The Helium Focusing Cone of the Local Interstellar Medium Close to the Sun

Abstract

The Solar and Heliospheric Observatory (SOHO) Ultraviolet Coronagraph Spectrometer is used to observe the interplanetary He focusing cone within 1 AU. Taken over 2 yr and from differing orbit positions, the series of observations includes measurements of He i 584 A ˚ and Ly intensities. The cone itself is spatially well defined, and the He i intensity within the cone was $45 R in 1996 December, compared with $1 R for lines of sight outward from 1 AU. Between 1996 December and 1998 June, the focusing cone dimmed by a factor of 3.3 as the level of solar activity rose. This is the first time that interstellar helium is observed so near the Sun. Measured intensities are compared to a detailed temperature and density model of interstellar helium in the solar system. The model includes EUV ionization but does not include ionization by electron impact from solar wind electrons. There are several features in the data model comparison that we attribute to the absence of electron impact ionization in the model. The absolute maximum intensity of 45 R first measured in 1996 December calls for an ionization 45% more intense than the EUV photoionization alone as measured by the Solar EUV Monitor/Charge, Element, and Isotope Analysis System (SEM/CELIAS) on SOHO. Important day-today variations of the intensity are observed, as well as a general decrease as the solar activity rises (both absolute and divided by a model with a constant ionization). This general decrease is even larger than predicted by a model run with the SEM/CELIAS photoionization rate alone, in spite of a factor of 1.5 increase of this rate from 1996 December to 1998 June. At this time, an additional ionization rate of 0:56 Â 10 À7 s À1 (compared with 1:00 Â 10 À7 s À1 from solar EUV) is required to fit the measured low intensity. We attribute this additional rate to solar wind electron impact ionization of the atoms. This shows that the helium intensity pattern is a very sensitive indicator of the electron density and temperature near the Sun. Subject headings: interplanetary medium-solar wind-ultraviolet: solar system