arXiv subjects

Dejin Wu

Publications and source records attributed to Dejin Wu.

8 recordsLinked to original sources

arXiv2025

Small-scale inhomogeneity effects on coherent solar radio emission

Coherent radio emission mechanism of solar radio bursts is one of the most complicated and controversial topics in solar physics. To clarify the mechanism(s) of different types of solar radio bursts, (radio) wave excitation by energetic electrons in homogeneous plasmas has been widely studied via particle-in-cell (PIC) code numerical simulations. The solar corona is, however, inhomogeneous over almost all spatial scales. Inhomogeneities of the plasma could influence the emission properties of solar radio bursts. In this paper, we, hence, investigate effects of inhomogeneity (in the magnetic field, plasma density and temperature) of plasmas in the solar corona on radio wave emission by ring-beam distributed energetic electrons utilizing 2.5-dimensional PIC simulations. Both the beam and electron cyclotron maser (ECM) instabilities could be triggered with the presence of the energetic ring-beam electrons. The resultant spectrum of the excited electromagnetic waves presents a zebra-stripe pattern in the frequency space. The inhomogeneous density or temperature in plasmas influences the frequency bandwidth and location of these excited waves. Our results can, hence, help to diagnose the plasma properties at the emission sites of solar radio bursts. Applications of our results to the solar radio bursts with zebra-stripe pattern are discussed.

astro-ph.SR↗

arXiv2023

Weak Solar Radio Bursts from the Solar Wind Acceleration Region Observed by Parker Solar Probe and Its Probable Emission Mechanism

The Parker Solar Probe (PSP) provides us the unprecedentedly close approach observation to the Sun, and hence the possibility of directly understanding the "elementary process" which occurs in the kinetic scale of particles collective interactioin in solar coronal plasmas. We reported a kind of weak solar radio bursts (SRBs), which are detected by PSP when it passed a low-density magnetic channel during its second encounter phase. These weak SRBs have low starting frequecny $\sim 20$ MHz and narrow frequency range from a few tens MHz to a few hundres kHz. Their dynamic spectra display a strongly evolving feature of the intermediate relative drift rate decreasing rapidly from above 0.01/s to below 0.01/s. Analyses based on common empirical models of solar coronal plasmas indicate that these weak SRBs originate from the heliocentric distance $\sim 1.1-6.1~R_S$ (the solar radius), a typical solar wind acceleration region with a low-$β$ plasma, and indicate that their soruces have a typic motion velociy $\sim v_A$ (Alfvén velocity) obviously lower than that of fast electrons required by effectively exciting SRBs. We propose that solitary kinetic Alfvén waves with kinetic scales can be responsible for the generation of these small-scalevweak SRBs, called solitary wave radiation (SWR).

astro-ph.SR↗

arXiv2023

The Radial Distribution of Ion-scale Waves in the Inner Heliosphere

Determining the mechanism responsible for the plasma heating and particle acceleration is a fundamental problem in the study of the heliosphere. Due to efficient wave-particle interactions of ion-scale waves with charged particles, these waves are widely believed to be a major contributor to ion energization, and their contribution considerably depends on the wave occurrence rate. By analyzing the radial distribution of quasi-monochromatic ion-scale waves observed by the Parker Solar Probe, this work shows that the wave occurrence rate is significantly enhanced in the near-Sun solar wind, specifically 21%$-$29% below 0.3 au, in comparison to 6%$-$14% beyond 0.3 au. The radial decrease of the wave occurrence rate is not only induced by the sampling effect of a single spacecraft detection, but also by the physics relating to the wave excitation, such as the enhanced ion beam instability in the near-Sun solar wind. This work also shows that the wave normal angle $θ$, the absolute value of ellipticity $ε$, the wave frequency $f$ normalized by the proton cyclotron frequency $f_{\mathrm{cp}}$, and the wave amplitude $δB$ normalized by the local background magnetic field $B_0$ slightly vary with the radial distance. The median values of $θ$, $|ε|$, $f$, and $δB$ are about $9^\circ$, $0.73$, $3f_{\mathrm{cp}}$, and $0.01B_0$, respectively. Furthermore, this study proposes that the wave mode nature of the observed left-handed and right-handed polarized waves corresponds to the Alfvén ion cyclotron mode wave and the fast-magnetosonic whistler mode wave, respectively.

astro-ph.SR↗

arXiv2022

Cosmic-Ray Convection-Diffusion Anisotropy

Under nonuniform convection, the distribution of diffusive particles can exhibit dipole and quadrupole anisotropy induced by the fluid inertial and shear force, respectively. These convection-related anisotropies, unlike the Compton-Getting effect, typically increase with the cosmic-ray (CR) energy, and are thus candidate contributors for the CR anisotropy. In consideration of the inertial effect, CR observational data can be used to set an upper limit on the average acceleration of the local interstellar medium in the equatorial plane to be on the order of 100 $ μ\text{m}/\text{s}^2 $. Using Oort constants, the quadrupole anisotropy above 200 TeV may be modeled with the shear effect arising from the Galactic differential rotation.

astro-ph.HE↗

arXiv2021

Electromagnetic Proton Beam Instabilities in the Inner Heliosphere: Energy Transfer Rate, Radial Distribution, and Effective Excitation

