Evolution of solar wind structures from 0.72 to 1 AU

Understanding the evolution of solar wind structures in the inner heliosphere as they approach the Earth is important to space weather prediction. From the in situ solar wind plasma and magnetic field measurements of Pioneer Venus Orbiter (PVO) at 0.72 AU (1979–1988), and of Wind/Advanced Composition Explorer (ACE) missions at 1 AU (1995–2004), we identify and characterize two major solar wind structures, stream interaction regions (SIRs) and interplanetary coronal mass ejections (ICMEs). The average percentage of SIRs occurring with shocks increases significantly from 3% to 24% as they evolve from 0.72 to 1 AU. The average occurrence rate, radial extent, and bulk velocity variation of SIRs do not change from 0.72 to 1 AU, while peak pressure and magnetic field strength both decrease with the radial evolution of SIRs. Within the 0.28 AU distance from the orbit of Venus to that of Earth, the average fraction of ICMEs with shocks increases from 49% to 66%, and the typical radial extent of ICMEs expands by about a fraction of 1.4, with peak pressure and magnetic field strength decreasing significantly. The mean occurrence rate and expansion velocity of ICMEs do not change from 0.72 to 1 AU.     

Analysis of NASA’s DSN Venus Express radio occultation data for year 2014

The Venus Express Radio Science Experiment (VeRa) was part of the scientific payload of the Venus Express (VEX) spacecraft and was targeted at the investigation of Venus’ atmosphere, surface, and gravity field as well as the interplanetary medium. This paper describes the methods and the required calibrations applied to VEX-VeRa raw radio occultation data used to retrieve vertical profiles of Venus’ ionosphere and neutral atmosphere. In this work we perform an independent analysis of a set of 25 VEX, single-frequency (X-band), occultations carried out in 2014, recorded in open-loop at the NASA Deep Space Network. Our temperature, pressure and electron density vertical profiles are in agreement with previous studies available in the literature. Furthermore, our analysis shows that Venus’ ionosphere is more influenced by the day/night condition than the latitude variations, while the neutral atmosphere experiences the opposite. Our scientific interpretation of these results is based on two major responsible effects: Venus’ high thermal inertia and the zonal winds. Their presence within Venus’ neutral atmosphere determine why in these regions a latitude dependence is predominant on the day/night condition. On the contrary, at higher altitudes the two aforementioned effects are less important or null, and Venus’ ionosphere shows higher electron density peaks in the probed day-time occultations, regardless of the latitude.     

Hybrid simulations of the O ion escape from Venus: Influence of the solar wind density and the IMF x component

As an initial effort to study the evolution of the Venus atmosphere, the influence of the solar wind density and the interplanetary magnetic field (IMF) x component (the x-axis points from Venus towards the Sun) on the O⁺ ion escape rate from Venus is investigated using a three-dimensional quasi-neutral hybrid (HYB-Venus) model. The HYB-Venus model is first applied to a case of the high-density (100 cm⁻³) solar wind interaction with Venus selected from the Pioneer Venus Orbiter observations to demonstrate its capability for the study. Two sets of simulations with a wide range of solar wind densities and different IMF x components are then performed. It is found that the O⁺ ion escape rate increases with increasing solar wind density. The O⁺ ion escape rate saturates when the solar wind density becomes high (above 100 cm⁻³). The results also suggest that the IMF x component enhances the O⁺ ion escape rate, given a fixed IMF component perpendicular to the x-axis. Finally, the results imply a higher ion loss rate for early-Venus, when solar conditions were dramatically different.     

地球同步轨道高能电子增强事件预报方法

分析了地球同步轨道高能电子通量增强事件的发生规律及其与太阳风和行星际磁场参数的关系，并在此基础上建立了基于人工神经网络的高能电子增强事件模式，经实测数据检验，预报模式可以对未来1天的高能电子通量进行预报，误差为8．2％，达到了较高水平．     

Electromagnetic waves observed by Venus Express at periapsis: Detection and analysis techniques

The magnetometer on Venus Express was designed to be able to obtain 128 Hz samples of the magnetic field from two sensors in a gradiometer configuration. This mode is used around periapsis to determine whether the signals reported at low altitudes near 100 Hz, had the properties of electromagnetic waves generated by electric discharges in the Venus atmosphere. The lack of a magnetic cleanliness program and the shortness of the magnetometer boom make this a challenging measurement. Fortunately the signals are sufficiently strong that they can be easily resolved with rather straightforward analysis techniques.     

