Local potential analysis of MHD instability

In many astrophysical problems, the study of the stability of an atmosphere in the presence of a magnetic field is of importance. In most cases the MHD instabilities of atmospheres are studied by energy principle of Bernsteinet al. (1958). In this paper, a general method for studying the stability of a system subject to MHD equations of conditions has been proposed. This is based on the local potential concept put forward by Glansdorff and Prigogine (1964). The scheme for securing stability criteria has been demonstrated in two particular cases.     

Visibility of Prominences Using the He <Emphasis Type="SmallCaps">i</Emphasis> D<Subscript>3</Subscript> Line Filter on the PROBA-3/ASPIICS Coronagraph

We determine the optimal width and shape of the narrow-band filter centered on the He?i D₃ line for prominence and coronal mass ejection (CME) observations with the ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun) coronagraph onboard the PROBA-3 (Project for On-board Autonomy) satellite, to be launched in 2020. We analyze He?i D₃ line intensities for three representative non-local thermal equilibrium prominence models at temperatures 8, 30, and 100 kK computed with a radiative transfer code and the prominence visible-light (VL) emission due to Thomson scattering on the prominence electrons. We compute various useful relations at prominence line-of-sight velocities of 0, 100, and 300 km?s^?1 for 20 Å wide flat filter and three Gaussian filters with a full-width at half-maximum (FWHM) equal to 5, 10, and 20 Å to show the relative brightness contribution of the He?i D₃ line and the prominence VL to the visibility in a given narrow-band filter. We also discuss possible signal contamination by Na?i D₁ and D₂ lines, which otherwise may be useful to detect comets. Our results mainly show that i) an optimal narrow-band filter should be flat or somewhere between flat and Gaussian with an FWHM of 20 Å in order to detect fast-moving prominence structures, ii) the maximum emission in the He?i D₃ line is at 30 kK and the minimal at 100 kK, and iii) the ratio of emission in the He?i D₃ line to the VL emission can provide a useful diagnostic for the temperature of prominence structures. This ratio is up to 10 for hot prominence structures, up to 100 for cool structures, and up to 1000 for warm structures.     

Northern Adriatic phytoplankton response to short Po River discharge pulses during summer stratified conditions

A 275‐km‐long transversel Northern Adriatic profile from the mouth of the Po River (Italian Adriatic coast) to the Kvarner region (Croatian coastal island area) was investigated in three successive case studies in August 2008, 2009 and 2010. The short Po River pulses in August result in the surface advection of riverine water, nutrients and phytoplankton from the western to the eastern side of the Adriatic. This surface spreading exhibits inter‐annual variability depending on the riverine discharge in the preceding period. The Po River discharge pulse in August 2010 in particular resulted in an extraordinary tongue‐like advection of riverine water, nutrients, and phytoplankton towards the Eastern Adriatic coast. The phenomenon was detected using both satellite imagery and classical oceanographic measurements. In the advective water, toxic dinoflagellates were most abundant in August 2010, when the influence of the Po was greatest.     

基于微地貌特征的西太平洋菲律宾海海底稳定性评价

随着海洋战略地位的逐步提高,深远海海底资源开发和海洋工程建设将面临巨大挑战,主要是受限于技术手段,高精度调查资料难以获取,无法全面准确地进行海底稳定性评价。针对这一问题,本文提出了基于微地貌特征的深远海海底稳定性评价方法。基于已有的研究工作,本文选取西太平洋菲律宾海中南部某区域为研究区,利用ArcGIS平台建立研究区DEM（Digital Elevation Model）,提取宏、微观地貌因子,结合全球地震数据、研究区底质类型和潜在地质灾害分布特征,运用模糊数学方法评价研究区海底稳定性,并绘制海底稳定性区划图。结果显示,对区域3220个评价单元进行稳定性分析,可将研究区海底稳定性划分为5个等级,包括基本稳定、较稳定、中等稳定、较不稳定和不稳定。其中,稳定区主要集中在较为平坦的中北部,不稳定区多发育在九州-帕劳海岭、海山、山间盆地等大规模地貌单元发育区,分析揭示,研究区海底稳定性与微地貌特征密切相关。因此,本文提出的基于微地貌特征的海底稳定性评价方法,能够很好地服务于深远海海底稳定性评价。     

