1. The solar wind: in situ data
Johan De Keyser
Belgian Institute for Space Aeronomy
15 February 2011
SOTERIA Capacity Building Workshop

2. SOTERIA Capacity Building Workshop
Overview
This interactive session will cover:
What is the solar wind?
Why do we study it?
How do we observe it?
Basic structure
Fast and slow wind
Interplanetary magnetic field
Dynamical phenomena
Corotating interaction regions
Interplanetary plasma clouds
15 February 2011
SOTERIA Capacity Building Workshop

3. Magnetic and electric forces
The gas in the solar corona is very hot, and therefore ionized to a very large degree. The gas therefore is a plasma.
A particle with charge q and speed v subject to a magnetic field B and an electric field E experiences a force
F = q ( E + v x B )
Uniform B: a particle experiences a sideways force: it spirals along field lines.
If, additionally, E is present
ￌ B: plasma has a perpendicular drift Vdrift = - E x B / B²
║ B: particles are accelerated
15 February 2011
SOTERIA Capacity Building Workshop

4. SOTERIA Capacity Building Workshop
What is the solar wind?
The Sun’s atmosphere – the corona – is so hot (106 K) that some of the plasma particles can escape the Sun’s gravity, forming a continuous stream of plasma: the solar wind.
Particles must be on “open” magnetic field lines.
Particles must overcome electric forces:
Ions and electrons must escape together for charge neutrality.
Mass/temperature differences lead to a parallel potential difference between the lower corona and the interplanetary medium.
15 February 2011
SOTERIA Capacity Building Workshop

5. Source in the solar atmosphere
Chromosphere
Flows in spicules
Lifetime minutes
Speed : 20 km/s
Corona
Closed field lines
Local loops
Trapped heated particles
Low latitudes
Open field lines
Radially outward field lines
Escaping colder particles, coronal holes
High latitudes
15 February 2011
SOTERIA Capacity Building Workshop

6. SOTERIA Capacity Building Workshop
The heliosphere
The Sun blows a “bubble” in the local interstellar medium, the heliosphere. This bubble is dominated by
the solar wind,
the interplanetary magnetic field,
interstellar neutral gas (in the outer regions).
As the Sun moves relative to the interstellar medium,
The heliosphere is deformed.
A collision zone is formed, with a bow shock, the heliopause, and the termination shock.
The Voyager spacecraft (launched 1977) have reached the heliopause (around 100 AU).
15 February 2011
SOTERIA Capacity Building Workshop

7. Why do we study the solar wind?
The Sun is a star. All stars produce a stellar wind to some extent:
Young stars may produce strong stellar winds, which affect star forming regions.
Old stars of medium mass go through an episode of intense mass loss prior to producing a planetary nebula.
Probing the solar wind with spacecraft provides us with ground truth about stellar wind dynamics, and thereby it tells us a lot about astrophysical objects that are too far away to allow in situ measurement.
15 February 2011
SOTERIA Capacity Building Workshop

8. SOTERIA Capacity Building Workshop
Gas collision zones
One example of wind dynamics are gas collision zones. Consider a star producing a wind that expands with a constant radial speed and temperature. A collision zone is formed with the ambient interstellar medium in the form of a spherical shell with higher density, bounded by forward and reverse shock fronts. In the middle is the contact surface that separates stellar and interstellar material.
Example: termination shock – heliopause – bow shock
15 February 2011
SOTERIA Capacity Building Workshop

9. Solar wind – magnetosphere interaction
A second reason for studying the solar wind is that it interacts with planetary environments – such as the Earth’s magnetosphere.
In fact, this is again an example of the plasma collision zone: bow shock – magnetopause – no termination shock as the magnetosphere is dominated by the strong geomagnetic field.
The solar wind exerts a variable pressure, which compresses the magnetosphere and thereby modulates the solar wind–magnetosphere interaction.
15 February 2011
SOTERIA Capacity Building Workshop

10. How do we study the solar wind?
In situ observations
Send a spacecraft out there and measure!
Very good plasma and field diagnostics, but is local.
In ecliptic: Pioneer 6, IMP-8, ACE, Wind, SOHO, Stereo-A/B, Voyager …
Out of ecliptic: Ulysses
Remote sensing
More recent, limited diagnostics, but large spatial coverage: Stereo, IBEX
15 February 2011
SOTERIA Capacity Building Workshop

