Guo-Qing Zhao

Fast electron beams (FEBs) and coherent radio bursts both are very common plasma activity phenomena in the cosmos. The electron cyclotron maser (ECM) instability driven by FEBs trapped in magnetic fields can directly amplify radio wave radiations. It has been applied successfully to explain coherent radio bursts from a planet’s magnetosphere, such as the auroral kilometric radiation at Earth and the decametric radiation at Jupiter. On the other hand, however, there is still significant controversy when applying it to solar radio bursts (SRBs) due to some heavy difficulties in the conventional ECM instability. Taking account of the electrodynamics effects of FEBs during their passage through the ambient plasma, we present a novel mechanism for the ECM instability from FEBs, in which the Alfvén wave (AW) generated by the electric current instability of the FEB, called the self‐generated AW, plays an important role. The enhanced AW not only influences considerably the motion of individual electrons of the FEB, but also changes significantly the physical characteristics of the ECM emission as a collective coherent mode excited by the FEB. In particular, this novel ECM instability can well overcome the main difficulties of the conventional theory, which are often encountered in the explanation of SRBs. The present results imply that this novel ECM instability may be applied to coherent radio bursts in wider magnetic plasma environments from the solar atmosphere and planetary magnetospheres in the solar system to exoplanets and other astrophysical objects outside the local cosmos.

Coronal heating remains a long-standing problem in solar and astrophysical plasmas. Alfvén waves (AWs) can carry sufficient energy into the solar atmosphere, but how this energy is dissipated into particle heating is not well understood. In this work, based on the ubiquitous inhomogeneity in the solar magnetically structured atmosphere, we propose a self-consistent mechanism for heating the solar magnetic atmosphere by kinetic Alfvén waves (KAWs), which involves the resonant mode conversion (RMC) of AWs to KAWs. Our finding reveals that KAWs can be effectively generated via the RMC of AWs in the solar magnetically structured atmosphere, with the RMC efficiency E _KAW / E _AW ∼ 0.01–0.1. Based on the estimation of the AW intensity in different environments of the solar magnetic atmosphere, the KAWs generated by the RMC of AWs are sufficient to sustain the heating of this atmosphere. This work not only bridges the gap between the large-scale AW energy flux and the small-scale kinetic processes essential for heating, but also establishes a unified mechanism for heating the solar magnetic atmosphere by KAWs.

Ion-scale waves are believed to play an important role in heating of the solar corona and wind, though their generation mechanism remains unclear. Based on Parker Solar Probe (PSP) observations, this Letter investigates the occurrence of ion-scale waves in the near-Sun solar wind with heliocentric distances between 0.1 and 0.2 au. Results show that the occurrence rate of left-handed polarized waves significantly depends on the plasma beta ( β ) and cross helicity ( σ _c ). The occurrence rate rapidly increases with decreasing β and increasing σ _c . Overall, the occurrence rate exceeds 45% when β < 0.2 and σ _c > 0.7 are satisfied. These observations are consistent with the direct predictions of the helicity barrier theory that suggests the generation of ion cyclotron waves via imbalanced magnetized turbulence.

Kinetic Alfvén waves (KAWs) are ubiquitous in space and solar plasmas and are believed to be crucial for energy transfer and particle energization. Existing studies on KAWs primarily focus on the low-frequency approximation, where the wave frequency is much smaller than the proton cyclotron frequency (i.e., ω ≪ ω _cp ). However, the wave properties of high-frequency KAWs with ω ≳ ω _cp remain unclear. In this work, based on the two-fluid theory, we derive a general dispersion relation for KAWs spanning low-frequency to high-frequency regimes, and examine this dispersion relation and the electromagnetic properties of both low-frequency and high-frequency KAWs. Our findings reveal that, compared to low-frequency KAWs, high-frequency KAWs exhibit several distinct features: higher wave frequency ( ω ≳ ω _cp ), propagation angle over a broader oblique angle range, significantly larger ratio of parallel to perpendicular electric field, and the magnetic helicity and magnetic compressibility that are highly sensitive to the plasma beta. The enhanced parallel electric field highlights the pivotal role of high-frequency KAWs in field-aligned particle acceleration. This work extends KAW theory to the high-frequency domain, providing key insights into KAW properties and the particle acceleration process in space and solar plasmas.

