• General Properties• Internal Structure• Solar Atmosphere

– Photosphere, chromosphere, and corona

• Surface Features and Magnetic Fields• Solar Activities• Solar Cycle• Sun-Earth Connection

The Surface of the Sun…

• The rapid decrease of the density within a short distance is the reason that we see a sharp edge…

• The surface layer is where sunlight are generated. It is referred to as the Photosphere.

In images of the Sun, we see a sharp edge, which we perceive as the surface of the Sun. However, like the surfaces of the Jovian planets (the Gas Giants), it is not a firm, solid, thick surface that we can stand on like on the Earth…

• Density of the atmosphere on the surface of the Earth is– 1.3 kg/m3, or about – 1 1025 N2 molecules per cubic meter, and

• Density of the atmosphere on the surface of the Sun is– 1 1023 particles per cubic meter, or about

only 1% the density of the Earth’s atmosphere.• Density of the solar atmosphere just a few

thousand kilometer above the ‘surface’, or in the solar corona– 1 1014 particles per cubic meter

Chromosphere The Chromosphere is a thin, irregular layer above the photosphere in which the

temperature rises up from 5,800 K to about 20,000 K. This layer is usually observed in the red wavelength of the Hydrogen absorption line. It is therefore termed Chromosphere, meaning color-sphere,

Bright patches (the Plages) and dark spots (sunspots) are related to higher magnetic field regions.

The Sun in Calcium absorption line in blue (393 nm) wavelength.

The Sun in Hydrogen absorption line in red (656 nm) wavelength.

The Sun in UV and X-Ray – Corona

The Sun in X-ray shows the structure of the very hot (1,000,000 K) corona• Recall that high temperature blackbody produces radiation with

shorter wavelength. X-ray are produced by blackbody with million degree temperature.

• We don’t know why the coronal temperature is so high…• The Sun in one of the emission spectra of Helium in the UV (30.4 nm) shows

the structure of a cool regions of the corona.

X-ray image of the Sun. UV image of the Sun in He II 30.4

The Solar Corona in ‘White Light’

This is an image of total solar eclipse. • The radiation are

reflection of sunlight by the electrons in the corona.

• A radial gradient has been removed from the image to enhance the coronal features.

• The streamers are where slow solar wind leave the Sun.

• The coronal holes are where fast solar wind leave the Sun.

The Many Faces of the Sun

By observing the Sun simultaneously at many different wavelengths (or colors), we can see different layer of the solar atmosphere, and get a better understanding of what’s going on…

MDI MAGNETOGRAM 01-May-2005 20:48 UT

MDI WHITE LIGHT 01-May-2005 20:48 UT

BBSO GHN H 01-May-2005 20:48 UT

EIT FeX 195 A 01-May-2005 20:48 UT

EIT FeXV 284 A 01-May-2005 20:48 UT

GOES SOFT X-RAY 01-May-2005 20:48 UT

The Coronal Heating Problems

• The temperature of the Sun is the highest in its core, about 15 million degrees.

• The temperature decreases as we move outward toward the surface, dropping to 6,000 K at the photosphere.

• The temperature then rises to about 20,000 K in the chromosphere, just a few thousand km above the photosphere.

• The temperature than rises rapidly to 1to 2 million degrees in the corona.

• We do not understand how the corona is heated, and this is one of the important unresolved questions of solar physics.

• General Properties• Internal Structure• Solar Atmosphere• Surface Features and Magnetic Fields

– Sunspots, Granulation, Filaments and Prominences, Coronal Loops…

• Solar Activities• Solar Cycle• Sun-Earth Connection

High-Resolution View of the Solar Surface

This is what the surface of the Sun looks like with high resolution…we see• Sunspot

– Umbra– Penumbra

• Solar Granulation

Solar Granulation

Image of solar granulation. The bright center of the cells are where hot gas rise to the surface. The narrow dark lanes are where cold gas sink to the ‘bottom’. • Each cell is about 1,000 km in

size

On the surface of the Sun, we can see the action of convection…

SunspotsSunspots are dark features on the surface of the Sun. Sunspots are strongly magnetized region on the surface of the Sun. They appear dark because the presence of very strong magnetic fields helps the plasma inside the sunspot to balance the pressure of the plasma outside of the sunspot. Therefore, the thermal pressure (related to the temperature of the plasma) of the plasma inside the sunspot is lower, leading to lower temperature, and lower intensity (darker compared with the surrounding area).

