Understanding the Hydrogen Alpha Solar Image Display

The hydrogen-alpha images of the Sun provide us with a look into one of the most active regimes of the Sun's atmosphere: the chromosphere. The chromosphere exists above the visible photosphere of the Sun, which can be viewed using any telescope that projects an image of the Sun onto a white sheet of paper (looking at the Sun directly of course may result in permanent blindness). The chromosphere can only be observed by permitting only a very narrow band of light to pass through to our eyes. This band of light, known as the hydrogen-alpha or h-alpha line, permits us to see the hydrogen of the Sun. This band also allows us to see powerful magnetic explosions known as solar flares and other types of interesting phenomena such as prominences and filaments, etc.

To help those who are unfamiliar with the features that can be seen on the Sun in the light of hydrogen, we have assembled below a set of two sample images with descriptions of what can be seen, as well as one four-frame image containing a portion of a movie showing what a typical solar flare looks like.

Note the time of the image in the upper-left-hand corner. This time is in UTC (Universally Coordinated Time, also known as Greenwich Mean Time [GMT] or Zulu time [Z]) relative to Greenwich England. For example, in the image above, the UTC date is 29 February 2000 and the UTC time is 17:44 UTC, which is equivalent to 1:44 pm Eastern Daylight Savings Time (EDT), or 12:44 pm Eastern Standard Time (EST).

Filaments are visible as dark stringy features that meander around. Filaments can erupt into space. When this occurs, they disappear from the surface of the Sun over a time spanning from a few minutes to perhaps several hours. Such events are known as disappearing filaments and are the sources of coronal mass ejections (CMEs) that can produce disturbed geomagnetic and auroral activity at the Earth several days later (if the filament that was ejected is directed toward the Earth). Large dark filaments that disappear generally produce larger interplanetary disturbances more capable of impacting the Earth than small filament disappearances.

To the untrained eye, sunspots usually look quite a bit different in the light of hydrogen alpha. They are usually less pronounced and harder to see in the light of hydrogen than in the broad-band white-light continuum that we see through a projected image of the Sun through a simple telescope. Several sunspot groups are shown in the image above. Each group may consist of many individual sunspots, although often only the largest are clearly visible in the light of hydrogen. Regions of enhanced hydrogen emission that are not associated with any visible sunspots are known as plage regions. These regions of higher-temperature hydrogen gas are often the sites of dead or dying sunspot regions. The plage associated with an active sunspot group usually survives long after the individual sunspots have vanished.

This is another sample image showing different views of the same types of activity described in the first image above. An active sunspot region is visible near the west limb of the Sun (the sun rotates from east to west, or from the left toward the right in the above images). A large and dark sunspot can be seen in this west-limb region (see pointer).

A solar filament is identical to a solar prominence. They are different words for the same type of phenomenon. The only difference is that a solar filament is observed against the brighter and hotter background of the solar disk while a prominence is a filament that is observed above the limb of the Sun (with the blackness of space as its background). Solar filaments and prominences are huge regions of cooler hydrogen gas that are suspended above the surface of the visible Sun by powerful magnetic fields. When these cooler gases are viewed against the brighter and hotter surface of the Sun, they appear dark. When they appear on the solar limbs where the blackness of space is the only background, they appear to be bright prominences. You will notice as a filament rotates onto the limbs of the Sun that they become visible as suspensions of gas above the surface of the Sun as the image above illustrates in several places (several other weaker prominences are also visible that have not been outlined).

Over time, specks of dust can contaminate the telescopic optics used to produce images of the Sun. Such a dust speck has been identified in the image above. Without knowing what to look for, it may be easy to mistaken a dust speck as a sunspot. The way to differentiate between dust specks and sunspots is most easily done by viewing movies of the Sun. Dust specks will not move relative to the frame of the image. Sunspots will change position from day to day as the Sun rotates. Such rotation is visible in the movies that are produced. If a black speck doesn't rotate over time, it's almost certainly a dust speck.

Some of the most powerful magnetic fields in the solar system originate within sunspots. When sunspots become particularly complex, their associated magnetic fields can become twistsed and tangled. This can result in enormous quantities of pent-up energy, much as energy is stored in an elastic band that is stretched and twisted to near its breaking point. If the right conditions exist, these magnetic fields may suddenly release their pent-up energy (just as the rubber band releases it's pent-up energy violently when it breaks or is released). The resulting magnetic explosion is known as a solar flare. Large solar flares can release many thousands of times more energy than all of the worlds nuclear weapons being simultaneously exploded. They are the most powerful natural explosions in the solar system. They can superheat the hydrogen gases around the site of the explosion to many times the normal background temperature. They can accelerate atomic particles to very high velocities that can bombard the Earth with radiation that can be hazardous to men and satellite equipment in space (life on the surface of the Earth is saved by the shielding effects of the Earth's magnetic field and our thicker atmosphere). Solar flares are perhaps one of the most interesting forms of energetic phenomena visible on the Sun.

The following sequence of four images shows the evolution of a slightly above-average solar flare in the active sunspot region outlined in the last sample image above. This sequence of images was taken from one of our regularly produced movies of the Sun using imagery from the Hilltop Dome Hydrogen Alpha Flare Patrol Telescope at Sacramento Peak, Sunspot, New Mexico.

This flare occurred on 02 May 2000 between 14:44 and 15:06 UTC (10:44 am and 11:06 am, EDT). The first image in the upper-left corner shows what the region looked like 1 to 2 minutes before the flare occurred. The flare occurred very rapidly, reaching a maximum intensity within 2 minutes and decaying back toward normal levels about 20 minutes later.

The solar flare in the sequence of images shown above resulted in a coronal mass ejection, shown below.

This image was obtained from the Solar and Heliospheric Observatory (SOHO) spacecraft using the on-board LASCO instrument. This instrument uses an occulting disk to block the bright light of the Sun from the sensitive camera that is used to image the much fainter light of the Sun's corona (which itself is visible as streaks of light radiating away from the Sun). The coronal mass ejection is visible in the boxed area as an irregular outflow of mass.

Other interesting features of this image are also outlined, such as the location of the planets Jupiter, Saturn and Mercury. The camera of the LASCO instrument is even sensitive enough to image background stars, which are visible as bright specks of light dotting the image. Unuusal streaks or dots of light that are visible in the image are caused by stray cosmic rays, which produce momentary flashes of light on the CCD of the camera when the high-energy cosmic ray particle strikes it. The less common solar proton flare which can flood the space around the Earth with high-energy protons, can significantly increase the number of streaks and dots of light that plague these LASCO images.

Notice also the dark area that connects the occulting disk in the center of the image with the lower-left corner of the image. This area of darkness is produced by a small arm that hols the occulting disk in-place.

The resources from which these images were taken may be found at the following locations:

Questions or comments should be directed to: STD@Solar.Spacew.Com, or to: COler@Solar.Stanford.Edu.