Dependence of Venus ionopause altitude and ionospheric magnetic field on solar wind dynamic pressure

The shape of the dayside Venus ionopause, and its dependence on solar wind parameters, is examined using Pioneer Venus Orbiter field and particle data. The ionopause is defined here as the altitude of pressure equality between magnetosheath magnetic pressure and ionospheric thermal pressure; its typical altitudes range from ～300 km near the subsolar point to ～900 km near the terminator. A strong correlation between ionopause altitude and magnetosheath magnetic pressure is demonstrated; correlation between magnetic pressure and the normally incident component of solar wind dynamic pressure is also evident. The data support the hypothesis of control of the ionopause altitude by solar wind dynamic pressure, manifested in the sheath as magnetic pressure. The presence of large scale magnetic fields in the ionosphere is observed primarily when dynamic pressure is high and the ionopause is low.     

极端太阳风条件下的磁层顶位形

基于极端太阳风条件下的三维MHD数值模拟数据, 构建了一种极端太阳风条件下的三维非对称磁层顶位形模型. 所提出的模型考虑了行星际南向磁场(IMF) B_z日下点距离侵蚀的饱和效应, 太阳风动压B_d对磁层顶张角影响的饱和效应, 赤道面、昼夜子午面磁层顶的不对称性以及极尖区的内凹结构和内凹中心的移动, 并利用Levenberg-Marquart多参量非线性拟合方法拟合了模型参数. 数值模拟研究表明, 在极端太阳风条件下, 随B_d增大, 磁层顶日下点距离减小, 磁层顶磁尾张角几乎不变; 随南向(IMF)B_z增大, 磁层顶日下点距离略有减小, 磁层顶磁尾张角减小, 极尖区内凹中心向低纬移动. 通过对2010年8月1日太阳风暴事件验证发现, 本文所建立的模型能够描述极端太阳风条件下的三维磁层顶位形.      

磁层顶穿越事件自动识别算法

磁层顶是太阳风与磁层进行质量、动量、能量交换的关键区域.磁层顶穿越事件(MCEs)可通过对卫星探测到的粒子能谱和磁场数据图进行人工分析的方式来识别.因太阳风动压和行星际磁场的易变性,位于磁层顶附近的卫星经过长期观测可能会经历成千上万次的磁层顶穿越.人工分析的方法工作量巨大,而且识别速度慢.本文发展了一种新的日下点附近M...     

EUV imaging of near-Venus space

Japan’s Venus Climate Orbiter (the Planet-C spacecraft) will be launched in 2008 and will reach an orbit in the ecliptic plane around Venus in 2009. We propose two eXtreme UltraViolet (XUV) imagers to take global two-dimensional snapshots of near-Venus space, including the Venus ionosphere and the interaction region between the solar wind plasma and the Venus ionospheric plasma. The imagers detect the resonantly scattering emissions of oxygen ions (O II 83.4 nm) and atoms (O I 130.3 nm), neutral helium (He I 58.4 nm), and hydrogen (H Ly-α 121.6 nm). Scientific goals are to investigate mechanisms of momentum and mass transfer across the ionopause, of convection in the upper atmosphere and ionosphere, and of atmospheric escape. Especially, we emphasize that sequential images of the O II 83.4-nm emission will enable us to understand temporal evolution of the vortex produced by the Kelvin–Helmholtz (K–H) instability. Though the wave structure due to the K–H instability is generated also at the terrestrial magnetopause, oxygen ions are too tenuous to detect the emission. On the other hand, at the Venus ionopause oxygen ions have enough density to image the resonance emission, i.e., the Venus ionosphere plays a role as a space laboratory for plasma physics.     

A Theoretic Interpretation of Movement of the Cusp Equatorward Boundary

A preliminary model is proposed to describe quantitatively the position and movement of cusp equatorward boundary. This integrated model, consisting of an empirical model of the magnetopause and a compressed dipolar model of Open/Closed field line, connects quantitatively the solar wind conditions, subsolar magnetopause and cusp equatorward boundary. It is shown that the increasing solar wind dynamic pressure and the increasing southward Interplanetary Magnetic Field (IMF) component drive the magnetopause to move inward and the cusp equatorward. This model is adopted to interpret quantitatively the cusp movement of August 14, 2001 observed by Cluster. The results show that the subsolar magnetopause moved earthward from 10.7 He to 9.0 Re during the period of 002300-002800 UT, and correspondingly the cusp equatorward boundary shifted equatorward. The observations of Cluster C1 and C4 show the cusp equatorward boundary that Cluster Cl and C4 were crossing during same interval moved equatorward by 4.6°. The cusp equatorward boundary velocity computed in the theoretical model (10.7km/s) is in good agreement with the observed value (9.4km/s) calculated from the data of CIS of Cluster C4 and C1.     

