Jets Downstream of Collisionless Shocks
Ferdinand Plaschke
Institut für Kommunikationsnetze und Satellitenkommunikation, TU Graz
16:15 - 17:15 Tuesday 30 June 2020 https://tugraz.webex.com/tugraz/j.php?MTID=mff871d6844cd79fb5533d7846f787e50 Join via Webex
The Earth’s magnetic field constitutes an obstacle to the solar wind plasma flow; their interaction results in a highly structured and dynamic near-Earth space. Earth’s magnetic field is thereby confined to the (inner) magnetosphere, its outer boundary being the magnetopause. The solar wind has to flow around that region. As it is super-magnetosonic in the Earth’s frame of reference, a bow shock has to form upstream of the magnetopause, at which the solar wind is decelerated, compressed, and heated. The region between the bow shock and the magnetopause is called magnetosheath.
The magnetosheath is oftentimes permeated by high-speed plasma jets. These jets are localized plasma structures featuring strong enhancements in dynamic pressure. In the subsolar magnetosheath, their velocities are found to be almost always super-Alfvénic, sometimes even super-magnetosonic, and their total pressure in anti-sunward direction is found to be twice as large, on average, in comparison to the ambient plasma [1]. Hence, jets can develop secondary shocks as they approach the magnetopause, stir plasma, and modify magnetic fields in the magnetosheath [2,3]. They can also cause severe boundary indentations when impacting the magnetopause. Their importance with respect to inner-magnetospheric effects is exacerbated by the fact that large scale, geoeffective jets (with cross-sectional diameters larger than 2 Earth radii) hit the magnetopause every couple of minutes under favorable quasi-radial interplanetary magnetic field (IMF) conditions [4]. Under these conditions, the quasi-parallel shock and the magnetically connected upstream foreshock region are both located in the subsolar region. The quasi-parallel shock is defined by low angles between the upstream magnetic field and the local bow shock normal; it is continuously formed and reformed by upstream foreshock structures and can be regarded as patchy and rippled. Most jets are consistent with being formed at bow shock ripples [5]. A minority fraction of jets is found to be related to IMF discontinuities [6].
Jets should be universally occurring downstream of collisionless shocks. Favorable conditions in the form of strong shocks and large system sizes are found at Jupiter and Saturn, whereas quasi-radial IMF conditions are much more prevalent closer to the Sun, i.e. at Mercury. There, the small magnetospheric system size seems to prevent strong rippling of the bow shock surface. Nevertheless, transient enhancements in dynamic pressure may still enter the magnetosheath in the form of solar wind magnetic holes. These are localized magnetic depressions, pressure-balanced by increases in density. Magnetic holes are close relatives of shock-generated jets and may provide for similarly strong effects to environments that are not propitious for local jet generation [7].
[1] Plaschke et al., Ann. Geophys., 31, 1877-1889, doi:10.5194/angeo-31-1877-2013, 2013.
[2] Plaschke and Hietala, Ann. Geophys., 36, 695-703, doi:10.5194/angeo-36-695-2018, 2018.
[3] Plaschke et al., Ann. Geophys., 38, 287-296, doi:10.5194/angeo-38-287-2020, 2020.
[4] Plaschke et al., J. Geophys. Res., 121, 3240-3253, doi:10.1002/2016JA022534, 2016.
[5] Hietala and Plaschke, J. Geophys. Res., 118, 7237-7245, doi:10.1002/2013JA019172, 2013.
[6] Plaschke et al., J. Geophys. Res., 122, 10, 10157-10175, doi:10.1002/2017JA024471, 2017.
[7] Plaschke et al., Astron. Astrophys., 618, A114, doi:10.1051/0004-6361/2018333300, 2018.
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