Using Energetic Electron And Ion Observations to Investigate Solar Wind Structures and Infer Solar Wind Magnetic Field Configurations

Report from ISSI Team #469 Using Energetic Electron And Ion Observations To Investigate Solar Wind Structures And Infer Solar Wind Magnetic Field Configurations led by G. Li and L. Wang

Coronal mass ejections (CMEs) represent some of the most energetic processes in the entire solar system. They are often associated with Solar Energetic Particle Events (SEP events) and are major concerns of space weather studies. When CMEs happen, they drive shock waves in front of them and charged particles are accelerated at the shock front through the diffusive shock acceleration mechanism. Protons and ions can be accelerated to the energy beyond 1 GeV/nuc in some of the most energetic SEP events. Understanding how particles are accelerated in these events and how these accelerated particles propagate to the Earth has been a central problem for space plasma physics.

Members in ISSI Team #469, including team leader Dr. G. Li and team member Dr. L. Zhao has recently won a National Science Foundation (NSF) grant through the ANSWERS program. This four-year, $2.301 million grant from the NSF started in July 2022 and will assist the PI and his team to develop a comprehensive scientific model to understand and predict how CMEs influence the energetic particle radiation environment in the inner solar system and Earth’s magnetosphere, and compare those results with measurements at the Earth’s surface. The grant supports a multidisciplinary team including UAH, the University of Michigan, the University of Wisconsin-River Falls, and the National Solar Observatory. Dr Li and Dr. Zhao, from UAH and UM are PI and Co-PI of this grant. The propagation of energetic protons and ions in the solar wind follow the same interplanetary magnetic field (IMF) lines as those energetic electrons, and studying the configuration of the IMFs is the goal of the ISSI team 469. We expect our ISSI study will be of great value to the newly funded NSF ANSWERS program.

Press Release of the University from Alabama in Huntsville >> 

Newly selected International Teams in Space and Earth Sciences 2022

Twenty-five International Teams have been selected by the ISSI Science Committee for implementation from the proposals received in response to the 2022 call. 

As one of ISSI’s and ISSI-BJ’s tools, International Teams of up to 15 scientists address specific self-defined problems in the Space and Earth Sciences, analyzing data and comparing these with models and theories. The teams  work together in an efficient and flexible format with typically 2-3 one-week meetings over two years. The results of the studies are published in the peer-reviewed literature.  

The next call for proposals will be issued in January 2023.

New International Teams 2022 >>

Galactic Cannibalism on Small Scales

Report from ISSI/ISSI-BJ Team #444 “Chemical Abundances in the ISM: The Litmus Test of Stellar IMF Variations in Galaxies Across Cosmic Time” led by D. Romano and  Z.-Y. Zhang

Astronomers have known for a long time that large galaxies grow through accretion and merging of smaller systems. A recent study published in Nature Astronomy demonstrates that this fundamental pattern of structure formation also applies to galactic satellites on small scales. A team of Italian researchers and members of the ISSI/ISSI-BJ Team #444 has discovered an old star cluster in the Large Magellanic Cloud (LMC) whose chemical composition is unambiguously pointing to an external origin.

The Large Magellanic Cloud. In the inset: zooming in the globular cluster NGC 2005. Image Credits: A. Mucciarelli / University of Bologna / Italian National Institute for Astrophysics (INAF)

How do galaxies grow? It is well known that large galaxies become larger and larger by accreting small satellite systems. The team has now found evidence that this is true also on small scales, for the satellites themselves. By examining the chemical properties of a sample of 11 globular clusters in the Large Magellanic Cloud (LMC), they noticed that one of those old groupings of stars had a very different chemical composition than the others. Using high-resolution stellar spectra obtained with the ESO (European Southern Observatory) Very Large Telescope and with the Magellan Telescope in Las Campanas Observatory, Chile, the astronomers discovered that NGC 2005’s stars have less silicon, calcium, copper and zinc than any other star in the targeted LMC clusters. By performing numerical simulations, they demonstrated that the peculiar chemical composition observed in NGC 2005 is best explained if this cluster is the remnant of an ancient, small stellar system that was swallowed by the LMC long ago.

