Summary of space weather worst-case environments (2nd revised edition)

1. Space weather may be described as disturbances of the upper atmosphere and near-Earth space that disrupt a wide range of technological systems -and, in a few cases, pose a direct threat to human health.2. The systems at risk are very diverse and include power grids, GNSS, many aspects of spacecraft and aircraft operations, many types of radio communications and control systems.3. This note lists a number of these different systems and outlines what we currently know of: The space weather environment parameters that best summarise the threat to those systems; A reasonable worst case for those parameters, together the quality of the knowledge underpinning that estimate of the worst case and the formal provenance of that knowledge, e.g. in the peer reviewed literature; What can be done to improve the quality of that knowledge; Other useful information.This information is presented in a series of tables -with each table focusing on a specific class of space weather threat to each particular system. Context1.The ultimate source of space weather is the Sun (see Appendix 1) and intervals of enhanced space weather risk are to some extent predictable, based on solar and geophysical observations.The longest interval of severe space weather is likely to be of the order of two weeks, based on the time it would take for a large region of activity on the Sun's surface to rotate across the Sun-Earth line [see, for example, the extreme event scenarios used in the impact studies by Eastwood et al ( 2018) and Oughton et al. ( 2018)].2. During an interval of enhanced space weather risk, several different types of space weather can occur (see Appendix 1).The physical nature of space weather is extremely complex compared to terrestrial weather.This means that during an interval of enhanced space weather risk, it is extremely difficult to predict the order, size, and duration of individual space weather phenomena.3. Therefore, different systems could experience adverse impacts (a) simultaneously, (b) sequentially, or (c) unpredictably (i.e., effectively randomly).Furthermore, it is highly likely that these system failures will interact with each other to cause cascading failure modes that are fundamentally difficult to predict.Summary of space weather worst-case environments Version 4.3, 19/07/2020 2 3 Caveats 1.This is a revision of the summary published in May 2016 (http://tinyurl.com/ydy8lu5p).The changes reflect advances in understanding space weather impacts, e.g. the growing focus on the geoelectric field as a critical parameter in assessing space weather impacts on power grids; the growing range of studies of the GIC risk to power grids in UK and similar regions (Ireland, Scandinavia, Canada and New Zealand); the recognition that high-energy electrons can damage electronic systems on satellites, including solar arrays; the substantial progress in quantifying the likelihood of intense radiation and charging events in space, and of radiation events in the atmosphere, the solar radio burst in November 2015 that reminded us how these bursts can sometimes disrupt radar systems.2. While this document provides separate descriptions of different space weather risks, it must be remembered that many of these different risks will present themselves close together in time -because they have a common origin in phenomena on the Sun.The associations between the different risks are illustrated in the figure at the end of this document.3.This document focuses on the environmental aspects of space weather and does not discuss measures that can be taken to provide resilience against space weather, e.g.combined use of complementary technologies with different responses to space weather.