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Auroral Electric Fields From Satellite Observations and Numerical Modelling
KTH, Superseded Departments, Alfvén Laboratory.ORCID iD: 0000-0003-1270-1616
2001 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is about electrodynamics of thehigh-latitude/auroral region of near-Earth space. The work ismainly based on electric field measurements made by the doubleprobe instrument on the Freja satellite at altitudes ofapproximately 800-1700 km, together with measurements fromother instruments on the same satellite, and ground-basedinstrumentation. A useful tool for interpreting observationaldata is also numerical modelling, and this is the subject ofpart of the work. The electric field measurements address threesubjects. The first one is that of very intense, divergingelectric fields at Freja altitude. Statistics and case studiesshow that these structures have scale-sizes of the order of 1km, and are associated with regions devoid of electronprecipitation (which at times can be identified with theoptical phenomenon of black aurora), with downward currents,ion heating, density depletions, and upward acceleratedelectron beams. A numerical model is used to study theionospheric response to intense small-scale current systems. Itis shown that on time scales of the order of 1 minute, deepdensity cavities and enhancements of the electric fieldresults. The second subject is the electrodynamics of thewestward traveling surge (WTS) and other large-scale auroralspirals. Freja measurements show that the electric field isdirected towards the surge/spiral center, and that the surgehead is associated with extremely intense, small-scale,converging electric fields, and field-aligned currents. Datasuggest that a significant part of the upward current of thesurge head is closed by localized, downward-directed currents,whereas no clear signature of a Cowling channel feeding thecurrent to the surge head from distributed downward currents inthe wake of the surge is observed. The third type ofmeasurements concerns the phenomenon of subauroral ion drifts(SAID), or equivalently subauroral electric fields (SAEF). Acomprehensive statistics shows the distribution in local timeand latitude of the SAID, and their correlation withgeomagnetic activity. It is concluded that the SAID areassociated with the low-conductivity region of the mid-latitudetrough, and that they may be a consequence of closure ofsubstorm-related currents through this region, which may becomeeven more low-conducting by the ionospheric response to theapplied current and electric field. Finally a new formulationof a classification scheme of auroral arc-associated electricfields is given. It is shown that this generalization enables aflexible way of modelling and predicting the arc-associatedfield in various situations.

Keywords: Satellite measurements, electric fields,ionosphere, magnetosphere, diverging electric fields, downwardcurrent region, black aurora, westward traveling surge, auroralbulge, subauroral ion drifts (SAID), subauroral electric fields(SAEF), mid-latitude trough, numerical modelling, auroral arcclassification, auroral arc-associated electric field.

Place, publisher, year, edition, pages
Stockholm: KTH , 2001. , 35 p.
Keyword [en]
Satellite measurements, electric fields, ionosphere, magnetosphere, diverging electric fields, downward current region, black aurora, westward travelling surge, auroral bulge, subauroral ion drifts (SAID), subauroral electric fields (SAEF), mid-latit
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-3130OAI: oai:DiVA.org:kth-3130DiVA: diva2:8894
Public defence
2001-04-30, 00:00 (English)
Note
QC 20100624Available from: 2001-04-25 Created: 2001-04-25 Last updated: 2010-06-24Bibliographically approved
List of papers
1. A statistical study of intense low-altitude electric fields observed by Freja
Open this publication in new window or tab >>A statistical study of intense low-altitude electric fields observed by Freja
1996 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 23, 1005-1008 p.Article in journal (Refereed) Published
Abstract [en]

Extremely intense (up to 2 V/m) and fine-structured (1–5 km) electric fields have been observed by the Freja satellite at altitudes of 1400–1770 km. To study the occurrence and characteristics of these intense electric field events, a database was set up by searching 7 months' worth of Freja data for events with peak values greater than 200 mV/m. The intense electric field events are distributed over all local times, but they are mostly concentrated to the midnight and early morning sector of the auroral oval. The events are seen to be associated with low ambient ionospheric conductivities, but are not activity dependent. The most intense events (located in the midnight sector) are associated with the smallest scale sizes, whereas the less intense events on the day side have larger scale sizes.

