Publications
87 results found
Wang B, Nishimura Y, Hietala H, et al., 2022, Investigating the Role of Magnetosheath High-Speed Jets in Triggering Dayside Ground Magnetic Ultra-Low Frequency Waves, GEOPHYSICAL RESEARCH LETTERS, Vol: 49, ISSN: 0094-8276
Trotta D, Vuorinen L, Hietala H, et al., 2022, Single-spacecraft techniques for shock parameters estimation: a systematic approach, Frontiers in Astronomy and Space Sciences, Vol: 9, Pages: 1-16, ISSN: 2296-987X
Spacecraft missions provide the unique opportunity to study the properties of collisionless shocks utilising in situ measurements. In the past years, several diagnostics have been developed to address key shock parameters using time series of magnetic field (and plasma) data collected by a single spacecraft crossing a shock front. A critical aspect of such diagnostics is the averaging process involved in the evaluation of upstream/downstream quantities. In this work, we discuss several of these techniques, with a particular focus on the shock obliquity (defined as the angle between the upstream magnetic field and the shock normal vector) estimation. We introduce a systematic variation of the upstream/downstream averaging windows, yielding to an ensemble of shock parameters, which is a useful tool to address the robustness of their estimation. This approach is first tested with a synthetic shock dataset compliant with the Rankine-Hugoniot jump conditions for a shock, including the presence of noise and disturbances. We then employ self-consistent, hybrid kinetic shock simulations to apply the diagnostics to virtual spacecraft crossing the shock front at various stages of its evolution, highlighting the role of shock-induced fluctuations in the parameters’ estimation. This approach has the strong advantage of retaining some important properties of collisionless shock (such as, for example, the shock front microstructure) while being able to set a known, nominal set of shock parameters. Finally, two recent observations of interplanetary shocks from the Solar Orbiter spacecraft are presented, to demonstrate the use of this systematic approach to real events of shock crossings. The approach is also tested on an interplanetary shock measured by the four spacecraft of the Magnetospheric Multiscale (MMS) mission. All the Python software developed and used for the diagnostics (SerPyShock) is made available for the public, including an example of parameter estimation fo
Vuorinen L, Vainio R, Hietala H, et al., 2022, Monte Carlo simulations of electron acceleration at bow waves driven by fast jets in the Earth’s magnetosheath, The Astrophysical Journal: an international review of astronomy and astronomical physics, Vol: 934, Pages: 1-7, ISSN: 0004-637X
The shocked solar wind flows around the Earth’s magnetosphere in the magnetosheath downstreamof the Earth’s bow shock. Within this region, faster flows of plasma, called magnetosheath jets, arefrequently observed. These jets have been shown to sometimes exhibit supermagnetosonic speedsrelative to the magnetosheath flow and to develop bow waves or shocks of their own. Such jet-drivenbow waves have been observed to accelerate ions and electrons. We model electron acceleration bymagnetosheath jet-driven bow waves using test-particle Monte Carlo simulations. Our simulationssuggest that the energy increase of electrons with energies of a few hundred eV to 10 keV can beexplained by a collapsing magnetic trap forming between the bow wave and the magnetopause withshock drift acceleration at the moving bow wave. Our simulations allow us to estimate the efficiencyof acceleration as a function of different jet and magnetosheath parameters. Electron acceleration byjet-driven bow waves can increase the total acceleration in the parent shock environment, most likelyalso at shocks other than the Earth’s bow shock.
Trotta D, Pecora F, Settino A, et al., 2022, On the Transmission of Turbulent Structures across the Earth's Bow Shock, ASTROPHYSICAL JOURNAL, Vol: 933, ISSN: 0004-637X
Koller F, Temmer M, Preisser L, et al., 2022, Magnetosheath jet occurrence rate in relation to CMEs and SIRs, Journal of Geophysical Research: Space Physics, Vol: 127, ISSN: 2169-9380
Magnetosheath jets constitute a significant coupling effect between the solar wind (SW) and the magnetosphere of the Earth. In order to investigate the effects and forecasting of these jets, we present the first-ever statistical study of the jet production during large-scale SW structures like coronal mass ejections (CMEs), stream interaction regions (SIRs) and high speed streams (HSSs). Magnetosheath data from Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft between January 2008 and December 2020 serve as measurement source for jet detection. Two different jet definitions were used to rule out statistical biases induced by our jet detection method. For the CME and SIR + HSS lists, we used lists provided by literature and expanded on incomplete lists using OMNI data to cover the time range of May 1996 to December 2020. We find that the number and total time of observed jets decrease when CME-sheaths hit the Earth. The number of jets is lower throughout the passing of the CME-magnetic ejecta (ME) and recovers quickly afterward. On the other hand, the number of jets increases during SIR and HSS phases. We discuss a few possibilities to explain these statistical results.