We still don’t know why there is an umbra and a penumbra in sunspots. Neither do we know why there are such sharp boundary between different regions…

Early Clues of Sunspot Magnetic FieldSunspot group seen in H

(Hydrogen absorption line)

Bar Magnet The pattern formed by the small magnetized iron bars shows the magnetic field lines.

Sunspots are strongly magnetized region on the surface of the Sun. The brightness structure of a sunspot seen in the absorption line of hydrogen resemble the magnetic field lines surrounding a bar magnet.

Evidence of Magnetic Field in Sunspot

Spectra of magnetic field sensitive absorption lines from a slice (the dark vertical line at the center of the image on the left) of a sunspot.

The separation between the lines measures the strength of the magnetic fields

The presence of a magnetic field in the solar atmosphere can be seen in the Zeeman Effect of the spectral line on the right. Some spectral lines have three components…and magnetic field can change the energy level of two of them. Thus, the spectral line will be split into three lines when there is a strong magnetic field.

The Sun as a Magnetic Star

Today, we know that almost all the solar surface and coronal features (except for solar granulation, which is generated by convection) we talked about so fare are related to magnetic fields…• Sunspots• Filaments and Prominences• Coronal loops

Without the magnetic fields, the Sun would be a very boring star to look at…

Magnetic Field of the Whole Sun

• A magnetogram shows the magnetic field on the surface (the photosphere) of the Sun. The black and white patches show where the magnetic fields are strong. – White indicates magnetic field

pointing toward us.– Black indicates magnetic fields

pointed away from us.– The large patches of black and

white are due to sunspot and active regions with strong magnetic fields.

• The pepper-and-salt patterns outside of the active regions indicates that there are magnetic fields everywhere on the surface of the Sun.

Filaments and Prominences Filaments and prominences are cool and dense gas suspended high in the

solar atmosphere, and embedded in the very hot solar corona.• When they are observed on the solar surface, they appear as dark

absorption features…filaments!• When the are observed outside of the solar limb, they appears as bright

features because they reflect sunlight toward us…prominences!• How they can survive in the million-degree temperature corona, and stay

high up against gravity is still a mystery. We know magnetic field plays an important role, but the details is not well understood.

A huge solar prominence observed in 1946

The Grand Daddy Prominence

Coronal LoopsWe believe that the loops we see in the

solar corona trace the magnetic field lines. However, the magnetic fields are everywhere in the corona. we are not quite sure why only some of the field lines are bright…

High resolution image of the coronal obtained in the UV wavelength obtained by TRACE satellite.

• General Properties• Internal Structure• Solar Atmosphere• Surface Features and Magnetic Fields• Solar Activities

– Flares, CMEs, and Filament Eruptions

• Solar Cycle• Sun-Earth Connection

Solar Activities---FlaresA flare is defined as a sudden, rapid,

and intense variation in brightness. A solar flare occurs when magnetic energy that has built up in the solar atmosphere is suddenly released. Radiation is emitted across virtually the entire electromagnetic spectrum, from radio waves at the long wavelength end, through optical emission to x-rays and gamma rays at the short wavelength end. The amount of energy released is the equivalent of millions of 100-megaton hydrogen bombs exploding at the same time! Or, about a few percent of the total energy released by the Sun every second.

Filament Eruptions

Filament eruptions are usually associated with flares and coronal mass ejection. Exactly how they work is still under investigation...

• General Properties• Internal Structure• Solar Atmosphere• Surface Features• Magnetic Fields• Solar Activities• Solar Cycle• Sun-Earth Connection

Solar Cycle---Sunspot Numbers and the Butterfly Diagram

Butterfly diagram Sunspots appear at higher latitude at the beginning of the solar cycle, and migrate toward the equator as the cycle evolve. So, when we plot the latitude of the sunspots as a function of time, the patterns looks like a series of butterfly…therefore it is referred to as the butterfly diagram’

Solar Cycle The number of sunspots on the surface of the Sun follows a 11-year cycle.

Magnetic Field and X-Ray Variation Through one Solar Cycle

• The temperature of the solar corona a few million degrees (no explanation yet).