The findings are consistent with the interpretation that a majority of the intense electric field events observed by Freja are associated with the small-scale, east-west-aligned, low-conductivity bands devoid of auroral emissions, known as black auroral bands, or with the curls that may develop when these bands go unstable. Several observations of diverging electric fields of around 1 V/m at 800 km altitude, within the southern auroral oval, may serve as support for the prediction that intense electric fields should exist also at ionospheric altitudes.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 1996
National Category
Engineering and Technology Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-13710 (URN)10.1029/96GL00773 (DOI)A1996UK81400026 ()2-s2.0-0030141801 (Scopus ID)
Note

QC 20100624. QC 20160212

Available from: 2010-06-24 Created: 2010-06-24 Last updated: 2017-12-12Bibliographically approved
2. On low-altitude particle acceleration and intense electric fields and their relationship to black aurora
Open this publication in new window or tab >>On low-altitude particle acceleration and intense electric fields and their relationship to black aurora
1997 (English)In: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, ISSN 0148-0227, Vol. 102, no A8, 17509-17522 p.Article in journal (Refereed) Published
Abstract [en]

Recent findings by the Freja satellite have shown the existence of extremely intense (1-2 V/m) and small-scale (1 km) diverging electric fields which are interpreted to be associated with east-west aligned dark striations or black auroral curls. Precipitating or transversely energized ions, downward field-aligned currents carried by upward fluxes of ionospheric electrons and dropouts of energetic electron precipitation, are found to be characteristic features of such events. A comparison of these characteristics to those of the aurora point at a symmetry between the aurora and the black aurora, the aurora being associated with negative divergence of the electric field and the black aurora with positive divergence. The diverging field events typically occur during winter conditions within the midnight to early morning sector of the auroral oval. Estimates of the ambient conductivity due to solar EUV radiation for each of these events show a clear anticorrelation with the electric field magnitude. The black auroral structures are likely to be associated with localized ionospheric density depletions below that of the ambient density and caused by the upward flow of ionospheric electrons. The efficiency by which such density holes are created in regions of downward field-aligned current flow have recently been demonstrated in model studies. The electric field magnitudes are found to decrease with the scale size, not inversely as suggested in recent theoretical work but with a power law exponent of 0.6-0.8. At lower altitudes (around 800 km) the maximum intensities for a majority of the events are in the range of values that have been reported from rocket and radar measurements in the ionosphere, i.e., around 150-200 mV/m. However, close to magnetic midnight and during winter conditions small-scale diverging electric fields of 1 V/m are occasionally found to exist down to at least 800 km. We suggest that the diverging electric fields observed by Freja are associated with low-altitude and narrow ( approximate to 1-2 km) potential structures similar to the auroral potential structures at higher altitude but associated with a positive space charge and a downward parallel electric field. This is supported by Freja observations of narrow upward beams of 2 keV electrons in good agreement with a 2 kV positive peak in the electrostatic potential for a black aurora event. The existence of a downward parallel electric field at low altitudes is also supported by low-altitude observations by the S3-3 and Viking satellites. If such low-altitude potential structures do exist as our results suggest, an outstanding problem for future investigation is how they may be formed and maintained.

Keyword
FREJA
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-13711 (URN)A1997XP07100027 ()
Note
QC 20100624Available from: 2010-06-24 Created: 2010-06-24 Last updated: 2010-06-24Bibliographically approved
3. Observations of the electric field fine structure associated with the westward traveling surge and large-scale auroral spirals
Open this publication in new window or tab >>Observations of the electric field fine structure associated with the westward traveling surge and large-scale auroral spirals
Show others...
1998 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 103, no A3, 4125-4144 p.Article in journal (Refereed) Published
Abstract [en]

