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Auroral electrodynamics of plasma boundary regions
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The electrodynamic coupling between the auroral ionosphere and the magnetosphere is the main subject of this thesis. Satellite measurements of electric and magnetic fields and of charged particles are used to explore three distinct plasma boundaries, magnetically linked to the nightside auroral ionosphere. These boundaries are the inner edge of the plasma sheet (PS), and the inner and the outer edges of the plasma sheet boundary layer (PSBL).

Strong ionospheric electric fields with amplitudes up to 400 mV/m may be observed in the subauroral ionosphere, in the vicinity of the ionospheric projection of the PS inner edge. Intense and dynamic auroral electric fields with local magnitudes up to 150 mV/m associated with upward ion beams and field-aligned currents are observed for the events treated here, at the inner and outer boundaries of the PSBL at an altitude of about 4-5 Earth radii, well above the acceleration region. Subauroral and auroral electric fields are the two main subjects of this thesis.

Subauroral ion drifts (SAID) are associated with poleward electric fields, occurring predominantly in the premidnight region during the substorm recovery phase. The recently revealed abnormal subauroral ion drifts (ASAID) are associated with equatorward electric fields, occurring during extended periods of low auroral activity. The results indicate that the generation mechanism of SAID can neither be regarded as a pure voltage generator nor a pure current generator, but having certain characteristics of both generator types. Ionospheric feedback appears to play a major role for the development and maintenance of the SAID electric fields. The formation of ASAID is proposed to result from the proximity and interaction between different plasma boundaries of the innermost magnetosphere during extended periods of low auroral activity.

The auroral electric fields observed in the upward current region at the PSBL inner and outer edges are associated with upward parallel electric fields, which partially decouple the high-altitude electric fields from the ionosphere. This is in contrast to the subauroral electric fields which are coupled. Multi-point measurements provided by the Cluster mission show that the observed electric fields are highly variable in space and time, revealing various types of acceleration processes. However, they appear to be tied to the boundary where they are originally formed. A case is  presented where they are associated with large electromagnetic energy fluxes directed upward away from the ionosphere. The interaction between the magnetosphere and ionosphere, being more pronounced at plasma boundary regions, is important for the understanding of the formation and regulation of the highly structured auroral electric fields observed in the upward current region.

Place, publisher, year, edition, pages
Stockholm: KTH , 2009. , xii, 74 p.
Series
Trita-EE, ISSN 1653-5146 ; 2009:027
National Category
Fusion, Plasma and Space Physics
Identifiers
URN: urn:nbn:se:kth:diva-10446ISBN: 978-91-7415-317-0 (print)OAI: oai:DiVA.org:kth-10446DiVA: diva2:217435
Public defence
2009-05-29, Sal F3, KTH, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20100727Available from: 2009-05-14 Created: 2009-05-14 Last updated: 2010-07-27Bibliographically approved
List of papers
1. Investigation of subauroral ion drifts and related field-aligned currents and ionospheric Pedersen conductivity distribution
Open this publication in new window or tab >>Investigation of subauroral ion drifts and related field-aligned currents and ionospheric Pedersen conductivity distribution
2004 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 22, no 3, 923-934 p.Article in journal (Refereed) Published
Abstract [en]

