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).

“Age and Formation of the Moon” with Thorsten Kleine (Münster University, Germany)

The Moon has been Earth’s steady companion ever since about 4.4 billion years ago. The age of the Moon and just how it formed is still a matter of intense scientific debate and research. Soon after the return of the first Apollo lunar samples, early hypotheses of the formation of the Moon (co-accretion, fission, and capture) have mostly been discarded in favor of the giant impact model, whereby a collision between a Mars-sized body with proto-Earth led to vaporization of the outer rock layers, expansion of the vapor cloud, and, ultimately, cooling and accretion of a comparatively iron-poor planetary body in orbit around Earth. Geophysical evidence for a small iron core and petrological evidence for an early lunar magma ocean lend credibility to the hypotheses. The giant impact model also predicts that the Moon consists predominantly of impactor mantle material. This implies that the Moon should be isotopically distinct from Earth, but isotope data for lunar samples have convincingly shown that it is not. This “lunar isotopic crisis” has led to the development of a new generation of giant impact models, in which the Moon either formed mostly from Earth’s mantle or isotopically equilibrated with the Earth after the giant impact. However, whether or not these models can account for the high degree of isotopic homogeneity of the Earth and Moon remains a matter of debate. 

The analyses of lunar samples not only provide clues to how but also when the Moon formed. Although the giant impact cannot be dated directly, the age of the Moon can be determined by dating the solidification of the lunar magma ocean. Yet, there is debate about how the ages of the distinct crystallization products of the magma ocean can be tied to the age of the Moon. The most recent results provide a Moon formation age of 4.425 ± 0.025 billion years, which is remarkably similar to the combined hafnium-tungsten and uranium-lead age for core formation on Earth. This suggests that the Moon-forming impact triggered the last major core formation event on Earth, as predicted in the giant impact model. 

Thorsten Kleine is a professor for Planetology at the University of Münster, Germany. He is a member of the North-Rhine Westphalian Academy of Sciences, Humanities and Arts, a fellow of the Meteoritical Society, recipient of the F.W. Clarke Award of the Geochemical Society, of the Victor Moritz Goldschmidt Award of the German Mineralogical Society, and of the Nier Prize of the Meteoritical Society. Thorsten Kleine specializes in isotopic studies of meteorites, lunar, martian and terrestrial rocks. His work is highly cited and is supported by an ERC Consolidator Grant. Since 2020 he is speaker of the DFG-funded collaborative research center “Late Accretion onto the Terrestrial Planets”, which investigates the late growth history of the Earth and Moon.

This webinar was recorded on July 1, 2021

Reading Terrestrial Planet Evolution in Isotopes and Element Measurements

Volume 80 in the Space Sciences Series of ISSI

This volume takes an interdisciplinary approach to the evolution of terrestrial planets, addressing the topic from the perspectives of planetary sciences, geochemistry, geophysics and biology, and solar and astrophysics.
The review papers analyze the chemical, isotopic and elemental evolution of the early Solar System, with specific emphasis on Venus, Earth, and Mars. They discuss how these factors contribute to our understanding of accretion timescales, volatile delivery, the origin of the Moon and the evolution of atmospheres and water inventories of terrestrial planets. Also explored are plate tectonic formation, the origin of nitrogen atmospheres and the prospects for exoplanet habitability.The papers are forward-looking as well, considering the importance of future space missions for understanding terrestrial planet evolution in the Solar System and beyond. Overall, this volume shall be useful for academic and professional audiences across a range of scientific disciplines.

This volume is based on an interdisciplinary international Workshop organised by Europlanet and ISSI, which took place at ISSI, in Bern (Switzerland) during October 22 and 26, 2018 where about 48 leading scientists discussed the issues presented here.

