Solar atmospheric abundances in space and time

I will attempt to review and reflect on developments in the ponderomotive force model of the First Ionization Potential (FIP) effect, since its first presentation as a poster at the 2004 AAS meeting in Denver CO (May 30th – June 3rd), and as a talk at COSPAR in Paris later that summer (18th – 25th July). I will pay particular attention to how my understanding of the relevant wave physics has improved in that time. The ponderomotive force appears to have been a completely new means of ion-neutral separation in solar physics or astrophysics, though it was not new to science. Had I had my wits about me in 2004, I would have realised that this mechanism represents an analog in magnetohydrodynamics of the optical part of a magneto-optical trap. As it was, it wasn't until I read about Arthur Ashkin's Nobel Prize in 2018 in Physics Today that the penny dropped about the exact physics involved. The basic model, that the coronal magnetic field geometry dictates the properties of the Alfvén wave field through resonances and interference, and that these different wave properties give rise to the different FIP effects seen in different coronal structure still holds, though ideas about the wave origins have evolved. I will illustrate the varieties of FIP fractionation that can occur, how they relate to the underlying solar geometry, and how such phenomena may be used to investigate other aspects of solar physics.

Dr Martin Laming

Naval Research Laboratory, USA

Dr Martin Laming

Naval Research Laboratory, USA

Martin Laming has worked in the Space Science Division of the Naval Research Laboratory in Washington DC since 1990, becoming a federal employee in 1999, following a DPhil in atomic physics at Oxford University and a post-doctoral fellowship held at the Smithsonian Astrophysical Observatory in Cambridge MA. His research interests moved from highly charged ions in the laboratory to highly charged ions in space, notably the solar corona and wind, and in supernova remnants. He works on problems connected with elemental abundances and particle acceleration in both areas. He has been interested in wave-particle interactions throughout his career, beginning with his doctoral work measuring Lamb shifts to the current focus on MHD waves that accelerate the solar wind and cause FIP fractionation.

The enrichment of coronal loops and the slow solar wind with elements that have low First Ionisation Potential, known as the FIP effect, has often been interpreted as the tracer of a common origin. A current explanation for this FIP fractionation rests on the influence of ponderomotive forces and turbulent mixing acting at the top of the chromosome. The implied wave transport and turbulence mechanisms are also key to wave-driven coronal heating and solar wind acceleration models. In this talk, I will first show to use a shell turbulence model to assess the ponderomotive force in coronal loops and open field lines. With this model, we find that the turbulence power necessary to heat the corona and accelerate the solar wind is also able to generate the FIP effect. However, we do not find significant differences in the fractionation due to the geometry of the large scale magnetic field. Consequently, the second part of the talk will be focused on dynamic reconnection events in smaller scale loops, typical of coronal bright points. Using full 2.5D resistive MHD simulations, we shall investigate the ponderomotive force due to the reconnection induced perturbations in the coronal loops and the resulting FUP fractionation.

Dr Victor Reville

French National Centre for Scientific Research, France

Dr Victor Reville

French National Centre for Scientific Research, France

Victor Réville is an astrophysicist working at Institute de Recherche en Astrophysique et Planétologie (IRAP) in Toulouse, France. He is interested in solar and stellar winds, space plasmas, MHD turbulence and reconnection. He uses numerical simulations and theory to explain the in situ and remote sensing observations of space observatories like Parker Solar Probe and Solar Orbiter.

Departures from the single-fluid approximation can be expected in some regions of the solar atmosphere, eg the chromosphere is partially ionized and goes from a collisional to a weakly collisional regime. Similarly, some species behave as a magnetized fluid while others are unmagnetized in the lower chromosphere. In addition, species with different ionization potential populations vary depending on the regions in the corona, providing unique constraints for the drivers of coronal heating. Finally, but not least, there are observational indications of multi-fluid effects in the chromosphere, transition region, and high energetic events in the corona. We have been developing a new multi-fluid and multi-species numerical code: Ebysus. The code treats each excited/ionized level for each desired species as a separate fluid self-consistently, including physical processes such as thermal conclusion, ion-coupling, non-equilibrium ionization/recombination, collisions, and radiation. In this work, we briefly describe the capabilities of Ebysus and the first results of the FIP effect and chemical fractionation.

