Publications
106 results found
Plaschke F, Hietala H, Archer M, et al., 2018, Jets downstream of collisionless shocks, Space Science Reviews, Vol: 214, ISSN: 0038-6308
The magnetosheath flow may take the form of large amplitude, yet spatially localized, transient increases in dynamic pressure, known as “magnetosheath jets” or “plasmoids” among other denominations. Here, we describe the present state of knowledge with respect to such jets, which are a very common phenomenon downstream of the quasi-parallel bow shock. We discuss their properties as determined by satellite observations (based on both case and statistical studies), their occurrence, their relation to solar wind and foreshock conditions, and their interaction with and impact on the magnetosphere. As carriers of plasma and corresponding momentum, energy, and magnetic flux, jets bear some similarities to bursty bulk flows, which they are compared to. Based on our knowledge of jets in the near Earth environment, we discuss the expectations for jets occurring in other planetary and astrophysical environments. We conclude with an outlook, in which a number of open questions are posed and future challenges in jet research are discussed.
Wang B, Nishimura Y, Hietala H, et al., 2018, Impacts of Magnetosheath High-Speed Jets on the Magnetosphere and Ionosphere Measured by Optical Imaging and Satellite Observations, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 123, Pages: 4879-4894, ISSN: 2169-9380
Plaschke F, Hietala H, 2018, Plasma flow patterns in and around magnetosheath jets, ANNALES GEOPHYSICAE, Vol: 36, Pages: 695-703, ISSN: 0992-7689
Karlsson T, Plaschke F, Hietala H, et al., 2018, Investigating the anatomy of magnetosheath jets - MMS observations, ANNALES GEOPHYSICAE, Vol: 36, Pages: 655-677, ISSN: 0992-7689
We use Magnetosphere Multiscale (MMS) mission data to investigate a small number of magnetosheath jets, which are localized and transient increases in dynamic pressure, typically due to a combined increase in plasma velocity and density. For two approximately hour-long intervals in November, 2015 we found six jets, which are of two distinct types. (a) Two of the jets are associated with the magnetic field discontinuities at the boundary between the quasi-parallel and quasi-perpendicular magnetosheath. Straddling the boundary, the leading part of these jets contains an ion population similar to the quasi-parallel magnetosheath, while the trailing part contains ion populations similar to the quasi-perpendicular magnetosheath. Both populations are, however, cooler than the surrounding ion populations. These two jets also have clear increases in plasma density and magnetic field strength, correlated with a velocity increase. (b) Three of the jets are found embedded within the quasi-parallel magnetosheath. They contain ion populations similar to the surrounding quasi-parallel magnetosheath, but with a lower temperature. Out of these three jets, two have a simple structure. For these two jets, the increases in density and magnetic field strength are correlated with the dynamic pressure increases. The other jet has a more complicated structure, and no clear correlations between density, magnetic field strength and dynamic pressure. This jet has likely interacted with the magnetosphere, and contains ions similar to the jets inside the quasi-parallel magnetosheath, but shows signs of adiabatic heating. All jets are associated with emissions of whistler, lower hybrid, and broadband electrostatic waves, as well as approximately 10 s period electromagnetic waves with a compressional component. The latter have a Poynting flux of up to 40 µW m−2 and may be energetically important for the evolution of the jets, depending on the wave excitation mechanism. Only one of th
Kiehas SA, Runov A, Angelopolos V, et al., 2018, Magnetotail Fast Flow Occurrence Rate and Dawn-Dusk Asymmetry at <i>X</i><sub>GSM</sub> ∼-60 <i>R<sub>E</sub></i>, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 123, Pages: 1767-1778, ISSN: 2169-9380
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- Citations: 28
Hietala H, Phan TD, Angelopoulos V, et al., 2018, In situ observations of a magnetosheath high-speed jet triggering magnetopause reconnection, Geophysical Research Letters, Vol: 45, Pages: 1732-1740, ISSN: 0094-8276
Magnetosheath high‐speed jets—localized dynamic pressure enhancements typically of ∼1 Earth radius in size—impact the dayside magnetopause several times per hour. Here we present the first in situ measurements suggesting that such an impact triggered magnetopause reconnection. We use observations from the five Time History of Events and Macroscale Interactions during Substorms spacecraft in a string‐of‐pearls configuration on 7 August 2007. The spacecraft recorded magnetopause in‐and‐out motion during an impact of a magnetosheath jet (VN∼−300 km/s along the magnetopause normal direction). There was no evidence for reconnection for the preimpact crossing, yet three probes observed reconnection after the impact. We infer that the jet impact compressed the originally thick (60–70 di), high magnetic shear (140–160° magnetopause until it was thin enough for reconnection to occur. Magnetosheath high‐speed jets could therefore act as a driver for bursty dayside reconnection.