“The chemical composition of a star is like its DNA and there is no way to change it” says Professor Alessio Mucciarelli, first author of the study. “Therefore, the chemistry of a star tells us about the genealogy of this star, namely, the chemical composition of the gas from which it formed. Our discovery demonstrates that NGC 2005 was born outside the LMC.”

Donatella Romano, co-author of the study and co-leader of the ISSI/ISSI-BJ team “Chemical abundances in the ISM: the litmus test of stellar IMF variations in galaxies across cosmic time”, performed the numerical simulations. “NGC 2005 must have been born in a peculiar environment, characterized by a low-level star formation rate and by a stellar initial mass function (IMF) skewed against the most massive stars,” she says. “A clear indication in favor of this interpretation is provided by the extremely low zinc abundance of NGC 2005’s stars: zinc is produced mostly by hypernovae that have stellar progenitors 30-40 times as massive as the Sun, or even more. Hence, a low abundance of this element strongly suggests that the formation of its main stellar producers was somehow suppressed or, at least, strongly reduced.”


Mucciarelli, A., Massari, D., Minelli, A. et al. A relic from a past merger event in the Large Magellanic Cloud. Nat Astron (2021).

Preprint in

Spotting Hard-To-Detect Coronal Mass Ejections from the Sun

Report from ISSI Team #415 Understanding the Origins of Problem Geomagnetic Storms led by N. V. Nitta and T. Mulligan

Coronal mass ejections (CMEs) are large eruptions from the Sun that are often powerful drivers of space weather effects at Earth. Being able to predict their behaviour in interplanetary space is one of the main goals of space weather forecasting. However, there is a class of CMEs that are particularly hard to observe and, therefore, forecast. These eruptions are known as “stealth CMEs” and they were first reported by Robbrecht et al. [2009], who used the twin STEREO spacecraft (in orbit around the Sun) that were separated by ~50° in longitude to observe a clear ejection off the solar limb from one perspective, but no corresponding eruptive signatures against the solar disc from the other. The lack of indications that an eruption has occurred makes it particularly challenging to establish whether a CME is Earth-directed, especially when imagery from secondary viewpoints is not available. Nitta & Mulligan [2017] analysed a number of stealth CMEs that, in fact, caused unexpected space weather effects at Earth, also known as “problem geomagnetic storms”.

New research from Palmerio et al. [2021] aims to explore techniques that may be useful to identify and analyse eruptions that are elusive when viewed against the solar disc. The authors revisited four well-known stealth CMEs that were characterised by off-limb observations from either one or both STEREO spacecraft, enabling knowledge of their approximate source region. They first applied different image-processing techniques to these events (see example in Figure 1), noting that the most prominent changes that can be attributed to eruptive signatures are evident in long-separation difference data (where to one image is subtracted a preceding one, from e.g. 12 hours prior). Once large-scale changes in the structure of the solar corona are singled out, more refined analysis using “plain” intensity images can be applied to interpret the identified structures, and data produced with more advanced processing techniques can be used to zoom-in on the source region and inspect the eruption in deeper detail.


Figure 1. Example of different image processing techniques applied to a stealth CME that erupted on 4 February 2012. The arrows in the last column point to the faint eruptive signatures (in terms of dimmings and brightenings) found in “plain” intensity images, difference images, and images processed with the wavelet packet equalisation and multi-scale gaussian normalisation techniques. Figure from Palmerio et al. [2021].

Since the events studied were characterised by two or three simultaneous observations of the Sun and its corona, the authors also applied several geometric techniques to reconstruct the eruptions in 3D and connect them to a more-or-less defined location on the solar disc (see example in Figure 2). They concluded that the efficacy of these methods strongly depends on the propagation direction of a CME with respect to the observers and the relative spacecraft separation, since it is not unusual for CMEs to deflect in latitude and/or longitude when they are only a few solar radii away from the surface.


Figure 2. Example of different geometric techniques applied to the same stealth CME shown in Figure 1, which erupted on 4 February 2012. The top row shows reconstructions applied to solar disc imagery using the tie-point technique, the middle row shows reconstructions applied to coronagraph data using the graduated cylindrical shell model, and the bottom row shows results from both methods. Figure from Palmerio et al. [2021].

