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This is a follow-up to an earlier question. My intent here is to work out the details, and explore the potential consequences, of one of the many excellent answers I received. Relevant questions have been asked before, but these have generally focused on solar flares and electronics, or were otherwise distinct.


The Question

My goal is to design a habitable planet with a sun that's like ours, except for that once a decade it undergoes a change that inspires a period of religious terror and awe. To that purpose, in this new question I want to explore the consequences and possibilities of a sun with a much more extreme solar cycle, with a high solar maximum. I'd also like to handwave, for this question, whether such an extreme cycle could happen naturally in a yellow star like our own, and instead focus on the practical consequences for an inhabited world in its orbit.

So in other words:

  • How far can I crank up the solar cycle of an inhabited world, as a flashy special effect, before I also accidentally obliterate all of civilization with solar radiation?

Tinkering with the atmosphere, orbit, etc. to keep the world alive is encouraged, so long as it retains basically earth-like conditions. The tech level is late 1700s, so there are no significant electronics to consider. Finally, though I have a very poor understanding of the science at work, I've attempted to cover my bases by adding some optional prompts below.

Extra Details

Light

  • Color: How big could I make the sunspots? Could they become large enough to change the color of the sun to an angry red? If so, how would that change the sun's luminosity?

Magnetism

  • Magnetism: Solar activity causes geomagnetic storms, which on earth can disrupt compass activity. Could these storms be made powerful enough to effect ferrous metals, even limiting construction and tool use?

  • Electricity: Geomagnetically induced currents can invest ground-level conductors with surges of electricity, which on earth means power grids and piping. If these currents were stronger, could they hit anything else? Going out on bit of a limb, could an extreme effect impact regions with large natural metal deposits?

Climate

  • Atmosphere: I've read that solar radiation can temporarily damage the atmosphere. Would that damage be visible to the eye? Would there be lasting effects for a few years after, such as higher cancer rates, for the people and animals below?

  • Weather: Would intense solar activity alter the climate, weather patterns, winds and cloud formation, etc.?

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  • $\begingroup$ For how long has this decennial extreme solar maximum occurred (which can be anything from the beginning of the solar system to within memory of young parents)? Heck, there can even be variations in how often the extreme solar maxima occur. $\endgroup$ – RonJohn May 14 '17 at 19:12
  • $\begingroup$ @RonJon: The plot point I was considering was that the sun used to be like ours, but has been gradually getting worse over the course of the last several thousand (2-5) years. The extreme maxima now, ideally, occur once every ten years, for 1-2 months. $\endgroup$ – Era May 14 '17 at 19:19
  • $\begingroup$ Ok. Well, the Earth's magnetosphere does a darned good job of protecting us from solar storms. en.wikipedia.org/wiki/Solar_storm_of_1859 In the solar storm of 1859 -- which lasted for 2 days -- the aurora borealis reached down to Cuba. Even then, and by definition, the vast majority of the CME was deflected by the magnetosphere. In order to kill off enough lower flora and fauna for most humans to die, the magnitude and frequency of such CME would have to be ginormous (especially since the *vast majority of CMEs would miss Earth). $\endgroup$ – RonJohn May 14 '17 at 19:30
  • $\begingroup$ Also, "Would that damage be visible to the eye?" That's the aurora borealis, which do destroy the ozone layer. $\endgroup$ – RonJohn May 14 '17 at 19:36
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By your rules, no limit really. As long as the level is consistent, you balance the increased energy with other changes as you mentioned.

The real issue is the variability and the time scale over which it is variable. But again, just change the ecosystem to match: maybe they have 20 year “superseasons” corresponding to the sun’s cycle, and the trees evolve to rely on that rather than finding it to be a problem.

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