# Why doesn't the solar wind disrupt the planets?

The sun creates this heliosphere by sending a constant flow of particles and a magnetic field out into space at over 670,000 miles per hour, which is also known as solar wind. If the speed of the wind is that great, why doesn't it disrupt the planets in our solar system? Does it just prevent foreign particles from interstellar space entering the heliosphere?

• Why the downvote? This seems a perfectly good question, and the answer would involve calculating the momentum flux of the Solar wind, which isn't a trivial calculation. – John Rennie Oct 25 '15 at 7:16
• 2 reasons: The title is actually not very relevant to the question and secondly the wording of the question. – Tamoghna Chowdhury Oct 25 '15 at 7:28
• It's not so much the solar wind that prevents many cosmic rays and particles from entering the solar system, it's the Sun's Heliosphere (it's fed by the solar wind, but it's technically not the solar wind). en.wikipedia.org/wiki/Heliosphere – userLTK Oct 25 '15 at 8:03
• Also, @tammy chong, the magnetic field does not propagate through space at 670,000 mph, it's effects are felt in space at the speed of light, nearly 180,000 mps(miles per second) – Tamoghna Chowdhury Oct 25 '15 at 9:32
• The comment from @Michael is the first place I've seen "km" as an abbreviation for "thousand miles". – Andreas Blass Oct 26 '15 at 19:30

The Solar wind does indeed exert a force on the planets, however it turns out that the force is so small that it has no measurable effect.

The force can be calculated using the fact that force is equal to the rate of change of momentum. Suppose the total mass of all the Solar wind particles hitting the Earth per second is $M$, and the average velocity of the particles is $v$, then the force the solar wind exerts on the Earth is simply:

$$F = Mv$$

Off-hand I don't know what the mass flux and velocity are, but the Wikipedia article on the solar wind reports the pressure, $P$, produced by the wind at the Sun-Earth distance to be 1 to 6 nano-Pascals. The total force on the Earth is this pressure multiplied by the cross sectional area $\pi r^2$. The radius of the Earth is about 6,371,000 metres, so we get:

$$F = P \times \pi r^2 \approx 130 \,\text{to}\, 800 \,\text{kN}$$

To see why this is negligible, let's compare it with the gravitational force between the Sun and the Earth. This is given by Newton's law of gravity:

$$F = \frac{GM_\text{Sun}M_\text{Earth}}{r^2}$$

and it works out to be:

$$F \approx 3.54 \times 10^{22} \,\text{N}$$

so the force from the Solar wind is only about 0.000000000000001% ($10^{-15}\%$) of the gravitational force.

• Comments are not for extended discussion; this conversation has been moved to chat. – Manishearth Oct 27 '15 at 1:02
• I'm puzzled by the $F = Mv$ formula (even if it doesn't matter for the rest of the answer). I know $F = Ma$ and $p = Mv$: how do you get to $F = Mv$? – OxTaz Oct 27 '15 at 12:56
• @OxTaz: force = rate of change of momentum. $M$ is the mass flux i.e. the amount of mass that hits the Earth per second. So assuming the particles stop when they hit the Earth $Mv$ is the momentum change per second. – John Rennie Oct 27 '15 at 12:58

The solar wind does disrupt the planets.

If a planet does not have a magnetic field (for reasons described later), the solar wind can strip an atmosphere through a process called sputtering. Without a magnetic field, the solar wind is able to hit the planet's atmosphere directly. The high-energy solar wind ions can accelerate atmosphere particles at high altitudes to great enough speeds to escape.

The relative importance of this effect compared to other forms of atmospheric escape is a topic of active research. The NASA Maven probe is one of the latest tools to answer this question:

Scientists have thought that Mars lost much of its atmosphere through a process known as stripping, when the solar wind pushed a lighter isotope (type) of hydrogen out into space, leaving a heavier isotope called deuterium behind. As hydrogen escaped, the atmosphere thinned. This could account for why water stopped flowing on the Martian surface billions of years ago.

So, although not all scientifics agree (Wikipedia contains an unsourced claim that those NASA Maven scientists are in error!), to claim that the solar wind does not disrupt the planets, is currently premature.