The careful, multi-step analysis presented in Palmerio et al. [2021] suggests that stealth CMEs can in principle be successfully identified even if they look “invisible” at first glance, thus allowing their inclusion in space-weather forecasting models and predictions.


More information can be found here: “New method predicts ‘stealth’ solar storms before they wreak geomagnetic havoc on Earth” >>



Nitta, N. V., and Mulligan, T.: Earth-affecting Coronal Mass Ejections without Obvious Low Coronal Signatures, Solar Physics, 292:125. doi:10.1007/s11207-017-1147-7, 2017.

Palmerio, E., Nitta, N. V., Mulligan, T., Mierla, M., O’Kane, J., Richardson, I. G., Sinha, S., Srivastava, N., Yardley, S. L., and Zhukov, A. N.: Investigating remote-sensing techniques to reveal stealth coronal mass ejections, Frontiers in Astronomy and Space Sciences, 8:695966, doi:10.3389/fspas.2021.695966, 2021.

Robbrecht, E., Patsourakos, S., and Vourlidas, A.: No Trace Left Behind: STEREO Observation of a Coronal Mass Ejection without Low Coronal Signatures, The Astrophysical Journal, 701, 283–291, doi:10.1088/0004-637X/701/1/283, 2009.

Moving Langmuir Waves and the Most Intense Radio Sources in the Sky

Report from the ISSI Team #408  Low Frequency Imaging Spectroscopy with LOFAR – New Look at Non-Thermal Processes in the Outer Corona led by E. Kontar

The combination of kinetic simulations with LOFAR telescope observations published  in a paper in Nature Astronomy shows that the fine structures are caused by the moving intense clumps of Langmuir waves in a turbulent medium.

The Sun routinely produces energetic electrons in its outer atmosphere that subsequently travel through interplanetary space. These electron beams generate Langmuir waves in the background plasma, producing type III radio bursts that are the brightest radio sources in the sky (Suzuki & Dulk, 1985).

These solar radio bursts also provide a unique opportunity to understand particle acceleration and transport which is important for our prediction of extreme space weather events near the Earth. However, the formation and motion of type III fine frequency structures is a puzzle but is commonly believed to be related to plasma turbulence in the solar corona and solar wind. 

Recent work by Reid & Kontar, Nature Astronomy, combines a theoretical framework with kinetic simulations and high-resolution radio type III observations using the Low Frequency Array and quantitatively demonstrates that the fine structures are caused by the moving intense clumps of Langmuir waves in a turbulent medium. These results show how type III fine structure can be used to remotely analyse the intensity and spectrum of compressive density fluctuations, and can infer ambient temperatures in astrophysical plasma, both significantly expanding the current diagnostic potential of solar radio emission.

Image shows dynamic spectra (left) and associated radio contours of solar type III radio bursts observed by LOFAR (right).  The LOFAR contours at 75% of the peak flux of the type III bursts going from 40 MHz to 30 MHz in the colour sequence white-blue-green-yellow-red. The LOFAR beam contour at 75% for 30 MHz is shown in the top left corner in white. The background is the Sun in EUV at 171 Angstroms observed by AIA. Image from Reid & Kontar, Nature Astronomy, 2021.


Suzuki, S. & Dulk, G. A. Bursts of Type III and Type V 289–332 (Cambridge Univ. Press, 1985)

Reid, H.A.S., Kontar, E.P. Fine structure of type III solar radio bursts from Langmuir wave motion in turbulent plasma. Nat. Astron. (2021).

Electromagnetic Power of Lightning Superbolts from Earth to Space

Report from the ISSI Team #477 “Radiation Belt Physics From Top To Bottom: Combining Multipoint Satellite Observations And Data Assimilative Models To Determine The Interplay Between Sources And Losses” led by led by J.-F. Ripoll (CEA, France), G. D. Reeves (Los Alamos National Laboratory, USA) & D. L. Turner (Applied Physics Laboratory, USA)

Lightning superbolts are the most powerful and rare lightning events with intense optical emission, first identified from space by the Vela satellites at the end of the 70s. Recently, radio frequency superbolts were geographically localized by the very low frequency (VLF) ground stations of the World-Wide Lightning Location Network (WWLLN). Interestingly, the distribution of superbolt locations and occurrence times was not equivalent to that of ordinary lightning: instead, superbolts were found to occur over oceans and seas at a much higher rate, and more often in winter [Holzworth et al., 2019].

