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missing-modes.rst

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Missing Modes

For adiabatic oscillation calculations using gyre, the radial order $\numpg$ of modes found should be monotonic-increasing1. Departures from this behavior can occur for a number of reasons.

Insufficient Frequency Resolution

If the frequency grid <freq-grids> has insufficient resolution, then gyre can skip modes during the bracketing phase, as discussed in the numerical-limits section. The signature of insufficient frequency resolution is that an even number of consecutive modes is missed --- most often, an adjacent pair of modes.

To fix this problem, first check that the distribution of points in the frequency grids matches (approximately) the expected distribution of mode eigenfrequencies:

  • In the asymptotic limit of large radial order, p modes are uniformly distributed in frequency (see, e.g., :ads_citealp:aerts:2010). Hence, to search for these modes set :nml_n:grid_type=:nml_v:'LINEAR' in the :nml_g:scan namelist group(s).
  • Likewise, in the asymptotic limit of large radial order, g modes are uniformly distributed in period. Hence, to search for these modes set :nml_n:grid_type=:nml_v:'INVERSE' in the :nml_g:scan namelist group(s).
  • For rotating stars, the asymptotic behaviors mentioned apply in the co-rotating reference frame, not in the inertial reference frame. So, be sure to also set :nml_n:grid_frame =:nml_n:'COROT_I'|:nml_n:'COROT_O' in the :nml_g:scan namelist group.

Next, try increasing the number of points in the frequency grids, simply by increasing the :nml_n:n_freq parameter in the :nml_g:scan namelist group(s).

Tip

A good rule of thumb is that :nml_n:n_freq should be around 5 times larger than the number of modes expected to be found.

Insufficient Spatial Resolution

If the spatial grid <freq-grids> has insufficient resolution, then certain modes can simply be absent from the (finite) set of distinct numerical solutions, as discussed in the numerical-limits section. The signature of insufficient spatial resolution is that modes that are found have radial orders comparable to the number of grid points N in the grid; and that the eigenfunctions of these modes are barely resolved (cf. fig-eigenfuncs-N7).

To fix this problem, first check that the :nml_n:w_osc, :nml_n:w_exp and :nml_n:w_ctr weighting parameters in the :nml_g:grid namelist group are set to reasonable values (see the spatial-grids-rec section). If that doesn't improve things, try gradually increasing both :nml_n:w_osc and :nml_n:w_ctr.

Non-adiabatic Effects

When undertaking non-adiabatic calculations <non-ad-osc>, modes can be mis-classified or completely missed. The former situation arises because the expectation of monotonic-increasing $\numpg$ formally applies only to adiabatic oscillations; while it can also work reasonably well for weakly non-adiabatic cases, there are no guarantees. If mis-classification does occur, then it must be fixed manually by determining which adiabatic mode the problematic non-adiabatic mode corresponds to.

Missing modes occur for a different reason: if a mode has a large growth rate, then the usual adiabatic method <non-ad-ad> for establishing initial trial roots can fail to find it. In such cases, the alternative contour method <non-ad-contour> performs very well.

Footnotes

  1. The sole exception is ℓ = 1 modes, where $\numpg=0$ is skipped due to the way the :ads_citealt:takata:2006b classification scheme is set up.↩