The Solar Wind: Source,
Structure and Dynamics
Robert T. Wicks
r.wicks@ucl.ac.uk
Overview
•
A brief history of solar physics and space weather
•
What is the solar wind?
•
Solar source: where does the solar wind come from?
•
Solar wind dynamics and evolution
•
How to identify features in solar wind data
•
Things to look out for: upcoming events and future
missions.
A brief history of solar physics
Sunspots:
Observed for hundreds of years. Galileo made regular
observations of the Sun in the 17th century, drawing the location
of dark spots.
1843: Samuel Heinrich Schwabe first to observe sunspots for long
enough to discover the solar cycle. Spots are more plentiful at
“maximum” and sometimes disappear completely at “minimum”,
this cycle repeats on an 11 year timescale.
1852: Link to geomagnetic activity. Edward Sabine, Rudolf Wolf,
Jean-Alfred Gaultier and Johann von Lamont correlate sunspot
number with the variability of magnetic fields measured in their
laboratories. The sunspots were affecting the Earth’s magnetic
field!
1859: Richard C. Carrington and Richard Hodgson make first
observations of a solar flare. 3 days later there was one of the
largest geomagnetic disturbances ever recorded. Aurora were
visible in the sky all over the world and were so bright they woke
people from their sleep. The new telegraph system in the USA and
in Europe failed, catching fire and giving operators electric
shocks.
A brief history of solar physics
Comet tails:
Comets have been signs of the changing nature
of outer space for thousands of years. The tails
of comets point away from the Sun.
1910: Observations of comets led Arthur
Eddington to postulate the existence of particles
traveling away from the Sun and hitting the
comet.
Comet tails split in two, we now know this is
because one tail is the “ion tail”, made of
charged ions picked up by the solar wind, and
the other is the “dust tail” of larger dust particles
Space plasma: basic facts
Composition: The solar wind has a very similar composition to
the Sun. By number: 95% H, 4-5% He, traces of heavier
elements. By mass: 80% H, 20% He.
Charged particles: All are highly ionised, the temperature
(>1MK) means that most electrons are stripped from the atoms
to make protons, ions and electrons.
Moving charges form a current and, just like in an electric
motor, a current in a magnetic field is forced to rotate. This
means that all of the particles rotate around the magnetic field
direction. Positive and negative charges rotate in opposite
directions!
Magnetic field: The Sun has a strong magnetic field. When the
conductivity of the plasma is very high, i.e. electric currents are
driven by any motion of the magnetic field, the magnetic field
becomes “frozen” into the plasm. The field cannot move
relative to the particles around it.
This means that if the field is strong it stops the particles from
moving, this is what makes sunspots dark. If the field is weak,
then the particles can pull the field around as they move.
A brief history of solar physics
The solar wind:
Measurements from the 1930s to 1950s
showed that the outer atmosphere of the
Sun, or corona, was at least 106 K.
1958: The solar wind is predicted
theoretically by Eugene Parker. Parker
calculated that such a hot plasma would
be able to escape the gravity of the Sun
and travel supersonically away from the
Sun.
1959: USSR probe Luna 1 measures
supersonic ions in outer space for the
first time.
1962: Mariner 2, launched by NASA
repeats and confirms the observations of
the solar wind.
First Solar Wind Observations
From the very earliest observations of interplanetary magnetic field B and
velocity V (Mariner 5, 1967) large scale Alfven waves!
Hundhausen, Space Sci.
Rev.8, 1968
Belcher & Davis, JGR,
1971
What is the Solar Wind?
Plasma originating from the Sun.
The plasma drags out the Sun’s
magnetic field with it.
Protons have temperature around
106 K
Thermal and magnetic field
pressure is about equal at the
Earth.
B ~ 5 nT, n ~ 5 cm-3, V ~450 km/s
Many different plasma phenomena
occur in the solar wind:
- shocks
- reconnection
- turbulence
- pick up of neutral ions
What is the Solar Wind?
When Coronal Mass Ejections (CMEs)
blast out into the solar wind at very high
speeds they generate shocks. These
shocks are some of the most violent
events in the solar wind. Shocks
accelerate particles to relativistic speeds
and if they hit the Earth can cause
geomagnetic storms.
The bright parts of the image show
where hot or dense plasma is located,
the plasma traces out the shape of the
magnetic field.
Radiation is generated in solar flares and
by the shockwave of the supersonic
CME in two different processes:
reconnection, and shock acceleration.
Solar Wind Source
Viewing the Sun in EUV wavelengths lets
us see different temperatures of plasma.
Brightness due to temperature and
mostly density.
The corona is hot, but not uniform.
Dark areas are coronal holes, almost
always over the poles of the Sun but
often in other places, can be any
latitude.
Dark because plasma has low density
=> open field lines allow plasma to
escape easily.
Solar Wind Source
Ulysses spacecraft is the only mission to fly over
the poles of the Sun. Provided unique insight into
the solar wind.
