On ultralow frequency waves in the lobes of the Earth's magnetotail

Abstract

Motivated by Thermal Catastrophe Model (TCM) of Goertz and Smith (1989), we turn attention to ultralow frequency (ULF) waves in the lobes, where waves are usually of relatively low amplitude. Remarkably little attention has been paid to characterizing the lobe spectra and the relation of lobe wave power to geophysical processes. In this paper we examine ULF waves (0.26 to 5 mHz or 3.2‐ to 64‐min periods) in the Earth's magneto tail lobe region (−10 ≲ XGSM ≲ −23 RE) using data from the ISEE 2 flux gate magnetometer. The studies used all the tail data from the year 1978. Data from the ISEE 2 Fast Plasma Experiment (FPE) enabled us to exclude from the analysis all intervals not in the tail lobe region. The relation between the substorm activity, characterized by the AE index, and the wave power is also described. We find that the wave amplitudes at periods of ∼3 to 16 min (∼1 to 5 mHz) are below 0.8 nT for the transverse magnetic field components and below 0.6 nT for the compressional components ∼90% of the time. The amplitudes of lower frequency waves are somewhat larger. For waves with periods 16 to 64 min (∼0.26 to 1 mHz) the amplitudes are below 1 nT for both the compressional and transverse components ∼90% of the time. The wave amplitudes and the AE index, which characterizes substorm activity, are weakly correlated, the correlation coefficients being ∼0.55 and ∼0.35, for the compressional and transverse components respectively. The correlation coefficients between the transverse wave amplitudes and the AE index change little when AE is taken for 1 hour prior to or subsequent to the hour in which the wave amplitude is measured. For the compressional component the correlation coefficient remains unchanged when AE is measured for the subsequent or simultaneous hour but decreases when AE is taken for the previous hour. A primarily statistical study of frequency‐weighted power of ULF waves at frequencies of 1 to 5 mHz is provided. The probability is 50% (5%) of observing frequency‐weighted power for the frequency range 1 to 5 mHz in excess of 10−4 nT² Hz (10−3 nT² Hz). Three extended intervals in the lobe were selected for case studies. The largest wave amplitude we observed was about 5 nT which occurred when the AE index was higher than 1500 nT. The period corresponding to the largest wave amplitudes was ∼35 min (∼0.48 mHz). Power spectra from three different substorm intervals are provided as examples of wave activity in the lobes. The power spectral density of all components falls off as ƒ−2 or faster. Statistical study by itself does not provide an appropriate test of the requirement on wave power for the TCM. Additional case studies using ground magnetometers to relate wave power to substorm phases will be needed to test the assumptions of the TCM.