## Session 8 - Radiation Environments

Rami Vainio (Univ. of Turku); Yuri Shprits (GFZ/UCLA)
Tuesday 19/11, 11:15-12:30 & 17:15-18:30
Thursday 21/11, 11:15-12:30
Mozane 789

Particles trapped in the radiation belts and carrying the ring current are hazardous to satellite electronics and can produce surface charging and deep dielectric charging.

Traditional models of the Earth’s radiation belts are divided into physics-based models describing the transport, acceleration, and loss processes and specification models based on a variety of historical data sets. New efforts are looking to merge these approaches to arrive at a more complete description of the radiation belt environment for the purposes of specification, now-casting, and forecasting.

Solar Energetic Particle (SEP) events are sporadic outbursts of particle radiation from the Sun and constitute the most prominent source of energetic proton radiation at MeV to GeV energies in interplanetary and near-Earth space, outside the Earth's radiation belts.

The standard SEP environment models are empirical probabilistic models that give the cumulative or worst-case fluence or proton peak flux for a mission with specified length at a user-specified confidence level.

Recent developments in SEP environment modelling have extended the energy range of protons beyond 300 MeV to the GeV and for ion species to cover from hydrogen to heavy ions. SEP modelling efforts have also extended from probabilistic modelling from over the duration of the mission towards short-term predictions of the occurrence and fluxes of SEP events, based on various solar observations. The third component of the radiation environment is Galactic Cosmic Rays (GCRs), which varies slowly with the solar cycle and more rapidly as a result of Forbush Decreases resulting from Interplanetary Coronal Mass Ejections (ICMEs).

The session will cover a broad range of topics related to the near-Earth and interplanetary radiation environment, and its effects on satellites.
We invite contributions related especially to:
• Specification of the radiation belt, ring current, GCR and SEP environments;
• Describing the new data sources or tools for data processing and analysis;
• Predictive models that can be adopted for operations;
• Data assimilation models allowing to reconstruct the evolution in the past.

Talks
Tuesday November 19, 11:15 - 12:30, Mosane 789
Tuesday November 19, 17:15 - 18:30, Mosane 789
Thursday November 21, 11:15 - 12:30, Mosane 789