• The high temperature causes it to emit photons mostly in the UV and X-ray wavelengths (high energy photons).

• The activities in the solar corona also follow the solar cycle.

• In fact, the level of almost every aspect of solar activities (flares, coronal mass ejections, etc.) follows the solar cycle.

The black-and-white patterns show the surface magnetic field variation through one sunspot cycle (11 years). Notice the reversal of the ordering at the beginning and the end of the cycle.

How Does Solar Cycle Work? The magnetic field of the Sun is postulated to be generated at the bottom of the

convection zone. This magnetic field then rises up to the surface and expand into the corona, to produce the magnetic features we see. • Since the magnetic field of the Sun reverse its orientation every 11 years, the

solar cycle is really a 22-year magnetic cycle. In comparison, the Earth’s magnetic field direction is stable.– The number of sunspot only depends on the strength of the solar

magnetic activities, but not the orientation of the magnetic fields. Therefore, sunspot number cycle is half that of the magnetic field cycle.

• How does the Sun changes its magnetic field orientation every 22 years? We don’t have a complete theory yet. However, there are a few clues. For example, we believe that the differential rotation of the Sun must play a role in changing the magnetic field configuration from that of a dipole (like a bar magnet) to that of a torus (shaped like a doughnut).

The exact mechanism of the solar cycle is still unknown!

Differential Rotation of the Sun The Sun does not rotate like a solid

body. It rotates every 25 days at the equator and takes progressively longer to rotate one revolution at higher latitudes, up to 35 days at the poles. This is known as differential rotation. • You can pick a few sunspots

located at different latitude from the movie on the right, and trace them as they rotate across the solar disk. Using the time information at the lower-left-hand corner of the images, you can calculate the rate of rotation of the Sun at different latitudes.

• You should find that sunspots near the equator rotate faster than those at higher latitude.

Effects of Differential Rotation

At the surface of the Sun, and deeper in the interior, when we move the solar plasma, the magnetic fields embedded in the plasma will move with the plasma. This is referred to as the frozen-in magnetic fields. • So, the effect of differential

rotation is the stretching of the magnetic fields that was originally in the north-south direction to make them runs along the east-west direction…

• Sometimes a small section of the magnetic field would pop up through the surface. This will make a sunspot, and it happens more frequently during solar maximum…

Solid (dashed) lines represents magnetic field lines above (under) the surface.

~5 years later…

Evolution of Solar Magnetic Field During the Solar Cycle

Solar Minimum • Dipole Magnetic Field • No Sunspot

Solar Maximum • Toroidal Magnetic Field • Many Sunspots

But, this is only half of the story!

11 years later…

The magnetic field configuration of the Sun evolves with a 22 year cycle.

S

N

22 years later…

The Solar Cycle Problem and the Sunspot Phenomenon

• At this point, we don’t have a satisfactory theory for the solar cycle. Neither do we have a complete understanding of the sunspot phenomenon. These are two more important problems of solar physics that needs to be solved.

• General Properties• Internal Structure• Solar Atmosphere• Surface Features• Magnetic Fields• Solar Activities• Solar Cycle• Sun-Earth Connection

Sun-Earth ConnectionHow do Solar Activities Affect Earth?In short time scale (compared with the lifetime of the Sun) • Space Weather and Geomagnetic Storm

Flares and Coronal Mass Ejections (CME) bombard the Earth with high energy charged particles, causing interruption to communications, and power grids

• Solar Irradiance Variations and Possible climate change The solar energy input determines the temperature on the surface of the Earth. If the Sun is to increase its luminosity by 1%, it will have significant effect on Earth’s temperature. It will increase by about 0.75 K.

In long time scale • The Sun will eventually evolve into a red giant star, increases its energy

output and its physical size. Earth may eventually be engulfed by this enlarged Sun, and life will be extinguished on Earth Next chapter…

Space WeatherSpace Weather (from NASA SoHO Space Weather Website)

Space weather happens with a solar storm from the Sun travels through space and impacts the Earth’s magnetosphere. Studying space weather is important to our national economy because solar storms can affect the advanced technology we have become so dependent upon in our everyday lives. Energy and radiation from solar flares and coronal mass ejections can • Harm astronauts in space • Damage sensitive electronics on orbiting spacecraft? • Cause colorful auroras, often seen in the higher latitudes? • Create blackouts on Earth when they cause surges in

power grids.

http://sohowww.nascom.nasa.gov/spaceweather/lenticular/

From Coronal Mass Ejections…

Space weather starts with Coronal Mass Ejection on the Sun…Coronal mass ejections and flares are due to the changes in the magnetic field structures in the solar corona. However, details mechanism is still not clear, and we cannot predict when flares and CME are going to occur yet!

