IAL 8: The Sun

Don't Panic

Sections

  1. Introduction to New Astronomy
  2. Introduction to the Sun
  3. Some Basic Sun Facts
  4. Solar Luminosity and the Solar Constant
  5. Nuclear Fusion in the Sun
  6. Controlled Fusion and Fusion Power
  7. The Outward Flow of Energy
  8. The Photosphere
  9. The Chromosphere
  10. The Corona
  11. The Solar Wind
  12. Solar Activity and Magnetic Fields (Reading Only)
  13. Sunspots (Reading Only)
  14. Dark Filaments and Prominences (Reading Only)
  15. Flares (Reading Only)
  16. Coronal Mass Ejections



  1. Introduction to New Astronomy

  2. In the first part of intro astro, we did old astronomy---positions of astro-bodies, their appearances on the sky (like the angular diameters illustrated in the figure above/below, calendar usage, etc---astronomy whose inner core goes back millennia.


    Then we did a
    foray into physics.

    Now we are ready for new astronomy or astrophysics in which we consider the physical nature of the astro-bodies.

    This physical nature must be determined by a combination of observation and modeling.

    Actually, a quasi-endless cycle of observation and modeling in order to improve our understanding---the hoary old scientific method, in fact. See figure below (local link / general link: sci_method.html).


    We primarily focus on what has been determined with only tidbits about the how---but the tidbits are important too.

    We start with the Sun and work outward.

    The traditional ordering of topics in intro astro classes---which IAL mostly follows---is Sun, Solar System, exoplanets (a new addition to the traditional ordering), stars, galaxies, cosmology---farther and farther out---with a last return to smaller scale for SETI (Search for Extra-Terrestrial Intelligence).

    The figure below illustrates our journey to infinity and eternity.

    For further impressions of the The Voyage Out, see Universe zoom videos below.


  3. Introduction to the Sun

  4. In this lecture, we consider the Sun as it is today. See figure below (local link / general link: sun_white_light.html).

    Later we will consider its birth, evolution, and death in IAL 9: The Life of the Sun.


    1. The Sun Today:

      By considering the Sun today (i.e., at our moment in cosmic time), we are just considering a snapshot in the Sun's lifetime, but its a snapshot that is roughly valid for most of it's 10 or 11 Gyr lifetime as a nuclear burning object. (For nuclear burning, see section Nuclear Fusion in the Sun below.)

      The Sun is currently 4.6 Gyr old (see Wikipedia: Formation and evolution of the Solar System: Timeline of Solar System evolution).

      The Sun is a main-sequence star. The main sequence being a narrow band of stars on a Hertzsprung-Russell diagram as we show in the figure below (local link / general link: star_hr_lum.html).

      Stars on the main sequence (of stars) burn hydrogen to helium (in a nuclear burning sense: see section Nuclear Fusion in the Sun below) in their cores and are fairly stable and unchanging.

      The main sequence phase of a star is the longest phase of a star's nuclear burning lifetime.

      The Sun is a typical star of its stellar classification.

      The range of star behavior is so broad that the idea of an average star is of little use.

      The average star of a particular stellar classification is of use because the stellar classifications are sufficiently narrow in range of behavior.

    2. The Spectral Classification of the Sun:

      The Sun is, in fact, a G2 V star in the OBAFGKM spectral classification.

      The figure below (local link / general link: star_hr_lum.html) of a Hertzsprung-Russell diagram gives us a preview of stellar classification including the OBAFGKM spectral classification.


      We will now mention or recapitulate some absolutely, important points about
      main-sequence stars:

      1. The spectral types are subdivided into spectral subtypes labeled by numbers running 0 through 9.

      2. There is the Main-sequence rule (local link / general link: star_main_sequence_rule.html):

      The OBAFGKM sequence was originally ABC ... according to hydrogen line strength, but hydrogen line strength turned out NOT to be monotonic with photosphere temperature.

        The photosphere is that layer of a star from which visible light escapes to infinity---so it's the primary layer that we see.

        Hydrogen line strength first increases with photosphere temperature, but then starts decreasing about 10,000 K which is about the highest photosphere temperature of the A star class.

      Instead of relabeling the classes---which they should have done----the old spectroscopists---the good old spectroscopists---just reordered photosphere temperature sequence in decreasing order.

        Somehow some letters got omitted for good like C, D, H, etc. in the reordering.

      OBAFGKM can be remembered from the mnemonic "O be a fine girl/guy kiss me." (Wikipedia: Stellar classification)---which is sometimes the only sensible thing to say.

      The Roman numeral V in the classification G2 V star for the Sun designates the luminosity class (which is shown in the figure above (local link / general link: star_hr_lum.html).

      Here it suffices to say that V essentially stands for main sequence.

      For more on stellar classification, see Stellar classification videos below.

    3. Magnetic Fields in Astrophysics:

      Just to forewarn, many of the Sun's behaviors are magnetic. We can measure the magnetic fields on the Sun using spectroscopy, but we will NOT go into how this is done.

      It's also true that other stars must have a lot of magnetic phenomena, but since we do NOT see those other stars close up, we notice their magnetic phenomena far less.

      In fact, magnetic fields are extremely important in many fields of astrophysics.

      But treating magnetic fields in depth is beyond our scope, and so we largely skirt aspects where they are important.


  5. Some Basic Sun Facts

  6. The Sun is a giant ball of hot gas---but NOT just a giant ball of hot gas.

    It's more complicated than that.

    1. Differential Rotation:

      The Sun is rotating, but NOT all at the same speed. It is NOT a solid, and so can rotate DIFFERENTIALLY: i.e., it has differential rotation.

