[1] We investigate the origin of waves leading to current disruption and dipolarization observed by a THEMIS satellite at X GSM $ À8 R E in the magnetotail near a substorm expansion onset on 29 January 2008. Continuous wavelet transform of the magnetic activity associated with current disruption shows a clear inverse cascade feature of waves starting at frequencies slightly below the ion cyclotron frequency to waves at low frequencies that correspond to the time scale of dipolarization. There was no other lowfrequency wave associated with the time scale of dipolarization. On the basis of this inverse cascade feature, we provide a preliminary theoretical analysis to propose that the initial excited waves causing eventual dipolarization in this event may originate from the drift-driven electromagnetic ion cyclotron instability, which is an electromagnetic version of the Weibel instability, or an ordinary mode instability similarly driven by the cross-field ion drift.
Electromagnetic stability of modified Harris model, featuring a bifurcated current density profile, against lower‐hybrid drift perturbation is investigated. The bifurcated current sheet is characterized by temperature and cross‐field flow velocity profiles on different spatial scales across the current sheet. It is found that the eigenmodes of unstable solutions are mainly confined to the inner slopes of the bifurcated current sheet. That is, the fluctuations are trapped within the two current layers. This finding shows that the source of anomalous resistivity is focused near the neutral sheet, thus indicating that the excitation of electromagnetic lower‐hybrid drift instability in a bifurcated current sheet may be a major cause of magnetic reconnection onset.
[1] Identification of wave characteristics during current disruption events in the near-Earth geomagnetic tail region is important for understanding the substorm onset mechanism. The present paper discusses a linear stability analysis in the ion cyclotron frequency range of a plasma possessing the temperature anisotropy and cross-field flow. It is found that the ion cyclotron drift waves propagating in quasi-perpendicular direction with respect to the ambient magnetic field are characterized by low frequencies (w ] 0.5W i ), while quasi-parallel waves have frequencies close to the ion cyclotron frequency (w ∼ W i ). This finding is consistent with the observation by THEMIS spacecraft of a current disruption event in which similar high-and low-frequency band structures are present. The interpretation of the low-frequency mode as the quasi-perpendicular mode is consistent with a recent data analysis. The present paper shows that the high-frequency quasi-parallel mode can simultaneously be excited with the low-frequency quasi-perpendicular mode.
Laser-driven Coulomb explosion can induce stimulated Raman scattering in cluster-embedded plasmas. The propagation and scattering of electromagnetic waves have been studied to show that Coulombic expansion of atomic clusters significantly modifies the scattering properties. When the cluster plasma collision is negligible, the cutoff frequency occurs due to the resonance, and this cutoff frequency is lowered as the cluster size increases. On the other hand, when there are collisions, the electron-ion collisions inhibit cluster electrons from absorbing laser energy resonantly, in which case the electromagnetic wave dispersion relation is not much affected by the presence of the cluster. Enhancement in the resonance absorption of laser energy is achieved when laser frequency becomes inversely proportional to the cubic root of the normalized cluster radius. The expansion of a dense cluster into the ambient plasma density is shown to accompany a swift decrease in the plasma wave frequency. The variation of the growth rate of the Raman backscattered wave with respect to the wave number, plasma density, and sizes of the clusters shows that, in a high-density cluster plasma, only long-wavelength modes survive until the end of the cluster expansion. The short wave scattering mode, which initially grows faster, quickly damps out in the early phase of the expansion.
Coulombic expansion of an atomic cluster can significantly affect the growth of stimulated Raman backscattering in cluster embedded plasmas. The pulse attenuation and the collisional absorption occur during the early phase of cluster expansion, competing against each other in determining the Raman threshold intensity in a cluster plasma. The threshold laser intensities estimated for various wave numbers of the electron plasma waves are found to be consistent with the previous result [P. K. Tiwari et al., Phys. Plasmas 14, 103101 (2007)]: Collisional threshold intensities are to be of 1∼105W∕cm2 at the electron-ion collisional frequency ≈1013s−1, and the pulse attenuated threshold intensities are to be of ∼1013W∕cm2 for short pulse length ∼20fs. The pulse attenuation and collisional absorption of Raman threshold intensities are found to be inversely proportional to both the pulse duration and attenuation length.
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