Foreshock Compressive Structures as the Cause of Magnetosheath Jets

Magnetosheath jets are a class of structures in the Earth's magnetosheath usually defined by an enhancement of the dynamic pressure of the plasma. Magnetosheath jets have been observed by several different spacecraft over the past few decades, but their origin and formation mechanism have remained unclear. The aim of this thesis is to use data from a global simulation to investigate the origin of magnetosheath jets. We defined two different kinds of structures, magnetosheath jets and foreshock compressive structures (FCS), and collected a database of individual jets and FCSs from 4 Vlasiator global hybrid-Vlasov simulation runs, all of which simulate only the ecliptic plane. We then conducted a statistical analysis of the properties of jets and FCSs, and their occurrence rates as a function of the definition of the FCS criterion. Jets were separated into two categories: jets that form in contact with FCSs (FCS-jets), and those that do not (non-FCS-jets). We found that up to 75% of magnetosheath jets form in association with an FCS impacting the Earth's bow shock. We also found that FCS-jets penetrate deeper into the magnetosheath than non-FCS-jets. Finally, we found no conclusive explanation for the formation of non-FCS-jets. The properties of both jets and FCSs agree qualitatively and to some extent quantitatively with spacecraft observations and other simulations in the literature. The formation of jets from FCSs impacting the bow shock is similar to the proposed theory that jets are linked to Short Large-Amplitude Magnetic Structures (SLAMS). In the future, we will study magnetosheath jets and FCSs in polar plane simulation runs as well, and ultimately in full 3D simulation runs. If made possible by new simulations, the effects of electron kinetic effects on jets and FCSs will also be studied. Comparison studies with spacecraft observations of jet formation from FCSs will also be conducted, if and when such observations are found and become available

Magnetosheath jet evolution as a function of lifetime : global hybrid-Vlasov simulations compared to MMS observations

Magnetosheath jets are regions of high dynamic pressure, which can traverse from the bow shock towards the magnetopause. Recent modelling efforts, limited to a single jet and a single set of upstream conditions, have provided the first estimations about how the jet parameters behave as a function of position within the magnetosheath. Here we expand the earlier results by doing the first statistical investigation of the jet dimensions and parameters as a function of their lifetime within the magnetosheath. To verify the simulation behaviour, we first identify jets from Magnetosphere Multiscale (MMS) spacecraft data (6142 in total) and confirm the Vlasiator jet general behaviour using statistics of 924 simulated individual jets. We find that the jets in the simulation are in quantitative agreement with the observations, confirming earlier findings related to jets using Vlasiator. The jet density, dynamic pressure, and magnetic field intensity show a sharp jump at the bow shock, which decreases towards the magnetopause. The jets appear compressive and cooler than the magnetosheath at the bow shock, while during their propagation towards the magnetopause they thermalise. Further, the shape of the jets flatten as they progress through the magnetosheath. They are able to maintain their flow velocity and direction within the magnetosheath flow, and they end up preferentially to the side of the magnetosheath behind the quasi-parallel shock. Finally, we find that Vlasiator jets during low solar wind Alfven Mach number M-A are shorter in duration, smaller in their extent, and weaker in terms of dynamic pressure and magnetic field intensity as compared to the jets during high M-A.Peer reviewe

