A theorem is proved that the minimum Nyquist compatible cadence Delta tau for subsequent model free determinations of current density J by magnetospheric multiscale (MMS) instrumentation is the cadence of the ion plasma subsystem DIS of the fast plasma investigation (FPI). A sample of published discoveries and signatures of magnetic dissipation associated with collisionless magnetic reconnection at the noon magnetopause are reviewed that utilize observations of the variable J(plasma)(t) reported at the electron DES subsystem cadence which is always Delta tau/5. Nyquist's theorem precludes a unique interpolation of DIS observations to the DES cadence, casting a serious shadow on rumored interpolation strategies said to allow measurement of J(plasma) at the DES cadence. The theorems and corollary are properties only of the FPI flight hardware and do not depend on peculiar plasma conditions for their conclusions. Determinations of J with measured scale lengths require a still slower cadence, twice that of the DIS subsystem. A rationale is still missing for the present MMS approach for determining J(plasma) at the DES cadence.
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