Dr Juan Martinez-Sykora

SETI Institute, USA

Dr Juan Martinez-Sykora

SETI Institute, USA

Juan Martinez-Sykora obtained his BSc, followed by his master's at the University of La Laguna (Spain) and continued his PhD in Solar physics at the University of Oslo. After that, he moved to Lockheed Martin Solar and Astrophysics Lab and Stanford University as a postdoc. Very soon he became a researcher at BAERI and LMSAL and adjunct associated professor at the University of Oslo. Nowadays, he is a senior researcher at SETI and LMSAL. Juan Martinez-Sykora's research focus on developing model for the solar atmosphere and comparing them to observations. In particular, he is interested in the effects of neutrals on chromospheric physics. Consequently, he is part of the Bifrost development team and the lead of the multi-fluid multi-species numerical code Ebysus. In addition, he has extensive experience performing IRIS planning, analysing observations, and comparing with numerical codes, including observatories such as SDO, Hinode, IRIS, ALMA, and ground-based observatories such as SST. He is highly involved in the development of MUSE (NASA MIDEX program) as science deputy lead. He has published more than 70 scientific papers in peer-reviewed journals.

It has been well-known for many years that elemental abundances vary across the solar atmosphere, both in space and time, in quiescent, active, and flaring loops as well as in the solar wind. To date, however, models of the solar atmosphere have assumed that the abundances are fixed in space and time, in direct contradiction with those observations. Typically, non-flaring loop and solar wind models have assumed coronal abundances, while flare models have assumed photospheric abundances. As a first test case, time-variable elemental abundances were introduced to the 0D ebtel++ code, modifying the total radiative losses, to show that the cooling and draining timescales of loops are strongly impacted by this assumption. This work has been expanded to the field-aligned HYDRAD code, solving a continuity equation for the abundance factor to track its spatiotemporal evolution. Coronal condensations (eg coronal rain) form as a natural consequence to this modification of radiation, even in impulsively heated loop simulations, whereas previous work has only found condensations in simulations with steady foot-point heating. This work is now being further extended by introducing a ponderomotive acceleration to study the rate at which loops fractionate, to better understand the time-scales of the FIP effect. This requires the introduction of a momentum equation for low FIP elements in addition to the continuity equation. The implications for radiation in the solar atmosphere, for interpretation of spectral lines, and for understanding the FIP effect will all be discussed.

Dr Jeffrey Reep

University of Hawai'i at Mānoa, USA

Dr Jeffrey Reep

University of Hawai'i at Mānoa, USA

Dr Reep graduated from Cornell University and received his PhD from Rice University, focusing on the modelling of solar coronal loops and solar flares. He has worked extensively on understanding the transport of energy between the chromospheric-coronal interface in the solar atmosphere. He completed postdoctoral positions at Cambridge University and the US Naval Research Laboratory (NRL), before being hired as an astrophysicist at NRL. He has recently joined the faculty at the University of Hawaii’s Institute for Astronomy, located on Maui, the site of the Daniel K Inouye Solar Telescope. 

Magnetic waves associated with MHD waves have been found to be linked to FIP bias regions in the corona, consistent with models based on the ponderomotive force. 

In this contribution, Dr Stangalini will show how spectropolarimetric measurements in the low chromosphere can be exploited to reveal crucial information for identifying wave modes potentially linked to FIP bias, and discuss the associated challenges. Finally, he will explore potential future steps in this field enabled by forthcoming high-resolution instrumentation.

Dr Marco Stangalini

Italian Space Agency, Italy

Dr Marco Stangalini

Italian Space Agency, Italy

Marco Stangalini is a researcher at the Italian Space Agency. His main scientific interest is the study of MHD waves in the lower atmosphere, based on high-resolution spectropolarimetry. He co-founded the international working group WaLSA (Waves in the Lower Solar Atmosphere). He is involved in the development of several upcoming solar space missions, including Solar-C and MUSE.

Elemental abundances in the solar atmosphere, typically measured from EUV and X-ray observations, are often different from the solar photospheric abundances. The first ionization potential (FIP) of the element appears to be play a significant role in the observed fractionation, which is thought to be linked to the processes responsible for heating the solar atmosphere. In the solar atmosphere, elements with low FIP often show a relative enrichment with respect to elements with high FIP (FIP effect). The extent of such FIP effect is observed to vary broadly across solar features, with some solar features (such as eg most flares) showing an inverse FIP effect. Despite the widespread presence of chemical fractionation, and its potential importance for understanding the processes governing the solar atmosphere, this phenomenon and its original are still poorly understood.

We used coordinated coronal spectral observations taken with Hinode/EIS, and chromospheric and transition region spectral observations taken with IRIS, to investigate the presence of a footprint of the chemical fractionation process in the lower atmosphere. Such a footprint for the FIP effect is expected, given that the lower atmosphere is where most elements get ionized. We study the spatial and temporal properties of coronal chemical composition and corresponding chromospheric properties, for a variety of solar features. We discuss intriguing correlations between coronal abundances and chromospheric properties, which can potentially constrain models of chemical fractionation.

