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 can’t 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 Sun’s 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 1600’s, 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, Galileo’s point of view eventually won out, and he was right.

7. Movie Made from Galileo’s Sunspot Drawings for June 2 - July 8, 1613 • Drawings were made at the same time each day, so orientation of the Sun’s rotation axis was the same for each drawing. • Can follow Sun’s 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 Magnetic Fields Optical Images 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.

11. 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 don’t 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 Magnetic Fields Optical Images 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 Sun’s differential rotation and its global magnetic field. (That’s an hour’s talk in itself.) Latitude on the Sun

15. 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. What’s 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 sun’s 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 camera’s 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 Earth’s 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 Earth’s 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 Medium:Magnetic 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 Earth’s Orbit? • Here we have a sketch of the full heliosphere; it’s complicated. • Magnetic fields get wrapped up to become almost circumferential as the solar wind carries them outward.

24. What Else Happens Beyond Earth’s 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 (it’s ‘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 Earth’s.

30. 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 we’re 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 don’t 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 sun’s 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 sun’s atmosphere. It therefore stores a lot of energy. • Sometimes the magnetic field gets stretched beyond its breaking point (not quite right -- but that’s 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 didn’t 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 CME’s 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 it’s 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.