Harmonics of low-amplitude anisotropic wave trains in cosmic rays

Abstract

The purpose of this work is to investigate the first three harmonics of low-amplitude anisotropic wave trains (LAEs) of cosmic ray intensity and their association with solar and heliospheric parameters. The significant behaviour of these events is that the amplitude remains low for the first harmonic and high for the second/third harmonics, whereas direction of the anisotropy shift is towards earlier hours for the first harmonic and towards later hours for the second/third harmonic compared to annual average anisotropy. The first two harmonics are found to correlate well with the solar activity cycle during these LAEs. The amplitude and the direction of the first two harmonics do not show any significant association with the polarity change of the Bx/By component of the interplanetary magnetic field during LAEs. However, the third harmonic (amplitude and phase) shows some positive correlation with the Bx and negative correlation with the By component. The occurrence of LAEs is dominant for the positive polarity of Bx and the negative polarity of By. The occurrence of LAEs is dominant during the period of average solar wind velocity but their occurrence during high-speed solar wind streams cannot be overlooked. The frequency of occurrence of these LAEs is more during co-rotating streams. The amplitude of first and second harmonic shows deviations for different values of geomagnetic activity index Ap. However, the amplitude of second harmonic and direction of all the three harmonics do not show any significant association with the Ap-index. The Ap-index consistently remains in the range 14⩽Kp⩽31 during these events. The amplitude of first and third harmonic and the direction of first harmonic show deviations for different values of proton density. However, the amplitude of the second harmonic and the direction of the second and third harmonics do not show any significant association with proton density. The occurrence of LAEs is dominant when proton density remains ⩽20. The cosmic ray intensity during LAEs has good anti-correlation with interplanetary magnetic field strength ( B) and its Bx component, whereas it shows a good correlation with its By component. However, it shows significant anti-correlation with sunspot number, the product ( R× V) and ( R× B).