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Solar AstronomySolar AstronomyThe Desert Sun

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The Sun, our active The Sun, our active starstar

“Astronomers love stars, and we have a fine one right near us. It may be studied in the daytime, a great advantage for those who, like me, have trouble adjusting to all-night observing.” -- Hal Zirin, Astrophysics of the Sun.

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Solar StructureSolar Structure

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A Small Physics A Small Physics LessonLesson

We primarily observe the light of the Sun.

Light is a kind of electromagnetic radiation.

Light can be considered a wave. The waves have wavelength and frequency. The different wavelengths produce different colors.

Ultraviolet radiation, infrared, radio, x-rays, and so forth are just like visible light, except that the wavelength makes these radiations invisible to us.

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Solar SpectrumSolar Spectrum

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Kirchhoff’s LawsKirchhoff’s LawsA gas at high pressure, a liquid, or a solid, if heated to incandescence, will glow with a continuous spectrum.

A hot gas under low pressure will produce only certain bright colors, called emission lines.

A cool gas at low pressure, if placed between the observer and a hot continuous-spectrum source, absorbs certain colors, causing absorption lines in the observed spectrum.

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Later on, it was found that it is possible for the absorption lines to originate in the same gas as the continuum, since some atoms may be going from bound to free states (forming the continuum) while others are transitioning from one bound state to another (forming the lines).

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Commonly used Commonly used spectral lines in visible spectral lines in visible

lightlight

Hα (Fraunhofer’s C line)

Ca II H and K

Na D

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Solar AtmosphereSolar Atmosphere

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““White light” White light” (photospheric) (photospheric)

FeaturesFeaturesSunspots

Pores

Faculae

Granulation

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Limb darkeningLimb darkening

The brightness of the Sun decreases as we move to the limbs (edges). This occurs because some light is absorbed above the “surface” of the Sun, and as we move to the limb the path length through these upper layers is longer. The “surface” is thus a little higher. The temperature in this higher layer is lower than at disk center, so less light is seen.

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BBSO White light Full disk July 30,1999

Photosphere

5700 Kelvin

Radius 695970 Km

Sunspots

Sunspots

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BBSO CakII

Fulldisk July 30,1999

Chromosphere

6,000-20,000 Kelvin

CaKII

3933 Angstroms

Active Region

Active Region

Sunspot

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BBSO H-alpha

Fulldisk July 30,1999

H-alpha

6563 Angstroms

Sun’s Chemical Composition

90% Hydrogen 9% Helium 1% Other

Active Region

Active Region

Filament

Polar Crown Filament

Prominence

Sunspot

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SunspotsSunspots

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HHαα Features Features

Prominences

Filaments

Plage

Surges

Fibrils

Spicules

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Active Region Active Region Magnetic Magnetic

ConfigurationsConfigurationsHale-Nicholson law: Magnetic field of leading spots in a region will be the same for each solar hemisphere, currently south (negative) leading in northern solar hemisphere and north (positive) leading in southern solar hemisphere.

Joy’s law: Trailing portion of a region is at higher latitude than leader. Tilt is normally small (~5º).

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What is the sunspot cycle?What is the sunspot cycle?Heinrich Schwabe, an amateur astronomer, began drawing sunspots in 1826. After doing this for seventeen years, he began to suspect there was a regular cycle in the number of sunspots, with a period around 11 years. In 1843 he announced his discovery.

Rudolf Wolf then devised in 1848 the sunspot index (the Zurich sunspot number) we still use today.

Since then the spot number records have been extended back to 1610.

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Sunspot Number (R)Sunspot Number (R)R = k (10g + f), where g is number of groups, f is number of individual spots, and k is Wolf’s “reduction factor” to compensate for different observers.

What counts as an individual spot? For the sunspot number, a spot must be > 3 arcseconds. In other words, the spot must be visible with a 50mm telescope under good seeing. Each umbra > 3 arcseconds is counted as a spot.

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Sunspots and PoresSunspots and Pores

There is no uniform definition of the difference between a pore and a sunspot. Most people in the field will consider any spot without a penumbra a pore. This does not reconcile with the definition for the sunspot number.

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The sunspot cycle.The sunspot cycle.

