225 research outputs found
Electrodynamics of an omega-band as deduced from optical and magnetometer data
We investigate an omega-band event that took place above northern Scandinavia
around 02:00–02:30 UT on 9 March 1999. In our analysis we use ground based
magnetometer, optical and riometer measurements together with satellite based
optical images. The optical and riometer data are used to estimate the
ionospheric Hall and Pedersen conductances, while ionospheric equivalent
currents are obtained from the magnetometer measurements. These data sets are
used as input in a local KRM calculation, which gives the ionospheric
potential electric field as output, thus giving us a complete picture of the
ionospheric electrodynamic state during the omega-band event.
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The overall structure of the electric field and field-aligned current (FAC)
provided by the local KRM method are in good agreement with previous studies.
Also the <I><B>E</B></I>×<I><B>B</B></I> drift velocity calculated from the local
KRM solution is in good qualitative agreement with the plasma velocity
measured by the Finnish CUTLASS radar, giving further support for the new
local KRM method. The high-resolution conductance estimates allow us to
discern the detailed structure of the omega-band current system. The highest
Hall and Pedersen conductances, ~50 and ~25 S, respectively, are
found at the edges of the bright auroral tongue. Inside the tongue,
conductances are somewhat smaller, but still significantly higher than
typical background values. The electric field shows a converging pattern
around the tongues, and the field strength drops from ~40 mV/m found at
optically dark regions to ~10 mV/m inside the areas of enhanced
conductivity. Downward FAC flow in the dark regions, while upward currents
flow inside the auroral tongue. Additionally, sharp conductance gradients at
the edge of an auroral tongue are associated with narrow strips of intense
FACs, so that a strip of downward current flows at the eastern (leading) edge
and a similar strip of upward current is present at the western (trailing)
edge. The Joule heating follows the electric field pattern, so that it is
diminished inside the bright auroral tongue
Spatio-temporal development of large-scale auroral electrojet currents relative to substorm onsets
During auroral substorms, the electric currents flowing in the ionosphere change rapidly, and a large amount of energy is dissipated in the auroral ionosphere. An important part of the auroral current system is the auroral electrojets whose profiles can be estimated from magnetic field measurements from low-earth orbit satellites. In this paper, we combine electrojet data derived from the Swarm satellite mission of the European Space Agency with the substorm database derived from the SuperMAG ground magnetometer network data. We organize the electrojet data in relation to the location and time of the onset and obtain statistics for the development of the integrated current and latitudinal location for the auroral electrojets relative to the onset. The major features of the behaviour of the westward electrojet are found to be in accordance with earlier studies of field-aligned currents and ground magnetometer observations of substorm temporal statistics. In addition, we show that, after the onset, the latitudinal location of the maximum of the westward electrojet determined from Swarm satellite data is mostly located close to the SuperMAG onset latitude in the local time sector of the onset regardless of where the onset happens. We also show that the SuperMAG onset corresponds to a strengthening of the order of 100 kA in the amplitude of the median of the westward integrated current in the Swarm data from 15 min before to 15 min after the onset.Peer reviewe
Volume cross section of auroral radar backscatter and RMS plasma fluctuations inferred from coherent and incoherent scatter data: a response on backscatter volume parameters
Norway and Finland STARE radar measurements in the eastward auroral
electrojet are combined with EISCAT CP-1 measurements of the electron
density and electric field vector in the common scattering volume to
investigate the variation of the auroral radar volume cross section (VCS)
with the flow angle of observations (radar look direction with respect to
the <I><B>E</B></I>×<I><B>B</I></B> electron drift). The data set available consists of ~6000 points
for flow angles of 40–85° and electron drifts between 500
and 2000 m s<sup>−1</sup>. The EISCAT electron density <I>N(h)</I>-profile data are used
to estimate the effective electron density, aspect angle and thickness of
the backscattering layer. It is shown that the flow angle variation of the
VCS is rather weak, only ~5 dB within the range of the considered
flow angles. The VCS values themselves respond almost linearly to the square
of both the electron drift velocity magnitude and the effective electron
density. By adopting the inferred shape of the VCS variation with the flow
angle and the VCS dependence upon wavelength, the relative amplitude of
electrostatic electron density fluctuations over all scales is estimated.
Inferred values of 2–4 percent react nearly linearly to the electron drift
velocity in the range of 500–1000 m s<sup>−1</sup> but the rate of increase slows
down at electron drifts >1000 m s<sup>−1</sup> and density fluctuations of ~5.5
percent due to, perhaps, progressively growing nonlinear wave losses
Auroral electrojets during deep solar minimum at the end of solar cycle 23
We investigate the auroral electrojet activity during the deep minimum at the end of solar cycle 23 (2008–2009) by comparing data from the IMAGE magnetometer chain, auroral observations in Fennoscandia and Svalbard, and solar wind and interplanetary magnetic field (IMF) observations from the OMNI database from that period with those recorded one solar cycle earlier. We examine the eastward and westward electrojets and the midnight sector separately. The electrojets during 2008–2009 were found to be weaker and at more poleward latitudes than during other times, but when similar driving solar wind and IMF conditions are compared, the behavior in the morning and evening sectors during 2008–2009 was similar to other periods. On the other hand, the midnight sector shows distinct behavior during 2008–2009: for similar driving conditions, the electrojets resided at further poleward latitudes and on average were weaker than during other periods. Furthermore, the substorm occurrence frequency seemed to saturate to a minimum level for very low levels of driving during 2009. This analysis suggests that the solar wind coupling to the ionosphere during 2008–2009 was similar to other periods but that the magnetosphere-ionosphere coupling has features that are unique to this period of very low solar activity.Peer reviewe
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