Based on Astrid-2 satellite data, results are presented from a statistical study on subauroral ion drift (SAID) occurrence. SAID is a subauroral phenomenon characterized by a westward ionospheric ion drift with velocity greater than 1000 m/s, or equivalently, by a poleward-directed electric field with intensity greater than 30 mV/m. SAID events occur predominantly in the premidnight sector, with a maximum probability located within the 20:00 to 23:00 MLT sector, where the most rapid SAID events are also found. They are substorm related, and show first an increase in intensity and a decrease in latitudinal width during the expansion phase, followed by a weakening and widening of the SAID structures during the recovery phase. The potential drop across a SAID structure is seen to remain roughly constant during the recovery phase. The field-aligned current density and the height-integrated Pedersen conductivity distribution associated with the SAID events were calculated. The results reveal that the strongest SAID electric field peaks are associated with the lowest Pedersen conductivity minimum values. Clear modifications are seen in the ionospheric Pedersen conductivity distribution associated with the SAID structure as time evolves: the SAID peak is located on the poleward side of the corresponding region of reduced Pedersen conductivity; the shape of the regions of reduced conductivity is asymmetric, with a steeper poleward edge and a more rounded equatorward edge; the SAID structure becomes less intense and widens with evolution of the substorm recovery phase. From the analysis of the SAID occurrence relative to the mid-latitude trough position, SAID peaks are seen to occur relatively close to the corresponding mid-latitude trough minimum. Both these features show a similar response to magnetospheric disturbances, but on different time scales - with increasing magnetic activity, the SAID structure shows a faster movement towards lower latitudes than that of the mid-latitude trough. From the combined analysis of these results, we conclude that the SAID generation mechanism cannot be regarded either as a pure voltage generator or as a pure current generator, applied to the ionosphere. While the anti-correlation between the width and the peak intensity of the SAID structures with substorm evolution indicates a magnetospheric source acting as a constant voltage generator, the ionospheric modifications and, in particular the reduction in the conductivity for intense SAID structures, are indicative of a constant current system closing through the ionosphere. The ionospheric feedback mechanisms are seen to be of major importance for sustaining and regulating the SAID structures.

Keyword
ionosphere, mid-latitude ionosphere, electric fields and currents, ionosphere magnetosphere interactions
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-14244 (URN)000220762200018 ()2-s2.0-1942472099 (Scopus ID)
Note

QC 20100727

Available from: 2010-07-27 Created: 2010-07-27 Last updated: 2014-12-11Bibliographically approved
2. Temporal and spatial evolution of discrete auroral arcs as seen by Cluster
Open this publication in new window or tab >>Temporal and spatial evolution of discrete auroral arcs as seen by Cluster
Show others...
2005 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 23, no 7, 2531-2557 p.Article in journal (Refereed) Published
Abstract [en]

Two event studies are presented in this paper where intense convergent electric fields, with mapped intensities up to 1350 mV/m, are measured in the auroral upward current region by the Cluster spacecraft, at altitudes between 3 and 5 Earth radii. Both events are from May 2003, Southern Hemisphere, with equatorward crossings by the Cluster spacecraft of the pre-midnight auroral oval. Event 1 occurs during the end of the recovery phase of a strong substorm. A system of auroral arcs associated with convergent electric field structures, with a maximum perpendicular potential drop of about similar to 10 kV, and upflowing field-aligned currents with densities of 3 mu A/m(2) (mapped to the ionosphere), was detected at the boundary between the Plasma Sheet Boundary Layer (PSBL) and the Plasma Sheet (PS). The auroral arc structures evolve in shape and in magnitude on a timescale of tens of minutes, merging, broadening and intensifying, until finally fading away after about 50 min. Throughout this time, both the PS region and the auroral arc structure in its poleward part remain relatively fixed in space, reflecting the rather quiet auroral conditions during the end of the substorm. The auroral upward acceleration region is shown for this event to extend beyond 3.9 Earth radii altitude. Event 2 occurs during a more active period associated with the expansion phase of a moderate substorm. Images from the Defense Meteorological Satellite Program (DMSP) F13 spacecraft show that the Cluster spacecraft crossed the horn region of a surge-type aurora. Conjugated with the Cluster spacecraft crossing above the surge horn, the South Pole All Sky Imager recorded the motion and the temporal evolution of an east-west aligned auroral arc, 30 to 50 km wide. Intense electric field variations are measured by the Cluster spacecraft when crossing above the auroral arc structure, collocated with the density gradient at the PS poleward boundary, and coupled to intense upflowing field-aligned currents with mapped densities of up to 20 mu A/m(2). The surge horn consists of multiple arc structures which later merge into one structure and intensify at the PS poleward boundary. The surge horn and the associated PS region moved poleward with a velocity at the ionospheric level of 0.5 km/s, following the large-scale poleward expansion of the auroral oval associated with the substorm expansion phase.