Forecasting magnetopause crossing locations by using Neural Networks

Given the highly complex and nonlinear nature of Near Earth Space processes, mathematical modeling of these processes is usually difficult or impossible. In such cases, modeling methods involving Artificial Intelligence may be employed. We demonstrate that data driven models, such as the Neural Network based approach, shows promise in its ability to forecast or predict the behavior of these processes. In this paper, modeling studies for forecasting magnetopause crossing locations are summarized and a Neural Network algorithm is presented to describe the nonlinear time-dependent response of the subsolar region of the magnetopause to varying solar wind conditions. In our approach the past history of the solar wind has, for the first time to the best knowledge of the authors, been included in forecasting the subsolar region of the magnetopause. It is proposed that the data driven approach is a valid approach to understanding and modeling the physical phenomena of Near Earth Space. The only basic requirement for the data driven approach is the availability of representative data for the phenomena. The objective of this paper is to demonstrate that by using WIND and GEOTAIL satellite data a Neural Network based model can be adapted to the modeling of the Earth’s magnetopause.     

偶极倾角对弓激波日下点距离和尾部张角的影响

通过对IMP 8,Geotail,Magion 4和Cluster 1四颗卫星弓激波穿越数据的统计及拟合分析,定量研究了偶极倾角对弓激波日下点距离和尾部张角的影响.结果表明:弓激波日下点距离随偶极倾角绝对值的增大而增大,且偶极倾角为负值时比其为正值时日下点距离增大的幅度更大;弓激波尾部张角随偶极倾角绝对值增大而减小;当偶极倾角由负变为正的时候,弓激波向地球一侧移动,同时尾部张角增大.研究结果为进一步建立包含偶极倾角效应的弓激波模型奠定了基础.      

Plasma measurements in the Venus near wake

A study of the plasma measurements conducted with the Mariner 5, Venera 9 and 10, and the Pioneer Venus spacecraft in the Venus ionosheath and near wake is presented. The data available indicate that in the inner ionosheath, downstream from the terminator, the density and the velocity of the plasma are significantly smaller than those measured further outside. The slower particle fluxes detected near the ionopause also exhibit higher plasma temperatures and show a tendency to move towards the nightside hemisphere. The observation of high plasma temperatures in the inner ionosheath indicates that the interaction of the solar wind with the Venus ionospheric/exospheric material is dominated by dissipative phenomena, and that its entry into the wake is due to local thermal expansion processes.     

The Venus ionosphere

Physical properties of the Venus ionosphere obtained by experiments on the US Pioneer Venus and the Soviet Venera missions are presented in the form of models suitable for inclusion in the Venus International Reference Atmosphere. The models comprise electron density (from 120 km), electron and ion temperatures, and relative ion abundance in the altitude range from 150 km to 1000 km for solar zenith angles from 0° to 180°. In addition, information on ion transport velocities, ionopause altitudes, and magnetic field characteristics of the Venus ionosphere, are presented in tabular or graphical form. Also discussed is the solar control of the physical properties of the Venus ionosphere.     

Spatial and temporal variations of the ion velocity measured in the venus ionosphere

The average velocity of the Venus ionosphere is nightward and approximately symmetric about the sun-Venus axis. We report here on temporal and spatial deviations from this average flow and their effects on the ionosphere. Temporal variability of the ion flux affects the main ionization source on the nightside. The influence of the solar wind is seen in the correlation between nightside ion density and ionopause height. Spatial asymmetries include a low-altitude superrotation (v-dawn < v-dusk) component related to superrotation of the neutral atmosphere, and a high-altitude prograde (v-dawn > v-dusk) component that is shown to be the result of asymmetric pressure gradients on the dayside.     

Comparative analysis of Venusian ionosphere dual-frequency radio soundings with the satellites Venera-9, 10 and Pioneer-Venus

A comparison of the Venusian ionosphere electron density profiles obtained by the dual-frequency radio occultation method for the satellites Venera-9,10 (1975) and the Pioneer-Venus orbiter (1978–1979) has been carried out. It is shown that the general nature of the profiles, the main maximum heights and electron densities, the ionopause height positions determined with the satellites Venera-9, 10 on the one hand and Pioneer-Venus orbiter on the other hand are in good agreement. This fact testifies to the reliability of the experimental dual-frequency radio occultation data. An attempt to compare the radio occultation data with the direct measurements points out the essential contradictions between them, which need detailed analysis and discussion.     