### Talks : Time schedule

Tuesday November 19, 11:15 - 12:30, Mosane 789
 11:15 Spectra and angular distribution of relativistic SEP particles derived using neutron monitor data Mishev, A et al. Oral [1,2] A. Mishev, [1,2] I. Usoskin [1] Space Climate Research Unit, University of Oulu, Finland, [2] Sodankylä Geophysical Observatory, University of Oulu, Finland Systematic study solar energetic particles (SEPs) provides basis for understanding their acceleration and propagation in the interplanetary space, as well as allow one to assess their space weather impact, specifically exposure to radiation at flight altitudes in the polar region. Usually the maximum energy of SEPs is several MeV/nucleon, but in some cases it is exceeding 100MeV/nucleon or even reaches several GeV/nucleon, which is high enough to generate an extensive air shower, whose secondary particles reach the ground and thus are registered by ground based detectors, specifically neutron monitors (NMs). This particular class of events is known as ground level enhancements (GLEs). Historically the strongest GLE under number five was registered on 23 February 1956, with increase of the count rate of NMs above 5000 %. In solar cycle 23, among the several strong GLEs, was produced the second largest event in the observational history - on 20 January 2005. In this study, we derived the spectral and angular features of the two strongest GLEs using NM records. We model the global NM network response including the particle propagation in the Earth’s magnetosphere and atmosphere and on the basis of optimization between modelled and measured NM responses we derived the SEP spectra and pitch angle distributions in their dynamical development throughout the events. 11:30 Propagation of relativistic protons from solar eruptive events Dalla, S et al. Oral S. Dalla[1], G. de Nolfo[2], J. Giacalone[3], A. Bruno[2], M. Battarbee[4], T. Laitinen[1] and S. Thomas[5] [1]University of Central Lancashire, UK, [2]NASA Goddard Space Flight Center, USA, [3]University of Arizona, USA, [4]University of Helsinki, Finland, [5] University of Reading, UK Flare/Coronal Mass Ejection (CME) events originating in solar Active Regions can accelerate protons to relativistic energies. These particles may travel to Earth and be detected by neutron monitors as Ground Level Enhancements (GLEs), associated with significant radiation increases. Interest in the acceleration and propagation through interplanetary space of protons in the ~100 MeV - ~1 GeV energy range has been renewed by recent observations by PAMELA and FERMI LAT. While traditional proton propagation models used to interpret GLE events solve a spatially 1D focused transport equation, there is evidence that at relativistic energies 3D effects are important. By means of a 3D test particle model we integrate trajectories of relativistic solar protons through interplanetary space and demonstrate that drift and heliospheric current sheet effects strongly influence the spatial extent of the event at 1 AU. We discuss the dependence of the number of times particles cross 1 AU on the polarity of the interplanetary magnetic field, for a given mean free path. We also simulate particle spectra and intensity profiles and compare them with PAMELA observations for the GLE event of 2012 May 17. 11:45 What will the intensity-time profiles of SEP events look like? An answer from the ESA’s SAWS-ASPECS project Aran, A et al. Oral Angels Aran[1], Rami Vainio [2], Miikka Paassilta[2], Osku Raukunen [2], Athanasios Papaioannou[3], Anastasios Anastasiadis[3], Sigiava Aminalragia-Giamini[4], Piers Jiggens[5] [1]Dept. of Quantum Physics and Astrophysics, Institute of Cosmos Sciences (ICCUB), Universitat de Barcelona, [2]Department of Physics and Astronomy, University of Turku, [3]Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing (IAASARS), National Observatory of Athens, [4]Space Applications & Research Consultancy (SPARC), [5] European Space Research and Technology Centre (ESTEC), European Space Agency Once the occurrence of a solar energetic particle (SEP) event has been forecasted, what will its intensity-time profiles look like for different energy thresholds? When will the peak intensities be reached? These are the most challenging questions we can pose for any SEP event forecasting system. In the framework of the ESA’s Advanced Solar Particle Event Casting System (ASPECS), we provide an answer to these questions. We have developed a tool that provides the community with different time-window forecasts of integral intensity-time profiles for > 10 MeV, > 30 MeV, > 100 MeV, and > 300 MeV solar energetic protons. The forecasted SEP events are modelled by using two different approaches. On the one hand, based on previous work by Kahler and Ling (2017) modified Weibull functions are used to predict the SEP intensity-time profiles. On the other hand, the predicted profiles are obtained by using the Solar Particle Engineering