Äärimmäiset avaruussäämyrksyt, niiden vaikutukset ja varautuminen

Tässä hankkeessa kerättiin tietoa äärimmäisen voimakkaiden avaruussää-myrskyjen vaikutuksista erilaisiin teknisiin järjestelmiin. Selvitykseen osallistuivat Ilmatieteen laitos, Helsingin yliopisto (HY, Avaruusfysiikan tutkimus) ja Change in Momentum -yritys. Raportissa esitellään laajasta kirjallisuustutkimuksesta kerättyä tietoa voimakkaista myrskyistä ja tietokonesimulaatioiden tuloksia. Raportin loppuosassa käsitellään avaruussäämyrskyihin liittyviä suoria ja välillisiä yhteiskunnallisia riskejä, kuvataan verrokkimaiden kansallisten riskiarvioiden tuloksia jaesitellään varautumisharjoituksiin soveltuva äärimmäisen avaruusmyrskyn skenaario. Kirjallisuustutkimuksessa kiinnitettiin erityistä huomiota avaruus-säämyrskyjen aiheuttamiin ongelmiin sähkönjakelujärjestelmissä niiden laajojen kerrannaisvaikutusten vuoksi. Nopeat vaihtelut Maan magneetti-kentässä synnyttävät jakelujärjestelmiin haitallisia geomagneettisesti indusoituneita (GI) virtoja. Äärimmäisten myrskyjen aikaan saattaa esiintyä jopa kolme kertaa suurempia magneettikentän aikaderivaattoja Euroopassa mitattuihin arvoihin verrattuna. Haittavaikutuksille altis alue ulottuu Keski- ja Etelä-Eurooppaan asti. Meidän tulee siis varautua myrskyjen aiheuttamiin välillisiin vaikutuksiin esim. kansainvälisten toimitusketjujen ongelmien seurauksena, vaikka Suomen sähkönjakelujärjestelmän GI-virtojen sietokyvyn tiedetään olevan hyvä. Koska geomagneettisia myrskyjä riittävän tarkasti kuvaavat aikasarjat ovat verrattain lyhyitä (<150 vuotta), tilastollisissa arvioissa esiintymistodennäköisyyksille esiintyy vielä paljon vaihtelua. Kirjallisuudessa annetut arviot yleisesti vertailukohteena käytetyn vuoden 1859 Carrington-myrskyn kaltaisen ääritapahtuman todennäköisyy-delle seuraavan 10 vuoden sisällä vaihtelevat välillä 0,5–20 %. Hankkeessa testattiin ensimmäistä kertaa Helsingin yliopiston Vlasiator-simulaatiota avaruussäämyrskyjen mallinnuksessa erityisesti satelliittien toimintaympäristöön liittyen. Suurteholaskentaa vaativa Vlasiator on maailman ainoa mallinnustyökalu, joka kattaa koko lähiavaruuden ja kuvaa tarkasti avaruusplasman ionien vaikutuksen myrskyjen kehittymiseen. Simulaatiot osoittivat, että äärimmäisten myrskyjen aikaan geostationaariset ja navigointi-satelliitit menettävät ajoittain magnetosfäärin antaman suojan Auringon hiukkaspurkauksia vastaan. Geostationaaristen satelliittien hiukkasmittausten perusteella HY:n tutkijat arvioivat myös, että korkeaenergiaisten elektronien vuot saattavat olla äärimmäisissä tilanteissa 1–3 kertaluokkaa suuremmat kuin aiempien myrskyjen aikana mitatut satelliittiteknologialle ongelmia aiheuttaneet vuot. Tässä hankkeessa vuosina 2021–2022 tehtyjä Vlasiatorajoja ja muuta tutkimustyötä tarkennetaan ja laajennetaan Suomen Akatemian rahoittamassa “Preparing for the most extreme space weather” -hankeessa vuosina 2020–2023.In this project, information about extreme space weather storms and their impacts on different technological systems was collected by Finnish Meteorological Institute, University of Helsinki (UoH, Space Physics Research), and Change in Momentum company. The report presents findings from a wide literature study addressing strong space weather storms and results from computer simulations. Latter part of the report discusses direct and indirect societal risks due to extreme storms, describes national risk assessments of some reference countries, and presents a storm scenario that can be used in tabletop exercises. The literature study focused on storm impacts on power transmission as they imply indirect effects in several other systems of high societal importance. Rapid variations in Earth’s magnetic field drive harmful Geomagnetically Induced Currents (GIC) into power networks. During extreme storms the time derivatives of geomagnetic field can be even 3 times larger than measured values in Europe. The impacted area covers also Mid- and South-European latitudes. Although Finland’s power transmission system in known to be resilient against GIC, we should be prepared to tackle indirect GIC-impacts that may appear e.g. as breaks in critical supply chains. Statistical estimates for the occurrence rates of extreme storms show large variability, because time series of accurate enough magnetic field measurements are relatively short (<150 years). Literature suggests that the occurrence probability for a storm of similar intensity as the famous Carrington extreme storm in 1859 to happen in forthcoming 10 years is in the range of 0,5–20%. The Vlasiator high performance computing code of UoH was tested for the first time in simulations of extreme space weather activity in this project. Simulation results were analysed with the focus on satellites’ plasma environment. Vlasiator is the only modelling tool in the world that can handle near-Earth plasma processes globally and describe accurately the crucial ion physics contribution to storm evolution. Simulation results show that geostationary and navigation satellites can occasionally loose the protection by Earth’s magnetosphere against hostile solar particle bursts during extreme activity. Furthermore, statistical analysis of geostationationary particle data by the UoH team suggests that fluxes of high energy electrons can be 1–3 orders of magnitude larger than the fluxes which have caused problems for satellites during previous storms. Vlasiator-simulations and other research conducted in this project (2021–2022) will be continued and expanded in the project “Preparing for the most extreme space weather” funded by the Academy of Finland (2020–2023)