Telloni D, Scolini C, Moestl C, et al., 2021, Study of two interacting interplanetary coronal mass ejections encountered by Solar Orbiter during its first perihelion passage Observations and modeling, Astronomy and Astrophysics: a European journal, Vol: 656, ISSN: 0004-6361
Context. Solar Orbiter, the new-generation mission dedicated to solar and heliospheric exploration, was successfully launched on February 10, 2020, 04:03 UTC from Cape Canaveral. During its first perihelion passage in June 2020, two successive interplanetary coronal mass ejections (ICMEs), propagating along the heliospheric current sheet (HCS), impacted the spacecraft.Aims. This paper addresses the investigation of the ICMEs encountered by Solar Orbiter on June 7−8, 2020, from both an observational and a modeling perspective. The aim is to provide a full description of those events, their mutual interaction, and their coupling with the ambient solar wind and the HCS.Methods. Data acquired by the MAG magnetometer, the Energetic Particle Detector suite, and the Radio and Plasma Waves instrument are used to provide information on the ICMEs’ magnetic topology configuration, their magnetic connectivity to the Sun, and insights into the heliospheric plasma environment where they travel, respectively. On the modeling side, the Heliospheric Upwind eXtrapolation model, the 3D COronal Rope Ejection technique, and the EUropean Heliospheric FORecasting Information Asset (EUHFORIA) tool are used to complement Solar Orbiter observations of the ambient solar wind and ICMEs, and to simulate the evolution and interaction of the ejecta in the inner heliosphere, respectively.Results. Both data analysis and numerical simulations indicate that the passage of two distinct, dynamically and magnetically interacting (via magnetic reconnection processes) ICMEs at Solar Orbiter is a possible scenario, supported by the numerous similarities between EUHFORIA time series at Solar Orbiter and Solar Orbiter data.Conclusions. The combination of in situ measurements and numerical simulations (together with remote sensing observations of the corona and inner heliosphere) will significantly lead to a deeper understanding of the physical processes occurring during the CME-CME interaction.
Vuorinen L, Hietala H, Plaschke F, et al., 2021, Magnetic field in magnetosheath jets: a statistical study of B-Z near the magnetopause, Journal of Geophysical Research: Space Physics, Vol: 126, ISSN: 2169-9380
Magnetosheath jets travel from the bow shock toward the magnetopause, and some of them eventually impact it. Jet impacts have recently been linked to triggering magnetopause reconnection in case studies by Hietala et al. (2018, https://doi.org/10.1002/2017gl076525) and Nykyri et al. (2019, https://doi.org/10.1029/2018ja026357). In this study, we focus on the enhancing or suppressing effect jets could have on reconnection by locally altering the magnetic shear via their own magnetic fields. Using observations from the years 2008–2011 made by the Time History of Events and Macroscale Interactions during Substorms spacecraft and solar wind OMNI data, we statistically study for the first time urn:x-wiley:21699380:media:jgra56695:jgra56695-math-0002 within jets in the Geocentric Solar Magnetospheric coordinates. We find that urn:x-wiley:21699380:media:jgra56695:jgra56695-math-0003 opposite to the prevailing interplanetary magnetic field (IMF) urn:x-wiley:21699380:media:jgra56695:jgra56695-math-0004 is roughly as common in jets as in the non-jet magnetosheath near the magnetopause, but these observations are distributed differently. 60–70% of jet intervals contain bursts of opposite polarity urn:x-wiley:21699380:media:jgra56695:jgra56695-math-0005 in comparison to around 40urn:x-wiley:21699380:media:jgra56695:jgra56695-math-0006 of similar non-jet intervals. The median duration of such a burst in jets is 10 s and strength is urn:x-wiley:21699380:media:jgra56695:jgra56695-math-0007nT. We also investigate the prevalence of the type of strong urn:x-wiley:21699380:media:jgra56695:jgra56695-math-0008nT pulses that Nykyri et al. (2019, https://doi.org/10.1029/2018ja026357) linked to a substorm onset. In our data set, such pulses were observed in around 13% of jets. Our statistical results indicate that jets may have the potential to affect local magnetopause reconnection via their magnetic fields. Future studies are needed to determine whether such effects can be ob
LaMoury AT, Hietala H, Plaschke F, et al., 2021, Solar wind control of magnetosheath jet formation and propagation to the magnetopause, Journal of Geophysical Research: Space Physics, Vol: 126, Pages: 1-15, ISSN: 2169-9380
