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Observations of the electric field fine structure associated with the westward traveling surge and large-scale auroral spirals
KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.ORCID iD: 0000-0002-1594-1861
KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.ORCID iD: 0000-0003-1270-1616
KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
KTH, Superseded Departments, Alfvén Laboratory. KTH, School of Electrical Engineering (EES), Space and Plasma Physics.ORCID iD: 0000-0001-5617-9765
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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.

Place, publisher, year, edition, pages
1998. Vol. 103, no A3, 4125-4144 p.
National Category
Fusion, Plasma and Space Physics
URN: urn:nbn:se:kth:diva-13712DOI: 10.1029/97JA00558OAI: diva2:326729
QC 20100624 NR 20140804Available from: 2010-06-24 Created: 2010-06-24 Last updated: 2012-03-19Bibliographically approved
In thesis
1. Auroral Electric Fields From Satellite Observations and Numerical Modelling
Open this publication in new window or tab >>Auroral Electric Fields From Satellite Observations and Numerical Modelling
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.
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
urn:nbn:se:kth:diva-3130 (URN)
Public defence
2001-04-30, 00:00 (English)
QC 20100624Available from: 2001-04-25 Created: 2001-04-25 Last updated: 2010-06-24Bibliographically approved

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