The characteristics of the fine scale electric field associated with the westward traveling surge and large-scale auroral spirals and surges are investigated using high-resolution electric field, magnetic field, particle and UV imager observations from four eveningside auroral oval crossings by the Freja satellite. Three of the crossings were associated with signatures of auroral substorms and one crossing went directly through the head of a surge close in time and space to substorm onset. Three passes were adjacent to auroral spiral formations, one poleward of and one equatorward of such forms and one through the multiple arc region near the front of an extended region of auroral activity. The ambient electric field was found to intensify in the direction toward the spiral head (or the center of the auroral activity region) over a region comparable to the size of the visible auroral forms. These results confirm previous findings that the spiral or surge head is associated with negative space charge and an intense upward field-aligned current. The fourth pass, directly through the surge head reveals a very complicated structure of the surge region. Narrowly structured, intense (up to 700 mV/m) and mostly converging electric fields associated with intense electron precipitation (of both high and medium energies) and balanced field-aligned currents (up to 30 μA/m2) are seen near the edge of the surge head and adjacent to auroral structures in the wake. These narrow regions are embedded within more extended regions of intense high-energy electron precipitation but very weak electric fields and field-aligned currents. According to some existing models of the surge, a pronounced westward electric field component and a southward polarisation electric field is expected within the entire high-conductivity region but evidence in support of this was not found in the data. Rather, these suggest that a significant part of the upward surge current is closed by distributed downward field-aligned currents from the near surroundings. The Freja electric field is typically seen to intensify at the edges of or in-between bright auroral structures and to decrease within the arcs similar to what is observed in the ionosphere. The surge electric field is, however, much more intense than previously observed or anticipated at these altitudes with characteristics rather similar to those observed in the auroral acceleration region. Since the particle data indicate that most of the acceleration takes place above Freja altitudes, it seems as if Freja traversed the lower part of the auroral acceleration region associated with the surge.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-13712 (URN)10.1029/97JA00558 (DOI)
Note
QC 20100624 NR 20140804Available from: 2010-06-24 Created: 2010-06-24 Last updated: 2017-12-12Bibliographically approved
4. Subauroral electric fields observed by the Freja satellite: A statistical study
Open this publication in new window or tab >>Subauroral electric fields observed by the Freja satellite: A statistical study
1998 (English)In: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, ISSN 0148-0233, Vol. 103, no A3, 4327-4341 p.Article in journal (Refereed) Published
Abstract [en]

Over 12 months of Freja electric field data have been scanned for subauroral electric fields (SAEF) to enable a comprehensive study of the ionospheric signatures of such electric fields. SAEF are encountered from 1800 to 0200 MLT in agreement with an earlier study. However, a large majority of the SAEF are encountered at a time slightly premidnight (2200 - 2300 MLT), with rather few occurrences before 2000 MLT and after 2400 MLT. Furthermore, the strength of the subauroral electric field is generally much larger for events close to 2200 MLT than for other events. The data confirm that SAEF occur during the substorm recovery phase but also show that SAEF occur earlier during recovery when located close to 2200 MLT than at other local times. The dependence on season and geomagnetic activity is studied, and it is found that the SAEF are more commonly observed at times of high activity when the subauroral electric fields are also generally stronger, except close to winter solstice, when strong electric fields are observed during low activity. The potentials associated with the SAEF and the relation to interplanetary magnetic field B-y are also studied. The observations are discussed in context with substorm-related field-aligned currents and the midlatitude trough, and we present evidence that support and refine one of the proposed production mechanisms.

Keyword
AURORAL ION PRECIPITATION, ALIGNED CURRENTS, F-REGION, CONVECTION, MODEL, IONOSPHERE, LATITUDES, BOUNDARY, DRIFTS, TROUGH
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-13713 (URN)000072401200032 ()
Note
QC 20100624 NR 20140804Available from: 2010-06-24 Created: 2010-06-24 Last updated: 2012-03-19Bibliographically approved
5. Simulations of effects of small-scale auroral current closure in the return current region
Open this publication in new window or tab >>Simulations of effects of small-scale auroral current closure in the return current region
1998 (English)In: Physics of space plasmas, Vol. 15, 401- p.Article in journal (Other academic) Published
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-13714 (URN)
Note

QC 20100624

Available from: 2010-06-24 Created: 2010-06-24 Last updated: 2014-04-17Bibliographically approved
6. On electric field patterns associated with night-side discrete auroral arcs: A generalization of an auroral arc classification scheme
Open this publication in new window or tab >>On electric field patterns associated with night-side discrete auroral arcs: A generalization of an auroral arc classification scheme
2001 (English)Report (Other academic)
Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2001. 8 p.
Series
Trita-ALP, ISSN 1103-6613 ; 2001:01
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-13715 (URN)
Note
QC 20100624Available from: 2010-06-24 Created: 2010-06-24 Last updated: 2012-06-04Bibliographically approved

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  • apa
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  • modern-language-association-8th-edition
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Output format
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