Recent results on the parameters of the interstellar helium from the ULYSSES/GAS experiment
Recent results on the parameters of the interstellar helium from the ULYSSES/GAS experiment
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ULYSSES/GAS 实验中星际氦参数的最新结果
DOI：
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发表时间：
1996
期刊：
影响因子：
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通讯作者：
H. Rosenbauer
中科院分区：
文献类型：
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作者：
M. Witte;M. Banaszkiewicz;H. Rosenbauer
星际氦流的速度和方向以及它的温度和密度已经从ULYSSES/GAS实验在两个不同时期的测量中确定：在ULYSSES的黄道路径期间，代表太阳的最大条件，以及在南极到北极的过渡期间（11/94-6/95），接近太阳的最小条件。在改进后的误差条内，数值与先前发表的结果一致。在内太阳系观测中确定日球层边界处星际氦的密度n∞，需要了解粒子在到达观测者的途中所经历的损失过程。对到达观测器的“直接”和“间接”轨道的氦粒子的同时观测提供了一种工具，可以直接确定损失过程的影响，假定损失过程主要是光致电离和-对于在太阳附近运动的粒子-高能太阳风电子的电子碰撞电离，1995年2月用ULYSSES/GAS仪器进行了这种观测。在太空探测器通过近日点之前从这些测量值的损失率和星际密度可以得出。假设光致电离是唯一的损失过程，得到了电离率β = 1.1 · 10−7 s−1和密度n∞ = 1.7 · 10−2 cm−3时与观测值的合理拟合。此外，如果再加上电子碰撞电离，光电离β = 0.6 · 10−7 s−1就足以符合这两个观测值，得到密度n∞ = 1.4 · 10−2 cm−3。
Velocity and direction of the flow of the interstellar helium and its temperature and density have been determined from the measurements of the ULYSSES/GAS experiment for two different epochs: during the in-ecliptic path of ULYSSES, representing solar maximum conditions, and during the south to the north pole transition (11/94-6/95), close to the solar minimum conditions. Within the improved error bars the values are consistent with results published earlier.The determination of the density n∞ of the interstellar helium at the heliospheric boundary from observations in the inner solar system requires knowledge about the loss processes experienced by the particles on their way to the observer. The simultaneous observation of the helium particles arriving on “direct” and “indirect” orbits at the observer provides a tool to directly determine the effects of the loss processes assumed to be predominantly photoionization and — for particles travelling close to the Sun — electron impact ionization by high-energy solar wind electrons.Such observations were obtained with the ULYSSES/GAS instrument in February 1995, before the spaceprobe passed its perihelion. From these measurements values for the loss rates and the interstellar density could be derived. Assuming photoionization to be the only loss process reasonable fits to the observations were obtained for an ionization rate β = 1.1 · 10−7 s−1 and a density n∞ ≈ 1.7 · 10−2 cm−3. Including, in addition, electron impact ionization, a photoionization β = 0.6 · 10−7 s−1 was sufficient to fit both observations, resulting in a density n∞ ≈ 1.4 · 10−2 cm−3.