A global view of Pc3 wave activity in near-Earth space : Results from hybrid-Vlasov simulations

Ultra-low frequency (ULF) waves in the Pc3 range, with periods between 10-45 s, are routinely observed in Earth's dayside magnetosphere. They are thought to originate in the foreshock, which extends upstream of the quasi-parallel bow shock and is populated with shock-reflected particles. The foreshock is permeated with ULF waves generated by ion beam instabilities, most notably the "30-s " waves whose periods match those of the Pc3 waves and which are carried earthward by the solar wind flow. However, the global picture of Pc3 wave activity from the foreshock to the magnetosphere and its response to changing solar wind conditions is still poorly understood. In this study, we investigate the global distribution and properties of Pc3 waves across near-Earth space using global simulations performed with the hybrid-Vlasov model Vlasiator. The simulations enable us to study the waves in their global context, and compare their properties in the foreshock, magnetosheath and dayside magnetosphere, for different sets of upstream solar wind conditions. We find that in all three regions the Pc3 wave power peaks at higher frequencies when the interplanetary magnetic field (IMF) strength is larger, consistent with previous studies. The Pc3 wave power is significantly enhanced in all three regions for higher solar wind Alfven Mach number. As this parameter is known to affect the shock properties but has little impact inside the magnetosphere, this brings further support to the magnetospheric waves originating in the foreshock. Other parameters that are found to influence the foreshock wave power are the solar wind density and the IMF cone angle. Inside the magnetosphere, the wave power distribution depends strongly on the IMF orientation, which controls the foreshock position upstream of the bow shock. The wave power is largest when the angle between the IMF and the Sun-Earth line is smallest, suggesting that wave generation and transmission are most efficient in these conditions.Peer reviewe

Connection Between Foreshock Structures and the Generation of Magnetosheath Jets : Vlasiator Results

Earth’s magnetosheath consists of shocked solar wind plasma that has been compressed and slowed down at the Earth’s bow shock. Magnetosheath jets are pulses of enhanced dynamic pressure in the magnetosheath. Jets have been observed by numerous spacecraft missions, but their origin has remained unconfirmed, though several formation mechanisms have been suggested. In this study, we use a method for automatically identifying and tracking jets as well as foreshock compressive structures (FCSs) in four 2D runs of the global hybrid-Vlasov simulation Vlasiator. We find that up to 75% of magnetosheath jets are caused by FCSs impacting the bow shock. These jets propagate deeper into the magnetosheath than the remaining 25% of jets that are not caused by FCSs. We conduct a visual case study of one jet that was not caused by FCSs and find that the bow shock was not rippled before the formation of the jet.Earth's magnetosheath consists of shocked solar wind plasma that has been compressed and slowed down at the Earth's bow shock. Magnetosheath jets are pulses of enhanced dynamic pressure in the magnetosheath. Jets have been observed by numerous spacecraft missions, but their origin has remained unconfirmed, though several formation mechanisms have been suggested. In this study, we use a method for automatically identifying and tracking jets as well as foreshock compressive structures (FCSs) in four 2D runs of the global hybrid-Vlasov simulation Vlasiator. We find that up to 75% of magnetosheath jets are caused by FCSs impacting the bow shock. These jets propagate deeper into the magnetosheath than the remaining 25% of jets that are not caused by FCSs. We conduct a visual case study of one jet that was not caused by FCSs and find that the bow shock was not rippled before the formation of the jet. Plain Language Summary The space around Earth is filled with plasma, the fourth state of matter. Earth's magnetic field shields our planet from the stream of plasma coming from the Sun, the solar wind. The solar wind plasma is slowed down at the Earth's bow shock, before it flows against and around the Earth's magnetic field in the magnetosheath. Sometimes, pulses of high density or velocity can occur in the magnetosheath that have the potential to disturb the inner regions of near-Earth space where many spacecraft orbit. We call these pulses magnetosheath jets. Magnetosheath jets have been observed by many spacecraft over the past few decades, but how they form has remained unclear. In this study, we use the Vlasiator model to simulate plasma in near-Earth space and investigate the origins of magnetosheath jets. We find that the formation of up to 75% of these jets can be explained by compressive structures in the foreshock, a region populated by intense wave activity extending sunward of the quasi-parallel bow shock, where interplanetary magnetic field lines allow shock-reflected particles to travel back toward the Sun.Peer reviewe

