The terrestrial bow shock is a shock front formed in front of the magnetosphere where the supersonic solar wind abruptly decelerates to subsonic velocities. Downstream of the bow shock lies a turbulent region of warm dense plasma -the magnetosheath. In the magnetosheath, localized dynamic pressure enhancements are often observed examples (Amata et al., 2011;Archer et al., 2012;Karlsson et al., 2012). These pressure enhancements have been referred by many different names, for example, plasmoids (Gunell et al., 2012(Gunell et al., , 2014, antisunward high-speed jets (Plaschke et al., 2020), and supermagnetosonic subsolar magnetosheath jets (Hietala et al., 2012). In this article, we will refer to them as jets, or impacting jets.Geoeffective jets, which are jets causing disturbances to the magnetosphere-ionosphere system, have raised attention. For instance, Hietala et al. (2012) investigated several jets measured during a period of one day by Cluster spacecraft. They compared the jet observations with simultaneous observations from the geostationary orbit by Geostationary Operational Environmental Satellite (GOES) and from the ionosphere by the Super Dual Aurora Network (SuperDARN) radars. Hietala et al. (2012) reported that during times with observations of jets, irregular pulsations in the magnetic field at the geostationary orbit could be seen. Furthermore, at the same time, localized enhanced ionospheric flows were observed by SuperDARN. Archer, Horbury, et al. (2013) performed a similar study using the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft observations and ground-based magnetometer (GMAG) data as well as SuperDARN observations during a time period of 12 h. Also they could observe enhanced ionospheric flows associated with the periods with jet observations. Archer, Horbury, et al. (2013) characterized these flows as traveling convection vortices (TCVs, cf. (Glassmeier et al., 1989)). Further, Archer et al (2013) suggested that the magnetopause filters away pressure variations on timescales shorter than a few minutes, yielding a low-pass filter effect. Dmitriev and Suvorova (2012) conducted a case study for a single jet using THEMIS spacecraft observations and GMAG data. They reported that the magnetosheath jet in their event impacted the magnetopause causing it to distort in an "expansion-compression-expansion" sequence, which lasted for ∼15 min. At the same time, the Chapman-Ferraro currents at the magnetopause were simi-Abstract Localized dynamic pressure pulses in the magnetosheath, or jets, have been a popular topic for discussion in recent decades. Studies show that they can propagate through the magnetosheath and impact the magnetopause, possibly showing up as geoeffective elements at ground level. However, questions still remain on how geoeffective they can be. Previous studies have been limited to case studies during few days and with only a handful of events. In this study we have found 65 cases of impacting jets using observations from the Multisca...
<p>Plasmoids, defined as plasma entities with a higher anti-sunward velocity component than the surrounding plasma, have been observed in the magnetosheath in recent years. Among other denominations, plasmoids are also called &#8220;magnetosheath jets&#8221; and can be classified by transient localized enhancements in dynamic pressure. Propagating through the magnetosheath, jets do not only affect the magnetopause and magnetosphere. Jets pushed slower ambient magnetosheath plasma out of their way. As a result, plasma moves around the jets, and it is slowed down or could even be pushed in the sunward direction. Consequently, jets may create anomalous flows and be a source of additional turbulence. Using the magnetosheath measurements by the Magnetospheric Multiscale (MMS) and THEMIS spacecraft, and comparing several criteria, we have identified several thousand events in the wide range of bow shock distances. Previous statistical studies have shown that jet occurrence is almost exclusively controlled by the angle between the IMF and the Earth&#8211;Sun line (cone angle), and jets are predominantly observed when this cone angle is small. However, high-speed jets downstream of the quasi-perpendicular bow shock are very common. Our statistical analysis shows differences of jets evolution in the quasi-parallel and quasi-perpendicular magnetosheath regions. We discuss their properties, nature and relation to anomalies regions in the magnetosheath.</p>
In this report we present spacecraft and ground-based observations of magnetosheath jets impacting the magnetosphere, both as a case study and a statistical study. In the case study, jets were detected in the magnetosheath by the Magnetospheric MultiScale mission, MMS. By utilizing a data-based magnetospheric model (Tsyganenko T96 [29]), we estimated which jets were likely to impact the magnetopause and where they would do so. We examined ground based magnetometers, GMAGs, at the expected foot-point to the affected magnetic field line and compared this with the spacecraft observations. Theoretical transfer times for a jet to be detectable by GMAGs have been estimated and compared with the observed time delay, from detection to GMAG response, and they were in good agreement for all cases. The times found for this geoeffective response were found to be around 1-2 min, and the response in the GMAGs was in the form of a pulse with an amplitude of around 50 nT. We suggest that jets of a long enough time duration can be geoeffective in a way that they are detectable at ground level by GMAGs. It was also found that GMAGs fluctuate more during periods containing many detected jets.We performed our statistical study with the intention of comparing fluctuations in GMAG observations during Interplanetary Magnetic Field, IMF, configurations which is suggested to be favorable for jet creation. The IMF observations was provided by the THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft. This was done by selecting periods of steady IMF with different orientations, and examining the GMAG variations. GMAGs were selected based on a region where most of our foot-points were found in our previous case study. We performed this study over a three year interval, and found that GMAGs observe about 2 nT higher variation, according to their standard deviations, during radial IMF compared to northward IMF. During northward IMF we expect less effects from magnetopause phenomena, thus making it suitable to compare with radial IMF. Our statistical investigation support our findings that magnetosheath jets can be geoeffective in a way that GMAGs can detect them.ii Acknowledgements I would like to show my appreciations to a couple of people who have helped me in various ways during my University studies and this final project. Firstly, I would like to thank my friends from the engineering physics program, if you can translate the English hockey term "grind" into Swedish, all persons concerned will know who I mean. All of you have made these last five years a some the most entertaining and I have experienced. I would also like to thank my girlfriend Felicia, for encouraging and supporting me greatly during both my studies, this project and my future plans. I also owe much gratitude to Oleksandr and Maria for incredibly valuable comments and suggestions for this project, both in regards of the physics and the writing. The entire space physics group at Umeå University have been very helpful and welcoming during ...
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