Subject: [ids.analysis.forum] surface forces
From: "John C. Ries" <[email-masked]>
To: "[email-masked]" <[email-masked]>, Cerri Luca <[email-masked]>, Eelco Doornbos <[email-masked]>, Marek Ziebart <[email-masked]>, Frank Lemoine <[email-masked]>, Scott Luthcke <[email-masked]>, Michiel Otten <[email-masked]>, Eric Leuliette <[email-masked]>, "Mercier Flavien (contact)" <[email-masked]>, "[email-masked]" <[email-masked]>, Steve Klosko <[email-masked]>, [email-masked], Nikita Zelensky <[email-masked]>
Date: Wed, 3 Mar 2010 22:23:16 -0600

Hi,

This thread is expanding to more and more recipients. I apologize if 
you get this multiple times.

>
>I would call it two-beta-prime, instead of half-beta-prime, since 
>the frequency is twice that of beta-prime. But anyway.

Indeed, I meant half the period but said frequency.

>
>With one-beta-prime we think about all the effects related to solar 
>pressure in which the sign of beta-prime matters, whether the sun is 
>above or below the orbital plane. I can see that that certainly is 
>the case of the z-shift.
>Note that the one-beta-prime frequency is virtually the same as the 
>aliased S1 frequency. This is logical, since S1 is related to local 
>solar time, and hence to beta-prime.
>
>The two-beta-prime is also related to solar radiation or heating, 
>but in a sense where it does not matter whether the sun is above or 
>below the orbital plane. For heating it really does not matter much: 
>it is the amount of sunlight the satellite gets, whether it comes 
>from left or right. Then with the satellite flipping around when the 
>beta-prime changes sign, the two-beta-prime dependency starts to 
>make sense.
>Again, two-beta-prime has the same frequency as S2.

If it's heating, it has to be something that cares about the yaw-flip 
to get the two-beta-prime frequency. I don't see the direct heating 
as caring very much about that, while the heat dumping would. But the 
effect has to be something that is 1/rev modulated by the beta prime. 
If it's heat being dumped, why would the modulation be strong during 
high beta prime when the satellite heating ought to be the most 
steady? For that matter, the changes in the sun-satellite orientation 
should be minimal at high beta prime, so I'm wondering if any direct 
or indirect effects from the sun make sense.

>
>>  2) There was also the mention of S2, whose alias is 58.6 days. How 
>>much did T/P influence the estimation of S2 in the tide models 
>>being used? Could S2 have absorbed a T/P perturbation that is now 
>>showing up in Jason 1 and 2 (and the same in both)? A tide modeling 
>>error will show up 100% in the sea level time series but will be 
>>diluted in the orbit differences. For what it's worth, one of the 
>>most prominent error aliases in the GRACE estimates for C20 is S2.
>
>This is also something Eelco suggested, and I had a hunch about that 
>too. Either we are all equally suspicious, or there is something too 
>it, it certainly is worth considering. However, any error in the S2 
>tides should affect TX, J1 and J2 the same during their tandem 
>missions (their time intervals are very small). And since we see the 
>58-day variation in the TX-J1 differences, it can hardly be tidal 
>gravity.

It would be rather unlucky, but my argument would be that a 
systematic error in T/P has biased the S2 tide model, so that the 
effect on T/P cancels out by construction while J1 now has to deal 
with a biased S2 tide. But this assumes that T/P had an enormous 
influence on the S2 tide estimates, something the tide modelers would 
have to tell us.

I also note that not only does S2 show up as a tidal alias in C20 
from GRACE, so does S1. S1 would be expected to be small, unless it 
is also biased by T/P. On the other hand, K2 and K1 also show up as 
strong aliases. So this argument may not hold much water.

Regarding the table Nikita sent about the solar array warp.... We did 
look at that in an earlier study, and we could not find anything 
convincing. But orbit accuracies were not nearly as good as now. This 
effect would be once/rev, modulated by beta prime, but again I would 
expect it to be maximum at small beta prime, not large (as noted 
above).

See ya,
JR