3He-rich Solar Energetic Particles Observations at Parker Solar Probe

Report from ISSI Team #425 Origins of 3He-Rich Solar Energetic Particles led by R. Bucik and J.F. Drake 

Left: Mass histogram for 3He-rich SEP event on 2019 April 20 shows small but clear 3He peak. Right: Jet observations at the west limb from the SDO/AIA. Adapted from Wiedenbeck et al. (2020).

3He-rich solar energetic particles (SEPs) are one of the most peculiar and least explored particle populations in the heliosphere with a tremendously enhanced abundance of the 3He nuclide and ultra-heavy elements (e.g., Pb) by a factor up to 104 above the solar corona or solar wind. One reason for the current lack of understanding of 3He-rich SEPs is the small size of these events. Recently launched Parker Solar Probe (PSP) is able to approach the solar sources of 3He rich SEPs at distances (~0.05 au; 1 au ~ 1.5⨉108 km) that have never been reached before. On 2019 April 20-21, the IS⊙IS energetic particle suite on PSP made its first observations of 3He-rich SEPs. 3He-rich SEPs were observed at energies near 1 MeV/nuc in association with energetic protons, heavy ions, and electrons. At the time of 3He-rich SEP observations, the spacecraft was near 0.46 au. The event was also detected by ULEIS and EPAM on Advanced Composition Explorer (ACE) spacecraft, located near Earth, at 0.99 au from the Sun. The average intensity at ~ 1 MeV/nuc was a factor ~4 greater at PSP than at ACE, which might be attributable to a 1/r2 dependence of the fluence, where r is a distance from the Sun. At that time, PSP and ACE were both magnetically connected to a location near the west limb of the Sun. Remote sensing measurements showed the presence of a type III radio burst and also a helical unwinding jet from this region of the Sun. This activity, which is commonly associated with 3He-rich SEP acceleration on the Sun, originated from the active region number AR 12738. We also searched for smaller 3He-rich SEP events that are not observable near the Earth but might have been detectable closer to the Sun because of the expected strong radial dependence of the intensities of SEP events impulsively released from localized sources. Although no such events were detected during the first two orbits of PSP, this search will be continuing as PSP moves progressively closer to the Sun, and as solar activity increases. These observations should enable IS⊙IS to make significant progress in understanding small 3He-rich SEP events.


Animation of the Jet observations at the west limb from the SDO/AIA. Adapted from Wiedenbeck et al. (2020).



Wiedenbeck M.E., Bucik R., Mason G.M., Ho G.C., Leske R.A. et al., 3He-rich Solar Energetic Particle Observations at Parker Solar Probe and Near Earth, Astrophys. J. Suppl. Ser. 246, 42, 2020.

Saturn’s Huge Moon Titan Drifting Away Faster Than Previously Thought

Report from ISSI Team #411 The ENCELADE Team: Constraining the Dynamical Timescale and Internal Processes of the Saturn and Jupiter Systems from Astrometry led by V. Lainey

Did you know that the Earth’s moon distance is increasing at about 3.8 cm/year because of the tides the Moon raises on our own planet? Thanks to Newton and his law of gravity, entailed the proper explanation of tides: the side of the Earth that is closer to the Moon is more attracted than its opposite side. As a consequence, the Earth takes an elongated shape like a football. Distance increase comes then as a consequence of friction inside the oceans essentially. Both shape distortion and orbital variation rate are expected to get significantly lower with distance, since tides are a consequence of gravitation.

A giant of a moon appears before a giant of a planet undergoing seasonal changes in this natural color view of Titan and Saturn from NASA’s Cassini spacecraft. (Image Credit: NASA/JPL-Caltech/Space Science Institute)

Since tidal theory is universal, researchers have applied it over the last 50 years to predict the orbital evolution of many moons. From the evolution of the four big Galilean satellites of Jupiter to the small moons of Mars, Phobos and Deimos, the same theory was used underneath. Recently and in the context of the Cassini mission, the ISSI ENCELADE Team led by Valery Lainey in collaboration with a team from the University of Bologna tried to quantify from observations the orbital expansion of Titan, the largest Saturnian moon, under Saturn’s tides. Surprisingly, they found that Titan is escaping Saturn’s gravity at a large pace of 11 cm/year, more than a hundred times faster than expected from theoretical models. Even more surprising, such expansion rate is larger than for moons closer to Saturn, in complete contradiction with classical tidal theory! But the study demonstrates a perfect agreement with the prediction of a new tidal mechanism, suggested only four years ago by Jim Fuller (Caltech) and co-authors. Such so-called “tidal lock mechanism” suggests that Titan may have formed way closer to Saturn, than commonly believed. This result brings an important new piece of the puzzle for the highly debated question of the age of the Saturnian system.

Like the classical tidal theory for terrestrial objects, tidal lock mechanism is a universal physical mechanism for giant planets. In principle, it could be at play in way other systems were giant planets are involved, starting with the Jupiter system itself.

More Information can be found here: News Release June 8, 2020, NASA JPL Caltech >>

Lainey, V., Casajus, L.G., Fuller, J. et al. Resonance locking in giant planets indicated by the rapid orbital expansion of Titan, Nature Astronomy, 2020. https://doi.org/10.1038/s41550-020-1120-5