|
|
 |
|
|
Ann. Geophys., 22, 2463-2472, 2004
www.ann-geophys.net/22/2463/2004/
© European Geosciences Union 2004
Separating spatial and temporal variations in auroral electric and magnetic fields by Cluster multipoint measurements
T. Karlsson1, G. T. Marklund1, S. Figueiredo1, T. Johansson1, and S. Buchert2
1Department of Plasma Physics, Alfvén Laboratory, KTH, SE-100 44 Stockholm, Sweden
2Swedish Institute of Space Physics , Box 537, SE-751 21 Uppsala, Sweden
Abstract. Cluster multipoint measurements of the electric and magnetic fields from a
crossing of auroral field lines at an altitude of 4RE 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 300mV/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
Alfvén waves. The periods of these waves are 20-40s, which is not
consistent with periods associated with either the Alfvénic 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 Alfvén waves to be of the
order of 120km, 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 Alfvén waves are ducted within a wave guide,
where the quasi-static structures are associated with the gradients making up
this waveguide.
Full Article in PDF (2278 KB) |
|
|