In our new study just published in Nature Communications (Ripoll et al. 2021), we show for the first time superbolt very low frequency (VLF) electromagnetic (EM) power density in space from the measurements of the NASA Van Allen Probes mission. We combine space and ground-based measurements of superbolt from CEA, WWLLN, and Météorage ground-based stations in a unique manner to follow electromagnetic superbolt signals from Earth to space over thousands of kilometers. We succeed to widely characterize their VLF electric and magnetic wave power density in space and on Earth, to compute ground-space transmitted power ratio, and to extract various statistical electromagnetic properties of lightning superbolts never before reported.

We find superbolts transmit 10-1000 times more powerful very low frequency waves into space than typical strokes, revealing their extreme nature in space. We conclude that superbolts exhibit several properties that differ from ordinary lightning (Ripoll et al., 2020), other than their geographic and seasonal distribution, deepening the mystery associated with these extreme events. They have, for instance, a more symmetric first ground-wave peak due to a longer rise time, larger peak current, weaker decay of electromagnetic power density in space with distance, and a power mostly confined in the very low frequency range. Reasons are not yet established. Our study should guide modelling and understanding of lightning electrodynamics, atmospheric discharges, and wave transmission from Earth to space, with applications in remote sensing, and wave modeling in space for radiation belt physics. Simultaneous optical and electromagnetic observations should be critical to help reveal more mysteries of superbolts.

Image showing wave power in space: electromagnetic (EM) signature of a 1.2 MJ superbolt measured from the Van Allen Probes. (a) burst mode electric field power spectral density (PSD in V2/m2/Hz) versus time, (b) the evolution of the measured electric field power and estimated wave power of WWLLN-detected lightning strokes in the time window (green circles #2-#10 and superbolt with red contour).

The studies on lightning (Ripoll et al., 2020) and on superbolts (Ripoll et al., 2021) electromagnetic power have been conducted by scientists from CEA in France, the University of Colorado, the Los Alamos National Laboratory, the university of Iowa, the University of Minnesota, and the Météorage Company.



Open Access: Ripoll, J.F., Farges, T., Malaspina, D.M. et al. Electromagnetic power of lightning superbolts from Earth to space. Nat Commun 12, 3553 (2021).

Ripoll, J.‐F., Farges, T., Malaspina, D. M., Lay, E. H., Cunningham, G. S., Hospodarsky, G. B., et al. (2020). Analysis of electric and magnetic lightning‐generated wave amplitudes measured by the Van Allen Probes. Geophysical Research Letters, 47, e2020GL087503. 10.1029/2020GL087503

Holzworth, R. H., McCarthy, M. P., Brundell, J. B., Jacobson, A. R., & Rodger, C. J. (2019). Global distribution of superbolts. Journal of Geophysical Research: Atmospheres, 124, 9996–10,005,


From the Interstellar Medium to Comets: The Case of Hydroxylated Silicates in 67P/Churyumov–Gerasimenko

Report from ISSI Team #397 Comet 67P/Churyumov-Gerasimenko Surface Composition as a Playground for Radiative Transfer Modeling and Laboratory Measurements” led by M. Ciarniello

Recent investigations of the surface composition of comet 67P/Churyumov-Gerasimenko, by means of observations provided by the VIRTIS imaging spectrometer onboard the Rosetta mission, revealed the presence of aliphatic organics and ammonium salts, which characterize the ubiquitous 3.2 µm absorption band in the comet’s infrared spectrum. (See ISSI Team Report from April 9, 2020)

Here we report of a further laboratory investigation, which indicates that hydroxylated magnesium-rich amorphous silicates have spectral properties compatible with the infrared absorption observed on the comet 67P/Churumov-Gerasimenko. They can be an additional constituent of the comet’s surface. Hydroxylated amorphous silicates are formed upon interaction of hydrogen atoms with amorphous silicates. Such process can take place in the interstellar medium (ISM), and the presence of hydroxylated silicates on a cometary nucleus would represent an evolutionary linkbetween the ISM and the primitive objects in the Solar System. The link is consistent with the evolution of aliphatic organics, which also originate in the ISM.