      On the surface, its period at the equator is 25 days (implying relatively fast angular rotation) and near the poles, 36 days (implying relatively slow angular rotation). The deep interior seems to rotate as if rigid and has a period of 30 days as is known from helioseismology which otherwise we will NOT discuss, except maybe fleetingly.

      For the solar rotation curves deduced from helioseismology, see the figure below (local link / general link: solar_rotation.html).


    2. Solar Composition, Primordial Solar Nebula Composition, Cosmic Composition:

      The Sun is made of matter---no surprise. By the by, matter is usually taken to be stuff with rest mass and volume.

      The solar composition is illustrated in the figure below (local link / general link: solar_composition.html).


      The composition of the
      rocky bodies and rocky-icy bodies in the solar system (i.e., Earth, Moon, Mercury, Venus, Mars, moons, and small Solar System bodies (asteroids, meteoroids, and trans-Neptunian objects)) is much like that of the Sun, except for much smaller amounts of H and He.

    3. Plasma (AKA Ionized Gas):

      An atom contains a small nucleus made of protons and neutrons: size scale 10**(-15) m which is 10**5 times smaller than the atomic size scale.

      The protons and neutrons make up nearly all the mass and they are bound by the strong nuclear force.

      Electrons surround the nucleus in quantum mechanical orbitals (the quantum mechanical analogs of orbits) and are bound to the nucleus by the electric force. They make up only a tiny bit of the mass, but give the atom its size scale of 10**(-10) m.

      Atoms and molecules with equal numbers of protons and electrons are electrically neutral or unchanged. This is the usual state for atoms and molecules under relatively low temperatures and sufficiently high densities.

      If an atom or molecule becomes charged by losing or gaining electrons, it is called an ion. The losing of electrons is called ionization.

      Heat energy and other processes can cause ionization.

      A gas with a significant fraction of atoms and molecules ionized is a state of matter is called a plasma.

        Gases with negative ions are also called plasmas, but they are relatively less important than positive-ion plasmas and we will NOT discuss them further here, except as needed for correct statements.

      Note yours truly like many others just consider a plasma to be a kind of gas: i.e., an ionized gas.

      Most plasmas we encounter in experiments are low-ionization plasmas where the atoms have lost (or gained) only a few electrons.

      Deep in the interior of stars, one gets an extreme plasma where all an atom's electrons are stripped off (i.e., the atom is completely ionized) and one just has a gas consisting of unbound electrons and bare atomic nuclei. The surface of stars are usually only partially ionized.

      The figure below (local link / general link: plasma_types.html) illustrates the range of plasmas that occur in nature---but it may NOT be an entirely reliable diagram.


      A dramatic,
      everyday-life plasma spectacular lightning is illustrated in the figure below (local link / general link: lightning.html)


    4. Basic Solar Parameters:

      The figure below (local link / general link: sun_basics.html) gives some basic solar parameters (i.e., controlling variables).


      The diagram and table in the figure below (
      local link / general link: sun_model_interior.html) illustrate the interior structure of the Sun.


      The matter gas pressure and radiation pressure are both strongly temperature dependent.

      As temperature increases, they increase; as temperature decreases, they decrease.

      If the temperature of the Sun were to drop to zero, the Sun would collapse to become a white dwarf which is a star of order Earth size, of enormous density, held up by a quantum mechanical degeneracy pressure of electrons.

      Becoming a white dwarf is the ultimate fate of the Sun when it runs of out hydrogen and helium fuel as we will discuss in Intro Astro Lecture 9: The Life of the Sun.



  7. Solar Luminosity and the Solar Constant

  8. Stars are dense in photons relative to space.

    So the free flow of photons between the two is overwhelmlingly from stars to space.

    In a thermodynamics description, this flow to space increases entropy in obedience to the 2nd law of thermodynamics.

    A prime piece of evidence for the comparative low density of photons (i.e., electromagnetic radiation (EMR)) in space is the darkness of the night sky.

    1. Electromagnetic Radiation in Space:

      As a preview of beyond the Solar System, the figure below (local link / general link: diffuse_extragalactic_background_radiation.html) explicates the EMR in space that does NOT come directly from compact sources: i.e., the diffuse extragalactic background radiation (DEBRA).


    2. Stars Lose Heat Energy to Space:

      Because of the net flow of EMR to space, the Sun (and all other stars) are constantly losing heat energy. They radiate it away as EMR.

      There are other less-important ways that stars lose energy to space: stellar winds and neutrinos. We will NOT discuss the cause of stellar winds. The flow of neutrinos to space is discussed in subsection Nuclear Burning in Stars and the Sun.

      The rate of heat energy outflow in EMR is called luminosity.

      For the Sun, the luminosity is the solar luminosity L_☉ = 3.828*10**26 W. Compare that to a 100-watt light bulb.

    3. Solar Constant:

      How much of that energy from the Sun do we get at Earth?

      The important parameter is the solar constant which is explicated in the figure below (local link / general link: solar_constant.html).


      As said in the figure above (
      local link / general link: solar_constant.html), solar constant is NOT absolutely constant---its name is a slight misnomer.

      The figure below illustrates the variation in the solar constant.

    4. The Secular Increase of the Solar Constant:

      In addition to the periodic variation of the solar constant, there are also secular (i.e., long-term) variations.

      The most important one is a long-term increase in the solar constant due to a long-term increase in the Sun's luminosity (WB-106). See the figure below (local link / general link: sun_evolution.html).

