Session CD3 - Lessons from Space Climate: Extreme solar events

Ilya Usoskin, onsite (University of Oulu, Finland), Silvia Dalla (University of Central Lancashire, UK), Florian Mekhaldi (University of Lund, Sweden)

Space Weather studies and tools are largely based on the recent decades of direct and indirect observations of solar/geomagnetic effects, where the strength of the events was not representative of the entire possible range. On the other hand, we know from Space Climate that extreme events, several orders of magnitude stronger than everything we have observed directly during the last 80 years, can take place on the Sun on the secular time scale. This includes extreme solar flares and particle storms, as well as enormous geomagnetic disturbances. The consequences of such extreme events can be dramatic for the modern technological society but this cannot be quantified based upon the existing direct experience. Extreme events are studied by indirect proxy datasets but these measurements are quite robust because of the enormous strength of the events. Here we propose to discuss current knowledge of extreme solar events and their application to modern Space Weather problems, viz. in the context of their parameters, physical origin and consequences. In this session, results related to extremely strong rare events will be presented. This will form new observational constraints and theoretical bounds for the practical Space Weather studies.

Poster Viewing
Thursday October 27, 08:30 - 13:30, Poster Area

Talks
Thursday October 27, 16:30 - 17:45, Fire Hall

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Talks : Time schedule

Thursday October 27, 16:30 - 17:45, Fire Hall

16:30	Nature of extreme solar events: Black swans vs. Dragon kings	Usoskin, I et al.	Oral
	Ilya Usoskin
	University of Oulu, Oulu, Finland
	Solar energetic processes, such as flares and coronal mass ejections, are well studied during the past decades using direct observations and measurements. However, the upper limit of the strength of such processes is not constrained by direct data because of a too short period of their observation and the range is narrow. On the other hand, extreme solar events are known to occur on the Sun over the last millennia, thanks to cosmogenic-proxy data, and also on sun-like stars, thanks to high-precision stellar photometry. Despite many new datasets for the extreme solar/stellar eruptive events being obtained, their nature is still unclear and the question is still open whether they are similar to ‘normal’ SEP events but stronger, representing a tail of the distribution, the so-called ‘Black swans’, or a new unknown type of solar events requiring new knowledge – the so-called ‘Dragon kings’. Here we summarize the existing pieces of knowledge and try to make a distinction between the Black-swan and Dragon-king scenarios of the extreme events.
16:45	Archival reanalyses of the Carrington event in 1859	Hayakawa, H et al.	Oral
	Hisashi Hayakawa[1], Heikki Nevanlinna[2], Ciaran Beggan[3], Yusuke Ebihara[4], Sabrina Bechet[5], Ellen Clarke[3], Ankush Bhaskar[6], Sean P. Blake[7], Yoshizumi Miyoshi[8]
	[1]Nagoya University and Rutherford Appleton Laboratory; [2]Finnish Meteorological Institute; [3]British Geological Survey; [4]Kyoto University; [5]Royal Observatory of Belgium; [6]Vikram Sarabhai Space Centre; [7]NASA (previously); [8]Nagoya University
	Solar eruptions occasionally launch geo-effective interplanetary coronal mass ejections (ICMEs) and cause geomagnetic storms and extend auroral ovals. In the last two centuries, the Carrington storm in September 1859 has been considered one of their benchmark in terms of its flare magnitude, ICME velocity, geomagnetic disturbance, and equatorward auroral extension. However, the limited data availability has made the actual magnitudes and temporal evolutions have remained somewhat controversial. Here, we have overcome these difficulties following archival investigations especially in the British archives (partially on the basis of: Hayakawa et al., 2022, ApJ, 928, 32). This presentation will revise the disturbance H variation at Colaba, approximate the minimum Dst intensity, newly derive disturbance Y and Z variations in Colaba (~ 8° MLAT), visualise the temporal evolutions of the Carrington event in higher time cadence, and revise the ICME profiles. We also tentatively analyse Carrington's solar drawings and his white-light flare sketch. Our result provides an archival basis for further quantitative analyses of the Carrington storm and comparisons with other extreme space weather events.
17:00	Ground-Level Enhancements and the Solar Cycle	Owens, M et al.	Oral
	Mathew J. Owens[1], Luke A. Barnard[1], Benjamin J. S. Pope[2], Mike Lockwood[1], Ilya Usoskin[3], Eleanna Asvestari[4]
	[1]University of Reading, UK; [2]University of Queensland, Australia; [3]University of Oulu, Finland; [4]University of Helsinki, Finland