Neutral atmosphere and crustal magnetization as factors determining differences in plasma convection and magnetic fields distribution in the ionospheres of Mars and Venus

The pattern of the magnetic field/plasma convection can be, to some extent, recovered from the magnetic field measurements by employing either theoretical or numerical models. We use the MAG/ER day-time measurements of the magnetic field at the altitudes from 90 to 180 km during the elliptical orbits of MGS. Analysis of the altitude variation of the characteristics of the large-scale magnetic fields, which were measured some distance away from strong crustal magnetic anomalies, is summarized. The low density of the Martian atmosphere together with the crustal magnetization result in critical differences in plasma convection which are followed by remarkable differences of the magnetic field features within the ionosphere of Venus and Mars (even in its northern hemisphere where the crustal magnetization is, on the average, low) and distribution of currents.     

磁鞘等离子体密度统计建模

利用理想磁流体LFM模型的模拟数据,基于非参数统计方法对2004年11月14日03:00UT-07:00UT磁暴恢复相期间磁鞘等离子体平均密度进行建模.分析磁鞘等离子体平均密度与上游太阳风参数、行星际磁场参数及地磁扰动参数的统计关系,建立基于数据降维的经验模型.结果表明,电离层扰动强度因子、太阳风-磁层耦合强度因子和日地空间因果链耦合强度因子是影响磁鞘等离子体平均密度的三个主要方面.磁暴恢复相期间电离层上行离子是磁层环电流和磁尾等离子体的重要离子来源.建模分析过程表明,利用经验模型对空间物理过程开展建模,数据的严重多重共线性通常会导致模型的精度较差.而利用SIR和LPR建立的磁鞘等离子体平均密度随相关参数变化的经验模型可以有效解决该问题,具有较好的预测精度,统计特征显著.      

A solar wind driven model of geosynchronous plasma moments

We describe a tabular specification model of the density and temperature of ions and electrons at geosynchronous orbit as a function of magnetic local time and solar wind parameters. This model can be used to provide boundary conditions for numerical ring current models. Unlike previous specification models of geosynchronous plasma moments, this model is parameterized by upstream solar wind conditions. We find that solar wind parameters are a better predictor of geosynchronous ion density than magnetospheric indices, and as upstream parameters they are often more appropriate as model inputs since they causally precede the model outputs. Of the upstream parameters that were tested, the best predictors of geosynchronous conditions were the solar wind flow pressure and the magnitude and Z-component of the interplanetary magnetic field.     

High resolution measurements of nightside ion troughs at venus: Evidence of electrodynamic perturbations

The Bennett rf ion mass spectrometer (OIMS) on the Pioneer Venus Orbiter was particularly designed to provide variable temporal resolution for measurements of thermal ion composition and density. An Explore-Adapt mode is used to obtain priority for measurement of most prominent ion species, and in a 2/16 configuration, the two dominant ions within the available range of 16 species are selectively sampled at the highest rate of 0.2 sec/sample. The high resolution measurements are combined with independent observations from the magnetic field (OMAG), neutral mass spectrometer (ONMS), and electron temperature (OETP) experiments to investigate sharply structured troughs in the low altitude nightside ion concentrations. The results indicate a close correlation between the structure in the ion distributions and the structured configuration of the magnetic field which is draped about the planet. In the regions of the ion depletions, sharp fluctuations in electron temperature and anomalous increases in the density of neutral gases indicate that the ion depletion may be associated either with dynamic perturbation in the ion and neutral flows, and/or local joule heating. The configuration of the ion flow/magnetic field draping and consequent electric fields for these events must be analyzed in detail to understand the relationships.     

Revised upper limit on the internal magnetic moment of Venus

Over four Venus years of low altitude nightside PVO magnetometer observations are used to establish a new upper limit for the magnetic moment of Venus. Improvements over previous studies include data coverage and new instrument calibration information. The upper limit on an internal dipole moment is determined to be 8.4 × 10¹⁰ T m³.     

