HSTOF ENA observations and energetic ion distributions in the heliosheath

Abstract

Context. Launched in 1996, HSTOF on board SOHO was the first instrument to detect the energetic neutral atoms (ENA) from the heliosheath. After mid 2003, the field of view of HSTOF was restricted to the flank sectors of the heliosheath, in which region the energetic ion distributions are still unknown. Interpretation of these data requires understanding of the energetic ion transport in the inner heliosheath. Aims. We update the HSTOF ENA hydrogen and helium spectra by adding the results from the recent (2006−2010) measurements. We calculate the energetic ion distributions in a numerical model of the heliosheath and use them as a basis for interpreting the HSTOF ENA observations, in particular those of the flank sectors. Methods. The hydrogen and helium ENA spectra were derived from the HSTOF observations of two 90 • -wide ecliptic longitude sectors around the crosswind directions. The energetic ion distributions in the heliosheath were calculated in a simple model of the heliosphere, assuming that the main acceleration occurs at the termination shock. The effects of different processes (charge-exchange loss, adiabatic acceleration, parallel diffusion and escape across the boundary) were determined with different assumptions about the boundary conditions and the transport parameters. The resulting ion distributions were used to calculate the ENA fluxes, which can be compared with the observations by HSTOF and other instruments. Results. The energetic ion density in the flank sectors of the heliosphere is lower than in the forward (upwind) sector. As a result, the contribution of the flanks of the heliosheath to the production of ENA is not directly proportional to the thickness of the heliosheath, but instead is comparable to the contribution of the forward sector, which agrees with the HSTOF data. The HSTOF ENA flux intensity is nevertheless significantly lower than the model calculations. Near the heliopause there appears a region of low energetic ion density caused by the charge-exchange and the escape losses.

Galactic cosmic rays measured by UVS on Voyager 1 and the end of the modulation

“…This implies that the thickness of the heliosheath beyond the S/C location in August 2012 is very small if not null. Such a conclusion is in line with other results based on Voyager 1 data suggestive of an entrance in the ISM: (i) the heliosheath energetic particle disappearance (Krimigis et al 2013); (ii) the abrupt increases in the GCRs and the new spectra (Stone et al 2013;Webber & McDonald 2013;Webber et al 2013a); (iii) the solar wind stagnation since 2010 Krimigis et al 2011); (iv) the comparisons between the Cassini, SOHO/HSTOF and IBEX data, on one hand, and Voyager particle data, on the other hand, which provided estimates of the heliosheath thickness in the Voyager 1 direction (21 ± 6, Hsieh et al 2010;Czechowski et al 2012 and 31(+31, −18) AU Roelof et al 2012); (v) the constancy of the IPGH proton flux (plus UVS, if corrected for anisotropies) for more than a year now, which has never happened since at least 1992; and (vi) the recent measurements of plasma oscillations that imply a high density at V1 ).…”

Heliosheath Processes and the Structure of the Heliopause: Modeling Energetic Particles, Cosmic Rays, and Magnetic Fields

“…The first ENA observation of the heliosheath were performed with the HSTOF sensor of the CELIAS instrument on the SOHO mission for hydrogen ENAs in the energy range from 55-80 keV [54]. In [55], the final analysis of these data is presented for hydrogen and helium ENAs originating in the heliosheath. At lower energies, in the range from 400 eV to 5 keV, the first hydrogen observations were done by the ASPERA-3 and ASPERA-4 ENA instruments on the Mars Express and Venus Express spacecraft [56].…”

“…By considering the charge exchange cross-sections, the energy spectra of protons in the heliosheath were derived, extending the range covered by the instrumentation of the Voyager spacecraft. Before IBEX measurements, the covered energy range spanned from 400 eV to 80 keV [56,58,55,59], including the HENA data from the IMAGE mission, with the most recent compilation of the ENA energy spectra given in [57]. From the fit of the proton spectra, which were derived from these ENA energy spectra, to the in situ proton spectra from Voyager at higher energies, which is the only fit parameter, the thickness of the heliosheath in the upwind direction can be estimated to be between 35-70 AU.…”

Solar wind He pickup ions as source of tens-of-keV/n neutral He atoms observed by the HSTOF/SOHO detector

“…The last term, L esc,He + = f He + /τ esc , describes diffusive escape through the heliopause with escape time τ esc . It was added following Czechowski et al (2012). Time τ esc corresponds to a random walk over the shortest distance l HP separating the considered point from the heliopause.…”