Keyword
ionosphere, ionosphere-magnetosphere interacctions, electric fields and currents, particle acceleration
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-14245 (URN)000233568900025 ()2-s2.0-27844609325 (Scopus ID)
Note
QC 20100727Available from: 2010-07-27 Created: 2010-07-27 Last updated: 2010-07-27Bibliographically approved
3. Magnetosphere-ionosphere coupling during periods of extended high auroral activity: A case study
Open this publication in new window or tab >>Magnetosphere-ionosphere coupling during periods of extended high auroral activity: A case study
Show others...
2008 (English)In: Annales Geophysicae, ISSN 0992-7689, Vol. 26, 583-591 p.Article in journal (Refereed) Published
Abstract [en]

Results are presented from a case study of a plasma boundary crossing by the Cluster spacecraft during an extended period of high auroral activity. The boundary between the magnetotail lobe region of the Southern Hemisphere and the plasma sheet boundary layer, was characterized by intense electric and magnetic field variations, structured upward accelerated ion beams, narrow-scale large field-aligned Poynting fluxes directed upward away from the ionosphere, and a relatively sharp plasma density gradient. The observations are shown to be consistent with the concept of a multi-layered boundary with temporal and/or spatial variations in the different layers. H+ and O+ ion beams are seen to be accelerated upwards both by means of a field-aligned electric field and by magnetic pumping caused by large-amplitude and low-frequency electric field fluctuations. The peak energy of the ion beams may here be used as a diagnostic tool for the temporal evolution of the spatial structures, since the temporal changes occur on a time-scale shorter than the times-of-flight of the detected ion species. The case study also shows the boundary region to be mainly characterized by a coupling of the detected potential structures to the low ionosphere during the extended period of high auroral activity, as indicated by the intense field-aligned Poynting fluxes directed upward away from the ionosphere.

Keyword
aurora; boundary layer; coupling; electric field; ionosphere; magnetic field; magnetosphere; plasma; Southern Hemisphere; spatial variation; temporal variation
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-14246 (URN)000255364900016 ()2-s2.0-41449084279 (Scopus ID)
Note
QC 20100727Available from: 2010-07-27 Created: 2010-07-27 Last updated: 2010-12-06Bibliographically approved
4. Statistical study on the occurrence of ASAID electric fields
Open this publication in new window or tab >>Statistical study on the occurrence of ASAID electric fields
2010 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 28, no 2, 439-448 p.Article in journal (Refereed) Published
Abstract [en]

The first statistical results on the occurrence of abnormal subauroral ion drifts (ASAID) are presented based on electric and magnetic field measurements from the low-altitude Astrid-2 satellite. ASAID are narrow regions of rapid eastward ion drifts observed in the subauroral ionosphere. They correspond to equatorward-directed electric fields with peak amplitudes seen to vary between 45 mV/m and 185 mV/m, and with latitudinal extensions between 0.2 degrees and 1.2 degrees Corrected Geomagnetic Latitude (CGLat), reaching in some cases up to 3.0 degrees CGLat. Opposite to subauroral ion drifts (SAID) that are known to be substorm-related, ASAID are seen to occur predominantly during extended periods of low substorm activity. Our results show that ASAID are located in the vicinity of the equatorward edge of the auroral oval, mainly in the postmidnight sector between 23:00 and 03:00 magnetic local time. They are associated with a local current system with the same scale-size as the corresponding ASAID, composed by a region of downward field-aligned currents (FACs) flowing in the ASAID poleward side, and a region of upward flowing FACs in the equatorward side. The FACs have densities between 0.5 and 2.0 mu A/m(2). The data suggest that ASAID do not contribute significantly to the reduction of the ionospheric conductivity. ASAID are seen to have life times of at least 3.5 h. A discussion on possible mechanisms for the generation of ASAID is presented. We speculate that the proximity of the electron to the ion plasma sheet inner boundaries and of the plasmapause to the ring current outer edge, during extended quiet times, is an important key for the understanding of the generation of ASAID electric fields.

Keyword
Ionosphere, Electric fields and currents, Ionosphere-magnetosphere interactions, Mid-latitude ionosphere
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-14247 (URN)10.5194/angeo-28-439-2010 (DOI)000274995800012 ()2-s2.0-76749156148 (Scopus ID)
Note
QC 20100727Available from: 2010-07-27 Created: 2010-07-27 Last updated: 2011-01-21Bibliographically approved

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Citation style
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Output format
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