Negative VTEC with spherical harmonic expansion model under different solar activity conditions

The Earth’s ionosphere can be described by a spherical harmonic (SH) expansion up to a specific degree. However, there exist negative vertical total electron content (VTEC) values in the global ionosphere map (GIM) with the SH expansion model. In this contribution, we specifically investigated the negative VTEC values that are induced by the SH expansion model and validated the performance of the inequality-constrained least squares (ICLS) method in eliminating the negative VTEC values. The GPS data from 2004 to 2017 was selected to cover one solar cycle and the experiments under different solar activity conditions were analyzed. The results in our work show that the occurrence of the negative VTEC values is attributed to the deficiency of the SH expansion model when the VTEC itself is small instead of the unevenly distribution of the GNSS stations. The negative VTEC values appear periodically in the temporal domain, showing apparently one year and half year periods. During one year, two peaks in June and December can be observed in the time series of the negative VTEC values. The number of negative VTEC values in June is obvious larger than that in December. During one solar cycle, the number of negative VTEC values under quiet solar activity condition is obvious larger than that under strong solar activity condition. In the spatial domain, the appearance of the negative VTEC values is strongly related with the movement of the subsolar point. In the latitude of the subsolar point has the largest magnitude, the negative values will appear on the opposite hemisphere and the further from the subsolar point the more negative values. The maximum number of the negative VTEC values in the southern hemisphere appears in June, while the peak value in the northern hemisphere appears in December. The maximum number of negative VTEC values in the southern hemisphere is generally larger than that in the northern hemisphere. In addition, the negative VTEC values are distributed both at middle latitude and high latitude in the southern hemisphere, while they are mainly distributed at high latitude in the northern hemisphere. When the ICLS method is used, the negative VTEC values can be eliminated efficiently and it has nearly no influence on the positive VTEC values. The ICLS method can also improve the receiver’s differential code bias (DCB) and significantly decrease the unreasonable negative slant TEC (STEC) values along the lines of sight. Using the final GIM product of the Jet Propulsion Laboratory (JPLG) as a reference, the root mean square (RMS) of the ICLS solution shows maximum 25%, 20% and 45% improvement relative to the least squares (LS) solution at northern high latitude, southern middle latitude and southern high latitude, respectively.     

MGS magnetic fields and electron reflectometer investigation: Discovery of paleomagnetic fields due to crustal remanence

The MGS MAG/ER investigation provides vector measurements of the magnetic field and the ionospheric electron density near periapsis. The instrumentation consists of twin triaxial fluxgate magnetometers and a “top hat” electrostatic analyzer of electrons in the energy range of 1 eV to 20 keV. Results include extensive mapping of the detached bow shock wave in the solar wind, the nature and structure of the ionopause and magnetic pile-up boundaries. The most surprising result is the discovery of localized magnetic fields of planetary origin, presumably due to remanent crustal magnetization. Maximum fields measured while below the ionosphere exceed 300 nT. We present an initial study of one of these very localized fields and extrapolations to the Martian surface in the Acidalia planitia.     

Influence of lunar topography on simulated surface temperature

The surface temperature of the Moon is one of the essential parameters for the lunar exploration, especially to evaluate the Moon thermophysical features. The distribution of the temperature is heavily influenced by the Moon topography, which, however, is rarely studied in the state-of-art surface temperature models. Therefore, this paper takes the Moon topography into account to improve the surface temperature model, Racca model. The main parameters, such as slopes along the longitude and latitude directions, are estimated with the topography data from Chang’E-1 satellite and the Horn algorithm. Then the effective solar illumination model is then constructed with the slopes and the relative position to the subsolar point. Finally, the temperature distribution over the Moon surface is obtained with the effective illumination model and the improved Racca model. The results indicate that the distribution of the temperature is very sensitive to the fluctuation of the Moon surface. The change of the surface temperature is up to 150 K in some places compared to the result without considering the topography. In addition, the variation of the surface temperature increases with the distance from the subsolar point and the elevation, along both latitude and longitude directions. Furthermore, the simulated surface temperature coincides well with the brightness temperature in 37 GHz observed by the microwave sounder onboard Chang’E-2 satellite. The corresponded emissivity map not only eliminates the influence of the topography, but also hints the inherent properties of the lunar regolith just below the surface. Last but not the least, the distribution of the permanently shadowed regions (PSRs) in the lunar pole area is also evaluated with the simulated surface temperature result.     