Fast solar wind comes from ‘open’ field lines, i.e.
the poles of the Sun. Slow wind comes from
‘closed’ field around the equator.
Open field lines (called coronal holes) often
appear close to the equator when the Sun is
more active. This gives rise to fast and slow
streams in the solar wind. Where fast streams
catch up to slow streams shocks can form.
www.ccmc.gsfc.nasa.gov
Solar activity
Solar activity varies over a roughly 11 year
period. Solar minimum has few sunspots, a
very dipolar magnetic field, and few flares
and CMEs.
Maximum occurs when there are many
sunspots visible at once. The solar
magnetic field is very complex and there
are many flares and CMEs, sometimes
several a day.
We are currently around solar maximum!
Sunspots are areas of intense magnetic
field in the photosphere. Sunspots transport
magnetic flux aiding in the ‘flipping’ of
polarity of the solar field over each cycle.
As the cycle progresses spots appear
closer to the equator.
Solar Wind Source
Solar activity: what is a solar flare?
SDO videos from 2/5/15
There are several flashes of bright light and rapid movements of plasma. These are
solar flares.
I highly recommend: http://sdo.gsfc.nasa.gov
Solar Wind Dynamics
http://ccmc.gsfc.nasa.gov/missionsupport/
NewHorizons_odstrcil.php
Solar Wind Dynamics
Solar Wind Dynamics
Roughly speaking, there are 3 ‘types’ of
solar wind:
‘Slow’ wind
‘Fast’ wind
CMEs
A better description is:
Streamer belt / closed field line wind
(usually slow)
Coronal hole wind (often fast)
Ejecta (CMEs, and others!)
Solar Wind Dynamics
CIR = Co-rotating Interaction Region
SIR = Stream Interaction Region (for when you are not quite sure)
MIR = Merged Interaction Region (when several things bump into one another
Solar Wind Dynamics
Definition of a CME is complex, some
features that may be seen:
density blob
speed change
composition change
magnetic field flux rope
shock
Solar activity and cosmic rays
Cosmic rays are high energy radiation from deep
space that arrive isotropically and form part of the
background radiation of life on Earth. The flux of
cosmic rays is modulated by the solar activity cycle.
Small scale dynamics: turbulence
The fluctuations in B and V “look similar” over a wide range of scales.
More rigorously, scales are statistically related: the probability distribution
at one scale can be re-scaled to match the probability distribution at other
scales.
What is turbulence?
The fluctuations in B and V “look similar” over a wide range of scales.
More rigorously, scales are statistically related: the probability distribution
at one scale can be re-scaled to match the probability distribution at other
scales.
What is turbulence?
Typically there is a large scale source of energy, non-linear interaction
cascades the energy from large scales to smaller until it is dissipated,
heating the fluid.
Outer
scale
What is turbulence?
Typically there is a large scale source of energy, non-linear interaction
cascades the energy from large scales to smaller until it is dissipated,
heating the fluid.
The solar wind is a plasma, in the simples approximation an
incompressible magneto-fluid:
Dissipation
(compressing field)
Wave propagation
Non linear interaction
Dissipation (heating)
Solar Wind Turbulence: Spectra
The amplitude of fluctuations in the magnetic field changes with scale,
larger scales have larger fluctuations.
Do fluctuations in different directions relative to B look different?
What is a “mean magnetic field”?
How to access
spacecraft data archives
at NASA
Go to CDAWeb:
http://cdaweb.gsfc.nasa.gov
Click on current
missions
You will see current issues
/ alerts on the first page
Below this you will see a list of current
missions with data archives and
types of data. If you know what
mission you want, just select that.
Click ‘submit’
Strongly recommend to
click CLEAR ALL
Now you see a complete list of data
products from the Wind spacecraft. If
you click on the blue text you will find
a short description of the data set,
some have links to other resources to
explain the data further. A lot of data
does not have much documentation
and can be very hard to use /
understand at first. All of the data have
a contact who you can email to learn
more about the data, this is strongly
recommended if you are trying to do
something complex with the data or if
you think you see something unusual.
To begin with we will use OMNI data as an example. OMNI is a cleaned and averaged data set with 1 minute
cadence constructed from many different spacecraft covering more than 30 years.
The other data product names follow the convention:
Two letter spacecraft name (WI for Wind) - data type H, K, or other designation (H is best, processed data)
numbers specify the level of processing, 0 is basic, higher numbers have more refinement - instrument name
For data specifically from Wind, the best choices are:
WI_H0_MFI and WI_H2_MFI (magnetic field, this has 0.092s, 3s,1 minute and 1 hour options)
WI_H1_SWE (92s proton distribution fits, e.g. speed, density, temperature).
Select as many or as few different data sources as you want, we will see the data set options on the next page.
For simplicity I will only select OMNI_HRO_1MIN but feel free to select everything or anything.
You can use any data, just be careful to read the caveats and talk to the PI if you have problems. There are
higher time resolution proton fits from 3DP, but these are subject to more noise than the slower SWE
observations. There are also electron observations, energetic protons and radio waves. All kinds of stuff, you can
play around and find data that you think are interesting on the next page.
