Reply [to “Comment on ‘On double current layers in the polar cusp’ by A. Bahnsen, N. D'Angelo, and A. Mencke Hansen”

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Bahnsen et al. [1975] have suggested that the double current the observations. However, in this context we feel that the layers reported by Fredricks et al. [1•73] are associated with specific form of the Kelvin-Helmholtz instability discussed the Kelvin-Helmholtz instability driven by velocity shears in by Bahnsen et al. [1975] cannot be tised to explain the Ogo 5 the polar cusp. This type of mechanism certainly can operate observations of large amplitude magnetic perturbations for in t,he polar cusp, given the proper conditions, but we do not the high-• (• = •e + /• • 10-•'-10 -x) conditions that were agree with the suggestion of Bahnsen et al. [1975] that this prevalent during the November 1, 1968, cusp encounters. mechanism explains the observat!ons f Fredricks et al. Bahnsen et al. [1975] discussed a specific orm of the Kelvin[1973]. There are several reas of disagreement wi h the model Helmholtz instability and related laboratory experiments carinvolving the number of field-aligned current encounters, the ried out by D'Angelo and yon Goeler [1966]; the experiments nature of the 'waves' ir• the cusp, and the inferred stability of were designed to test a theory developed by D'Angelo [1965]. the cusp to Kelvin-Helmholtz waves at the time of the Ogo 5 However, as D'Angelo and von Goeler pointed out, this theobservations. We examine these points below. ory was only appropriate for a shear that was sufficiently The first study of the ULF and VLF waves in the polar cusp smooth so that the WKB method could be used. Moreover, showed intense magnetic ULF waves and VLF electrostatic the theoretical treatment neglected ion viscosity, Landau emissions [Russell et al., 1971]. Two types of ULF power spectra were observed: one with power inversely proportional to the square of tha frequency both above and below the ion gyrofrequency arid one with a cutoff at the ion gyrofrequency. Subsequent examination of the high-resolution magnetic records revealed that the occurrences of the former spectra were damping, and large • effects, and all of these conditions were not really fulfilled in the laboratory expe.ri•ents of D'Angelo and von Goeler. In fact, Smith and yon Go1⁄2ler [1968] demonstrated that the simpli,fied fluid treatment of D'Angelo needs considerable modificati9n to account for the complel;e plasma effects, and they noted that in the realistic case the maximum associated with irregular purely transverse disturbances in the growth rate mode is inaccessible to an approximate heoretical magnetic field, consistent with either st•.iated or impulsive treatment. In addition, Dobrowolny [1972] showed that the • field-aligned currents, and that occurrences of the latter spectra were associated with the occurrence of ion cyclotron. waves [Scarlet al., 1972; Fredricks et al., 1973; Fredricks and Russell, 1973; Kivelson et al., 1973; Russell et al., 1974]. Of importance to the present discussion is the fact that ma. ny of these sheets of striated or impulsive currents were seen per cusp encounter. For example, during the first cusp crossing on November 1, 1968, Ogo 5 encountered more than 12 such current bursts (see Table 1 of Fredricks et al, [1973]). While Imp 5 plasma measurements [Frank, 1971] and Heos 2 plasma measurements [Paschmann et al., 1974] reveal cusp-aligned flows, the velocity shear layer should occur at the cusp boundary and not repeatedly throughout he cusp. Even the remarkable simultaneous appearance of both upward and downward flowing cusp plasma observed on Heos 2 was detected only once per cusp encounter upon entry to the cusp. Thus simply the number of current encounters observed by Ogo 5 tends to argue against the validity of any velocity shear model as an explanation of the Ogo 5 data. Nevertheless, since the cusp boundary is a nonuniform plasma region, it is certainly of interest to consider if any generalized kinds of Kelvin-Helmholtz instabilities can explain