Discovery of the Heliosphere

1
Discovery of the Heliosphere

• What is it?
• Why does it matter?
• R. Bruce McKibben
• Research Professor
• Space Science Center and Dept. of Physics, UNH

2
What Traditional Astronomers See(All-sky view
from Mt. Graham, AZ)

• Looking at the night sky, we are looking right
  through the heliosphere. Yet it seems that
  nothing is between us and the stars of the Milky
  Way.
• What is the Milky Way?
• What is the heliosphere and how did we find out
  about it?

3
A Sister to the Milky Way Two Views of NGC 7331

• NOAO Optical Spitzer Infrared
• A Galaxy is an assembly of, typically, about
  100,000,000,000 stars
• Many, like the Milky Way and NGC7331 have a
  spiral form.
• In the Milky Way, the Sun is a star in one of
  the spiral arms, about 30,000 light years from
  the center, or about half way out.

4
Sketch of the Heliosphere

• The Heliosphere is the local region of the galaxy
  dominated by plasmas and magnetic fields from the
  Sun. (Plasma is very hot gas, so hot that
  electrons have been stripped off of the atoms.
  More about that later.)
• The nearest star is about 300 times further away
  than the nose of the bow shock. The heliosphere
  is therefore very local.
• We cant see any of this, so how did we learn
  about it?

5
The Sun-Earth Connection

• The Sun has more effects on Earth than simply
  providing heat and light and something to orbit
  around.
• The Sun is in some sense a variable star, with a
  regular cycle of activity of about 11 years.
• Long before the space age, variations in the
  Suns activity were observed to produce effects
  at Earth, the first clue to the existence of a
  medium in interplanetary space through which
  disturbances can propagate from the Sun to the
  Earth.
• So what is the solar activity cycle?

6
The Sun, a Not-So-Perfect Orb

• Until the 1600s, most people in Europe followed
  Aristotle (4th Century BC) in considering the
  Sun and the Heavens to be perfect and unchanging.
• After the invention of the telescope in 1608,
  Galileo and others observed the Sun and
  discovered spots on its face. He saw them rotate
  across the Sun and concluded that they were on
  the Sun, perhaps clouds in its atmosphere.
• A Jesuit priest, Christoph Scheiner, also studied
  sunspots, but believed in the perfect orb concept
  of the Sun, so he argued they were satellites of
  the Sun. In the discussion that followed,
  Galileos point of view eventually won out, and
  he was right.

7
Movie Made from Galileos Sunspot Drawings for
June 2 - July 8, 1613

• Drawings were made at the same time each day, so
  orientation of the Suns rotation axis was the
  same for each drawing.
• Can follow Suns rotation (every 27 days) and see
  the evolution of spots from day to day.

8
Close-up of a Sunspot(from the MDI Instrument on
the NASA/ESA SOHO Spacecraft)

• Sunspots are very large, and show complex
  structure.

9
Sunspots Have Strong Magnetic Fields
Optical Images
Magnetic Fields
White and black represent oppositepolarities (N
S)
Magnetic field linestraced out by hot gases.
10
What are Magnetic Fields?
In the familiar bar magnet, lines of magnetic
force are considered, by convention, to leave the
North Pole and re-enter the South Pole to form a
loop through the Magnet. If there are two
magnets, the north pole of one is attracted to
the south pole of the other. Magnets attract
things made of iron because each individual iron
atom behaves like a tiny magnet.
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Why are Magnetic Fields Important?
Magnetic fields store energy. It takes energy to
increase the strength of a magnetic field or to
bend a field line from its natural shape, and
energy is released when a field decreases or
relaxes to its natural shape. Magnetic fields
affect electrically charged particles. Particles
with electric charge move easily along magnetic
fields, but are bent into a circular path around
the field lines if they try to cross them.
12
Plasmas and Magnetic Fields
Atoms have no electric charge. The negative
charge of the electrons exactly balances the
positive charge of the nucleus. Therefore atoms
dont see magnetic fields except in subtle ways.
Hydrogen andHelium Atoms
When a gas becomes a plasma, electrons are
separated from the nuclei of atoms in the
gas. When an electron is removed from an atom,
as in a plasma, both the electron and the
remaining ion have an electric charge, and both
are affected by magnetic fields. Because the
particles in a plasma have electric charges and
cannot move easily across magnetic fields,
magnetic fields and plasmas are strongly
connected to each other.
13
Sunspots Have Strong Magnetic Fields
Optical Images
Magnetic Fields
White and black represent oppositepolarites (N
S)
Magnetic field linestraced out by hot gas
(plasma).
14
The Sunspot Cycle

