  The principal aim of observational helioseismology is to determine mode
parameters of the solar oscillations as accurately as possible. Yet estimates
of the mode parameters are subject to many sources of noise, including
inevitable gaps in data, stochastic nature of excitation processes,
interferences among modes. Another uncertainty in frequency determination
results from a modulation of the oscillation frequency in itself. It is well
known that variation in the mean strength of the solar magnetic field modulates
the frequency. We consider effects of the solar magnetic field variation on the
spectral behavior of the power spectrum using a simple model. We show that the
solar magnetic field variation can cause unwanted sidelobes beside the main
peak in the power spectrum, and consequently show that the effect of the
frequency modulation due to the solar magnetic field variation can lead a bias
in frequency determinations. This effect should be considered seriously
particularly when the $ l \ne 0$ $p$-mode multiplets are analyzed to measure
the rotational splitting since the separations could be comparable. In addition
to a bias in frequency estimates, the line width is also likely to be
overestimated due to a line broadening. We therefore suggest that determination
of the mode parameters should be derived with due care considering the magnetic
field variation when a long data set is analyzed. The frequency modulation
should be taken into account in the analysis to make a solid conclusion on any
subtle dependence of the mode parameters, such as, that on the solar cycle. We
conclude by pointing out that a new method is required to accommodate the
stochastic force and the phase variation.
