Transcript MPE Science Highlight

Energy Conversion in the Auroral Magnetosphere
The energy dissipated in the auroral ionosphere is believed to come from generator regions located far away in the
magnetosphere. Several studies addressed the auroral generator by using analytical (e.g. Rostoker and Boström,
1976), semi-analytical (e.g. Lysak, 1985; Vogt et al., 1999), and numerical (e.g. Birn et al., 1996; Birn and Hesse,
1996) tools. To our knowledge, however, the experimental investigation of the generator region is missing, as far as
the evaluation of the power density, E•J, is concerned. The reason might be that even for a strong aurora the energy
flux into the ionosphere is consistent with quite small power densities in the generator region. For nightside auroras
(which are stronger) the electric field and current density close to the tail midplane (a reasonable location of the
generator) are E≈1mV/m and J≈1nA/m2, close to the instrumental error margin.
The examination of conjugated nightside data from the CLUSTER fleet (at an altitude of ~20RE) and FAST (at
~4000km) provides a good opportunity for the investigation of the auroral generator (Hamrin et al., 2004). Because
of its multispacecraft character, CLUSTER makes possible the complete evaluation of J. In addition, each spacecraft
is equipped with 3 instruments able to measure the electric field (EFW, EDI, and CIS), which improves on the
reliability of the E estimates (Marghitu et al., 2004). At the same time FAST offers an ‘instantaneous’ view over the
auroral electron precipitation and energy flux into the ionosphere.
The figure presents an example of conjugated data from CLUSTER and FAST. A remarkable feature is the negative
tendency of E•J (in the two bottom CLUSTER panels), at 02:20 – 02:30. At about the same time FAST measures
energetic inverted-V electrons. Although the error of E•J is probably substantial, its negative tendency is presumably
not an artefact: As seen in the ion spectrograms, the negative E•J shows up close to the boundary between the plasma
sheet and the lobe (the plasma sheet boundary layer), a region where generator processes are expected to occur. The
association (within the error due to the mapping along the magnetic field line) of the negative E•J with an energetic
inverted-V at low altitude renders further support to the interpretation that this signature is real.
CLUSTER
FAST
(a)
(b)
(c)
Left: CLUSTER data. Ion energy spectrograms
(a, b, c); average magnetic field (d); electric
field (blue) and current density (green)
components in a magnetic field aligned
reference system (x along B) close to GSE (e, f,
g); contributions to E•J from the y and z
directions (h, i); E•J (j) and its cumulative sum
along CLUSTER path (k).
Right: FAST data. Electron energy (a) and pitchangle (b) spectrograms; electron energy flux into
the ionosphere (c). The conjunction time is
indicated in both plots with vertical lines.
References:
• Birn, J., et al., J. Geophys. Res., 101, 12939, 1996
• Birn., J., and M. Hesse, J. Geophys. Res. 101, 15345, 1996.
• Hamrin, M., et al., Energy transfer in the auroral
magnetosphere as derived from CLUSTER and FAST data,
1st EGU General Assembly, Nice, 2004.
• Lysak, R.L., J. Geophys. Res., 90, 4178, 1985.
• Marghitu, O., et al., CLUSTER electric field measurements
in the magnetotail, 1st EGU General Assembly, Nice, 2004.
• Rostoker, G., and R. Boström, J. Geophys. Res. 81, 235,
1976.
• Vogt, J., et al., J. Geophys. Res. 104, 269, 1999.
O. Marghitu, M. Hamrin, B. Klecker, M. André, J.P. McFadden, H. Vaith