The three near-infrared (NIR) instruments on JWST (NIRCam, NIRSpec, and NIRISS) are known to experience large cosmic-ray events termed ‘snowballs’ that can affect hundreds to thousands of pixels. These events cause large amounts of charge to be deposited in the primary impact site that is released gradually over time, resulting in artifacts that can limit the ability of observations to reach the detector noise floor. More information about these snowball artifacts and the related mid-infrared ‘shower’ phenomena can be found on JDox.

Correcting for these artifacts within the JWST pipeline has been an ongoing focus of development throughout Cycle 2 and as a result, typical NIR data can now be mostly cleaned of snowball artifacts. Starting in Build 10.0, no snowball correction was made in the default pipeline, leaving significant residuals from both the saturated cores and extended snowball halos. In Build 10.1, a snowball detection algorithm was enabled by default for most NIR observing modes, removing the faint outer halos. Now the current Build, 10.2, also removes persistence from saturated cores that could significantly bias data taken with multiple integrations. These improvements are demonstrated in Figure 1. 

Additional improvements are still under development, including corrections for fainter and asymmetrical snowballs (to be included in Build 11.0, expected late summer 2024) and for persistence of snowball cores between exposures.

Users seeking more information about the impact of such artifacts on their observations are encouraged to reach out to the JWST Help Desk.