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- Johlander, Andreas, 1990-, et al.
(författare)
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Ion Acceleration Efficiency at the Earth's Bow Shock : Observations and Simulation Results
- 2021
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 914:2
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Tidskriftsartikel (refereegranskat)abstract
- Collisionless shocks are some of the most efficient particle accelerators in heliospheric and astrophysical plasmas. Here we study and quantify ion acceleration at Earth's bow shock with observations from NASA's Magnetospheric Multiscale (MMS) satellites and in a global hybrid-Vlasov simulation. From the MMS observations, we find that quasiparallel shocks are more efficient at accelerating ions. There, up to 15% of the available energy goes into accelerating ions above 10 times their initial energy. Above a shock-normal angle of similar to 50 degrees, essentially no energetic ions are observed downstream of the shock. We find that ion acceleration efficiency is significantly lower when the shock has a low Mach number (M ( A ) < 6) while there is little Mach number dependence for higher values. We also find that ion acceleration is lower on the flanks of the bow shock than at the subsolar point regardless of the Mach number. The observations show that a higher connection time of an upstream field line leads to somewhat higher acceleration efficiency. To complement the observations, we perform a global hybrid-Vlasov simulation with realistic solar-wind parameters with the shape and size of the bow shock. We find that the ion acceleration efficiency in the simulation shows good quantitative agreement with the MMS observations. With the combined approach of direct spacecraft observations, we quantify ion acceleration in a wide range of shock angles and Mach numbers.
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| 2. |
- Kajdic, P., et al.
(författare)
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ULF Wave Transmission Across Collisionless Shocks : 2.5D Local Hybrid Simulations
- 2021
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 126:11
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Tidskriftsartikel (refereegranskat)abstract
- We study the interaction of upstream ultralow frequency (ULF) waves with collisionless shocks by analyzing the outputs of 11 2D local hybrid simulation runs. Our simulated shocks have Alfvenic Mach numbers between 4.29 and 7.42 and their theta BN angles are 15 degrees, 30 degrees, 45 degrees, and 50 degrees. The ULF wave foreshocks develop upstream of all of them. The wavelength and the amplitude of the upstream waves exhibit a complex dependence on the shock's MA and theta BN. The wavelength positively correlates with both parameters, with the dependence on theta BN being much stronger. The amplitude of the ULF waves is proportional to the product of the reflected beam velocity and density, which also depend on MA and theta BN. The interaction of the ULF waves with the shock causes large-scale (several tens of upstream ion inertial lengths) shock rippling. The properties of the shock ripples are related to the ULF wave properties, namely their wavelength and amplitude. In turn, the ripples have a large impact on the ULF wave transmission across the shock because they change local shock properties (theta BN, strength), so that different sections of the same ULF wavefront encounter shock with different characteristics. Downstream fluctuations do not resemble the upstream waves in terms the wavefront extension, orientation or their wavelength. However, some features are conserved in the Fourier spectra of downstream compressive waves that present a bump or flattening at wavelengths approximately corresponding to those of the upstream ULF waves. In the transverse downstream spectra, these features are weaker.
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| 4. |
- Suni, J., et al.
(författare)
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Connection Between Foreshock Structures and the Generation of Magnetosheath Jets : Vlasiator Results
- 2021
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 48:20
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Tidskriftsartikel (refereegranskat)abstract
- 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. Key Points We investigated magnetosheath jets in four hybrid-Vlasov simulation runs with different solar wind parameters Foreshock compressive structures can explain the formation of up to 75% of magnetosheath jets The foreshock compressive structure-caused jets travel deeper into the magnetosheath than the 25% of the jets whose origins remain unclear
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