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 Å 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-to-day 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.

Solar cycle dependence of the helium focusing cone from SOHO/UVCS observations

“…The Ultraviolet Coronograph on board SOHO (UVCS) has observed the 58.4 nm glow of the interplanetary He focusing cone at regular intervals since 1996. The intensity decrease with time already observed during the first two years (Michels et al 2002) has dramatically amplified during the solar activity increase. Intensities seem to reach a plateau in 2001.…”

Synopsis of the interstellar He parameters from combined neutral gas, pickup ion and UV scattering observations and related consequences

“…The cone structure was then modeled using two independent hot models of the He inflow. All the UV scattering observations were simulated as described by Michels et al (2002), with a model that goes back to Dalaudier et al (1984). For the pickup ion observations a model by Ruciński & Fahr (1989) was used that goes back to Wu & Judge (1979).…”

“…At the same time also continuous monitoring of crucial interplanetary parameters, such as ionization rates and solar line profiles, had become available for the first time or had been greatly improved. Therefore, several observation campaigns for near Earth spacecraft were initiated during the Earth's crossing of the He focusing cone over consecutive years, which were combined (Michels et al 2002;Lallement et al 2004a), based on a hot model of the interstellar gas. A line-of-sight through the near-Sun cone, as obtained with SOHO UVCS in December, is indicated in orange.…”

“…While the shape of the cone as observed with these methods can be reproduced within the observational uncertainties, the peak height or the absolute scattering intensity of the cone appears to be consistently overestimated, when only the observed photo ionization rates are used to compute the local He neutral density. While the SOHO UVCS observations of the cone at solar minimum can be modeled with the observed photo ionization rates ) and a contribution from electron impact ionization according to Ruciński & Fahr (1989), the amount of electron impact ionization has to be increased by a factor of 3.5 for solar maximum observations, when photo ionization only increases by a factor of about 2 (Michels et al 2002;Lallement et al 2004a). After including an electron ionization rate that is adapted to the solar maximum observations and varies linearly with solar activity according to the observed solar EUV flux, Lallement et al (2004a) have modeled the near Sun cone observations with the same set of interstellar He parameters over the entire observation period that spans almost from solar minimum to solar maximum.…”

Imprints from the solar cycle on the helium atom and helium pickup ion distributions

“…Hence, the information obtained from measurements of the interstellar helium gas has been used for quite some time as a valuable reference to study other interstellar elements inside the heliosphere and some element-specific processes (like charge exchange) to which these are subjected in the heliospheric interface. The LIC helium gas has been studied by numerous space-borne observations of the interplanetary glow in the He-584Å line, starting with the Nike sounding rocket observations by Paresce et al (1974a,b) and STP satellite observations by Weller and Meier (1974), followed by second generation experiments (Weller and Meier, 1981;Burgin et al, 1983;Dalaudier et al, 1984;Chassefière et al, 1986) and presently by the most recent EUVE and SOHO satellite observations, respectively by Flynn et al (1998) and Michels et al (2002). The first proposal to observe the EUV-resonance glow of the derivative products of the LIC helium, i.e.…”