      Alas, the long-term increase spells the doom of life on Earth. But that's a story for another day. See the discussion in IAL 11: The Earth.




  9. Nuclear Fusion in the Sun

  10. Since the Sun is emitting EMR, it would cool off relatively rapidly if heat energy (properly internal energy) were NOT continually being resupplied.

    The resupply comes from nuclear fusion of hydrogen (H) to helium (He) in the core of the Sun which extends from the center to ∼ 0.2 R_☉. This is illustrated in the figure below (local link / general link: sun_structure_cutaway.html).


    Note that astrophysicists usually refer to self-sustaining nuclear reaction chains as
    nuclear burning with it understood that chemical burning is NOT meant.

    The Sun's center is hot (T=15.7*10**6 K) and dense (density=150 g/cm**3) (Cox-342).

    Under such conditions, the matter is completely ionized: i.e., there are no electrons bound to nuclei and both nuclei and electrons bounce around as free particles.

    As we explained above in subsection Plasma (AKA Ionized Gas), this state of matter is an extreme plasma (i.e., a maximally ionized gas).

    Now let's look at nuclear physics and nuclear burning in stars and the Sun.

    1. Nuclear Physics:

      We need a tiny bit of nuclear physics.

      The figure below (local link / general link: ernest_rutherford_lab.html) illustrates where nuclear physics all began.


      Now
      nuclei are made out of positively charged protons and neutral neutrons.

      Protons and neutrons are collectively classed as nucleons.

      Nuclei are much smaller than atoms.

      The number of protons determines the chemical species.

      Species with the same number of protons and different numbers of neutrons are isotopes of each other.

      In regard to chemical reactions, different isotopes of an atom are nearly identical.

      There are minute differences because of the differences in atomic mass.

      But in regard to nuclear reactions, the isotopes can be quite different in behavior.

      For example, the hydrogen nucleus usually just consists of a single proton. A proton and neutron nucleus is a heavy hydrogen nucleus which has a special name deuteron.

      For another example, consider the helium (He) nucleus. It comes in two stable isotopes (see Wikipedia: Helium: Isotopes): He-3 (which the much less abundant isotope) and He-4 (which the abundant isotope).

      "Stable" in this context means the isotope will NOT spontaneously radioactive decay to a different species.

      The He-4 atom and nucleus are illustrated in the figure below (local link / general link: atom_he_4.html).


    2. The Nuclear Strong Force:

      Nuclei are held together against the electric force repulsion of the protons by the strong nuclear force.

      The electric force is explicated a bit in the figure below (local link / general link: electric_force_coulombs_law.html).


      The
      strong nuclear force is a very strong force, but it is very short range.

      It acts only over a distance of about 1 fermi (10**(-15) m). Recall 1 fermi is 10**5 times smaller than atom size.

      The length range of the strong nuclear force is what sets the size scale of nuclei at about 1 to 10 fermis.

    3. The Weak Nuclear Force:

      There are 4 known fundamental interactions (i.e., the four fundamental forces) in physics: gravity, the electromagnetic force, the strong nuclear force, and the weak nuclear force. In a sense, the last 3 are united in the standard model of particle physics (see Wikipedia: Fundamental interaction: The Standard Model). Hopefully, one day gravity (including quantum gravity) will be united with the other 3 in the theory of everything (TOE).

      But at the moment, the question is what does the weak nuclear force do?

      It transforms protons into neutrons or vice versa in order to stabilize the nucleus. The process is called beta decay. In nuclear burning, beta decay often occurs as part of overall nuclear reactions. Beta decay also happens as relatively isolated events in which case it classified as a form of radioactivity---in fact, the most usual kind of radioactivity.

      Beta decay is important in hydrogen burning in stars as we discuss just below in subsection Nuclear Burning in Stars and the Sun.

    4. Nuclear Burning in Stars and the Sun:

      Now H nuclei (which are just single protons usually) strongly repel by the electric force because they are like-charged particles.

      In stars, only in the cores is it sufficiently hot and dense that the electric force repulsion can be overcome and the H nuclei can collide closely enough that the strong nuclear force can bind them (i.e., fuse them).

      The interaction of protons is somewhat explicated in the figure below (local link / general link: nuclear_burning_pp.html).


      Now the
      deuteron is a reactive nucleus compared to ordinary hydrogen and it burns to He-3 (two protons and one neutron in the nucleus) comparatively quickly.

      But the final product in stellar hydrogen burning is the very stable He-4 nucleus.

      There are several H-to-He-4 burning processes in stars. The figure below (local link / general link: stellar_nuclear_burning_processes.html) illustrates the two dominant ones: the proton-proton chain reaction and the CNO cycle.


      The dominant
      hydrogen burning (i.e., H-to-He-4 burning) process in the Sun is pp chain reaction in the pp I branch and pp II branch. The pp I branch is illustrated in the figure below (local link / general link: nuclear_burning_ppi_chain.html).


      How long can the
      Sun do hydrogen burning: i.e., what is its main sequence lifetime? For the answer, see the figure below (local link / general link: sun_lifetime_estimate.html).


    5. Star Nuclear Burning is Stable:

      Star nuclear burning is STABLE for main-sequence stars and mostly for post-main-sequence stars.

      Main-sequence stars (including especially the good old Sun) are NOT just going turn off NOR do a thermonuclear runaway and blow up like a giant bomb.

      What does STABILITY mean exactly in this context?

      The cartoon in the figure below (local link / general link: stability_mechanical.html) illustrates STABILITY in general via a mechanics analogue.


      To be general, say you have a
      system which is in a steady state (i.e., an unchanging state).