Code-2 (SOLPENCO2) tool, based on the results of a physics-based model to describe the SEP particle transport and interplanetary shock propagation (Crosby et al. 2015; Pomoell et al. 2015). In this presentation we will briefly introduce the different methods used to forecast the SEP event profiles and we will show how this tool works. Acknowledgement. The ASPECS tool was developed, receiving funding through the ESA activity “Solar Energetic Particle (SEP) Advanced Warning System (SAWS)”. ESA Contract No. 4000120480/NL/LF/hh. 12:00 SEP Scoreboard Mays, L et al. Oral M. Leila Mays[1], Masha Kuznetsova[1], Joycelyn Jones[1], Eddie Semones[2], Kerry Lee[2], Janet Barzilla [3,2], Steve Johnson[2], Kathryn Whitman[4,2], Phillip Quinn [3,2], Christopher Mertens [2], Ian Richardson [6,1], Mark Dierckxsens [7], Mike Marsh [8] [1] NASA Goddard Space Flight Center, [2] NASA Johnson Space Center [3] Leidos Exploration and Mission Support [4] University of Houston [5] NASA Langley Research Center [6] University of Maryland, [7] BIRA-IASB, [8] UK Met Office The CCMC has been facilitating real-time forecast verification projects led by the international space weather community to test predictive capabilities before event onset. These "Scoreboards" allow a consistent real-time comparison of various operational and research forecasts. The scoreboards also enable world-wide community involvement in real-time predictions, foster community validation projects, and ultimately help researchers improve their forecasts. The SEP scoreboard captures SEP onset, duration, peak flux, probability, all-clear, and overall profile. Recently in 2018, Johnson Space Center's Space Radiation Analysis Group has become involved in the SEP scoreboard as part of a 3-year project called ISEP. As part of this project variety of SEP models will become available on the SEP Scoreboard display in real-time in support of upcoming human exploration missions. Here we present the goals of the scoreboard and demo the first prototype of the interactive SEP scoreboard display. All SEP forecast modelers and experts worldwide are invited to advise or participate in this community-wide effort. (https://ccmc.gsfc.nasa.gov/challenges/sep.php) 12:15 ISEP: A Joint SRAG/CCMC Collaboration to Improve Space Weather Prediction for Crew Protection during Near-Term Lunar Surface and Cis-Lunar Missions Lee, K et al. Oral Janet Barzilla[1,2], Kerry Lee[1], Eddie Semones[1], Steve Johnson[1,2], Katie Whitman[1,3], Phillip Quinn[1,2], M. Leila Mays[4], Masha Kuznetsova[4], Joycelyn Jones[4], Christopher Mertens[5] [1]NASA Johnson Space Center, Houston, Texas, USA, [2]Leidos Innovations Corporation, Houston, Texas, USA, [3]University of Houston, Houston, Texas, USA, [4]NASA Goddard Space Flight Center, Greenbelt, MD, [5]NASA Langley Research Center, Hampton, Virginia, USA As human spaceflight goals extend from Low-Earth Orbit (LEO) missions like the International Space Station to the moon, Mars and beyond, the Space Radiation Analysis Group (SRAG) at Johnson Space Center needs to update their approach for mitigation of crew radiation exposure due to large Solar Particle Events (SPEs). Some concerns for exo-LEO missions include the lack of protection offered by the Earth’s geomagnetic field as well as limited communication capability between the crew and the ground. Although vehicle shielding is an important aspect of radiation exposure protection, NASA requires monitoring and prediction of the space weather environment in case of a need for the crew to take corrective action (i.e., seek shelter); to this end, SRAG maintains a console position in Mission Control with 24/7 mission support capability. SRAG’s concept of operations for exo-LEO missions will transition from nowcasting to an emphasis on improving forecasting capabilities which will provide the Flight Control Team with more information when responding to a space weather event. The Integrated Solar Energetic Proton Event Alert/Warning System (ISEP) represents a collaboration between SRAG and the Community Coordinated Modeling Center (CCMC) at Goddard Space Flight Center to bring state-of-the-art space weather models from research and development at universities and small businesses to operational use at NASA (R2O). These models will have a user interface in the form of a model Scoreboard that will allow the SRAG console operator to view and compare the results from several different models simultaneously; this approach also encourages the console operator to understand the background and associated caveats of each model in order to formulate the best crew response to changes in the space weather environment. The ISEP team is incorporating an R2O approach to improve space radiation exposure mitigation capabilities in the exo-LEO mission era. Here we present the various tools that the ISEP project is focused on for the improvement of space weather forecasting for the near term human exploration missions.

