Return the cross-spectrum of the spherical harmonic coefficients as a function of spherical harmonic degree.

## Usage

`array`

= cross_spectrum (`clm1`

, `clm2`

, [`normalization`

, `degrees`

, `lmax`

, `convention`

, `unit`

, `base`

])

## Returns

`array`

: ndarray, shape (len(degrees))- 1-D ndarray of the spectrum.

## Parameters

`clm1`

: ndarray, shape (2,`lmax`

+ 1,`lmax`

+ 1)- ndarray containing the first set of spherical harmonic coefficients.
`clm2`

: ndarray, shape (2,`lmax`

+ 1,`lmax`

+ 1)- ndarray containing the second set of spherical harmonic coefficients.
`normalization`

: str, optional, default = ‘4pi’- ‘4pi’, ‘ortho’, ‘schmidt’, or ‘unnorm’, for geodesy 4pi normalized, orthonormalized, Schmidt semi-normalized, or unnormalized coefficients, respectively.
`lmax`

: int, optional, default = len(clm[0,:,0]) - 1.- Maximum spherical harmonic degree to output.
`degrees`

: ndarray, optional, default = range(`lmax`

+1)- Array containing the spherical harmonic degrees where the spectrum is computed.
`convention`

: str, optional, default = ‘power’- The type of spectrum to return: ‘power’ for power spectrum, ‘energy’ for energy spectrum, and ‘l2norm’ for the l2 norm spectrum.
`unit`

: str, optional, default = ‘per_l’- If ‘per_l’, return the total contribution to the spectrum for each spherical harmonic degree l. If ‘per_lm’, return the average contribution to the spectrum for each coefficient at spherical harmonic degree l. If ‘per_dlogl’, return the spectrum per log interval dlog_a(l).
`base`

: float, optional, default = 10.- The logarithm base when calculating the ‘per_dlogl’ spectrum.

## Description

This function returns either the cross-power spectrum, cross-energy spectrum, or l2-cross-norm spectrum. Total cross-power is defined as the integral of the clm1 times the conjugate of clm2 over all space, divided by the area the functions span. If the mean of the functions is zero, this is equivalent to the covariance of the two functions. The total cross-energy is the integral of clm1 times the conjugate of clm2 over all space and is 4pi times the total power. The l2-cross-norm is the sum of clm1 times the conjugate of clm2 over all angular orders as a function of spherical harmonic degree.

The output spectrum can be expresed using one of three units. ‘per_l’ returns the contribution to the total spectrum from all angular orders at degree l. ‘per_lm’ returns the average contribution to the total spectrum from a single coefficient at degree l, which is equal to the ‘per_l’ spectrum divided by (2l+1). ‘per_dlogl’ returns the contribution to the total spectrum from all angular orders over an infinitessimal logarithmic degree band. The contrubution in the band dlog_a(l) is spectrum(l, ‘per_dlogl’)*dlog_a(l), where a is the base, and where spectrum(l, ‘per_dlogl’) is equal to spectrum(l, ‘per_l’)*l*log(a).