Browsing by Author "Kilpua, E.K.J."

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Statistical study of the alteration of the magnetic structure of magnetic clouds in the Earth's magnetosheath
(2017-03-01) Turc, L.; Fontaine, D.; Escoubet, C. P.; Kilpua, E.K.J.; Dimmock, A. P.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The magnetosheath plays a central role in the solar wind-magnetospheric coupling. Yet the effects of its crossing on solar wind structures such as magnetic clouds (MCs) are generally overlooked when assessing their geoeffectivity. Using 82 MCs observed simultaneously in the solar wind and the magnetosheath, we carry out the first statistical study of the alteration of their magnetic structure in the magnetosheath. For each event, the bow shock properties are obtained from a magnetosheath model. The comparison between the model results and observations shows that in 80% of cases, the MHD-based model captures well the magnetosheath transition; the other events are discussed separately. We find that just downstream of the bow shock the variation of the magnetic field direction shows a very good anticorrelation (r =− 0.91) with the angle between the upstream magnetic field and the shock normal. We then focus on the magnetic field north-south component Bz because of its importance for geoeffectivity. Although the sign of Bz is generally preserved in the magnetosheath, we also find evidence of long-lasting intervals of opposite Bz signs in the solar wind and the magnetosheath during some events, with a |Bz| reversal >10 nT at the magnetopause. We find that these reversals are due to the draping of the field lines and are associated with predominant upstream By. In those cases, the estimated position of the regions of antiparallel fields along the magnetopause is independent of the sign of the upstream Bz. This may have strong implications in terms of reconnection.
Temperature variations in the dayside magnetosheath and their dependence on ion-scale magnetic structures: THEMIS statistics and measurements by MMS
(2017-06-01) Dimmock, A. P.; Osmane, A.; Pulkkinen, T. I.; Nykyri, K.; Kilpua, E.K.J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The magnetosheath contains an array of waves, instabilities, and nonlinear magnetic structures which modify global plasma properties by means of various wave-particle interactions. The present work demonstrates that ion-scale magnetic field structures (∼0.2–0.5 Hz) observed in the dayside magnetosheath are statistically correlated to ion temperature changes on orders 10–20% of the background value. In addition, our statistical analysis implies that larger temperature changes are in equipartition to larger amplitude magnetic structures. This effect was more pronounced behind the quasi-parallel bow shock and during faster solar wind speeds. The study of two separate intervals suggests that this effect can result from both local and external drivers. This manuscript presents two separate case studies, one from using THEMIS (Time History of Events and Macroscale Interactions during Substorms) data and another from Magnetospheric Multiscale; these measurements are then supported by extensive THEMIS statistical observations. These results could partly explain the 10–20% dawn-favored asymmetry of the magnetosheath ion temperature seed population and contribute to the dawn-favored asymmetry of cold component ions in the cold dense plasma sheet.
Unraveling the drivers of the storm time radiation belt response
(2015) Kilpua, E.K.J.; Hietala, H.; Turner, D.L.; Koskinen, H.E.J.; Pulkkinen, Tuija I.; Rodriguez, J.V.; Reeves, G.D.; Claudepierre, S.G.; Spence, H.E.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä