All Figures

$\begin{figure} \par\includegraphics[width=15.8cm,clip]{3345f1.eps} \end{figure}$ Figure 1: The upper panel compares the mean spectrum of HR 3831 (thin line) and magnetic spectrum synthesis calculations (thick line) in the 6116-6156 Å wavelength region. The strongest spectral lines are identified. The length of the lines under the identification is proportional to the strength of the corresponding spectral features. The lower panel shows the standard deviation spectrum.
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In the text

$\begin{figure} \par\includegraphics[width=10cm,clip]{3345f2a.eps}\par\vspace*{3mm} \includegraphics[width=10cm,clip]{3345f2b.eps}\par \end{figure}$ Figure 2: Illustration of the rapid variation of the Nd III 6145.07 Å line in the spectrum of HR 3831. The upper panels show LPV recorded on the night of 5 Feb. 2001 and the lower panels those obtained on 7 Feb. 2001. Each of the six horizontal subpanels presents the average spectrum of HR 3831 at the specified rotation phase and time sequence of the difference between the average and 50 individual time-resolved observations obtained during 74 min and covering roughly 6 pulsation periods of HR 3831. Consecutive difference spectra are shifted in the vertical direction. The bottom part of the horizontal subpanels shows the standard deviation for each pixel of the observed spectra.
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In the text

$\begin{figure} \par\includegraphics[width=15.5cm,clip]{3345f3.eps} \end{figure}$ Figure 3: Illustration of the Nd III 6145.07 Å line profile variation at different rotation phases of HR 3831. Each of the greyscale images is based on groups of 23 time-resolved spectra covering roughly 3 oscillation cycles of HR 3831 at specified rotation phases. The upper plot in each panel shows residuals from the mean line profile. The greyscale plots show time evolution of the residuals, phased with the main pulsation frequency $\nu=1427.9958~\mu$ Hz and using the scale of $\pm$ 2.5% of the continuum intensity. The six columns of panels show LPV recorded during six different observing nights. The UT date is specified at the top of each column. Evolution of the variability pattern during observing nights is illustrated by three subpanels in each column which display spectra obtained at the beginning, middle, and end of the continuous monitoring.
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In the text

$\begin{figure} \par\includegraphics[width=13cm,clip]{3345f4.eps} \end{figure}$ Figure 4: Radial velocity variation of the Nd III 6145.07 Å line in the spectrum of HR 3831. Symbols show individual measurements phased with the rotation period P=2.851976 d. The smooth curve illustrates a Fourier fit of the rotational modulation of the average RV.
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In the text

$\begin{figure} \par\includegraphics[width=17cm,clip]{3345f5.eps} \end{figure}$ Figure 5: Frequency analysis of the RV variation of the Nd III 6145.07 Å line in the spectrum of HR 3831. The left panels show low-resolution amplitude spectra. The vertical dashed lines mark the fundamental pulsation frequency ( $\nu=1427.9958~\mu$ Hz) and its first harmonic ( $2\nu=2855.916~\mu$ Hz). a) The amplitude spectrum computed from the raw radial velocities shown in Fig. 4; b) the amplitude spectrum after prewhitening of the rotational modulation. The right panels present high-resolution amplitude spectra in the vicinity of the main pulsation frequency. The vertical dashed lines show the expected position of the triplet around the main frequency: $\nu -\nu _{\rm rot}, \nu , \nu +\nu _{\rm rot}$ (where $\nu _{\rm rot}=4058.265$ nHz). c) The amplitude spectrum after prewhitening of the rotational modulation; d) the corresponding CLEANed amplitude spectrum.
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In the text

$\begin{figure} \par\includegraphics[width=14cm,clip]{3345f6.eps} \end{figure}$ Figure 6: Radial velocity curve for all observations of the Nd III 6145.07 Å line in the spectrum of HR 3831. Symbols show individual RV measurements, with the rotational modulation of RV removed using the Fourier fit illustrated in Fig. 4. The solid curve shows the RV predicted by the frequency solution discussed in the text. Individual panels correspond to the data obtained during each of the six observing nights. The number in the lower left of each panel gives HJD -2 451 900 at the start of spectroscopic monitoring. The x-axis is labelled with UT (in hours) and rotation phases.
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In the text

