On the Magnetic Dip Ahead of the Dipolarization Fronts

Abstract

Dipolarization front (DF), a transient structure that is associated with the earthward plasma flow, is widely considered as a result of magnetic reconnection. It is often characterized with a sharp increase in the northward magnetic field Bz preceded by a minor decrease of it. However, the small magnetic dip ahead of the DF is not always present, the reason of which is not well known. By analyzing in site Magnetospheric MultiScale spacecraft measurements at the magnetotail, we present two events of DF with and without a dip ahead of it. It is found that the magnetic dip ahead of DF is accompanied by a guide field By. To investigate the physical mechanism that determines the appearance of the dip, we perform kinetic simulations of symmetric single X line magnetic reconnection under various guide fields. The simulation results show that the dip is generated due to the separation of the gyro‐motion between ions and electrons under the influence of the guide field. Ions are reflected at the DF and further deviated by the guide field away from the neutral sheet, while more electrons remain in the neutral sheet due to their small gyroradius. In the absence of a compensating ion current, a dawnward current thus is formed, carried by the electrons, and generates the dip. Our results bring in a new perspective on the formation of the dip in single X line reconnection.

The Effect of Energetic Ion Dispersionless Injections on the Ring Current Dynamics

“…The gradual increase of dB H and inclination indicates the dipolarization of the magnetic fields. The dip, also reported in several studies (Liu et al, 2016;Motoba et al, 2021;Ohtani et al, 2004), can be interpreted as diamagnetic effect due to reflected and accelerated thermal plasma sheet ions (Huang et al, 2022;Zhou et al, 2014). The injections build up plasma thermal pressure in the ring current, and the magnetic pressure thus reduces, which results in the decrease of dB H (e.g., He et al, 2017;Yin et al, 2021;Yin et al, 2022).…”

Magnetic Hump Associated with Electron Vortex at Dipolarization Front

“…As the plasma boundary between two distinct plasma populations, DFs and their adjacent regions should host abundant kinetic-scale processes. There sometimes exist kinetic-scale magnetic dips prior to the DFs (Yao et al 2013), attributed to dawnward current carried by either reflected ions (Zhou et al 2014) or electrons (Huang et al 2022). DFs and their adjacent regions are closely associated with various types of plasma waves, such as whistler waves (Hwang et al 2014;Viberg et al 2014;Chen et al 2021Chen et al , 2022, lower hybrid drift waves (Zhou et al 2009;Divin et al 2015;Chen et al 2021), and high-frequency electrostatic waves (Zhou et al 2009;Zhang & Angelopoulos 2014;Yang et al 2017;Chen et al 2022), which can heat and accelerate electrons, and result in energy conversion (Huang et al 2015;Khotyaintsev et al 2017;Liu et al 2023).…”

Energy Conversion in the Dip Region Preceding Dipolarization Front

“…Another region which can potentially host strong energy conversion is the magnetic field dip usually observed ahead of the fronts. Previous studies have shown that the dips may be associated with current formation (Schmid et al., 2019), plasma compression (Yao et al., 2015), wave generation (Wei et al., 2021; Zhao et al., 2016), and ion reflection (Huang et al., 2022; Pan et al., 2015; Zhou, Angelopoulos, et al., 2014; Zhou, Ni, et al., 2014). Therefore, the dip region hosts various processes which can be directly related to energy exchange between fields and particles.…”