Dynamical friction in an isentropic gas

When a gravitating object moves across a given mass distribution, it creates an overdense wake behind it. Here, we performed an analytical study of the structure of the flow far from object when the flow is isentropic and the object moves subsonically within it. We show that the dynamical friction force is the main drag force on the object and by using a perturbation theory, we obtain the density, velocity and pressure of the perturbed flow far from the mass. We derive the expression of the dynamical friction force in an isentropic flow and show its dependence on the Mach number of the flow and on the adiabatic index. We find that the dynamical friction force becomes lower as the adiabatic index increases. We show analytically that the wakes are less dense in our isentropic case in comparison to the isothermal ones.     

Coronal Density Structures and CMEs: Superior Solar Conjunctions of Mars Express, Venus Express, and Rosetta: 2004, 2006, and 2008

Coronal radio-sounding experiments were carried out using the S-band (2.3 GHz) and X-band (8.4 GHz) signals of the ESA Mars Express, Venus Express, and Rosetta spacecraft during five superior conjunctions occurring in 2004, 2006 (3×), and 2008/2009. Differential frequency and propagation delay (ranging) observations were recorded during these opportunities over the better part of a solar cycle, yielding information on the large-scale structure of the coronal electron-density distribution and its variations, including fluctuations on time scales from seconds to hours. These results concern primarily regions of slow solar wind because the radio propagation path is generally confined to the low heliolatitude regions by the conjunction. The mean frequency fluctuation and total electron content are determined as a function of heliocentric distance, and, with a few exceptions caused by streamers and CMEs, are found to be consistent with previous results from experiments on Ulysses. Dense coronal streamers and several coronal mass ejection (CME) events were identified in the radio-frequency data, some of which were observed in white light by the LASCO coronagraphs onboard SOHO. For those events with sufficient mutual coverage, good correlations are found between the electron-content fluctuations and structure imaged by the LASCO instrument.     

Spatio-temporal analysis of temperature variability,trend, and magnitude in the Hindu Kush region using Monte Carlo and Sen’s slope approaches

The present study explores the spatial and temporal deviations in temperature using Monte Carlo (MC) and Sen’s slope (SS) approaches in the Hindu Kush (HK) region. Climate change holds sturdy association against the temperature trend that has generated adverse impacts in the form of floods. In this attempt, for trend analysis, temperature has been selected as a meteorological parameter. This study mainly focuses on exploring the tendency in average temperature with respect to time and the consequential flood recurrences in the region. For the current study, data regarding temperature were typically collected from Pakistan Meteorological Department. In the study region, there are a total of seven meteorological station falls namely Dir, Chitral, Drosh, Saidu, Malam Jabba, Kalam, and Timergara. The temperature time series data was calculated and analyzed using MC and SS approaches for trend detection in order to demonstrate the kind of fluctuation in the Hindu Kush region. The resultant analysis further revealed that in the meteorological station of Dir, a more significant positive trend (α?=?0.0001) was found in mean monthly maximum, minimum, and monthly normal temperature. Likewise, at Drosh, a positive trend is detected in mean monthly maximum (α?=?0.04), monthly minimum (α?=?0.003), and monthly average (α?=?0.0005). Moreover, at Saidu met station, there is also a trend detected in temperature sub-variables such as monthly maximum (α?=?0.0001) and monthly minimum (α?=?0.001). In addition to these, at Kalam, there is a temperature trend noted for monthly minimum (α?=?0.01) and monthly average (α?=?0.02). Furthermore, the analysis demonstrates that there is no trend detected in the remaining stations, i.e., Chitral, Malam Jabba, Drosh, and Timergara. The overall analysis discovered that there is a sturdy relationship between climate change phenomenon and temperature variability. After using SS test to the temperature data of mean monthly maximum (TMMMax), the results explored that Kalam station grips the highest magnitude, i.e., Q?=?0.76; however, Timergara shows the lowermost, i.e., Q?=???0.34. For the monthly minimum temperature (TMMMin), at Kalam again, the highest value (Q?=?0.005) was detected; however, other stations revealed a negative trend, except Drosh which express no change in terms of magnitude. Similarly, in terms of monthly normal temperature (TMNor), Timergara station (Q?=???0.4) verified a negative trend magnitude and Malam Jabba station again trendless. Among all, the met station of Malam Jabba which holds an altitude of 2591 m is a hilly station just followed by Kalam having 2103 m height; however, Dir holds 1375 m height and the rest of the met stations show low elevation. The main reason for the temperature difference is the altitude of the study region.     