The investigation took advantage, among other authors, of the collaboration of the ISSI Team “Comet 67P/Churyumov-Gerasimenko Surface Composition as a Playground for Radiative Transfer Modeling and Laboratory Measurements” led by M. Ciarniello and has been published in the paper

Hydroxylated Mg-rich Amorphous Silicates: A New Component of the 3.2 μm Absorption Band of Comet 67P/Churyumov–Gerasimenko by V. Mennella et al., 2020, The Astrophysical Journal Letters, Volume 897, Number 2, DOI:

ISSI Tuesday Tea(m) Time 1551

Report by Joachim Wambsganss, ISSI Director

Following the interruption of the usual activities at ISSI in Bern due to the Corona crisis, the ISSI directorate discussed other/new/additional ways to promote and enable international space science. We came up with three new initiatives, the first of which started on July 14, 2020: ‘ISSI Tuesday Tea(m) Time 1551’ and will be presented here.

The idea behind it is as follows: Due to national and international travel restrictions, regular full physical meetings at ISSI Bern are presently difficult. This is of particular concern to the ISSI International Teams. International Teams consist of 8 to 15 scientists and typically meet in Bern at the ISSI premises two or three times within roughly two years. At any given time, about 50 International Teams are “active”. Since mid-March, no meeting of an International Team has taken place at ISSI although the first physical meetings will likely resume in September.  

To provide support to International Teams, the ISSI directorate came up with the suggestion to meet “virtually” in form of video conferences. In order to structure this and to give ISSI scientists a chance to participate, we fixed a weekly slot, Tuesdays at 15:51 o’clock Bern time (i.e. CEST or CET, respectively). So ideally, every Tuesday a different team shall meet within the next so many months. The slightly odd-looking time-of-day was chosen because the numerals 1551 look very similar to the letters ISSI and hence have a visual connection and can be easily remembered as well. Since on one hand, this afternoon time is when many cultures celebrate a cup of tea, on the other hand we want each ISSI Team to use this opportunity, we call this new activity: 

ISSI Tuesday Tea(m) Time 1551

(or in short ISSI TTT 1551). Most of the International Teams responded positively, some embraced this new opportunity of a soon-to-be-held team meeting enthusiastically.

The first actual “ISSI TTT 1551”-event took place on Tuesday this week, July 14, 2020. The ISSI International Team An Exploration of the Valley Region in the Low altitude Ionosphere: Response to Forcing from Below and Above and Relevance to Space Weather lead by Jorge Chau (Leibniz Institute of Atmospheric Physics, Rostock, Germany) met online via the ZOOM system, the session had been prepared by ISSI.

Screenshot of the first Tuesday Tea(m) Meeting with the J. Chau Team and ISSI Staff Members














This “ISSI TTT 1551”-premiere worked very well. After a few introductory words by ISSI representatives, the team chair Koki Chau presented a draft agenda for the meeting. First some administrative issues were discussed, e.g. whether the next envisioned physical meeting team meeting at ISSI Bern – foreseen for end of September – could or should be held, maybe combined as a hybrid meeting with remote participation possible. Then the team discussed how to proceed with another new ISSI activity, namely an ISSI@25 video, meant to celebrate 25 years of ISSI with a 25 second video per International Team (this will be reported about in the near future with a separate spotlight). Following a brief status of activities, three short science talks (10 min each) were presented by team members followed by a Q&A period. A general discussion concluded the meeting.

This first ISSI TTT 1551 was an excellent realization of our vision at ISSI of how such a meeting should work. Five ISSI staff members participated, at least for part of the time. They enjoyed the opportunity to meet the team and get an excellent impression on what their science is all about as well as of the enthusiasm of the team members. This team was an excellent pioneer and did extremely well from our ISSI perspective. We certainly hope that the team enjoyed their Tea Time as well. We look forward to many more interesting and exciting ISSI TTT 1551 events with other active ISSI International Teams in the coming months!

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.