Field line distribution of density at L =4.8 inferred from observations by CLUSTER
Field line distribution of density at L =4.8 inferred from observations by CLUSTER
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根据 CLUSTER 观测推断 L =4.8 处的密度场线分布
DOI：
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发表时间：
2009
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通讯作者：
J. Goldstein
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作者：
R. Denton;P. Décréau;M. Engebretson;F. Darrouzet;J. Posch;C. Mouikis;L. Kistler;C. Cattell;Kazue Takahashi;S. Schäfer;J. Goldstein
抽象的。对于星团航天器观测到的两个事件，从Alfven波谐频率推断了质量密度ρ的场线分布，并与等离子体波数据得到的电子密度Ne和离子组成实验得到的氧密度NO+进行了比较。在一种情况下，平均离子质量M≈ρ/ne约为5amu(2002年10月28日)，而在另一种情况下约为3amu(2002年9月10日)。这两起事件都发生在星系团1(C1)航天器在等离子体中的时候。然而，第一次事件(Ne=8 cm−3)的电子密度Ne明显低于第二次事件(Ne=22 cm−3)，这似乎是导致M值不同的主要差异。对于第一次事件(2002年10月28日)，我们能够以前所未有的精度测量八次谐波的Alfven波频率，因此推断的质量密度的误差可能是由频率不确定性以外的因素(如磁场模型和理论波动方程)主导的。对于磁纬度|MLAT|≲20°，这个场线分布(L=4.8时)非常平坦，但对于|MLAT|≳40°，则相对于|MLAT|非常陡峭地增加。ρ的总变化约为四个数量级，大的|MLAT|值与电离层的值大致一致。对于第二次事件(2002年9月10日)，在磁赤道附近有一个小的局部最大质量密度。推算的质量密度在|MLAT|=15.5°时比赤道值低最小23%，然后随着人们沿着场线向电离层移动，质量密度急剧增加。对于这一事件，我们还能够使用所有四个星团航天器对Ne的测量来检查电子密度的空间相关性。我们的分析表明，在L~5处，Ne的密度随L的变化而变化，与L的−4大致相同，Ne也在磁赤道处有局部峰值，但峰值较小。在|MLAT|=12.5°时，Ne达到一个密度极小值，比赤道值低约6%，然后随着|MLAT|的增大，Ne急剧增加。据我们所知，这是在磁赤道出现局部体电子密度峰值的第一个证据。我们的结果表明，磁震学可以用来确定近地点星团质量密度的场线分布，也可以通过多个航天器的测量来推断电子密度的分布。
Abstract. For two events observed by the CLUSTER spacecraft, the field line distribution of mass density ρ was inferred from Alfven wave harmonic frequencies and compared to the electron density ne from plasma wave data and the oxygen density nO+ from the ion composition experiment. In one case, the average ion mass M≈ρ/ne was about 5 amu (28 October 2002), while in the other it was about 3 amu (10 September 2002). Both events occurred when the CLUSTER 1 (C1) spacecraft was in the plasmatrough. Nevertheless, the electron density ne was significantly lower for the first event (ne=8 cm−3) than for the second event (ne=22 cm−3), and this seems to be the main difference leading to a different value of M. For the first event (28 October 2002), we were able to measure the Alfven wave frequencies for eight harmonics with unprecedented precision, so that the error in the inferred mass density is probably dominated by factors other than the uncertainty in frequency (e.g., magnetic field model and theoretical wave equation). This field line distribution (at L=4.8) was very flat for magnetic latitude |MLAT|≲20° but very steeply increasing with respect to |MLAT| for |MLAT|≳40°. The total variation in ρ was about four orders of magnitude, with values at large |MLAT| roughly consistent with ionospheric values. For the second event (10 September 2002), there was a small local maximum in mass density near the magnetic equator. The inferred mass density decreases to a minimum 23% lower than the equatorial value at |MLAT|=15.5°, and then steeply increases as one moves along the field line toward the ionosphere. For this event we were also able to examine the spatial dependence of the electron density using measurements of ne from all four CLUSTER spacecraft. Our analysis indicates that the density varies with L at L~5 roughly like L−4, and that ne is also locally peaked at the magnetic equator, but with a smaller peak. The value of ne reaches a density minimum about 6% lower than the equatorial value at |MLAT|=12.5°, and then increases steeply at larger values of |MLAT|. This is to our knowledge the first evidence for a local peak in bulk electron density at the magnetic equator. Our results show that magnetoseismology can be a useful technique to determine the field line distribution of the mass density for CLUSTER at perigee and that the distribution of electron density can also be inferred from measurements by multiple spacecraft.