Comparative outline of the region of interaction of the solar wind with the Ionosphere of Venus and Mars

The general features of the region of interaction of the solar wind with the ionosphere of Venus and Mars are compared using data obtained with the Mariner 5 and the Pioneer Venus Orbiter (PVO) spacecraft for Venus and with the Phobos II, the Mars Global Surveyor (MGS) and the Mars Express spacecraft for Mars. Despite the overall weak intrinsic global magnetic field that is present in both planets there are significant differences in the manner in which the interplanetary magnetic field accumulates and is organized around and within their ionosphere. Such differences are unrelated to the crustal magnetic field remnants inferred from the MGS measurements around Mars. In fact, while in Venus and Mars there is a region in which the magnetic field becomes enhanced as it piles up in their plasma environment it is shown that such a region exhibits different regimes with respect to changes in the ion composition measured outside and within the ionosphere. At Venus the region of enhanced magnetic field intensity occurs in general above the ionopause which represents the boundary across which there is a change in the ion composition with dominant solar wind protons above and planetary O⁺ ions below. At Mars the region of enhanced magnetic field is located below a magnetic pileup boundary across which there is also a comparable change in the ion composition (solar wind protons above and planetary O⁺ ions below). It is argued that this difference in the relative position of the region of enhanced magnetic field with respect to that of a plasma boundary that separates different ion populations results from the peculiar response of the ionosphere of each planet to the oncoming solar wind dynamic pressure. While at Venus the peak ionospheric thermal pressure is in general sufficient to withhold the incident solar wind kinetic pressure there is a different response in Mars where the peak ionospheric thermal pressure is in general not large enough to deviate the solar wind. In this latter case the ionosphere is unable to force the solar wind to move around the ionosphere and as a result the oncoming electron population can reach low altitudes where it is influenced by neutral atmospheric particles (the solar wind proton population is replaced at the magnetic pileup boundary which marks the upper extent of the region where the interplanetary magnetic field becomes enhanced). Peculiar conditions are expected near the magnetic polar regions and over the terminator plane where the solar wind is directed along the sides of the planet.     

WIND observations of coherent electrostatic waves in the solar wind

The time domain sampler (TDS) experiment on WIND measures electric and magnetic wave forms with a sampling rate which reaches 120 000 points per second. We analyse here observations made in the solar wind near the Lagrange point L1. In the range of frequencies above the proton plasma frequency f_pi and smaller than or of the order of the electron plasma frequency f_pe, TDS observed three kinds of electrostatic (e.s.) waves: coherent wave packets of Langmuir waves with frequencies f ≃ f_pe, coherent wave packets with frequencies in the ion acoustic range f_pi ≥ f ≥ f_pe, and more or less isolated non-sinusoidal spikes lasting less than 1 ms. We confirm that the observed frequency of the low frequency (LF) ion acoustic wave packets is dominated by the Doppler effect: the wavelengths are short, 10 to 50 electron Debye lengths λ_D. The electric field in the isolated electrostatic structures (IES) and in the LF wave packets is more or less aligned with the solar wind magnetic field. Across the IES, which have a spatial width of the order of ≃25_D, there is a small but finite electric potential drop, implying an average electric field generally directed away from the Sun. The IES wave forms, which have not been previously reported in the solar wind, are similar, although with a smaller amplitude, to the weak double layers observed in the auroral regions, and to the electrostatic solitary waves observed in other regions in the magnetosphere. We have also studied the solar wind conditions which favour the occurrence of the three kinds of waves: all these e.s. waves are observed more or less continuously in the whole solar wind (except in the densest regions where a parasite prevents the TDS observations). The type (wave packet or IES) of the observed LF waves is mainly determined by the proton temperature and by the direction of the magnetic field, which themselves depend on the latitude of WIND with respect to the heliospheric current sheet.     

太阳风在地球激波前兆区减速的统计研究

本文首次利用完全相同两颗卫星（CLUSTER C1和C3）的数据对地球激波前兆区太阳风的减速和偏转特性进行了统计研究.结果表明,在激波前兆坐标系中,太阳风减小的速度随观测点到激波的距离D_BS增大而减小,随行星际磁场与激波法向夹角θ_BN增大也减小,在ULF波动区深度D_WS小于6R_e(R_e为地球半径)的范围内最为显著;伴随着太阳风减速的另外一个现象——太阳风的偏转,也存在相似的规律.其最大减速和最大偏转角度分别为10 km/s和3°.太阳风减速和偏转,以及随之变化的太阳风动压,可能会引起地球磁层顶位置和形状发生改变,同时也为激波前兆区弥散（diffuse）离子的起源及加热提供了一种可能的机制.     

