160-minute solar cycle

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The 160-minute solar cycle was an apparent periodic oscillation in the solar surface which was observed in a number of early sets of data collected for helioseismology.

Helioseismology inferring the internal structure of the Sun from the propagation of seismic waves

Helioseismology, a term coined by Douglas Gough, is the study of the structure and dynamics of the Sun through its oscillations. These are principally caused by sound waves that are continuously driven and damped by convection near the Sun's surface. It is similar to geoseismology, or asteroseismology, which are respectively the studies of the Earth or stars through their oscillations. While the Sun's oscillations were first detected in the early 1960s, it was only in the mid-1970s that it was realised that the oscillations propagated throughout the Sun and could allow scientists to study the Sun's deep interior. The modern field is separated into global helioseismology, which studies the Sun's resonant modes, and local helioseismology, which studies all the waves propagating at the Sun's surface.

Contents

The presence of a 160 minute cycle in the Sun is not substantiated by contemporary solar observations, and the historical signal is considered by mainstream scientists to occur as the redistribution of power from the diurnal cycle as a result of the observation window and atmospheric extinction.

Sun Star at the centre of the Solar System

The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometers, or 109 times that of Earth, and its mass is about 330,000 times that of Earth. It accounts for about 99.86% of the total mass of the Solar System. Roughly three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.

History

The birth of helioseismology occurred in 1976 with the publications of papers from Brookes, Isaak and van der Raay [1] and Severny, Kotov and Tsap, [2] both of which reported upon the observation of a 160-minute solar oscillation with an amplitude of approximately two metres per second.

It was rapidly realised that this frequency corresponded to one-ninth of a day, and therefore the authenticity of this signal was in some doubt. If a non-sinusoidal oscillation is present in a time-series then power will be seen in a periodogram at not only the frequency of the oscillation, but also harmonics at integer multiples of this frequency. A re-analysis of data obtained over the period of 1974–1976 by Brookes et al. [3] showed that the evidence for a stable, phase-coherent 160 minute oscillation at a constant amplitude was far from conclusive. Although the signal could be detected the amplitude appeared variable and was lower than first reported.

In signal processing, a periodogram is an estimate of the spectral density of a signal. The term was coined by Arthur Schuster in 1898. Today, the periodogram is a component of more sophisticated methods. It is the most common tool for examining the amplitude vs frequency characteristics of FIR filters and window functions. FFT spectrum analyzers are also implemented as a time-sequence of periodograms.

A re-affirmation of the 160 minute signal was obtained by analysis of data from groups in Crimea and Stanford over a long period of time. It was found that the phase showed a steady drift, indicative that the frequency being used in analysis differed slightly from that in the data. This implied that a period of 160.01 minutes [4] produced a better fit to the data. Evidence also emerged that multiple sets of observations were phase-coherent. These facts contributed to impressions that the origin of the observed signal was stellar and not terrestrial in origin.

In 1989 as higher-quality multiple-year datasets from a single site became available it was shown by Elsworth et al. that the period of the 160 minute signal was indeed 160.00 minutes, and the amplitude was dependent upon both the length and quality of data obtained in a season, with the signal more prominent at time where atmospheric condition were worse. The group were able to demonstrate that the signal may be simulated by a slightly distorted diurnal sine-wave such as may be obtained by differential atmospheric extinction. [5]

Although claims of the presence of a 160-minute period in the Sun were still presented by Kotov et al. in 1990, [6] and 1991, [7] the mainstream scientific establishment had moved on.

Contemporary observations

There are currently two solar-observation networks, the BiSON and GONG networks which consist of a global network of stations, as well as space based instruments such as the GOLF instrument aboard the SOHO spacecraft. These are able to keep the Sun under near-continuous observation, and so largely eliminate the influence of diurnal signals. Data from these instruments shows no oscillation at 160 minutes.

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References

  1. ↑ "Observation of free oscillations of the sun", Nature, vol. 259, January 15, 1976, pp. 92–95
  2. ↑ "Observations of solar pulsations", Nature, vol. 259, January 15, 1976, pp. 87–89
  3. ↑ The search for solar oscillations, 1974 to 1976, Royal Astronomical Society, Monthly Notices, vol. 184, September 1978, pp. 759–767
  4. ↑ Further evidence of solar oscillations with a period of 160 minutes, Astrophysical Journal, Part 2 – Letters to the Editor, vol. 237, May 1, 1980, pp. L97, L98
  5. ↑ The 160 minute solar oscillation – an artifact? Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 338, March 1, 1989, pp. 557–562
  6. ↑ 160 minute solar variations and the 22 year cycle, Solar Physics (ISSN 0038-0938), vol. 128, July 1990, p. 269–280
  7. ↑ 160-min pulsation of the sun – New observational results, Solar Physics (ISSN 0038-0938), vol. 133, May 1991, p. 95–102
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