Transverse oscillations of coronal loops not only influence coronal dynamics, but also provide a new perspective for coronal detection. To enhance their exploitation and utilization, accurately estimating the oscillation parameters is crucial. The least chi-squares fitting is a widely employed method for this estimation. However, due to the highly nonlinear nature of the fitting process, there is no established theory to provide the exact error of the fitted parameters. In this study, we systematically investigated the accuracy and precision of the fitted parameters of transverse coronal oscillations through a series of forward simulations. Our investigation indicates that the accuracy and precision of fitted oscillation parameters are influenced not only by the standard deviation of the measurement error, but also by the temporal resolution and the characteristics of the signal itself. Based on these findings, we conducted additional forward simulations to estimate the influences on the real observation examples. The results clearly show that (1) the fitted damping timescale should be overestimated in a significant number of transverse coronal loop oscillations, and (2) errors estimated by the general method may be biased. In response, we have developed a forward simulation-based schema to achieve a self-consistent error estimation for the fitted oscillation parameters. These contributions not only offer valuable insights into the error estimation of fitted transverse coronal oscillation parameters, but also establish a robust foundation for obtaining accurate and precise transverse oscillation parameters of the corona.

It is generally believed that coronal mass ejections (CMEs) have magnetic flux rope structures because of their helical shapes. However, only about 30%–40% of interplanetary CMEs (ICMEs) have a local magnetic flux rope structure. The usual explanations are that the spacecraft only crossed the flank of the ropes and failed to detect the complete magnetic flux rope structure or that some processes destroyed these magnetic flux rope structures. Several studies suggest that some ICMEs inherently possess disordered magnetic fields and consequently exhibit no magnetic flux-rope structures. We introduce a special kind of ICME which has a low magnetic field magnitude and stable magnetic field direction, relatively fast expansion speed, and lower proton temperature and density. All three of the measured magnetic field components are relatively stable. We want to know whether these ICMEs also have magnetic flux rope structures or not. We identified 20 special ICMEs and analyzed their evolution based on their observed characteristics. We took a special ICME as an example, which had an apparent rope configuration at 1 au but evolved to a special ICME at 5.4 au, to illustrate that this kind of ICME could come from magnetic clouds (MCs) whose rope structure had been being stretched due to expansion. We inferred that the missing obvious flux rope structure may be due to the expansion of MCs, not the flank crossing effect. However, more than 50% of the events were associated with the dominant x-component of the magnetic field, which indicates a leg crossing. Therefore, the detection of part of these special ICMEs may also be the result of the leg-crossing effect.

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.

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.

Solar wind heating is an outstanding issue that has been discussed for decades. Research on the connection between solar wind particle temperatures and turbulence may provide insight into this issue. Based on Parker Solar Probe observations, this paper investigates the properties of solar wind proton and electron temperatures in relation to turbulent magnetic energy, via the calculation of correlation coefficients (CCs) between particle temperatures and magnetic energy. The calculations are regulated by the spatial scale, plasma beta (β), and the angle between the solar wind velocity and background magnetic field, where the plasma beta is the ratio of plasma thermal to magnetic pressure. Results show that the correlation between proton temperature and magnetic energy is positive and can be strong with a CC exceeding 0.8. The strong correlation preferentially occurs at ion scales, with the wind velocity and background magnetic field quasi-perpendicular and over a wide beta range (β < 3.0). On the other hand, the correlation between electron temperature and magnetic energy is commonly negative, often with an intermediate or negligible CC, accordingly. The CC with an amplitude up to 0.8 can arise at larger scales with the wind velocity and background magnetic field quasi-(anti)parallel and in the low-beta case (β < 0.6). The implication of these findings on the physics of turbulent heating in the solar wind is discussed.