• The number of sunspots varies with about an 11
  year cycle, but sometimes it breaks down (for
  example, 1650-1710)
• The first spots of a cycle appear at high
  latitude, and the last spots are at low latitudes
  (Butterfly Diagram).
• The Sunspot cycle reflects a global change in
  some basic characteristic of the Sun.
• We now believe the Sunspot cycle is caused by an
  interaction between the Suns differential
  rotation and its global magnetic field. (Thats
  an hours talk in itself.)

Latitude on the Sun
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Other things than sunspots vary with the solar
cycle The solar corona
Sunspot Minimum
Sunspot Maximum

• The corona is the very hot (1 million degrees)
  outer atmosphere of the Sun. Until spacecraft
  measurements, it was generally observable only
  during total eclipses. These are eclipse photos.

16
Other things than sunspots vary with the solar
cycle Cosmic Ray Intensities
Reminder Cosmic Rays are very high energy
atomic nuclei (and therefore charged particles)
that are probably produced by supernovae in the
galaxy. They arrive at Earth after traveling
through the galaxy for several million years.
17
Other things than sunspots vary with the solar
cycle Aurorae
Large auroral displays often occur a day or so
after large solar flares. Whats a solar flare?
18
Other things than sunspots vary with the solar
cycle Solar Flares

• This movie shows an Ultra-violet view of a large
  solar flare that occurred July 14, 2000.
• Notice the hot gases trapped in magnetic fields
  above the rim of the sun. This is the lower
  corona - the suns hot outer atmosphere.
• Notice the snow in the picture after the flare
  has gone off. These are protons accelerated by
  shockwaves from the flare that are hitting the
  cameras CCD.

19
Summary What was known in1955

• The sun is an active star.
• The activity, first and most easily measured by
  the number of sunspots, varies with an 11 year
  cycle. The frequency of explosive solar flares
  and the shape of the corona also vary with the
  sunspot cycle.
• Things at Earth also vary with the 11 year
  cycle
• Frequency of large auroral displays
• These displays can be associated with large
  variations in the Earths magnetic field, even at
  ground level -- compasses can go crazy.
• Intensity of galactic cosmic rays hitting the
  atmosphere
• Long-term variations related to the general level
  of solar activity
• Short-term sudden decreases often observed a day
  or so after large solar flares
• Conclusion There is something that provides a
  direct connection between events on the Sun and
  events on Earth.
• Most people thought it could all be explained
  by individual gusts of plasma ejected by specific
  events on the sun.

20
First Glimmer of the Heliosphere Particles
from the Feb. 1956 Flare (Meyer, Parker, and
Simpson)
Neutron Monitor Intensity
Suggested Structure of Interplanetary SpaceThe
First Heliosphere Drawing

• Fast rise of intensity suggested no barriers to
  propagation of the flare particles between the
  Sun and the Earth
• Slow decay suggested something impeding the
  particles on their way out of the solar system.
• Note that the outer barrier, if it changes in
  response to solar activity, can also help explain
  the variation in cosmic ray intensity over the
  solar cycle.

21
Prediction of the Solar Wind

• In 1958, Eugene Parker of the University of
  Chicago was attempting to solve the equations
  that would describe the hot outer atmosphere of
  the Sun - The Corona. He could find no solution
  that allowed a stable atmosphere.
• All the solutions he found predicted that the top
  of the corona would blow off, escaping the Sun in
  the form of a high speed plasma wind. He
  calculated that it should be blowing with a speed
  of several hundred kilometers per second, a
  supersonic speed.
• His ideas were not widely accepted. However in
  1961 Explorer 6, the first spacecraft that got
  outside the region of space shielded by Earths
  magnetic field with an instrument capable of
  measuring the wind found the wind, exactly as
  Parker had predicted.
• The average speed near Earth is 400 km/s, though
  there have been brief periods (hours) when the
  wind has almost stopped, and others where the
  speed has approached 2000 km/s (after a large
  flare.)
• On average the density of the wind near Earth is
  about 10 ions per cubic centimeter. So
  interplanetary space is still a much better
  vacuum than we can make in a lab on Earth.

