Browsing by Author "DImmock, Andrew P."

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    The impact on global magnetohydrodynamic simulations from varying initialisation methods: Results from GUMICS-4
    (2017-08-01) Lakka, Antti; Pulkkinen, Tuija I.; DImmock, Andrew P.; Osmane, Adnane; Honkonen, Ilja; Palmroth, Minna; Janhunen, Pekka; Department of Electronics and Nanoengineering; Finnish Meteorological Institute; University of Helsinki
    We investigate the effects of different initialisation methods of the GUMICS-4 global magnetohydrodynamic (MHD) simulation to the dynamics in different parts of the Earth's magnetosphere and hence compare five 12 h simulation runs that were initiated by 3 h of synthetic data and followed by 9 h of solar wind measurements using the OMNI data as input. As a reference, we use a simulation run that includes nearly 60 h of OMNI data as input prior to the 9 h interval examined with different initialisations. The selected interval is a high-speed stream event during a 10-day interval (12-22 June 2007). The synthetic initialisations include stepwise, linear and sinusoidal functions of the interplanetary magnetic field with constant density and velocity values. The results show that the solutions converge within 1 h to give a good agreement in both the bow shock and the magnetopause position. However, the different initialisation methods lead to local differences which should be taken into consideration when comparing model results to satellite measurements.
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    Statistical analysis of mirror mode waves in sheath regions driven by interplanetary coronal mass ejection
    (2018-05-24) Ala-Lahti, Matti M.; Kilpua, Emilia K.J.; DImmock, Andrew P.; Osmane, Adnane; Pulkkinen, Tuija; Souček, Jan; Department of Electronics and Nanoengineering; University of Helsinki; Czech Academy of Sciences
    We present a comprehensive statistical analysis of mirror mode waves and the properties of their plasma surroundings in sheath regions driven by interplanetary coronal mass ejection (ICME). We have constructed a semi-automated method to identify mirror modes from the magnetic field data. We analyze 91 ICME sheath regions from January 1997 to April 2015 using data from the Wind spacecraft. The results imply that similarly to planetary magnetosheaths, mirror modes are also common structures in ICME sheaths. However, they occur almost exclusively as dip-like structures and in mirror stable plasma. We observe mirror modes throughout the sheath, from the bow shock to the ICME leading edge, but their amplitudes are largest closest to the shock. We also find that the shock strength (measured by Alfvén Mach number) is the most important parameter in controlling the occurrence of mirror modes. Our findings suggest that in ICME sheaths the dominant source of free energy for mirror mode generation is the shock compression. We also suggest that mirror modes that are found deeper in the sheath are remnants from earlier times of the sheath evolution, generated also in the vicinity of the shock.