Following the report of a decrease in count rates in the longest-wavelength channels of the MIRI Medium Resolution Spectrometer, the team has worked on carefully quantifying the effect of this loss on science data. In the short-wavelength channels of the MRS (Channels 1 and 2), the impact is negligible and no mitigation is required. In Channels 3 and 4, the loss increases with wavelength from 1-10%, and 15-50%, respectively. The issue is not seen at present in the MIRI Imager, although efforts are ongoing to characterize whether any impact is observed at substantially lower levels.

The MRS calibration from count rate to physical units derived during JWST commissioning no longer provides the same accuracy as at the start of the mission (see Figure 1 below).

To compensate for the change in the spectrophotometric response, the MIRI team is producing an update to the spectrophotometric calibration reference files that adds a wavelength- and time-dependent flux correction factor to the data. This update will be included in the next major pipeline update (Build 9.3), scheduled for release in summer 2023. The benefit of this updated reference file is demonstrated in Figure 2 below.

In Channels 4A and 4B, the achieved (continuum) SNR relative to commissioning has decreased by about 5-10% and 10-20%, respectively, but is still comparable to current ETC predictions. In Channel 4C, the SNR loss is roughly 20-30%, lower than predicted by the current ETC. Line sensitivities follow a similar trend. The rate of loss has shallowed substantially in recent months and will continue to be monitored carefully. The ETC release for the upcoming Cycle 3 call for proposals will be updated accordingly.
 
MIRI Instrument contact scientists will proactively reach out to PIs of Cycle 2 accepted programs whose science may be affected to make individualized recommendations. Investigator teams of Cycle 1 programs who have concerns about meeting planned scientific objectives that rely on Channel 4 should reach out to the JWST Help Desk. Similarly, observers with questions about MRS data or the execution of upcoming MIRI programs are encouraged to issue Help Desk tickets.

The root cause of this issue is still under investigation. MIRI hardware is not at risk and the instrument continues executing observations normally.