Browsing by Subject "ULF FORESHOCK"

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  • Blanco-Cano, Xochitl; Battarbee, Markus; Turc, Lucile; Dimmock, Andrew P.; Kilpua, Emilia K. J.; Hoilijoki, Sanni; Ganse, Urs; Sibeck, David G.; Cassak, Paul A.; Fear, Robert C.; Järvinen, Riku; Juusola, Liisa; Pfau-Kempf, Yann; Vainio, Rami; Palmroth, Minna (2018)
    In this paper we present the first identification of foreshock cavitons and the formation of spontaneous hot flow anomalies (SHFAs) with the Vlasiator global magnetospheric hybrid-Vlasov simulation code. In agreement with previous studies we show that cavitons evolve into SHFAs. In the presented run, this occurs very near the bow shock. We report on SHFAs surviving the shock crossing into the down-stream region and show that the interaction of SHFAs with the bow shock can lead to the formation of a magnetosheath cavity, previously identified in observations and simulations. We report on the first identification of long-term local weakening and erosion of the bow shock, associated with a region of increased foreshock SHFA and caviton formation, and repeated shock crossings by them. We show that SHFAs are linked to an increase in suprathermal particle pitch-angle spreads. The realistic length scales in our simulation allow us to present a statistical study of global caviton and SHFA size distributions, and their comparable size distributions support the theory that SHFAs are formed from cavitons. Virtual spacecraft observations are shown to be in good agreement with observational studies.
  • Turc, L.; Roberts, O. W.; Archer, M. O.; Palmroth, M.; Battarbee, M.; Brito, T.; Ganse, U.; Grandin, M.; Pfau-Kempf, Y.; Escoubet, C. P.; Dandouras, I. (2019)
    The foreshock, extending upstream of Earth's bow shock, is a region of intense electromagnetic wave activity and nonlinear phenomena, which can have global effects on geospace. It is also the first geophysical region encountered by solar wind disturbances journeying toward Earth. Here, we present the first observations of considerable modifications of the foreshock wave field during extreme events of solar origin called magnetic clouds. Cluster's multispacecraft data reveal that the typical quasi-monochromatic foreshock waves can be completely replaced by a superposition of waves each with shorter correlation lengths. Global numerical simulations further confirm that the foreshock wave field is more intricate and organized at smaller scales. Ion measurements suggest that changes in shock-reflected particle properties may cause these modifications of the wave field. This state of the foreshock is encountered only during extreme events at Earth, but intense magnetic fields are typical close to the Sun or other stars.
  • Turc, L.; Ganse, U.; Pfau-Kempf, Y.; Hoilijoki, S.; Battarbee, M.; Juusola, L.; Järvinen, R.; Brito, T.; Grandin, M.; Palmroth, M. (2018)
    In this paper, we present a detailed study of the effects of the interplanetary magnetic field (IMF) strength on the foreshock properties at small and large scales. Two simulation runs performed with the hybrid-Vlasov code Vlasiator with identical setup but with different IMF strengths, namely, 5 and 10 nT, are compared. We find that the bow shock position and shape are roughly identical in both runs, due to the quasi-radial IMF orientation, in agreement with previous magnetohydrodynamic simulations and theory. Foreshock waves develop in a broader region in the higher IMF strength run, which we attribute to the larger growth rate of the waves. The velocity of field-aligned beams remains essentially the same, but their density is generally lower when the IMF strength increases, due to the lower Mach number. Also, we identify in the regular IMF strength run ridges of suprathermal ions which disappear at higher IMF strength. These structures may be a new signature of the foreshock compressional boundary. The foreshock wave field is structured over smaller scales in higher IMF conditions, due to both the period of the foreshock waves and the transverse extent of the wave fronts being smaller. While the foreshock is mostly permeated by monochromatic waves at typical IMF strength, we find that magnetosonic waves at different frequencies coexist in the other run. They are generated by multiple beams of suprathermal ions, while only a single beam is observed at typical IMF strength. The consequences of these differences for solar wind-magnetosphere coupling are discussed. Plain Language Summary Our solar system is filled with a stream of particles escaping from the Sun, called the solar wind. The Earth is shielded from these particles by its magnetic field, which creates a magnetic bubble around our planet, the magnetosphere. Because the solar wind flow is supersonic, a bow shock forms in front of the magnetosphere to slow it down. The outermost region of the near-Earth space is called the foreshock. It is a very turbulent region, filled with particles reflected off the Earth's bow shock, and with a variety of magnetic waves. These waves can be transmitted inside the magnetosphere and create disturbances in the magnetic field on the Earth's surface. In this work, we use supercomputer simulations to study how the foreshock changes when the solar magnetic field, carried by the solar wind, intensifies. This happens in particular during solar storms, which create stormy space weather at Earth and can have adverse consequences on, for example, spacecraft electronics and power grids. We find that the foreshock properties are very different during these events compared to normal conditions and that these changes may have consequences in the regions closer to Earth.
