Backgrounds, radiation damage, and spacecraft orbits Catherine E. Grant, Eric D. Miller, Mark W. Bautz (MIT Kavli Institute for Astrophysics and Space Research)

Summary SPace ENVironment Information System

• Charged particle background and radiation-induced damage • Supported by ESA, developed by consortium led by Royal Belgium Institute can limit scientific utility of space-based observatories • Web interface to library of space environment models • Commonly used for simulating particle background and radiation damage in • SPENVIS simulations of radiation environment are often used proposed missions to support choices about orbit selection • User inputs spacecraft orbit parameters or trajectory • We present general properties and a few simulation results for • SPENVIS outputs include: - Trapped proton and electron fluxes from the Earth’s radiation belts example orbits - Solar proton fluence and Galactic Cosmic Ray (GCR) fluxes, corrected for • We discuss limitations of SPENVIS simulations, particularly geomagnetic shielding - Radiation dose for simple shielding geometries Flux of trapped electrons (> 1 MeV) and protons (> 10 MeV) at 500 km altitude outside Earth’s trapped radiation belts using NASA AE-8 and AP-8 models. Images from SPENVIS manual. • www.spenvis.oma.be

Common types of orbits Particle radiation sources

• Low-Earth Orbit (LEO) • Trapped protons and electrons in the Earth’s radiation belts - P ~ 90 minutes, substantial Earth occultation - Primary particle source in LEO; minimized for low inclination - Mostly below Earth’s radiation belts - Higher energy particles in inner belts, lower further outward - Exposure to trapped radiation (South Atlantic Anomaly & - For HEO, only important during perigee passages; dominated by high latitude) depends on orbit inclination soft protons which can be easily stopped by shielding - Geomagnetic shielding of GCR and solar proton events also - High radiation levels during SAA (LEO) and perigee (HEO) depends on orbit inclination passages prevent science observations, lowering efficiency - Examples: NuSTAR (i ~ 6°), Swift (i ~ 20°), Suzaku (i ~ 31°), ISS-NICER (i ~ 52°) • Solar proton events: flares and coronal mass ejections SOHO (ESA & NASA) NASA/CXC/SAO - Direct impact shielded in LEO; can energize radiation belts • High-Earth Orbit (HEO) • - More frequent near solar maximum Earth-Sun L2 Solar min - Highly elliptical orbit, P ~ 2 - 3 days - No exposure to trapped radiation except - Large distribution in duration and particle spectrum - Transits outer radiation belt during perigee passages low-density magnetotail - Instruments shut down during worst storms - Exposure to trapped radiation in outer radiation belts - No geomagnetic shielding - Outside of perigee, environment should be similar to L2 - Examples: eROSITA, Athena • Galactic Cosmic Rays (GCR) or lunar resonance orbit - Heliosphere magnetic shielding, anti-correlated with solar cycle • Lunar resonance orbit - Low energy proton flares – observed but not well - In LEO, receive geomagnetic shielding; less effective at high - Long period ~ 7 - 14 days Solar max understood latitudes - Similar environment to L2 - Examples: XMM-Newton, Chandra, INTEGRAL - Primary source of quiescent instrument background in HEO - Example: TESS, Arcus

Radiation damage Backgrounds SPENVIS limitations

• Non-ionizing particle interactions create defects in the CCD • Instrument background in science data caused by particle-induced • Generally, reality is more complicated (temporally, spatially, etc.) than models • Increasing Charge Transfer Inefficiency (CTI) and dark current events indistinguishable from X-ray events • Trapped radiation models degrades performance • Background can be modeled or subtracted from data - Models are observation-based; less certain where data is sparse such as high - Stable or well-modeled background is preferable inclination HEO • Instruments are not taking data during SAA and radiation belt - Cannot follow shorter timescale variation (days – years) passages, so they do not contribute to observed background • Solar proton models • In LEO, background is correlated with geomagnetic cut-off rigidity - Fluence is binary; zero during solar minimum (COR). High COR = more geomagnetic shielding - During solar maximum, assumes a distribution of solar event fluences based on - Equatorial orbit receives the most shielding data from 3 solar cycles - In reality, large variation between solar cycles, continued activity outside of (Data from Tawa+ 2008, PASJ 60, S11) defined solar max HEO Chandra • Galactic Cosmic Ray models

2 - − Not included in simple dose calculations – small but may not be negligible cm 1 − • These are all important for modeling lunar-resonance and L2 orbits! keV 1 −

0.01 0.1 • Crowley+ 2016, “Radiation effects on the Gaia CCDs after 30 months at L2” Counts s LEO Suzaku - Radiation damage much less than expected from SPENVIS simulations SPENVIS simulation of annual CTI increase for four orbit types as a - Unusually low solar activity; damage from GCR not negligible 3 function of the shielding thickness. − 10

0.1 1 10 Energy (keV) • For LEO, orbit inclination is important - Equatorial orbit has much lower accumulated radiation Left: LEO Suzaku 5-12 keV background as a function of geomagnetic shielding. Right: Background spectrum for LEO Suzaku and HEO Chandra. damage than Suzaku (i ~ 31°) Acknowledgements - Particle exposure is primarily in the SAA • L2 and HEO should have similar background levels outside of radiation • This work was supported by NASA contracts NAS 8- LEO particles are higher energy than HEO, detector shielding belt passages 37716 and NAS 8-38252. SPENVIS is developed by is less effective at minimizing damage, flatter curve - No geomagnetic shielding -> higher background than LEO a consortium led by the Royal Belgian Institute for • Additional degradation of HEO over L2 is from low energy Space Aeronomy for ESA’s Space Environments and • Soft proton flares observed in HEO, source population not understood Effects Section through its General Support particles in the outer belts. Minimal detector shielding is - Anti-correlated with altitude, may not be important at L2? Technology Programme. sufficient to stop these particles.