Observations of the Crab pulsar at 327 MHz were made at the Toyokawa Observatory of the Institute for Space-Earth Environmental Research, during the solar occultation in mid-June 2018 to investigate the coronal plasma density in the weak sunspot cycle, Cycle 24. The dispersion measurements (DMs) were determined using giant pulses detected from observations of the Crab pulsar. The systematic increase in DM over the background level, observed during the period of the closest approach of the Crab pulsar’s line-of-sight to the Sun, was ascribed to the effect of the coronal plasma. A coronal density model assuming spherical symmetry was determined by fitting it to the DM data, and was compared with those determined in past solar cycles. The best-fit model had large errors, and indicated a systematically higher value than those derived from past observations. The results obtained here are likely to be significantly affected by latitude/longitude variation in coronal plasma density, the time variation of the interstellar medium, mainly the Crab nebula, and increased measurement errors due to the reduced occurrence of giant pulses. Hence, further observations are needed to derive conclusions about a change of coronal density in the current cycle.

中文翻译：

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2018 年使用蟹状脉冲星的巨脉冲对日冕进行无线电探测测量

在 2018 年 6 月中旬的太阳掩星期间，空间地球环境研究所丰川天文台对 327 MHz 的蟹状脉冲星进行了观测，以研究第 24 周期弱太阳黑子周期中的日冕等离子体密度。测量值 (DM) 是使用从蟹状脉冲星观测中检测到的巨脉冲确定的。在蟹状脉冲星最接近太阳的视线期间观察到的 DM 超过背景水平的系统性增加归因于日冕等离子体的影响。假设球对称的日冕密度模型是通过将其拟合到 DM 数据来确定的，并将其与过去太阳周期中确定的数据进行比较。最佳拟合模型有很大的误差，并表明比从过去观察得出的值系统地更高。这里获得的结果很可能受到日冕等离子体密度的经纬度变化、星际介质（主要是蟹状星云）的时间变化以及由于巨脉冲出现减少而增加的测量误差的显着影响。因此，需要进一步观察以得出有关当前周期日冕密度变化的结论。