第22太阳活动周峰年的ULF波观测与研究

本文介绍第２２太阳活动周峰年期间利用ＵＬＦ波的观测和分析，对磁层近地空间电磁环境的研究，文中列举了这峰年期间地磁脉动的主要成果。有些成果是国际上首次提出和发表的，目前，地磁脉动的观测和研究已经成为监测近地空间电磁环境的重要手段。     

通量传输事件磁场的理论计算及其与观测的比较

根据涡旋诱发重联理论,对通量传输事件（FTEs）磁场分布特性作了计算.结果表明,卫星测到的FTEs的不同磁场分布形态,是取决于通量管的运动方向及卫星穿越通量管的部位.在北半球,当通量管由低纬向高纬（由南向北）直向运动时,不论卫星通过什么部位,绝大多数情况下观测到先正后负的B_x,变化（即正FTE）,个别部位观测到先负后正的B_x变化（即反FTE）;B_z是单峰分布形式,表现为V型、倒V型或是U型和倒U型.当通量管在x方向有正或负速度分量即斜向运动时,大部分部位测到的B_x呈不规则变化,B_z表现为双极分布.与61个FTEs的观测实例作了对比,理论计算与观测符合得较好.     

1998年5月磁暴磁层电流体系的地磁效应分析

低纬度地区地磁场的短时变化主要由以下电流体系产生:电离层发电机电流(IDC)、对称环电流(SRC)以及由部分环电流和Ⅱ区场向电流及其电离层回路组成的内磁层三维电流体系(PRFI).此外,由Ⅰ区场向电流及其电离层回路组成的电流体系(FACI)所产生的低纬地磁场也是不可忽略的.本文针对1998年5月1-6日的大磁暴,首先利用多个同子午线台站对的数据分离并消去由IDC电流产生的Sq场.然后,通过线性建模分离其他电流体系产生的磁场成分.结果表明:(1)发生在5月1-6日的磁暴可以分为两个过程,PRFI和FACI电流体系在1-3日不明显,在4-5日伴随着亚暴强烈发生.(2)SRC的变化情况在第一阶段同Dst指数相似,在第二阶段明显滞后于Dst指数.(3)在5月4-5日,PRFI在SRC之前增强,随着PRFI和FACI的恢复,SRC开始增强.这一结果为我们了解环电流和场向电流的形成以及它们的关系提供线索.     

上行离子密度分布特性的理论研究

采用动力学方程，求解了定态情况下磁层中上行离子沿磁力线的分布函数，针对不同的Ｋ_ｐ指数，分别对北半球极光带区起源的上行离子Ｏ_＋、Ｈ_＋和Ｈｅ_＋在子午面内沿磁力线的密度分布及其特性进行了研究．结果表明：１．沿磁力线向外，上行离子密度在近地空间呈急剧下降趋势，在远地空间呈缓慢下降趋势；２．重离子或初始能量较小的离子，其密度沿磁力线向外下降较快；３．Ｋ_ｐ指数越大，离子进入磁层的空间范围越大；４．离子的投掷角分布对密度分布的影响甚小；５．离子密度沿磁力线的下降程度随到Ｘ轴距离的增大而呈增大趋势；６．在典型参数条件下，求得上行离子Ｏ_＋在等离子体片边界附近的密度为１０^－３－１０^－２个ｃｍ^－３，这与观测结果相一致．     

Global force balance of region 1 current system

At times of strong solar wind forcing such as those that produce major magnetic storms, the region 1 current system dominates over the Chapman–Ferraro current system in mediating the transfer of force between the solar wind and the terrestrial system. The global force balance can be broken into two components, one involving the high-altitude part of the region 1 current system that is in contact with the solar wind (labeled here the HRS) and the other involving the low-altitude part of the region 1 current system that lies in the ionosphere (the LRS). Both communicate their J×B force to the geomagnetic dipole via a gradient in the magnetic field that they generate. In the HRS case the force acts to push the dipole away from the sun. This is the region 1 analog of the Chapman–Ferraro mechanism for transmitting the solar wind's force to the Earth. However, in the LRS case, the force (which is much stronger than in the HRS case) acts to push the dipole toward the sun, seemingly paradoxically. The LRS balances the ‘paradoxical’ sunward force on the dipole with an opposite force on the atmosphere. This paper uses MHD simulations to demonstrate the presence of both the normal force-transmitting gradient generated by the Chapman–Ferraro and the counter-Chapman–Ferraro gradient in the magnetic field generated by the region 1 current system.     

The structure and time variability of the ring atmosphere and ionosphere

The saturnian system is subject to constant bombardment by interplanetary meteoroids and irradiation by solar UV photons. Both effects release neutral molecules from the icy ring particles either in the form of impact water vapor or gas emission in the form of H₂O, O₂ and H₂. The observations of the Cassini spacecraft during its orbit insertion have shown the existence of molecular and atomic oxygen ions. Subsequent modeling efforts have led to the picture that an exospheric population of neutral oxygen molecules is probably maintained in the vicinity of the rings via photolytic-decomposition of ice and surface reactions. At the same time, ionized products O⁺ and ions move along the magnetic field lines and, depending on the optical local thickness rings, can thread through the ring plane or impact a ring particle, the ion principal sink. In addition, collisional interactions between the ions and neutrals will change the scale height of the ions and produce a scattered component of O₂ molecules and O atoms which can be injected into Saturn’s upper atmosphere or the inner magnetosphere. The ring atmosphere, therefore, serves as a source of ions throughout Saturn’s magnetosphere. If photolysis of ice is the dominant source of O₂, then the complex structure of the ring atmosphere/ionosphere and the injection rate of neutral O₂ will be subject to modulation by the seasonal variation of Saturn along its orbit. In this work, we show how the physical properties of the ring oxygen atmosphere, the scattered component, and the magnetospheric ion source rate vary as the ring system goes through the cycle of solar insolation. In particular, it is shown that the magnetopheric ions should be nearly depleted at Saturn’s equinox if O₂ is produced mainly by photolysis of the ring material.     

