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What is the mechanism of solar flares that causes degradation of spacecraft orbits? The kind that deorbited Skylab?

The proton storm is fairly thin, its pressure shouldn't affect the kinetic energy. What other effects can cause that? Electrodynamic drag?

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Density of the proton wind coming from the Sun is few ten particles per cubic centimeter (atmospheric density is orders of magnitude higher in LEO), thus not influencing a satellite by drag. Actually, the proton flow could also add tiny amounts of speed to the satellite when hitting it from the back. The solar storm is accompanied by a tiny magnetic field (up to 10 nT, less than Earth magnetic field) which could cause some electrodynamic drag, but not very much.

There are two possible ways how increased solar activity can degrade a orbit in indirect ways: First, the protons (as far as they do penetrate the Earth magnetic field at all) can cause radiation influenced damage to on-board electronics. This could lead to a failure of the guidance system and subsequently to unplanned orbital maneuvers. Fortunately, these systems are usually redundant and therefore save from such errors.

As you explicitly mention Skylab - its faster reentry was due to increased solar activity over a longer period. The activity heats up the upper atmosphere and increases its density. Some nice plots can be found in this paper: Empirical Modeling of the Thermosphere.

As you can see, solar activity can change the density of the medium in LEO by more than a factor 10. Any increase in density proportionally increases the drag and decreases orbital lifetime. It seems that in the Skylab case the estimate of atmospheric density was about half of the actual density, therefore halving the time before a reboost of the station was necessary.

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  • $\begingroup$ Interesting graphs! It seems the drag at 400km orbit rises to equivalent of a 250km orbit during high solar activiy - and the density scale being exponential, that's over an order of magnitude. And speed->altitude loss is an exponential function again (craft entering "naturally" exponentially more dense atmosphere and losing velocity faster.) So, we're facing an $O(e^{e^x})$ scale phenomenon. $\endgroup$ – SF. May 12 '16 at 6:09
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It is an indirect effect; increased solar activity affects the Earth's atmosphere in such a way to cause a net density increase at a given orbital height. This increases the drag on objects in LEO. See the entry section of this article.

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