linear algebra - Hermitian matrices with prescribed number of positive and negative eigenvalues

Let $H$ be a linear subspace of the space of Hermitian $ntimes n$ matrices. Is there a good characterization of those $H$ such that every $Ain H$ has at least $k$ positive and $k$ negative eigenvalues?

For $k=1$ a nice characterization is the following: there is a positive definite matrix $B$ orthogonal to $H$ (w.r.t. the scalar product $(A,B)=mathrm{tr}(AB)$), or equivalently there exists a basis of $mathbb C^n$ such that all matrices in $H$ have zero trace.

Even for $k=2$ I was not able to find any good characterization.

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