# Missing Modes

For adiabatic oscillation calculations using **gyre**, the
radial order \(\npg\) of modes found should be
monotonic-increasing[1]. Departures from this behavior can
occur for a number of reasons.

## Insufficient Frequency Resolution

If the frequency grid has insufficient resolution,
then **gyre** can skip modes during the bracketing phase, as
discussed in the Limitations of the Numerical Method 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., Aerts et al., 2010). Hence, to search for these modes set

`grid_type`

=`'LINEAR'`

in the`&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

`grid_type`

=`'INVERSE'`

in the`&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

`grid_frame`

=`'COROT_I'`

|`'COROT_O'`

in the`&scan`

namelist group.

Next, try increasing the number of points in the frequency grids,
simply by increasing the `n_freq`

parameter in the
`&scan`

namelist group(s).

Tip

A good rule of thumb is that `n_freq`

should be around 5
times larger than the number of modes expected to be found.

## Insufficient Spatial Resolution

If the spatial grid has insufficient resolution, then certain modes can simply be absent from the (finite) set of distinct numerical solutions, as discussed in the Limitations of the Numerical Method 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. 5).

To fix this problem, first check that the `w_osc`

,
`w_exp`

and `w_ctr`

weighting parameters in the
`&grid`

namelist group are set to reasonable values (see the
Recommended Values section). If that doesn’t improve things, try
gradually increasing both `w_osc`

and `w_ctr`

.

## Non-adiabatic Effects

When undertaking non-adiabatic calculations, modes can be mis-classified or completely missed. The former situation arises because the expectation of monotonic-increasing \(\npg\) 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 for establishing initial trial roots can fail to find it. In such cases, the alternative contour method performs very well.

Footnotes