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An unusual activity minimum.An unusual activity minimum.We are currently in a minimum of sunspot activity.

Such a minimum occurs as part of every activity cycle.

In April 2007 a group of solar astronomers made predictions for the new sunspot cycle. There were two opposing camps, one predicting a very strong cycle and the other predicting a weaker cycle. Both groups predicted that sunspot numbers should begin to rise in early 2008.

Both groups were wrong!

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Sunspot number beginning to rise?Sunspot number beginning to rise?

It is now late 2009 and the number of sunspots is still very low. Just now it’s possible the spots are beginning to increase.

How unusual is this? No sunspot minimum has been this long during the last 50 years, but minima this long did occur early in the 20th century (1913 being the last one).

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Development of Active Development of Active RegionsRegions

Pores and/or Faculae (Plage)

Arch Filaments, Brightening of Plage (EFR)

Sunspots with Penumbrae

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Solar FlaresSolar Flares

Solar flares are sudden brightenings on the Sun.

Flares occur mainly in active regions, along magnetic neutral lines.

Flares may last from a few minutes up to an hour.

Flares are normally observed in spectral lines (particularly Hα), rarely in white light.

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Flare EffectsFlare EffectsParticles such as protons, electrons and alpha particles may be ejected from flares and arrive at Earth about 15 minutes after a flare begins.

UV and X-ray emission from a flare may intensify the Earth’s ionosphere and cause shortwave “fade-outs”.

Lower energy particles may arrive a day or two after the flare and cause auroral displays and geomagnetic storms.

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CMEs and Filament CMEs and Filament EruptionsEruptions

Similar low-energy particle streams are observed from some coronal mass ejections (CMEs) and filament eruptions.

These particle streams are also observed from coronal holes.

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Conditions Indicating Conditions Indicating Flare PotentialFlare Potential

Growth within an established region. Arch filaments and bright plage in an established region.

Mixed magnetic configuration (βγ, particularly δ).

Magnetic configuration violates Hale’s and/or Joy’s law.

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Eclipses and the Eclipses and the CoronaCorona

The other major light source in our sky, besides the Sun, is the Moon. Most astronomers, who study faint galaxies, consider the Moon a pesky nuisance that takes away half the nights each month for serious observing. However, for us solar astronomers it has one very useful feature. It so happens that the Moon has about the same apparent size in our sky as the Sun, and the Moon’s orbit is such that the apparent position of the Moon sometimes coincides with that of the Sun. Thus solar eclipses can occur.

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Equipment and Equipment and Techniques for Solar Techniques for Solar

ObservingObserving

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TelescopesTelescopes

Refractors are generally best for solar use. However, reflectors can also be very effectively used.

In solar work, the filter is perhaps even more important than the telescope. Frequently the telescope must be chosen to fit the filter being used. The f-ratio is often most important.

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ResolutionResolutionResolution given by 206265 * (λ/D).

Diameter (mm)

Resolution at Hα

(arcsec)

50 2.7

90 1.5

150 0.9

200 0.7

Mt. WilsonThe Snow telescope (1904)

was the first large, specially-built permanent

solar telescope. It was followed by the 60-foot

(focal length) and 150-foot tower telescopes in 1908

and 1912.The tower design was

intended to put the aperture of the telescope above the

layer of the air with the most severe turbulence from ground heating.

The 60-foot and 150-foot towers are still in use today.

Einstein Tower, 1924Einstein Tower, 1924

McMath-Pierce, 1962McMath-Pierce, 1962

Dunn Tower, 1969Dunn Tower, 1969

Swedish Solar Telescope, Swedish Solar Telescope, 20022002

Gregor, 2010Gregor, 2010

Big Bear Solar Observatory1970 Version

BBSO of 1970Large tube holds four

telescopes:2 10-inch refractors with

narrow-band filters.A 16-inch off-axis reflector to

feed the spectrograph.A 9-inch reflecting

coronagraph (which never worked out).

Shortly after the observatory was completed, an 8.5-inch telescope was added. This was an H-alpha telescope used to train the Skylab

astronauts, and was built by the Singer-Link company.

