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Oxygen Ion Flow Reversals in Earth's Magnetotail: A Cluster Statistical Study
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.ORCID-id: 0000-0003-1167-8055
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.ORCID-id: 0000-0002-2043-4442
Vise andre og tillknytning
2019 (engelsk)Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, nr 11, s. 8928-8942Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

We present a statistical study of magnetotail flows that change direction from earthward to tailward using Cluster spacecraft. More precisely, we study 318 events of particle flux enhancements in the O+ data for which the pitch angle continuously changes with time, either from 0 degrees to 180 degrees or from 180 degrees to 0 degrees. These structures are called "Pitch Angle Slope Structures" (PASSes). PASSes for which the pitch angle changes from 0 degrees to 180 degrees are observed in the Northern Hemisphere while those for which the pitch angle changes from 180 degrees to 0 degrees are observed in the Southern Hemisphere. These flux enhancements result in a reversal of the flow direction from earthward to tailward regardless of the hemisphere where they are observed. Sometimes, several PASSes can be observed consecutively which can therefore result in oscillatory velocity signatures in the earth-tail direction. The PASS occurrence rate increases from 1.8% to 3.7% as the AE index increases from similar to 0 to similar to 600 nT. Also, simultaneously to PASSes, there is typically a decrease in the magnetic field magnitude due to a decrease (increase) of the sunward component of the magnetic field in the Northern (Southern) Hemisphere. Finally, based on the 115 (out of 318) PASSes that show energy-dispersed structures, the distance to the source from the spacecraft is estimated to be typically R-E along the magnetic field line. This study is important as it sheds light on one of the causes of tailward velocities in Earth's magnetotail.

sted, utgiver, år, opplag, sider
Wiley-Blackwell, 2019. Vol. 124, nr 11, s. 8928-8942
Emneord [en]
Earth's magnetotail, oxygen ions, velocity reversal, pitch angle distribution, Cluster, field aligned
HSV kategori
Identifikatorer
URN: urn:nbn:se:umu:diva-166575DOI: 10.1029/2019JA027054ISI: 000497604100001OAI: oai:DiVA.org:umu-166575DiVA, id: diva2:1380617
Tilgjengelig fra: 2019-12-19 Laget: 2019-12-19 Sist oppdatert: 2020-05-04bibliografisk kontrollert
Inngår i avhandling
1. There and back again... An Earth magneto-tale: understanding plasma flows in the magnetotail
Åpne denne publikasjonen i ny fane eller vindu >>There and back again... An Earth magneto-tale: understanding plasma flows in the magnetotail
2020 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Alternativ tittel[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.

sted, utgiver, år, opplag, sider
Umeå: Umeå University, 2020. s. 54
Emneord
Plasma dynamics, Earth magnetosphere, magnetotail, plasma sheet flows, oscillatory flows, burst bulk flows, tailward flows
HSV kategori
Forskningsprogram
rymdfysik; rymd- och plasmafysik
Identifikatorer
urn:nbn:se:umu:diva-170347 (URN)978-91-7855-290-0 (ISBN)978-91-7855-289-4 (ISBN)
Disputas
2020-06-02, Umeå University, N360, Umeå, 09:00 (engelsk)
Opponent
Veileder
Merknad

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

Thesis: Lacking information regarding digital/print ISBN.

New location for the defence.

Tilgjengelig fra: 2020-05-12 Laget: 2020-05-04 Sist oppdatert: 2020-05-26bibliografisk kontrollert

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