Magnetosheath jets are localized high-dynamic pressure pulses originating at Earth's bow shock and propagating earthward through the magnetosheath. Jets can influence magnetospheric dynamics upon impacting the magnetopause; however, many jets dissipate before reaching it. In this study we present a database of 13,096 jets observed by the Time History of Events and Macroscale Interactions during Substorms spacecraft from 2008 to 2018, spanning a solar cycle. Each jet is associated with upstream solar wind conditions from OMNI. We statistically examine how solar wind conditions control the likelihood of jets forming at the shock, and the conditions favorable for jets to propagate through the magnetosheath and reach the magnetopause. We see that, for each solar wind quantity, these two effects are separate, but when combined, we find that jets are over 17 times more likely to reach and potentially impact the magnetopause when the interplanetary magnetic field (IMF) orientation is at a low cone angle, and approximately 8 times more likely during high speed solar wind. Low IMF magnitude, high Alfvén Mach number, and low density approximately double the number of jets at the magnetopause, while urn:x-wiley:21699380:media:jgra56749:jgra56749-math-0001 and dynamic pressure display no net effect. Due to the strong dependence on wind speed, we infer that jet impact rates may be solar cycle dependent as well as vary during solar wind transients. This is an important step towards forecasting the magnetospheric effects of magnetosheath jets, as it allows for predictions of jet impact rates based on measurements of the upstream solar wind.
Desai RT, Freeman M, Eastwood J, et al., 2021, Interplanetary shock-induced magnetopause motion: Comparison between theory and global magnetohydrodynamic simulations, Geophysical Research Letters, Vol: 48, Pages: 1-11, ISSN: 0094-8276
The magnetopause marks the outer edge of the Earth’s magnetosphere and a distinct boundary between solar wind and magnetospheric plasma populations. In this letter, we use global magneto-hydrodynamic simulations to examine the response of the terrestrial magnetopause to fast-forward interplanetary shocks of various strengths and compare to theoretical predictions. The theory and simulations indicate the magnetopause response can be characterised by three distinct phases; an initial acceleration as inertial forces are overcome, a rapid compressive phase comprising the majority of the distance travelled, and large-scale damped oscillations with amplitudes of the order of an Earth radius. The two approaches agree in predicting subsolar magnetopause oscillations with frequencies2–13 mHz but the simulations notably predict larger amplitudes and weaker damping rates. This phenomenon is of high relevance to space weather forecasting and provides a possible explanation for magnetopause oscillations observed following the large interplanetary shocks of August 1972 and March 1991.
Runov A, Grandin M, Palmroth M, et al., 2021, Ion distribution functions in magnetotail reconnection: global hybrid-Vlasov simulation results, ANNALES GEOPHYSICAE, Vol: 39, Pages: 599-612, ISSN: 0992-7689
Woodham L, Horbury T, Matteini L, et al., 2021, Enhanced proton parallel temperature inside patches of switchbacks in the inner heliosphere, Astronomy and Astrophysics: a European journal, Vol: 650, Pages: 1-7, ISSN: 0004-6361
Context. Switchbacks are discrete angular deflections in the solar wind magnetic field that have been observed throughout the helio-sphere. Recent observations by Parker Solar Probe(PSP) have revealed the presence of patches of switchbacks on the scale of hours to days, separated by ‘quieter’ radial fields. Aims. We aim to further diagnose the origin of these patches using measurements of proton temperature anisotropy that can illuminate possible links to formation processes in the solar corona. Methods. We fit 3D bi-Maxwellian functions to the core of proton velocity distributions measured by the SPAN-Ai instrument onboard PSP to obtain the proton parallel, Tp,‖, and perpendicular, Tp,⊥, temperature. Results. We show that the presence of patches is highlighted by a transverse deflection in the flow and magnetic field away from the radial direction. These deflections are correlated with enhancements in Tp,‖, while Tp,⊥remains relatively constant. Patches sometimes exhibit small proton and electron density enhancements. Conclusions. We interpret that patches are not simply a group of switchbacks, but rather switchbacks are embedded within a larger-scale structure identified by enhanced Tp,‖that is distinct from the surrounding solar wind. We suggest that these observations are consistent with formation by reconnection-associated mechanisms in the corona.
