2023 THEMIS SCIENCE NUGGETS
Magnetosheath jet formation influenced by parameters in solar wind structures
Florian Koller
Institute of Physics, University of Graz, Austria
Introduction
The solar wind, the plasma that is ejected from the Sun, moves through interplanetary space with very high velocities. However, upon encountering Earth's magnetic field, it experiences a sudden deceleration and compression, leading to the formation of the magnetosheath. This region is turbulent and susceptible to various phenomena, one of which are so-called magnetosheath jets. These jets are characterized by increased dynamic pressure due to higher density or velocity compared to the surrounding plasma. They usually get formed at the quasi-parallel bow shock and move through the magnetosheath, lasting for several tens of seconds. Magnetosheath jets can significantly impact Earth's magnetic field and thus can be geoeffective as a result.
The generation is mostly linked to the orientation of the magnetic field in the solar wind: If the angle between the Sun-Earth line and the magnetic field (called the cone angle) is low, the bow shock between Earth and Sun is in the quasi-parallel condition. Here, particles get scattered back in the direction of the Sun along the field lines due to the high Mach number of the shock. Those particles interact with the incoming solar wind causing plasma waves, which are then growing and carried back to the bow shock by the solar wind. This region is called the foreshock and it causes an irregular, non-smooth surface of the bow shock. The majority of jets get generated at this region.
The interplanetary plasma coming from the Sun is far from uniform: there can be large-scale structures moving through it. Coronal Mass Ejections or CMEs are burst of ejected plasma clouds that have their own magnetic structure inside, often described as a magnetic flux rope. CMEs can form a shock and sheath region in front of it when they move fast through the solar wind, comparable to a snowplow. Stream interaction regions or SIRs are interplanetary compression regions caused by fast solar wind streams from the Sun. These structures can last for days and significantly change the condition of the bow shock and the Earth’s magnetosheath. We investigate how the parameters in these structures influence the occurrence of jets.
Results
We searched for jets in the magnetosheath using THEMIS data from 2008 to 2021. We search for jets as sudden enhancements in the dynamic pressure of the plasma every time THEMIS moves through the magnetosheath. For each event we have the corresponding solar wind condition by OMNI. Thus, we know, which condition in the solar wind caused each jet. Figure 1 shows a time interval of THEMIS measurements as well as the corresponding solar wind data during a high-speed stream. Several jets were detected within this time range.
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| Figure 1. Example of a jet detecting by THEMIS A in the magnetosheath (lower panels) with corresponding solar wind plasma measurements by OMNI (upper panels). |
We focus on the following parameter in the OMNI data: cone angle and Alfvén Mach number. Those two parameters are directly connected to jet occurrence and can significantly change within solar wind structures. Figure 2 (left) shows the jet occurrence distribution in percent (color-coded) depending on the cone angle and Alfvén Mach number. This is a result of dividing the solar wind conditions during jet detection by the overall solar wind distribution. We see that low cone angle increase jet occurrence as expected, while high cone angle and low Mach numbers result in the lowest jet occurrence.
We then compare this distribution with the overall conditions that we statistically measure in individual solar wind structures. Looking at the right panels in Figure 2: 2 a) shows the distribution inside CME-sheaths, 2 b) the CME-magnetic ejecta, 2 c) SIR compression regions and 2d d) fast solar wind (high speed streams). CME- sheath and CME- magnetic ejecta distributions match well with the conditions most unfavorable for of jet occurrence, while high speed stream have a larger overlap with high jet occurrence.
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| Figure 2. 2D histograms of parameter distribution using cone angle and Alfvén Mach number. The left Plot shows the Jet occurrence depending on the overall solar wind distribution. The 4 panels on the right show the overall statistical distribution measured in different solar wind structures. Contour lines indicate available data in hours. |
Conclusion
The analysis connects the previous knowledge on jet occurrence and proposed jet generation mechanisms with the conditions we find in solar wind structures. We conclude that the conditions found in CMEs are unfavorable for jet generation due to statistically high cone angles and low Mach numbers. The latter is an effect mostly resulting from the strong magnetic field inside CMEs. The extended analysis shows that high speed streams have statistically the highest probability to generate jets, because all parameters are favorable for jet generation compared to quiet solar wind and other solar wind structures.
This could give hint to infer jets existence at the magnetosheath of other planets: at Mercury we often find low cone angles, but also rather low Mach numbers, which could hinder the generation of jets.
Further analysis will investigate the influence on jet parameter in each solar wind structure using the rich database of THEMIS plasma measurements from the past 15 years.
References
Koller, F., Temmer, M., Preisser, L., Plaschke, F., Geyer, P., Jian, L. K., et al. (2022). Magnetosheath jet occurrence rate in relation to CMEs and SIRs. Journal of Geophysical Research: Space Physics, 127, e2021JA030124. https://doi.org/10.1029/2021JA030124Koller, F., Plaschke, F., Temmer, M., Preisser, L., Roberts, O. W., and Vörös, Z. (2023). Magnetosheath jet formation influenced by parameters in solar wind structures. Journal of Geophysical Research: Space Physics, 128, e2023JA031339. https://doi.org/10.1029/2023JA031339
Plaschke, F., Hietala, H., Archer, M. et al. Jets Downstream of Collisionless Shocks. Space Sci Rev 214, 81 (2018). https://doi.org/10.1007/s11214-018-0516-3
Biographical Note
Florian Koller is a PhD student at the Institute of Physics, University of Graz, Austria. Coming from stellar CME research with a main background in solar physics, he now specializes on the connection of solar wind structures with planetary magnetic fields and magnetosheaths, connecting both heliospheric and magnetospheric plasma physics research.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu