One of the limiting factors affecting performance of the MIRI Medium Resolution Spectrometer (MRS) in the faint-source regime are the “shower” artifacts produced by cosmic rays. Unlike regular cosmic rays, these large artifacts affect many hundreds of pixels and can produce large-scale correlated noise that can hide faint spectral features, particularly in the deepest exposures.

Build 11.3 of the JWST Science Calibration Pipeline (released May 20, 2025) introduced an optional method to correct for these artifacts by adding the clean_showers option in the straylight step of the calwebb_spec2 pipeline. An example of the performance of this new algorithm for MIRI MRS observations of a high-redshift galaxy is shown below. This correction is not used by default to produce the data products available in the MAST archive, as it is not recommended for bright sources.

An overview of this algorithm and its use can be found on the MRS Known Issues and Shower and Snowball pages on JDox. An example of how to enable this correction in the JWST Science Calibration Pipeline for offline reductions of data is provided by the latest MIRI MRS Pipeline Notebook. Observers are encouraged to contact the JWST Help Desk with any questions.