Tuesday November 19, 17:15 - 18:30, Mosane 789
 1 To soft gamma-rays variations in the atmosphere during precipitations Balabin, Y et al. p-Poster Yury Balabin[1], Aleksey Germanenko[1], Igor Yankovsky[2] [1] Polar Geophysical Institute, Apatity, Russia, [2] Kabardin-Balkarian State University, Nalchik, Russia Some years ago increase events of electromagnetic radiation are revealed in Polar Geophysical Institute. These events have amplitude up to 100 % above background, are connected with precipitations and absent in charged particle part. Later measurement of differential spectrum of soft gamma-rays (0.2-8 MeV) was added to monitoring case. Crystal NaI(Tl) 150x100 mm is used, accumulation time of one spectrum is 30 minute. Differential measurement of gamma-ray spectrum is important hitch to studying this phenomenon. Some features of soft gamma-ray spectra are found under a detailed analysis. Firstly, there are absent any radionuclide lines. All increase events have an upper limit of energy about 3 MeV. Secondly during increase events a total spectrum consists of power law and exponential forms. The power law one is the same of fine weather, the exponential form corresponds to increasing during precipitations. Some other features of differential spectra of increase events were carried out too. 2 Mobile complex for registration of some components of SCR Balabin, Y et al. p-Poster Balabin Yury, Mikhalko Evgenia, Gvozdevsky Boris, Maurchev Evgeny, Germanenko Aleksey Polar Geophysical Institute A mobile unit for cosmic ray monitoring was designed in the Cosmic Ray laboratory of Polar Geophysical Institute. The unit includes three detectors: a charged particle detector (set of Geiger-Muller counters), gamma ray detector with NaI scintillation crystal and a detector of moderate energy neutrons (set of helium SNM-18 counters). Hence the unit is able to detect main components of secondary cosmic rays. The gamma ray detector has 20-400 keV energy range, moderate neutron one is sensitive to neutrons with energies E < 1 MeV. Also the unit has auxiliary equipment including temperature and pressure sensor, GPS-receiver to correct time, fix geographic coordinates and altitude. All data have 1 minute time resolution. The small size, low power consumption and the ability to write data on flash drives are suitable to use the unit isolated places or in expedition; can be used in aircraft. Now the unit set near the conventional neutron monitor, fix the same variation of cosmic rays and reveals stabile working. Special feature of the unit is high precision of time appearance of neutron detector pulses. It is 2 microseconds. Due to it one can find that the neutron detector count rate consists of two part. The first part (about 90 %) is single pulses, interval between them corresponds to the average count rate. Pulses of the second part are in small (double, triple and so on) groups with intervals less than 100 mcs. We consider it to be traces of atmospheric showers of particles. 3 On the cause of relativistic electron acceleration in the outer Van Allen belt Katsavrias, C et al. p-Poster Christos Katsavrias[1,2], Ioannis A. Daglis[1,2,3], Wen Li[4], Ingmar Sandberg[5], Elena Podladchikova[6], Constantinos Papadimitriou[5] and Sigiava Aminalragia-Giamini[5] [1]Department of Physics, National and Kapodistrian University of Athens, Athens, Greece, [2]Institute of Accelerating Systems and Applications, National and Kapodistrian University of Athens, Athens, Greece, [3]Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens, Penteli, Greece, [4]Center for Space Physics, Boston University, Boston, USA, [5]Space Applications and Research Consultancy (SPARC), Athens, Greece, [6]Solar–Terrestrial Centre of Excellence, Royal Observatory of Belgium, Brussels, Belgium We investigate the response of the outer Van Allen belt electrons to various types of solar wind and internal magnetospheric forcing, to geospace magnetic storms of different intensities and to intense magnetospheric substorms. We use electron phase space density (PSD) calculations as well as concurrent Pc5 and chorus wave activity observations in the outer belt during the Van Allen Probes era to compare 20 electron enhancement and 8 depletion events. Our results indicate that the combined effect of magnetopause shadowing and outward diffusion driven by Pc5 waves is present in both groups of events. Furthermore, in the case of enhancement events, the synergy of enhanced seed population levels and chorus activity – due to the enhanced substorm activity – can effectively replenish the losses of relativistic electrons while, inward diffusion can further accelerate them. 4 The UTU-SEP Products in ESA's Space Radiation Expert Service Centre Vainio, R et al. p-Poster Osku Raukunen[1], Miikka Paassilta[1], Timo Eronen[1], Esa Riihonen[1], Rami Vainio[1], Mark Dierckxsens[2], Norma Crosby[2] [1]University of Turku, Finland, [2]BIRA-IASB, Brussels, Belgium UTU-SEP is a collection of tools available through ESA's Space Radiation Expert Service Centre (R-ESC), which are aimed at assessment of the high-energy proton radiation environment over long time periods, like space missions. It consists of four products, which will be presented in this poster. The UTU-SEP product R.128 is a tool for modeling solar energetic proton fluence at high energies. It provides the fluence-probability curve for a selected energy channel, and the fluence energy spectrum at a selected confidence level, both for a user-specified mission duration. The updated