Solar cycle dependence of interplanetary Lyman α emission and solar wind anisotropics derived from NOZOMI/UVS and SOHO/SWAN observations

We have investigated the distributions of neutral interplanetary hydrogen Lyman a emission and their solar cycle dependence from observations with the ultraviolet imaging spectrometer (UVS) onboard the NOZOMI spacecraft and the solar wind anisotropy instrument (SWAN) onboard the solar and heliospheric observatory (SOHO) spacecraft. The SOHO/SWAN observation covers the period from the solar minimum in 1996 to the solar maximum in 2001, while the NOZOMI/UVS observation covers the period of 1999-2001 near the solar maximum. The most distinct feature of the full sky Lyman α intensity maps near the solar maximum in 2000 is a structure with a single bright region extending across the ecliptic plane on the upwind side of the interstellar wind. This structure is entirely different from the structure observed by SWAN at the solar minimum in 1996, which shows an intensity depression called a "groove" and bright lobes above and below the groove centered near the ecliptic plane. From the fitting of model calculations to the observational data, it is confirmed that the latitudinal distribution of the ionization rate of the interplanetary hydrogen due to the charge exchange with solar wind protons becomes isotropic toward the solar maximum. However, an important result is that the condition for producing anisotropic ionization rate due to the slow, high-density solar wind around the equator exists by October 1999, although the theoretical model predicts isotropic ionization rate throughout 1999.