行星际结构与垂直无碰撞激波的相互作用

应用一维混合模拟方法数值研究了两种行星际结构──反向磁场和高密度等离子团与垂直无碰撞激波的相互作用．结果表明，随着激波上游区磁场的反向，下游区磁场将逐渐改变符号，且等离子体密度和速度分别呈现较强的湍动．激波上游和下游的物理量依然满足Rankine-Hugonoit关系．当高密度的等离子体团通过垂直无碰撞激波时，部分质子被激波反射，部分质子被加速并进入下游区域．由于质子速度分布为非Maxwell分布，在激波下游也激发出较强的湍动．     

Solar wind governing the response of Venusian atmosphere and ionosphere

The ionosphere of Venus is primarily formed by photoionization of a gaseous blanket around Venus. The impact ionization by energetic solar charged particles also plays an important role in the variability of Venusian ionospheric ion, electron density and their temperature profiles. The microscopic variations in the solar wind velocity, particle flux and orientations of frozen-in interplanetary magnetic field determine the solar wind interaction with the Venusian ionosphere. The ion and electron density profiles obtained by Pioneer Venus Orbiter and Pioneer Venus Entry Probes have been analysed in the light of simultaneous solar wind velocity and particle flux. Marked changes in height profiles of ion, electron densities and their temperatures have been found to correlate with the simultaneous changes in the solar wind velocity and particle flux. It is shown that the solar wind plays a more important role in controlling the physical properties and behavior of daytime as well as nighttime ionosphere of Venus, whereas the solar xuv sustains the primary ionization process.     

月球尾迹的二维MHD模拟

采用理想的二维单流体MHD方程,对太阳风通过月球时所形成的尾迹结构进行数值模拟,得到了太阳风尾迹的粒子分布及磁场分布.模拟结果表明,在月球背阳面的本影区,太阳风粒子密度急剧下降,行星际磁场增强.当行星际磁场与太阳风流动方向平行时,尾迹被拖得很长,而磁场与太阳风流动方向垂直时,尾迹较短.      

Saturn radio emission and the solar wind: Voyager-2 studies

Voyager 2 data from the Plasma Science experiment, the Magnetometer experiment and the Planetary Radio Astronomy experiment were used to analyze the relationship between parameters of the solar wind/interplanetary medium and the nonthermal Saturn radiation. Solar wind and interplanetary magnetic field properties were combined to form quantities known to be important in controlling terrestrial magnetospheric processes.The Voyager 2 data set used in this investigation consists of 237 days of Saturn preencounter measurements. However, due to the immersion of Saturn and the Voyager 2 spacecraft into the extended Jupiter magnetic tail, substantial periods of the time series were lacking solar wind data. To cope with this problem a superposed epoch method (CHREE analysis) was used. The results indicate the superiority of the quantities containing the solar wind density in stimulating the radio emission of Saturn — a result found earlier using Voyager 1 data — and the minor importance of quantities incorporating the interplanetary magnetic field.     

Fluctuations of cosmic rays and IMF in the vicinity of interplanetary shocks

Fluctuations of cosmic rays and interplanetary magnetic field upstream of interplanetary shocks are studied using data of ground-based polar neutron monitors as well as measurements of energetic particles and solar wind plasma parameters aboard the ACE spacecraft. It is shown that coherent cosmic ray fluctuations in the energy range from 10 keV to 1 GeV are often observed at the Earth’s orbit before the arrival of interplanetary shocks. This corresponds to an increase of solar wind turbulence level by more than the order of magnitude upstream of the shock. We suggest a scenario where the cosmic ray fluctuation spectrum is modulated by fast magnetosonic waves generated by flux of low-energy cosmic rays which are reflected and/or accelerated by an interplanetary shock.     

