Zilong Zhang

The Z‐to‐O mode conversion at the density gradient is the prevailing mechanism of narrowband (NB) radio emission in planetary magnetosphere. Most previous numerical models were for NB emission observed in the Earth magnetosphere, using the cold plasma fluid approximation that excluded any kinetic effect. Here we investigate the Z‐to‐O conversion process underlying the Saturn's 20 kHz NB emission, using the fully kinetic and electromagnetic particle‐in‐cell (PIC) simulation. We simulate the whole process starting from the pumping of the Z mode, to its propagation and reflection, and further conversion into the O mode radiation. The energy conversion rate of the Z‐to‐O process is estimated to be 10%–20%. This provides the first quantitative estimate of such rate with PIC simulations.

Ring-beam energetic electrons can drive two different kinetic instabilities, including the bump-on-tail instability by the beam component and the electron cyclotron maser instability by the ring component. It is critical to understand how the two instabilities develop and interact, and how their competition for free energy affects the wave modes and further plasma emission process. Here, we present fully kinetic particle-in-cell simulations of the ring-beam interaction with weakly magnetized plasmas of coronal conditions. We found that both the beam-Langmuir (BL) mode and the upper-hybrid (UH) mode can be excited efficiently, along with significant fundamental and harmonic plasma emissions. In comparison with the corresponding pure-beam case, in the ring-beam case, the BL mode has a suppressed range of k⊥, slower damping and thus stronger intensity, and the fundamental emission is considerably stronger, while the harmonic emission has comparable intensity yet being more directional. We further investigated the effect of the pitch angle (α) of the ring-beam distribution, found that the (1) BL mode is more limited in the range of k⊥ with larger α and (2) the UH mode is excited in localized blobs that can be clustered with a straight line whose slope is ∼cot α. Most of these intriguing results can be interpreted with the linear kinetic theory.

According to the standard scenario of plasma emission, escaping radiations are generated by the nonlinear development of the kinetic bump-on-tail instability driven by a single beam of energetic electrons interacting with plasmas. Here, we conduct fully-kinetic electromagnetic particle-in-cell simulations to investigate plasma emission induced by the ring-distributed energetic electrons interacting with overdense plasmas. Efficient excitations of the fundamental (F) and harmonic (H) emissions are revealed with radiation mechanism(s) different from the standard scenario: (1) The primary modes accounting for the radiations are generated through the electron cyclotron maser instability [for the upper-hybrid (UH) and Z modes] and the thermal anisotropic instability [for the whistler (W) mode]; the F emission is generated by the nonlinear coupling of the Z and W modes and the H emission by the nonlinear coupling of the UH modes. This presents an alternative mechanism of coherent radiation in overdense plasmas.

According to the standard scenario of plasma emission, escaping radiations are generated by the nonlinear development of the kinetic bump-on-tail instability driven by a single beam of energetic electrons interacting with plasmas. Here, we conduct fully-kinetic electromagnetic particle-in-cell simulations to investigate plasma emission induced by the ring-distributed energetic electrons interacting with overdense plasmas. Efficient excitations of the fundamental (F) and harmonic (H) emissions are revealed with radiation mechanism(s) different from the standard scenario: (1) The primary modes accounting for the radiations are generated through the electron cyclotron maser instability [for the upper-hybrid (UH) and Z modes] and the thermal anisotropic instability [for the whistler (W) mode]; the F emission is generated by the nonlinear coupling of the Z and W modes and the H emission by the nonlinear coupling of the UH modes. This presents an alternative mechanism of coherent radiation in overdense plasmas.

According to the standard scenario of plasma emission, escaping radiations are generated by the nonlinear development of the kinetic bump-on-tail instability driven by a single beam of energetic electrons interacting with plasmas. Here, we conduct fully-kinetic electromagnetic particle-in-cell simulations to investigate plasma emission induced by the ring-distributed energetic electrons interacting with overdense plasmas. Efficient excitations of the fundamental (F) and harmonic (H) emissions are revealed with radiation mechanism(s) different from the standard scenario: (1) The primary modes accounting for the radiations are generated through the electron cyclotron maser instability [for the upper-hybrid (UH) and Z modes] and the thermal anisotropic instability [for the whistler (W) mode]; the F emission is generated by the nonlinear coupling of the Z and W modes and the H emission by the nonlinear coupling of the UH modes. This presents an alternative mechanism of coherent radiation in overdense plasmas.

According to the standard scenario of plasma emission, escaping radiations are generated by the nonlinear development of the kinetic bump-on-tail instability driven by a single beam of energetic electrons interacting with plasmas. Here, we conduct fully-kinetic electromagnetic particle-in-cell simulations to investigate plasma emission induced by the ring-distributed energetic electrons interacting with overdense plasmas. Efficient excitations of the fundamental (F) and harmonic (H) emissions are revealed with radiation mechanism(s) different from the standard scenario: (1) The primary modes accounting for the radiations are generated through the electron cyclotron maser instability [for the upper-hybrid (UH) and Z modes] and the thermal anisotropic instability [for the whistler (W) mode]; the F emission is generated by the nonlinear coupling of the Z and W modes and the H emission by the nonlinear coupling of the UH modes. This presents an alternative mechanism of coherent radiation in overdense plasmas.

