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Astron. Astrophys. 364, 876-878 (2000)

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5. Results and discussion

Fig. 2 shows [FORMULA] as a function of radius for three colatitude values. According to the Eq. (8) criterion, the turbulent instability may occur at the equator ([FORMULA]) in the interval [FORMULA]; the turbulent layer is thinner at higher latitudes. In other words, the unstable region for the vertical shear instability, beneath the convection zone, measures barely 2 percent of the solar radius and is localized around the equator.

[FIGURE] Fig. 2. For the colatitudes [FORMULA] (full), [FORMULA] (dashed) and [FORMULA] (dash-dot-dash), stable and unstable regions beneath the tachocline located about at the limit Rd Cv between the convection zone and the radiative interior.

This instability should not be confused with the shear instability arising from the differential rotation in latitude, which is not inhibited by the vertical stratification, and which may play an important role in smoothing horizontal gradients in composition and angular velocity (Spiegel & Zahn 1992). But that instability does not contribute to the vertical transport.

We conclude that another physical process is needed to transport matter and angular momentum beneath the solar tachocline, which would operate more efficiently than the turbulence generated by the vertical [FORMULA]-gradient. A plausible mechanism is the meridional circulation which probably exists in the tachocline (Brun et al. 1999). A more powerful process is required also to establish the almost uniform angular velocity at greater depth, which could be magnetic torquing (Gough & McIntyre 1998) or transport of angular momentum by waves (Schatzman 1996; Kumar et al. 1999).

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001