	Severe geomagnetic storms appear to be ordered by the solar cycle in a number of ways: They occur more frequently close to solar maximum and declining phase, are more common in larger solar cycles and show different patterns of occurrence in odd- and even-numbered solar cycles. Our knowledge of the most extreme space weather events, however, comes from the spikes in cosmogenic-isotope ($^{14}$C, $^{10}$Be and $^{36}$Cl) records that are attributed to significantly larger solar energetic particle (SEP) events than have been observed during the space age. Despite both storms and SEPs being driven by solar eruptive phenomena, the event-by-event correspondence between extreme storms and extreme SEPs is low. Thus it should not be assumed \textit{a priori} that the solar cycle patterns found for storms also hold for SEPs and the cosmogenic-isotope events. In this study we investigate the solar cycle trends in the timing and magnitude of the 67 SEP ``ground-level enhancements'' (GLEs) recorded by neutron monitors since the mid 1950s. Using a number of models of GLE occurrence probability, we show that GLEs are around a factor four more likely around solar maximum than around solar minimum, and that they preferentially occur earlier in even-numbered solar cycles than in odd-numbered cycles. There are insufficient data to conclusively determine whether larger solar cycles produce more GLEs. Implications for putative space-weather events in the cosmogenic-isotope records are discussed. We find that GLEs tend to cluster within a few tens of days, likely due to particularly productive individual active regions, and with approximately 11-year separations, owing to the solar cycle ordering. But these timescales do not explain cosmogenic-isotope spikes which require multiple extreme SEP events over consecutive years.
17:15	Reconstructing the Sunspot Number's history: a necessary challenge to learn lessons from past solar activity	Bhattacharya, S et al.	Oral
	Laure Lefevre[1], Shreya Bhattacharya[1], Frédéric Clette[1]
	[1]Royal Observatory of Belgium
	We will present the international effort that led to the first-ever revision of the Sunspot and Group Numbers (let us call them "the Sunspot Series"), a well-known series that had never been put into question since its creation by Rudolf Wolf in 1849. We will review the process and its challenges (ISSI review paper, https://www.issibern.ch/teams/sunspotnoser/). At this point in time, the number of Group Number series available is still based on different techniques that enable the stitching of datasets of various quality over wildly different periods, and although they present an undeniable improvement over the original Group Number from Hayt & Schatten (1998), the resulting series remain to be extensively tested in order to be clearly validated by the scientific community. Since this first revision in 2015 (Solar Physics Topical Issue, 2016), the Sunspot Series have become living datasets that require constant monitoring since more source data are being recovered regularly (Arlt & Vaquero, 2020). With the team from ISSI, we are currently driving a large effort to gather raw data from all around the world. At the Royal Observatory of Belgium, within the WDC-SILSO (https://wwwbis.sidc.be/silso/) where the original Mittheilungen have been digitized (2017-2019) we also have 2 PhD students working on stitching historical and modern sunspot numbers and evaluating the quality of the reconstructed series through advanced statistical techniques. After this review of previous efforts, we will focus more specifically on the reconstruction of the International Sunspot Number from raw sunspot data (Bhattacharya et al., 2021, 2022, FARSUN belgian project) end present the potential impact of these revisions on end users (e.g. F10.7, Clette, 2021).
17:30	New reconstruction of extreme solar particle events fluences	Koldobskiy, S et al.	Oral
	Sergey Koldobskiy[1], Florian Mekhaldi[2], Gennady Kovaltsov[1], Ilya Usoskin[1]
	[1]University of Oulu, Finland, [2]British Antarctic Survey
	Extreme solar particle events (ESPE) are a special class of solar energetic particle (SEP) events characterized by huge SEP fluxes, orders of magnitude greater than ever observed directly. The first event of this class, dated back to 774 AD, was found by Miyake and coworkers in 2012 in cosmogenic radiocarbon records in tree rings and identified as an ESPE. Today, four ESPEs are independently found and confirmed in different isotopes, and we also have several event candidates registered in radiocarbon that are awaiting confirmation in other cosmogenic proxies. In this work, we report a new method of the ESPE fluence reconstruction which simultaneously accounts for different cosmogenic-isotope datasets. For evaluations of the spectral shape of ESPEs, we used the recent reconstruction of strong SEP events registered by neutron monitors (GLE events), where GLEs SEP fluences were fitted with the combination of power-laws with exponential cutoffs. For each of GLEs we calculated the expected production/deposition of cosmogenic isotopes and then we found the scaling factor, which allows us to simultaneously fit all available cosmogenic proxies signals for each ESPE. After that, we described the fluence for each of ESPEs with scaled fluences of separated GLEs, that allowed us to account for different SEP spectral shapes and hardnesses. The method also accounts for different sources of uncertainties (including changes in the geomagnetic field, the solar activity and measurement accuracy). In comparison to GLE #5 (23/02/1956), which was the hardest and the strongest directly registered GLE, ESPE events have a 30—70 times greater fluence.

