INTEGRAL Picture Of the Month
December 2019

INTEGRAL POM
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INTEGRAL part of multi-point Galactic cosmic-ray measurements across the Solar System

Recently, the ESA missions INTEGRAL, Rosetta, Proba-1, Herschel and Planck were part of a study of Galactic Cosmic Rays (GCRs) and their properties within the Solar System, as they all share one thing in common: they are/were equipped with a similar Standard Radiation Environment Monitor (SREM), which records the rates of highly energetic particles that can cause damage to the spacecraft's instruments. The monitors were cross-calibrated with INTEGRAL/SREM serving as the base line. From a radiation environment around the Earth, as INTEGRAL and Proba-1 experience it, the study reached out to the 2nd Lagrangian Point at around 1.5 million kilometres from Earth, where Herschel and Planck were operating, to far beyond Mars up to 4.5 astronomical units (AU) where Rosetta was on its way to chase comet 67P/Churyumov–Gerasimenko. Additionally, data were used from the High-Energy Neutron Detector (HEND) on NASA's Mars Odyssey spacecraft orbiting around the Red Planet.

The analysis of the radiation monitor data from these 6 spacecrafts at different locations allowed to observe the temporal evolution of the GCR within the inner Solar System during the eleven-year Solar cycle. With increasing interplanetary magnetic-field strength, GCRs tend to be stronger deflected away from the Sun, and, therefore, their flux is less as seen by the radiation monitors. The opposite occurs with decreasing interplanetary magnetic-field strength (see inset (a)).

In addition to the observed GCR gradient towards the outer Solar System, an unexpected, yet unexplained, 8% reduction of the GCR flux was found by the Rosetta/SREM in the vicinity of comet 67P/Churyumov–Gerasimenko. As the reduction was measured with respect to the interplanetary space radiation seen by INTEGRAL/SREM, the period after Rosetta's hibernation has been examined in more detail. The upper panel in inset (b) illustrates the orbits going counterclockwise with Rosetta approaching its closest point to the Sun on 13 August 2015, and INTEGRAL in the vicinity of Earth. The two bottom panels show close-ups of the upper panel and reveal the detailed two trajectories, with Rosetta chasing the comet while performing additional manoeuvres, and INTEGRAL periodically orbiting the Earth.

Inset (c) gives the complementary information on the measured GCR flux. The lower panel shows the above-mentioned 8% negative deviation of the GCR flux measured by Rosetta with respect to INTEGRAL, while the upper panel shows the actual measured GCR flux of Rosetta (blue) and INTEGRAL (green), as well as the simulated GCR flux Rosetta was expected to see under the assumption of the measured GCR gradient (yellow). While a number of possible explanations for GCR flux reduction are ruled out, further work needs to be done on the nature of the overall cometary coma characteristics, in order to quantify its potential impact, along with the heliospheric GCR modulation associated with the Solar polarity changes.

This study shows the enormous potential of using radiation monitor data for purely scientific purposes. A proper data processing and cross calibration can even result in a qualitative comparison, using different kinds of radiation monitors like HEND.

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