version of the model is based on a revised event list including GLEs and sub-GLEs observed between 1973 and 2017, and an improved modeling, which treats the parameters of the fluence spectra as model variables. The product R.129 models the solar energetic proton peak flux at very high energies. It provides the peak flux-probability curve for a selected energy channel, and the peak flux spectrum at a selected confidence level, both for a user-specified mission duration. The updated version of the model is based on a revised event list and reprocessed GOES/HEPAD fluxes observed between 1986 and 2017. The event occurrence is modeled as a Poisson process, and the peak fluxes are modeled with cut-off power law functions. The product R.130 is a SEP event catalogue of high energy proton events during 1997 to 2017. It provides detailed information of solar energetic ions and electrons and related solar phenomena. In the updated product the peak intensities and fluences were recalculated using SOHO/ERNE data which was cross-calibrated using COSTEP/EPHIN to get new sampling bias and dead time corrections. The peak intensities were derived using 15-minute sliding averages for each event. The product R.138 is a new tool for estimation of high-energy solar heavy ion fluences. It is based on ERNE observations between 1997 and 2015. The tool provides fluence-probability estimates for a user-selected ion species, energy channel, and mission duration. The energy range and number of channels depend on the ion species. The event occurrence is modeled as a Poisson process and the event fluences with cut-off power law functions. 5 Experimentally obtained time-intensity profiles of high energy protons in solar energetic particle events Vainio, R et al. p-Poster Miikka Paassilta[1], Rami Vainio[1], Osku Raukunen[1], Athanasios Papaioannou[2], Anastasios Anastasiadis[2], Angels Aran[3], Ingmar Sandberg[4] [1]University of Turku, Finland, [2]National Observatory of Athens, Greece, [3]University of Barcelona, Spain, [4]SPARC, Athens, Greece A study of solar particle events by Kahler & Ling (2017: Solar Physics 292:59) suggested that a simple Weibull function is a good model for the time–intensity profile of proton events. Thus, we investigated the events included in the NOAA-maintained listing (Solar Proton Events Affecting the Earth Environment; ftp://ftp.swpc.noaa.gov/pub/indices/SPE.txt) by fitting a modified Weibull function to proton intensity data recorded during the events (spanning the years 1976--2015; 218 cases in total). In an extension of earlier work (ESWW15: Poster 5, Session 6), we studied four integral energy channels, $>$10 MeV, $>$30 MeV, $>$100 MeV, and $>$300 MeV. The data were derived directly from the SEPEM reference dataset, with the exception of the $>$300 MeV channel, which was constructed using GOES/HEPAD observations and the SEPEM dataset. HEPAD channels P8, P9, and P10 were processed using a “bow-tie” method, which finds optimal values for effective channel energy and geometric factor based on calibrated response functions. Using the bow-tie method, the differential channels can be analysed as integral channels. The >300 MeV fluxes were calculated by assuming a power-law spectrum between the 166 MeV SEPEM channel and the differential bow-tie P9 channel (473 MeV before 1995, 457 MeV after), integrating this from 300 MeV to the integral bow-tie P10 energy ($>$486 MeV / $>$462 MeV) and adding the integral bow-tie P10 flux. We find that a statistically meaningful fit can be obtained for between some 63\% ($>$10 MeV) and 20\% ($>$300 MeV) of the studied cases, with this proportion diminishing as the channel energy increases. This can be attributed to shorter event durations and smaller intensity enhancements in high particle energies than in the lower energies, leading to poorer statistics. The function as applied by us has two free parameters, $\alpha$ and $\beta$; $\alpha$ controls the shape of the initial flux increase and $\beta$ the duration of the event. The results suggest at least a weak dependence between the parameters and the solar longitude of the event-related flare. This tends to be roughly quadratic, but a considerable amount of scatter is present. The events with an eastern solar origin show a great deal of variety in the shapes of their time profiles. This work was supported by the European Space Agency (ESA) contract No. 4000120480/17/NL/LF/hh, "Solar Energetic Particle (SEP) Advanced Warning System (SAWS)". 6 Energetic electrons in Van Allen radiation belts: Linking with geospheric conditions Niemelä celeda, A et al. p-Poster A. Niemelä-Celeda[1], V. Lanabere[1], S. Dasso[1,2,3], M. Colazo[4] [1]Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Departamento de Ciencias de la Atmósfera y los Océanos. Buenos Aires, [2]CONICET - Universidad de Buenos Aires, Instituto de Astronomía y Física del Espacio (IAFE), Buenos Aires, Argentina, [3]Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Departamento de Física. Buenos Aires, Argentina, [4] Comision Nacional de Actividades Espaciales, Argentina. The Van Allen radiation belts contain ultrarelativistic electrons, with energies that range from several keVs to several MeVs. Since their discovery in the early stages of the space era they have been object of a large number of studies, and many of the processes that occur are not fully understood. Due to their high time-space variability, specially during magnetospheric storms, it is crucial to understand these physical processes and its effects