Geoeffectiveness of Magnetosheath Jets

Abstract

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 ...