Abstract
Abstract The interplanetary Lyman α backscattered emission is an effective tool for remote sensing of the global structure of the solar wind proton flux. This paper reports an attempt to derive the latitudinal dependence of the solar wind density by combining the interplanetary Lyman α measurements of the Nozomi spacecraft for the period 1999–2002 with the solar wind speed data derived from interplanetary scintillation measurements. This approach successfully revealed the slow and dense solar wind over the poles during the period of the solar maximum. Data on the polar solar wind density indicate a significant growth from the middle of 2000, and the polar values of solar wind density are close to those of the equatorial values as a result of the disappearance of the coronal hole. A marked density depletion occurred in the middle of 2001, which can be ascribed to the development of fast winds from the polar coronal hole. To evaluate the remote sensing method, we considered solar wind density data from in situ measurements obtained by the Ulysses spacecraft. We conclude that our method basically agrees with in situ measurements, although we found a significant (a factor of 2) difference between these in the middle of 2001.
Highlights
The Sun is located in a warm (∼6500 K) and partly ionized cloud of interstellar gas, called the local interstellar cloud (LIC), and the heliosphere surrounding the Sun is formed by the interaction between the expanding solar wind and the components of the LIC (Fig. 1)
The global three-dimensional structure of the solar wind is little understood owing to the shortage of observational data—with the exception of the recent in situ observations of the Ulysses spacecraft (e.g., Phillips et al, 1995; Marsden et al, 1996; McComas et al, 2002, 2003) and interplanetary scintillation (IPS) observations that indicated the changes in the global structure of the solar wind speed according to the solar activity cycle (e.g., Kojima and Kakinuma, 1987, 1990)
In this study, the interplanetary Lyman α emission observations of the Nozomi spacecraft are analyzed with the aim of investigating the latitudinal variation of the ionization rate of interplanetary hydrogen and its long-term evolution during 1999–2002
Summary
The Sun is located in a warm (∼6500 K) and partly ionized cloud of interstellar gas, called the local interstellar cloud (LIC), and the heliosphere surrounding the Sun is formed by the interaction between the expanding solar wind and the components of the LIC (Fig. 1) This interaction has a complex structure in which the solar wind, interstellar neutral atoms, galactic and anomalous cosmic rays, and pickup ions all play roles (Fahr, 1996; Zank, 1999; Fahr et al, 2000; Izmodenov, 2004; Baranov, 2006; Izmodenov and Baranov, 2006). The global three-dimensional structure of the solar wind is little understood owing to the shortage of observational data—with the exception of the recent in situ observations of the Ulysses spacecraft (e.g., Phillips et al, 1995; Marsden et al, 1996; McComas et al, 2002, 2003) and interplanetary scintillation (IPS) observations that indicated the changes in the global structure of the solar wind speed according to the solar activity cycle (e.g., Kojima and Kakinuma, 1987, 1990)
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