UHW.

Figure 3a presents the profile of the growth rate of UHW, as well as bulk parameters, which parametrically determine the property of (Dokgo et al., 2020). The broadening angle Δ and the beam density increase from the very edge of the EDR where only the crescent electrons having the highest energy can reach, while the beam speed decreases. Because rapidly decreases in proportion to (Dokgo et al., 2020), has the maximum value at some location between the very edge and the center of the EDR.

Figures 3b and 3c show how the local UHW activities can change profiles of plasma quantities in the EDR. We plotted , , , and in Figure 3b for the low density beam case and Figure 3c for the high density beam case. We assume that the changes of and are simply proportional to for the low‐density beam and − for the high‐density beam according to our simulation results. Solid/dashed lines mean profiles with/without UHW activities, respectively. Thus, dashed lines could be considered as an equilibrium state without UHWs or 2‐D reconnection case. Note that we assumed a constant total electric field for the case without UHW as in previous studies (Hesse et al., 1999; Zenitani et al., 2011). is derived from − to show only the component coming from .

As shown in Figure 3b, when the beam density is low, is enhanced where UHW occurs. Then the gradient of also increases. Hence, intensifies slightly, and the stiff gradient of is formed where its sign is reversed. Meanwhile, decreases where UHW occurs. Consequently, the profile of the larger‐scale energy dissipation is changed. When the beam density is high, is diminished where UHW occurs, as shown in Figure 3c. Thus, a concave profile of can be formed where maximizes. This reversal behavior associated with the gradient of makes reversed and profiles. However, the total is similar with the low‐beam density case because the current is dominant in this diagram.

Discussion and Conclusion

In previous studies, several locations and structures were suggested to cause the dissipation during the reconnection process, such as the out‐of‐plane currents near the EDR (Burch, Torbert, et al., 2016), field‐aligned currents at separatrices (Hesse et al., 2018; Shay et al., 2016; Swisdak et al., 2018), and the current filaments at O‐lines (Fu et al., 2017). Even though it is very important to understand which one is the most crucial mechanism dissipating energy, it is not clearly understood yet. Thus, it has been investigated actively how the electron‐, intermediate‐, and ion‐scale structures are related in the energy dissipation process during the magnetic reconnection.

In this context, the present letter considered in detail the roles of UHW in the EDR as they relate to the dissipation via local wave‐particle interaction. The energy dissipation rate in our simulation is consistent with the MMS observation measured by when UHW is observed. Our simulation result implies that UHW affect the energy dissipation process near the EDR not only via local and rapid wave‐particle interaction but also by changing profiles of large‐scale energy dissipation . In particular, the thinner the EDR structure in N direction, the larger the change of −, hence and accordingly.

At this stage, we could not calculate the change of total energy dissipation because the kinetic equilibrium profiles of plasma parameters including pressure tensor in the EDR are unknown yet. However, Figure 3 shows that the decrease of current as a result of local UHW‐particle interaction leads a decrease of large‐scale energy dissipation. Therefore, the small‐scale energy dissipation via UHW‐particle interaction can compensate the decrease of large‐scale energy dissipation. Future study of Vlasov or Vlasov‐Maxwell equilibrium of the EDR (or thin current sheet), including pressure tensors, is required to understand further quantitative analysis and compare with other dissipation processes.

is an important and effective parameter to quantify the gyrotropy of particle distributions, and it is very closely related to as its definition. These two quantities are thought to vary in sync because a fixed core electron distribution is usually assumed when the contribution of agyrotropic electrons on and is considered. As shown in our simulations, however, the core and the agyrotropic electrons can interact with each other via UHW‐particle interaction that energizes the core electrons significantly. The increase of and from the core contribution makes decrease despite the enhancement of . As a consequence, we deem that would not be a very good proxy to describe dynamical phenomena in which multiple species or components participate simultaneously.

In summary, we have investigated the effects and roles of UHW near the EDR in quantifiable terms, which has not been done before. The measured as determined from MMS observation agrees with the energy dissipation rate via wave‐particle interaction in our simulation, which can be explained by the scattering or diffusion of electrons. As a result of UHW activities, the pressure tensor and the current are changed, which determine energy dissipation in the EDR. Thus, our simulation results mean that UHW plays roles not only to energize electron via the local and rapid wave‐particle interaction but also to affect the large‐scale in the EDR by changing the profiles of underlying plasma parameters.