To Geomagnetic Storm…Flares and coronal mass ejection send high energy charged particles (electrons, protons) into space. If the direction and speed of these particles are just right, they can reach the Earth. These high energy particles are harmful to life on Earth. They can also cause damages to satellites operating in space, as well as power grids. • Charged particles travel along the magnetic field lines • We are protected by Earth’s magnetic field, which directs the majority of

the high energy charged particles toward the north and south poles to produce the aurora borealis and aurora Australis.

Charged particles spiral around the magnetic field lines.

Effects of Geomagnetic Storm• In October 31, 2003, a series of strong geomagnetic

storms damaged two satellites, caused the power grid in Sweden to shutdown, cutting power to 20,000 customers, disrupted radio communication and broadcast systems, and forced the airlines to change flight plans. – The large variation of the Earth’s magnetic fields can induce

strong, uncontrolled electric current in the power lines, causing the power grid to overload and shutdown…

– The high energy charged particles from the Sun are a health risk to astronauts in space orbit and passengers in jets flying at high altitudes over the north and south polar regions .

• Aurora were reported as far south as New Mexico, Texas, and Florida.

http://www.space.com/scienceastronomy/power_outage_031031.html

Solar Irradiance VariationsModern measurements showed that the solar constant is really not a constant.

The energy output of the Sun is modulated by the magnetic activity. But details of how this happens is still under study…Nevertheless, we know that

• Solar irradiance is higher when the surface magnetic field is stronger (when ther are more sunspots)…

• The amplitude of the solar irradiance variation is about 2 W/m2, or about 0.1%.

• This variation is too weak to cause climate change.

• But, if solar magnetic activities was significantly reduced or enhanced for a long period of time, it can change the climate of the Earth…for example, did the Sun caused the Little Ice Age? Sunspot Maximum

Solar constant measurements from several satellite experiments

Little Ice Age (1650-1700)• During a period that lasted

approximately 50 years from the mid 1650s to the early 1700, the temperatures in northern Europe had their lowest values for the past millennium, with winter temperatures being on average 1 to 2 degrees colder than in later periods.

• This period has been called the Little Ice Age. During this period, access to Greenland was largely cut off by ice from 1410 to the 1720s. At the same time, canals in Holland routinely froze solid, glaciers advanced in the Alps, and sea-ice increased so much that no open water was present in any direction around Iceland in 1695.

Aert van der Neer, Dutch, 1603-1688Winter Scene with Frozen Canal

Was The Sun Responsible for the Little Ice Age?

Maunder Minimum, the period with reduced sunspot number around 1,650 AD, was coincident with the little ice age of western Europe.

• Does the reduced sunspot number imply reduced solar energy output, causing the temperature on Earth to drop?

• Given the small amplitude of the total solar irradiance variation (0.1%), it is unlikely that the total solar irradiance variation is responsible for the global warming trend we have seen in the last 100 years. But the amplitude of the UV irradiance variation is much larger…

• Solar UV radiation interact with Earth’s upper atmosphere. What’s the effect of solar UV variations on Earth’s climate?

Solar UV Variation and Global Warming?

From Haberreiter et al., Advances in Space Research 35 (2005) 365-369…1. Introduction

It is known that the variability of the solar UV irradiance has a considerable effect on the terrestrial atmosphere. Recently, Egorova et al. (2004) have shown that the introduction of solar UV flux into a spectral Global Circulation Model (GCM) with a chemistry transport model leads to an intensification of the polar vortex and a statistically significant warming of up to 1.2 K over North America and Siberia. Due to a missing longterm record of the solar UV irradiance with a sufficient temporal resolution,

• Like the effect of the increasing CO2

content in Earth’s atmosphere is still not clear, we do not understand the mechanisms that are causing the variations of the solar irradiance, nor do we understanding the details of how solar input affect the global climate.