This page allows the selection of time period
and specific data sets. There are some
preset times, or you can specify a time of
your choice. Select a time period less than a
month for best viewing. I picked 20 days in
summer 2010.
Initially I always look at the data in plot form
to check:
that there is some data worth looking
1.
at.
that I have selected the time period I
2.
wanted.
I selected all the variables I wanted
3.
and didn’t forget anything.
Later, having validated that we have chosen
the correct options, we can return to this
page to download the data in whatever
format we prefer
It is worth clicking on the data filter when
making plots so that a few spurious points
don’t make the plots useless. The filtered
points will not be filtered from the data file
you download, just the plots.
Lower down this page is a long list of all
the different data products that we can
plot. I have chosen magnetic field (B),
velocity (V), the plasma density,
temperature, electric field and the
plasma beta and Alfven mach number.
These parameters will be plotted on the
next screen.
If you clicked “download original CDFs”
on the previous page then you won’t see
any of this data, and in fact the data file
will contain all of the data listed on this
page regardless of whether you
selected it or not.
If you clicked “List data” or “Create new
CDFs” you will only get the data that you
select here.
Once you have selected everything you
want, click “submit”
If you select a long time period or lots of
different data it may take some time for
the next page to load, however it is not
normally more than a few seconds.
This is the result. We can see:
B
Some things to note, see if
you can figure out why!
A CME flux rope
V
Slow wind
Shock(s)
High amplitude turbulence
n
T
|E|
beta
Ma
Fast wind
Heliospheric current sheet
crossing(s)
ANSWERS (not definitive, just best guesses)
Some things to note, see if you can figure out why!
A CME flux rope: strong B, N shaped field reversal in Bx, filled with
cold plasma with density enhancements before and after indicating
ejected material.
Slow wind: It is slow (< 400 km/s) but also note the lower amplitude of
magnetic fluctuations with coarser structures, lower average
temperature, and higher average density.
Shock(s)
High amplitude turbulence
Fast wind
Heliospheric current sheet crossing(s)
Some things to note, see if you can figure out why!
A CME flux rope
Slow wind
Shock(s): sudden jumps in V associated with dense blobs and high B
and temperatures. Around which you can also see….
High amplitude turbulence: particularly visible in the components of B
and V and associated with high temperatures.
Fast wind
Heliospheric current sheet crossing(s)
Some things to note, see if you can figure out why!
A CME flux rope
Slow wind
Shock(s)
High amplitude turbulence
Fast wind: it is faster than around 500 km/s but also has high T, high
amplitude turbulence, it is bounded by a shocks where is catches up
to slower wind and low density.
Heliospheric current sheet crossing(s): Bx and By flip directions (+ve
to -ve) and there are associated changes in Vy and Vz, there are also
associated T and n peaks (ideally we would also see |B| ~ 0 but data
resolution may not be high enough) Current sheet crossings are often
‘swept up’ in interaction regions where shocks form when fast wind
catches slow but are more easily seen in the ‘trailing edges’ of fast
streams, where fast wind slows down.
If you like this OMNI data you can now go back to the data selection
page and download the original CDFs to make your own analysis.
CDF is the NASA default data format, so it is worth figuring out how to
use this format in Matlab or IDL (both have standard functions for
handling CDF, although you may need to update the libraries as there
has been a recent update to CDF).
Alternatively select some of the other Wind data, or even other
spacecraft.
Not all data is contained with CDAWeb, for example, new data formats
are continually being added, and non-standard data products can be
requested from the instrument teams directly.
Solar Wind Observations
NASA has a fleet of spacecraft investigating the
Sun, solar wind, and the response of the Earth
to the ever-changing space weather conditions.
Voyager: Furthest manmade objects from the
Earth, oldest operational spacecraft, entering
interstellar space.
ACE / Wind: Monitor conditions just upstream
of Earth. Longest continual dataset of solar wind
observations at 1 AU.
SDO: Highest resolution images of the Sun from
space. 100 million images taken in 5 years.
IBEX: Observing the boundary of the solar wind
and interstellar space from Earth orbit.
Discovered a ‘ribbon’ of energetic particles
encircling the nose of the Heliosphere.
STEREO: First 3D pictures of the Sun and solar
wind. Allow views of the far-side of the Sun and
help give early warning of impending solar
storms
MMS: Next major science mission to be
launched by NASA Heliophysics division. 4
spacecraft will provide most detailed
observations of the magnetosphere ever made.
Future missions
Solar Probe Plus:
Go as close to the Sun as possible (<10 solar radii). Measure the acceleration and heating of
the solar wind and investigate the complex phenomenology of the solar corona. Launch in 2018.
Solar Orbiter:
Carries cameras, particle instruments (built at MSSL / UCL!) and magnetic and electric field
probes. Mission: How is the solar wind accelerated? Can we connect the solar wind directly to
source regions of the Sun? How does the solar wind evolve as it travels outwards? Launch in
2018.
Conclusions