11. Spacecraft that study the solar wind
Where to find info:
NSSDC
Spacecraft and data set information is contained in the NSSDC master catalog that is found at
More specific space physics info can be found at the Space Physics Data Facility
Orbits can be displayed with the VSPO 4D viewer that you can find there at
Choose Stereo-A and B and ACE
Time interval :00 to :00
You see how ACE circles Sun-Earth L1 and how Stereo-A and –B move inward/outward to go faster/slower than the Earth – simple orbital mechanics at work!
15 February 2011
SOTERIA Capacity Building Workshop

12. SOTERIA Capacity Building Workshop
Instruments
Magnetometers:
fluxgate (3 orthogonal coils)
range nT down to a few nT
magnetic cleanliness requirements
Plasma spectrometers:
exploit rotation to scan the sky
energy selection
particle velocity distributions
Electric field / wave instruments:
wire antennae on rotating sc
Remote sensing cameras:
avoid stray light, max contrast
15 February 2011
SOTERIA Capacity Building Workshop

13. Basic structure of the solar wind
Open field lines give rise to fast solar wind.
Closed field lines produce slow solar wind near the open/closed boundary
15 February 2011
SOTERIA Capacity Building Workshop

14. Basic structure of the solar wind: data
Data source: start your exploration at the Space Physics Data Facility
which links to various databases.
Alternatively, use Coordinated Data Analysis Web for downloading data.
An interesting site is Cohoweb, giving 1-hour average solar wind properties from various spacecraft:
Take a look at the following data
Ulysses spacecraft
1990/01/ /01/01 daily averaged data
Plot R λ Br Bt Bn |B| Vsw Np Na
You can learn a lot from that plot!
15 February 2011
SOTERIA Capacity Building Workshop

15. SOTERIA Capacity Building Workshop
15 February 2011
SOTERIA Capacity Building Workshop

16. Fast and slow solar wind
15 February 2011
SOTERIA Capacity Building Workshop

17. Interplanetary magnetic field
The solar wind drags the solar magnetic field out into interplanetary space.
The quiet Sun gives rise to a regularly shaped heliospheric current sheet. A consequence of that are the regular sector boundary crossings by Earth.
The magnetic field takes the shape of an Archimedean spiral, the so-called Parker spiral, as the plasma stays on its magnetic field line, and as the radially outward speed is rather constant.
15 February 2011
SOTERIA Capacity Building Workshop

18. Dynamic phenomena in the solar wind
Because of the inhomogeneity of the solar wind – both in space and time – the expanding flow may be subject to important dynamic changes.
Spatial inhomogeneity:
fast and slow solar wind streams create corotating interaction regions
Space-time inhomogeneity:
coronal mass ejections leading to major interplanetary perturbations
15 February 2011
SOTERIA Capacity Building Workshop

19. Corotating interaction regions
When fast wind overtakes slower wind in front of it – which may happen right at sector boundaries if the heliospheric current sheet is tilted – then a collision zone is created with forward shock, contact surface, and reverse shock (rarefaction wave).
Since these spatial structures may exist for many months, this interaction zone is encountered repeatedly during several solar rotations (~27 day recurrence), hence the name corotating interaction region.
15 February 2011
SOTERIA Capacity Building Workshop

20. Coronal mass ejections
Coronal mass ejections produce interplanetary magnetic clouds. These are basically flux tubes that carry the ejected material outward. In front of such a cloud, a collision zone is formed as well, with a bow shock that compresses the ambient solar wind. The helical magnetic field topology can be discerned easily. Depending on the magnetic field orientation, the effect of the field on planetary bodies may be different.
15 February 2011
SOTERIA Capacity Building Workshop

21. Interplanetary magnetic cloud signature
15 February 2011
SOTERIA Capacity Building Workshop

22. SOTERIA Capacity Building Workshop
Conclusion
Although the heliosphere is our immediate neighbourhood, much remains to be learned about the solar wind. Particular challenges are
Understand local plasma diagnostics.
Relate them to the overall configuration of the solar wind.
This will require bringing together data from various sources, as well as modelling results. Only such a combination can help us to grasp the physics behind the wealth of phenomena.
The implications are tremendous:
We will better understand stellar winds in astrophysical objects.
This understanding is a basis for space weather prediction.
15 February 2011
SOTERIA Capacity Building Workshop