Mejnertsen L, Eastwood J, Hietala H, et al., 2017, Global MHD simulations of the Earth's bow shock shape and motion under variable solar wind conditions, Journal of Geophysical Research: Space Physics, Vol: 123, Pages: 259-271, ISSN: 2169-9380
Empirical models of the Earth's bow shock are often used to place in situ measurements in context and to understand the global behavior of the foreshock/bow shock system. They are derived statistically from spacecraft bow shock crossings and typically treat the shock surface as a conic section parameterized according to a uniform solar wind ram pressure, although more complex models exist. Here a global magnetohydrodynamic simulation is used to analyze the variability of the Earth's bow shock under real solar wind conditions. The shape and location of the bow shock is found as a function of time, and this is used to calculate the shock velocity over the shock surface. The results are compared to existing empirical models. Good agreement is found in the variability of the subsolar shock location. However, empirical models fail to reproduce the two-dimensional shape of the shock in the simulation. This is because significant solar wind variability occurs on timescales less than the transit time of a single solar wind phase front over the curved shock surface. Empirical models must therefore be used with care when interpreting spacecraft data, especially when observations are made far from the Sun-Earth line. Further analysis reveals a bias to higher shock speeds when measured by virtual spacecraft. This is attributed to the fact that the spacecraft only observes the shock when it is in motion. This must be accounted for when studying bow shock motion and variability with spacecraft data.
Palmroth M, Hoilijoki S, Juusola L, et al., 2017, Tail reconnection in the global magnetospheric context: Vlasiator first results, Annales Geophysicae, Vol: 35, Pages: 1269-1274, ISSN: 0992-7689
The key dynamics of the magnetotail have beenresearched for decades and have been associated with eitherthree-dimensional (3-D) plasma instabilities and/or magneticreconnection. We apply a global hybrid-Vlasov code, Vlasiator, to simulate reconnection self-consistently in the ion kinetic scales in the noon–midnight meridional plane, including both dayside and nightside reconnection regions withinthe same simulation box. Our simulation represents a numerical experiment, which turns off the 3-D instabilities butmodels ion-scale reconnection physically accurately in 2-D.We demonstrate that many known tail dynamics are presentin the simulation without a full description of 3-D instabilities or without the detailed description of the electrons. Whilemultiple reconnection sites can coexist in the plasma sheet,one reconnection point can start a global reconfiguration process, in which magnetic field lines become detached and aplasmoid is released. As the simulation run features temporally steady solar wind input, this global reconfiguration isnot associated with sudden changes in the solar wind. Further, we show that lobe density variations originating fromdayside reconnection may play an important role in stabilising tail reconnection.
Kajdic P, Hietala H, Blanco-Cano X, 2017, Different Types of Ion Populations Upstream of the 2013 October 8 Interplanetary Shock, ASTROPHYSICAL JOURNAL LETTERS, Vol: 849, ISSN: 2041-8205
Plaschke F, Karlsson T, Hietala H, et al., 2017, Magnetosheath high-speed jets: internal structure and interaction with ambient plasma, Journal of Geophysical Research: Space Physics, Vol: 122, Pages: 10157-10175, ISSN: 2169-9380
For the first time, we have studied the rich internal structure of a magnetosheath high‐speed jet. Measurements by the Magnetospheric Multiscale (MMS) spacecraft reveal large‐amplitude density, temperature, and magnetic field variations inside the jet. The propagation velocity and normal direction of planar magnetic field structures (i.e., current sheets and waves) are investigated via four‐spacecraft timing. We find structures to mainly convect with the jet plasma. There are indications of the presence of a tangential discontinuity. At other times, there are small cross‐structure flows. Where this is the case, current sheets and waves overtake the plasma in the jet's core region; ahead and behind that core region, along the jet's path, current sheets are overtaken by the plasma; that is, they move in opposite direction to the jet in the plasma rest frame. Jet structures are found to be mainly thermal and magnetic pressure balance structures, notwithstanding that the dynamic pressure dominates by far. Although the jet is supermagnetosonic in the Earth's frame of reference, it is submagnetosonic with respect to the plasma ahead. Consequently, we find no fast shock. Instead, we find some evidence for (a series of) jets pushing ambient plasma out of their way, thereby stirring the magnetosheath and causing anomalous sunward flows in the subsolar magnetosheath. Furthermore, we find that jets modify the magnetic field in the magnetosheath, aligning it with their propagation direction.