BBSO NST, 2009BBSO NST, 2009

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New Solar Telescope Light New Solar Telescope Light PathPath

BBSO HBBSO Hαα Telescope Telescope

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Aperture 98mm

Focal length 1625mm, image scale 1.005 arcsec/pixel

Filter: 1/4Å Lyot birefringent filter made by Zeiss. Camera 2048x2048 12-bit by Pulnix.

BBSO HBBSO Hαα Telescope Telescope

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MountsMounts

Altazimuth

Equatorial

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FiltersFilters

Absorption (colored glass)

Reflection (“half-silvered”)

Interference

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Interference FiltersInterference Filters

Filters use two partially-reflecting layers separated by a different medium. The thickness of the medium is an integral number of wavelengths. The waves are reflected from the surfaces of the medium and either add together (“constructive interference” or cancel each other out “destructive interference”). Constructive interference occurs at the desired wavlength, and at other wavelengths evenly spaced around it.

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InterferenInterference Filterce Filter

path difference p = 2nl cosθ

phase difference δ = (2π/λ)pPeak transmission occurs if p is

integral multiple of λ.

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The ratio δλ/Δλ is called the finesse of the etalon.

The peak transmission wavelength can be changed by changing:

the spacing of the plates

the index of refraction of the material between the plates.

tilting the etalon.

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The index of refraction can be changed by:

Changing the temperature of the etalon.

Changing the air pressure inside the etalon (for an air-spaced etalon).

Tilting the etalon shifts the wavelength to the blue.

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Ca II K FiltersCa II K Filters

Ca II K filters are used to show chromospheric features such as plage.

Bandpass of usual etalons is 5Å to 2Å. The width of the K line is greater than Hα and the core of the line is much darker. In order to see upper chromospheric features such as filaments a very narrow bandpass (<0.3Å) must be used. Such filters are not readily obtainable.

As we age, our ability to see the K line decreases.

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The filter is built with additional elements to block out the other wavelengths, and also a broad-band filter is used to block out wavelengths away from the desired one.

The medium can be a crystal like mica or just air. This determines how the filter is built.

Etalons require a “slow” beam. If the entrance angle is too large the bandpass becomes broader and non-uniform.

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Bandpass can be between 5 or 10 Å down to 0.5Å.

Cost depends on the bandwidth and the diameter of the filter (increases rapidly with diameter).

Narrow-band interference filters are often called Fabry-Perot etalons, but this term can be applied to any interference filter.

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Birefringent FiltersBirefringent FiltersBirefringent filters are based on a different principle than interference filters. They use birefringent materials, polarizers and waveplates to select a series of bandpasses, similar to an etalon.

Advantages: Very stable and uniform. Can be narrower bandpass than available from etalons. Can be mechanically tuned.

Disadvantages: Extremely expensive -- nearly impossible to buy (>$100K). Low transmission. Very heavy (25 lbs.). Large (about a foot long).

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ProjectionProjectionProjection of the solar image is a traditional and safe (for you, not necessarily for the telescope!) observing method.

Only small refractors should be used for projection. Schmidt-Cassegrain and Maksutov telescopes have baffles inside which will MELT if used for projection.

Eyepieces used should be cheap and simple since they can be damaged.

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Sunspot DrawingsSunspot Drawings

Making sunspot drawings is still quite useful. Drawings can show spots smaller than can be seen in most photographs, and spot positions are easier to obtain from drawings. Drawings are often done using projection (if your mount is big enough you can trace the spots on your projection plate). Drawings can also be done using direct viewing.

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Sunspot PositionsSunspot Positions

It is fairly simple to find the heliographic (physical) positions of spots and active regions. The coordinate system used was devised by Carrington (from studies of sunspot rotation using drawings) in 1853. In doing so, the tilt of the poles of the Sun must be accounted for. The easiest way to do this is by using Stonyhurst disks.

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CamerasCameras

Digital cameras -- two types

Digital SLR

Specialized CCD cameras

Any camera needs to matched to your telescope and the desired field of view.

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SafetySafety

The Sun is not to be trifled with. It can be very dangerous.

Use only filters designed for use on the Sun. Saving a little money is not worth your eyesight.

