tokens: dynamo than precession. We must point out that this conclusion is valid only at this Ekman number. At a smaller Ekman number, the precessing flow is more complex, which favors the onset of the dynamo, and this conclusion may not hold any longer (we leave the large-scale simulations at smaller Ekman numbers for other researchers who will be interested in the result of this work). The comparison between Figures 1 (a) and 2 (a) suggests that not only the mean poloidal kinetic energy but also the fluctuation amplitude of the precessing flow are much higher than those of the convective flow. This implies that the precessional instabilities are more vigorous than the convective instabilities in the two successful dynamos. The dominant azimuthal mode in the convective flow is m = 3 (Table 1 ), indicating a shorter length scale than the dominant spin-over mode m = 1 in the precessing flow. Figure 2 (b) shows the time evolution of the magnetic energy. Compared to Figure 1 (b), the magnetic energy of the convection dynamo becomes noticeable at the time ≈5000, which is much later than the time ≈400 of the precession dynamo. It is not surprising that the magnetic energy of the convection dynamo is also much lower than that of the precession dynamo (Table 1 ) because of the lower Rm of the former. Figure 2. Convection dynamo at . Here, we show the time evolution of the poloidal kinetic energy (a) and the magnetic energy (b). After studying the dynamos driven by precession alone and by convection alone, we study the combined effect of precession and convection. In the above two dynamos, the dynamo driven by precession alone cannot be maintained at Po = −0.2 and the dynamo driven by convection alone cannot be maintained at . We test whether the preceission-convection dynamo can be maintained at Po = −0.2 and . This dynamo works! It indicates that the combined effect of precession and convection can indeed facilitate the onset of the dynamo. As we discussed, the conventional Ra differs by more than one million, which indicates that precession greatly helps the onset of the convection dynamo. Moreover, it is interesting that the Rm = 677 of the precession-convection dynamo is lower than the Rm = 694 of the failed precession dynamo, as shown in Table 1 . This suggests that the combination of precession and convection has some non-trivial effect and triggers the dynamo action at a lower Rm of the pure precession dynamo. This non-trivial effect probably arises from the resonance of precessional instability and convective instability. Figure 3 shows the time evolution of this precession-convection dynamo and its time-averaged values are listed in Table 1 . Although the of the precession-convection dynamo is lower than the of the convection dynamo, the Nu of the former is higher (Table 1 ). This is because the poloidal flow driven by precession contributes more to heat transfer than convection, i.e., of the precessing flow at Po = −0.2 is already much higher than
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