Posters

1	The dominant fraction in atmospheric 10Be transport	Golubenko, K et al.	Poster
	K.Golubenko[1], E. Rozanov[2], G. Kovaltsov[1], M. Baroni[3] and I. Usoskin[1]
	[1]Space climate research unit and Sodankyl ̈a Geophysical Observatory, University of Oulu, Oulu, 9057, Finland [2]Physikalisch-Meteorologisches Observatorium Davos and World Radiation Center, Davos Dorf, 7260, Switzerland [3] Centre Europ ́een de Recherche et d’Enseignement des G ́eosciences de l’Environnement, Universit ́e Aix-Marseille, Aix-en-Provence, France
	This study explores the dominant fraction of the atmospheric transport of $^{10}$Be produced by galactic cosmic rays (GCR) and solar cosmic rays (SCR). The main region of interest is the polar region due to the uncertainties of existing models. As part of this study, the model was verified using real measurements of ice cores for Antarctic and Greenland stations. We pay attention to all possible sources of cosmogenic $^{10}$Be in the atmosphere using the SOCOL-AER2-BE chemical climate model (CCM) in combination with the CRAC model for the period 1980\,--\,1992, which corresponds to one solar cycle. We found that for polar stations, the dominant source of $^{10}$Be produced by GCR is located in the low stratosphere (25\,--\,40 km) roughly between 0 and 30 latitudes. While for $^{10}$Be produced by SCR, the dominant source is located in the low stratosphere between 60 and 90 latitudes. The longitudinal position of the source is insignificant in all cases.
2	The role of the horizontal displacements of the photospheric magnetic features in the strongest flares of solar cycle 23	Romano, P et al.	Poster
	Paolo Romano[1], Abouazza Elmhamdi[2]
	[1] Istituto Nazionale di Astrofisica - Osservatorio Astrofisico di Catania, [2] King Saud University - Department of Physics and Astronomy
	We analyzed the vector magnetograms taken by the Helioseismic and Magnetic Imager onboard of the Solar Dynamics Observatory some hours before the occurrence of the strongest solar flares of the cycle 23. We highlighted a possible link between the horizontal displacements of the main photospheric magnetic features and the energy delivered by the subsequent flares. We used the horizontal velocity field obtained by the DAVE4VM technique to compute the Poynting fluxes and measure the energy injected into the corona by the shearing motions and the magnetic field emergence. These events can also be adopted as a hint for a possible interpretation of the strong activity of young G-type stars, recently reported by the Kepler mission. We argue that a possible explanation of the acceleration of huge numbers of particles producing white-light emission on the Sun, as well as during white-light superflares in young Sun-like stars, might be attributed to the special accompanying conditions of the occurrence of magnetic reconnection at low altitudes of their atmospheres.
3	Space weather effects during extreme GLEs: a new assessment	Mishev, A et al.	Poster
	Alexander Mishev[1], Ilya Usoskin[1], Sanja Panovska[2]
	[1] University of Oulu, Finland, [2] GFZ, Potsdam, Germany
	An important topic in the field of space weather is the quantification of cosmic-ray-induced effects in the Earth’s atmosphere, specifically the exposure to radiation and atmospheric ionization. For a realistic assessment of these effects, it is necessary to possess precise information on the solar energetic particles (SEPs) spectra. Here we present a full chain procedure, namely an analysis of SEPs using neutron monitor (NM) records, that is derivation of their spectra, and application of numerical models to compute the exposure to radiation and atmospheric ionization. The method is employed to study several strong SEPs, and here we focuse on a study of the strongest directly recorded event, namely GLE 5 of 23-Feb-1956. Subsequently, employing a convenient rescaling of GLE 5 to the strongest indirectly reconstructed historical extreme SEP event of 774 AD we discuss the space weather effect during this extreme event. The worst-case scenario, namely a combination of a geomagnetic excursion, that is the Laschamp excrusion ca. 42 kyr ago and the 774 AD like event is also discussed.
4	Uncertainties determination on the multi-century Sunspot Number Series - An important historical perspective on the solar cycle.	Bhattacharya, S et al.	Poster
