For the purpose of studying pulsations, a criterion for
selecting pulsation events from the entire data set is needed. When
continuous pulsations are observed on the ground, they usually
have a clear enhancement at a particular frequency. An
enhancement in the spectrum can be identified by comparing the
wave power in the neighboring frequency bands, but before this
comparison is made, the ``trend'' in the spectrum should be removed.
The trend can be represented by the six median
values shown in Figure 2, except for the sixth (highest) frequency
band because of its strong noise component.
Figure 4 shows the details
of the Q-Q plot for , where the wave component and the
noise component are fitted linearly. The median of *P _{6}* is located in
the noise component, and it would have been a smaller value if the
noise component were absent. If we assume that the wave
component has a linear structure in the Q-Q plot, the median of the
wave power data can be estimated by the small square in Figure 4.
In the following, this estimation is used as the median
wave power for the sixth frequency band.

Figure 4. Q-Q plot of log P_{6}. The two straight lines are the linear
fits of the ``wave'' component and the ``noise'' component. Small
square shows the estimation of the median of wave power
if noise is absent. |

If is the median power of the *i*th frequency band,
the ``detrended'' wave power is defined as

(1) |

(2) |

The strong noise component in *P _{6}* can certainly affect the
selection of the