According to the standard scenario of plasma emission, escaping radiations are generated by the nonlinear development of the kinetic bump-on-tail instability driven by a single beam of energetic electrons interacting with plasmas. Here, we conduct fully-kinetic electromagnetic particle-in-cell simulations to investigate plasma emission induced by the ring-distributed energetic electrons interacting with overdense plasmas. Efficient excitations of the fundamental (F) and harmonic (H) emissions are revealed with radiation mechanism(s) different from the standard scenario: (1) The primary modes accounting for the radiations are generated through the electron cyclotron maser instability [for the upper-hybrid (UH) and Z modes] and the thermal anisotropic instability [for the whistler (W) mode]; the F emission is generated by the nonlinear coupling of the Z and W modes and the H emission by the nonlinear coupling of the UH modes. This presents an alternative mechanism of coherent radiation in overdense plasmas.

Context. Electron-cyclotron maser emission (ECME) is the favored mechanism for solar radio spikes and has been investigated extensively since the 1980s. Most studies relevant to solar spikes employ a loss-cone-type distribution of energetic electrons, generating waves mainly in the fundamental X/O mode (X1/O1), with a ratio of plasma oscillation frequency to electron gyrofrequency (ω_pe/Ω_ce) lower than 1. Despite the great progress made in this theory, one major problem is how the fundamental emissions pass through the second-harmonic absorption layer in the corona and escape. This is generally known as the escaping difficulty of the theory. Aims. We study the harmonic emissions generated by ECME driven by energetic electrons with the horseshoe distribution to solve the escaping difficulty of ECME for solar spikes. Methods. We performed a fully kinetic electromagnetic particle-in-cell simulation with ω_pe/Ω_ce = 0.1, corresponding to the strongly magnetized plasma conditions in the flare region, with energetic electrons characterized by the horseshoe distribution. We also varied the density ratio of energetic electrons to total electrons (n_e/n₀) in the simulation. To analyze the simulation result, we performed a fast Fourier transform analysis on the fields data. Results. We obtain efficient amplification of waves in Z and X2 modes, with a relatively weak growth of O1 and X3. With a higher-density ratio, the X2 emission becomes more intense, and the rate of energy conversion from energetic electrons into X2 modes can reach ∼0.06% and 0.17%, with n_e/n₀ = 5% and 10%, respectively. Conclusions. We find that the horseshoe-driven ECME can lead to an efficient excitation of X2 and X3 with a low value of ω_pe/Ω_ce, providing novel means for resolving the escaping difficulty of ECME when applied to solar radio spikes. The simultaneous growth of X2 and X3 can be used to explain some harmonic structures observed in solar spikes.

Plasma emission (PE), i.e., electromagnetic radiation at the plasma frequency and its second harmonic, is a general process occurring in both astrophysical and laboratory plasmas. The prevailing theory presents a multi-stage process attributed to the resonant coupling of beam-excited Langmuir waves with ion-acoustic waves. Here, we examine another possibility of the fundamental PE induced by the resonant coupling of Z-mode and whistler (W) waves. Earlier studies have been controversial in the plausibility and significance of such process in plasmas. In this study, we show that the matching condition of three-wave resonant interaction (Z + W → O) can be satisfied over a wide regime of parameters based on the magnetoionic theory, demonstrate the occurrence of such process, and further evaluate the rate of energy conversion from the pumped Z or W mode to the fundamental O mode with particle-in-cell simulations of wave pumping. The study presents an alternative form of the fundamental PE, which could possibly play a role in various astrophysical and laboratory scenarios with both Z and W modes readily excited through the electron cyclotron maser instability.

Plasma emission (PE), i.e., electromagnetic radiation at the plasma frequency and its second harmonic, is a general process occurring in both astrophysical and laboratory plasmas. The prevailing theory presents a multi-stage process attributed to the resonant coupling of beam-excited Langmuir waves with ion-acoustic waves. Here, we examine another possibility of the fundamental PE induced by the resonant coupling of Z-mode and whistler (W) waves. Earlier studies have been controversial in the plausibility and significance of such process in plasmas. In this study, we show that the matching condition of three-wave resonant interaction (Z + W → O) can be satisfied over a wide regime of parameters based on the magnetoionic theory, demonstrate the occurrence of such process, and further evaluate the rate of energy conversion from the pumped Z or W mode to the fundamental O mode with particle-in-cell simulations of wave pumping. The study presents an alternative form of the fundamental PE, which could possibly play a role in various astrophysical and laboratory scenarios with both Z and W modes readily excited through the electron cyclotron maser instability.