行星际结构与准平行无碰撞激波的相互作用

应用一维混合模拟方法数值研究了高密度等离子体团和行星际激波与准平行无碰撞激波的相互作用．结果表明，由于推平行无碰撞激波上游的大振幅低频波动的散射，除了在通过激波过渡区时稍有压缩外，等离子体团从激波的上游开始就一直是不断弥散的．行星际激波在向准平行无碰撞激波靠近的过程中，会在其上游产生大振幅的低频波动，同时行星际激波的强度不断增加，最后和准平行无碰撞激波会并成一个新的激波，在新激波前继续有大振幅的低频波动产生     

Ionospheric storm due to solar Coronal mass ejection in September 2017 over the Brazilian and African longitudes

Coronal mass ejection (CME) occurs when there is an abrupt release of a large amount of solar plasma, and this cloud of plasma released by the Sun has an intrinsic magnetic field. In addition, CMEs often follow solar flares (SF). The CME cloud travels outward from the Sun to the interplanetary medium and eventually hits the Earth’s system. One of the most significant aspects of space weather is the ionospheric response due to SF or CME. The direction of the interplanetary magnetic field, solar wind speed, and the number of particles are relevant parameters of the CME when it hits the Earth’s system. A geomagnetic storm is most geo-efficient when the plasma cloud has an interplanetary magnetic field southward and it is accompanied by an increase in the solar wind speed and particle number density. We investigated the ionospheric response (F-region) in the Brazilian and African sectors during a geomagnetic storm event on September 07–10, 2017, using magnetometer and GPS-TEC networks data. Positive ionospheric disturbances are observed in the VTEC during the disturbed period (September 07–08, 2017) over the Brazilian and African sectors. Also, two latitudinal chains of GPS-TEC stations from the equatorial region to low latitudes in the East and West Brazilian sectors and another chain in the East African sector are used to investigate the storm time behavior of the equatorial ionization anomaly (EIA). We noted that the EIA was disturbed in the American and African sectors during the main phase of the geomagnetic storm. Also, the Brazilian sector was more disturbed than the African sector.     

Interplanetary coronal mass ejections during the descending cycle 23: Sheath and ejecta properties comparison

We have used Omniweb data in order to identify the sheath and the ejecta boundaries of 67 shock-driving interplanetary coronal mass ejections during the time period 2003–2006. We examine and compare their statistical properties (speed, magnetic field strength, proton density and temperature, proton plasma beta), with those of the typical solar wind. We also calculate their passage time and radial width. We study the correlation between the ejecta and sheath characteristics.     

The solar wind control of the equatorial ionosphere dynamics during geomagnetic storms

The interplanetary magnetic field, geomagnetic variations, virtual ionosphere height h′F, and the critical frequency foF2 data during the geomagnetic storms are studied to demonstrate relationships between these phenomena. We study 5-min ionospheric variations using the first Western Pacific Ionosphere Campaign (1998–1999) observations, 5-min interplanetary magnetic field (IMF) and 5-min auroral electrojets data during a moderate geomagnetic storm. These data allowed us to demonstrate that the auroral and the equatorial ionospheric phenomena are developed practically simultaneously. Hourly average of the ionospheric foF2 and h′F variations at near equatorial stations during a similar storm show the same behavior. We suppose this is due to interaction between electric fields of the auroral and the equatorial ionosphere during geomagnetic storms. It is shown that the low-latitude ionosphere dynamics during these moderate storms was defined by the southward direction of the B_z-component of the interplanetary magnetic field. A southward IMF produces the Region I and Region II field-aligned currents (FAC) and polar electrojet current systems. We assume that the short-term ionospheric variations during geomagnetic storms can be explained mainly by the electric field of the FAC. The electric fields of the field-aligned currents can penetrate throughout the mid-latitude ionosphere to the equator and may serve as a coupling agent between the auroral and the equatorial ionosphere.     

利用BP神经网络预测AE指数

采用的预报模式是一种全连接的BP网络模型,利用太阳风及行星际磁场的观测数据预报AE指数.神经网络输入选用ACE卫星数据,取5 min平均值,通过比较,选用4个预报参量.构造了预报参量时续为20 min,40 min和60 min依次递增的三个网络,分别进行训练和预测,并对行星际参量对AE指数影响的时续性进行了探讨.预报结果表明,全连接BP神经网络在AE指数的短期预报中是比较有效的,同时还提出了需要进一步改进的环节.