Model of the all-sky He II 30.4 nm solar flux F. Auche `re a, * , J.W. Cook b , J.S. Newmark b , D.R. McMullin c , R. von Steiger d , M. Witte e a Institut dÕAstrophysique Spatiale, Solar Physics Group, Universite ´ Paris-Sud, Ba ˆ timent 121, 91405 Orsay, France b Naval Research Laboratory, Washington, DC 20375, USA c Praxis, Inc., 2200 Mill Road, Alexandria, VA 22314-5380, USA d International Space Science Institute, Hallerstrasse 6, Bern 3012, Switzerland e Max Planck Intitut fu ¨ r Aeronomie, Max-Planck-Strasse 2, Katlenburg-Lindau 37191, Germany Received 30 October 2004; received in revised form 14 February 2005; accepted 14 February 2005 Abstract Because of the orbit characteristics of the vast majority of spacecraft, the solar flux has been generally measured at Earth or in the plane of the ecliptic. So far, most published studies did not consider the fact that the extreme ultraviolet (EUV) solar flux is largely anisotropic. Indeed, in the EUV, the distribution of very contrasted bright and dark features at the surface of the Sun produces both the obvious rotational (longitudinal) modulation of the flux, but also a strong latitudinal anisotropy. Although largely ignored up to now, the latitudinal anisotropy affects the physical conditions in the corona and heliosphere. We describe an empirical model of the all-sky He II 30.4 nm flux based on EIT/SOHO data. The 30.4 nm flux was found to be strongly aniso- tropic. The anisotropy I pol /I eq between the fluxes computed for viewpoints located above the solar poles and within the solar equatorial plane ranges from 0.9 at solar minimum to 0.6 at solar maximum. A 20% asymmetry was also discovered between the north and south polar fluxes. Ó 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Sun: UV radiation; Sun: flux activity 1. Introduction In the extreme ultraviolet (EUV), the Sun displays very contrasted structures. At solar minimum, the polar regions are darkened by large coronal holes, and throughout the solar cycle, bright active regions are scattered around the solar globe at mid to low helio- graphic latitudes. Clearly, one can expect this non- uniform distribution of very contrasted dark and bright structures to produce a pronounced latitudinal anisot- ropy in the EUV solar flux at any given date and in its variability over the solar cycle. To date, due to the lack of off-ecliptic measurements, very few attempts have been made to investigate these variations. However, the anisotropy of the solar EUV flux has a significant impact on several physical processes in the solar corona and in the heliosphere. For example, preliminary results suggest that the GAS/Ulysses measurements of the inter- stellar helium density are significantly biased by the anisotropy of the solar EUV emission. Indeed, the he- lium photoionization rates needed to reduce the data are currently computed from in-ecliptic measurements of the EUV flux. Neglecting the anisotropy leads to an overestimation of the photoionization rates at high lati- tudes, which artificially increases the deduced interstellar helium density. The compensation of this bias is a major motivation for the modeling of the 3D EUV irradiance (Witte et al., 2004). 0273-1177/$30 Ó 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2005.02.036 * Corresponding author. Tel.: +33169858733; fax: +33169858701. E-mail address: frederic.auchere@ias.u-psud.fr (F. Auche `re). www.elsevier.com/locate/asr Advances in Space Research 35 (2005) 388–392