Lou-Chuang Lee

This paper presents a conducting channel model aimed at elucidating the generation of high-energy particles within a plasma chamber. Initially, the chamber is charged with neutral hydrogen gas at a density of approximately ~3.3×10²²/m³, equivalent to 1 torr at 300K under ideal gas conditions. A Townsend discharge (dark discharge), driven by an externally imposed electric potential (500-1000V) across the cathode and anode, is utilized to induce partial ionization of the hydrogen gas. Once a stable conducting channel with a high conductivity is established, a low electric potential (e.g., 100V-500V) is introduced to sustain the current in the conducting channel. Our investigation then delves into the impact of a high electron emissivity cathode, such as lanthanum hexaboride (LaB6) during an arc discharge. We develop a theoretical model of the conducting channel that may emerge under these conditions. As the cathode surface undergoes heating, emitted thermionic electrons form a localized layer of negative charge density, leading to an electric potential dip. Our multi-fluid simulations unveil the emergence of electron-ion two-stream instability owing to the high-density electron layer, leading to the appearance of multiple potential peaks and dips, each measuring several to tens of kV. We delineate a set of conditions conducive to the formation of these potential peaks and dips within the conducting channel. Our proposed scenario furnishes a framework for elucidating electron and ion acceleration within a weakly ionized plasma chamber.

A synthesis of various processes associated with auroral substorms is attempted by considering medium intensity substorms. In this paper, we consider that the magnetosphere-ionosphere system is an electric current system. When the solar wind-magnetosphere dynamo power starts to increase above 10¹¹ W, the current cannot flow in a quiet (low conductive) ionosphere, so that the current is initially blocked, causing energy accumulation of about 10¹⁶ J and inflation of the inner magnetosphere during the growth phase. This initial anomaly is removed by the development of a specific instability of the cross-tail current and subsequent disruption, causing deflation and the so-called dipolarization in the inner magnetosphere. These processes result in the development of the earthward electric field, which generates a new current system of sheet field-aligned current together with the double layer, greatly increasing the ionization of the ionosphere and establishing the expansion phase, the sudden brightening of an arc. This sequence of processes allows finally the disrupted cross-tail current to flow in the ionosphere (the auroral electrojet), dissipating the accumulated magnetic energy manifested by auroral substorms, so that the whole system can function finally like a normal current system, the recovery phase.

This study conducts a comprehensive analysis of observations related to the M_L6.2 Hualien earthquake that struck eastern Taiwan on 6 February 2018, focusing particularly on gamma-ray emissions and total electron content (TEC) as earthquake precursors. Prior research has shown that significant gamma-ray enhancements are frequently detected at the YMSG (Yangmingshan gamma-ray) station prior to major earthquakes in eastern and northeastern Taiwan, suggesting that gamma-ray anomalies may serve as reliable indicators for identifying seismic precursors in this area. Our findings reveal a significant rise in gamma-ray emissions at the YMSG station from 19 January to 4 February 2018, which corresponds to a precursor period of approximately 18 days before the Hualien earthquake. Positive and negative TEC anomalies were observed in Taiwan on 20–21 January and 5 February, respectively, and may be considered as ionospheric precursors to the earthquake. Additionally, deep-learning techniques applied to TEC data facilitate the detection of ionospheric precursors associated with the Hualien earthquake, enabling forecasts of an approaching seismic event. Collectively, these observations indicate that all identified anomalies are regarded as short-term precursors, explicable through the theoretical framework of lithosphere–ionosphere coupling (LIC).

Kinetic Alfvén waves (KAWs) and ion cyclotron waves (ICWs) are believed to play a significant role in the solar wind heating and acceleration and are pervasive in space and astrophysical plasmas. However, the generation mechanism for the coexistence of both wave modes remains unclear. The present work proposes a novel generation scenario of KAWs and parallel ICWs in a homogeneous solar wind plasma using hybrid simulations. Our numerical study reveals that the emissions of KAWs and parallel ICWs can be triggered by the ion beam (IB) mode in a homogeneous alpha or proton beam plasma with a beam velocity of ≥1.2v _A for alpha particles or ≥1.6v _A for proton beams, where v _A represents the local Alfvén velocity. The growth rates of both KAWs and parallel ICWs are significantly higher than that of the IB mode. Moreover, the initial background proton beta exhibits an inverse correlation with the growth of KAWs. Ultimately, the saturation energy of these triggered emissions could rival that of the IB mode. Given the prevalent occurrence of IBs in the solar wind, this triggering process provides a credible explanation for the origin of KAWs and parallel ICWs and their coexistence within the beam plasma environments.

