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Modeling the interstellar-interplanetary helium 58.4 nm resonance glow: Towards a reconciliation with particle measurements

Abstract
Pioneering observations of the diffuse HeI-58.4 nm background radiation were performed with a series of satellites in the 70's. Today, their published results on the flow of interstellar helium atoms in the heliosphere are still in contradiction with (i) the results of the particle experiments, i.e. in situ detection of neutrals and pickup ions; (ii) expectations from heliospheric models and comparison with the hydrogen flow; (iii) results of the recent helium glow observations with the Extreme Ultraviolet Explorer (EUVE). Here we discuss these data sets and their modeling, together with the EUVE data and the first coronographic observations of the helium glow obtained with the Ultraviolet Coronographic Spectrometer (UVCS) on board SOHO. We show how they can all be made compatible, and reconciled with in situ data.

We have reanalysed the Prognoz data and we derive an updated and higher value of the background noise level. Based on this we can now fit the data satisfactorily with the same set of helium parameters as that one derived from recent EUVE and in situ data. We suggest that other early data sets could be reanalyzed in the same way.

Using this updated analysis, EUVE and SOHO-UVCS measurements, we find that all glow data are compatible with the interstellar parameters  km s ^-1,  K,  deg, 6.0 deg (downwind axis, ecliptic coordinates), as well as with the solar parameters derived from SOHO CELIAS-SEM, SUMER and CDS observations, i.e. the helium photoionisation rate, the 58.4 nm irradiance, and the 58.4 nm Doppler width, found to be between 60 and 90 mÅ (30 and 45 km s ^-1). The density is the least constrained parameter from the glow measurements. Prognoz lateral scans, EUVE LWS and SOHO UVCS data are compatible with an interstellar helium density  n₀ in the range 0.013-0.016 cm ^-3. Prognoz anti-solar data and EUVE scanner data lead to a 40% lower value, suggesting uncertainties in the calibrations. A large part of the contradictions between particle and remote sensing results are thus removed, since the above parameters are very similar to those derived from in situ data.

The high electron impact rates inferred from the UVCS remote sensing observations imply high fluxes of newly produced helium pickup ions, which can possibly explain in part the observed correlation between H ⁺ and He ⁺ pickup fluxes, and the inverse correlation between He ⁺ fluxes and solar wind velocity.