Freja studies of the current‐voltage relation in substorm‐related events

Olsson, A.; Andersson, L.; Eriksson, Anders; Clemmons, J. H.; Erlandsson, R. E.; Reeves, G. D.; Hughes, T. J.; Murphee, J. S.

doi:10.1029/97ja00560

Cited by 42 publications

(32 citation statements)

References 38 publications

(16 reference statements)

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“…In 971103 the determination of the ®eld-aligned current is more reliable and both FAC and potential have a bipolar signature. However, in both these cases the potentials are small (1±2 kV) compared to what we typically found for low altitudes in this study (5±15 kV) or what has been found in earlier studies (Lin and Ho man, 1979;Olsson et al, 1996Olsson et al, , 1998.…”

Section: Discussioncontrasting

confidence: 52%

How does the U-shaped potential close above the acceleration region? A study using Polar data

et al. 1999

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Abstract. We present a statistical study of Polar electric ®eld observations using auroral oval passes over Scandinavia above the acceleration region. We are especially interested in seeing whether we can ®nd large perpendicular electric ®elds associated with an upward extended classical U-shaped potential drop for these passes, during which Polar is in the northern hemisphere usually at about 4 R E altitude. We also use Polar magnetic ®eld data to infer the existence of a ®eld-aligned current (FAC) and conjugate ground-based magnetometers (the IMAGE magnetometer network) to check whether the event is substorm-related or not. We ®nd several events with a FAC but only weak perpendicular electric ®elds at Polar. In those rare cases where the Polar electric ®eld was large, its direction was mostly found to be incompatible with the U-shaped potential model, or it was associated with disturbed conditions (substorms), where one cannot easily distinguish between inductive and static perpendicular electric ®elds. We found only two cases which are compatible with the upward extended U-shaped potential picture, and even in those cases the potential value is quite small (1±2 kV). To check the validity of the analysis method we also estimate the perpendicular electric ®eld on the southern hemisphere, where Polar¯ies within or below the acceleration region, and we found a large number of inverted-V-type signatures as expected from previous studies. To explain the lack of perpendicular electric ®elds at high altitudes we suggest an O-shaped potential model instead of the U-shaped one.

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Section: Discussioncontrasting

confidence: 52%

How does the U-shaped potential close above the acceleration region? A study using Polar data

et al. 1999

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“…It is very common, however, that the pitch angle scattering is incomplete for the electrons. This is seen, e.g., in the Freja study by Olsson et al (1997), where the estimated e ective source plasma densities were much lower than the true plasma density can possibly be.…”

Section: Kappa Distributionmentioning

confidence: 87%

The current-voltage relationship revisited: exact and approximate formulas with almost general validity for hot magnetospheric electrons for bi-Maxwellian and kappa distributions

Janhunen

Olsson

1998

Ann Geophysicae

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Abstract. We derive the current-voltage relationship in the auroral region taking into account magnetospheric electrons for the bi-Maxwellian and kappa source plasma distribution functions. The current-voltage formulas have in principle been well known for a long time, but the kappa energy¯ux formulas have not appeared in the literature before. We give a uni®ed treatment of the bi-Maxwellian and kappa distributions, correcting some errors in previous work. We give both exact results and two kinds of approximate formulas for the current density and the energy¯ux. The ®rst approximation is almost generally valid and is practical to compute. The ®rst approximation formulas are therefore suitable for use in simulations. In the second approximation we assume in addition that the thermal energy is small compared to the potential drop. This yields even simpler linear formulas which are suitable for many types of event studies and which have a more transparent physical interpretation than the ®rst approximation formulas. We also show how it is possible to derive the ®rst approximation formulas even for those distributions for which the exact results can not be computed analytically. The kappa ®eld-aligned conductance value turns out always to be smaller than the corresponding Maxwellian conductance. We also verify that the obtained kappa current density and energy¯ux formulas go to Maxwellian results when j 3 I.

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“…Event 869 is a surge event; event 907 is a quiet arc event. Since these events have been studied in Olsson et al (1997a), we will not discuss the geophysical context any further in the present paper. On the surge head of event 869 the energies were so high that the MATE instrument had to be used.…”

Section: Observationsmentioning

confidence: 99%

Field-aligned conductance values estimated from Maxwellian and kappa distributions in quiet and disturbed events using Freja electron data

Olsson

Janhunen

1998

Ann. Geophys.

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Abstract. We study the question of what dierence it makes for the derived ®eld-aligned conductance (u) values if one uses Maxwellian or kappa distributions for the ®tting of low-orbiting satellite electron¯ux spectra in the auroral region. This question has arisen because sometimes a high-energy tail is seen in the spectra. In principle, the kappa ®ts should always be better, because the kappa distribution is a generalization of the Maxwellian. However, the physical meaning of the parameters appearing in the Maxwellian is clearer. It therefore makes sense to study under which circumstances it is appropriate to use a Maxwellian. We use Freja electron data (TESP and MATE) from two events. One of the events represents quiet magnetospheric conditions (stable arc) and the other represents disturbed conditions (surge). In these Freja events, at least, using kappa rather than Maxwellian ®tting gives a better ®t to the observed distribution, but the dierence in u values is not large (usually less than 20%). The dierence can be of either sign. However, sometimes even the kappa distribution does not provide a good ®t, and one needs a more complicated distribution such as two Maxwellians. We investigate the relative contributions of the two Maxwellians to the total ®eld-aligned conductance value in these cases. We ®nd that the contribution of the highenergy population is insigni®cant (usually much less than 20%). This is because u is proportional to na i p , where n is the source plasma density and i is the characteristic energy.

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Freja studies of the current‐voltage relation in substorm‐related events

Cited by 42 publications

References 38 publications

How does the U-shaped potential close above the acceleration region? A study using Polar data

How does the U-shaped potential close above the acceleration region? A study using Polar data

The current-voltage relationship revisited: exact and approximate formulas with almost general validity for hot magnetospheric electrons for bi-Maxwellian and kappa distributions

Field-aligned conductance values estimated from Maxwellian and kappa distributions in quiet and disturbed events using Freja electron data

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