Liu TZ, Lu S, Angelopoulos V, et al., 2017, Fermi acceleration of electrons inside foreshock transient cores, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 122, Pages: 9248-9263, ISSN: 2169-9380
Liu TZ, Angelopoulos V, Hietala H, et al., 2017, Statistical study of particle acceleration in the core of foreshock transients, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 122, Pages: 7197-7208, ISSN: 2169-9380
Liu TZ, Angelopoulos V, Hietala H, 2017, Energetic ion leakage from foreshock transient cores, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 122, Pages: 7209-7225, ISSN: 2169-9380
Mistry R, Eastwood JP, Phan TD, et al., 2017, Statistical properties of solar wind reconnection exhausts, Journal of Geophysical Research: Space Physics, Vol: 122, Pages: 5895-5909, ISSN: 2169-9380
The solar wind provides an excellent opportunity to study the exhausts that form as a result of symmetric guide field reconnection, where spacecraft rapidly cross the exhausts far downstream of the X line. We study the statistical properties of solar wind exhausts through a superposed epoch analysis of 188 events observed at 1 AU using the Wind spacecraft. These events span a range of guide fields of 0 to 10 times the reconnecting magnetic field and inflow region plasma beta of 0.1 to 6.6. This analysis reveals that the out-of-plane magnetic field is enhanced within solar wind exhausts. Furthermore, the amount by which the plasma density and ion temperature increase from inflow region to exhaust region is found to be a function of the inflow region plasma beta and reconnection guide field, which explains the lack of these enhancements in a subset of previous observations. This dependence is consistent with the scaling of ion heating with inflow region Alfven speed, which is measured to be consistent with previous observations in the solar wind and at the magnetopause.
Dorfman S, Hietala H, Astfalk P, et al., 2017, Growth rate measurement of ULF waves in the ion foreshock, GEOPHYSICAL RESEARCH LETTERS, Vol: 44, Pages: 2120-2128, ISSN: 0094-8276
Hoilijoki S, Ganse U, Pfau-Kempf Y, et al., 2017, Reconnection rates and X line motion at the magnetopause: Global 2D-3V hybrid-Vlasov simulation results, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 122, Pages: 2877-2888, ISSN: 2169-9380
Hietala H, Artemyev AV, Angelopoulos V, 2017, Ion dynamics in magnetotail reconnection in the presence of density asymmetry, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 122, Pages: 2010-2023, ISSN: 2169-9380
Han D-S, Hietala H, Chen X-C, et al., 2017, Observational properties of dayside throat aurora and implications on the possible generation mechanisms, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 122, Pages: 1853-1870, ISSN: 2169-9380
Artemyev AV, Angelopoulos V, Hietala H, et al., 2017, Ion density and temperature profiles along (X-GSM) and across (Z(GSM)) the magnetotail as observed by THEMIS, Geotail, and ARTEMIS, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 122, Pages: 1590-1599, ISSN: 2169-9380
Pfau-Kempf Y, Hietala H, Milan SE, et al., 2016, Evidence for transient, local ion foreshocks caused by dayside magnetopause reconnection, Annales Geophysicae, Vol: 34, Pages: 943-959, ISSN: 0992-7689
We present a scenario resulting in time-dependent behaviour of the bow shock and transient, local ion reflection under unchanging solar wind conditions. Dayside magnetopause reconnection produces flux transfer events driving fast-mode wave fronts in the magnetosheath. These fronts push out the bow shock surface due to their increased downstream pressure. The resulting bow shock deformations lead to a configuration favourable to localized ion reflection and thus the formation of transient, travelling foreshock-like field-aligned ion beams. This is identified in two-dimensional global magnetospheric hybrid-Vlasov simulations of the Earth's magnetosphere performed using the Vlasiator model (http://vlasiator.fmi.fi). We also present observational data showing the occurrence of dayside reconnection and flux transfer events at the same time as Geotail observations of transient foreshock-like field-aligned ion beams. The spacecraft is located well upstream of the foreshock edge and the bow shock, during a steady southward interplanetary magnetic field and in the absence of any solar wind or interplanetary magnetic field perturbations. This indicates the formation of such localized ion foreshocks.
