Remote diagnostic of the hydrogen wall through measurements of the backscattered solar Lyman alpha radiation by Voyager 1/UVS in 1993–2003

Abstract

We perform a new analysis of the Lyman alpha data obtained by Voyager 1 during the spatial scans in 1993–2003 while Voyager 1 was at 53–88 AU from the Sun. These data are the important source of information on the hydrogen distribution in the outer heliosphere. A sophisticated global kinetic‐MHD model of the heliospheric interface and a radiative transfer model are used for the analysis. It is shown for the first time that the ratio of the Lyman alpha intensities detected in the downwind and upwind lines of sight in the outer heliosphere is sensitive to the configuration (peak value and location) of the hydrogen wall. The hydrogen wall is a source of Doppler‐shifted backscattered Lyman alpha photons, so it can be seen from inside the heliosphere. Therefore, Voyager 1/ultraviolet spectrometer (UVS) Lyman alpha data can be used for remote sensing of the hydrogen wall. We show that our current global model of the outer heliosphere, which is consistent with many other measurements including Lyman alpha data from both Voyager 1 and 2 in 1980–1993, provides a systematically larger downwind to upwind intensity ratio compared with the UVS data in 1993–2003. In order to decrease the ratio, a higher and/or closer hydrogen wall is needed.

Calibration of the Voyager ultraviolet Spectrometers and the Composition of the Heliosphere Neutrals: Reassessment

“…In addition, a heliospheric origin of the UVS Lyman-α excess (Ben-Jaffel et al 2000;Ratkiewicz et al 2008) leads to properties of the LISM magnetic field orientation that are now accurately confirmed by SOHO/SWAN, Voyager plasma, Voyager radio emission, IBEX ribbon, and HST Lyman-α absorption measurements (Lallement et al 2005;Schwadron et al 2009;Heerikhuisen & Pogorelov 2011;Ben-Jaffel & Ratkiewicz 2012;Ben-Jaffel et al 2013;Fuselier & Cairns 2013;Mccomas et al 2012;Wood et al 2014;Zank 2015;Zirnstein et al 2016). Furthermore, recent sophisticated 3D MHD-kinetic and RT modeling of the outer heliosphere observations obtained by UVS during the 1993-2003 period led to the important result that a high hydrogen density is required to fit the data (Katushkina et al 2016), a finding that is consistent with the high density level derived by Puyoo et al (1997); Puyoo & Ben-Jaffel (1998) with the original UVS calibration, and confirmed here for the inner heliosphere (< 10 AU).…”

Voyager 1/UVS Lyman α Measurements at the Distant Heliosphere (90–130 AU): Unknown Source of Additional Emission

“…Previously (Katushkina et al, ), we have shown that our state‐of‐the‐art heliospheric model predicts a systematically larger ratio of Lyman α intensities at the nose and tail directions compared to the Voyager 1/UVS data obtained in the scans regime in 1993–2003. It was suggested that one possible solution of this problem is to increase the hydrogen wall by, fir example, changing the number densities of the LISM protons and hydrogen atoms.…”

“…The height, width, and location of the hydrogen wall are substantially different for these three models. The same models were considered before by Katushkina et al (), and it was shown that they give significantly different results for the upwind to downwind ratio of Lyman α intensities, which were also measured by Voyager‐1 in 1993–2003. However, Figure a shows that all three models give approximately the same results for the radial dependence of the intensity measured by Voyager in 2003–2014, and this dependence is qualitatively different from the Voyager data.…”

“…Therefore, emission backscattered at the hydrogen wall is less scattered inside the heliosphere and should be visible from a much larger distance from the scattering point than the normal heliospheric glow. Katushkina et al () have shown that emission from the hydrogen wall can be identified in the Voyager 1/UVS data obtained in the scans‐regime in 1993–2003. In principle, the H wall should lead to a flattening of a radial dependence of Lyman α intensities measured in the distant heliosphere.…”

“…Voyager 1/UVS collected a huge amount of data during 37 years. Observations can be divided into three periods: (1) 1979–1993 (∼7–53 AU), observations for a variety of lines of sight with a few rolls over the sky (Fayock et al, ; Hall et al, ; Lallement et al, ; Quémerais et al, ); (2) 1993–2003 (∼54–89 AU), systematic scans over the sky from the upwind to downwind directions (e.g., Katushkina et al, , Lallement et al, , Quémerais et al, ); (3) 2003–2014 (∼90–130 AU), Voyager 1's scan platform was not moved and all measurements were performed for approximately the same line of sight (LOS) close to the upwind direction.…”