• Can you give an example about how the Sun wind velocity influences to atmospheric escape? I mean what if it doesn't influence at all, as long as the Magnetic field is strong enough? I mean analogy to gravity on the answer of Rennie. It's like standing beside a train track, you would be killed by the collision of Train. -But you are not in the track, so a conversation about the Train velocity has no relevance. What if the Solar wind ie. increases the amount of atmospheric gases, ie. through Aurora? Can you even answer if the impact of the solar wind is positive or negative? Wiki says "belief". – Jokela Oct 25 '15 at 14:10
• @JokelaTurbine I don't know, and I'm not sure if domain experts know. I'm not a specialist and my knowledge is quite limited. – gerrit Oct 25 '15 at 17:54
• @JokelaTurbine - The upper atmosphere is ionized and thus subject to $\mathbf{E} \times \mathbf{B}$-drifts. In this case, one can approximate $\mathbf{E} \sim -\mathbf{V}_{sw} \times \mathbf{B}$, where $\mathbf{V}_{sw}$ is the solar wind velocity. Thus, the solar wind velocity is not irrelevant (my comment is in regards to a non-magnetized planet... see my answer for issues with magnetized planets). – honeste_vivere Oct 26 '15 at 10:48

The solar wind is a flow of charged particles, called a plasma, constantly eminating from the sun. Plasmas can exhibit an interesting property called frozen-in whereby the magnetic field and bulk flow are locked together (well, technically it's a flux freezing condition but...). The magnetic field does not really move, rather the sources move, but that's a rather nuanced point. The point is that a magnetized plasma is constantly bombarding planetary magnetospheres.

The answer to your question is that the solar wind does in fact affect planetary atmospheres and certainly their magnetospheres. Below I will describe one example for how this can affect our atmosphere. John already explained that they dynamic pressure is quite low, thus the "wind" itself is not really an issue. So I will focus on the electromagnetic effects.

The Aurora
Through a process called magnetic reconnection, energy and momentum can transport across the magnetopause into the magnetosphere. The reconnection process results in a reconfiguration of the magnetic field topology and new stress/strains are imposed on the field in different places within the magnetosphere.

One of those places is the geomagnetic tail (i.e., anti-sunward side of the planet), where enhanced solar wind flows and dayside reconnection can lead to "stretching" of the topology. The stretched fields in the tail can also reconnect. One of the consequences of the reconnection process is similar to that of a slingshot. When magnetic field lines are stretched and then released, they can react in manner similar to a relaxing rubber band. Since charged particles generally do not like to move across the magnetic field, due to the Lorentz force, the relaxation of the magnetic field can result in significant particle acceleration/energization.

So when the tail fields relax, they can accelerate particles toward the planet. As the particles near the planet, encountering different plasma densities and field intensities, they can be further accelerated by several other processes. I discussed one of those processes here. Some of these particles will gain enough energy and have a small enough pitch-angle that they can enter the planetary atmosphere. The deposition of energetic particles can excite neutral atoms and lead to the emission of light, called the aurora.

Other Effects
There are several other effects of the solar wind on planets including the rate of polar outflow (i.e., process by which charge particles "leak" out of an atmosphere along the magnetic field), ground induced currents, effects on total electron content, etc.

So in general, there are many ways in which the solar wind can affect a planet, whether it is internally magnetized or not.

I can recall 2 points as to this question:

1) The Solar Wind, as you call it, is not very dense. Even if it is very fast, it doesn't carry enough kinetic energy to, say, knock asteroids out of orbit.

But note that the solar wind not being dense is applicable only beyond the Sun's coronal extent. Within the coronal region, the solar wind is ( reasonably ) dense ( as regards space gas ) and does influence the Sun's heliosphere/corona by pure kinetic disruptions/collisions alone.

2) The solar wind's maximum disruptive effect does not come from its kinetic energy, but the electrical disruption produced by the charged particles it carries. However, all planets with a reasonably thick atmosphere ( approx. 300km extent not including exosphere ) have a magnetoshpere generated by tidal currents in their ferrous liquid cores which repel these charged particles away and prevent them from eroding the atmosphere by ionising the gas and then repelling them away. Case in point: Mars apparently lost much of its magnetospheric strength to protect itself from the solar wind due to some unknown catastrophe of cosmic scales some 2-3 billion years ago. Earth still has its at near full strength, so we are alive. But even on Earth, above 70 degrees latitude, you can see the aurorae, which occur due to electrical interactions between ionospheric particles and charged particles from the solar wind. This is because of the fact that the Earth's magnetic field is in continuous flux and is somewhat weaker at higher latitudes ( beyond Canada ). So, to answer your questions:

The solar wind does disrupt the planets of the solar system, but electrically, not (significantly) kinetically. Even this electrical disruption is reduced due to our protective magnetosphere. It also retards ( and prevents entry if they have low energy or are highly charged ) the entry of interstellar particles (cosmic rays) into the system. So all the cosmic rays we observe on Earth have very high energy even after penetrating through the atmosphere. This is also a reason why we extremely rarely find things like alpha particles in cosmic rays.