Coronal mass ejections (CMEs) are one of the most energetic explosions in the solar system. It is generally accepted that CMEs result from eruptions of magnetic flux ropes, which are dubbed as magnetic clouds in interplanetary space. The composition (including the ionic charge states and elemental abundances) is determined prior to and/or during CME eruptions in the solar atmosphere, and does not alter during magnetic cloud propagation to 1 AU and beyond. It has been known that the composition is not uniform within a cross section perpendicular to magnetic cloud axis, \textbf{and the distribution of ionic charge states within a cross section provides us an important clue to investigate the formation and eruption processes of flux ropes due to the freeze-in effect.} The flux rope is a three-dimensional magnetic structure intrinsically, and it remains unclear whether the composition is uniform along the flux rope axis as most magnetic clouds are only detected by one spacecraft. In this paper we report a magnetic cloud that was observed by ACE on 1998 March 4--6 and Ulysses on March 24--28 sequentially. At these times, both spacecraft were located around the ecliptic plane, and the latitudinal and longitudinal separations between them were $\sim$2.2$^{\circ}$ and $\sim$5.5$^{\circ}$, respectively. \textbf{It provides us an excellent opportunity to explore the axial inhomogeneity of flux rope composition, as both spacecraft almost intersected the cloud center at different sites along its axis.} Our study shows that the average values of ionic charge states exhibit significant difference along the axis for carbon, and the differences are relatively slight but still obvious for charge states of oxygen and iron, as well as the elemental abundances of iron and helium. Besides the means, the composition profiles within the cloud measured by both spacecraft also exhibit some discrepancies. We conclude that the inhomogeneity of composition exists along the cloud axis.

中文翻译：

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沿磁云轴的成分不均匀性

日冕物质抛射（CME）是太阳系中能量最高的爆炸之一。人们普遍认为 CME 是由磁通量绳的喷发引起的，磁通量绳被称为行星际空间中的磁云。组成（包括离子电荷态和元素丰度）是在太阳大气中 CME 喷发之前和/或期间确定的，并且在磁云传播到 1 AU 及以上期间不会改变。已知在垂直于磁云轴的截面内成分不均匀，\textbf{和截面内离子电荷态的分布为我们研究磁链的形成和喷发过程提供了重要线索。到冻结效应。}磁通绳本质上是一个三维磁性结构，由于大多数磁云只能被一艘航天器探测到，因此仍不清楚沿磁链轴的成分是否均匀。在本文中，我们报告了 ACE 在 1998 年 3 月 4--6 日和 Ulysses 在 3 月 24--28 日依次观测到的磁云。此时，两艘航天器都位于黄道面附近，它们之间的纬度和纵向间隔分别为 $\sim$2.2$^{\circ}$ 和 $\sim$5.5$^{\circ}$。\textbf{它为我们提供了探索磁链组成的轴向不均匀性的绝佳机会，因为两艘航天器几乎沿其轴在不同位置与云中心相交。}我们的研究表明，离子电荷态的平均值沿着碳的轴，氧和铁的电荷态以及铁和氦的元素丰度差异相对较小但仍然明显。除了均值外，两艘航天器测量的云内成分剖面也表现出一些差异。我们得出结论，沿云轴存在成分的不均匀性。