The Splitting Or Disappearance of the Solar 160-Min Mode?

Abstract

The ‘CrAO-WSO’-network experiment was designed for detection of low-degree oscillations of the Sun representing either its normal g -modes or those driven by, e.g., rapid (hypothetical) rotation of the central solar core. The Doppler-shift measurements were made in 1974–1995 at both sites during about 13600 hr, in all. Taking into account the upper limit (≈0.08 m s^-1) for amplitudes of potential g-modes, attention is paid to the Sun's behaviour at frequencies near the 9th daily harmonic (period P ≈160.

The two main issues follow from analysis of the combined CrAO-WSO data: (a) in 1974–1982 the primary period of solar pulsation was P ₀160.0099 ± 0.0016 ± 0.0016 min, but (b) during the last 13 yr it attained a new value, P ₁ ≈ 159.9654 ± 0.0010 min, which happens to be a near-annual sidelobe of P ₀. We find therefore that the phase stability of the 160-min mode is no longer present: it appears to be splitted at least into a pair of oscillations,P ₀ and P ₁, having perhaps different physical origins. But the most striking is the fair coincidence of the strongest peaks in the two data sets: CrAO (1974–1995): P = 159.9662 ±0.0006 min, WSO (1977–1994): P = 159.9663 ± 0.0007 min. The existence of two frequencies,P ^-1 ₀ and P ^-1 ₁, with their separation corresponding to ≈1-yr period, seems to be difficult to explain in terms of gravity g modes.