Whistler Waves Associated With Electron Beams in Magnetopause Reconnection Diffusion Regions

Abstract

Magnetic reconnection involves explosive energy conversion from electromagnetic fields to plasmas. In this process, highly structured particle distribution functions are formed that are unstable to various instabilities and waves to help dissipate energies and accelerate particles. The whistler wave is one example. It has 𝐴𝐴 𝐴𝐴𝑟𝑟 < 𝐴𝐴𝑐𝑐𝑐𝑐 with right-hand polarization, since it is coupled to the electron gyro-motion, where the wave frequency is expressed as 𝐴𝐴 𝐴𝐴= 𝐴𝐴𝑟𝑟 + 𝑖𝑖𝑖𝑖 . Whistlers can be excited through two modes. (a) The cyclotron mode is excited when the electron distribution has a perpendicular anisotropy that overcomes cyclotron damping (e.g., Gurnett & Bhattacharjee, 2005;Kennel, 1966), and the cyclotron resonant velocity is 𝐴𝐴 𝐴𝐴‖,𝑟𝑟𝑟𝑟𝑟𝑟 ≡ 𝑉𝑉𝑔𝑔𝑔𝑔𝑟𝑟𝑔𝑔 = (𝜔𝜔𝑟𝑟 ± 𝜔𝜔𝑐𝑐𝑟𝑟) ∕𝑘𝑘‖ . (b) The Landau mode requires a positive slope in the distribution along v || ( 𝐴𝐴 𝐴𝐴𝐴𝐴 ∕𝐴𝐴𝑑𝑑‖ > 0 ), and the resonant velocity is 𝐴𝐴 𝐴𝐴‖,𝑟𝑟𝑟𝑟𝑟𝑟 ≡ 𝑉𝑉𝑝𝑝𝑝,‖ = 𝜔𝜔𝑟𝑟∕𝑘𝑘‖ .The whistler wave is commonly observed in magnetopause reconnection, and a few wave excitation mechanisms have been analyzed (e.g., review in Khotyaintsev et al., 2019). The first instability source is the anisotropy of hot magnetospheric electrons (

Ion‐Scale Magnetic Flux Rope Generated From Electron‐Scale Magnetopause Current Sheet: Magnetospheric Multiscale Observations

“…3). While the generation mechanisms of these intermediate-frequency waves (Khotyaintsev et al, 2019) are beyond the scope of the present paper and need further investigation, it is probable that they were driven by bi-directional electron beams (Wang et al, 2022), as shown in Figures 8k and 8l. A number of simulations and observations have shown that electron beam-driven or two-stream instabilities and resulting electrostatic waves (EWs), electrostatic solitary waves (ESWs or electron holes), or whistler waves can be responsible for electrostatic and electromagnetic fluctuations around 𝐴𝐴 𝐴𝐴ce in separatrix regions (Choi et al, 2022;Fujimoto, 2014;Goldman et al, 2014;Holmes et al, 2019;Steinvall et al, 2019).…”

A comparison of energetic particle energization observations at MMS and injections at Van Allen Probes

“…A strong, sustained tailward flow without a notable earthward flow before it does not match the damped sine wave-type flow seen by Panov et al (2010), so this is likely not a flow rebound. Another candidate is that these energetic particles are electrons beams associated with whistler waves near the reconnection site (Wilder et al, 2016;Wang et al, 2022). However, in the pitch angle distribution of the thermal electrons, any beam-like signature is very short-lived (less than 10 s) and after the energization has already begun, so this also does not fit the case study.…”

Electrostatic Waves Around a Magnetopause Reconnection Secondary Electron Diffusion Region Modulated by Whistler and Lower‐Hybrid Waves

“…In this case, we interpret that the background anisotropy excites whistler, and the beam is produced or at least modulated by Landau resonance with whistler, while beams are unlikely to play a significant role in exciting whistler waves. The DC structure of reconnection can also generate uni‐directional electron beams due to changes of the magnetic field topology plus additional acceleration, which may excite whistler waves (e.g., Wang et al., 2022, and references therein). Such reconnection‐generated beams are magnetosheath electrons expected to have high intensities and move toward the magnetosphere (anti‐parallel in this case), in a region very close to the X‐line.…”

“…A series of electrostatic waves are present and modulated in the whistler wave in the vicinity of the current sheet mid-plane (B L ∼ 0, Figures 2a and 2h). Whistler observed by MMS1 occurs at B L < 0 with frequencies slightly below f ce /2 (Figure 2c), associated with the perpendicular anisotropy of energized magnetosheath electrons (seen in Figure 2n) slightly downstream of the central EDR (Wang et al, 2022). We select whistler waves as bins in FFT spectrograms that have magnetic field powers >10 times of the background noise level, degree of polarization >0.7, and ellipticity >0.5 (using the spectral analysis (Samson & Olson, 1980)).…”