Experimental determination of the dominant wave mode in the active near‐Earth magnetotail

Observations of the electric field at frequencies from 2‐128 Hz, using the burst mode of the spherical double probe on ISEE‐1, have been examined for a time period previously identified as containing the passage of a near‐earth neutral line past the satellite. Most of the power occurred at frequencies less than 20 to 50 Hz. Intense waves (3‐>30 mV/m) at approximately half the lower hybrid frequency, were observed throughout the plasma sheet from the neutral sheet to the high latitude boundary, but only during the period of the large dc electric field and E×B velocity associated with the substorm neutral line. The wavenumber obtained by including a Doppler shift term in linear fits of the frequency of the peaks of the power spectra was comparable to the inverse electron gyroradius (k ρ_e<1). These results are consistent with the lower hybrid drift instability and inconsistent with ion beam driven instabilities (which may explain the much weaker waves observed at higher frequencies). The largest amplitude waves occurred at the neutral sheet when the southward component of the magnetic field was 6 gammas, in contrast to theoretical work which has suggested that the instability would be suppressed there. The observed waves could provide an anomalous resistivity of ∼3×10⁻⁷ to 1×10⁻⁴ s (compared to the classical value of 1×10⁻¹⁸ s).