Mass-loading at Venus: Theoretical expectations

Previous theoretical studies of the solar wind interaction with Venus have generally involved gas dynamic models of flow past the impenetrable ionopause obstacle. However, because these studies neglected the effects of planetary ion production or mass-loading in the magnetosheath, they did not accurately reproduce certain observed features such as the position of the bow shock. Recent numerical calculations include a source term in the gas dynamic equations to represent the photoionization of the hot atomic oxygen corona. These produce results which agree more closely with both the measured flared shape of the bow shock and properties of the magnetosheath plasma and magnetic field. The solar wind interaction with Mars is also expected to be affected by mass-loading. These findings emphasize the similarity between the unmagnetized planets and comets.     

土星磁层顶的Kelvin-Helmholtz不稳定性和磁流体表面波

本文采用可压缩磁流体力学近似并选取Voyager-1的观测数据,讨论了土星磁层顶的Kelvin-Helmholtz不稳定性和磁流体表面波.计算表明,土星磁层随星体自转同磁鞘流之间的速度差,可以在地方时午前、中午和距中午不远的午后磁层顶激发不稳定性;午前和午后区有很强的非对称性.计算所得的波的特性与观测一致.      

Electron densities and temperatures in the Venus ionosphere: Effects of solar EUV,solar wind pressure and magnetic field

The Venus ionosphere is influenced by variations in both solar EUV flux and solar wind conditions. On the dayside the location of the topside of the ionosphere, the ionopause, is controlled by solar wind dynamic pressure. Within the dayside ionosphere, however, electron density is affected mainly by solar EUV variations, and is relatively unaffected by solar wind variations and associated magnetic fields induced within the ionosphere. The existence of a substantial nightside ionosphere of Venus is thought to be due to the rapid nightward transport of dayside ionospheric plasma across the terminator. Typical solar wind conditions do not strongly affect this transport and consequently have little direct influence on nightside ionospheric conditions, except on occasions of extremely high solar wind dynamic pressure. However, both nightside electron density and temperature are affected by the presence of magnetic field, as in the case of ionospheric holes.     

Empirical model of the composition of the Venus ionosphere: Repeatable characteristics and key features not modeled

In-situ measurements of positive ion composition of the ionosphere of Venus are combined in an empirical model which is a key element for the Venus International Reference Atmosphere (VIRA) model. The ion data are obtained from the Pioneer Venus Orbiter Ion Mass Spectrometer (OIMS) which obtained daily measurements beginning in December 1978 and extending to July 1980 when the uncontrolled rise of satellite periapsis height precluded further measurements in the main body of the ionosphere. For this period, measurements of 12 ion species are sorted into altitude and local time bins with altitude extending from 150 to 1000 km. The model results exhibit the appreciable nightside ionosphere found at Venus, the dominance of atomic oxygen ions in the dayside upper ionosphere and the increase in prominence of atomic oxygen and deuterium ions on the nightside. Short term variations, such as the abrupt changes observed in the ionopause, cannot be represented in the model.     

Evidence for mass-loading of the Venus magnetosheath

The observed magnetic field configuration in the Venus magnetosheath contains information about the solar wind mass-loading processes occurring as a result of the extension of the neutral atmosphere into the magnetosheath. In this paper, magnetic field signatures of various mass-loading processes are discussed and experimental results from the Pioneer Venus Orbiter magnetometer experiment are examined for evidence of these signatures. The data suggest that the ?$\text{V}$X$\text{B}$ acceleration process, stochastic pickup of ionospheric ions, and $\text{J}$X$\text{B}$ force “scavenging” at the ionopause all occur at various times.     

A mathematical description of the annual variation of the temperature-height profile of the stratosphere at Berlin

Since 1958, daily temperature-height profiles have been measured up to 35–40 km at Berlin by means of radiosondes. An attempt is made here to describe these profiles as a function of the noon solar zenith angle, χ. It is shown that the basic annual variation of the measured profiles, T(h), can be presented as T(h) = T_o(h) cos^n(h)_χ. The subsolar temperature, T_o(h), and exponent, n(h), have been determined empirically from the summer and autumn data when the radiative balance is obviously dominant. Neither term depends on the solar cycle. Warmings in winter and coolings in spring are treated as disturbances in the annual variation, due to dynamics, and are described separately as ± ΔT(h).