Best Of SOHO / LASCO CD Vol. 1 April 1998

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Table of Contents


The SOHO satellite, a joint project of NASA and ESA (see acronyms) was launched in December 1995 to a position about 1.5 million kilometers (900,000 mi.) sunward of the Earth. SOHO orbits the Sun just like the Earth does, once per year. Gas jets are fired to keep the satellite along the Sun-Earth line. From this position, SOHO is always in sunlight which is ideal for studying the Sun.

SOHO is just one of a vast network of satellites, space probes, and ground sensors which together form the International Solar Terrestrial Physics (ISTP) program. One of the goals of ISTP is to monitor approaching interplanetary storms and to observe their possible effects on the Earth's space environment. In the past, some solar storms have affected spacecraft in orbit, short-wave communications and electric power grids. The SOHO spacecraft carries 12 experiments which can be divided into 3 categories - those that study helioseismology, the corona, or the solar wind. LASCO is one of the coronal experiments and is a collection of three coronagraphic telescopes, called C1, C2, and C3.

During a solar eclipse, the Moon moves in front of the Sun, blocking the solar disk and providing us an opportunity to see the solar corona with our eyes. However, an eclipse generally occurs only once a year. In the 1930s, Bernard Lyot invented the optical design of the coronagraph to observe the corona regularly by blocking the Sun. The three LASCO coronagraphs utilize similar techniques to observe the solar corona from 1.1 (70,000 km from the surface) out to 32 solar radii. Since the intensity of the corona becomes fainter further from the Sun, the field of view of each coronagraph is restricted to reduce the range of intensities in a single telescope.

LASCO uses CCD cameras to record the image. A CCD records an image electronically. When light strikes the silicon surface of the CCD, electronic charge is generated in an amount proportional to the amount of light falling on the CCD. One of the properties of the CCDs is that when a pixel is saturated the charge is too much for a single pixel to hold and it "bleeds" along the columns of the CCD filling up the pixels until the remaining charge finally can be held. This causes streaks that run from left to right in C2 and C3 and up and down in C1. Energetic particles such as cosmic rays pass through the instrument case and can strike the CCDs causing electrons to be generated along the track of the cosmic ray through the CCD. The track can appear as a single pixel or as a line. Since they only occur in one image at a given location, it gives the effect of a twinkling light.

The coronagraph images show the plasma around the Sun. Plasma is ionized gas that contains a magnetic field. We normally think of space as being a vacuum, but it really isn't absolutely empty. There is a solar wind blowing away from the Sun containing particles and magnetic field. By the time the solar wind reaches earth, the density, mostly ionized hydrogen atoms (protons), is only about 5 particles per cubic centimeter (about 80 per cubic inch). That is compared to about a thousand million particles per cubic centimeter at the base of the corona. The Sun has a complex magnetic field close to the surface, but further from the Sun, the dipole component is the only one that persists. The Earth's magnetic field and the field of a simple bar magnet are examples of dipole magnetic fields.

As the wind escapes from the Sun, the particles follow the field lines which curve toward the equator until about 2.5 solar radii where the field lines are almost radial and the flow is almost radial. LASCO measures the density of the plasma summed up along the line of sight. So where the intensity is higher, the plasma density is higher. We call the regions of higher density streamers if they are in the equatorial region of the corona and plumes if they are in the polar regions. We think that these structures arise from different physical processes. Occasionally, a violent eruption from the Sun occurs and a complex mass of dense material and magnetic fields is ejected. This phenomena is called a coronal mass ejection. Huge amounts of material are involved, a million million kilograms. It isn't clear what causes them, but we know that if they are aimed at Earth, serious consequences at Earth can occur when they hit about three days later. Exactly how they propagate toward Earth and their consequences are several questions that the ISTP program is trying to investigate.

This CDROM contains a collection of some of the movies that we have made from individual images of the solar corona. In all of them the Sun is at the center of the occulting disk which is used to block the light from the solar disk itself. In some C3 movies the pylon holding the occulting disk can be seen extending from the occulter to the lower left. In some of the frames a square or rectangular block is blackened. This occurs when data is missing due to an error in receiving the data from the spacecraft. In the images there is a twinkle of bright dots or streaks. This is caused by cosmic rays, particles of very high energies, which pass through the instrument box and strike the CCDs. Stars, planets, dust particles, and comets are other objects which can be seen in some of the movies. The color tables of the movies are artificial and have been created to be highlight certain features of each movie.