Basin structure and water quality of the Oligocene aquifer in the Sebkhet El Behira basin (central Tunisia)

ABSTRACT

The Sebkhet El Behira–Garaat El Majdoul multilayer aquifer system is composed of Mio-Plio-Quaternary and Oligocene groundwater exploited in Sidi Bouzid and Kairouan governorates. Annual withdrawal volumes from El Behira Oligocene groundwater were about 0.37 hm³ in 2005 and 0.36 hm³ in 2008. The present study of the Oligocene reservoir in the Sebkhet El Behira–Garaat El Majdoul basin is based on various data including 2D seismic sections, petroleum wells, field geological cross-sections and geochemical analysis. The gathered data allowed us to establish the tectonic framework and to define its influence on the structure of the aquifers seated in the deep Oligocene sandstone reservoirs. Three Oligocene sub-basin aquifers are defined showing different depth, thickness and petrophysical characteristics: lower salinity (<1.5 g/L) and higher porosity (30%). Two field sections in the northern and in the southern parts of Cherahil anticline exhibit the presence of four to five sandstone levels. The anticlines limit the Sebkhet El Behira–Garaat El Majdoul basin. In addition, the morphostructural configuration controls the piezometry of underground flows in the Oligocene and Mio-Plio-Quaternary unconfined aquifers.

Editor D. Koutsoyiannis; Associate editor M. Besbes     

The Late Eocene and Late Miocene fronts of the Atlas Belt in eastern Maghreb: integration in the geodynamic evolution of the Mediterranean Domain

Regional cross sections at the scale of the eastern Maghreb based on subsurface and field data allow presenting the structural styles related to the Middle-Late Eocene compressional events. The structural cross sections depict that the Late Eocene front of the Atlas Belt extends far through the Northern Africa plate margin comparatively to the Late Miocene front cropping out in the Eastern Tunisian Atlas. The sections allow proposing a new subsurface front for the Atlas belt encompassing a large part of the Pelagian-Sirt basins. The consequences of this particular configuration are discussed at the scale of the south Tethys margin and replaced in the frame of the geodynamic evolution of the Mediterranean Domain.     

Mars high resolution gravity fields from MRO, Mars seasonal gravity, and other dynamical parameters

With 2 years of tracking data collection from the MRO spacecraft, there is noticeable improvement in the high frequency portion of the spherical harmonic Mars gravity field. The new JPL Mars gravity fields, MRO110B and MRO110B2, show resolution near degree 90. Additional years of MGS and Mars Odyssey tracking data result in improvement for the seasonal gravity changes which compares well to global circulation models and Odyssey neutron data and Mars rotation and precession (). Once atmospheric dust is accounted for in the spacecraft solar pressure model, solutions for Mars solar tide are consistent between data sets and show slightly larger values (k₂ = 0.164 ± 0.009, after correction for atmospheric tide) compared to previous results, further constraining core models. An additional 4 years of Mars range data improves the Mars ephemeris, determines 21 asteroid masses and bounds solar mass loss (dGM_Sun/dt < 1.6 × 10⁻¹³ GM_Sun year⁻¹).     

云南程海真光层深度的时空分布及其影响因子

程海位于云南省丽江市永胜县,是高原封闭型深水湖泊的典型代表.为了揭示程海真光层深度的时空分布及其影响因子,于2016年3月-2017年2月在程海布设9个点位开展逐月调查.结果显示：光合有效辐射漫射衰减系数、真光层深度的年均值分别为1.06±0.25 m^-1、4.64±1.27 m;空间变化方面,无论"北、中、南部"还是"西、中、东部",全年、逐月及雨旱季的真光层深度均无显著差异;时间变化方面,5月的真光层深度最高（6.49±1.03 m）、7月的最低（3.63±0.48 m）,且全年、雨季、旱季的各月份间差异显著;回归分析发现,浮游植物生物量是程海真光层深度的最主要影响因子,悬浮物浓度次之（主要在雨季）,有色可溶性有机物的影响甚微.为进一步识别程海真光层深度的间接影响因子,本文还分析了浮游植物生物量与生态因子的相关性.本研究可为程海的保护和治理积累数据并提供借鉴.     

抚仙湖有色可溶性有机物的来源组成与时空变化

基于2017年1-12月在抚仙湖开展的逐月观测,利用紫外-可见吸收光谱和三维荧光光谱技术探讨该湖有色可溶性有机物（CDOM）的来源组成及时空变化特征.12个月CDOM吸收值a（254）的均值为3.47±0.57 m^-1,范围为1.82~5.22 m^-1,说明CDOM丰度较低.平行因子分析结果给出了2种类酪氨酸荧光组分（C1和C3）、1种类色氨酸荧光组分（C2）、1种类腐殖质荧光组分（C4）,12个月内源组分（C1+C3）对总荧光强度的平均贡献为65.81%±15.38%,外源组分（C2+C4）的平均贡献为34.19%±15.38%;荧光指数FI的均值为1.73±0.14,腐殖化指数HIX的均值为1.02±0.37,生源化指数BIX的均值为1.23±0.27,说明CDOM主要为微生物内源产生.时空变化方面,春（3-5月）、夏（6-8月）、秋（9-11月）和冬（1、2、12月）季的a（254）分别为3.20±0.47、3.76±0.64、3.67±0.50和3.23±0.38 m^-1,夏季和秋季均显著高于冬季和春季;CDOM丰度及内外源组分的空间分布具有季节异质性,可能与流域土地利用、河流输入、降雨、温度、光辐射等因素有关.