Latitudinal variation of the solar He I 58.4 nm irradiance from the optical observation of the interplanetary He I emission

Details Latitudinal variation of the solar He I 58.4 nm irradiance from the optical observation of the interplanetary He I emission Latitudinal variation of the solar He I 58.4 nm irradiance from the optical observation of the interplanetary He I emission

Jun 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006jgra..11106106y&link_type=abstract

Journal of Geophysical Research, Volume 111, Issue A6, CiteID A06106

Astronomy and Astrophysics

Astrophysics

Interplanetary Physics: Interstellar Gas, Solar Physics, Astrophysics, And Astronomy: Solar Irradiance, Solar Physics, Astrophysics, And Astronomy: Ultraviolet Emissions, Interplanetary Physics: Heliosphere/Interstellar Medium Interactions

Scientific paper

The Extreme Ultraviolet (XUV) scanner on board the Planet-B spacecraft observed the He I 58.4-nm emission of the local interstellar gas in interplanetary space during solar maximum. The spacecraft was in an interplanetary orbit with a perihelion of 1 AU and an aphelion of 1.5 AU and encountered the helium focusing cone at a longitude different from the position of the Earth during March and April of 2000. Numerical simulations of the XUV observation were carried out based on a hot model of the interstellar gas (Wu and Judge, 1979) and an empirical solar flux model (SOLAR2000) (Tobiska et al., 2000). In comparison with the XUV observations, we obtain an ionization rate of (2.3 +/- 0.27) × 10-7/s and a line width of 0.0098 +/- 0.0014 nm, corresponding to a velocity of 50 +/- 7.2 km/s. However, the distribution of the interplanetary He I intensity indicates that the solar He I irradiance has an anisotropy in longitude and latitude. From the comparison between the XUV observation and alternative simulations with an anisotropy in the solar He I irradiance and with no anisotropy in the total ionization rate, the He I flux ratio toward the pole to within the ecliptic plane is estimated to be 0.61 +/- 0.24. Therefore it is suggested that the ionization rate of helium atoms has similar anisotropies as the solar He I flux. Since the relationship between anisotropies of the ionization rate and the helium density is not linear, a three-dimensional and time-dependent approach is required for full understanding on the helium distribution in interplanetary space.

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