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Susie Slavney

Negative temperatures in Diviner data

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Some users of LRO Diviner data have asked why there are some data products with negative temperature values. For example: "I'm exploring the temperature data of the Diviner, I have downloaded TB (Brightness Temperature) data for 6,7,8, and 9 channels and I found negative values (in K) in a few of the images after conversion (TB= (DN * scaling factor) + Offset). This negative value is not possible in K, I'm unable to understand this."

Answers from Diviner team members:

"The shorter wavelength channels are noisy at low temperatures. Each channel has a minimum temperature below which it becomes noisy. These minimum temperature cutoffs go down with longer wavelength channels, so channels 8 and 9 are really the only ones that return useful data in the PSRs. When the channel gets below this minimum temperature, it starts returning negative radiance values. This would explain why channel 4 is giving you negative temperatures, while channel 9 does not. I've attached the Supporting Material from the Paige et al 2010 Science paper [see attachment Paige.SOM.pdf]. The table gives the minimum temperatures for the IR channels (3-9)." (Dr. Jean-Pierre Williams, UCLA, 2019-03-11)

"The negative temperatures are actually "good" data. When the net signal at the detectors after calibration gets close to zero, Diviner records both small  positive and small negative radiances. When the negative radiances are turned into brightness temperatures, they are reported as brightness temperatures. Zero-ing out the negative radiances would be completely inappropriate, because it is possible to pull signal out of the noise by averaging. You should see more negative temperatures in the shorter wavelength channels (3-6), as well as all channels in the coldest places near the poles.

When the thermal emission from the moon is so small that there is no measurable signal, there's a 50% probability that a small negative signal will be measured. We convert these to brightness temperatures, and flag them as negative brightness temperatures, with the understanding that negative temperatures are not possible.

In isolation, you can't convert negative temperatures into meaningful temperatures.  What we generally do is pick a threshold below which all temperatures should be ignored. This will vary depending on which Diviner channel you are using." (Dr. David Paige, UCLA, 2020-03-24)

Paige.SOM.pdf

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