Symplectic criteria for elliptic curves, revisited

Let $\mathrm{\ell }$ and $p\ge 3$ be different primes. Let $E\mathrm{/}{\mathbb{Q}}_{\mathrm{\ell }}$ and $E^{\mathrm{\prime }}\mathrm{/}{\mathbb{Q}}_{\mathrm{\ell }}$ be elliptic curves with isomorphic $p$-torsion. Assume that $E$ has potentially multiplicative reduction. We classify when all $G_{{\mathbb{Q}}_{\mathrm{\ell }}}$-isomorphisms $\phi :E[p]\to E^{\mathrm{\prime }}[p]$ have the same symplectic type and prove two new criteria to determine the type in that case. In particular, when both curves have multiplicative reduction, our results cover the case of unramified $p$-torsion which is not covered by the original criterion due to Kraus and Oesterlé. We also give a variant of a symplectic criterion for the case when both $E$ and~$E^{\mathrm{\prime }}$ have good reduction and provide an algorithm to apply it. As an application, we determine the symplectic type of all the mod $p\ge 5$ congruences between rational elliptic curves with conductor $\le 500000$ that satisfy the hypothesis of either of our criteria at some prime~$\mathrm{\ell }$.