Spatial filtering in a 6D hybrid-Vlasov scheme to alleviate adaptive mesh refinement artifacts : a case study with Vlasiator (versions 5.0, 5.1, and 5.2.1)

Numerical simulation models that are used to investigate the near-Earth space plasma environment require sophisticated methods and algorithms as well as high computational power. Vlasiator 5.0 is a hybrid-Vlasov plasma simulation code that is able to perform 6D (3D in ordinary space and 3D in velocity space) simulations using adaptive mesh refinement (AMR). In this work, we describe a side effect of using AMR in Vlasiator 5.0: the heterologous grid approach creates discontinuities due to the different grid resolution levels. These discontinuities cause spurious oscillations in the electromagnetic fields that alter the global results. We present and test a spatial filtering operator for alleviating this artifact without significantly increasing the computational overhead. We demonstrate the operator's use case in large 6D AMR simulations and evaluate its performance with different implementations.Peer reviewe

Enabling technology for global 3D + 3V hybrid-Vlasov simulations of near-Earth space

We present methods and algorithms that allow the Vlasiator code to run global, three-dimensional hybrid-Vlasov simulations of Earth's entire magnetosphere. The key ingredients that make Vlasov simulations at magnetospheric scales possible are the sparse velocity space implementation and spatial adaptive mesh refinement. We outline the algorithmic improvement of the semi-Lagrangian solver for six-dimensional phase space quantities, discuss the coupling of Vlasov and Maxwell equations' solvers in a refined mesh, and provide performance figures from simulation test runs that demonstrate the scalability of this simulation system to full magnetospheric runs.Peer reviewe

Magnetospheric responses to solar wind Pc5 density fluctuations : Results from 2D hybrid Vlasov simulation

Ultra-low frequency (ULF) waves are routinely observed in Earth's dayside magnetosphere. Here we investigate the influence of externally-driven density variations in the near-Earth space in the ULF regime using global 2D simulations performed with the hybrid-Vlasov model Vlasiator. With the new time-varying boundary setup, we introduce a monochromatic Pc5 range periodic density variation in the solar wind. A breathing motion of the magnetopause and changes in the bow shock standoff position are caused by the density variation, the time lag between which is found to be consistent with propagation at fast magnetohydrodynamic speed. The oscillations also create large-scale stripes of variations in the magnetosheath and modulate the mirror and electromagnetic ion cyclotron modes. We characterize the spatial-temporal properties of ULF waves at different phases of the variation. Less prominent EMIC and mirror mode wave activities near the center of magnetosheath are observed with decreasing upstream Mach number. The EMIC wave occurrence is strongly related to pressure anisotropy and beta(||), both vary as a function of the upstream conditions, whereas the mirror mode occurrence is highly influenced by fast waves generated from upstream density variations.Peer reviewe

Lower-thermosphere–ionosphere (LTI) quantities: current status of measuring techniques and models

The lower-thermosphere-ionosphere (LTI) system consists of the upper atmosphere and the lower part of the ionosphere and as such comprises a complex system coupled to both the atmosphere below and space above. The atmospheric part of the LTI is dominated by laws of continuum fluid dynamics and chemistry, while the ionosphere is a plasma system controlled by electromagnetic forces driven by the magnetosphere, the solar wind, as well as the wind dynamo. The LTI is hence a domain controlled by many different physical processes. However, systematic in situ measurements within this region are severely lacking, although the LTI is located only 80 to 200 km above the surface of our planet. This paper reviews the current state of the art in measuring the LTI, either in situ or by several different remote-sensing methods. We begin by outlining the open questions within the LTI requiring high-quality in situ measurements, before reviewing directly observable parameters and their most important derivatives. The motivation for this review has arisen from the recent retention of the Daedalus mission as one among three competing mission candidates within the European Space Agency (ESA) Earth Explorer 10 Programme. However, this paper intends to cover the LTI parameters such that it can be used as a background scientific reference for any mission targeting in situ observations of the LTI.Peer reviewe