Turner DL, Wilson LB, Goodrich KA, et al., 2021, Direct multipoint observations capturing the reformation of a supercritical fast magnetosonic shock, The Astrophysical Journal Letters, Vol: 911, Pages: 1-11, ISSN: 2041-8205
Using multipoint Magnetospheric Multiscale (MMS) observations in an unusual string-of-pearls configuration, we examine in detail observations of the reformation of a fast magnetosonic shock observed on the upstream edge of a foreshock transient structure upstream of Earth's bow shock. The four MMS spacecraft were separated by several hundred kilometers, comparable to suprathermal ion gyroradius scales or several ion inertial lengths. At least half of the shock reformation cycle was observed, with a new shock ramp rising up out of the "foot" region of the original shock ramp. Using the multipoint observations, we convert the observed time-series data into distance along the shock normal in the shock's rest frame. That conversion allows for a unique study of the relative spatial scales of the shock's various features, including the shock's growth rate, and how they evolve during the reformation cycle. Analysis indicates that the growth rate increases during reformation, electron-scale physics play an important role in the shock reformation, and energy conversion processes also undergo the same cyclical periodicity as reformation. Strong, thin electron-kinetic-scale current sheets and large-amplitude electrostatic and electromagnetic waves are reported. Results highlight the critical cross-scale coupling between electron-kinetic- and ion-kinetic-scale processes and details of the nature of nonstationarity, shock-front reformation at collisionless, fast magnetosonic shocks.
Omelchenko YA, Roytershteyn V, Chen L-J, et al., 2021, HYPERS simulations of solar wind interactions with the Earth's magnetosphere and the Moon, Journal of Atmospheric and Solar: Terrestrial Physics, Vol: 215, ISSN: 1364-6826
The hybrid simulations, where the ions are treated kinetically and the electrons as a fluid, seek to describe ion microphysics with maximum physical fidelity. The hybrid approach addresses the fundamental need for space plasma models to incorporate physics beyond magnetohydrodynamics. Global hybrid simulations must account for a wide range of both kinetic ion and whistler/Alfvén wave spatio-temporal scales in strongly inhomogeneous plasmas. We present results from two three-dimensional hybrid simulations performed with a novel asynchronous code, HYPERS designed to overcome computational bottlenecks that typically arise in such multiscale simulations. First, we demonstrate an excellent match between simulated lunar wake profiles and observations. We also compare our simulations with two other simulations performed with conventional (time-stepped) hybrid codes. Second, we investigate the interaction of the solar wind with the Earth's dayside magnetosphere under conditions when the orientation of the interplanetary magnetic field is quasi-radial. In this high-resolution simulation we highlight three-dimensional properties of foreshock perturbations formed by the backstreaming ions.
Robertson SL, Eastwood JP, Stawarz JE, et al., 2021, Electron trapping in magnetic mirror structures at the edge of magnetopause flux ropes, Journal of Geophysical Research: Space Physics, Vol: 126, Pages: 1-17, ISSN: 2169-9380
Flux ropes are a proposed site for particle energization during magnetic reconnection, with several mechanisms proposed. Here, Magnetospheric Multiscale mission observations of magnetic mirror structures on the edge of two ion‐scale magnetopause flux ropes are presented. Donut‐shaped features in the electron pitch angle distributions provide evidence for electron trapping in the structures. Furthermore, both events show trapping with extended 3D structure along the body of the flux rope. Potential formation mechanisms, such as the magnetic mirror instability, are examined and the evolutionary states of the structures are compared. Pressure and force analysis suggest that such structures could provide an important electron acceleration mechanism for magnetopause flux ropes, and for magnetic reconnection more generally.
