# What are the atmospheric/planetary effects of having no solar wind?

Suppose we have an earthlike world orbiting an artificial star which produces light but no solar wind. I would like to understand the implications that a lack of solar wind would have for the planet. (I'm not currently concerned with the feasibility of an artificial star or how one could or could not work)

So, for the purposes of the question: assume a solar system which is a perfect copy of the Sol system, with the exception that Sol itself has been replaced with an artificial star of equal mass and with light output of equal spectrum/intensity/etc, which does not by any mechanism (aside from emitting light) spew any of its mass into space.

What are the differences between this system and ours that would affect Earth?

I can think of a couple differences I would expect:

• There would be no aurorae borealis/australis.
• We would not suffer the effects of any "solar storms", because of a lack of coronal mass ejections.
• Hydrogen and helium (how much?) would be accumulating in the upper atmosphere instead of being blown into space.

A good answer to this question will:

• Confirm, amend, correct or refute my existing expectations,
• Enumerate any additional effects I am missing,
• Outline any obvious/large scale implications of these differences (eg. "because of the accumulation of hydrogen in the atmosphere, you will have..."; "because of the lack of solar wind, the interstellar medium will permeate the system and..." )
• You'd have to deal with the interstellar medium for one, there being no helisphere, as well as interstellar radiation – Madman May 4 at 18:09
• I'm definitely interested in the effects of the interstellar medium, yes! – Qami May 4 at 18:10
• No more comet tails :.( – DWKraus May 5 at 14:12
• @DWKraus more like comets will only have one tail, instead of the two they have now. The dust tail should be vanishingly thin, but the gas tail is propelled mostly by direct radiation pressure, not solar wind. – PcMan May 5 at 18:01

This is a great question; I hope I can write a half-decent answer. Some of these points have already been made by others, but I wanted to go into more detail.

## Atmospheric escape

I actually don't think that the atmospheric composition will change significantly, because the solar wind isn't the primary driver of atmospheric escape. Rather, photons - particularly, ultraviolet photons - are the main energizers of molecules. You can certainly transfer energy from solar wind particles via a sort of analog to sputtering, but it's not the dominant mechanism. If you're not convinced, then consider the "luminosity", so to speak, of the solar wind - the change in kinetic energy imparted by the Sun onto the wind. It's $$L_{\text{wind}}=\frac{1}{2}\dot{M}v_{\infty}^2$$ with $$\dot{M}$$ the mass-loss rate and $$v_{\infty}$$ the speed. Assuming $$\dot{M}\sim10^{-14}M_{\odot}\text{ yr}^{-1}$$ and $$v_{\infty}\approx500\text{ km s}^{-1}$$, I get that $$\frac{L_{\text{wind}}}{L_{\odot}}\approx2\times10^{-7}$$ meaning that most of the energy the Sun impacts on molecules in the atmosphere is transferred via photons, not the solar wind. Therefore, removing the solar wind wouldn't affect thermal escape, the dominant category of mechanisms of atmospheric escape.

## Interplanetary medium vs. interstellar medium

It's hard to compare the interplanetary medium to the interstellar medium (ISM) because the ISM is a very complicated thing (which, of course, astronomers are more than happy to ignore from time to time!). There are three stable phases (McKee & Ostriker 1977) and a myriad of structures, including molecular clouds, galactic cirrus, H II regions, and more. They vary drastically in temperature (from $$10\text{ K}$$ to $$\sim10^6\text{ K}$$) and density (from $$\sim10^{-2}\text{ cm}^{-3}$$ to $$\sim10^3\text{ cm}^{-3}$$).

The Solar System is (likely) embedded in a structure called the Local Interstellar Cloud (LIC), a structure inside the larger Local Bubble, so let's say that your system is, too. The LIC has a temperature of $$7000\text{ K}$$ and a number density of $$n\sim0.1\text{ cm}^{-3}$$. This makes it roughly an order of magnitude cooler and a factor of 50 less dense than the interplanetary medium. In other words (as others have said), if the interplanetary medium was replaced with the ISM immediately outside the Solar System, nothing would change for the planets - and that assumes that the Sun wouldn't just heat it up again, which seems rather likely.

