Important notice about STEREO Behind

How STEREO Views the Entire Sun

Starting in February 2011, and continuing on for the next eight years, mankindnow has itsfirst ever 360 degree view of a star - our own Sun. By combining images from NASA'sSolar TerrestrialRelations Observatory(STEREO) Ahead and Behind spacecraft, together with images from NASA'sSolar DynamicObservatory(SDO) satellite, a complete map of the solar globe can be formed. Previous tothe STEREO mission, astronomers could only see the side of the Sun facingEarth, and had little knowledge of what happened to solar features after theyrotated out of view. Would active regions grow larger, and affect the spaceweather environment when they rotated back again two weeks later, or would theydecay away? What about new active regions forming on the far side of the Sun,waiting to surprise us? With STEREO's 360 degree view of the entire Sun, thatwill no longer happen.

Rotating solar globe combining images from the STEREO-Ahead, STEREO-Behind, and SDO, taken on 4 January 2011 in the Helium II emission line at 304 Angstroms. The small black wedge on the far side of the Sun was filled in starting February 2011.

ThisQuicktime movie shows the wedge closing between February and June 2011 as STEREO A and B moved further towards the farside of the Sun.

STEREO's Orbit

STEREO is able to accomplish this feat because of the unique orbits of its two spacecraft. Each spacecraft is in its own orbit about the Sun (aheliocentric orbit) with orbital parameters that differ just slightly from those of Earth. It's these slight differences that make all the difference. The STEREO-Ahead spacecraft has an orbit that is a little bit closer to the Sun than Earth, and therefore orbits a little bit faster. STEREO-Behind, on the other hand, has an orbit just slightly outside Earth's, and is thus a little bit slower. The end result is that each spacecraft seems to slowly drift in opposite directions away from Earth by about 22 degrees per year, as illustrated below.

Although, as seen from Earth, the two spacecraft seem to be going in oppositedirections, they're really going in the same direction, just at differentspeeds. ThisQuicktime movieshows the STEREO orbits as they would be seen by a hypothetical observer abovethe solar system. The green dot represents Earth, and the red "A" and blue "B"represent the STEREO "Ahead" and "Behind" spacecraft respectively. The yellowdot represents the Sun. The orbits of Mercury, Venus, and Mars are also shown.

Note that the orbits of the two STEREO spacecraft differ not only in theirorbital distances, but also by how much that distance varies over the orbit.This property is described by a parameter known as the orbitaleccentricity, denoted with the symbole. A perfectlycircular orbit would havee=0. STEREO-Ahead's orbit is very close tocircular, withe=0.006, while that of STEREO-Behind is more eccentric(e=0.042). The eccentricity of Earth's orbit falls somewhere inbetween (e=0.017).

STEREO Orbital Insertion

Getting the STEREO spacecraft into orbit around the Sun was not simple. Itinvolved using the Moon's gravity to "slingshot" the spacecraft in their properorbits. Both spacecraft were originally launched together on a single Delta IIrocket on 26 October 2006. Immediately after launch they are placed intohighly elliptical orbits that range from just a few hundred kilometers aboveEarth's surface out to a little beyond the distance of the Moon. Over the nextfew weeks the two spacecraft slowly separated from each other, and the MissionOperations carefully adjusted the orbits of each to line them up for when bothflew by the Moon a few minutes apart on 15 December 2006. The Moon's gravitygrabbed both spacecraft, and flung STEREO-Ahead completely away from Earth intoits orbit about the Sun. STEREO-Behind was also flung out, but not completely,and came back to swing by the Moon again on 21 January 2007, when it was thencompletely flung away in the opposite direction into its own orbit about theSun. These motions are demonstrated in thisQuicktime moviewhere the green and grey dots represent Earth and the Moon respectively, andthe red "A" and blue "B" represent the STEREO "Ahead" and "Behind" spacecraftas before.

How the images are combined

The first step in making a map of the solar globe is to find two STEREO imagestaken at the same time and in the same wavelength. This is fairly simple,because the observing schedules of the two spacecraft are coordinated so thatboth should be doing the same thing at the same time. There are a couple ofsmall effects, however, that need to be taken into account. Since theSTEREO-Ahead spacecraft orbits closer in, light from the Sun reachesit earlier than it reaches STEREO-Behind. This is taken into account bydelaying the STEREO-Behind images by an appropriate amount so that we are imaging the same moment on the Sun. Depending onwhere the two spacecraft are in their orbits, this delay can be anything from afew seconds up to as much as a minute. However, the amount of delay is known,and it's quite simple to match the images from one spacecraft with the other.The other effect that needs to be taken into account is that the amount ofavailable telemetry for each spacecraft varies from day to day depending on thescheduling of the ground stations. Thus, on any given day, one spacecraftmight be able to take more images than the other. However, this is taken intoaccount in the scheduling so that there's always a subset of images that arecoordinated between the spacecraft.

Next, the position on the solar surface of each pixel in the image is computed.To do this, we need to know both where each spacecraft is, and how it ispointed. The position of each STEREO spacecraft over time is carefully trackedby the NASA Flight Dynamics Facility. A number of different coordinate systemsare used for tracking spacecraft in the solar system, but the easiest todescribe is theecliptic system. The ecliptic plane is defined ascontaining Earth's orbit, and the two STEREO spacecraft also orbit close tothis plane, with only slight inclinations (0.13 degrees for STEREO-Ahead, and0.29 degrees for STEREO-Behind). Thus, we can describe the position of eachspacecraft by how far along it is in its orbit (the ecliptic longitude), howmuch it's above or below the ecliptic plane (the ecliptic latitude), andby how far away it is from the Sun.