Solar wind-magnetosphere interaction as simulated by a 3-D em particle code

We have studied the solar wind-magnetosphere interaction using a 3-D electromagnetic particle code. The results for an unmagnetized solar wind plasma streaming past a dipole magnetic field show the formation of a magnetopause and a magnetotail, the penetration of energetic particles into cusps and radiation belt and dawn-dusk asymmetries. The effects of interplanetary magnetic field (IMF) have been investigated in a similar way as done by MHD simulations. The simulation results with a southward IMF show the shrunk magnetosphere with great particle entry into the cusps and nightside magnetosphere. This is a signature of a magnetic reconnection at the dayside magnetopause. After a quasi-stable state is established with an unmagnetized solar wind we switched on a solar wind with an northward IMF. In this case the significant changes take place in the magnetotail. The waving motion was seen in the magnetotail and its length was shortened. This phenomena are consistent with the reconnections which occur at the high latitude magnetopause. In our simulations kinetic effects will determine the self-consistent anomalous resistivity in the magnetopause that causes reconnections.Deceased January 24, 1993; R. Bunemanet al. 1993.     

Ion and neutral sources and sinks within Saturn's inner magnetosphere: Cassini results

Using ion-electron fluid parameters derived from Cassini Plasma Spectrometer (CAPS) observations within Saturn's inner magnetosphere as presented in Sittler et al. [2006a. Cassini observations of Saturn's inner plasmasphere: Saturn orbit insertion results. Planet. Space Sci., 54, 1197-1210], one can estimate the ion total flux tube content, N_IONL², for protons, H⁺, and water group ions, W⁺, as a function of radial distance or dipole L shell. In Sittler et al. [2005. Preliminary results on Saturn's inner plasmasphere as observed by Cassini: comparison with Voyager. Geophys. Res. Lett. 32(14), L14S04), it was shown that protons and water group ions dominated the plasmasphere composition. Using the ion-electron fluid parameters as boundary condition for each L shell traversed by the Cassini spacecraft, we self-consistently solve for the ambipolar electric field and the ion distribution along each of those field lines. Temperature anisotropies from Voyager plasma observations are used with (T_⊥/T_∥)_W⁺∼5 and (T_⊥/T_∥)_H⁺∼2. The radio and plasma wave science (RPWS) electron density observations from previous publications are used to indirectly confirm usage of the above temperature anisotropies for water group ions and protons. In the case of electrons we assume they are isotropic due to their short scattering time scales. When the above is done, our calculation show N_IONL² for H⁺ and W⁺ peaking near Dione's L shell with values similar to that found from Voyager plasma observations. We are able to show that water molecules are the dominant source of ions within Saturn's inner magnetosphere. We estimate the ion production rate S_ION∼10²⁷ ions/s as function of dipole L using N_H⁺, N_W⁺ and the time scale for ion loss due to radial transport τ_D and ion-electron recombination τ_REC. The ion production shows localized peaks near the L shells of Tethys, Dione and Rhea, but not Enceladus. We then estimate the neutral production rate, S_W, from our ion production rate, S_ION, and the time scale for loss of neutrals by ionization, τ_ION, and charge exchange, τ_CH. The estimated source rate for water molecules shows a pronounced peak near Enceladus’ L shell L∼4, with a value S_W∼2×10²⁸ mol/s.     

The dayside magnetospheric boundary layer at Mercury

Magnetic field and plasma data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft on the outbound portions of the first (M1) and second (M2) flybys of Mercury reveal a region of depressed magnetic field magnitude and enhanced proton fluxes adjacent to but within the magnetopause, which we denote as a dayside boundary layer. The layer was present during both encounters despite the contrasting dayside magnetic reconnection, which was minimal during M1 and strong during M2. The overall width of the layer is estimated to be between 1000 and 1400 km, spanning most of the distance from the dayside planetary surface to the magnetopause in the mid-morning. During both flybys the magnetic pressure decrease was ∼1.6 nPa, and the width of the inner edge was comparable to proton gyro-kinetic scales. The maximum variance in the magnetic field across the inner edge was aligned with the magnetic field vector, and the magnetic field direction did not change markedly, indicating that the change in field intensity was consistent with an outward plasma-pressure gradient perpendicular to the magnetic field. Proton pressures in the layer inferred from reduced distribution observations were 0.4 nPa during M1 and 1.0 nPa during M2, indicating either that the proton pressure estimates are low or that heavy ions contribute substantially to the boundary-layer plasma pressure. If the layer is formed by protons drifting westward from the cusp, there should be a strong morning–afternoon asymmetry that is independent of the interplanetary magnetic field (IMF) direction. Conversely, if heavy ions play a major role, the layer should be strong in the morning (afternoon) for northward (southward) IMF. Future MESSENGER observations from orbit about Mercury should distinguish between these two possibilities.