We study both theoretically and numerically two-dimensional magnetohydrodynamic turbulence at infinite and zero magnetic Prandtl number
$\mathit{Pm}$ (and the limits thereof), with an emphasis on solution regularity. For
$\mathit{Pm}= 0$ , both
$\Vert \omega \Vert ^{2} $ and
$\Vert j\Vert ^{2} $ , where
$\omega $ and
$j$ are, respectively, the vorticity and current, are uniformly bounded. Furthermore,
$\Vert \boldsymbol{\nabla} j\Vert ^{2} $ is integrable over
$[0, \infty )$ . The uniform boundedness of
$\Vert \omega \Vert ^{2} $ implies that in the presence of vanishingly small viscosity
$\nu $ (i.e. in the limit
$\mathit{Pm}\rightarrow 0$ ), the kinetic energy dissipation rate
$\nu \Vert \omega \Vert ^{2} $ vanishes for all times
$t$ , including
$t= \infty $ . Furthermore, for sufficiently small
$\mathit{Pm}$ , this rate decreases linearly with
$\mathit{Pm}$ . This linear behaviour of
$\nu \Vert \omega \Vert ^{2} $ is investigated and confirmed by high-resolution simulations with
$\mathit{Pm}$ in the range
$[1/ 64, 1] $ . Several criteria for solution regularity are established and numerically tested. As
$\mathit{Pm}$ is decreased from unity, the ratio
$\Vert \omega \Vert _{\infty } / \Vert \omega \Vert $ is observed to increase relatively slowly. This, together with the integrability of
$\Vert \boldsymbol{\nabla} j\Vert ^{2} $ , suggests global regularity for
$\mathit{Pm}= 0$ . When
$\mathit{Pm}= \infty $ , global regularity is secured when either
$\Vert \boldsymbol{\nabla} \boldsymbol{u}\Vert _{\infty } / \Vert \omega \Vert $ , where
$\boldsymbol{u}$ is the fluid velocity, or
$\Vert j\Vert _{\infty } / \Vert j\Vert $ is bounded. The former is plausible given the presence of viscous effects for this case. Numerical results over the range
$\mathit{Pm}\in [1, 64] $ show that
$\Vert \boldsymbol{\nabla} \boldsymbol{u}\Vert _{\infty } / \Vert \omega \Vert $ varies slightly (with similar behaviour for
$\Vert j\Vert _{\infty } / \Vert j\Vert $ ), thereby lending strong support for the possibility
$\Vert \boldsymbol{\nabla} \boldsymbol{u}\Vert _{\infty } / \Vert \omega \Vert \lt \infty $ in the limit
$\mathit{Pm}\rightarrow \infty $ . The peak of the magnetic energy dissipation rate
$\mu \Vert j\Vert ^{2} $ is observed to decrease rapidly as
$\mathit{Pm}$ is increased. This result suggests the possibility
$\Vert j\Vert ^{2} \lt \infty $ in the limit
$\mathit{Pm}\rightarrow \infty $ . We discuss further evidence for the boundedness of the ratios
$\Vert \omega \Vert _{\infty } / \Vert \omega \Vert $ ,
$\Vert \boldsymbol{\nabla} \boldsymbol{u}\Vert _{\infty } / \Vert \omega \Vert $ and
$\Vert j\Vert _{\infty } / \Vert j\Vert $ in conjunction with observation on the density of filamentary structures in the vorticity, velocity gradient and current fields.