It seems as though the range increases we've seen with turning on the SR3 ring heater are at least partially real and not just due to miscalibrations, according to Georgia and Dan.  This alog attempts to explain what about the DARM plant changes when the SR3 is heated, and why it wins us megaparsecs.

It appears that turning on the SR3 ring heater from 0 W to 5 W flips the sign of the DARM detuning, as witnessed by the DARM optical spring.
Otherwise the DARM plant has ~1% changes in the optical gain and DARM pole between on and off states.

As a reminder, DARM is set for resonant sideband extraction (RSE), meaning the nominal phase of the DARM coupled cavity is 90 degrees.  This is accomplished by setting the SRC length such that carrier is antiresonant.  "Detuning" refers to the fact that the SRC is not actually tuned to exactly 90 degrees, and this tuning can change the DARM response.  The DARM optical spring relies on the sign of the detuning: in one case, the optical spring is restoring like a normal spring.  In the other, we have an antispring behavior where the further we are off resonance, the antispring pushes us away even harder.  For years at LIGO Hanford, we have operated with an nontrivial DARM antispring. 

I analyzed the DARM plant measurements during the SR3 heater turn on from two nights ago. The DARM plant measurements were taken every twenty minutes, starting when we turned on the SR3 heater to 5 W, then continuing through three hours later when turning the SR3 heater back to 0 W. (SR3 Heater turned on = 2019-03-16 09:18:03 UTC, SR3 Heater turned off = 2019-03-16 12:17:59 UTC)

I recovered 18 DARM plants, and 18 DARM OLGs.  The DARM plant clearly goes through a large change at low frequencies.  DARM UGF = 54 Hz. (Attachments 4 and 5)

I modified my DARM plant model to allow for both positive and negative detuning (see Appendix D of Hall for more), and fit each measurement via MCMC.  (Attachment 2, or the PDF)  
Somewhere between 20 and 40 minutes after turning on the heater, the DARM optical spring flips sign.  

I also plotted the DARM fit parameters as a function of time. (Attachment 3)
While the SR3 heater is hot (at least 40 minutes after turning on the heater):
- The optical gain of the DARM plant increases by about 1%.  
- The DARM pole decreases from 425 Hz to about 418 Hz.
- The delay increases by 7% (from 43 to 46 μs)
- The optical spring changes sign from 2.7 to -3.6 Hz.
- The optical spring Q has no discernible change given its noise. 

The increase in optical gain will help kill shot noise everywhere, while the detuning sign flip increases the DARM response significantly (~8% at 20 Hz) at low frequency.  The decrease in the cavity pole will decrease DARM response at frequencies beyond the DARM pole.  Overall, the net effect is a significant gain in range.

Since we are now running with the SR3 heater on all the time, and are potentially going to power up to 36 W, our DARM model which assumes an antispring detuning may need to be updated.