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SpecificsSpecificsUV light is absorbed by the cornea, lens and interior of the eye. IR light is focused by the lens on the retina, but the retina does not detect it.

UV & excessive visible light damages cells; IR cooks them!

The EU recommends that a safe solar filter should transmit less than 0.0032% (density ~4.5) of visible light 380 nm to 780 nm, no more than 0.027% (density ~3.8) of infrared light between 780 and 1400 nm and no more than 0.003% (density ~4.5) between 280 and 380 nm (UVB, UVA).

Filter density is logarithmic, D = -log T (T is transmission of filter).

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Thousand Oaks, Baader, and other manufacturers make safe solar filters.

If the telescope is above 150mm diameter, it should not be used for full disk direct viewing unless it is stopped down.

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Practical PointersPractical Pointers

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Seeing and the Best Seeing and the Best Times to ObserveTimes to Observe

Resolution on the Sun is normally limited by turbulence in the Earth’s atmosphere (seeing).

Most daytime seeing is caused by the Sun heating up the ground consequently causing air convection.

Seeing is less bad early in the morning or late in the afternoon at most locations.

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Locations covered by grass or bushes are better than bare ground.

Asphalt or concrete covered areas should be avoided. Looking over the roof of a nearby building is also bad.

Typical daytime seeing is 2 arcseconds or worse from most locations.

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Setting Up EquipmentSetting Up Equipment

Finding north -- since you are working in the daytime, Polaris is not available. A compass can be used, provided you know the difference between true and magnetic north. Road maps can also help initially. If you have the time and an ephemeris program the azimuth of the Sun can be used.

Getting the time -- it’s important if you are recording your observations that the time be accurate since the Sun is constantly changing. Also useful for the telescope mount.

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Always remember to have all your filters in place BEFORE you put the telescope on the Sun!

Finder scopes must be covered, removed, or equipped with their own solar filters. Find the Sun by using the shadow of your telescope. Special solar finders exist; they are generally some variation of a pinhole and a screen. They are safe and effective, but not absolutely necessary.

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If you are using a sub-aperture filter, the filter mount must be completely opaque and cover the entire aperture of your telescope.

Filters should be checked for damage before each time you observe.

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Useful Things to Bring Useful Things to Bring with Youwith You

Table and chair, perhaps a sunshade

Hat, sunglasses and sunblock (unless you have a dome!)

Water

Printout from www.solarmonitor.org

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Capturing ImagesCapturing ImagesDigital cameras -- two types

Digital SLR

Specialized CCD cameras

Optical setups

prime focus

eyepiece projection (reimaging)

afocal

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Processing ImagesProcessing Images

Frame selection

Frame “stacking”

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ResourcesResourcesStonyhurst disks

A selection of Stonyhurst disks can be found here:http://www.petermeadows.com/html/stonyhurst.html . This site contains good information on making spot drawings and how to find spot positions.

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Current Solar Current Solar Observations on the Observations on the

WebWebSDO: http://sdo.gsfc.nasa.gov/data/

Various (mostly SOHO): http://www.solarmonitor.org

Mt. Wilson: http://obs.astro.ucla.edu/intro.html

NSO: http://www.nso.edu/current_images.html

Today’s Space Weather: http://www.swpc.noaa.gov/today.html

BBSO: http://www.bbso.njit.edu

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BooksBooks

Kitchin, C., 2002, Solar Observing Techniques, Springer.

Beck, R. et al., 1995, Solar Astronomy Handbook, Willmann-Bell.

Macdonald, L., 2003, How to Observe the Sun Safely, Springer.

Zirin, H., 1988, Astrophysics of the Sun, Cambridge.

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SoftwareSoftware

http://www.zerobyzero.ca/Starstruck/?page_id=248 Index of mostly Macintosh software for amateurs.

Keith’s Image Stacker -- probably the most useful image processing program for amateurs on the Mac.

http://keithwiley.com/software/keithsImageStacker.shtml

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IDL -- expensive software, but the standard for professionals. A programming language rather than a pre-assembled application. For solar work this is used with the SSW library, a very large set of procedures and functions for analyzing solar images from spacecraft and ground-based observatories.

GDL -- a limited free version of IDL. For Linux only.