Interplanetary scintillation observations of interaction regions in the solar wind

Co-rotating interaction regions (CIRs) between fast and slow streams of plasma are a prominent feature of the solar wind. Measurements of interplanetary scintillation (IPS) using the three widely separated antennas of the EISCAT facility have been used to detect the compression regions at the leading edges of interaction regions and to determine the location and velocity of the structure. Observations show that interaction regions have developed as close to the Sun as 25–30 solar radii, a result supported by theoretical modelling which shows that the conditions needed for CIRs to develop exist inside 30 solar radii.     

近日太阳风中磁场回弯结构的MHD模拟

帕克太阳探针(Parker Solar Probe,PSP)在太阳附近发现大量磁力线回弯结构,通常还伴随有太阳风速度增加.这些磁力线回弯的产生机制到目前为止有多种解释,其中有代表性的一种是由慢太阳风中的喷流引起的.我们首先对PSP的就地观测数据进行了统计分析并给出了发生率和空间尺度随径向距离的演化情况,然后使用简化的1.5维磁流体动力学(magnetohydrodynamics,MHD)模型对喷流在太阳风中的演化进行了模拟,其中太阳风被简化为位于黄道面的球对称流.模拟结果表明喷流的确可以导致太阳附近磁力线发生偏转,验证了喷流可以对磁场方向改变有贡献的图景.不过喷流形成的原因还需要进一步研究.

     

等离子体片离子与太阳风及地磁条件的关联研究

本文根据搭载于Cluster卫星的CIS/CODIF和RAPID仪器的观测数据,统计研究了等离子体片中的H+、O+离子在磁暴期间的时间变化特性,及其对太阳风条件的响应.观测结果表明：(1) 磁暴开始前,O+离子（0~40 keV）数密度保持在较低水平.随着磁暴的发展,O+数密度缓慢上升,其峰值出现在Dst极小值附近;H+离子(0~40 keV)数密度在磁暴开始之前的较短时间迅速增加并达到峰值,在磁暴开始之后迅速降低,并在整个主相和恢复相期间保持在相对较低水平.更高能量的离子则在磁暴开始后迅速增多,并在低能O+离子达到峰值之前达到峰值.因此我们推测磁暴初期从等离子体片注入环电流的主要是H+离子,主相后期O+离子可能扮演更为重要的角色.(2)在地磁活动时期,太阳风密度和动压强与等离子体片中的H+、O+数密度存在一定相关性.等离子体片中的H+离子对北向IMF Bz较为敏感,而IMF Bz南向条件下更有利于太阳风参数对等离子体片中O+数密度的影响.在地磁活动平静期,太阳风条件对等离子体片中的离子没有明显影响.     