      If small perturbations cause the system to change in a permenant way, the steady state is UNSTABLE.

      But if there is a restoring force or analogous restoring process that damps out the effects of the perturbations and continually restores the system toward the steady state, then the steady state is STABLE.

      Virtually all long-lasting states are STABLE.

      There is a restoring force or analogous restoring process that prevents significant permanent change due to small perturbations.

      But there are always perturbations big enough to cause permanent change

      The building we are in is STABLE---small vibrations won't collapse it---but an earthquakes will.

      Hydrogen burning in the Sun and all main-sequence stars is STABLE due to the process discussed in the figure below (local link / general link: sun_hydrogen_burning_stability.html).

      The steady input of nuclear energy in the core of the Sun allows the Sun to have a steady output of EMR which is good for life on Earth.



  11. Controlled Fusion and Fusion Power

  12. As a societally useful digression, we will consider controlled fusion and fusion power.

    Down here on Earth we would like to have STABLE hydrogen burning or, as it is called, controlled fusion for fusion power.

    Controlled fusion and fusion power are explicated in the figure below (local link / general link: nuclear_fusion_deuteron_triton.html).


    For
    nuclear weapons proliferation, see the countries to which nuclear weapons have proliferated in the figure below.

    For radioactive waste, we have Yucca Mountain---you've heard of it all your lives---only 130 km from Las Vegas. See Yucca Mountain in the figure below (local link / general link: nuclear/yucca_mountain.html).




  13. The Outward Flow of Energy

  14. How does heat energy flow from deep in the Sun and to outside the Sun?

    The abstract driver is the 2nd law of thermodynamics, of course: random processes leading to increased disorder in the overall Sun-space system.

    But what are the actual heat transfer proceses?

    Well oneth by radiative transfer and twoeth by convection---as illustrated in the figure below (local link / general link: sun_interior_heat_transfer.html).


    1. Radiative Transfer:

      From the center to ∼ 0.71 R_☉, the dominant energy transfer process is radiative transfer.

      This is the radiative zone.

      In a simplified model, one can picture photons executing a random walk in which they fly along straight lines between matter interactions.

      The interactions often actually destroy the photons, but others are created in the same place feeding off the energy of the destroyed ones. The created ones fly off in random directions.

      Despite the random photon flight directions, there is a net flow outward since random walking photons must eventually wander to the surface and escape forever.

      There is a bias that speeds up the process relative to a hypothetical Sun that was homogeneous in matter properties:

        The density decreases outward, and so in the outward direction the flights are longer. This creates a bias toward outward flow.

        NOT returning. The surface must be colder than the interior. %%% This second idea seems to be redundant from an Monte Carlo perspective.>

      Above the radiative zone is the convection zone---see the figure above (local link / general link: sun_interior_heat_transfer.html) and the figure below (local link / general link: sun_structure_cutaway.html).


    2. Convection:

      In the Sun, the convection zone extends from ∼ 0.71 R_☉ to the solar photosphere (Cox-342). Of course, radiative transfer goes on in the convection zone too---but it is NOT the dominant process there.

      Convection is a universally important, macroscopic heat transfer process in regions with gravitational fields and sufficiently steep temperature gradients. It occurs in:

      1. most stars including the Sun.
      2. the interior of the Earth. It is the driver of the plate tectonics of the Earth's crust as we'll see later.
      3. in the Earth's atmosphere.

        Question: What is a common, everyday, obvious example of convection?

        1. The freezing of water.
        2. Usually in boiling of water in a pot.
        3. The churning of butter.











        Answer 2 is right.

        You can see the convection flows even.

        Note boiling isn't convection. The two processes are happening at the same time.

      What is and why for convection? First see the figure below (local link / general link: convection.html) for "what is" and then the text description below that for "why for".


      Second, "why for convection in the Sun?"

      In the Sun, the gas relatively near the solar photosphere is NOT fully ionized, and in this case, that makes it more opaque to photons. Thus there is a higher insulation barrier for heat flow and the temperature gradient steepens relative to otherwise.

      The steepening creates an buoyancy instability for hot gas and convection is the upshot. We won't go into the conditions needed for convective instability---but they occur pretty commonly since convection is pretty common as mentioned in the figure above.

      At the solar photosphere, the Sun becomes sufficiently transparent that some photons can just escape to space.

      The escaping photons are how the blobs of hot convecting gas deposit their heat. Then they can break up (?) and sink as cold gas.

      We see the hot blobs at the solar photosphere as solar granules (see below the section The Photosphere).

    3. Convection in Stars in General:

      The figure below (local link / general link: star_convection.html) illustrates as a preview convection in stars in general.




  15. The Photosphere

  16. The solar photosphere is where the Sun becomes transparent: i.e., where at about half the radially-traveling photons escape to infinity.

    It is the layer of the Sun that is usually called the surface. But actually the Sun extends outward without a sharp break at all.

    We tend to call the photosphere the surface because that is where we see most light coming from.

    The photosphere is the first of the outer layers of the Sun. For the outer layers of the Sun, see the figure below (local link / general link: sun_outer_layers_cartoon.html).


    The
    photosphere is about 500 km in thickness: this is probably partially by definition since there are no sharp boundaries???.

    The temperature in the photosphere is about 6000 K, but it varies a bit.

    A blackbody radiator equivalent to the photosphere has temperature of 5772 K (Wikipedia: Sun), and so that is a characteristic solar photosphere temperature.

