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The powerlaw X-ray spectra of active galactic nuclei at moderate to highaccretion rates normally appear softer when they brighten, for which theunderlying mechanisms are yet unclear. Utilizing XMM-Newton observations andexcluding photons $<$ 2 keV to avoid contamination from the soft excess, inthis work we scrutinize the powerlaw spectral variability of NCG 4051 from twonew aspects. We first find that a best-fit "softer-when-brighter" relation isstatistically insufficient to explain the observed spectral variabilities, andintervals deviated from the empirical relation are clearly visible in the lightcurve of 2 -- 4 keV/4 -- 10 keV count rate ratio. The deviations are seen notonly between but also within individual XMM-Newton exposures, consistent withrandom variations of the corona geometry or inner structure (with timescales asshort as $\sim$ 1 ks), in addition to those behind the smooth"softer-when-brighter" trend. We further find the "softer-when-brighter" trendgradually weakens with the decreasing timescale (from $\sim$ 100 ks down to 0.5ks). These findings indicate that the powerlaw spectral slope is not solelydetermined by its brightness. We propose a two-tier geometry, includingflares/nano-flares on top of the inner disc and an embedding extended corona(heated by the flares, in analogy to solar corona) to explain the observationstogether with other observational clues in literature. Rapid spectralvariabilities could be due to individual flares/nano-flares, while slow onesare driven by the variations in the global activity of inner disc region (akinto the variation of solar activity, but not the accretion rate) accompaniedwith heating/cooling and inflation/contraction of the extended corona.

More than softer-when-brighter: the X-Ray powerlaw spectral variability in NGC 4051