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Dust Storm
Dalhousie
post Jun 10 2018, 11:01 PM
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QUOTE (djellison @ Jun 10 2018, 09:34 PM) *
Moreover - it's not dust 'fall' that hits the arrays - it's just blowing around and sometimes you get some. Tilting at a large angle may actually put your arrays facing upwind and cause more harm than good.


Dust collects on upwind slopes, not downwind.
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ngunn
post Jun 10 2018, 11:11 PM
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Sure, my question was not about managing the situation right now, so academic in that sense. That's good information on the sandy stuff sliding on dangerous slopes, thanks. Just one thing I'd like to add: the solar panels however they are supported must have some some natural vibration frequencies. If it were possible to drive those natural oscillations the amplitude might be enhanced to the point where more dust slides.
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marsophile
post Jun 10 2018, 11:38 PM
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Is it still true that the prevailing wind is upslope?
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djellison
post Jun 11 2018, 12:30 AM
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QUOTE (ngunn @ Jun 10 2018, 03:11 PM) *
If it were possible to drive those natural oscillations the amplitude might be enhanced to the point where more dust slides.


They wobble - yes - and that's caused sand to slide down them driving on dangerously steep slopes in the past.
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djellison
post Jun 11 2018, 12:43 AM
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QUOTE (Dalhousie @ Jun 10 2018, 04:01 PM) *
Dust collects on upwind slopes, not downwind.


That's a rather simplified view of what really occurs

Papers like this : https://onlinelibrary.wiley.com/doi/pdf/10.1002/esp.1272 : go into it in more detail.

Moreover - the rover itself presents a topographic feature to the oncoming wind, whatever angle it comes from.
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pioneer
post Jun 11 2018, 12:55 AM
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Thanks. I thought I saw an image taken by one of the MER rovers during a dust storm.
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James Sorenson
post Jun 11 2018, 01:15 AM
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Oppy as well as Spirit did take some images of the ground to monitor pebble movement and images of the horizon during the 07 global storm when power conditions allowed it.
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serpens
post Jun 11 2018, 03:49 AM
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Yep. Spirit provided a sequence of images that provided a short movie of dust ripples in motion, but I think this was taken towards the end of the storm as the intensity of the dust diminished.
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djellison
post Jun 11 2018, 04:23 AM
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This mosaic is from Opportunity
https://www.nasa.gov/images/content/182691m...er-20070719.jpg
during the 2007 storm.

They were able to execute simple Pancam observations briefly during that storm. However - even if the tau were the same now, Opportunity would be in worse shape for all sorts of reasons (11 year older batteries, on a slope, it's not high summer ).

However - the tau is a lot LOT worse right now. We've seen both record high tau's, and record low power. The vehicle is facing conditions we just have no experience with.
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James Sorenson
post Jun 11 2018, 04:36 AM
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I am curious if the full frame wide range tau images that have been coming down shows any trace of the solar disk in the original data products, Doug? And if so, can a current estimated Tau be worked out from them, heard anything? smile.gif Mark's site was last updated on 5106 when the last visible solar disk images were taken.
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Deimos
post Jun 11 2018, 11:49 AM
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Hoping to see some new values added to that site soon. The combination of data from which tau may not be automatically extracted and a newsworthy event may require some permission to restart the updates.
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serpens
post Jun 11 2018, 12:15 PM
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QUOTE (djellison @ Jun 11 2018, 04:23 AM) *
However - the tau is a lot LOT worse right now. We've seen both record high tau's, and record low power. The vehicle is facing conditions we just have no experience with.

The Tau is about double the worst experienced by Opportunity during the last storm. With the current orientation what is the correlation between tau and Whr generated? Since the attenuation of sunlight through the atmosphere is essentially exp(-tau) the observed light intensity with tau around 10.8 must be negligible.
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Deimos
post Jun 11 2018, 07:32 PM
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I suppose since the news is here, it is on the plot here.

I did a very simple 2-stream model, and you can approximate total insolation (proportional to array W-hrs/sol) as varying with exp(-tau*gamma), where gamma is 0.35-0.40 or so. If you compare two sols, that's exp(-delta_tau*gamma). A lot of the extinction goes into sky light, but total light is still much reduced. At these taus, tilt is a small perturbation, I'd guess. (A chunk of sky is blocked, but that chunk is the source of a small fraction of light, and the tilt hasn't changed.)
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fredk
post Jun 11 2018, 07:47 PM
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QUOTE (Deimos @ Jun 11 2018, 08:32 PM) *
A lot of the extinction goes into sky light, but total light is still much reduced.

I guess there isn't much data from the ground during dust storms to say very accurately how much reduced the total light is. Technically the tau itself tells you how much darker the sun has become, but as Deimos says some of that will be scattered to the ground via the sky.

Anyway of course we don't need to do full scattering calculations, since what really matters is array power, which is known (if not publicly).
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MoreInput
post Jun 11 2018, 07:49 PM
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I just can't believe how fast the tau increased. Over the years it oscillated just below 1, and now within 8 sols it increased by 18 times.


--------------------
Need more input ...
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