    Cooler gas in the lower chromosphere just above photosphere probably creates the absorption line spectrum. We discussed absorption line spectra in IAL 7: Spectra. See the model of the Sun's atmosphere in the figure below (local link / general link: sun_atmosphere_model.html) for the probable region of absorption line spectrum formation.


    The convective blobs that reach up into the
    solar photosphere are called granules because they look granular.

    The granules are brighter than their surroundings (which look like dark lanes) because the granules are hotter.

    Recall if you just tone down all parts of a bright image equally, the less bright parts can become dark.

    The darker surroundings of the granules is the sinking convective gas.

    Granules are illustrated in the two figures below (unlinked; local link / general link: solar_sunspots_granules.html).




  17. The Chromosphere

  18. The chromosphere is a low density layer above the solar photosphere that is about 10,000 km thick.

    The chromosphere is illustrated in the figure below (local link / general link: sun_outer_layers_cartoon.html).


    The
    chromosphere temperature rises from a low of about 4000 K to about 100,000 K at the top.

    The lower cooler chromosphere is where the absorption line spectrum of the Sun forms.????

    The hotter upper chromosphere has low density and an emission line spectrum which is NOT seen in integrated Sun spectra, but can be seen with special techniques.

    Chromo means color and the name probably arises from the pink color the chromosphere would show to the naked eye.

    But, the chromosphere is probably never seen by the naked eye under ordinary conditions. However, solar prominences which we discuss below are chromospheric in color (Se-160) and are visible during total solar eclipses.

    Nowadays, the chromosphere is often observed from space through narrow filters centered on emission lines where it is bright.

    The Solar and Heliospheric Observatory (SOHO, (1995--2022?) has provided some good extreme UV images from the 0.0304 micron line of singly-ionized He (AKA He II) which is the strongest singly-ionized He line. For example, see the figure below.


  19. The Corona

  20. Above the chromosphere is the corona.

    The corona is illustrated in the figure below (local link / general link: sun_outer_layers_cartoon.html).


    The
    corona is that milky white, tenuous, wispy gas seen around the Sun in total solar eclipses.

    The wispy structure is because the ions tend to spiral around the magnetic field lines of the Sun. This is the effect of the magnetic force. We discuss magnetic fields further below.

    The corona reaches from the chromosphere outward until it makes a transition into the solar wind. There is no sharp transition.

    The corona can be traced out to 30 R_☉ (0.14 AU) (Se-151) which is still well within Mercury's mean distance to the Sun of 0.38709893 AU (Cox-294).

    The corona's temperature is of order 10**6 K, and so it is much hotter than the solar photosphere and chromosphere.

    But it is so dilute that it radiates much less than the solar photosphere.

    It can be seen from Earth at eclipse times??? and from space at non-eclipse times by masking out the Sun. See the space image of the corona in the figure below (local link / general link: corona_soho.html).


    Why are the outer
    chromosphere and corona hotter than the solar photosphereorbit?

    Some mechanism pumps heat to them and it is NOT EMR from the photosphere. There is certainly enough photospheric EMR to do it, but the chromosphere and corona are too transparent to capture much of that EMR.

    The most popular idea is that somehow magnetic field energy generated in the interior is then dumped as heat energy above the photosphere.

    Now I wave my hands here. Sometimes I will just say magnetic field energy and NOT bother explaining---since I don't know---how that energy gets converted into other forms.

    There is certainly much more elaboration in the theory, but the definitive answer is NOT yet in (Wikipedia: Corona: Coronal heating problem).

    See Solar corona videos below (local link / general link: solar_corona_videos.html):

      EOF


  21. The Solar Wind

  22. As it goes outward, the corona morphs into the solar wind.

    The solar wind is an expanding stream of protons (ionized hydrogen atoms) and electrons and other particles coming off the corona.

    We will NOT go into the causes of the solar wind: they may arise from magnetic effects????, but the instructor admits to plain ignorance on the subject.

    1. Where the Solar Wind Starts:

      The solar wind mostly comes off from coronal holes: places where the Sun's magnetic field lines DO NOT close trapping the particles on closed loops (Ni-130).

      Recall charged particles have a strong tendency to helix around magnetic field lines (see IAL 6: Electromagnetic Radiation), and thus if the those field lines return to the Sun, the particles have difficulty escaping to infinity.

      A cartoon of the Sun's magnetic field lines is shown in the figure below (local link / general link: sun_magnetic_dipole_cartoon.html).


      The
      Sun's magnetic field is essentially dipolar like the Earth's and a bar magnet. It switches polarity every 11 years on average for a total solar cycle of 22 years on average. (Dipole means two poles: a north and a south pole.)

      Additionally, there are complex magnetic field structures that are time variant.

      There are permanent coronal holes at the Sun's axial poles (Ni-130) which are also its magnetc poles or close to them (???). Coronal holes can occur at other latitudes in a time dependent fashion (Ni-131).

      The 3 figures below illustrate coronal holes and the start of the solar wind.












        sun_xray_coronal_hole.jpg

        Caption: X-ray images of a boot-shaped coronal hole from Skylab 1973.

        The images are from about 2 days apart. The rotation of the Sun is clear. Coronal holes seem to be magnetic field free areas or areas of outwardly open magnetic field lines in the corona that allow more free-streaming solar wind sort of like the nozzle of the hose whipped around.

        Credit/Permission: NASA, 1973 / Public domain.
        Download site: NASA. Alas, a dead link.
        Image link: Itself.



        ./surface/nasa_solar_wind.jpg

        Caption: The Ulysses spacecraft's (1990--2009) map of solar wind speed.

        This the wind speed close to the Sun I guess. Near the Earth, the speed is slower and near 400 km/s.

