<p>When plasma moves in a magnetic field then the charged particles follow helical paths around the field lines, due to the <span class="math-container">$q\vec{v}\times \vec{B}$</span> Lorentz force. If the radius of gyration is small, then the plasma is effectively tied (or frozen) to the field lines.</p>

<p>However to decide whether it is the motion of the magnetic field that enforces motion of the plasma or vice versa, we must compare the magnetic pressure with the gas pressure (or equivalently, compare the magnetic and kinetic energy densities).</p>

<p>The magnetic energy density scales with magnetic field squared and the field drops further from the Sun (e.g. a dipole field falls as <span class="math-container">$r^{-3}$</span>), whereas the plasma kinetic energy density (which depends on the plasma density and temperature) could remain relatively high (the solar wind density drops only as <span class="math-container">$r^{-2}$</span> and the temperature is roughly constant).  There is a transition between the magnetic field dominating the plasma close to the Sun, to where the plasma dominates and effectively carries the field with it. This transition point is known as the Alfven radius and is usually at a few tenths of an au from the Sun.</p>

<p>Below this radius, the plasma (sort-of) co-rotates with the field, which is anchored to the photosphere. Beyond that the plasma is (sort-of) free to move radially outwards. This how the Sun loses angular momentum.</p>

CC BY-SA 4.0
2019-12-05T00:33:48.197Z
Oh I see. It's nothing like conserving angular momentum, the rotation mentioned is the Sun's rotation rate $\omega$ (circa 14 deg/day), which is definitely going to be unsustainable at some distance. This is much clearer now, *thanks!*
uhoh
CC BY-SA 4.0
2019-12-05T00:04:42.593Z
2019-12-05T00:04:42.593Z
ProfRob
<p>Between <code>02:01</code> and <code>02:29</code> the new NASA Goddard video <a href="https://www.youtube.com/watch?v=ReQAUocScw0" rel="nofollow noreferrer">5 New Discoveries from NASA's Parker Solar Probe</a> explains</p>
<blockquote>
<p><strong>finding # 4, A Breaking Point:</strong></p>
<p>Before parker, scientists knew that the corona rotates with the visible surface below it. But they didn’t know how — or where — the solar wind switched to flowing straight by the time it reaches Earth.</p>
<p>Parker has finally spotted signs of this transition — and the changeover happens significantly farther out than expected.</p>
</blockquote>
<p>Naively I'd apply conservation of angular momentum and assume that the particles are in individual ballistic orbits, but the solar wind contains charged particles as well as neutrals and so even though of low density and long mean free paths they can interact collectively.</p>
<p>But I don't understand why the solar wind would suddenly stop rotating beyond some radius from the Sun well inside Earth's orbit, rather than beyond the heliopause.</p>
<p>Why does this happen?</p>

CC BY-SA 4.0
2019-12-05T02:12:36.697Z
What do you mean by flowing straight? Radially outwards as opposed to having an azimuthal velocity component? 'Straight' in outer space is not very well defined.
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-12-05T02:29:04.943Z
@AtmosphericPrisonEscape apparently NASA Goddard and I both understand it to mean along a straight line away from the Sun. I've simply adopted the same term from the video, as shown in the quote. Neither of us is discussing flow far from the ecliptic, since Parker is in the ecliptic.
uhoh
CC BY-SA 4.0
2019-12-05T02:32:31.140Z
I'm happy that your understanding and NASA's align so well. Many of those coincidences these days, seemingly. Anyways, so you mean radially away from the sun, thanks, that's vocabulary that your fellow scientist can understand.
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-12-05T02:37:13.367Z
And what about the fact that the Sun, Earth and Parker are all in the ecliptic at the same time? Too many coincidences for comfort!
uhoh
CC BY-SA 4.0
2021-08-31T15:03:33.763Z
Just a quick note:  The solar wind leaves the Sun with some radial and azimuthal velocity components.  Near to the Sun, the flow roughly co-rotates with the magnetic "foot point" of any given solar wind parcel.  Beyond some radial distance, called the Alfven point, the co-rotation breaks down but the azimuthal component of the solar wind doesn't disappear at this point.  It's just no longer following, in longitude, the surface of the Sun.
honeste_vivere
CC BY-SA 4.0
2021-08-31T15:06:36.383Z
@honeste_vivere I see, thanks! As I just mentioned elsewhere these simple, clear explanations are most welcomed!
uhoh
CC BY-SA 4.0
2021-08-31T15:07:39.833Z
Yes, I just figured I would clarify something though I think Rob's answer is more than sufficient (and correct, as per usual).
honeste_vivere
<magnetic-field><solar-wind><corona><parker-solar-probe>
Why does the solar wind switch to flowing straight by the time it reaches Earth?
uhoh
# Why does the solar wind switch to flowing straight by the time it reaches Earth?