$\begin{figure} \par\includegraphics[width=17cm,clip]{3345f7.eps} \end{figure}$ Figure 7: Amplitude spectra of the pulsational variability of (from top to bottom) the equivalent width $W_\lambda$ and the first three moments - the radial velocity $\langle V \rangle$ , the second moment $\langle V^2 \rangle$ , and the third moment $\langle V^3 \rangle$ - measured for the Nd III 6145.07 Å line. The left panels show low-resolution amplitude spectra. The vertical dashed lines mark the main pulsation frequency and its first harmonic. The right sequence of panels illustrate amplitudes of the components of the fundamental septuplet ( $\nu-3\nu_{\rm rot}, \ldots, \nu, \ldots, \nu+3\nu_{\rm rot}$ ) derived with the least-squares analysis (see Table 2). The horizontal dashed line corresponds to the $3\sigma$ uncertainty of the fitted amplitudes. The lower amplitude spectra in the left panels show the amplitude spectra after the frequency solutions illustrated in the respective right panels have been removed from the data. These residual amplitude spectra are shown at the same scale as the original spectra and are shifted downward for display purposes.
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In the text

$\begin{figure} \par\includegraphics[width=15.5cm,clip]{3345f8.eps} \end{figure}$ Figure 8: Rotational modulation of the pulsation amplitude ( left panels) and phase ( right panels) of the variability in moments of the Nd III 6145.07 Å line in the spectrum of HR 3831. The panels show (from top to bottom) measurements of the equivalent width $W_\lambda$ , radial velocity $\langle V \rangle$ , the second moment $\langle V^2 \rangle$ , and the third moment $\langle V^3 \rangle$ . Individual points in these diagrams were calculated by fitting the main pulsation frequency ( $\nu=1427.9958~\mu$ Hz) to the subsets of 38 spectroscopic observations (about 5 pulsation cycles or 56 min). The smooth curves show rotational modulation expected from the frequency solution for the fundamental septuplet given in Table 2 and illustrated in Fig. 7.
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In the text

$\begin{figure} \par\includegraphics[width=11.3cm,clip]{3345f9.eps}\par \end{figure}$ Figure 9: Amplitude spectra of the RV variation observed for spectral features other than the Nd III 6145.07 Å line. The vertical dashed lines mark the fundamental pulsation frequency ( $\nu=1427.9958~\mu$ Hz) and its first harmonic ( $2\nu=2855.916~\mu$ Hz).
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In the text

$\begin{figure} \par\includegraphics[width=14.7cm,clip]{3345f10.eps} \end{figure}$ Figure 10: Comparison of the observed rotational modulation of the pulsation amplitude and phase of the $\langle V \rangle$ and $\langle V^2 \rangle$ moments of the Nd III 6145.07 Å line (symbols) with predictions of the oblique pulsator model discussed in the text. The solid line shows results obtained with the best-fit combination of the non-axisymmetric dipole ( $\ell =1$ , m=-1,0,1) and axisymmetric octupole ( $\ell =3$ , m=0) pulsation components, whereas the dashed line illustrates the moment modulation expected from the dipole components alone.
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In the text

$\begin{figure} \par\includegraphics[width=15.5cm,clip]{3345f11.eps} \end{figure}$ Figure 11: Depth-dependence of the magnetoacoustic wave properties in the atmosphere of HR 3831. a) (A_-1+A₊₁)/2 characterises variation with depth of the average amplitude of the $\nu \pm \nu _{\rm rot}$ components, b) depth-dependence of the respective phases of the $\nu +\nu _{\rm rot}$ (open circles) and $\nu -\nu _{\rm rot}$ (filled circles) pulsation, c) the (A_-1+A₊₁)/A₀ parameter characterises strength of the main frequency component relative to the $\nu \pm \nu _{\rm rot}$ components, d) A_-1/A₊₁ measures asymmetry of the $\nu \pm \nu _{\rm rot}$ components. The symbols show results obtained for individual spectral lines, whereas the dashed line corresponds to the photometric pulsation behaviour in a Johnson B filter.
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In the text

$\begin{figure} \par\includegraphics[width=15.8cm,clip]{3345f1.eps} \end{figure}$	Figure 1: The upper panel compares the mean spectrum of HR 3831 (thin line) and magnetic spectrum synthesis calculations (thick line) in the 6116-6156 Å wavelength region. The strongest spectral lines are identified. The length of the lines under the identification is proportional to the strength of the corresponding spectral features. The lower panel shows the standard deviation spectrum.
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$\begin{figure} \par\includegraphics[width=13cm,clip]{3345f4.eps} \end{figure}$	Figure 4: Radial velocity variation of the Nd III 6145.07 Å line in the spectrum of HR 3831. Symbols show individual measurements phased with the rotation period P=2.851976 d. The smooth curve illustrates a Fourier fit of the rotational modulation of the average RV.
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$\begin{figure} \par\includegraphics[width=11.3cm,clip]{3345f9.eps}\par \end{figure}$	Figure 9: Amplitude spectra of the RV variation observed for spectral features other than the Nd III 6145.07 Å line. The vertical dashed lines mark the fundamental pulsation frequency ( $\nu=1427.9958~\mu$ Hz) and its first harmonic ( $2\nu=2855.916~\mu$ Hz).
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