Separating spatial and temporal variations in auroral electric and magnetic fields by Cluster multipoint measurements
2004 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 22, no 7, 2463-2472 p.Article in journal (Refereed) Published
Cluster multipoint measurements of the electric and magnetic fields from a crossing of auroral field lines at an attitude of 4 R-E are used to show that it is possible to resolve the ambiguity of temporal versus spatial variations in the fields. We show that the largest electric fields (of the order of 300 mV/m when mapped down to the ionosphere) are of a quasi-static nature, unipolar, associated with upward electron beams, stable on a time scale of at least half a minute, and located in two regions of downward current. We conclude that they are the high-altitude analogues of the intense return current/black auroral electric field structures observed at lower altitudes by Freja and FAST. In between these structures there are temporal fluctuations, which are shown to likely be downward travelling Alfven waves. The periods of these waves are 20-40 s, which is not consistent with periods associated with either the Alfvenic ionospheric resonator, typical field line resonances or substorm onset related Pi2 oscillations. The multipoint measurements enable us to estimate a lower limit to the perpendicular wavelength of the Alfven waves to be of the order of 120 kin, which suggests that the perpendicular wavelength is similar to the dimension of the region between the two quasi-static structures. This might indicate that the Alfven waves are ducted within a wave guide, where the quasi-static structures are associated with the gradients making up this waveguide.
Place, publisher, year, edition, pages
2004. Vol. 22, no 7, 2463-2472 p.
magnetospheric physics, auroral phenomena, electric fields, plasma waves and instabilities, particle-acceleration, region, beams, arc, efw
IdentifiersURN: urn:nbn:se:kth:diva-23691ISI: 000223620300018ScopusID: 2-s2.0-4344648106OAI: oai:DiVA.org:kth-23691DiVA: diva2:342390
QC 20100525 QC 201110282010-08-102010-08-102011-10-28Bibliographically approved