        The diagram is NOT well captioned. I'm guessing that the image is in the X-ray (and hence false color). IMF probably stands for "something magnetic field". I assume that all these speeds were measured at the circular orbit of Ulysses, but I can't track that information right now.

        Credit/Permission: NASA, before or circa 2009 / Public domain.
        Download site: NASA. Alas, a dead link.
        Image link: Itself.


      The mass loss rate by the solar wind isn't large: it's only about 2*10**9 kg/s (Se-152, but note the values need some correction.)

      If the rate kept steady---which it won't---how long until the Sun is exhausted?

      First, let us convert to solar masses lost per year.

      
        2*10**9 kg/s x (1 M_☉ / 2*10**30 kg) x (3*10**7 s / 1 year)
      
        =  approx 3*10**(-14) M_☉/yr  ,
      
      Then from the ordinary exhaustion formula
      
         Amount/Rate =  1 M_☉ / 3*10**(-14) M_☉/yr
      
                     ≅ 3*10**13 yr  = 3*10**4 Gyr .
      
      

        Question: What is the estimated lifetime of the Sun?

        1. 3*10**4 Gyr.
        2. 1 Gyr.
        3. 10 Gyr.











        Answer 3 is right.

        Since the Sun's lifetime is only about 10 Gyr, the Sun will NOT lose significant mass because of the current solar wind.

        In its post-main-sequence life, the Sun will have stronger solar winds and will probably end up with only about 70 % of its current mass when it becomes a white dwarf (CK-329).

      The figure below illustrates the solar wind as it flows through the Solar System.

      See Solar wind videos below:

    2. The Heliopause:

      The solar wind extends out to of order 120 AU and then runs into the interstellar medium (ISM) at a surface called the heliopause. Then the wind merges with the interstellar medium in some way. The behavior at the heliopause is illustrated in the figure below.

      The Voyager Program probes launched in 1977 reached the vicinity of the heliopause at of order 120 AU from the Sun in the 2010s. To be exact, the Voyager 1 spacecraft crossed the heliopause 2012 Aug25 121 AU from the Sun (see Wikipedia: Voyager 1: Heliopause). The Voyager 2 spacecraft crossed the heliopause 2018 Nov05 (see Wikipedia: Voyager 2: Interstellar mission).

      Hopefully, the in situ information from the Voyager Program probes and other information further elucidate the heliopause and surroundings.

      It's likely to be a rather complicated story, and so we will NOT go into it further.

      Note that the heliopause is NOT the edge of the Solar System in yours truly's opinion since astronomical objects gravitationally bound to the Sun include the Oort cloud which extends outward to somewhere between 50,000 and 200,000 AU (0.8--3.2 ly).

    3. The Solar Wind at the Earth and the Aurora:

      At the distance of the Earth from the Sun, the solar wind speed is about 400 km/s. This is much faster than the low-Earth orbital velocity of 8 km/s. However, you wouldn't feel the solar wind if you were exposed to it. The ram pressure at 1 AU is typically (1 to 6)**(-9) Pa (i.e., N/m**2) (see Wikipedia: Solar wind: Pressure). Recall air pressure at sea level is about 1 standard atmosphere (atm) = 101325 Pa exactly ≅ 10**5 Pa).

      The Earth is protected from the solar wind mostly by the Earth's magnetic field: a distorted dipole field which forms what is called the magnetosphere---although it isn't spherical. See the figure below.

      The solar wind particles mostly can't force their way to the Earth. The Earth's magnetic field tends to make them slide around the magnetosphere.

      The protection by the magnetosphere is probably necessary. The solar wind probably blew away part of Mars's atmosphere (Se-480).

      The solar wind particles can also act as dangerous ionizing radiation for life and electronic systems. The magnetosphere largely protects astronauts and satellites, but large solar storms (see below) can cause solar wind particles to penetrate the magnetosphere and be more dangerous than ordinarily. See UCAR's Effects at Earth of Space Weather Events.

      Some particles do get trapped in the reservoirs in the magnetosphere. These reservoirs are called the Van Allen belts. They are donut-shaped or toroidal and there are 3 of them (PF-99): the inner 3rd belt was discovered circa 2000. The particles in the Van Allen belts may have other causes besides the solar wind. See the figure below.

      Some of the solar wind particles can helix into the Earth's atmosphere near the poles. The tend to come down in a ring called an AURORAL RING.

      In fact, during strong gusts of solar wind (e.g., coronal mass ejections: see section Coronal Mass Ejections below), the particles can helix in at lower latitudes and one can get aurora there and strong aurora in many places.

      Through a rather complex process, the solar wind particles result in currents in the Earth's atmosphere of ions and ions and electrons.

      The collisions of the of ions and ions and electrons of the currents with with air molecules excites air molecules (i.e., gives them internal energy).

      When the air molecules de-excite they emit light. This is same process as in a neon light that generates ultraviolet light that is then by another process converted in to visible light.

      The result in the atmosphere is the aurora.

      The aurora, in fact, have an emission line spectrum as illustrated in the figure below (local link / general link: /noaa_aurora_line_spectrum.html).


        Question: Can the aurora ever be seen in Las Vegas?

        1. Never.
        2. Frequently.
        3. Rarely.











        Answer 3 is right.

        My late colleague at UNLV Lon Spight remembers seeing an aurora circa 1970, but I think they must have been seen more recently than that.

