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.
References:
"Multi-point galactic cosmic ray measurements between 1 and 4.5 AU over a
full solar cycle",
Thomas Honig, et al.,
2019, Ann. Geophys., 37, 903–918, 2019
https://doi.org/10.5194/angeo-37-903-2019