分类： 地球科学 >> 空间物理学 分类： 其他 提交时间： 2024-06-08
摘要： 469219 Kamo`oalewa is selected as one of the primary targets of Tianwen-2 mission, which is currently believed to be the most stable quasi-satellite of Earth. Here we derive a weak detection of the Yarkovsky effect for Kamo`oalewa, giving $A_2 = -1.075 pm0.447 times 10^{-13} rm{au/d}^2$, with the available ground-based optical observations from Minor Planet Center and a relatively conservative weighting scheme. Due to the quasi-satellite resonance with Earth, we show that the detection of Yarkovsky effect by orbital fitting with astrometric observations becomes difficult as its orbital drift shows a slow oscillatory growth resulting from the Yarkovsky effect. In addition, we extensively explore the characteristics of orbital uncertainty propagation and find that the positional uncertainty mainly arises from the geocentric radial direction in 2010-2020, and then concentrates in the heliocentric transverse direction in 2020-2030. Furthermore, the heliocentric transverse uncertainty is clearly monthly dependent, which can arrive at a minimum around January and a maximum around July as the orbit moves towards the leading and trailing edges, respectively, in 2025-2027. Finally, we investigate a long-term uncertainty propagation in the quasi-satellite regime, implying that the quasi-satellite resonance with Earth may play a crucial role in constraining the increase of uncertainty over time. Such interesting feature further implies that the orbital precision of Kamo`oalewa is relatively stable at its quasi-satellite phase, which may also be true for other quasi-satellites of Earth.
分类： 地球科学 >> 空间物理学 分类： 其他 分类： 其他 分类： 其他 提交时间： 2024-06-03
摘要： The Closeby Habitable Exoplanet Survey (CHES) is dedicated to the astrometric exploration for habitable-zone Earth-like planets orbiting solar-type stars in close proximity, achieving unprecedented micro-arcsecond precision. Given the elevated precision, thorough consideration of photocenter jitters induced by stellar activity becomes imperative. This study endeavors to model the stellar activity of solar-type stars, compute astrometric noise, and delineate the detection limits of habitable planets within the astrometric domain. Simulations were conducted for identified primary targets of CHES, involving the generation of simulated observed data for astrometry and photometry, accounting for the impact of stellar activity. Estimation of activity levels in our samples was achieved through chromospheric activity indices, revealing that over 90% of stars exhibited photocenter jitters below 1 $ mu mathrm{as}$. Notably, certain proximate stars, such as $ alpha$ Cen A and B, displayed more discernible noise arising from stellar activity. Subsequent tests were performed to evaluate detection performance, unveiling that stellar activity tends to have a less pronounced impact on planetary detectability for the majority of stars. Approximately 95% of targets demonstrated a detection efficiency exceeding 80%. However, for several cold stars, e.g., HD 32450 and HD 21531, with the habitable zones close to the stars, a reduction in detection efficiency was observed. These findings offer invaluable insights into the intricate interplay between stellar activity and astrometric precision, significantly advancing our understanding in the search for habitable planets.
分类： 地球科学 >> 空间物理学 提交时间： 2017-03-31
摘要： Ulysses magnetic and plasma data are used to study hourly-scale Alfv{\'e}nic fluctuations in the solar polar wind. The calculated energy ratio $R^2_{v_A}$(cal) of inward to outward Alfv{\'e}n waves is obtained from the observed Wal{\'e}n slope through an analytical expression, and the observed $R^2_{v_A}$(obs) is based on a direct decomposition of original Alfv{\'e}nic fluctuations into outward- and inward-propagating Alfv{\'e}n waves. The radial variation of $R^2_{v_A}$(cal) shows a monotonically increasing trend with heliocentric distance $r$, implying the increasing local generation or contribution of inward Alfv{\'e}n waves. The contribution is also shown by the radial increase in the occurrence of dominant inward fluctuations. We further pointed out a higher occurrence ($\sim 83\%$ of a day in average) of dominant outward Alfv{\'e}nic fluctuations in the solar wind than previously estimated. Since $R^2_{v_A}$(cal) is more accurate than $R^2_{v_A}$(obs) in the measurement of the energy ratio for dominant outward fluctuations, the values of $R^2_{v_A}$(cal) in our results are likely more realistic in the solar wind than previously estimated and than $R^2_{v_A}$(obs) in our results. The duration ratio $R_T$ of dominant inward to all Alfv{\'e}nic fluctuations increases monotonically with $r$, and is about two or more times that from {\it Voyager 2} observations at $r \geq 4$~AU. Finally, from the variation trend in our results, a higher (lower) occurrence rate is expected at $r 4$~AU) for dominant outward Alfv{\'e}nic fluctuations, and opposite variations are expected for dominant inward fluctuations. Simultaneously, $R^2_{v_A}$(cal) and $R_T$ will be expected to be smaller at $r 4$~AU. These results reveal new qualitative and quantitative features of Alfv{\'e}nic fluctuations therein compared with previous studies and put constraints on modelling the variation of solar wind fluctuations.
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