Differential flows among different ion species are often observed in the solar wind, and such ion differential flows can provide the free energy to drive Alfvén/ion-cyclotron and fast-magnetosonic/whistler instabilities. Previous works mainly focused on ion beam instability under the parameters representative of the solar wind nearby 1 au. In this paper we further study proton beam instability using the radial models of the magnetic field and plasma parameters in the inner heliosphere. We explore a comprehensive distribution of proton beam instability as functions of the heliocentric distance and the beam speed. We also perform a detailed analysis of the energy transfer between unstable waves and particles and quantify how much the free energy of the proton beam flows into unstable waves and other kinds of particle species (i.e., proton core, alpha particle, and electron). This work clarifies that both parallel and perpendicular electric fields are responsible for the excitation of oblique Alfvén/ion-cyclotron and oblique fast-magnetosonic/whistler instabilities. Moreover, this work proposes an effective growth length to estimate whether the instability is efficiently excited or not. It shows that oblique Alfvén/ion-cyclotron instability, oblique fast-magnetosonic/whistler instability, and oblique Alfvén/ion-beam instability can be efficiently driven by proton beams drifting at the speed $\sim$ 600-1300 km s$^{-1}$ in the solar atmosphere. In particular, oblique Alfvén/ion-cyclotron waves driven in the solar atmosphere can be significantly damped therein, leading to the solar corona heating. These results are helpful for understanding proton beam dynamics in the inner heliosphere and can be verified through in situ satellite measurements.

astro-ph.SR↗

arXiv2020

Electron Temperature Anisotropy and Electron Beam Constraints From Electron Kinetic Instabilities in the Solar Wind

Electron temperature anisotropies and electron beams are nonthermal features of the observed nonequilibrium electron velocity distributions in the solar wind. In collision-poor plasmas these nonequilibrium distributions are expected to be regulated by kinetic instabilities through wave-particle interactions. This study considers electron instabilities driven by the interplay of core electron temperature anisotropies and the electron beam, and firstly gives a comprehensive analysis of instabilities in arbitrary directions to the background magnetic field. It clarifies the dominant parameter regime (e.g., parallel core electron plasma beta $β_{\mathrm{ec\parallel}}$, core electron temperature anisotropy $A_{\mathrm{ec}}\equiv T_{\mathrm{ec\perp}}/T_{\mathrm{ec\parallel}}$, and electron beam velocity $V_{\mathrm{eb}}$) for each kind of electron instability (e.g., the electron beam-driven electron acoustic/magnetoacoustic instability, the electron beam-driven whistler instability, the electromagnetic electron cyclotron instability, the electron mirror instability, the electron firehose instability, and the ordinary-mode instability). It finds that the electron beam can destabilize electron acoustic/magnetoacoustic waves in the low-$β_{\mathrm{ec\parallel}}$ regime, and whistler waves in the medium- and large-$β_{\mathrm{ec\parallel}}$ regime. It also finds that a new oblique fast-magnetosonic/whistler instability is driven by the electron beam with $V_{\mathrm{eb}}\gtrsim7V_{\mathrm{A}}$ in a regime where $β_{\mathrm{ec\parallel}}\sim0.1-2$ and $A_{\mathrm{ec}}<1$. Moreover, this study presents electromagnetic responses of each kind of electron instability. These results provide a comprehensive overview for electron instability constraints on core electron temperature anisotropies and electron beams in the solar wind.

physics.space-ph↗

arXiv2018

Observations on a series of merging magnetic flux ropes within an interplanetary coronal mass ejection

Coronal mass ejections (CMEs) are intense solar explosive eruptions. CMEs are highly important players in solar-terrestrial relationships, and they have important consequences for major geomagnetic storms and energetic particle events. It has been unclear how CMEs evolve when they propagate in the heliosphere. Here we report an interplanetary coronal mass ejection (ICME) consisting of multiple magnetic flux ropes measured by WIND on March 25-26, 1998. These magnetic flux ropes were merging with each other. The observations indicate that internal interactions (reconnections) within multi-flux-rope CME can coalesce into large-scale ropes, which may improve our understanding of the interplanetary evolution of CMEs. In addition, we speculated that the reported rope-rope interactions may also exist between successive rope-like CMEs and are important for the space weather forecasting.

physics.space-ph↗

arXiv2014

Excitation of Langmuir waves by the lower energy cutoff behavior of power-law electrons

Langmuir waves (LWs), which are believed to play a crucial role in the plasma emission of solar radio bursts, can be excited by streaming instability of energetic electron beams. However, solar hard X-ray observations imply that the energetic flare electrons usually have a power-law energy distribution with a lower energy cutoff. In this paper, we investigate LWs driven by the power-law electrons. The results show that power-law electrons with the steepness cutoff behavior can excite LWs effectively because of the population inversion distribution below the cutoff energy ($E_c$). The growth rate of LWs increases with the steepness index ($δ$) and decreases with the power-law index ($α$). The wave number of the fastest growing LWs ($kλ_D$), decreases with the characteristic velocity of the power-law electrons ($v_{c}=\sqrt{2E_{c}/m_{e}}$) and increases with the thermal velocity of ambient electrons ($v_T$). This can be helpful for us to understand better the physics of LWs and the dynamics of energetic electron beams in space and astrophysical plasmas.

astro-ph.SR↗