Equivalent-linear site response analysis on the site of the historical Trakošćan Castle,Croatia, using HVSR method

This paper presents a major extension of seismic vulnerability research project on the site of Trako??an Castle based on the initial horizontal-to-vertical-spectral-ratio (HVSR) results from Stanko et al. (2016). The estimated HVSR site frequencies and HV amplification at Trako??an Castle can only be used as an indication of the initial soil site frequency and amplification, so-called natural soil model, corresponding to the subsoil profile without the influence of an earthquake. The equivalent-linear (EQL) site response analysis has been carried out for different earthquake scenarios for a maximum input rock peak ground acceleration (PGA_ROCK) that corresponds to return periods of 95 (0.08 g), 475 (0.18 g) and 1000 years (0.31 g). The aim of the research is to evaluate structural seismic design responses and to determine type and degree of damage caused by local site effect, which is the result of an alluvial basin and topographic influences. The main objective of this research is the formation of local microseismic zones based on an EQL analysis: surface spectral acceleration and amplification maps at the predominant frequency. Based on the HVSR frequency response of the core structure of Trako??an Castle and the Tower itself (fundamental and higher frequency modes), maps of surface spectral acceleration and soil amplification at different frequencies (3, 5 and 10 Hz) are developed for different input PGA_ROCK levels (0.08, 0.18 and 0.31 g) to evaluate seismic response of the Castle. Observed amplifications are correlated with ground motion polarization and directionality of the ground motion from the alluvial basin to the hilltop. Shortening of predominant frequencies (lengthening of the period), particularly in the alluvial basin, has been observed with higher input PGA_ROCK in the EQL analysis. This effect is not manifested in the Trako??an hill, and predominant frequencies match HVSR frequencies. The use of certain geophysical survey methods at historical sites is a big problem, because terrain features (e.g. steep hills, mountains, ridges, slopes, cliffs) create lack of space and make it impossible to carry out geophysical investigation. Microtremor measurements at historical sites can overcome this limitation and provide local seismic response and vulnerability behaviour of historical monuments without destroying their authenticity. Also, computational modelling can greatly improve the results. The EQL site response analysis on the site of Trako??an Castle has confirmed and improved the results of seismic response and vulnerability based on HVSR method.     

Hypoxia in the northern Gulf of Mexico: Does the science support the Plan to Reduce,Mitigate, and Control Hypoxia?

We update and reevaluate the scientific information on the distribution, history, and causes of continental shelf hypoxia that supports the 2001 Action Plan for Reducing, Mitigating, and Controlling Hypoxia in the Northern Gulf of Mexico (Mississippi River/Gulf of Mexico Watershed Nutrient Task Force 2001), incorporating data, publications, and research results produced since the 1999 integrated assessment. The metric of mid-summer hypoxic area on the LouisianaTexas shelf is an adequate and suitable measure for continued efforts to reduce nutrients loads from the Mississippi River and hypoxia in the northern Gulf of Mexico as outlined in the Action Plan. More frequent measurements of simple metrics (e.g., area and volume) from late spring through late summer would ensure that the metric is representative of the system in any given year and useful in a public discourse of conditions and causes. The long-term data on hypoxia, sources of nutrients, associated biological parameters, and paleoindicators continue to verify and strengthen the relationship between the nitratenitrogen load of the Mississippi River, the extent of hypoxia, and changes in the coastal ecosystem (eutrophication and worsening hypoxia). Multiple lines of evidence, some of them representing independent data sources, are consistent with the big picture pattern of increased eutrophication as a result of long-term nutrient increases that result in excess carbon production and accumulation and, ultimately, bottom water hypoxia. The additional findings arising since 1999 strengthen the science supporting the Action Plan that focuses on reducing nutrient loads, primarily nitrogen, through multiple actions to reduce the size of the hypoxic zone in the northern Gulf of Mexico.