LASCO/C3 CME of January 15, 1996 (3 min. between frames)

MPEG Movie 1.1MB

This movie shows the first LASCO observation of material being blown away from the Sun at supersonic speeds (800 km/s or 1.6 million miles per hour). The observations were taken with the C3 telescope on January 15, 1996. This type of solar phenomena is called a coronal mass ejection or CME. It occurs less frequently during the minimum phase of the 11 year solar activity cycle than it does at the maximum of solar activity.

On LASCO we have observed a CME about every 1-2 days during this minimum phase. The next solar maximum is expected to occur during 1999-2001. On some days we observe more than one whereas sometimes three or more days might go by without seeing a CME. These images were obtained early in the mission before the C1 and C2 telescopes were ready for use. Consequently, more telemetry time was available for the C3 coronagraph, which obtained images at the rate of about one every three minutes. This very high temporal resolution caused the CME to have a smooth, almost smoke-like appearance as it drifted outward from the Sun. The normal rate for C3 images is one every 60 or 90 minutes.

LASCO/C3 May 1996 - April 1998

MPEG Movie 100MB

Unlike movie 1, this movie was made with a low temporal resolution (only 4-6 frames per day), but extends for the much longer period of 2 years. Not only is the gradual evolution of the Sun's corona seen, including its 27-day rotation, but also several planets are visible moving either left-to-right or right-to-left across the field. Around Christmas of each year, the Milky Way drifts across the field. As always, a myriad of stars are visible moving left-to-right across the sky as the Sun moves through the consellations of the Zodiac. Sometimes recognizable star clusters (such as the Pleiades) and nebulae can also be seen. The planets saturate the CCD pixels causing the bright streaks to the left and right of the planet. This gives the planet a Saturn-like ring, but that is just an instrumental artifact. Another artifact occurs when the planets pass behind the pylon holding the occulting disk. Objects in the area around the pylon experience severe geometric distortion depending upon the distance of the object from the pylon. Another artifact that can be seen sometimes is due to sunlight being scattered by a single small dust particle located near to the spacecraft. This causes a wide track across the image. The width of the track depends upon how far the particle is from the instrument. If the particle is close, it is more out of focus and so the width is wide. The further the particle is from the instrument, the more in focus it is, and the width is smaller.

LASCO/C2 and LASCO/C3 Halo CME of April 7, 1997

MPEG Movie 0.6MB

This movie shows two CMEs, one that surrounds the occulting disk and one that is heading to the southeast (lower left). The one that surrounds the disk is called a halo, but we interpret it as seeing a CME head-on, that is, coming straight toward us. It would have a similar appearance if it were going away from us, but we know from other observations that it originated on the visible side of the Sun, and thus was coming toward us.

Notice the curved structures which are like the magnetic loops in the southeast CME seen from a very different perspective. This ejection of material impacted the Earth's environment a few days later, causing a major geomagnetic storm. Another CME can be seen traveling to the southeast. We believe that the two CMEs have similar origins, but the exact details of how they are connected are still not known. The images from C2 and C3 are combined by selecting the images closest in time and inserting the C2 image into the C3 image.

LASCO/C2 and LASCO/C3 proton "snowstorm" of November 6, 1997

MPEG Movie 4.8MB

This movie shows a huge CME traveling almost 1700 km/s (3.6 million miles per hour!) away from the west (right) limb of the Sun. The images from C2 and C3 are combined to show the full extent of the CME from 2 solar radii to 15 solar radii. Since the images cannot be taken at exactly the same time, we have chosen the two images closest in time to be combined. The mass ejection is visible for a few frames as it heads westward (to the right) in profile against the sky. About an hour later, energetic protons arrived at the SOHO spacecraft causing a "snowstorm" of interference that lasted the rest of the day. Such protons also affect Earth and its environment, where they penetrate deep into the polar atmosphere and cause short-wave radio fadeouts and other noticeable effects.

