From the Teams

Report from the ISSI Team #535 “Unraveling Surges: a joint perspective from numerical models, observations, and machine learning” led by D. Nóbrega-Siverio

A numerical experiment – performed by Daniel Nóbrega Siverio and Fernando Moreno Insertis – has shown for the first time how one of the most abundant structures in the solar atmosphere, the Coronal Bright Points, can be formed, acquire energy, and be disrupted through the action of solar granulation.

When the Sun is observed from space with X-ray or extreme ultraviolet detectors, its atmosphere is seen to be full of roundish bright points with sizes similar to our planet Earth. These Coronal Bright Points (CBPs) are found to be consisted of sets of bright magnetic arcs that confine very hot plasma and emit enormous amounts of energy for hours and even days, typically disappearing after a series of eruptive phenomena.

So far, the existing CBP models have been very idealized, missing crucial aspects of the physics of the Sun such as the energization of the magnetic structures by means of the solar granules and the radiative transfer to explain the coldest solar atmospheric layers. In a paper recently published in the journal Astrophysical Journal Letters, Daniel Nóbrega Siverio and Fernando Moreno Insertis have studied the CBPs using a state-of-the-art numerical code, the Bifrost code, that allows these astrophysicists to model the Sun with the necessary realism to include convective and radiative processes that fundamentally influence the heating of the solar atmosphere.

The two researchers demonstrate for the first time that the action of solar granulation on a magnetic structure of the type expected in many CBPs gives rise to hot and bright arcs, thus being able to explain different features observed with solar space missions for decades. In addition, in the paper, the authors show how new magnetic flux brought from the solar interior to the surface by the granular convective motions even in small scales is enough to destabilize and disrupt the CBP topology, leading to cool ejections (surges) and hot collimated ejections (coronal jets) as frequently detected in observations at the end of the CBP lifetime. The article also includes predictions about the cold regions underneath a CBP and about the small-scale structure that have not yet been approached from an observational point of view. These predictions will require very high-resolution data, such as those from the Swedish 1-m Solar Telescope (SST, in La Palma) and those from the recent Solar Orbiter space mission, in order to be confirmed.

The numerical experiment of this work has required thousands of hours of calculation in two of the most advanced supercomputing facilities in the world: Betzy (in Norway) and MareNostrum (in Spain). The work is also included within the project: “Unraveling Surges: a joint perspective from numerical models, observations, and machine learning”, this ISSI Team is devoted to analyze the cool chromospheric ejections commonly associated with eruptive phenomena in the Sun.

Reference

Nóbrega-Siverio, D. and Moreno-Insertis, F.: “A 2D Model for Coronal Bright Points: Association with Spicules, UV Bursts, Surges, and EUV Coronal Jets”, 2022, ApJL,
DOI: https://doi.org/10.3847/2041-8213/ac85b6 (Open Access)

Contact:
Daniel Nóbrega Siverio, Email:  

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 >> 

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.

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.”

Reference:

Mucciarelli, A., Massari, D., Minelli, A. et al. A relic from a past merger event in the Large Magellanic Cloud. Nat Astron (2021). https://doi.org/10.1038/s41550-021-01493-y

Preprint in ArXiv.org

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” >>

References

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.

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.

References

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). https://doi.org/10.1038/s41550-021-01370-8

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.

References

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). https://doi.org/10.1038/s41467-021-23740-6

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. https://doi.org/ 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, https://agupubs.onlinelibrary.wiley.com/doi/10.1029/JC082i018p02566

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: https://doi.org/10.3847/2041-8213/ab919e