The Secondary Stream of Interstellar Neutral Hydrogen Flow

The solar system is located in a low-density interstellar cloud. Neutral hydrogen and helium atoms of the interstellar medium can penetrate deep into the heliosphere without being completely interrupted by the solar wind plasma. Consequently, there is a uniform flow of interplanetary hydrogen and helium in the solar system. This neutral hydrogen and helium flow is called `interstellar wind'. Spacecraft data demonstrated that the upstream direction of the interplanetary neutral helium flow is (254.7°+-0.4°, 5.2°+-0.2°) in the ecliptic coordinate system [Witte, 2004; Vallerga et al., 2004; Gloeckler et al., 2004], while it is (252.5°+-0.5°, 8.8°+-0.5°) for the interplanetary neutral hydrogen [Lallement, et al., 2005; Quemerais et al., 1999]. The presence of this well-established primary stream leads the hydrogen and helium glows to symmetry with respect to the 74° / 254° ecliptic longitude axis. Meanwhile, the existence of a secondary stream of the neutral wind inside the heliosphere arriving from a direction between about 260° and 290° ecliptic longitude, about 10° - 40° different from the upstream primary interstellar neutral flow direction, has been proposed recently by a synthetic analysis of a wide variety of spacecraft observations [M. R. Collier, private communication]. In this study, we have performed a detailed analysis of the interstellar hydrogen resonance glow data obtained from ultraviolet imaging spectrometer (UVS) measurements onboard Nozomi spacecraft. Although the UVS instrument instantaneously points a certain direction with the field-of-view which is perpendicular to the spin axis controlled toward the Earth, spatial distributions of emissions are measured by using the spin and orbital motion of the Nozomi spacecraft. One year observations enable us to derive the full sky image of Lyman alpha emission. We plotted the data obtained in the ecliptic plane from 2000 to 2001 and compared them with the model calculations in cases of the upwind direction of 244, 254, 264, 284, and 304 in longitude. We have found evidence for a secondary stream in the data obtained in 2000, peaked at a longitude of 30 - 40 degrees different from the primary stream. However, the secondary peak has not been observed in 2001. We will discuss about these yearly variations of the secondary peak.