LASCO/C2 one month prior to the eclipse

MPEG Movie 3.2MB

This movie consists of images spaced about 1 hour apart for 30 days prior to the February 26, 1998 solar eclipse. It shows the bright streamer belt in varying perspectives as the Sun rotates on its axis. As seen from Earth, this period is 27 days, which is approximately the duration of the movie. The eclipse allowed us on the ground to observe a snapshot of the corona for just a few minutes.

LASCO/C3 Comet Hyakutake May 1996

MPEG Movie 1.2MB

This sequence monitors the continual outflow of material in the Sun's streamers as Comet Hyakutake passes through the field of view. This comet's closest approach to the Sun (perihelion) occurred when it was at the top of the field. The perihelion distance was 49 solar radii, which is within the orbit of the planet Mercury (65 solar radii), but seems even closer in projection against the sky. During its passage through the coronagraph's field of view, the comet's blue gas tail changes orientation, remaining more or less aligned with the radial outflow of the solar wind. Two other tails, including the dust tail, are also visible. The tail of the Comet appears to be much smaller than it was when the comet could be seen from Earth before and after perihelion. This is due to the tremendous foreshortening of the comet and its tail at perihelion.

LASCO/C3 "Christmas" movie

MPEG Movie 12.5MB

This movie shows the background sky around the Sun for December 22-28, 1996. As the SOHO spacecraft moves in step with Earth around the Sun, the Milky Way appears to drift across the field from left to right. Its dark rifts are also visible. On December 24, a sungrazing comet can be seen to approach close to the Sun and disappear behind the occulting disk at 4 solar radii. More than 40 such "sungrazers" have been seen so far by the LASCO telescope, but none of them reappeared later. Occasionally, a bright member of this group of comets can be seen from the ground (even in daylight when it is close to the Sun), as Comet Ikeya-Seki was in October 1965.

The Kreutz group of comets apparently came from the breakup of a single comet many thousands of years ago during its passage very close to the Sun. When the orbits of these comets are calculated, they appear to all lie on the same (or very similar) plane in space. We see them coming from different directions (always from the South), but that is simply the relative perspective of the comet's plane to the position of the Earth around the Sun. Because of its high sensitivity, LASCO has discovered more sungrazing comets than all previous observations of this type of comet. This is because the comets are not bright enough to be seen when they are far from the Sun. When they get close to the Sun, then an observer from the ground cannot see them because the Sun is too bright.

LASCO/C2 "Christmas" movie

MPEG Movie 1.2MB

This movie is similar to the C3 movie described above, except that its field of view lies closer to the Sun at 2-6 solar radii. The comet seems to fade as it nears the Sun, as if it were losing its dusty "skywriting" material. This may be why we seldom see the tail after perihelion. The much brighter sungrazers occasionally seen from the ground are traceable as they recede from the Sun.

LASCO/C3 Comet SOHO-46

MPEG Movie 0.7MB

This movie shows another comet discovered by LASCO. This comet, seen for two days in April of 1998 is not a member of the Kreutz group. Instead of disappearing into the Sun, this comet grows a tail as it sweeps around the South pole of the Sun from left to right. The comet then fades considerably but it can be seen after perihelion as a faint dot exiting the field of view to the north.

LASCO/C1 June - July 1996

MPEG Movie 12.4MB

This movie shows the evolution of the corona from 1.1 to about 2 solar radii for two months from May 15 to July 19, 1996. The movie is a composite of LASCO/C1 images and maps of the photospheric magnetic field observed from the Wilcox Solar Observatory of Stanford University. The C1 images were taken in the green coronal emission line of 13 times ionized iron, (written as Fe XIV) at a wavelength of 530.9 nm. This emission line is formed at a temperature of about 1.6 million degrees Kelvin. Inset into the position of the occulter is a map of the Sun's magnetic field. The 27-day solar rotation is easily visible, both in the magnetic map and in the coronal structures. A bright structure at one limb can be seen about 14 days later on the other limb. At times very sudden eruptions propagate outward and may be seen later in the C2 and C3 coronagraphs. At this minimum phase in the solar activity cycle, the green line emission is concentrated at lower latitudes and doesn't appear above the North and South poles of the Sun where the coronal holes are. Coronal holes are regions of reduced emission, reduced density, and increased speed. Occasional flashes of emission above the poles can be seen. These are due to very small deflections (about 1 millimeter) in the box that holds the LASCO optics.


Further Information

Address of Solar Physics Branch at NRL