      For examples of the aurora, see the figures below.


      alien_click_to_see_image click on image

      Caption: A 360 degree panorama at the South Pole with Constellations with aurora. A nifty gigapan from the deep, deep south:

      The military is very keen on developing snapshot gigapans for reasons of peeping into windows (see Can You See Me Now? A camera with a unique, spherical lens may bring single-shot gigapixel cameras closer to reality, 2011 March).

      Images:
      1. Credit/Permission: © David Jeffery, 2012 / Own work.
        Image link: alien_click_to_see_image.html: Image link direct: Itself.
      2. Credit/Permission: © Jeremy Johnson, 2009 / No permission.
        Image link: 360 degree pano South Pole with Constellations.

      To further illustrate aurora, see the auroral oval (AKA auroral ring) in the figure below (local link / general link: auroral_oval_film.html).


      See
      Earth aurora videos below (local link / general link: earth_aurora_videos.html):

        EOF


  23. Solar Activity and Magnetic Fields (Reading Only)

  24. Solar activity is essentially solar weather. But unlike the Earth's weather, solar weather is dominated by magnetic effects. But we won't go into the causes of solar activity which are NOT fully understood and are certainly too complex for our course.

    The Sun has an overall magnetic field that is dipolar with a north and south pole like a bar magnet and like the Earth (Ni-130). A cartoon of the Sun's magnetic field lines is shown in the figure below (local link / general link: sun_magnetic_dipole_cartoon.html).


    The polarity of the
    Sun's field reverses every 11 years on average for an overall cycle of 22 years on average (HI-300; FMW-296). The reversals occur at the solar minima of the solar cycle????.

    What causes the magnetic field of the Sun?

    Well the Sun is a plasma in its interior: i.e., all the particles are charged. It is also rotating differentially and has convection.

    Somehow, in way that is NOT fully understood yet, large electric currents must form. Electric currents generate a magnetic field: this is just a fundamental fact. And this must be what happens in the Sun.

    The process of generating a magnetic field this way is called the DYNAMO EFFECT (Se-157).

    In addition to the main dipole magnetic field structure their are smaller time varying structures associated with sunspots mainly.

    For further insight, see the Solar atmosphere videos below (local link / general link: solar_atmosphere_videos.html):

      EOF



  25. Sunspots (Reading Only)

  26. Sunspots have probably been occasionally noticed going back into prehistory.

    Certainly, they were observed and recorded for thousands of years in China (SRJ-357).

    But Galileo (1564--1642) and other early telescopic observers were the first to make their existence well known and to start systematic observations. Sunspots are illustrated in the figure below (local link / general link: sun_white_light.html).


    Sunspots are typically tens of thousands of kilometers in size scale and are typically twice the diameter of the
    Earth (Ni-123; Se-154).

    Sunspots are transient and may last a week or so.

    They tend to occur in groups of up to 100 members and the groups can last 2 months or more (Se-155).

    Sunspots occur in the photosphere and are colder than the surrounding photosphere.

    Their temperature at center is about 4000 K (Se-155), whereas the photosphere temperature is usually 6000 K.

    Because of the lower temperature, sunspots radiate less than the surrounding photosphere and appear dark in comparison in toned down images.

    The dark inner region of a sunspot is called the UMBRA and the less dark border the PENUMBRA. This is a different usage of "umbra" and "penumbra" than in eclipse phenomena (Se-155). See the figure below.

    Sunspots are comparatively cold because they are have strong magnetic fields that seem to suppress convection that is otherwise heating the photosphere.

    Sunspots are essentially a magnetic phenomenon.

    Frequently sunspots occur in pairs. One member being a north pole and the other a south pole. This illustrated in the figure below.

    Sunspots obey in 22-year cycle divided into two 11-year subcycles. The time lengths are averages. Subcycles as low as 8 years and as high as 16 years are known (FMW-296). See the figure below.

    The start of an 11-year subcycle of the solar cycle, the sunspots are rare: there can even be none: this is the solar minimum.

    At mid-subcycle there are typically of order 100 and the record is 254 (HI-299): this is the solar maximum.

    The sunspots first appear at about 35 degrees north and south latitude. As the subcycle progresses they move closer to the equator and at the end they are usually within 5 degrees of the equator.

    The latitude evolution is illustrated by a butterfly diagram that plots sunspot latitude versus time. See the figure below.

    There are theories of sunspot formation. But we won't discuss them here.

    In fact, the theories are inadequate in that the solar cycle CANNOT be predicted.

    If you CANNOT predict a basic fact of a phenomenon, then clearly you don't really know what is happening.

    Associated with sunspots are other solar activities: dark filaments, prominences, flares, and coronal mass ejections.


  27. Dark Filaments and Prominences (Reading Only)

  28. In the vicinity of sunspots are DARK FILAMENTS: comparatively dark narrow sinuous string-like regions in the solar photosphere---but I can't find a good image of one.

    They stretch for up to 100,000 km and are locations of zero magnetic field between regions of opposite magnetic field lines (PF-186).

    Filaments often underlie ???? solar prominences (PF-186). prominences are giant arcs of glowing gas that can be a fair fraction of the solar radius in size structure.

    The arc shape is determined by magnetic field lines. The charged particles helix around the field lines tracing the arc.

    The special strong time-varying magnetic field structure that shapes a prominence must be determined by internal currents and energy sources in the Sun---and that is all I'm going to say about cause.

    Prominences in the visible are pink or red from H alpha emission and resemble chromospheric conditions. They are of order 100 times denser than the corona and have temperatures of order 10,000 K (Ni-126).

    They are only seen by the naked eye during total solar eclipses.

    A QUIESCENT PROMINENCE can arise in hours and last weeks or months (Se-160).