	Shreya Bhattacharya[1], Laure Lefèvre[2], Maarten Jansen[3], Frèdèric Clette[4]
	[1] Royal Observatory of Belgium, Université Libre de Bruxelles, Bruxelles, Belgium, [2] Royal Observatory of Belgium, [3] Université Libre de Bruxelles, Bruxelles, Belgium, [4] Royal Observatory of Belgium
	The Sunspot Number series is one of the longest and most detailed available series in astrophysics. The series was first constructed in 1849 by Prof. Rudolf Wolf and a time series has been built in real time since then, involving a lot of observers who differ from each other in terms of their way of counting sunspots, different telescopes and eyesights. As an important step for the ongoing sunspot number recalibration throughout the entire solar physics community, (Lefevre et al., 2018), SN Version 2 was released in July, 2015 (Clette et al., 2016), which helped in shedding new light to long-term solar variations and instabilities of the 11-year solar cycle. However, uncertainties remain and errors in past historical data need to be further revised using the data at our disposal for a robust long-term series. Indeed, a key issue when building such a long-term record is to bring all observations to the same normalization scale, by diagnosing and compensating various inhomogeneity factors (instruments, observing techniques, etc.). The level of solar activity can then be compared on a constant scale across multiple centuries. A homogeneous Sunspot number series is necessary to study the evolution of long-term solar activity and its impact on the Earth (Carrasco, 2022). Moreover, Sunspot Number series plays an active role in the accurate determination of the total solar irradiance models, thus, sunspot number reconstruction with the errors is an absolute necessity for the community at present. The Royal Observatory of Belgium (https://www.astro.oma.be/en/), particularly the WDC-SILSO, conducted a mission between 2017 and 2019 to digitize all the data contained in the published Prof. Wolf’s Journals known as ‘the Mittheilungen’. Following these efforts, it is expected to reconstruct the sunspot number series from the raw original data of individual observers. These raw data can be largely found in the Mittheilungen, but recent studies have partly revised and added individual sunspot and group observations on the basis of archival investigations for original materials (Carrasco et al., 2020; Hayakawa et al., 2020) In this study, we introduce statistical techniques to stitch together all these recovered data and implement confidence bands or errors on daily Sunspot Numbers, an information that existing versions lack. The long-term aim is a complete reconstruction of the Sunspot Number series from the available raw data instead of a recalibration.
5	Solar cycle 25 records: the strongest, brightest and most distinct features as monitored by the ESA Space Weather Service Network Portal products	Palacios, J et al.	Poster
	Judit Palacios[1], Federico Da Dalt[2], Ralf Keil[3], Hannah Laurens[4], Alexi Glover[5], Juha-Pekka Luntama[6]
	[1] Rhea System GmbH for ESA-ESOC, [2] Rhea System GmbH for ESA-ESOC, [3] Rhea System GmbH for ESA-ESOC, [4] Rhea System GmbH for ESA-ESOC, [5] Space Weather Office - ESA, [6] Space Weather Office - ESA
	With the current focus of the ESWW2022 on observation and monitoring (“the importance of comprehensive space-weather monitoring”), the ESA Space Weather Service Network products provide a wide overlook for monitoring the current space weather conditions for all the stages of the Sun-Earth system. Making use of these user-focused products, alerts, and tools, we have pinpointed the space weather records corresponding to the most prominent events of the current solar cycle 25 rise phase so far: the strongest geomagnetic storm, the brightest flare, and the most noteworthy CME, monitored with a variety of observation and nowcast products. This highlights the current capabilities of the ESA Space Weather Service Network for timely information about disturbed space weather conditions for users with different interests including both downstream service users and the scientific community. The products and tools provided support a wide range of use cases including monitoring, forecasting and analysis.