Publications
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Palmroth M, Hietala H, Plaschke F, et al., 2018, Magnetosheath jet properties and evolution as determined by a global hybrid-Vlasov simulation, Annales Geophysicae: atmospheres, hydrospheres and space sciences, Vol: 36, Pages: 1171-1182, ISSN: 0992-7689
Abstract. We use a global hybrid-Vlasov simulation for the magnetosphere, Vlasiator, to investigate magnetosheath high-speed jets. Unlike many other hybrid-kinetic simulations, Vlasiator includes an unscaled geomagnetic dipole, indicating that the simulation spatial and temporal dimensions can be given without scaling. Thus, for the first time, this allows investigating the magnetosheath jet properties and comparing them directly with the observed jets within the Earth's magnetosheath. In the run shown in this paper, the interplanetary magnetic field (IMF) cone angle is 30°, and a foreshock develops upstream of the quasi-parallel magnetosheath. We visually detect a structure with high dynamic pressure propagating from the bow shock towards the magnetopause. The structure is confirmed as a jet using three different criteria, which have been adopted in previous observational studies. We compare these criteria against the simulation results. We find that the magnetosheath jet is an elongated structure extending Earthward of the bow shock by ~ 2.3 RE, while its size perpendicular to the direction of propagation is ~ 0.5 RE. We also investigate the jet evolution, and find that the jet originates due to the interaction of the foreshock Ultra Low Frequency (ULF) waves with the bow shock surface. The simulation shows that magnetosheath jets can develop also under steady IMF, as inferred by observational studies.
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.
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
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.
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.
Archer MO, Hartinger MD, Walsh BM, et al., 2017, Magnetospheric and solar wind dependences of coupled fast-mode resonances outside the plasmasphere, Journal of Geophysical Research: Space Physics, Vol: 122, Pages: 212-226, ISSN: 2169-9380
We investigate the magnetospheric and solar wind factors that control the occurrence probabilities, locations, and frequencies of standing Alfvén waves excited via coupled fast-mode resonances (cFMRs) in the outer magnetosphere's dawn and dusk sectors. The variation of these cFMR properties with the observed magnetospheric plasma density profiles and inputs to the semiempirically modeled magnetic field from the numerical cFMR calculations of Archer et al. (2015) are studied. The probability of cFMR occurrence increases with distance between the magnetopause and the Alfvén speed's local maximum. The latter's location depends on magnetospheric activity: during high activity it is situated slightly outside the plasmapause, whereas at low activity it is found at much larger radial distances. The frequencies of cFMR are proportional to the Alfvén speed near the magnetopause, which is affected by both density and magnetic field variations. The location of the excited resonance, however, depends on the relative steepness of the Alfvén speed radial profile. The steeper this is, the closer the resonance is to the outer boundary and vice versa. The variation of the density profiles with solar wind conditions and activity is also shown.
Archer MO, Hartinger MD, Walsh BM, et al., 2015, Frequency variability of standing Alfven waves excited by fast mode resonances in the outer magnetosphere, Geophysical Research Letters, Vol: 42, Pages: 10150-10159, ISSN: 0094-8276
Coupled fast mode resonances (cFMRs) in the outer magnetosphere, between the magnetopause and a turning point, are often invoked to explain observed discrete frequency field line resonances. We quantify their frequency variability, applying cFMR theory to a realistic magnetic field model and magnetospheric density profiles observed over almost half a solar cycle. Our calculations show that cFMRs are most likely around dawn, since the plasmaspheric plumes and extended plasmaspheres often found at noon and dusk can preclude their occurrence. The relative spread (median absolute deviation divided by the median) in eigenfrequencies is estimated to be 28%, 72%, and 55% at dawn, noon, and dusk, respectively, with the latter two chiefly due to density. Finally, at dawn we show that the observed bimodal density distribution results in bimodal cFMR frequencies, whereby the secondary peaks are consistent with the so-called “CMS” frequencies that have previously been attributed to cFMRs.
