Quasi-biennial and quasi-triennial oscillations in geomagnetic activity indices

R P Kane

doi:10.1007/s00585-997-1581-1

Abstract

Abstract. Data for geomagnetic activity index aa for 1868–1994 were subjected to spectral analysis for 12 intervals each of 11 consecutive years. In each interval, QBO and QTO (quasi-biennial and quasi-triennial oscillations) were observed at ~2.00, 2.15, 2.40, 2.70 y and ~3.20, 3.40 y, but not all in all intervals. These fluctuations are absent near (2–3 y before and after) the sunspot minima and are present only as 2 or 3 peaks in aa indices, one near or before the sunspot maximum and the other (one or two, generally the larger ones) in the declining phase of the sunspot cycle. Comparison with the solar wind (1965 onwards) showed a fairly good match, indicating that the aa variations were mostly due to similar variations in the solar wind, which must have their origin in solar physical processes. A few aa variations did not match with solar wind. When compared with terrestrial phenomena, no match was found with stratospheric low-latitude zonal wind QBO; but some QTO in aa matched QTO in ENSO (El Nino/Southern Oscillation). This may or may not be a chance coincidence and needs further exploration.

Highlights

After the initial work of Bartels (1963) discussing the time variation of geomagnetic activity indices Kp and Ap for 1932±1961, Fraser-Smith (1972) made an FFT analysis of 38 y (1932±1969) of Ap data and obtained very promonent peaks at 35.6, 16.1, 10.2, 0.5 y, prominent peaks at 7.04, 5.14, 4.10, 1.47, 1.09 y and 27.2, 27.6 days, moderate peaks at 54.0, 37.4, 30.5, 26.9, 18.7, 14.1, 13.7, 13.6, 9.39 days and very weak peaks for 2±9 days
Two mechanisms are proposed as possible explanations viz., axial mechanism related to the heliolatitude excursion of the Earth (7X2 to À7X2) and the other to the orientation of the Earth's magnetic dipole relative to the Earth-Sun line (Gonzalez et al, 1993 and references therein)
The detailed analysis of Gonzalez et al (1993) indicates multiple origins for these variations and, since the interest of the present study is in QBO and QTO, the seasonal variations are eliminated by calculating the 12monthly running means

Summary

Introduction

After the initial work of Bartels (1963) discussing the time variation of geomagnetic activity indices Kp and Ap for 1932±1961, Fraser-Smith (1972) made an FFT analysis of 38 y (1932±1969) of Ap data and obtained very promonent peaks at 35.6, 16.1, 10.2, 0.5 y, prominent peaks at 7.04, 5.14, 4.10, 1.47, 1.09 y and 27.2, 27.6 days, moderate peaks at 54.0, 37.4, 30.5, 26.9, 18.7, 14.1, 13.7, 13.6, 9.39 days and very weak peaks for 2±9 days. Legrand and Simon (1989) discussed the contribution to geomagnetic activity of shock waves and of the solar wind, while Gonzalez et al (1990) emphasized the role of the dual peak cycle distribution of intense geomagnetic storms The R component would be due to sporadic or short-lived solar events, while the I component would be due to longlived solar features such as coronal holes. Legrand and Simon (1989) discussed the contribution to geomagnetic activity of shock waves and of the solar wind, while Gonzalez et al (1990) emphasized the role of the dual peak cycle distribution of intense geomagnetic storms

Methods

Results

Conclusion