5.5 Low-degree acoustic mode splittings 1988 – 2002

The next several years were active ones for low-degree helioseismology, with the development of the BiSON (Birmingham-based) and IRIS (based in Nice) networks. Together with the IPHIR instrument that rode the PHOBOS spacecraft on its cruise phase to Mars, and the ground tests of the LOI (Luminosity Oscillations Imager) instrument that would later be mounted on the SOHO spacecraft, these brought a succession of estimates of the low-degree splitting, as summarized in Table 1 and Figure 14View Image. In addition to the MDI instrument for medium and high-degree observations, the SOHO spacecraft carried both LOI and GOLF (Global Oscillations at Low Frequencies) specifically for observing low-degree modes. Even though GOLF malfunctioned and could not be operated in its intended differential mode, instead being confined to making Doppler observations on one side of an absorption line, it provided some of the best available long-term, low-degree observations.
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Figure 14: l = 1 splitting estimates as a function of publication date.

Table 1: Summary of l = 1 splitting measurements, 1988 – 2002
Reference Project δν(μHz)


Pallé et al. (1988) Tenerife 0.75

Summers of 1981 – 1986

Toutain and Fröhlich (1992) IPHIR 0.563 ± 0.017

Intensity measurements on PHOBOS spacecraft

Loudagh et al. (1993Jump To The Next Citation Point) IRIS 0.494

Based on 3 low-frequency modes

Jiménez et al. (1994Jump To The Next Citation Point) Tenerife 0.4768 ± 0.0097

Solar maximum

Jiménez et al. (1994) Tenerife 0.525 ± 0.0127

Solar minimum

Toutain and Kosovichev (1994) IPHIR 0.452 ± 0.020

Elsworth et al. (1995Jump To The Next Citation Point) BiSON 0.42 ±0.02

Appourchaux et al. (1995) LOI 0.402 ± 0.031

l = 2

Chaplin et al. (1996aJump To The Next Citation Point) BiSON 0.415 ± 0.006

Lazrek et al. (1996Jump To The Next Citation Point) IRIS 0.456 ± 0.012

Lazrek et al. (1997) GOLF 0.452 ± 0.014

Gizon et al. (1997) IRIS 0.456 ± 0.010

Bertello et al. (2000Jump To The Next Citation Point) GOLF 0.436 ± 0.009

Bertello et al. (2000) MDI 0.447 ± 0.011

Asymmetric profile

Chaplin et al. (2001Jump To The Next Citation Point) BiSON 0.435 ± 0.0036

Gelly et al. (2002) GOLF 0.433 ± 0.002

The reported results show considerable variation, but apart from the early Tenerife result, which was based on much shorter and lower-duty-cycle observations than most of the others, they all cluster around the surface rotation rate, some (particularly the IRIS results) pointing to a core rotation faster than the surface rate and some (in particular the BiSON results) to one substantially below it, perhaps as low as zero. As we approach the present time and the observation and analysis improve, the values tend to converge on a splitting quite close to that which would correspond to the surface rate. Early in this period, there was room to speculate (e.g., Chaplin et al., 1996a) that the differences reflected a temporal variation, but this could not explain away all the discrepancies.

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Figure 15: Power spectrum from 10 years of BiSON data, 1992 – 2002; the insets show the low-frequency end of the five-minute band (blue) and a single, rotationally split l = 1 peak (red).

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