火星电离层电子密度对太阳辐射变化的响应

太阳辐射是火星电离层变化的重要控制因素.利用火星全球勘探者号(Mars Global Surveyor，MGS)电离层掩星探测数据，并结合一个火星电离层总电子含量(Total Electron Content，TEC)经验模型，研究了火星北半球高纬地区电离层电子密度对太阳辐射变化的响应特性.在考虑了火星掩星数据中电离层电子密度对太阳天顶角的依赖关系后，发现随着太阳辐射增强，火星电离层M₂层峰值密度增大，但增长偏离线性趋势，而M₂层峰值高度和大气标高没有很明显的变化趋势.从100~200 km高度区域掩星电子密度剖面积分得到的TEC及底部和顶部TEC也随太阳辐射增大而增大，但增长率有所减小，表明火星电离层可能存在类似地球电离层的饱和特征.MGS掩星TEC及其底部和顶部剖面的TEC与经验模型TEC的比值均与太阳辐射强度变化呈反相关特性，表明在强太阳辐射情形下200 km以上电子含量在TEC中占比增大.这一特性意味着太阳活动增强，在火星顶部电离层区域，动力学过程对电离层的控制逐渐超过光化学过程.

Response of Martian ionospheric electron density to changes in solar radiation

Changes in solar radiation is one of the main drivers in modulating variations of the Martian ionosphere. In this investigation, we report the response of the Martian ionospheric electron density to changes in solar radiation as revealed from the electron density profiles from radio occultation observations by Mars Global Surveyor (MGS) and an empirical model of the total electron content (TEC) of Martian ionosphere. After considering the solar zenith angle dependence of the ionospheric electron density, with increasing solar radiation, the peak density of the M2 layer in the Martian ionosphere tends to increase but with a decreased rate, while the peak height and neutral scale height of the M2 layer do not show a significant change. The TEC of the occultation electron density profiles and the bottomside and topside TEC in the 100~200 km altitude range also increase with solar radiation, but all with a slower rate, indicating that there may be a saturation feature in the Martian ionosphere, similar to the earth′s ionosphere. The ratios of bottomside and topside TEC to the TEC from the empirical model are inversely correlated with changes of solar radiation. This inverse relationship tells that with increasing solar radiation, electron densities at altitudes above 200 km account for a greater proportion of TEC. This fact means that the control of dynamic processes in the topside Martian ionosphere gradually exceeds that of the photochemical process with increasing solar activity.