Hydrogen and helium escape on Venus via energy transfer from hot oxygen atoms

ABSTRACT Due to the relatively strong gravity on Venus, heavy atmospheric neutrals are difficult to accelerate to the escape velocity. However, a variety of processes, such as the dissociative recombination of ionospheric O$_2^+$, are able to produce hot atoms which could deliver a significant amount of energy to light neutrals and drive their escape. In this study, we construct a Monte Carlo model to simulate atmospheric escape of three light species, H, H2, and He, on Venus via such a knock-on process. Two Venusian background atmosphere models are adopted, appropriate for solar minimum and maximum conditions. Various energy-dependent and species-dependent cross-sections, along with a common strongly forward scattering angle distribution, are used in our calculations. Our model results suggest that knock-on by hot O likely plays the dominant role in driving total atmospheric hydrogen and helium escape on Venus at the present epoch, with a significant portion contributed from regions below the exobase. Substantial variations are also revealed by our calculations. Of special interest is the modelled reduction in escape flux at high solar activities for all species, mainly associated with the enhancement in thermal O concentration near the exobase at high solar activities which hinders escape. Finally, model uncertainties due to several controlling factors, including the distribution of relevant light species in the background atmosphere, the plane-parallel approximation, and the finite O energy distribution, are evaluated.

Quantifying the redshift space distortion of the bispectrum I: primordial non-Gaussianity

ABSTRACT The anisotropy of the redshift space bispectrum contains a wealth of cosmological information. This anisotropy depends on the orientation of three vectors $\boldsymbol {k_1},\boldsymbol {k_2},\boldsymbol {k_3}$ with respect to the line of sight. Here, we have decomposed the redshift space bispectrum in spherical harmonics which completely quantify this anisotropy. To illustrate this, we consider linear redshift space distortion of the bispectrum arising from primordial non-Gaussianity. In the plane-parallel approximation, only the first four even ℓ multipoles have non-zero values, and we present explicit analytical expressions for all the non-zero multipoles, that is, upto ℓ = 6 and m = 4. The ratio of the different multipole moments to the real-space bispectrum depends only on β1 the linear redshift distortion parameter and the shape of the triangle. Considering triangles of all possible shapes, we have studied how this ratio depends on the shape of the triangle for β1 = 1. We have also studied the β1 dependence for some of the extreme triangle shapes. If measured in future, these multipole moments hold the potential of constraining β1. The results presented here are also important if one wishes to constrain fNL using redshift surveys.

KINEMATIC MODEL OF THE PARALLEL APPROXIMATION METHOD

В данной статье рассматривается модель задачи преследования методом параллельного сближения. Целью данной статьи является модификация метода параллельного сближения для того, чтобы учитывать случай, когда в момент начала преследования вектор скорости преследователя направлен не на цель. Кроме того, в рассматриваемой в статье модели, преследователь не может мгновенно изменять направление движения. То есть происходит наложение условия, что радиус кривизны траектории движения преследователя не может быть меньше определенной величины. Предлагаемый метод основан на том, преследователь, выбирая шаг на этапе итераций, будет стараться следовать прогнозируемым траекториям. По материалам статьи написана тестовая программа, которая рассчитывает траекторию преследователя, учитывая изложенные условия. Выполненное анимированное изображение визуализирует изменение координат преследователя, цели и прогнозируемых траекторий от времени.

Asymptotic boundary conditions for the analysis of hydrodynamic stability of flows in regions with open boundaries

A new approach to formulation of asymptotic boundary conditions for eigenvalue problems arising in numerical analysis of hydrodynamic stability of such shear flows as boundary layers, separations, jets, wakes, characterized by almost constant velocity of the main flow outside the shear layer or layers is proposed and justified. This approach makes it possible to formulate and solve completely the temporal and spatial stability problems in the locally-parallel approximation, reducing them to ordinary algebraic eigenvalue problems.