The papers are edited by H. Lammer, B. Marty, A. Zerkle, M. Blanc, H. O’Neill, T. Kleine

This volume is co-published in Space Science Reviews in the Topical Collection “Reading Terrestrial Planet Evolution in Isotopes and Element Measurements” (Partial Open Access) >>

Hard Cover Book >>


“Weather Disasters in a Changing Climate” with Stephen Belcher (MET Office, Exeter, UK)

Weather and climate extremes such as heatwaves, heavy rainfall, drought, wind storms, flooding and wildfires have huge socio-economic and environmental costs. And climate change is already driving changes in weather extremes. Since the Paris Agreement in December 2015 there is a focus on the transition to net zero emissions, in order to limit the damage from climate change. Nevertheless, we are committed to further changes on the climate system, and so there remains a need to understand the impacts of further changes to the climate and to build resilience. As a result, the focus of climate science research at centres such as the Met Office has shifted to reflect these changing drivers: moving from proving that climate change is happening, to understanding the nature of the change and helping design solutions. This presentation will survey some of the inspirational new work that is being done to rise to this enormous challenge, including new observations, new modelling for projections, and new partnerships that are needed to move to building solutions.

Stephen Belcher obtained his PhD in fluid dynamics from the University of Cambridge (UK) in 1990 and has subsequently published over 100 peer-reviewed papers on the fluid dynamics of atmospheric and oceanic turbulence. Having completed his PhD he became a research fellow at Stanford and Cambridge Universities. In 1994 he moved to the Department of Meteorology at the University of Reading (UK), where he served as Head of the School of Mathematical and Physical Sciences between 2007 and 2010.  In 2010 he became the Joint Met Office Chair in Weather Systems. This role gave him a taster of working closely with the Met Office, and in 2012 he joined the Met Office as Director of the Met Office Hadley Centre (UK).Stephen led the evolution of the Met Office Hadley Centre to focus on climate science and services: motivated by the need to provide governments, industry and society with actionable advice, i.e. ‘climate services’. He was a driving force behind the initiation of the Newton Fund Climate Science for Service Partnership China (CSSP China), in which scientists from both China and the UK are now working together to develop fundamental climate science and climate services.

This webinar was recorded on June 24, 2021

The Tidal Disruption of Stars by Massive Black Holes

New Topical Collection published in Space Science Reviews (partial Open Access)

A new collection of 14 review articles has been completed and is available online in Space Science Reviews, a printed book version will be published as volume 79 of the Space Sciences Series of ISSI.

For several decades, astronomers have speculated that a hapless star could wander too close to a super massive black hole (SMBH) and be torn apart by tidal forces. It is only with the recent advent of numerous wide-field transient surveys that such events have been detected in the form of giant-amplitude, luminous flares of electromagnetic radiation from the centers of otherwise quiescent galaxies. These discoveries span the entire electromagnetic spectrum, from γ-rays through X-rays, ultra-violet, optical, infrared, and radio. A small number of events launch relativistic jets. These tidal disruption events (TDEs) have caused widespread excitement as they can be used to study the properties of quiescent, otherwise undetectable, SMBHs; the populations and dynamics of stars in galactic nuclei; the physics of black hole accretion including the potential to detect relativistic effects near the SMBH; and the physics of (radio) jet formation and evolution in a pristine environment. For scientific questions concerning quiescent SMBHs, TDEs are unique probes beyond the local universe. TDEs can also occur around active galactic nuclei (AGNs), although uniquely identifying such an event on top of a bright AGN is difficult.

Currently, the diverse emission properties of flares associated with TDEs is not fully understood. This challenge is being addressed by a sharp increase in observational work and theoretical modelling. Over the next few years, the largest growth areas will likely be in the greatly expanded surveys of the transient sky, and in new numerical modeling techniques. Together these will reveal how SMBHs shine by ripping apart orbiting stars and swallowing the stellar debris.

In light of this foreseen growth, many new researchers are expected to enter the field. Therefore, the time was deemed ripe to compose a comprehensive overview of the state of the art in this rapidly-evolving field. This topical collection was planned and launched at a workshop held at the International Space Science Institute (ISSI) in Bern on 8–12 October, 2018.