第23太阳活动周期太阳风参数及地磁指数的统计分析

日冕物质抛射(Coronal Mass Ejection,简称CME)和共转相互作用区(Corotating Interaction Region,简称CIR)是造成日地空间行星际扰动和地磁扰动的两个主要原因,提供了地球磁暴的主要驱动力,进而显著影响地球空间环境.为深入研究太阳风活动及受其主导影响的地磁活动的时间分布特征,本文对大量太阳风参数及地磁活动指数的数据进行了详细分析.首先,采用由NASA OMNIWeb提供的太阳风参数及地磁活动指数的公开数据,通过自主编写matlab程序对第23太阳活动周期(1996-01-01—2008-12-31)的数据包括行星际磁场Bz分量、太阳风速度、太阳风质子密度、太阳风动压等重要太阳风参数及Dst指数、AE指数、Kp指数等主要的地磁指数进行统计分析,建立了包括269个CME事件和456个CIR事件列表的数据库.采用事例分析法和时间序列叠加法分别对两类太阳活动的四个重要太阳风参数(IMF Bz、太阳风速度、太阳风质子密度、太阳风动压)和三个主要地磁指数(Dst、AE、Kp)进行统计分析,并研究了其统计特征.其次,根据Dst指数最小值确定了第23太阳活动周期内的355个孤立地磁暴事件,并以Dst指数最小值为标准将这些磁暴进一步分类为145个弱磁暴、123个中等磁暴、70个强磁暴、12个剧烈磁暴和5个巨大磁暴.最后,采用时间序列叠加法对不同强度磁暴的太阳风参数和地磁指数进行统计分析.统计分析表明,对于CME事件,Nsw/Pdyn(Nsw表示太阳风质子密度,Pdyn表示太阳风动压)线性拟合斜率一般为正;对于CIR事件,Nsw/Pdyn线性拟合斜率一般为负,这可作为辨别CME和CIR事件的一种有效方法.从平均意义上讲,相较于CIR事件,CME事件有更大的南向IMF Bz分量、太阳风动压Pdyn、AE指数、Kp指数以及更小的Dstmin.一般情况下,CME事件有更大的可能性驱动极强地磁暴.总体而言,对于不同强度的地磁暴,Dst指数的变化呈现出一定的相似性,但随着地磁暴强度的增强,Dst指数衰减的速度变快.CME和CIR事件以及其各自驱动的地磁暴事件有着很多不同,因此,需要将CME事件驱动的磁暴及CIR事件驱动的磁暴分开研究.建立CME、CIR事件及地磁暴的数据库以及获取的统计分析结果,将为深入研究地球磁层等离子体片、辐射带及环电流对太阳活动的响应特征提供有利的帮助.     

Evolution of the Tharsis region of Mars: insights from magnetic field observations

Mars Global Surveyor (MGS) observations of crustal magnetic fields over Tharsis provide new constraints on models for the thermal and magmatic evolution of this region. We analyze the distribution of magnetic field anomalies over Tharsis surface units of Noachian, Hesperian and Amazonian age. These data suggest that early Noachian crust underlies the Tharsis province, and formed contemporaneously with the existence of a martian dynamo. This crust either pre-dates the formation of Tharsis, or formed during the earlier phases of Tharsis volcanism. The preservation of strong magnetic field anomalies over some of the earliest Noachian and topographically high units, together with the observation of magnetic field anomalies over Hesperian- and Amazonian-age surface units, indicate that a large fraction of the magnetized crust has remained cool (below the blocking temperature of the magnetic carrier) throughout the construction of Tharsis. Moreover, the distributions of magnetic anomaly amplitudes over Noachian, Hesperian, and Amazonian surface units suggest that the youngest units overlie sites of prolonged intrusion and have undergone a greater extent of thermal demagnetization. The absence of magnetic anomalies around the Tharsis Montes and Olympus Mons argues for strong, localized heating, as would be expected at volcanic centers. We show that end-member models for progressive thermal demagnetization of a Noachian magnetized crustal layer are consistent with the anomaly amplitude distributions. We integrate the magnetic field observations with constraints from tectonics, gravity, and topography, and present a revised scenario for the evolution of the Tharsis region.     

太阳风压力系数的研究

利用全球磁流体力学(MHD)的模拟结果,研究了太阳风压力系数与上游太阳风参数和日下点磁层顶张角的相关性.在识别出日下点附近磁层顶位置后,通过拟合得到日下点附近的磁层顶张角.在考虑上游太阳风中的磁压和热压以及磁层顶外侧的太阳风动压的情况下,计算了太阳风压力系数.通过分析行星际磁场不同方向时太阳风动压在日地连线上与磁压和热压的转化关系,详细研究了太阳风参数和日下点磁层顶张角对太阳风压力系数的影响,得到以下相关结论:(1) 在北向行星际磁场较大(Bz≥5 nT)时,磁层顶外侧磁压占主导,南向行星际磁场时磁层顶外侧热压占主导;(2) 太阳风压力系数随着行星际磁场的增大而增大,随着行星际磁场时钟角的增大而减小;并且在行星际磁场大小和其他太阳风条件相同时,北向行星际磁场时的太阳风压力系数要大于南向行星际磁场时的;北向行星际磁场时,太阳风压力系数随着太阳风动压的增大而减小,南向行星际磁场时,太阳风压力系数随着太阳风动压的增大而增大;以上结论是对观测结果的扩展;(3) 最后,我们还发现太阳风压力系数随着日下点磁层顶张角的增大而增大.     

