Working Group on "Cross-calibration of past FUV experiments"

Application to more than 30 years of interplanetary background data, including data from Voyager, Pioneer, Pioneer Venus, Cassini, Galileo, HST, SOHO, Mars-Express and Venus-Express


Eric Quémerais, Institut d'Astrophysique Spatiale and Service d'Aeronomie, France
Roger-Maurice Bonnet, International Space Science Institute (ISSI), Switzerland


Dimitra Koutroumpa, Service d'Aéronomie, France
Aurélie Rébérac, Service d'Aéronomie, France
Maciej Bzowski, Space Research Centre PAS, Warsaw, Poland
Wayne Pryor, Central Arizona College, USA
Vlad Izmodenov, Moscow State University and Space Research Institute (IKI), Russia
Dmitry Alexashov, Institute for Problems in Mechanics Russian Academy, Russia
John T. Clarke, Boston University, Boston, USA


The interplanetary background in the UV is due to the scattering of solar Lyman alpha photons by hydrogen atoms present in the interplanetary medium. This emission, known for more than four decades, is used to study the distribution of hydrogen atoms in the heliosphere. This study has recently found renewed interest for the following reasons.
Firstly, both voyager spacecraft crossed the termination shock of the solar wind, in 2004 and 2007 respectively, giving a new momentum to the study of the interface region where the expanding solar wind meets the interstellar medium. Lallement et al. (2005) also showed that the study of the interplanetary background could be used to derive the direction of the interstellar magnetic field beyond the heliopause which strongly affects the heliospheric interface.
Secondly, as shown in Quémerais et al. (2006) the analysis of the maps of interplanetary background measured by the SWAN instrument on SOHO can be used to derive hydrogen ionization rates at all ecliptic latitudes. Such maps will be used to compute the transfer function of Energetic Neutral Atoms between the outer heliosphere and the inner heliosphere. In July 2008, the IBEX mission will be launched. It will study the physical properties of the inner heliosheath, which is the region between the solar wind termination shock and the heliopause, by measuring the distributions of ENAs seen at Earth Orbit. For the slower ENAs, the knowledge of the transfer function is necessary to derive proper source terms in the outer heliosphere.
The proposed team will compile and analyse measurements of the interplanetary UV background, obtained by various space instruments over a period of more than 30 years. The aims of the team are twofold.
First, we wish to create a composite data set showing the time variations of the hydrogen distribution in the heliosphere. This data set will be obtained by compiling data from various space missions such as Voyager 1/2, Pioneer 10/11, Pioneer-Venus, Cassini, Galileo, HST, SOHO, Mars-Express and Venus-Express.
Second, by comparison with a time-dependent model of the hydrogen distribution used to fill data gaps in space and time coverage, we wish to cross-calibrate all those data sets. This method will allow us to get an independent check of the absolute calibration of all the UV instruments used in this survey. Among the data sets used here, we will include measurements of Voyager UVS and SWAN/SOHO. These instruments which are still used to study the interplanetary background show a relatively large uncertainty in their absolute calibration factor. By improving our knowledge of their calibration factors, we will enhance the scientific output of these instruments. It should also be noted that many of the instruments we will use in this survey have been used to study planetary atmospheres and exospheres. An independent check of their calibration will also enhance the results of these studies.
The work of this team is a natural continuation of the highly successful previous team on the breathing of the heliosphere led by V. Izmodenov (2006-2007).

Schedule: The last meeting of the Working Group was held from 25 to 29 May 2009 at ISSI.

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