The Solar EUV flux in the 3D Heliosphere
Abstract
The absolute measurement of solar EUV flux and its time dependence provide critical data on the solar driven photo chemistry which results in solar system objects. For example, the Solar HeII 30.4 nm emission is a major contributor to photo-ionization in the heliosphere, and thus 30.4 nm measurements provide the data required to determine the absolute photoionization rate of neutral interstellar helium flowing into our solar system. However, because of the orbit characteristics of the vast majority of spacecraft, the solar irradiance has mostly been measured at Earth or at least in the plane of the ecliptic. Therefore, the existing data ignore the fact that the angular distribution of the solar flux is largely anisotropic. Indeed, in the far and extreme ultraviolet, the chromosphere and the corona display very contrasted structures. At solar minimum, the polar regions are darkened by large coronal holes, and in the most active part of the solar cycle, bright active regions are scattered around the solar globe at low heliocentric latitudes. To date, due to the small number of off- ecliptic measurements, very few attempts have been made to investigate these variations. The implications of the anisotropy of the solar irradiance are diverse. For example, in the case of the 121.6 nm line of H I, the latitudinal anisotropy must be taken into account when modeling the Lyman alpha resonantly backscattered sky background. Identically, the anisotropy must be included in the modeling of the intensity of the HI 121.6 nm and He II 30.4 nm resonantly scattered coronal lines. We present here a generalization of an earlier work on the anisotropy of the 30.4 nm EUV flux. Our empirical model, is now able to synthethize solar spectra as seen from any point in the heliosphere in the 10 to 50 nm wavelength range. The model is based on a Differential Emission Measure resonstruction of the spectrum from reprojected EUV Carrington maps of the Sun. We present the results obtained on the 3D distribution of the Solar EUV flux during solar cycle 23. We discuss their consequences on the GAS/Ulysses measurements of the Helium abundance in the Local Interstellar Medium.
- Publication:
-
40th COSPAR Scientific Assembly
- Pub Date:
- 2014
- Bibcode:
- 2014cosp...40E.150A