火星快车日冕射电探测实验的VLBI数据处理
VLBI data processing on coronal radio-sounding experiments of Mars express
摘要：
欧空局的火星快车日冕实验是在2015年和2017年由欧洲VLBI网络的一些射电望远镜在两次太阳合点进行的。介绍了航天器无线电信号频率和相位起伏的测量方法，以及利用互相关技术研究等离子体湍流对单站和多站信号特性的影响。在4.9和76.3 $\rm R_{s}$之间观察到的频率波动的功率谱在频率间隔$ ν_{lo}< ν<ν_{up}$上具有接近于柯尔莫哥洛夫谱的幂律形状，其中$ν_{lo}$的标称值被设置为3 mHz并且$ν_{up}$在0.03 $\sim$ 0.15 Hz的范围内。频率波动的RMS作为日心距离的函数。此外，我们还分析了太阳偏移距为4.9 $\rm{R_{s}}$和9.9 $\rm{R_{s}}$时电子柱密度起伏的变化以及VLBI站间的互相关产物。不同台站之间的差分波动的功率密度在$ν< 0.01$ Hz处减小。最后，由$ν< 0.01$ Hz处的频率起伏的互相关，导出了太阳风$>700$ $\rm{km~s^{-1}}$的快速流动速度。太阳风的快速流动速度与无线电射线经过的日冕区域的高日纬度相对应。VLBI观测和分析方法可以通过提供地球与航天器之间的多条视线来研究内太阳风中多个空间点的电子柱密度波动和湍流。
英文摘要：
The ESA's Mars Express solar corona experiments were performed at two solar conjunctions in the years 2015 and 2017 by a number of radio telescopes in the European VLBI Network. This paper presents the methods to measure the frequency and phase fluctuations of the spacecraft radio signal, and the applications to study the characteristics of the plasma turbulence effects on the signal at a single station and at multiple stations via cross-correlation. The power spectra of the frequency fluctuations observed between 4.9 and 76.3 $\rm R_{s}$ have a power-law shape close to a Kolmogorov spectrum over the frequency interval $ ν_{lo}< ν<ν_{up}$, where the nominal value of $ν_{lo}$ is set to 3 mHz and $ν_{up}$ is in the range of 0.03 $\sim$ 0.15 Hz. The RMS of the frequency fluctuations is presented as a function of the heliocentric distance. Furthermore, we analyse the variations of the electron column density fluctuations at solar offsets 4.9 $\rm{R_{s}}$ and 9.9 $\rm{R_{s}}$ and the cross-correlation products between the VLBI stations. The power density of the differential fluctuations between different stations decreases at $ν< 0.01$ Hz. Finally, the fast flow speeds of solar wind $>700$ $\rm{km~s^{-1}}$ are derived from the cross-correlation of frequency fluctuations at $ν< 0.01$ Hz. The fast flow speeds of solar wind correspond to the high heliolatitude of the coronal region that the radio rays passed. The VLBI observations and analysis methods can be used to study the electron column density fluctuations and the turbulence at multiple spatial points in the inner solar wind by providing multiple lines of sight between the Earth and the spacecraft.
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