22
The Interplanetary MediumMagnetic Fields and
Plasma Wind

• The solar wind is plasma. Therefore it interacts
  with magnetic fields.
• As it leaves the Sun, it pulls coronal magnetic
  fields with it.
• Since the Sun is rotating as the solar wind
  leaves, the wind pulls the fields out into a
  spiral pattern - the garden sprinkler effect.

23
What Happens Beyond Earths Orbit?

• Here we have a sketch of the full heliosphere
  its complicated.
• Magnetic fields get wrapped up to become almost
  circumferential as the solar wind carries them
  outward.

24
What Else Happens Beyond Earths Orbit?

• The fields wrap up differently at different
  latitudes.
• Eventually the solar wind runs into the the
  plasmas and magnetic fields in the local
  interstellar medium and is slowed down.
• Since the wind is supersonic, it slows down by
  forming a standing shock wave, the Termination
  Shock.

25
Kitchen Sink Model of the Termination Shock

• In the center of the plate the water is flowing
  faster than the speed of water waves (its
  supersonic for water waves).
• The rim of the plate provides resistance, similar
  to the pressure from the Interstellar Medium
• A shock forms as the water slows down in response
  to the resistance.

26
And Finally?

• The Sun is moving through the local Interstellar
  Medium at about 25 km/sec
• Therefore all the slowed solar wind is swept
  downstream to form a heliotail.
• The heliopause is the boundary between
  interstellar material and the slowed solar wind.
• Depending on the properties of the Interstellar
  Medium (which are poorly known), there may also
  be a bow shock in front of the heliopause, like
  the bow wave in front of a fast moving boat.

27
How Big is it?

• The termination shock is expected to form about
  85 - 100 AU from the Sun, more than twice the
  distance to Pluto.
• At 400 km/s, it takes the solar wind a little
  over 14 months to reach 100 AU.
• At 25 km/s, it takes the heliosphere about 38
  years to move 200 AU, its own diameter, in the
  Interstellar Medium.

An AU is the distance from the Sun to the
Earth, or 150 million km.
28
How Do We Understand More About It?

• We need spacecraft
• Spacecraft going outwards
• Spacecraft going to different latitudes, for
  example a polar orbiter around the Sun
• Spacecraft to monitor conditions at one spot, for
  example near Earth
• Fortunately, we have them
• Voyager 1 and 2 are heading for the termination
  shock
• Ulysses is in an orbit that goes almost over the
  poles of the Sun at distances of 2 - 3 AU.
• Several spacecraft are near Earth ACE, SOHO,
  WIND.

An AU is the distance from the Sun to the
Earth, or 150 million km.
29
Spacecraft Investigating the Heliosphere

• Pioneer 10 and Pioneer 11 are no longer active.
  They made important contributions in setting the
  scale of the heliosphere but could not last the
  full trip to the termination shock.
• Data from Voyager 1 near 90 AU suggest it is
  near, and has maybe even crossed the shock.
• Ulysses orbit is invisible on this scale, as is
  Earths.

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Ulysses Orbit
31
Ulysses Solar Wind at Solar Minimum (1992-1997)

• During its first pass over the solar poles near
  solar minimum, Ulysses showed that the equatorial
  region where we live is a special region of the
  heliosphere. At minimum, the wind over the poles
  is twice as fast as it is near Earth.
• At solar maximum, Ulysses found a different
  story the wind was slow everywhere.
• How does this variability affect the structure of
  the Heliosphere? It does, but were still
  working on exactly how.
• The heliosphere has a dynamic structure.

32
Changes in the Corona from Solar Minimum to
Maximum The View from SOHO (1)
Minimum
Maximum

• The structure of the corona controls the
  structure of the solar wind throughout the
  heliosphere.
• Note the dark regions. They are coronal holes,
  regions where magnetic field lines stretch
  straight into space to let the coronal plasmas
  escape. In brighter regions, magnetic field
  lines close back on the sun, confining the
  coronal plasmas.

33
Changes in the Corona from Solar Minimum to
Maximum The View from SOHO (2)
Minimum
Maximum

• At solar minimum the solar magnetic field is very
  simple. The polar coronal holes correspond to
  North and South Magnetic poles.
• Fast wind (800 km/s) escapes from the polar
  holes, while at lower latitude magnetic fields
  resist escape of the plasma, and the wind that
  escapes is slower (400 km/s). Because fast and
  slow wind in general come from different
  latitudes they dont mix, and the solar wind flow
  is relatively smooth and simple.

