FIELD-ALIGNED CONDUCTANCE VALUES ESTIMATED FROM MAXWELLIAN AND KAPPA-DISTRIBUTIONS IN QUIET AND DISTURBED EVENTS USING FREJA ELECTRON DATA

We study the question of what difference it makes for the derived fiel d-aligned conductance (K) values if one uses Maxwellian or kappa distr ibutions for the fitting of low-orbiting satellite electron flux spect ra in the auroral region. This question has arisen because sometimes a high-energy tail is seen in the spectra. In principle, the kappa fits should always be better, because the kappa distribution is a generali zation of the Maxwellian. However, the physical meaning of the paramet ers appearing in the Maxwellian is clearer. It therefore makes sense t o study under which circumstances it is appropriate to use a Maxwellia n. We use Freja electron data (TESP and MATE) from two events. One of the events represents quiet magnetospheric conditions (stable are) and the other represents disturbed conditions (surge). In these Freja eve nts, at least, using kappa rather than Maxwellian fitting gives a bett er fit to the observed distribution, but the difference in K values is not large (usually less than 20%). The difference can be of either si gn. However, sometimes even the kappa distribution does not provide a good fit, and one needs a more complicated distribution such as two Ma xwellians. We investigate the relative contributions of the two Maxwel lians to the total field-aligned conductance value in these cases. We find that the contribution of the high-energy population is insignific ant (usually much less than 20%). This is because K is proportional to n/ root E-c, where it is the source plasma density and E-c is the cha racteristic energy.