| 1. |
- Dimmock, Andrew P., et al.
(författare)
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The GIC and Geomagnetic Response Over Fennoscandia to the 7-8 September 2017 Geomagnetic Storm
- 2019
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Ingår i: Space Weather. - 1542-7390. ; 17:7, s. 989-1010
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Tidskriftsartikel (refereegranskat)abstract
- Between 7 and 8 September 2017, Earth experienced extreme space weather events. We have combined measurements made by the IMAGE magnetometer array, ionospheric equivalent currents, geomagnetically induced current (GIC) recordings in the Finnish natural gas pipeline, and multiple ground conductivity models to study the Fennoscandia ground effects. This unique analysis has revealed multiple interesting physical and technical insights. We show that although the 7-8 September event was significant by global indices (Dst similar to 150 nT), it produced an unexpectedly large peak GIC. It is intriguing that our peak GIC did not occur during the intervals of largest geomagnetic depressions, nor was there any clear upstream trigger. Another important insight into this event is that unusually large and rare GIC amplitudes (>10 A) occurred in multiple Magnetic Local Time (MLT) sectors and could be associated with westward and eastward electrojets. We were also successfully able to model the geoelectric field and GIC using multiple models, thus providing a further important validation of these models for an extreme event. A key result from our multiple conductivity model comparison was the good agreement between the temporal features of 1-D and 3-D model results. This provides an important justification for past and future uses of 1-D models at Mantsala which is highly relevant to additional uses of this data set. Although the temporal agreement (after scaling) was good, we found a large (factor of 4) difference in the amplitudes between local and global ground models due to the difference in model conductivities. Thus, going forward, obtaining accurate ground conductivity values are key for GIC modeling.
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| 2. |
- Kilpua, E. K. J., et al.
(författare)
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Solar Wind Properties and Geospace Impact of Coronal Mass Ejection-Driven Sheath Regions : Variation and Driver Dependence
- 2019
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Ingår i: Space Weather. - 1542-7390. ; 17:8, s. 1257-1280
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Tidskriftsartikel (refereegranskat)abstract
- We present a statistical study of interplanetary conditions and geospace response to 89 coronal mass ejection-driven sheaths observed during Solar Cycles 23 and 24. We investigate in particular the dependencies on the driver properties and variations across the sheath. We find that the ejecta speed principally controls the sheath geoeffectiveness and shows the highest correlations with sheath parameters, in particular in the region closest to the shock. Sheaths of fast ejecta have on average high solar wind speeds, magnetic (B) field magnitudes, and fluctuations, and they generate efficiently strong out-of-ecliptic fields. Slow-ejecta sheaths are considerably slower and have weaker fields and field fluctuations, and therefore they cause primarily moderate geospace activity. Sheaths of weak and strong B field ejecta have distinct properties, but differences in their geoeffectiveness are less drastic. Sheaths of fast and strong ejecta push the subsolar magnetopause significantly earthward, often even beyond geostationary orbit. Slow-ejecta sheaths also compress the magnetopause significantly due to their large densities that are likely a result of their relatively long propagation times and source near the streamer belt. We find the regions near the shock and ejecta leading edge to be the most geoeffective parts of the sheath. These regions are also associated with the largest B field magnitudes, out-of-ecliptic fields, and field fluctuations as well as largest speeds and densities. The variations, however, depend on driver properties. Forecasting sheath properties is challenging due to their variable nature, but the dependence on ejecta properties determined in this work could help to estimate sheath geoeffectiveness through remote-sensing coronal mass ejection observations.
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| 3. |
- Werner, A. L. E., et al.
(författare)
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Modeling the Multiple CME Interaction Event on 6-9 September 2017 with WSA-ENLIL plus Cone
- 2019
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Ingår i: Space Weather. - : AMER GEOPHYSICAL UNION. - 1542-7390. ; 17:2, s. 357-369
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Tidskriftsartikel (refereegranskat)abstract
- A series of coronal mass ejections (CMEs) erupted from the same active region between 4-6 September 2017. Later, on 6-9 September, two interplanetary (IP) shocks reached LE creating a complex and geoeffective plasma structure. To understand the processes leading up to the formation of the two shocks, we model the CMEs with the Wang-Sheeley-Arge (WSA)-ENLIL+Cone model. The first two CMEs merged already in the solar corona driving the first IP shock. In IP space, another fast CME presumably interacted with the flank of the preceding CMEs and caused the second shock detected in situ. By introducing a customized density enhancement factor (dcld) in the WSA-ENLIL+Cone model based on coronagraph image observations, the predicted arrival time of the first IP shock was drastically improved. When the dcld factor was tested on a well-defined single CME event from 12 July 2012 the shock arrival time saw similar improvement. These results suggest that the proposed approach may be an alternative to improve the forecast for fast and simple CMEs. Further, the slowly decelerating kilometric type II radio burst confirms that the properties of the background solar wind have been preconditioned by the passage of the first IP shock. This likely caused the last CME to experience insignificant deceleration and led to the early arrival of the second IP shock. This result emphasizes the need to take preconditioning of the IP medium into account when making forecasts of CMEs erupting in quick succession.
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