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Oscillatory Flows in the Magnetotail Plasma Sheet: Cluster Observations of the Distribution Function
Umeå University, Faculty of Science and Technology, Department of Physics.ORCID iD: 0000-0003-1167-8055
Umeå University, Faculty of Science and Technology, Department of Physics.ORCID iD: 0000-0002-2043-4442
Umeå University, Faculty of Science and Technology, Department of Physics.
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2019 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, no 4, p. 2736-2754Article in journal (Refereed) Published
Abstract [en]

Plasma dynamics in Earth's magnetotail is often studied using moments of the distribution function, which results in losing information on the kinetic properties of the plasma. To better understand oscillatory flows observed in the midtail plasma sheet, we investigate two events, one in each hemisphere, in the transition region between the central plasma sheet and the lobes using the 2-D ion distribution function from the Cluster 4 spacecraft. In this case study, the oscillatory flows are a manifestation of repeated ion flux enhancements with pitch angle changing from 0 degrees to 180 degrees in the Northern Hemisphere and from 180 degrees to 0 degrees in the Southern Hemisphere. Similar pitch angle signatures are observed seven times in about 80 min for the Southern Hemisphere event and three times in about 80 min for the Northern Hemisphere event. The ion flux enhancements observed for both events are slightly shifted in time between different energy channels, indicating a possible time-of-flight effect from which we estimate that the source of particle is located similar to 5-25R(E) and similar to 40-107R(E) tailward of the spacecraft for the Southern and Northern Hemisphere event, respectively. Using a test particle simulation, we obtain similar to 21-46 R-E for the Southern Hemisphere event and tailward of X similar to - 65R(E) (outside the validity region of the model) for the Northern Hemisphere event. We discuss possible sources that could cause the enhancements of ion flux.

Place, publisher, year, edition, pages
AMER GEOPHYSICAL UNION , 2019. Vol. 124, no 4, p. 2736-2754
National Category
Fusion, Plasma and Space Physics
Identifiers
URN: urn:nbn:se:umu:diva-162349DOI: 10.1029/2018JA026116ISI: 000477707800027Scopus ID: 2-s2.0-85064673181OAI: oai:DiVA.org:umu-162349DiVA, id: diva2:1346224
Available from: 2019-08-27 Created: 2019-08-27 Last updated: 2023-03-23Bibliographically approved
In thesis
1. There and back again... An Earth magneto-tale: understanding plasma flows in the magnetotail
Open this publication in new window or tab >>There and back again... An Earth magneto-tale: understanding plasma flows in the magnetotail
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Bort och hem igen... En magnetosfärs äventyr : studier av plasmaflöden i jordens magnetsvans
Abstract [en]

On average, the Earth's magnetotail plasma sheet seems to be a calm region of the magnetosphere where the plasma moves slowly towards Earth. However, the plasma sheet actually hosts many phenomena, some of which can affect Earth. For example, high-speed flows of plasma with speed larger than 400 km/s are observed in the plasma sheet and they can lead to aurorae. Such dynamical phenomena and the impact they may have on Earth naturally makes the plasma sheet an important region of study. Even though these high-speed flows can affect Earth, they are observed less than 5% of the time, meaning that most of the time, other disturbances take place in the plasma sheet. Our aim is to investigate and better understand the plasma dynamics in the plasma sheet.

The plasma above and below the cross-tail current sheet was previously thought to convect in the same direction. However, we find that under clearly non-zero Interplanetary Magnetic Field (IMF)By (dawn-dusk component), the plasma has a tendency to convect in opposite dawn-dusk direction across the current sheet near the midnight sector depending on the sign of IMF By.

The high-speed plasma flows are known to be associated with an increase of the northward component of the magnetic field as they propagate toward Earth. Using simulations, we notice that the magnetic field lines are bent by the high-speed flows and dents can appear. The deformation of the magnetic field is such that it may be directed towards the tail above the cross-tail current sheet and towards the Earth below it. This is opposite to the expected orientation of the magnetic field thus making this feature important in order to properly identify the region in which a spacecraft is located.

At times, the plasma can be seen to move back and forth in an oscillatory manner. We investigate statistically such oscillatory behaviour and compare them to high-speed flows and to time periods when the plasma is calm. These oscillatory flows are observed about 8% of the time in the plasma sheet. They typically have a frequency of about 1.7 mHz (~10 min period) and usually last about 41 min.

Some oscillations of the plasma velocity are observed along the magnetic field. The particles measured by the satellite initially have a velocity parallel to the magnetic field and towards Earth. Gradually with time, the measured velocity of the particles turns around to first become more perpendicular to the magnetic field and then be along the magnetic field but away from Earth. These signatures are interpreted simply as being due to mirroring particles injected tailward of the satellite and move toward Earth. The particles are then reflected, and move away from Earth. We investigate the general features of such oscillations along the magnetic field and find that the source of the particles is typically located less than 25 RE (Earth's radii) tailward of the satellite.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2020. p. 54
Keywords
Plasma dynamics, Earth magnetosphere, magnetotail, plasma sheet flows, oscillatory flows, burst bulk flows, tailward flows
National Category
Fusion, Plasma and Space Physics
Research subject
Space Physics; Space and Plasma Physics
Identifiers
urn:nbn:se:umu:diva-170347 (URN)978-91-7855-290-0 (ISBN)978-91-7855-289-4 (ISBN)
Public defence
2020-06-02, Umeå University, N360, Umeå, 09:00 (English)
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Note

Posting sheet: Wrongly stated number of pages. Digital ISBN missing. 

Thesis: Lacking information regarding digital/print ISBN.

New location for the defence.

Available from: 2020-05-12 Created: 2020-05-04 Last updated: 2020-05-26Bibliographically approved

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De Spiegeleer, AlexandreHamrin, MariaGunell, HerbertPitkänen, Timo

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