Dr Paola Testa

Harvard Smithsonian Center for Astrophysics, USA

Dr Paola Testa

Harvard Smithsonian Center for Astrophysics, USA

Dr Paola Testa is an Astrophysicist at the Harvard-Smithsonian Center for Astrophysics (USA). She received her doctorate in Physics at the University of Palermo (Italy) on the topic of coronal activity in the Sun and other stars, and then held a post-doctoral appointment at the Kavli Institute for Astrophysics at the Massachusetts Institute of Technology. Her research focused on understanding the heating mechanisms and high-energy processes in the hot outer layers of the atmosphere of the Sun and other stars by combining observations with advanced numerical modelling. She is also involved in development, observations, and dissemination of data of space instrumentation: she is co-investigator of several solar missions, including the Hinode/X-ray telescope (XRT, the Interface Region Imaging Spectrograph (IRIS), and the Multi-Slit Solar Explorer (MUSE, NASA Heliophysics Medium-Class Explorer to be launched in 2027).

The elemental composition of the solar corona differs from the photosphere due to the FIP effect, where low first ionization potential (FIP) elements exhibit enhanced abundances. This fractionation is attributed to ponderomotive forces generated by magnetohydrodynamic (MHD) waves, particularly incompressible transverse waves. We investigate how chromospheric transverse MHD waves influence coronal abundance fractionation by analysing their spatial correlation and wave properties. Using Hα observations from the Fast Imaging Solar Spectrograph (FISS) and EUV spectra from the EUV Imaging Spectrometer (EIS)/Hinode, we detect transverse waves in the chromosphere and determine coronal abundances of Si X (low-FIP) and S X (high-FIP). Magnetic field extrapolations from SDO provide insight into the connectivity between the chromosphere and corona. We identify ~400 wave packets and characterise their properties, including period, velocity, amplitude, and propagation direction. These waves, predominantly incompressible, are concentrated near loop footpoints, particularly in sunspot penumbra and superpenumbral fibrils. Strong abundance fractionation is observed in regions with closed magnetic structures, indicating the key role of magnetic confinement. 43% of detected waves are low-frequency downward-propagating waves. Our findings support the hypothesis that ponderomotive forces from chromospheric transverse waves drive elemental fractionation, providing observational constraints on the FIP effect.

Dr Kyoung-Sun Lee

Seoul National University, Republic of Korea

Dr Kyoung-Sun Lee

Seoul National University, Republic of Korea

Kyoung-Sun Lee is a Research Associate Professor at Seoul National University, South Korea, conducting research in solar physics and space weather. She obtained her BSc, MSc, and PhD in Astronomy and Space Science from Kyung Hee University. She has held research positions at the Institute of Space and Astronautical Science (ISAS) at JAXA, the National Astronomical Observatory of Japan (NAOJ), and the University of Alabama in Huntsville. Her research focuses on solar eruptive phenomena such as flares, coronal mass ejections, and jets, as well as on the spectroscopic diagnostics of the solar atmosphere. She uses both space-based observatories, including Hinode, IRIS, SDO, and SOHO, and ground-based instruments such as the Fast Imaging Solar Spectrograph (FISS) on the Goode Solar Telescope. She has been actively involved in observational planning and data analysis for both Hinode/EIS and GST/FISS. Her work also incorporates deep learning techniques for the spectral inversion of chromospheric lines. Recently, she has been investigating the relationship between chromospheric wave dynamics and elemental abundances in the solar corona using multi-wavelength spectroscopic observations.

The plasma composition of the solar corona is characterised by a varying overabundance of elements with a low first ionization potential (FIP) compared to those with a high FIP – this is called the FIP effect. This variation is not observed in the solar photosphere, suggesting that the enhancement of low-FIP elements must take place in the chromosphere. Typically, the strongest enhancement is observed in active regions and is influenced by a series of factors which will be discussed in this talk. On large scales and long timescales (days to weeks), the FIP bias is correlated with the active region evolutionary stage and the magnetic flux density within the active region. On smaller sub active region scales and shorter timescales (hours), the FIP bias is influenced by magnetic flux emergence and the presence of wave activity with different properties.

Dr Teodora Mihăilescu

USRA & NASA Goddard Space Flight Center, USA

Dr Teodora Mihăilescu

USRA & NASA Goddard Space Flight Center, USA

Teodora "Teia" Mihailescu is a postdoctoral researcher at USRA based at the NASA Goddard Space Flight Center, where she is part of the Solar Orbiter SPICE team. She studied Physics BSc at Imperial College London and Space Science and Engineering MSc at University College London (UCL). She then carried out her PhD in Solar Physics at UCL’s Mullard Space Science Laboratory, which she completed in 2024. Her research focuses on the analysis of EUV spectroscopic observations (mainly from Hinode EIS and Solar Orbiter SPICE) to study variations in plasma composition and other plasma parameters in solar active regions and flares.