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
Horbury TS, Archer MO, Brown P, et al., 2015, The MAGIC of CINEMA: First in-flight science results from a miniaturised anisotropic magnetoresistive magnetometer, Annales Geophysicae, Vol: 33, Pages: 725-735, ISSN: 1432-0576
We present the first in-flight results from a novel miniaturised anisotropic magnetoresistive space magnetometer, MAGIC (MAGnetometer from Imperial College), aboard the first CINEMA (CubeSat for Ions, Neutrals, Electrons and MAgnetic fields) spacecraft in low Earth orbit. An attitude-independent calibration technique is detailed using the International Geomagnetic Reference Field (IGRF), which is temperature dependent in the case of the outboard sensor. We show that the sensors accurately measure the expected absolute field to within 2% in attitude mode and 1% in science mode. Using a simple method we are able to estimate the spacecraft's attitude using the magnetometer only, thus characterising CINEMA's spin, precession and nutation. Finally, we show that the outboard sensor is capable of detecting transient physical signals with amplitudes of ~ 20–60 nT. These include field-aligned currents at the auroral oval, qualitatively similar to previous observations, which agree in location with measurements from the DMSP (Defense Meteorological Satellite Program) and POES (Polar-orbiting Operational Environmental Satellites) spacecraft. Thus, we demonstrate and discuss the potential science capabilities of the MAGIC instrument onboard a CubeSat platform.
Archer M, Plaschke F, 2015, What frequencies of standing surface waves can the subsolar magnetopause support?, Journal of Geophysical Research: Space Physics, Vol: 120, Pages: 3632-3646, ISSN: 2169-9380
It is has been proposed that the subsolar magnetopause may support its own eigenmode, consisting of propagating surface waves which reflect at the northern/southern ionospheres forming a standing wave. While the eigenfrequencies of these so‐called Kruskal‐Schwarzschild (KS) modes have been estimated under typical conditions, the potential distribution of frequencies over the full range of solar wind conditions is not known. Using models of the magnetosphere and magnetosheath applied to an entire solar cycle's worth of solar wind data, we perform time‐of‐flight calculations yielding a database of KS mode frequencies. Under nonstorm times or northward interplanetary magnetic field (IMF), the most likely fundamental frequency is calculated to be urn:x-wiley:jgra:media:jgra51793:jgra51793-math-0001 mHz, consistent with previous estimates and indirect observational evidence for such standing surface waves of the subsolar magnetopause. However, the distributions exhibit significant spread (of order ±0.3 mHz) demonstrating that KS mode frequencies, especially higher harmonics, should vary considerably depending on the solar wind conditions. The implications of such large spread on observational statistics are discussed. The subsolar magnetopause eigenfrequencies are found to be most dependent on the solar wind speed, southward component of the IMF, and the Dst index, with the latter two being due to the erosion of the magnetosphere by reconnection and the former an effect of the expression for the surface wave phase speed. Finally, the possible occurrence of KS modes is shown to be controlled by the dipole tilt angle.
Hartinger MD, Plaschke F, Archer MO, et al., 2015, The global structure and time evolution of dayside magnetopause surface eigenmodes, GEOPHYSICAL RESEARCH LETTERS, Vol: 42, Pages: 2594-2602, ISSN: 0094-8276
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Brown P, Whiteside BJ, Beek TJ, et al., 2014, Space magnetometer based on an anisotropic magnetoresistive hybrid sensor, Review of Scientific Instruments, Vol: 85, ISSN: 1089-7623
Archer MO, Turner DL, Eastwood JP, et al., 2014, Global impacts of a Foreshock Bubble: Magnetosheath, magnetopause and ground-based observations, Planetary and Space Science, Vol: 106, Pages: 56-66, ISSN: 1873-5088
Using multipoint observations we show, for the first time, that Foreshock Bubbles (FBs) have a global impact on Earth׳s magnetosphere. We show that an FB, a transient kinetic phenomenon due to the interaction of backstreaming suprathermal ions with a discontinuity, modifies the total pressure upstream of the bow shock showing a decrease within the FB׳s core and sheath regions. Magnetosheath plasma is accelerated towards the intersection of the FB׳s current sheet with the bow shock resulting in fast, sunward, flows as well as outward motion of the magnetopause. Ground-based magnetometers also show signatures of this magnetopause motion simultaneously across at least 7 h of magnetic local time, corresponding to a distance of 21.5RE transverse to the Sun–Earth line along the magnetopause. These observed global impacts of the FB are in agreement with previous simulations and in stark contrast to the known localised, smaller scale effects of Hot Flow Anomalies (HFAs).