on technologies and human activities in space. With this information the satellite design teams are able to develop systems in order to counteract these adverse effects and preserve the integrity of such valuable devices. The purpose of this work is to characterize the electron population in the outer radiation belt, for energies ranging from 1.6 to 18.9 MeV, by analysing the measurements made by the Relativistic Electron-Proton Telescope (REPT) instrument on-board the Van Allen probes, launched into orbit in August 2012. These probes have a highly elliptical orbit, with a perigee of 618~km and an apogee of 30414~km, making them cover a wide range of distances in the Earth environment, from the inner to the outer radiation belt. In particular, we study the behaviour of the electron flux in the outer radiation belt during several strong geomagnetic storms and compare them with several geospheric properties (e.g., interplanetary, geomagnetic and ionospheric conditions). Also we make a comparison with the AE-8 radiation belt model, that is the most widely used model by the international community. 7 Radiation Environment and risks during human exploration of habitable sites on Mars during Solar maximum, minimum and under the October 2003 events Da pieve, F et al. p-Poster F. Da Pieve[1], G. Gronoff[2], E. Botek[1], V. Pierrard[1], J. Kohanoff[3], F. Cleri[4], B. Gu[3] and A.C. Vandaele[1] [1]Royal Belgian Institute for Space Aeronomy, BIRA-IASB, Brussels, Belgium; [2]NASA Langley Research Centre, Hampton, USA; [3]Queen's University of Belfast, Belfast, UK ;[4]IEMN and University of Lille, Lille, France Human exploration of Mars is a primary long-term objective in the international road map for Space exploration. Mars offers in situ resources in the form of ices, hydrated minerals, and CO2, allowing for a relatively sustainable human presence that would not depend heavily on frequent deliveries from Earth [1]. The first explorers will likely be involved in contextual surveys, search of candidate samples for astrobiology driven analysis, and supervised drilling operations at sites with high biosignature preservation potential, acting with the support of an Earth-based multidisciplinary science team [2]. Mars subsurface exploration lays the foundation for the search of extant/extinct life, but also for self-sufficient human settlements, providing an emerging potential for synergistic collaborations between the astrobiology and the human exploration planning community. Here we present an estimation of the radiation environment at Oxia Planum, Mawrth Vallis and Arcadia Planitia on Mars, via the Geant4-based code dMEREM [3]. The risks for human exploration (both tissue/deterministic effects and stochastic effects) are evaluated for different durations on the stay on Mars, as induced by Galactic Cosmic Rays during Solar maximum (January 2014), minimum (January 2009), and as induced by Solar Energetic Particles accelerated by the Halloween event of the 28th of October 2003. The results are compared to recent Geant4-modelling studies for Gale Crater [4] (using different physics lists in Geant4, such as the Bertini Cascade, the Liège Intra-nuclear Cascade, or using deterministic approaches to particle transport [5], to measurements from the radiation detector RAD on the Curiosity rover [6,7] and measurements during the cruise [7], and measurements at both high elliptical and lower capture orbit by the Liulin-MO onboard ExoMars Trace Gas Orbiter [8]. [1] V. Stamenković, L. W. Beegle, K. Zacny et al., Nature Astronomy 3, 116 (2019) [2] Brady, A.L., Kobs N., S.E., Hughes, S.S.,et al., Astrobiology 19, 347 (2019) [3] McKenna-Lawlor S, Gonalves P, Keating A, et al., Icarus, 218, 723 (2012) [4] D. Matthiae, D. M. Hassler, W. de Wet, et al, Life Sciences in Space Research 14, 18 (2017) [5] G. Gronoff, R. B.Norman, C.J.Mertens, Advances in Space Research 55, 1799 (2015) [6] D.M. Hassler, C. Zeitlin, B. Ehresmann, et al., Space Weather 16, 2018W001959 [7] C. Zeitlin, D.M. Hassler, J. Guo, et al. Geophysical Research Letters, 45, 5845 (2018) [8] J. Guo, C. Zeitlin, R. F. Wimmer-Schweingruber, et al., A&A 577, A58 (2015) [9] J.Semkova, R. Koleva,V. Benghinc et al, Icarus 303, 53 (2018) 8 Chorus wave interactions with ultra-relativistic electrons Allison, H et al. p-Poster Hayley J. Allison [1], Yuri Y. Shprits [1,2,3], Irina S. Zhelavskaya [1,2], and Dedong Wang [1] [1] GFZ German Centre for Geosciences, Potsdam Germany, [2] Institute of Physics and Astronomy, University of Potsdam, Germany, [3] Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA, USA The energy range of electron radiation belt flux enhancements, during active periods, is highly variable and can extend to ~7.7 MeV. The energisation mechanism by which electrons reach these ultra-relativistic energies is still unknown, but a potential candidate is local acceleration by whistler mode chorus waves. However, under usual circumstances, chorus wave acceleration reduces for multi-MeV populations. Here, electron plasma density data throughout 2015 is analysed to explore how density evolves during 7.7 MeV flux enhancement events. Densities lower than empirical models are observed during these periods, and the reduced density results in increased energy diffusion for >1 MeV electrons. Density reductions impact the energy diffusion coefficients more readily than the pitch angle diffusion coefficients and VERB 2-D model runs demonstrate that 7 MeV enhancements can be formed via chorus acceleration within observed timescales.