If I remember correctly, that’s due to thermal noise in the lowest PHA channels, which is periodic because of the differing sun exposure as the orbit changes.  I will see if I can find some plots that I made near the beginning of the mission which showed this.  It is why I discard the lowest PHA channels and never look much below the proton peak – the data are not valid down there.

--Mark

Mark D. Looper
Space Sciences Department
The Aerospace Corporation
M/S M2-260
P.O. Box 92957
Los Angeles, CA 90009-2957
Mobile: 310-529-3406
Voicemail: 310-336-6302

Publications: https://www.loopers.org/curvitae.html

From: Crater-team <crater-team-bounces@lists.sr.unh.edu>
Date: Monday, May 17, 2021 at 7:07 AM
To: crater-team@lists.sr.unh.edu <crater-team@lists.sr.unh.edu>
Subject: [Crater-team] Interesting Artifacts in CRaTER Data: 6-Month Periodicity in Low LET Fluxes (including a strange anti correlation)

All,

While working on the final processing of the observed CRaTER Flux vs. LET data, I have found an interesting artifact.

I have been looking at the Flux vs. LET vs Time data for the entire mission so far.

• For the smallest LET values, there seems to be a 6-month periodicity in the observed flux, see the ‘wavy’ red edge in Figure 1 below.
• When I divide everything by the flux observed in August 2009, I get the flux ratio shown in Figures 2 and 3 below. When looking at the flux ratio, the 6 month periodicity becomes very clear.
• One interesting thing I also notice is that it seems like there is an anti-correlation between the flux in lowest LET bins (< 0.25 keV/micron) and the slightly higher LET bins  (> 0.25 keV/micron).

Has anyone else observed this yet? Any thoughts about what is causing this?

Thanks,

Wouter

Figure 1: Flux vs. LET vs. Time

Figure 2: Flux Ratio vs. LET vs. Time (3-Dimensioal)

Figure 3: Flux Ratio vs. LET vs. Time (2-Dimensioal)