The editors would like to thank all authors and coordinators for their hard work in creating this collection, the staff at ISSI for their friendly and generous support, and the future readers of this topical collection who will no doubt answer many of the TDE puzzles that so far remain unresolved.

Peter G. Jonker, Iair Arcavi, E. Sterl Phinney, Elena M. Rossi, Nicholas C. Stone & Sjoert van Velzen (Guest Editors)


Introductory Article: 
Peter G. Jonker, Iair Arcavi, E. Sterl Phinney, Elena M. Rossi, Nicholas C. Stone & Sjoert van Velzen. Editorial to the Topical Collection: The Tidal Disruption of Stars by Massive Black Holes. Space Sci Rev 217, 62 (2021).

Complete Topical Collection in Space Science Reviews >>

Auroral Physics

New Topical Collection in Space Science Reviews (partial Open Access)

The new article collection on “Auroral Physics” is designed to provide a comprehensive review of our current scientific understanding of the terrestrial aurora, both observational and theoretical, with an emphasis on developments since a previous collection devoted to the terrestrial aurora, “Auroral Plasma Physics” (Space Science Reviews volume 103, 2002, by G. Paschmann, S. Haaland, and R. Treumann). That collection emphasizes introductory and background material which is not repeated in the current set. 

The term “aurora borealis”, or “northern dawn” dates back centuries and refers to emissions of light from the otherwise-dark nighttime atmosphere, usually occurring in the polar regions except during highly disturbed periods. Aurora is distinct from airglow, which is a weak and relatively unstructured emission in the thermosphere caused by chemical reactions and ionization driven primarily by solar UV illumination during the day. In contrast, auroral emissions are the result of excitation of neutral atoms and molecules in the upper atmosphere by collisions with charged particles, typically which originate in the magnetosphere and precipitate along geomagnetic field lines with energies of hundreds of eV to tens of keV.

Popular descriptions of the aurora including in the media, dictionaries, encyclopedias and even textbooks often claim that the aurora is caused by “particles from the sun striking the upper atmosphere”. It is well established in auroral science that such a description is not accurate except in very limited cases. While it is certainly true that particles from the sun – the solar wind – provide the energy that drives the aurora, the widely varying morphologies and behaviors of the aurora are the result of a complex chain of events that take place within Earth’s magnetosphere or at its boundary, the magnetopause. The end result of this chain – excitation of neutrals by charged particles – is also well-established fact, as are many other aspects described in this collection. However, due to the vast region of the magnetosphere that is magnetically conjugate to the auroral ionosphere, and the difficulty in sampling it with a single or even multiple spacecraft, the nature of the magnetospheric generator responsible for driving individual auroral forms still remains one of the most elusive aspects of the aurora.

Planning for this Topical Collection took place during a workshop hosted in August 2018 by the International Space Science Institute (ISSI), in Bern (Switzerland), and attended by more than 40 members of the international scientific community.

This Topical Collection will be reprinted as the Volume 78 in the Space Sciences Series of ISSI and is edited by David Knudsen, Joe Borovsky, Tomas Karlsson, Ryuho Kataoka and Noora Partmies.

Introductory article: 
David Knudsen, Joe Borovsky, Tomas Karlsson, Ryuho Kataoka and Noora Partmies. Editorial: Topical Collection on Auroral Physics, Space Sci Rev 217, 19 (2021).

Complete Topical Collection in Space Science Reviews >>


New selected International Teams in Space and Earth Sciences 2021

Twenty-five International Teams have been selected by the ISSI Science Committee for implementation from the proposals received in response to the 2021 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 2022.