(Asked by: username_0 on 2019-12-04)

Between `02:01` and `02:29` the new NASA Goddard video [5 New Discoveries from NASA's Parker Solar Probe](https://www.youtube.com/watch?v=ReQAUocScw0) explains

> **finding # 4, A Breaking Point:**
>
> Before parker, scientists knew that the corona rotates with the visible surface below it. But they didn’t know how — or where — the solar wind switched to flowing straight by the time it reaches Earth.
>
> Parker has finally spotted signs of this transition — and the changeover happens significantly farther out than expected.

Naively I'd apply conservation of angular momentum and assume that the particles are in individual ballistic orbits, but the solar wind contains charged particles as well as neutrals and so even though of low density and long mean free paths they can interact collectively.

But I don't understand why the solar wind would suddenly stop rotating beyond some radius from the Sun well inside Earth's orbit, rather than beyond the heliopause.

Why does this happen?

**Tags:** magnetic-field, solar-wind, corona, parker-solar-probe

> **Comments:**
> **username_1**: What do you mean by flowing straight? Radially outwards as opposed to having an azimuthal velocity component? 'Straight' in outer space is not very well defined.  
> **username_0**: @username_1 apparently NASA Goddard and I both understand it to mean along a straight line away from the Sun. I've simply adopted the same term from the video, as shown in the quote. Neither of us is discussing flow far from the ecliptic, since Parker is in the ecliptic.  
> **username_1**: I'm happy that your understanding and NASA's align so well. Many of those coincidences these days, seemingly. Anyways, so you mean radially away from the sun, thanks, that's vocabulary that your fellow scientist can understand.  
> **username_0**: And what about the fact that the Sun, Earth and Parker are all in the ecliptic at the same time? Too many coincidences for comfort!  
> **username_2**: Just a quick note:  The solar wind leaves the Sun with some radial and azimuthal velocity components.  Near to the Sun, the flow roughly co-rotates with the magnetic "foot point" of any given solar wind parcel.  Beyond some radial distance, called the Alfven point, the co-rotation breaks down but the azimuthal component of the solar wind doesn't disappear at this point.  It's just no longer following, in longitude, the surface of the Sun.  
> **username_0**: @username_2 I see, thanks! As I just mentioned elsewhere these simple, clear explanations are most welcomed!  
> **username_2**: Yes, I just figured I would clarify something though I think Rob's answer is more than sufficient (and correct, as per usual).  

---

## 1 answers

### Answer by username_3 on 2019-12-05. Score: 8 | Accepted answer

When plasma moves in a magnetic field then the charged particles follow helical paths around the field lines, due to the $q\vec{v}\times \vec{B}$ Lorentz force. If the radius of gyration is small, then the plasma is effectively tied (or frozen) to the field lines.

However to decide whether it is the motion of the magnetic field that enforces motion of the plasma or vice versa, we must compare the magnetic pressure with the gas pressure (or equivalently, compare the magnetic and kinetic energy densities).

The magnetic energy density scales with magnetic field squared and the field drops further from the Sun (e.g. a dipole field falls as $r^{-3}$), whereas the plasma kinetic energy density (which depends on the plasma density and temperature) could remain relatively high (the solar wind density drops only as $r^{-2}$ and the temperature is roughly constant). There is a transition between the magnetic field dominating the plasma close to the Sun, to where the plasma dominates and effectively carries the field with it. This transition point is known as the Alfven radius and is usually at a few tenths of an au from the Sun.

Below this radius, the plasma (sort-of) co-rotates with the field, which is anchored to the photosphere. Beyond that the plasma is (sort-of) free to move radially outwards. This how the Sun loses angular momentum.

> **Comments:**
> **username_0**: Oh I see. It's nothing like conserving angular momentum, the rotation mentioned is the Sun's rotation rate $\omega$ (circa 14 deg/day), which is definitely going to be unsustainable at some distance. This is much clearer now, *thanks!*  

---