The key point here is that there really wouldn't be any issues from the (quite small) influx of particles from the ISM - not by a long shot. We'd probably see some differences in elemental abundances, but there's still going to be mainly hydrogen and dust around.

## The auroras

I can't add anything here; you're quite right that we would no longer see auroras. Interactions between the solar wind and a planet's magnetosphere are crucial for their production, and without one of those . . . well, you don't get much of a light show.

## Comet tails

In the comments, DWKraus noted that comets would be affected. Comets have two main tails (as well as a fainter tail of escaping sodium), one composed of ions pushed away by magnetic fields and the charged particles of the solar wind, and the other composed of dust blown away by solar radiation pressure. Without the solar wind, the tail of ions would be gone, but the dust tail would remain.

• On the other hand regarding auroras: Hardly anyone sees them anyway (relative to the total percentage of people on earth). – Hobbamok May 5 at 13:24
• @Hobbamok That's true; I think they're worth mentioning because they're probably the easily the most visible consequence of the solar wind on Earth. – HDE 226868 May 5 at 13:58
• @HDE226868 Thank you for your research effort and analysis on this! – Qami 2 days ago
• @Qami I'm glad it helped! – HDE 226868 2 days ago

My guess? Surprisingly little.

The lack of a heliosphere would allow a higher galactic cosmic ray flux (about 2-10X depending on the source), but while the possible impact on the cloud cover (and hence on the climate) has long been thought to be a possible solution to the Young Sun paradox, it has recently been determined that this is very likely not the case.

The interstellar medium invading the Solar System wouldn't be a significant concern unless the Solar System crossed a molecular dust cloud, with densities above 1E+6 particles per cubic centimeter. In that case, some effect on the Earth orbit might be observable, albeit over a long period of time. I suspect that would be more than offset by the lack of momentum conversion into thermal energy that the Earth experiences due to the interaction between its magnetic field and the heliospheric current circuit.

Large aurorae would be replaced by a nearly constant, but way dimmer, auroral light caused by the onrush of the galactic wind (with the occasional OMG particle).

Hydrogen and helium would accumulate in the upper atmosphere, but this would not have a significant impact on anything, not on sub-geological timescales at least.

Lack of solar cycles would not greatly impact Earth, except for a slightly greater climate stability (so, also slower evolution). This too would not be perceptible on short time scales.

Most important effects would stem from the lack of a heliosphere The amount of just raw radiation in interstellar space, according to current human observations is about 70 rems per year, while the moon receives about 30 rems per year, so you can assume that the amount of radiation received is going to be around 2.3333333.... times higher than the earth, or so, not accounting for the magnetic field

As the planet pushes and pulls on the interstellar medium around the star through the magnetic field applying a prograde and retrograde force, due to the interstellar medium flowing one way, meaning the planet would be slowed down by drag against the interstellar medium then sped back up again, then slown down.... the consequence of this would be the planet's orbit would end up wobbling, as it pushed and pulled and pushed and pulled against the interstellar medium. Incidentally, this would also cause an Aurora Borealis to continuously happen on a yearly basis, so the term Estisios Borealis might be better fitting.

Other effects might be due to the accumulation of interstellar particles in the atmosphere the atmospheric pressure would end up fluctuating to a degree -on -a -predictable -regular basis, the amount of which depends on the amount of interstellar medium gathered and released, which in turn depends on how large the planets orbit is( a closer orbit causing more extreme fluctuations time-wise of a lesser degree, while farther out the fluctuations are larger due to more contact with the interstellar medium, meaning inherently that more gas is accumulated over a larger timespan.

which would also incidentally make the exaggerated increase and decrease of the planet's orbit even more extreme due to increasing and decreasing mass, which would also indicate a more extreme back and forth the farther the planet's net orbit got away from the sun. On top of having an circulular orbit. Stricly speaking you still can have a perfectly circular orbit, but the energy present based of off the relation to it's sun alone wouln't be.

Oh- and depending on the radiation strength and the magnetic field you range between having non of the effects of a solar storm ever, and having a solar storm all the time, nonstop