Electron kinetic effects in the nonlinear evolution of a driven ion-acoustic wave

The electron kinetic effects are shown to play an important role in the nonlinear evolution of a driven ion-acoustic wave. The numerical simulation results obtained (i) with a hybrid code, in which the electrons behave as a fluid and the ions are described along the particle-in-cell (PIC) method, are compared with those obtained (ii) with a full-PIC code, in which the kinetic effects on both species are retained. The electron kinetic effects interplay with the usual fluid-type nonlinearity to give rise to a broadband spectrum of ion-acoustic waves saturated at a low level, even in the case of a strong excitation. This low asymptotic level might solve the long-standing problem of the small stimulated Brillouin scattering reflectivity observed in laser-plasma interaction experiments

Electron and ion kinetic effects in the saturation of a driven ion acoustic wave

The role of ion and electron kinetic effects is investigated in the context of the nonlinear saturation of a driven ion acoustic wave(IAW) and its parametric decay into subharmonics. The simulations are carried out with a full–particle-in-cell (PIC) code, in which both ions and electrons are treated kinetically. The full-PIC results are compared with those obtained from a hybrid-PIC code (kinetic ions and Boltzmann electrons). It is found that the largest differences between the two kinds of simulations take place when the IAW is driven above the ion wave-breaking limit. In such a case of a strong drive, the hybrid-PIC simulations lead to a Berstein-Greene-Kruskal-like nonlinear IAW of a large amplitude, while in the full-PIC the IAW amplitude decays to a small level after a transient stage. The electron velocity distribution function is significantly flattened in the domain of small electron velocities. As a result the nonlinear frequency shift due to the electron kinetic effects compensates partly the nonlinear frequency shift due to the ion kinetic effects, allowing then for the parametric decay of the driven IAW into subharmonics. These observations lead to the conclusion that electron kinetic effects become important whenever the nonlinear effects come into play

Tsunami observations by coastal ocean radar

When tsunami waves propagate across the open ocean, they are steered by the Coriolis effect and refraction due to gentle gradients in the bathymetry on scales longer than the wavelength. When the wave encounters steep gradients at the edges of continental shelves and at the coast, the wave becomes nonlinear and conservation of momentum produces squirts of surface current at the head of submerged canyons and in coastal bays. High frequency (HF) coastal ocean radar is well conditioned to observe the surface current bursts at the edge of the continental shelf and give a warning of 40 minutes to 2 hours when the shelf is 50 to 200km wide. The period of tsunami waves is invariant over changes in bathymetry and is in the range 2 to 30 minutes. Wavelengths for tsunamis (in 500 to 3000m depth) are in the range 8.5 to over 200 km, and on a shelf where the depth is about 50m (as in the Great Barrier Reef (GBR)) the wavelengths are in the range 2.5 to 30 km. In the use of HF radar technology, there is a trade-off between the precision of surface current speed measurements and time resolution. It is shown that the phased array HF ocean surface radar being deployed in the GBR and operating in a routine way for mapping surface currents, can resolve surface current squirts from tsunamis in the wave period range 20 to 30 minutes and in the wavelength range greater than about 6 km. An advantage in signal-to-noise ratio can be obtained from the prior knowledge of the spatial pattern of the squirts at the edge of the continental shelf, and it is estimated that, with this analysis, the time resolution of the GBR radar may be reduced to about 2.5 minutes, which corresponds to a capability to detect tsunamis at the shelf edge in the period range 5 to 30 minutes. It is estimated that the lower limit of squirt velocity detection at the shelf edge would correspond to a tsunami with water elevation of about 2.5 cm in the open ocean. This means that the GBR HF radar is well conditioned for use as a monitor of small, as well as larger, tsunamis and has the potential to contribute to the understanding of tsunami genesis research

FITOPLANKTON SEBAGAI BIOINDIKATOR PENCEMARAN ORGANIK DI PERAIRAN SUNGAI MUSI BAGIAN HILIR SUMATRA SELATAN

Sungai Musi merupakan sungai terbesar dan terpanjang di Sumatra Selatan. Berkembangnya kegiatan penduduk di Daerah Aliran Sungai (DAS) Musi dapat berpengaruh terhadap kualitas air sungai dan dapat menyebabkan terjadinya pencemaran. Tingginya aktivitas industri maupun rumah tangga di sepanjang Sungai Musi menyebabkan menurunnya kualitas lingkungan di DAS Musi. Berdasarkan hal tersebut, maka perlu dilakukan penelitian lebih lanjut untuk mengetahui seberapa besar tingkat pencemaran yang terjadi di DAS Musi. Tujuan dari penelitian ini adalah untuk mengkaji dan mengetahui tingkat saprobitas di sepanjang DAS Musi bagian hilir berdasarkan nilai SI (Saprobik Indeks), serta mengetahui tingkat pencemaran air menggunakan penilaian saprobitas perairan. Penelitian ini menggunakan plankton sebagai bioindikator pencemaran organik perairan. Penelitian ini menggunakan rancangan eksplorasi dengan metode survei, dan penetapan stasiun pengambilan sampel dengan metode purposive sampling. Hasil penelitian menunjukkan kelimpahan fitoplankton di perairan Sungai Musi pada rentang 123-2581 sel/liter atau rata-rata sebesar 1397 sel/liter. Indeks Saprobik di perairan Sungai Musi berkisar antara 0,63-1, digolongkan pada fasesaprobik, yaitu β-Mesosaprobik, sehingga pada perairan Sungai Musi digolongkan pada tingkat pencemaran ringan.The Musi River is the largest and longest river in South Sumatra. The development of population activities in the Musi River Basin can affect river water quality and can cause pollution. The high level of industrial activity and households along the Musi River causes a decrease in environmental quality in the Musi River Basin. The declining quality of aquatic environment can be seen from the presence of phytoplankton. Based on this, further research is needed to determine the extent of pollution in the Musi River Basin. The purpose of this study is to assess saprobitas along the Musi River Basin based on SI (Saprobic Index) value and knowing the level of water pollution using saprobitas water assessment. This study uses plankton as a bioindicator of aquatic organic pollution. This study uses an exploratory design with survey methods, and the determination of sampling stations by purposive sampling method. The results showed abundance of phytoplankton in the waters of the Musi River in the range of 123 to 2581 cells.liter-1 or an average of 1397 cells.liter-1. The Saprobic index in the waters of the Musi River ranges from 0.631 to 1, classified in the phases of the microbial, namely β-Mesosaprobic, so that the waters of the Musi River are classified as mild