    A giant prominence is illustrated in the figure below.

    There are also ERUPTIVE PROMINENCES that occur on the order of hours (???) and eject matter into space.

    An ERUPTIVE PROMINENCE is illustrated in the figure below.


  29. Flares (Reading Only)

  30. FLARES are enormous explosions that are much stronger than eruptive prominences.

    FLARES are believed to be caused by MAGNETIC RECONNECTION. This is when tangled magnetic field lines quite suddenly become unstable and reform in a simpler pattern.

    When they do this they somehow dump magnetic field energy as heat energy, EMR, and kinetic energy.

    The analogy is often made that MAGNETIC RECONNECTION is like an elastic band snapping: potential energy stored in the stretched configuration is suddenly released as kinetic energy.

    A solar flare is illustrated in the figure below.

    The energy released is up to

    
       10**25 J = 2.5 * 10**9 megatons
    
                  ( 1 megaton TNT = about 4*10**15 J )
    
    and temperatures can reach
    
            5*10**6 K  which is much hotter than the chromosphere 
            or solar photosphere.
    


  31. Coronal Mass Ejections

  32. Coronal mass ejections (CMEs) are huge bubbles of coronal gas ejected from the Sun within a few hours.

    Magnetic effects seem responsible for coronal mass ejections---and that is all we'll say about cause.

    Coronal mass ejections often accompany solar flares or eruptive prominences, but can occur in the absence of either.

    1. What Coronal Mass Ejections Look Like:

      A false-color image of a coronal mass ejection is shown in the figure below.

      See Solar corona videos below (local link / general link: solar_corona_videos.html):

        EOF

    2. When Coronal Mass Ejections Hit the Earth:

      If coronal mass ejections hit the Earth in the form of strong gusts of solar wind, we can get strong aurora as mentioned above in subsection The Solar Wind at the Earth and the Aurora.

      Such strong gusts can also cause geomagnetic storms: i.e., cause the Earth's magnetosphere (AKA Earth's magnetic field) to vary rapidly in time.

      A cartoon of a coronal mass ejection hitting the Earth is shown in the figure below.

      Geomagnetic storms can cause blackouts (AKA power outages).

      A fundamental effect of electromagnetism is that a time-varying magnetic field causes an electromotive force (emf) (via Faraday's law of induction) which will drive an electrical current in an electrical conductor. This effect is the basis of electrical generator, and so is the basis of the electrical grid.

      In a geomagnetic storm Earth's magnetic field under goes rapid variations over large distances over the the Earth Now neither of Earth's magnetic field nor the variations are large, but they act over large distances. There is thus a cumulative large and unsafe emf in electrical power transmission wires. The large electrical currents induced can burn out electrical transformers which are everywhere in the electrical power grids. The big ones are in fenced-in areas that one frequently sees.

      Now if electrical transformers burn out, the electricity stops and there is a blackout. Powerful geomagnetic storms can cause major blackouts.

    3. Super Geomagnetic Storms:

      A super coronal mass ejection that impacts the Earth could cause a super geomagnetic storm that could potentially crash electrical power grids worldwide.

      It could take months or year to repair all the electrical transformers and other damage. A main difficulty is that repair procedures themselves depend on having electrical power grids.

      There would be other effects too.

      See the articles What If the Biggest Solar Storm on Record Happened Today?, Richard A. Lovet (2011), Solar storm researchers prepare for the 'big one' with new urgency, Tracey Regan (2016), and BBC: Lagrange: The early warning satellite.

      A massive electrical power grid failure caused by a super geomagnetic storm would be a massive worldwide catastrophe---except for those lucky people who don't depend on electrification for anything---i.e., the world's last hunter-gatherers.

      The rest of us would be like Puerto Rico after Hurricane Maria (2017)---but a lot worse.

      Could such a super geomagnetic storm happen?

      Yes. The super CMEs that cause them are NOT very rare: they just usually do NOT hit the Earth.

      In fact, super geomagnetic storm did happen occasionally in the pre-electrification world without anyone noticing---except for the great aurora.

      The largest known pre-electrification one may have been the Year 774--775 CE carbon-14 spike event. See also Jonathan O'Callaghan 2021, SciAm, "Solar 'Superflares' Rocked Earth Less Than 10,000 Years Ago---and Could Strike Again".

      However, one super CME did happen in 1859 when electrification was just beginning and of minor significance to society---there was just a little bit of electrical telegraphy.

      This super geomagnetic storm was the Solar storm of 1859 (AKA the Carrington event).

      We had a near miss in 2012 July. See the article Near Miss: The Solar Superstorm of July 2012, Tony Phillips (2014).

      Civilization was saved---for now.

      See the Geomagnetic storm videos below (local link / general link: geomagnetic_storm_videos.html):

        EOF

      Probably, we could prevent worldwide catastrophe just by shutting down the electrical power grids for a few hours if we saw a super CME was coming. That would be incredibly disruptive, but better than NOT shutting down.

      But can we see super CMEs coming early enough? Not yet, but people are working on it ...

      The European Space Agency (ESA) is planning two spacecraft, the Lagrange spacecrafts (est. 2020s--2030s), which will study solar weather (and space weather generally) and in particular give the early warning of CMEs, particularly dangerous super CMEs. One will be at L1 and and the other at L5 (see Wikipedia: Lagrange spacecrafts: Overview).

      The Lagrange points in general and the Lagrange points where the Lagrange spacecrafts (est. 2020s--2030s) will orbit are explicated in the figure below (local link / general link: lagrange_points.html).