Mistry R, Eastwood JP, Haggerty CC, et al., 2016, Observations of Hall reconnection physics far downstream of the X-line, Physical Review Letters, Vol: 117, ISSN: 1079-7114
Observations made using the Wind spacecraft of Hall magnetic fields in solar wind reconnection exhausts are presented. These observations are consistent with the generation of Hall fields by a narrow ion inertial scale current layer near the separatrix, which is confirmed with an appropriately scaled particle-in-cell simulation that shows excellent agreement with observations. The Hall fields are observed thousands of ion inertial lengths downstream from the reconnection X line, indicating that narrow regions of kinetic dynamics can persist extremely far downstream.
Eriksson E, Vaivads A, Graham DB, et al., 2016, Strong current sheet at a magnetosheath jet: Kinetic structure and electron acceleration, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 121, Pages: 9608-9618, ISSN: 2169-9380
Liu TZ, Hietala H, Angelopoulos V, et al., 2016, Observations of a new foreshock region upstream of a foreshock bubble's shock, GEOPHYSICAL RESEARCH LETTERS, Vol: 43, Pages: 4708-4715, ISSN: 0094-8276
Plaschke F, Hietala H, Angelopoulos V, et al., 2016, Geoeffective jets impacting the magnetopause are very common, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 121, Pages: 3240-3253, ISSN: 2169-9380
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- Citations: 50
Mistry R, Eastwood JP, Hietala H, 2015, Development of bifurcated current sheets in solar wind reconnection exhausts, Geophysical Research Letters, Vol: 42, Pages: 10513-10520, ISSN: 1944-8007
Petschek-type reconnection is expected to result in bifurcations of reconnection current sheets. In contrast, Hall reconnection simulations show smooth changes in the reconnecting magnetic field. Here we study three solar wind reconnection events where different spacecraft sample oppositely directed reconnection exhausts from a common reconnection site. The spacecraft's relative separations and measurements of the exhaust width are used to geometrically calculate each spacecraft's distance from the X line. We find that in all cases spacecraft farthest from the X line observe clearly bifurcated reconnection current sheets, while spacecraft nearer to the X line do not. These observations suggest that clear bifurcations of reconnection current sheets occur at large distances from the X line (~1000 ion skin depths) and that Petschek-type signatures are less developed close to the reconnection site. This may imply that fully developed bifurcations of reconnection current sheets are unlikely to be observed in the near-Earth magnetotail.
Turner DL, O'Brien TP, Fennell JF, et al., 2015, The effects of geomagnetic storms on electrons in Earth's radiation belts, GEOPHYSICAL RESEARCH LETTERS, Vol: 42, Pages: 9176-9184, ISSN: 0094-8276
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- Citations: 66
Palmroth M, Archer M, Vainio R, et al., 2015, ULF foreshock under radial IMF: THEMIS observations and global kinetic simulation Vlasiator results compared, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 120, Pages: 8782-8798, ISSN: 2169-9380
Hietala H, Drake JF, Phan TD, et al., 2015, Ion temperature anisotropy across a magnetotail reconnection jet, Geophysical Research Letters, Vol: 42, Pages: 7239-7247, ISSN: 1944-8007
A significant fraction of the energy released by magnetotail reconnection appears to go into ion heating, but this heating is generally anisotropic. We examine ARTEMIS dual-spacecraft observations of a long-duration magnetotail exhaust generated by anti-parallel reconnection in conjunction with Particle-In-Cell simulations, showing spatial variations in the anisotropy across the outflow far (> 100di) downstream of the X-line. A consistent pattern is found in both the spacecraft data and the simulations: Whilst the total temperature across the exhaust is rather constant, near the boundaries Ti,|| dominates. The plasma is well-above the firehose threshold within patchy spatial regions at |BX| ∈ [0.1, 0.5]B0, suggesting that the drive for the instability is strong and the instability is too weak to relax the anisotropy. At the mid-plane (|BX|0.1 B0), Ti,⊥ > Ti,|| and ions undergo Speiser-like motion despite the large distance from the X-line.
Kilpua EKJ, Hietala H, Turner DL, et al., 2015, Unraveling the drivers of the storm time radiation belt response, GEOPHYSICAL RESEARCH LETTERS, Vol: 42, Pages: 3076-3084, ISSN: 0094-8276
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- Citations: 78
Eastwood JP, Hietala H, Toth G, et al., 2015, What Controls the Structure and Dynamics of Earth's Magnetosphere?, SPACE SCIENCE REVIEWS, Vol: 188, Pages: 251-286, ISSN: 0038-6308
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- Citations: 36
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