Transformation and absorption of magnetosonic waves generated by solar wind in the magnetosphere

Resonant transformation of fast magnetosonic (FMS) wave flux into Alfven and slow magnetosonic (SMS) oscillations is investigated in the one-dimensionally inhomogeneous magnetosphere. Spatial distribution of energy absorption rate of FMS oscillations penetrating into the magnetosphere from the solar wind is studied. The FMS wave energy absorption rate caused by magnetosonic resonance excitation is shown to be several orders of magnitude greater than that caused by Alfven resonance excitation at the same surface. It is connected with the spectrum of incident FMS waves. The Kolmogorov spectrum is used in numerical calculations. Magnitude of the Fourier harmonics exciting resonant Alfven oscillations is much smaller than that of the harmonics driving lower-frequency magnetosonic resonance. It is shown that resonant transformation of FMS waves into SMS oscillations can be an effective mechanism of energy transfer from the solar wind to the magnetosphere.     

太阳高能粒子(SEP)传播数值模拟中的太阳风背景场研究

太阳高能粒子(SEP)事件是一类重要的空间天气灾害性事件,如能准确预报SEP事件,人们便可以采取必要的防护措施,保障卫星、星载设备以及航天员的安全,尽可能地降低经济损失.因此,其数值预报研究在空间天气预报研究中占有很重要的地位.SEP事件中的高能粒子在不同的时间尺度内被耀斑过程或者CME驱动的激波加速,并且在被扰动后的行星际太阳风中传输,这些过程都紧紧依赖于太阳风背景场.因此获取更加接近物理真实的太阳风背景场是模拟SEP事件的重要部分,也是提高SEP物理模式的关键因素之一.我们目前的工作基于张明等发展的SEP在行星际空间传播的模型,尝试将Parker太阳风速度解及WIND飞船观测的磁场实时数据融入模型中,研究不同的太阳风速度以及真实磁场分布对SEP在行星际空间中传播的影响.通过求解聚焦传输方程,我们的模拟结果表明:(1)快太阳风条件下,绝热冷却效应项发挥了更大的作用,使粒子能量衰减的更快,而慢太阳风对粒子的通量变化没有显著影响;(2)加入观测的磁场数据时,粒子的全向通量剖面发生了比较明显的变化,具体表现在:通量峰值推迟到达、出现多峰结构、各向异性也发生一些改变.分析表明真实磁场的极性对粒子在行星际空间中传播有着重要的影响.     

High-speed streams from coronal holes and the accelerating mechanism of the solar wind

Abstract

In this paper, the general Mach number equation is derived, and the influence of typical energy forms in the solar wind is analysed in detail. It shows that the accelerating process of the solar wind is influenced critically by the form of heating in the corona, and that the transonic mechanism is mainly the result of the adjustment of the variation of the crosssection of flowing tubes and the heat source term.

The accelerating mechanism for both the high-speed stream from the coronal hole and the normal solar wind is similar. But, the temperature is low in the lower level of the coronal hole and more heat energy supply in the outside is required, hence the high speed of the solar wind; while the case with the ordinary coronal region is just the opposite, and the velocity of the solar wind is therefore lower. The accelerating process for various typical parameters is calculated, and it is found that the high-speed stream may reach 800 km/sec.     

极点热层密度变化特征及其受太阳风扇形结构调制

本文基于2002年至2010年的GRACE卫星的观测密度统计分析南北极点的热层大气密度的世界时（即磁地方时）变化.研究发现：在9—11月份地球处于行星际磁场为背向太阳的扇区内（背向扇区）时,南极点热层密度在约17:00 UT（13:30 MLT）达到最大值,比日平均值高约22%;而在6—8月份,当地球处于行星际磁场为面向太阳的扇区内（面向扇区）时,北极点热层密度在06:00 UT（12:30 MLT）达到最大值,比日平均值高约13%.南极点的磁纬是-74°,其在15:30 UT处于磁地方时正午,恰与极尖区位置重合.北极点在5:30 UT处于磁地方时正午,此时北极点与极尖区位置最靠近.因此,极点热层大气密度的磁地方时变化可能是其周期性靠近极尖区的结果.南北极点热层密度的磁地方时变化分别在背向和面向扇区内更明显,这可能与行星际磁场B_y分量对南北半球密度的不同影响有关.统计结果还表明,极点热层大气密度的磁地方时变化在冬季半球内不明显.这可能是由于在冬季半球,沉降于极尖区的粒子相比夏季半球少、沉降高度低,因而能量沉降所引起的热层上部的密度增强较小.