Chen Y, Toth G, Hietala H, et al., 2020, Magnetohydrodynamic with embedded particle‐in‐cell simulation of the Geospace Environment Modeling dayside kinetic processes challenge event, Earth and Space Science, Vol: 7, Pages: 1-15, ISSN: 2333-5084
We use the MHD with embedded particle‐in‐cell model (MHD‐EPIC) to study the Geospace Environment Modeling (GEM) dayside kinetic processes challenge event at 01:50‐03:00 UT on 2015‐11‐18, when the magnetosphere was driven by a steady southward IMF. In the MHD‐EPIC simulation, the dayside magnetopause is covered by a PIC code so that the dayside reconnection is properly handled. We compare the magnetic fields and the plasma profiles of the magnetopause crossing with the MMS3 spacecraft observations. Most variables match the observations well in the magnetosphere, in the magnetosheath, and also during the current sheet crossing. The MHD‐EPIC simulation produces flux ropes, and we demonstrate that some magnetic field and plasma features observed by the MMS3 spacecraft can be reproduced by a flux rope crossing event. We use an algorithm to automatically identify the reconnection sites from the simulation results. It turns out that there are usually multiple X‐lines at the magnetopause. By tracing the locations of the X‐lines, we find the typical moving speed of the X‐line endpoints is about 70~km/s, which is higher than but still comparable with the ground‐based observations.
Plaschke F, Hietala H, Vörös Z, 2020, Scale sizes of magnetosheath jets, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-12, ISSN: 2169-9380
Magnetosheath jets are plasma entities that feature a significantly enhanced dynamic pressure with respect to the ambient plasma. They occur more often downstream of the quasi‐parallel bow shock. Jets can propagate through the entire magnetosheath and impact on the magnetopause. We reanalyze multi‐spacecraft data from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission to obtain the first unbiased distributions of scale sizes of the jets, in the directions parallel and perpendicular to their propagation direction. These distributions are log‐normal; they fit well to the observations. We argue that jet scales should be log‐normally distributed as they should result from multiplicative processes in the foreshock and in the magnetosheath. We find that typical jet scales are on the order of 0.1 Earth radii (RE), one order of magnitude smaller than previously reported. Median scale sizes of 0.12 RE and 0.15 RE in the perpendicular and parallel directions are obtained. The small scales may be related to the substructure of Short Large Amplitude Magnetic Structures (SLAMS) in the foreshock, or to the break up of larger jets within the magnetosheath. Use of the log‐normal distributions also allows for an analysis of impact rates of small‐scale jets: While previous results on large jets hitting the magnetopause several times per hour remain largely unchanged, we now find that hundreds to thousands of mostly small‐scale jets could potentially impact the dayside magnetopause every hour.
Liu TZ, Hietala H, Angelopoulos V, et al., 2020, Statistical study of magnetosheath jet‐driven bow waves, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-14, ISSN: 2169-9380
When a magnetosheath jet (localized dynamic pressure enhancements) compresses ambient magnetosheath at a (relative) speed faster than the local magnetosonic speed, a bow wave or shock can form ahead of the jet. Such bow waves or shocks were recently observed to accelerate particles, thus contributing to magnetosheath heating and particle acceleration in the extended environment of Earth’s bow shock. To further understand the characteristics of jet‐driven bow waves, we perform a statistical study to examine which solar wind conditions favor their formation and whether it is common for them to accelerate particles. We identified 364 out of 2859 (~13%) magnetosheath jets to have a bow wave or shock ahead of them with Mach number typically larger than 1.1. We show that large solar wind plasma beta, weak interplanetary magnetic field (IMF) strength, large solar wind Alfvén Mach number, and strong solar wind dynamic pressure present favorable conditions for their formation. We also show that magnetosheath jets with bow waves or shocks are more frequently associated with higher maximum ion and electron energies than those without them, confirming that it is common for these structures to accelerate particles. In particular, magnetosheath jets with bow waves have electron energy flux enhanced on average by a factor of 2 compared to both those without bow waves and the ambient magnetosheath. Our study implies that magnetosheath jets can contribute to shock acceleration of particles especially for high Mach number shocks. Therefore, shock models should be generalized to include magnetosheath jets and concomitant particle acceleration.
