Measuring Particle Current Density J on NASA's Magnetospheric Multiscale Mission (MMS)

A theorem is proved that the minimum Nyquist compatible cadence Δ τ 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 Δ τ /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.