  • Battarbee, Markus; Blanco-Cano, Xochitl; Turc, Lucile; Kajdic, Primoz; Johlander, Andreas; Tarvus, Vertti; Fuselier, Stephen; Trattner, Karlheinz; Alho, Markku; Brito, Thiago; Ganse, Urs; Pfau-Kempf, Yann; Akhavan-Tafti, Mojtaba; Karlsson, Tomas; Raptis, Savvas; Dubart, Maxime; Grandin, Maxime; Suni, Jonas; Palmroth, Minna (2020)
    The foreshock is a region of space upstream of the Earth's bow shock extending along the interplanetary magnetic field (IMF). It is permeated by shock-reflected ions and electrons, low-frequency waves, and various plasma transients. We investigate the extent of the He2+ foreshock using Vlasiator, a global hybrid-Vlasov simulation. We perform the first numerical global survey of the helium foreshock and interpret some historical foreshock observations in a global context. The foreshock edge is populated by both proton and helium field-aligned beams, with the proton foreshock extending slightly further into the solar wind than the helium foreshock and both extending well beyond the ultra-low frequency (ULF) wave foreshock. We compare our simulation results with Magnetosphere Multiscale (MMS) Hot Plasma Composition Analyzer (HPCA) measurements, showing how the gradient of suprathermal ion densities at the foreshock crossing can vary between events. Our analysis suggests that the IMF cone angle and the associated shock obliquity gradient can play a role in explaining this differing behaviour. We also investigate wave-ion interactions with wavelet analysis and show that the dynamics and heating of He2+ must result from proton-driven ULF waves. Enhancements in ion agyrotropy are found in relation to, for example, the ion foreshock boundary, the ULF foreshock boundary, and specular reflection of ions at the bow shock. We show that specular reflection can describe many of the foreshock ion velocity distribution function (VDF) enhancements. Wave-wave interactions deep in the foreshock cause de-coherence of wavefronts, allowing He2+ to be scattered less than protons.
  • Palmroth, Minna; Hietala, Heli; Plaschke, Ferdinand; Archer, Martin; Karlsson, Tomas; Blanco-Cano, Xochitl; Sibeck, David; Kajdic, Primoz; Ganse, Urs; Pfau-Kempf, Yann; Battarbee, Markus; Turc, Lucile (2018)
    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 in SI units 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 degrees, and a foreshock develops upstream of the quasi-parallel magnetosheath. We visually detect a structure with high dynamic pressure propagating from the bow shock through the magnetosheath. 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 from the bow shock by similar to 2.6 R-E, while its size perpendicular to the direction of propagation is similar to 0.5 R-E. We also investigate the jet evolution and find that the jet originates due to the interaction of the bow shock with a high-dynamic-pressure structure that reproduces observational features associated with a short, large-amplitude magnetic structure (SLAMS). The simulation shows that magnetosheath jets can develop also under steady IMF, as inferred by observational studies. To our knowledge, this paper therefore shows the first global kinetic simulation of a magnetosheath jet, which is in accordance with three observational jet criteria and is caused by a SLAMS advecting towards the bow shock.
  • Takahashi, Kazue; Turc, Lucile; Kilpua, Emilia; Takahashi, Naoko; Dimmock, Andrew P.; Kajdic, Primoz; Palmroth, Minna; Pfau-Kempf, Yann; Souček, Jan; Motoba, Tetsuo; Hartinger, Michael D.; Artemyev, Anton; Singer, Howard; Ganse, Urs; Battarbee, Markus (2021)
    We have examined the properties of ultralow-frequency (ULF) waves in space (the ion foreshock, magnetosheath, and magnetosphere) and at dayside magnetometer stations (L = 1.6-6.5) during Earth's encounter with a magnetic cloud in the solar wind, which is characterized by magnetic fields with large magnitudes (similar to 14 nT) and small cone angles (similar to 30 degrees). In the foreshock, waves were excited at similar to 90 m Hz as expected from theory, but there were oscillations at other frequencies as well. Oscillations near 90 mHz were detected at the other locations in space, but they were not in general the most dominant oscillations. On the ground, pulsations in the approximate Pc2-Pc4 band (5 mHz-120 mHz) were continuously detected at all stations, with no outstanding spectral peaks near 90 mHz in the H component except at stations where the frequency of the third harmonic of standing Alfven waves had this frequency. The fundamental toroidal wave frequency was below 90 mHz at all stations. In the D component spectra, a minor spectral peak is found near 90 mHz at stations located at L <3, and the power dropped abruptly above this frequency. Magnetospheric compressional wave power was much weaker on the nightside. A hybrid-Vlasov simulation indicates that foreshock ULF waves have short spatial scale lengths and waves transmitted into the magnetosphere are strongly attenuated away from noon.