A comparison of auroral currents measured by the Chatanika Radar With 50-MHz backscatter observed from Anchorage

Abstract

Auroral electrojet parameters measured by the Chatanika incoherent scatter radar have been compared with VHF backscatter observed in a comparable spatial area by the 50‐MHz auroral radar located at Anchorage. We find that the D region absorption, occurring in concert with the morning (westward) electrojet, can significantly decrease the observed scatter amplitude. If the amplitude is corrected for absorption effects, we find that the scatter varies approximately linearly with either eastward or westward current density with the same slope for both periods. A surprising result is that at times, relatively large north‐south current densities do not give rise to detectable backscatter. The Chatanika electric field measurements indicate that during the morning period, substantial backscatter amplitudes occur for southward fields of only 10 mV/m. In contrast, during the evening period the backscatter amplitude is zero or very low until the northward field is at least 25 mV/m. This indicates that most of the auroral backscatter that we observe in the evening period occurs under conditions when the two‐stream plasma instability may be operative. However, in the morning period the backscattering irregularities are strongly generated under conditions associated with either gradient drift or two‐stream instabilities.

Joint two-dimensional observations of ground magnetic and ionospheric electric fields associated with auroral zone currents. 2. Three-dimensional current flow in the morning sector during substorm recovery.

“…Strong experimental evidence for this relationship has been found recently by ECKLUND et al (1977) andCAHILL et al (1978). The only serious problem in observing auroral zone electric fields with this type of radar is that a threshold electric field apparently is needed to excite the plasma irregularities in the ionosphere (TSUNODA and PRESNELL, 1976;SIREN et al, 1977). CAHILL et al (1978) found threshold values of 15-20mVm-1 for the STARE radars.…”

“…GREENWALD (1979) has shown that in the auroral E layer the net drift velocity of the irregularities is nearly a pure EXB drift and that, therefore, the transverse ionospheric electric field vector is orthogonal and proportional to the measured drift velocity vector. Strong experimental evidence for this relationship has been found recently by ECKLUND et al (1977) andCAHILL et al (1978). The only serious problem in observing auroral zone electric fields with this type of radar is that a threshold electric field apparently is needed to excite the plasma irregularities in the ionosphere (TSUNODA and PRESNELL, 1976;SIREN et al, 1977).…”

Propagation of plasma wave energy in the auroral E region

“…Since these waves can grow to large amplitudes without saturating [Sudan et al, 1973], the growth times can be correspondingly large; numerical modeling for the equatorial electrojet [McDonald et al, 1974[McDonald et al, , 1975 suggest times of more than 1 s. From the times given in Table 2, it appears that if these waves are confined to the lower E region (as might be expected in the premidnight period) they may not reach saturation (or even the nonlinear regime) before encountering conditions of marginal stability, whereas the longer times available in the upper E region (a growth region in the post midnight period if the E region peak is low) would permit larger wave amplitudes. This might explain why it was in the morning sector that Siren et al [1977] observed auroral echoes coincident with electric fields of only 10 mV/m (which implies the involvement of the gradient-drift instability).…”

Recent advances in the study of electric and magnetic fields in the ionosphere and magnetosphere.

“…SIREN et al (1977) have compared the auroral electrojet parameters measured at Chatanika with the 50MHz auroral radar data from the backscatter radar located Note the large amplitude D-and Z-component oscillations. at Anchorage.…”