The SSC mosaicker, MOPEX, identifies energetic particle hits as follows. All pixels in BCDs that contribute to a given pixel in the final mosaic are identified, and significant outliers (a user-specified number of sigmas above or below the filtered mean of all overlapping pixels of overlapping BCDs) are rejected. This method is very similar to the outlier rejection performed by shifting and adding ground-based images. The rejected pixels can be inspected in the “rmask” output files (one per input image). Outlier rejection in MOPEX can be adjusted. The parameters used in the online pipeline-generated mosaics rely on three or more sightings of each sky pixel. In general, a coverage of at least five is necessary to produce optimal results with the multi-frame (standard) outlier rejection.
A special outlier rejection scheme can be used for sparse (2 - 4x) coverage; this “dual outlier” mode can be turned on using the namelist parameter file. Dual outlier rejection identifies pixels with flux values greater than a specified threshold above the background, groups these pixels and adjacent pixels above a threshold into objects, and compares the object to objects in overlapping frames. If the object overlaps with objects in other frames (in celestial coordinates), then it is not a cosmic ray. If the object is not detected in a user-specified fraction of overlapping images, it is flagged as a cosmic ray. This information is written into the rmask files used for mosaicking and source extraction.
Figure 7.23: The central 128 pixels × 128 pixels of IRAC 12-second images taken on January 20, 2005 during a major solar proton event. Channels 1 and 2 are top left and top right; channels 3 and 4 are bottom left and bottom right. Except for the bright star in channels 1 and 3, almost every other source in these images is a cosmic ray. These data are from observations in PID=3126.
The dual outlier method should also be used in conjunction with the multi-frame outlier rejection method. Multi-frame rejection may throw out data around bright sources depending on the thresholding, due to pixel phase effects between BCDs. Using the dual outlier rejection and the REFINE_OUTLIER = 1 option in MOPEX will prevent this.
Additionally, a single-frame radiation hit detector is run and produces bit 9 in the imask, but this bit is not used by the SSC post-BCD software and is not recommended because radiation hits cannot be uniquely separated from real sources in single images.
Cosmic rays for channels 3 and 4 are larger and affect more pixels than the channel 1 and 2 cosmic rays due to the larger width of the active layer of the Si:As detectors. Some tuning of cosmic ray detection parameters may be necessary when working with deep integrations, especially for channels 3 and 4.
Each IRAC array receives approximately 1.5 cosmic ray hits per second, with ≈ 2 pixels per hit affected in channels 1 and 2, and ≈ 6 pixels per hit affected in channels 3 and 4. The cosmic ray flux varies randomly by up to a factor of a few over time scales of minutes but does not undergo increases larger than that. Also, the cosmic ray flux is normally about a factor of two higher on average around solar minimum compared with solar maximum. Radiation hits do increase suddenly and dramatically during some major solar proton events. Historically, several such events have occurred over the course of the active part of a solar cycle.
Two major solar proton events occurred during IOC, so we gained experience in identifying them and their effects. Because of shielding around the instruments, only extremely energetic protons (> 100 MeV) of any origin appeared as cosmic ray hits in the data. Thus, many solar weather phenomena (“storms,” etc.) which do occasionally affect other spacecraft, or ground systems, were not of concern to Spitzer.
Radiation had very little effect on the IRAC arrays beyond elevating the counts in a given pixel. Some high energy cosmic rays caused persistent images, column pull-down, and muxbleed effects.