The dispersion of shock is universal in various media, and in plasmas, standing whistler waves represent the dispersion of collisionless shocks. These standing whistler waves are usually observed upstream of low Mach number planetary bow shocks and interplanetary shocks. However, our understanding of the plasma behavior and electric field properties within these waves remains limited. Using conjoint THEMIS and Magnetospheric Multiscale (MMS) observations, we report the first observation of standing whistler wave upstream of a fast shock in Earth's magnetosheath resulting from the interaction between a solar wind tangential discontinuity and the bow shock. High‐resolution MMS measurements provide unprecedented insights into these waves, characterizing their circular polarization, near‐parallel propagation to the shock normal, and fixed phase relative to the shock ramp. The observed field and particle characteristics are discussed in detail.

Solar wind observations in the space of the proton temperature anisotropy versus parallel proton beta always show a distorted rhomboid‐like pattern, where the boundaries of this plot are associated with several instabilities. However, the specific mechanism on the constraint of the proton temperature anisotropy is unclear in the low‐beta plasma. In this work, we study the kinetic instabilities driven by proton temperature anisotropy and alpha beam with the Vlasov theory and investigate the nonlinear evolution of these instabilities with hybrid simulations. We also compare the theoretical and simulation results with Wind observations. The alpha beam with drift velocity leads to a new kind of Alfvén/ion‐cyclotron instability (A/IC instability) in the region of . In the region of and , the nonlinear wave‐particle interaction through the A/IC instability can decelerate alpha beams to and result in the anti‐correlation between and . In the region of and , the nonlinear wave‐particle interaction through the A/IC instability further leads to slight increase of . The present results provide a potential mechanism for regulating the proton temperature anisotropy in the low‐beta plasma with in the solar wind.

The dispersion of shock is universal in various media, and in plasmas, standing whistler waves represent the dispersion of collisionless shocks. However, at present, our understanding of the plasma behavior and electric field properties within these waves remains limited. Using conjoint THEMIS and Magnetospheric Multiscale (MMS) observations, we report the first observation of standing whistler wave upstream of a fast shock in Earth’s magnetosheath resulting from the interaction between a solar wind tangential discontinuity and the bow shock. High-resolution MMS measurements provide unprecedented insights into these waves, characterizing their circular polarization, near-parallel propagation to the shock normal, and fixed phase relative to the shock ramp. Moreover, generated ion acoustic waves and wave‒particle interactions are observed in these waves. These findings highlight that the magnetosheath is a compelling region for investigating standing whistler wave properties.

Solar-terrestrial interaction is a dynamic process that manifests itself in the ionosphere. Interplanetary (IP) shocks or solar wind dynamic pressure pulses can generate enhanced brightening in dayside aurora. Foreshock transients are capable of inducing pressure changes, larger in magnitude than solar wind pressure pulses, which also contribute to intensifying dayside aurora. These pressure variations can accelerate particles into the ionosphere, generating field-aligned currents that produce magnetic impulse events and enhanced dayside auroral activity with periods of increased brightening. This study presents several dayside auroral brightening events that are not associated with IP shocks or solar wind dynamic pressure pulses. The dayside auroral brightening events are associated with a green (557.7 nm) to red (630.0 nm) ratio which is greater than 15. These extreme brightening events (EBEs) begin on the eastern or western end of a pre-existing dayside auroral arc. Periodic pulses of enhanced brightening are correlated with large sharp increases in the X-component (points toward the north-geographic pole) from ground magnetometers in the IMAGE network. EBEs occur predominately before magnetic noon and with X-component signatures from high-latitude stations. Ground-based data were obtained from the Kjell Henriksen Observatory in Longyearbyen and the IMAGE magnetometer network.

Explosive mass ejections triggered by magnetic activities are common on our Sun and other stars in the Universe. However, there is a lack of evidence for such explosive phenomena in magnetized or partially magnetized planets with atmospheres. Here we present direct evidence for explosive mass ejections from the Martian ionosphere, resulting from magnetic reconnections between strong crustal field regions with open magnetic fields. A plasma density cavity with signatures of magnetic reconnection that is directly evident for an eruptive mass ejection caught in the act indicates that a considerable amount of ionospheric mass has been rapidly ejected into space. Although Martian mass loss associated with magnetic reconnection has been reported previously, our results demonstrate that explosive mass ejections can occur even on partially magnetized planets without global magnetic fields. In this scenario, we suggest that strong localized magnetic fields extending above the exobase are needed.