Effects of large-scale magnetic fields in the Venus ionosphere

Theoretical models of the ionosphere of Venus have been constructed in the past without due consideration of the fact that the ionosphere is sometimes magnetized. This paper examines some differences between the magnetized and unmagnetized dayside Venus ionosphere using the Pioneer Venus Orbiter Langmuir probe and magnetometer data. Particular attention is given to the evaluation of the altitude profiles of the thermal electron heating and comparison of the magnitude of the magnetic force(¯vׯB) ׯB with other forces in the ionosphere. Several examples illustrate how heating profiles are different in the magnetized ionosphere with effective heating below ～200 km altitude reduced by orders of magnitude compared to the field-free ionosphere. The force associated with the magnetic field is comparable to other forces in the magnetized ionosphere. The measured plasma density, electron temperature and magnetic field thus suggest that large-scale magnetic fields should be included in future ionosphere models.     

The accuracy of present models of the high altitude polar magnetosphere

The Polar satellite has explored the high-latitude, high-latitude magnetosphere out to 9 Earth radii (Re). The magnetic field data returned from this mission can be used both to provide data for new empirical models and to test existing models. Tests include comparing the observed location of the polar cusp with its position in the empirical models and comparing the strength of the magnetic field in the surrounding region. Near the cusp the magnetosphere is quite sensitive to solar wind conditions. In particular the energy density of the cusp plasma depends on the pressure of the solar wind applied to the interface of the cusp and the sheath. The applied pressure in turn depends on the shape of the magnetopause and the orientation of that interface, both controlled by the direction of the interplanetary magnetic field. Magnetohydrodynamic (MHD) models provide a coarse picture of the magnetosphere at high latitudes. While generally quite realistic, these too require testing against observations because even the MHD models must make some simplifying assumptions.     

Decision program on asteroid threat mitigation

The Association of Space Explorers Committee on Near-Earth Objects (NEOs) and its Panel on Asteroid Threat Mitigation have prepared a decision program to aid the international community in organizing a coordinated response to asteroid impact threats. The program is described in the ASE's report, Asteroid Threats: A Call for Global Response, which will be considered by the United Nations Committee on the Peaceful Uses of Outer Space in its 2009 sessions. The findings and recommendations of this report are presented here as well as some of the major implications of the complex decision-making involved in developing a coordinated international response to the challenge of protecting the Earth from NEO impacts.     

卫星舱内两类剂量计测量结果的比较

本文利用1994年和1996年两次返回式卫星的搭载条件对舱内辐射剂量进行了对比测量．通过对比测量,研究不同掺杂、不同厚度LiF剂量计测量空间辐射剂量的特点;研究GM计数管计数和LiF剂量间的转换系数以及转换系数随屏蔽状况的变化;由剂量和GM计数研究粒子平均碰撞阻止本领的估计方法．结果表明,不同掺杂、不同厚度的LiF剂量计测量结果间无显著差异,而转换系数几乎不受舱内位置和屏蔽状态的影响．不同厚度LiF剂量计,不同屏蔽状态的GM计数管计数和剂量—计数转换系数的比较研究以及对粒子平均碰撞阻止本领的估计表明,舱内辐射剂量起决定作用的是高能粒子成分,其平均碰撞阻止本领估计约为5MeV／cm．     

Unusually strong magnetic fields in Titan’s ionosphere: T42 case study

Observations of unusually large magnetic fields in the ionosphere indicate periods of maximum stress on Titan’s ionosphere and potentially of the strongest loss rates of ionospheric plasma. During Titan flyby T42, the observed magnetic field attained a maximum value of 37 nT between an altitude of 1200 and 1600 km, about 20 nT stronger than on any other Titan pass and close to five times greater in magnetic pressure. The strong fields occurred near the corotation-flow terminator rather than at the sub-flow point, suggesting that the flow which magnetized the ionosphere was from a direction far from corotation and possibly towards Saturn. Extrapolation of solar wind plasma conditions from Earth to Saturn using the University of Michigan MHD code predicts an enhanced solar wind dynamic pressure at Saturn close to this time. Cassini’s earlier exits from Saturn’s magnetosphere support this prediction because the Cassini Plasma Spectrometer instrument saw a magnetopause crossing three hours before the strong field observation. Thus it appears that Titan’s ionosphere was magnetized when the enhanced solar wind dynamic pressure compressed the Saturnian magnetosphere, and perhaps the magnetosheath magnetic field, against Titan. The solar wind pressure then decreased, leaving a strong fossil field in the ionosphere. When observed, this strong magnetic flux tube had begun to twist, further enhancing its strength.