中文摘要：

表征EUV辐射通量的E10．7指数在越来越多的研究和应用中被用来代替传统的F10．7指数．X射线对地球D层和E层的电离起着重要作用，但由于D层观测数据的不足和E层电离源的多样性，难以被用来考虑X射线对电离层的影响．火星电离层下层的电离源几乎是单一的软X射线，这为研究X射线对电离层的作用提供了可能性．通过研究火星电离层下层的峰值电子浓度对E10．7的依赖关系，发现即便经过必要的修正，这种关系对不同的观测时段并不具备一致性．通过理论推导和数据分析，得到了一种特别用于描述太阳软X射线辐射通量的新指数，即Xs指数，用来替代E10．7指数．Xs指数在描述火星电离层下层对太阳辐射的依赖关系时，不同的观测时段有很好的一致性，表明五指数在表征太阳软X射线辐射强度方面比E10.7指数更加合适．

英文摘要：

The E10.7 index representing the solar EUV irradiance flux has been used to replace the traditional F10.7 index in more and more researches and applications. X-rays are important ionization source of the terrestrial D and E layer. Due to lack of enough observation to the D layer and versatile ionization sources of E layer, it is difficult to study the influence of X-rays to terrestrial ionosphere. The dominant ionization source for the secondary layer of Martian ionosphere is soft X-ray. This makes it possible to study the X-ray influence on the ionosphere. By studying the dependence of Martian ionospheric secondary layer peak density to E10.7, it is found that even after necessary corrections, the dependences are in disagreement for different data sets. After data and theoretical analysis, a new index Xs is introduced to represents the solar soft X-ray irradiance flux. After replacing E10.7 with this Xs index, the peak density dependences upon the solar irradiance are in good agreement for different data sets, indicating Xs index is a better measure of solar soft X-ray irradiance than E10.7 index.