Archer MO, Turner DL, Eastwood JP, et al., 2014, The role of pressure gradients in driving sunward magnetosheath flows and magnetopause motion, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 119, Pages: 8117-8125, ISSN: 2169-9380
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- Citations: 39
Archer MO, Horbury TS, Eastwood JP, et al., 2013, Magnetospheric response to magnetosheath pressure pulses: A low pass filter effect, Journal of Geophysical Research, Vol: 118, Pages: 5454-5466
Abstract. We present observations from the magnetopause to the ground during periods of large amplitude, transient dynamic pressure pulses in the magnetosheath. While individual magnetosheath pulses are sharp and impulsive, the magnetospheric response is much smoother with frequencies in the Pc5-6 range being excited in the compressional and poloidal components of the magnetic field. We show that the magnetopause acts like a low pass filter, suppressing timescales shorter than a few minutes. Further filtering appears to occur locally within the magnetosphere, which may be due to the unusual field line resonance frequency profile on this day. Ground magnetometer and radar data along with equivalent ionospheric currents show signatures of travelling convection vortices, similar to the response from pressure variations of solar wind origin. However, the signatures are associated with groups of magnetosheath pulses rather than individual ones due to the impulsive nature of the pressure variations. Thus the scale-dependent magnetospheric response to these transient pressure variations, results in coherent signatures on longer timescales than any individual pulse.
Archer MO, Horbury TS, 2013, Magnetosheath dynamic pressure enhancements: Occurrence and typical properties, Ann. Geophys., Vol: 31, Pages: 319-331
Archer MO, Hartinger MD, Horbury TS, 2013, Magnetospheric “magic” frequencies as magnetopause surface eigenmodes, Geophysical Research Letters, Vol: 40, Pages: 5003-5008
Abstract. The persistent so-called “magic” magnetospheric frequencies are thought to be either directly driven by monochromatic solar wind pressure fluctuations or resonantly excited global (cavity/waveguide) or magnetopause surface eigenmodes. We distinguish between these cases by statistically investigating, using simultaneous observations, the magnetospheric response to jets in the subsolar magnetosheath. The broadband jets do not exhibit discrete frequencies, but do drive waves at the discrete “magic” frequencies, with both direct and resonant driving. We show that the expected fundamental frequencies of magnetopause surface eigenmodes have two preferential values over a wide range of upstream conditions, corresponding to fast and slow solar wind, and that their harmonics are in good agreement with the “magic” frequencies. We also show that the waves are largely inconsistent with global modes outside the plasmasphere. Thus we conclude that these “magic” frequencies are most likely due to magnetopause surface eigenmodes.
Archer MO, Horbury TS, Eastwood JP, 2012, Magnetosheath pressure pulses: Generation downstream of the bow shock from solar wind discontinuities, Journal of Geophyical Research, Vol: 117, ISSN: 0148-0227
We present multipoint Time History of Events and Macroscale Interactions duringSubstorms (THEMIS) observations of transient dynamic pressure pulses in themagnetosheath 3–10 times the background in amplitude, due to enhancements in both theion density and velocity. Their spatial dimensions are of the order of 1 RE parallel tothe flow and 0.2–0.5 RE perpendicular to it, inferred from the difference in theamplitudes observed by the different spacecraft. For the first time, simultaneousobservations of the solar wind and foreshock are also shown, proving no similar dynamicpressure enhancements exist upstream of the bow shock and that the majority of pulsesare downstream of the quasi-parallel shock. By considering previously suggestedmechanisms for their generation, we show that the pressure pulses cannot be caused byreconnection, hot flow anomalies, or short, large-amplitude magnetic structures andthat at least some of the pressure pulses appear to be consistent with previoussimulations of solar wind discontinuities interacting with the bow shock. Thesesimulations predict large-amplitude pulses when the local geometry of the shock changesfrom quasi-perpendicular to quasi-parallel, while the opposite case should also producenotable pulses but typically of lower amplitude. Therefore, in a given region of themagnetosheath, some of the discontinuities in the solar wind should generate pressurepulses, whereas others are expected not to. There is also evidence that the pulses canimpinge upon the magnetopause, causing its motion.
Archer M, Horbury TS, Lucek EA, et al., 2005, Size and shape of ULF waves in the terrestrial foreshock, J. Geophys. Res., Vol: 110, Pages: A05208-A05208
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