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We present the first results of a search for transient hard X-ray (HXR)emission in the quiet solar corona with the \textit{Nuclear SpectroscopicTelescope Array} (\textit{NuSTAR}) satellite. While \textit{NuSTAR} wasdesigned as an astrophysics mission, it can observe the Sun above 2~keV withunprecedented sensitivity due to its pioneering use of focusing optics.\textit{NuSTAR} first observed quiet Sun regions on 2014 November 1, althoughout-of-view active regions contributed a notable amount of background in theform of single-bounce (unfocused) X-rays. We conducted a search for quiet Suntransient brightenings on time scales of 100 s and set upper limits on emissionin two energy bands. We set 2.5--4~keV limits on brightenings with time scalesof 100 s, expressed as the temperature T and emission measure EM of a thermalplasma. We also set 10--20~keV limits on brightenings with time scales of 30,60, and 100 s, expressed as model-independent photon fluxes. The limits in bothbands are well below previous HXR microflare detections, though not low enoughto detect events of equivalent T and EM as quiet Sun brightenings seen in softX-ray observations. We expect future observations during solar minimum toincrease the \textit{NuSTAR} sensitivity by over two orders of magnitude due tohigher instrument livetime and reduced solar background.

First NuSTAR Limits on Quiet Sun Hard X-Ray Transient Events