A MEASUREMENT OF THE ADIABATIC COOLING INDEX FOR INTERSTELLAR HELIUM PICKUP IONS IN THE INNER HELIOSPHERE
A MEASUREMENT OF THE ADIABATIC COOLING INDEX FOR INTERSTELLAR HELIUM PICKUP IONS IN THE INNER HELIOSPHERE
复制标题
内日球层星际氦拾取离子绝热冷却指数的测量
DOI：
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发表时间：
2009
期刊：
影响因子：
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通讯作者：
R. Kallenbach
中科院分区：
文献类型：
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作者：
L. Saul;P. Wurz;R. Kallenbach
星际中性气体进入内日光层，在那里被电离，成为太阳风的拾取离子群。人们通常认为，由于太阳风的发散流动，这些人口随后将绝热降温，就像膨胀的理想气体一样。在这里，我们报道了第一次从SOHO Celias在垂直行星际磁场期间对单电荷氦的测量中独立地测量出内日光层的有效绝热冷却指数。我们使用一个简单的星际拾取氦离子的绝热输运模型，该模型适用于内日光层的上风区。用CELIAS/CTOF测得的氦离子时均速度谱符合单一自由参数的模型，它代表了有效冷却速率，其幂指数γ=1.3 5±0.2。虽然这一平均值与γ=1.5的“理想气体”假设是一致的，但分析表明，这种假设并不普遍适用，而且由于观测限制，有必要进一步测量以限制冷却过程。讨论了理解内日光层的传输过程和改进这一测量技术的意义。
Interstellar neutral gas enters the inner heliosphere where it is ionized and becomes the pickup ion population of the solar wind. It is often assumed that this population will subsequently cool adiabatically, like an expanding ideal gas due, to the divergent flow of the solar wind. Here, we report the first independent measure of the effective adiabatic cooling index in the inner heliosphere from SOHO CELIAS measurements of singly charged helium taken during times of perpendicular interplanetary magnetic field. We use a simple adiabatic transport model of interstellar pickup helium ions, valid for the upwind region of the inner heliosphere. The time averaged velocity spectrum of helium pickup ions measured by CELIAS/CTOF is fit to this model with a single free parameter which indicates an effective cooling rate with a power-law index of γ = 1.35 ± 0.2. While this average is consistent with the “ideal-gas” assumption of γ = 1.5, the analysis indicates that such an assumption will not apply in general, and that due to observational constraints further measurements are necessary to constrain the cooling process. Implications are discussed for understanding the transport processes in the inner heliosphere and improving this measurement technique.