New International Teams 2021 >>

“Do We Know What the Sun is Made of? The Puzzle of the Solar Composition” with Sarbani Basu (Yale University, USA)

All stars, including the Sun, are predominantly made of hydrogen and helium. However, the very small amount of the remaining elements, or “metals” as they are called, has a profound effect on the structure and evolution of a star. This is mainly because metals impede radiation – the higher the metallicity, the more opaque stellar material is. Given the proximity and ease of observing the Sun, solar metallicity is used as the reference for the metallicity of other stars. However, for the last two decades, there is no consensus about what the solar metallicity is.

In the early 2000s, the solar composition was revised downwards by about 30%, a result of analyses using 3D atmospheric models and non-LTE effects. While under most circumstances, this result would have been hailed as a triumph of using the best physics to analyze spectra, it created a problem – solar models constructed with the older metallicities matched the structure of the Sun, the models with the newer metallicities are extremely discrepant.

The structure of the Sun can be determined in a model-independent manner by analyzing the frequencies with which the Sun oscillates. All such analyses have shown that the lower abundances produce models that are discrepant. Although the metallicity estimates have increased since the original low ones, they are not high enough to result in solar models whose structures agree with that of the Sun.

In this talk, the speaker will review the problem and discuss how helioseismology, i.e., the study of solar oscillations, can be used to examine solar metallicity. She will also discuss attempts that have been made to change, as well as test, inputs to solar models in an attempt to reconcile the low abundances with helioseismic results. The speaker will end by discussing how solar neutrinos might provide the answer to the question of what the Sun is made of.

Sarbani Basu is the William K. Lanman Jr. Professor and Chair of the Department of Astronomy, Yale University. She is a global authority in helioseismology, or the study of the structure and dynamics of the sun using solar oscillations, her research also uses the sun as a laboratory to study the physics within it. She received the George Ellery Hale Prize of the American Astronomical Society in 2018 for her contributions to the understanding of the internal structure and dynamics of the Sun and stars. She served as the Chair for the panel on Starts, the Sun and Stellar Populations for the Astro2020 decadal survey; the survey will determine the direction of US astrophysics for the next decade.

This webinar was recorded on June 17, 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,


“The Merger History of the Milky Way – What Gaia Revealed” with Eva Grebel (ZAH, Heidelberg University, Germany)

The ESA Cornerstone Mission Gaia is revolutionizing our understanding of the assembly history of the Milky Way. Cosmological models suggest that Milky Way-like galaxies are made up in part of stars that formed in situ and in part of stars that formed in other, smaller galaxies and that were subsequently accreted. Most of the more massive merger events should have occurred more than nine or ten billion years ago. Such galactic cannibalism is believed to be the typical mode of growth of more massive galaxies. The surviving satellite galaxies, which all host ancient stellar populations, hold important clues to the properties of those early building blocks.

In combination with massive ground-based photometric and spectroscopic surveys, Gaia is confirming and refining the cosmological picture. Gaia data have uncovered numerous stellar tidal streams in our Galaxy, not all of which have known progenitors. Many come from disrupted dwarf galaxies, others from dissolving globular clusters – Gaia permits us to trace the detailed assembly history of our Galaxy, revealing the type of objects, their numbers, their properties, and the time of accretion. The most spectacular discovery is arguably that of the fairly massive dwarf galaxy Gaia-Enceladus or Gaia Sausage, which merged with the Milky Way about 10 Gyr ago. This event contributed many globular clusters and likely triggered the formation of the thick disk. In fact, Gaia data suggest that possibly half of our globular clusters come from merger events. Also, Gaia reveals the orbits of the surviving satellites, providing clues to their origins and future merger history.

Eva Grebel is professor of astronomy at Heidelberg University and director of the “Astronomisches Rechen-Institut”. She leads a collaborative research center (Sonderforschungsbereich) on the Milky Way system funded by the German Research Foundation (DFG).  She is a member of the Heidelberg Academy of Sciences, of the Hector Fellow Academy, and of the German National Academy of Sciences (Leopoldina).  For her work on galactic archaeology, she has received a number of scientific awards. A more detailed CV of Prof. Grebel can be found at

This webinar was recorded on June 10, 2021