Liu TZ, Hietala H, Angelopoulos V, et al., 2020, Electron acceleration by magnetosheath jet‐driven bow waves, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-13, ISSN: 2169-9380
Magnetosheath jets are localized fast flows with enhanced dynamic pressure. When they supermagnetosonically compress the ambient magnetosheath plasma, a bow wave or shock can form ahead of them. Such a bow wave was recently observed to accelerate ions and possibly electrons. The ion acceleration process was previously analyzed, but the electron acceleration process remains largely unexplored. Here we use multi‐point observations by Time History of Events and Macroscale during Substorms from three events to determine whether and how magnetosheath jet‐driven bow waves can accelerate electrons. We show that when suprathermal electrons in the ambient magnetosheath convect towards a bow wave, some electrons are shock‐drift accelerated and reflected towards the ambient magnetosheath and others continue moving downstream of the bow wave resulting in bi‐directional motion. Our study indicates that magnetosheath jet‐driven bow waves can result in additional energization of suprathermal electrons in the magnetosheath. It implies that magnetosheath jets can increase the efficiency of electron acceleration at planetary bow shocks or other similar astrophysical environments.
Gieseler J, Oleynik P, Hietala H, et al., 2020, Radiation monitor RADMON aboard Aalto-1 CubeSat: First results, Advances in Space Research, Vol: 66, Pages: 52-65, ISSN: 0273-1177
Oleynik P, Vainio R, Punkkinen A, et al., 2020, Calibration of RADMON radiation monitor onboard Aalto-1 CubeSat, Advances in Space Research, Vol: 66, Pages: 42-51, ISSN: 0273-1177
Dimmock AP, Hietala H, Zou Y, 2020, Compiling magnetosheath statistical data sets under specific solar wind conditions: lessons learnt from the dayside kinetic southward IMF GEM challenge, Earth and Space Science, Vol: 7, Pages: 1-13, ISSN: 2333-5084
The Geospace Environmental Modelling (GEM) community offers a framework for collaborations between modelers, observers, and theoreticians in the form of regular challenges. In many cases, these challenges involve model‐data comparisons to provide wider context to observations or validate model results. To perform meaningful comparisons, a statistical approach is often adopted, which requires the extraction of a large number of measurements from a specific region. However, in complex regions such as the magnetosheath, compiling these data can be difficult. Here, we provide the statistical context of compiling statistical data for the southward IMF GEM challenge initiated by the “Dayside Kinetic Processes in Global Solar Wind‐Magnetosphere Interaction” focus group. It is shown that matching very specific upstream conditions can severely impact the statistical data if limits are imposed on several solar wind parameters. We suggest that future studies that wish to compare simulations and/or single events to statistical data should carefully consider at an early stage the availability of data in context with the upstream criteria. We also demonstrate the importance of how specific IMF conditions are defined, the chosen spacecraft, the region of interest, and how regions are identified automatically. The lessons learnt in this study are of wide context to many future studies as well as GEM challenges. The results also highlight the issue where a global statistical perspective has to be balanced with its relevance to more‐extreme, less‐frequent individual events, which is typically the case in the field of space weather.
Plaschke F, Jernej M, Hietala H, et al., 2020, On the alignment of velocity and magnetic fields within magnetosheath jets, Annales Geophysicae: atmospheres, hydrospheres and space sciences, Vol: 38, Pages: 287-296, ISSN: 0992-7689
Jets in the subsolar magnetosheath are localized enhancements in dynamic pressure that are able to propagate all the way from the bow shock to the magnetopause. Due to their excess velocity with respect to their environment, they push slower ambient plasma out of their way, creating a vortical plasma motion in and around them. Simulations and case study results suggest that jets also modify the magnetic field in the magnetosheath on their passage, aligning it more with their velocity. Based on Magnetospheric Multiscale (MMS) jet observations and corresponding superposed epoch analyses of the angles ϕ between the velocity and magnetic fields, we can confirm that this suggestion is correct. However, while the alignment is more significant for faster than for slower jets, and for jets observed close to the bow shock, the overall effect is small: typically, reductions in ϕ of around 10∘ are observed at jet core regions, where the jets' velocities are largest. Furthermore, time series of ϕ pertaining to individual jets significantly deviate from the superposed epoch analysis results. They usually exhibit large variations over the entire range of ϕ: 0 to 90∘. This variability is commonly somewhat larger within jets than outside them, masking the systematic decrease in ϕ at core regions of individual jets.