     

On the equatorward phase propagation of high-m ULF pulsations observed by radars

An often observed and still unexplained feature of the high-m Alfvén waves in the terrestrial magnetosphere is their equatorward phase motion, in contrast with low-m waves. We suggest an explanation of this fact in terms of a model of wave excitation by an azimuthally drifting particle inhomogeneity injected during substorm activity. The azimuthal direction of the phase velocity coincides with that of the cloud. If the drift velocity increases with the radial coordinate, the particle cloud is stretched into spiral in the equatorial plane which leads to a radial component of the phase velocity directed toward Earth, that is, an equatorward phase propagation.     

Initiation of the San Jacinto Fault and its Interaction with the San Andreas Fault: Insights from Geodynamic Modeling

The San Andreas Fault (SAF) is the Pacific-North American plate boundary, yet in southern California a significant portion of the relative plate motion is accommodated by the San Jacinto Fault (SJF). Here we investigate the initiation of the SJF and its interaction with the SAF in a three-dimensional visco-elasto-plastic finite-element model. The model results show that the restraining bend of the southern SAF causes strain localization along the SJF, thus may have contributed to its initiation. Slip on the SJF tends to reduce slip rate on the SAF and enhance deformation in the Eastern California Shear Zone. The initiation of the SJF and its interaction with the SAF reflect the evolving plate boundary zone as it continuously seeks the most efficient way to accommodate the relative plate motion.     

Erodibility of some crust forming soils/sediments from the Southern Aral Sea Basin as determined in a wind tunnel

Severe dust storms in the Southern Aral Sea Basin have become common with the desiccation of the sea. The high incidence of dust in the area has had severe ecological consequences. Within the framework of efforts to reduce this phenomenon, deflatability as well as deflatability‐related characteristics of some prominent soils/sediment surfaces in the Southern Aral Sea Basin were examined. The materials included a salt crust from a developed Solonchak, a Takyr crust and a Takyr‐like soil, and salt crusts from undeveloped Solonchaks formed on the exposed bottom of the Aral Sea. Characteristics determined were particle size distribution, dry aggregate size distribution and salt, carbonate and organic carbon contents. Deflatability was examined using a suction type wind tunnel with a SENSIT‐type sensor to detect airborne unconsolidated material, on materials treated to different moisture levels and with a chemical stabilizer, and on restored crusts created from the unconsolidated materials. Fine sand dominates in the materials, and in the Takyr crust and Takyr‐like soils is accompanied by significant amounts of silt and clay. All materials contain moderate amounts of carbonate and are low in organic matter. All soils/sediments contain salts, but in the salt crusts of the Solonchaks the salt fraction dominates. They all have more than 50 per cent PM₈₅₀ (particles with diameter <850 µm), indicating a relatively high deflatability potential. The materials from the Takyr crusts and Takyr‐like soil with a high proportion of fine aggregates had the lowest threshold friction velocities, while the salt crusts of the Solonchaks with a high proportion of coarse aggregates had the highest. This suggests that Takyrs and Takyr‐like soils are the most deflatable and Solonchak soils the least deflatable. These differences are attributed to the presence of salts that create stable, large aggregates in the Solonchak crusts. Wetting of the materials to three moisture levels considerably increased threshold friction velocity. The increase was most prominent in the salt‐rich materials, and was attributed to the rapid formation of surface films by drying in the course of the wind tunnel determinations. Applications of chemical stabilizers at two levels also considerably increased threshold friction velocity. On the restored crusts, threshold friction velocity dramatically increased, occasionally to non‐recordable values. This increase was monitored with both the salt crusts characteristic for the Solonchak soils and the fine‐grained crusts characteristic for the Takyr soils. The stability was attributed to the tightly packed salt particles in the salt crusts, and to the cohesive properties of the fine‐grained materials in the Takyr crusts. Once the crusts were ruptured, however, strong deflation commenced. These results suggest that by maintaining moisture in the soils/sediments (for example, by maintaining a high water table in the Amu‐Darya river flood plain) deflation can be reduced. By the same means, deflation can be reduced by creating new crusts or by preserving existing crusts. Copyright © 2005 John Wiley & Sons, Ltd.