[P-EM14] 太陽圏・惑星間空間

[PEM14-04] Solar wind density distribution derived from DM measurements of Crab pulsar

*前田 龍哉¹、徳丸 宗利¹、俵 海人¹、寺澤 敏夫² (1.名古屋大学宇宙地球環境研究所、2.東京大学宇宙線研究所)

キーワード：太陽風、パルサー

The pulsed radio wave emitted by a pulsar undergoes a delay by intervening plasma through propagation in the interstellar medium and the solar wind (SW). This delay depends on frequency, and the pulsar emission observed at the Earth exhibits the frequency dispersion. The magnitude of frequency dispersion is called dispersion measures (DM), and provides information of the plasma density integrated along the LOS from the pulsar to the observer. We have made observation of Crab pulsar since 2018 using 327MHz radio telescope (SWIFT) at Toyokawa. The LOS of Crab pulsar approaches the sun by ~5R_s (R_s: solar radius) in every mid-June. This enables to investigate the plasma density near the sun by using the DM of Crab pulsar. We derived the DM from Crab pulsar observations at Toyokawa by optimizing intensity height of the giant radio pulses (GRP), and determined the SW density distribution near the sun from derived DM data. In our previous study, we used the GRPs with a signal-to-noise ratio (S/R)>15. However, the number of GRP with S/N>15 observed in both 2018 and 2019 decreased as Crab pulsar's LOS approached to the sun, and this decrease significantly worsened the accuracy of DM measurements. In order to improve decrease in accuracy, we used GRPs with S/N>10 to derive DM in this study. A lower threshold for GRP intensity results in an increase of the number of GRP, and leading to better statistics even for the period when the number of GRPs greatly reduces. We found that errors and outliers of DM measurements were significantly reduced in the analysis with S/N>10. We determined the increase in DM (ΔDM) due to the solar wind by subtracting the effect of the interstellar medium. The effects of the interstellar medium were estimated from observations made for the period of a large solar elongation. The solar wind density model was fit to observed ΔDM. While the density model obtained here was almost the same as the one from the previous study, the estimation error was reduced by about one-third, and the accuracy of model determination was greatly improved.