However, what we really want to know is where each spacecraft is relative tothe Sun's own coordinate system, known asheliographic coordinates.Like other astronomical bodies, the Sun has a rotational axis with a north poleand a south pole. This rotational axis is inclined by about 7.3 degrees to theecliptic axis, so that part of the year we see the Sun's north pole tiltedtoward us, and sometimes the south pole. From the orbital position of eachspacecraft, we can calculate where it is in heliographic longitude andlatitude.

Image from STEREO-Ahead with lines of constant heliographic longitude andlatitude overplotted.

We also need to know the pointing of the spacecraft at the time of theobservation. This can be described as the position of Sun center in the image,and also by the orientation of solar axis in the image, known as the rollangle. In the above image, one can see that the solar axis is not quitestraight up-and-down. This is because the STEREO spacecraft are oriented tokeep their high gain antennas pointed toward Earth, effectively maintaining aconstant roll of about 0 in the ecliptic coordinate system. Thus, the solaraxis can be off by as much as +/- 7.2 degrees from straight up-and-down. (Thebrowse images on the STEREO website have been corrected for roll.) Like theorbital position, the orientation of the spacecraft as a function of time, bothin terms of the pointing and the roll, is also carefully tracked.

After the coordinates of the STEREO images have been determined, the images areconverted into heliographic maps such as in the examples below. Only pixelsthat are inside of the disk of the sun are used, since unique heliographiccoordinates cannot be calculated above thelimb (the term used to refer to edge of the Sun's disk). This would be fine if all theemission was coming from the surface, but it's clear from the above image thatthe Sun has an atmosphere (called thecorona) which extends well abovethe surface. Vertical features at points near the limb will tend to beprojected nearer to the limb than their actual heliographic position. In theheliographic maps this shows up as smearing near the edges of the map;this is expected.

Image from STEREO-Ahead converted into a heliographic map.Zero longitude represents the direction towards Earth.

Adding in SDO images

If we only had the STEREO images, we'd have a good view of the far side of theSun after February 2011, but would start to lose the parts of the globe on theside facing Earth. Fortunately, we can fill this part of the Sun with imagesfrom the SDO satellite. SDO observes the Sun in three of the four wavelengthsseen by STEREO: in the Helium II emission line at 304 Angstroms, representativeof plasma at about 80,000 degrees, the Iron IX line at 171 Angstroms (1.3million degrees), and the Iron XII line at 195 Angstroms (1.6 million degrees).SDO does not have a bandpass equivalent to the Iron XV line at 284 Angstroms (2million degrees) seen by STEREO, but the slightly cooler Iron XIV line at 211Angstroms is a reasonable substitute.

SDO images are not taken at exactly the same time as the STEREO images, but theSDO cadences are so high one can usually expect to find an image at the rightwavelength within a few seconds of the STEREO observation. Occasionally thisis not the case; for example there are calibration periods when suitable dataare unavailable. In such cases the nearest SDO image is found, and acorrection is made for the solar rotation. The Sun rotates about once every 25days (sidereal rate). However, because the Sun is gaseous and not asolid body, the rotation rate is not constant; it varies as a function oflatitude. This is known asdifferential rotation. Also, because thespacecraft are also orbiting about the Sun as it rotates, the apparent rotationrate is somewhat lower (synodic rate), about 27 days. When the SDOimages are corrected for rotation, both of these effects are taken intoaccount.

The same procedure applied to the STEREO images are also applied to the SDOimage, resulting in three heliographic maps which are then combined to make asingle map. Where the maps overlap, the one where the observation is closestto solar disk center is used; this minimizes the effect of smearing atthe map edges. Also, since the SDO telescopes are not identical to those onSTEREO, the brightnesses is not be exactly the same, so adjustment factorsare applied to the SDO maps to better blend in with the STEREO maps.

Heliographic map made with combined data from STEREO-Ahead, STEREO-Behind and SDO from Dec. 30, 2011.

Converting the maps back into a globe

Once a combined heliographic map is obtained, we can then reproject this backinto a three-dimensional globe viewable from any arbitrary direction, usingessentially the inverse of the processed used for making the maps in the firstplace. Of course only the solar surace is modeled, not the coronal emissionabove the limb, so it's not quite a true three-dimensional representation. Inthe rotating globe movie above, a series of 36 back projections were made,rotated by 10 degrees of longitude between each step, to give a full 360 degreeview.

Completeness

The separation between the STEREO Ahead and Behind spacecraft exceeded 180degrees on 6 February 2011. On this date the two spacecraft viewed the Sun fromcompletely opposite directions, thus viewing the entire Sun. After that date,the spacecraft started to approach each other on the far side of the Sun, and SDOdata were needed to fill in the Earth-facing side that STEREO no longersees.

Although one can say with honesty that the entire Sun was seen after 6 February2011, a small gap still persisted in the heliographic maps for several daysafter this. This effect is due to perspective. From a distance of about 150million kilometers one does not see exactly one half ofthemathematical solar surface. Instead, the solar horizon is beslightly smaller than 90 degrees away from Sun center. This is really alimitation in the way that the heliographic maps are formed, because the solaremission is not coming from a mathmatically flat surface, but from an extendedatmosphere with thickness above that theoretical surface. In the images,emission from this part of the Sun is seen above the limb, but that observedemission does not show up in the maps. The gap in the maps graduallydisappeared over the next several days, starting in the northern hemisphere, andwas be essentially gone by 12 February 2011, although some remnantpersisted in the regions near the poles.

Last Revised: Wednesday, 14-Sep-2016 15:34:08 EDT
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