Haaland S, Paschmann G, Øieroset M, et al., 2020, Characteristics of the flank magnetopause: MMS results, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-13, ISSN: 2169-9380
We have used a large number of magnetopause crossings by the Magnetospheric Multi Spacecraft (MMS) mission to investigate macroscopic properties of this current sheet, with emphasis on the flanks of the magnetopause. Macroscopic features such as thickness, location and motion of the magnetopause were calculated as a function of local time sector. The results show that the flanks of the magnetopause are significantly thicker than the dayside magnetopause. Thicknesses vary from about 650 km near noon to over 1000 km near the terminator. Current densities varies in a similar manner, with average current densities around noon almost twice as high as near the terminator. We also find a dawn‐dusk asymmetry in many of the macroscopic parameters; The dawn magnetopause is thicker than at dusk, while the dusk flank is more dynamic, with a higher average normal velocity.
Hietala H, Dimmock AP, Zou Y, et al., 2020, The challenges and rewards of running a geospace environment modeling challenge, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-5, ISSN: 2169-9380
Geospace Environment Modeling (GEM) is a community‐driven, National Science Foundation‐sponsored research program investigating the physics of the Earth's magnetosphere and its coupling to the solar wind and the atmosphere. This commentary provides an introduction to a Special Issue collating recent studies related to a GEM Challenge on kinetic plasma processes in the dayside magnetosphere during southward interplanetary magnetic field conditions. We also recount our experiences of organizing such a collaborative activity, where modelers and observers compare their results, that is, of the human side of bringing researchers together. We give suggestions on planning, managing, funding, and documenting these activities, which provide valuable opportunities to advance the field.
Vuorinen L, Hietala H, Plaschke F, 2019, Jets in the magnetosheath: IMF control of where they occur, ANNALES GEOPHYSICAE, Vol: 37, Pages: 689-697, ISSN: 0992-7689
Liu TZ, Hietala H, Angelopoulos V, et al., 2019, THEMIS Observations of Particle Acceleration by a Magnetosheath Jet-Driven Bow Wave, GEOPHYSICAL RESEARCH LETTERS, Vol: 46, Pages: 7929-7936, ISSN: 0094-8276
Palmroth M, Praks J, Vainio R, et al., 2019, FORESAIL-1 CubeSat Mission to Measure Radiation Belt Losses and Demonstrate Deorbiting, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 5783-5799
Kilpua EKJ, Turner DL, Jaynes AN, et al., 2019, Outer Van Allen Radiation Belt Response to Interacting Interplanetary Coronal Mass Ejectionsy, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 1927-1947, ISSN: 2169-9380
Archer MO, Hietala H, Hartinger MD, et al., 2019, Direct observations of a surface eigenmode of the dayside magnetopause, Nature Communications, Vol: 10, ISSN: 2041-1723
The abrupt boundary between a magnetosphere and the surrounding plasma, the magnetopause, has long been known to support surface waves. It was proposed that impulses acting on the boundary might lead to a trapping of these waves on the dayside by the ionosphere, resulting in a standing wave or eigenmode of the magnetopause surface. No direct observational evidence of this has been found to date and searches for indirect evidence have proved inconclusive, leading to speculation that this mechanism might not occur. By using fortuitous multipoint spacecraft observations during a rare isolated fast plasma jet impinging on the boundary, here we show that the resulting magnetopause motion and magnetospheric ultra-low frequency waves at well-defined frequencies are in agreement with and can only be explained by the magnetopause surface eigenmode. We therefore show through direct observations that this mechanism, which should impact upon the magnetospheric system globally, does in fact occur.
Turner DL, Kilpua EKJ, Hietala H, et al., 2019, The Response of Earth's Electron Radiation Belts to Geomagnetic Storms: Statistics From the Van Allen Probes Era Including Effects From Different Storm Drivers, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 1013-1034, ISSN: 2169-9380
This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.