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An unusual giant spiral arc in the polar cap region during the northward phase of a Coronal Mass Ejection
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
2007 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 25, no 2, 507-517 p.Article in journal (Refereed) Published
Abstract [en]

The shock arrival of an Interplanetary Coronal Mass Ejection (ICME) at ∼09:50 UT on 22 November 1997 resulted in the development of an intense (Dsi < -100 nT) geomagnetic storm at Earth. In the early, quiet phase of the storm, in the sheath region of the ICME, an unusual large spiral structure (diameter of ∼ 1000 km) was observed at very high latitudes by the Polar UVI instrument. The evolution of this structure started as a polewardly displaced auroral bulge which further developed into the spiral structure spreading across a large part of the polar cap. This study attempts to examine the cause of the chain of events that resulted in the giant auroral spiral. During this period the interplanetary magnetic field (IMF) was dominantly northward (Bz >25 nT) with a strong duskward component (By>15 nT) resulting in a highly twisted tail plasma sheet. Geotail was located at the equatorial dawnside magnetotail flank and observed accelerated plasma flows exceeding the solar wind bulk velocity by almost 60%. These flows are observed on the magnetosheath side of the magnetopause and the acceleration mechanism is proposed to be typical for strongly northward IMF. Identified candidates to the cause of the spiral structure include a By induced twisted magnetotail configuration, the development of magnetopause surface waves due to the enhanced pressure related to the accelerated magnetosheath flows aswell as the formation of additional magnetopause deformations due to external solar wind pressure changes. The uniqeness of the event indicate that most probably a combination of the above effects resulted in a very extreme tail topology. However, the data coverage is insufficient to fully investigate the physical mechanism behind the observations.

Place, publisher, year, edition, pages
2007. Vol. 25, no 2, 507-517 p.
Keyword [en]
dynamics, topology, extreme value, solar wind, deformation, surface waves, Magnetopause, Magnetosheath, Flow velocity, Plasma flow, Accelerated flow, Magnetospheric tail, Plasma layer, Interplanetary magnetic field, Polar Cap, High latitude, Diameter, Spiral structure, storms, Early phase, magnetic storms, Coronal mass ejection, Auroral arc
National Category
Physical Sciences
URN: urn:nbn:se:uu:diva-97172ISI: 000245543300014OAI: oai:DiVA.org:uu-97172DiVA: diva2:171993
Available from: 2008-04-29 Created: 2008-04-29 Last updated: 2011-04-07Bibliographically approved
In thesis
1. Energy Transfer and Conversion in the Magnetosphere-Ionosphere System
Open this publication in new window or tab >>Energy Transfer and Conversion in the Magnetosphere-Ionosphere System
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Magnetized planets, such as Earth, are strongly influenced by the solar wind. The Sun is very dynamic, releasing varying amounts of energy, resulting in a fluctuating energy and momentum exchange between the solar wind and planetary magnetospheres. The efficiency of this coupling is thought to be controlled by magnetic reconnection occurring at the boundary between solar wind and planetary magnetic fields. One of the main tasks in space physics research is to increase the understanding of this coupling between the Sun and other solar system bodies. Perhaps the most important aspect regards the transfer of energy from the solar wind to the terrestrial magnetosphere as this is the main source for driving plasma processes in the magnetosphere-ionosphere system. This may also have a direct practical influence on our life here on Earth as it is responsible for Space Weather effects. In this thesis I investigate both the global scale of the varying solar-terrestrial coupling and local phenomena in more detail. I use mainly the European Space Agency Cluster mission which provide unprecedented three-dimensional observations via its formation of four identical spacecraft. The Cluster data are complimented with observations from a broad range of instruments both onboard spacecraft and from groundbased magnetometers and radars.

A period of very strong solar driving in late October 2003 is investigated. We show that some of the strongest substorms in the history of magnetic recordings were triggered by pressure pulses impacting a quasi-stable magnetosphere. We make for the first time direct estimates of the local energy flow into the magnetotail using Cluster measurements. Observational estimates suggest a good energy balance between the magnetosphere-ionosphere system while empirical proxies seem to suffer from over/under estimations during such extreme conditions.

Another period of extreme interplanetary conditions give rise to accelerated flows along the magnetopause which could account for an enhanced energy coupling between the solar wind and the magnetosphere. We discuss whether such conditions could explain the simultaneous observation of a large auroral spiral across the polar cap.

Contrary to extreme conditions the energy conversion across the dayside magnetopause has been estimated during an extended period of steady interplanetary conditions. A new method to determine the rate at which reconnection occurs is described that utilizes the magnitude of the local energy conversion from Cluster. The observations show a varying reconnection rate which support the previous interpretation that reconnection is continuous but its rate is modulated.

Finally, we compare local energy estimates from Cluster with a global magnetohydrodynamic simulation. The results show that the observations are reliably reproduced by the model and may be used to validate and scale global magnetohydrodynamic models.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2008. vii, 54 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 430
Space and plasma physics, solar system, space physics, magnetospheric physics, plasma physics, magnetic storms, substorms, boundary layers, magnetic reconnection, energy conversion, space weather, Rymd- och plasmafysik
urn:nbn:se:uu:diva-8716 (URN)978-91-554-7192-7 (ISBN)
Public defence
2008-05-23, Häggsalen, Ångströmslaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15
Available from: 2008-04-29 Created: 2008-04-29Bibliographically approved

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Rosenqvist, LisaKullen, AnitaBuchert, Stephan
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