34
Changes in the Corona from Solar Minimum to
Maximum The View from SOHO (3)
Maximum
Minimum

• At solar maximum, the field becomes very complex,
  and is concentrated in sunspots and active
  regions that appear as bright spots in these
  pictures.
• Coronal holes are smaller and scattered all over
  the suns surface. The fast wind therefore
  escapes in isolated streams at all latitudes.
  Since the Sun rotates, these fast streams quickly
  run into slow wind that was emitted earlier, and
  the solar wind flow becomes very complicated.

35
Changes in the Corona from Solar Minimum to
Maximum The View from SOHO (4)
Minimum
Maximum

• At solar maximum, the magnetic field is
  concentrated in active regions and is strongly
  deformed from its equilibrium shape by plasma
  flows in the denser regions of the suns
  atmosphere. It therefore stores a lot of energy.
• Sometimes the magnetic field gets stretched
  beyond its breaking point (not quite right -- but
  thats the general idea) and it snaps back to a
  simpler configuration, releasing a lot of the
  stored energy. These explosive releases of
  energy are called solar flares.

36
November 4, 2001 Flare

• Top plot shows X-ray intensity, a sign of very
  hot plasma.
• Note complex twist of region before event,
  slightly simpler after.
• Note gas ejected towards the bottom right. This
  is the start of a coronal mass ejection (CME).

37
November 4, 2001 CME

• This movie is from another instrument on SOHO
  that looks at the distant corona.
• The flare ejected a large puff of plasma (the
  CME) directly towards Earth. This is called a
  halo CME since it appears to completely encircle
  the Sun.
• Note all the energetic particles that appear as
  snow as they hit the CCD

38
Cosmic Ray Effects McMurdo Sound Neutron Monitor

• Very high energy protons arrived promptly after
  the flare went off and caused an increase in the
  counting rate.
• Early on Nov. 6, the CME arrived at Earth. Once
  it passed beyond Earth it acted as a barrier to
  cosmic rays and caused a short-term (few days)
  decrease in the cosmic ray intensity.

39
Spectacular Aurorae were observed, this one over
Edinburgh Scotland on Nov. 6.
40
Summary from Nov. 6, 2001 Event

• The interplanetary medium, which fills the
  heliosphere, provides strong coupling between
  events on the Sun and events on Earth. As the
  disturbances propagate beyond Earth they continue
  to cause changes in the heliosphere all the way
  out to the termination shock.
• The Sun and heliosphere thus form a tightly
  coupled system that controls our space
  environment. Changes in this environment can
  have important effects on Earth satellites, radio
  propagation, power grids, etc.
• Study of these changes and their effects has
  become a specialty called Space Weather, which is
  now a high priority for NASA research.
• And all because of the heliosphere, something
  that 50 years ago we didnt even know existed.

41
Recent Period of Major Solar Activity Oct.-Nov.
2004

• In a period of about two weeks starting at the
  end of October, 17 major flares erupted on the
  Sun. One was the most powerful ever recorded.
• The series of events and their effects were
  observed by spacecraft throughout the
  heliosphere.

42
SOHO Monitored the Solar Corona

• This combined view shows images from three
  instruments that monitor the lower, middle, and
  outer corona.
• Notice the frequent increases in radiation. If
  astronauts are sent to Mars, protecting them from
  such radiation storms is one of the biggest
  problems that has to be solved.

43
The CMEs Continued Outward through the Heliosphere

• The large sunspot group was one of the active
  regions that produced the flares.
• Ulysses was very near Jupiter, Cassini was near
  Saturn, and Voyager 1 and 2 were approaching the
  Termination Shock when the events occurred.

44
The CMEs Interact with the Heliopause

• Even beyond the termination shock, the CME blast
  waves will continue to have effects.
• The CMEs may push the heliopause outwards by
  about 400 million miles.
• It will likely take a year or two for the
  heliopause to settle back to its normal position.

45
From Heliosphere to Astrosphere

• A Hubble photograph of the bow shock in front of
  the Astrosphere of LL Orionis, a young star
  with a strong wind embedded in the Orion Nebula.
• As well as showing us all the ways our
  heliosphere affects our own environment here on
  Earth, studying our heliosphere can help us
  understand how other stars interact with their
  surroundings.