STEREO observed stealth CME
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== Introduction == | == Introduction == | ||
| - | [http://en.wikipedia.org/wiki/Coronal_Mass_Ejection Coronal mass ejections (CMEs)] are discovered in the early seventies, long after flares and prominences had been observed. CMEs are much harder to observe than flares because of several reasons. The classical way of observing a CME is through a white light [http://en.wikipedia.org/wiki/Coronagraph coronagraph]. This instrument shows the faint corona in visible light, by mimicking an eclipse. In white light CMEs are seen through the process of [http://en.wikipedia.org/wiki/Thomson-scattered Thomson scattering]. Because the Thomson scattering's efficiency changes with direction, the same CME can | + | [http://en.wikipedia.org/wiki/Coronal_Mass_Ejection Coronal mass ejections (CMEs)] are discovered in the early seventies, long after flares and prominences had been observed. CMEs are much harder to observe than flares because of several reasons. The classical way of observing a CME is through a white light [http://en.wikipedia.org/wiki/Coronagraph coronagraph]. This instrument shows the faint corona in visible light, by mimicking an eclipse. In white light CMEs are seen through the process of [http://en.wikipedia.org/wiki/Thomson-scattered Thomson scattering]. Because the Thomson scattering's efficiency changes with direction, the same CME can appear differently from different viewpoints. This makes it hard to observe CMEs, especially when they are seen face-on. Luckily, the sun lets loose some other indicators that act as "smoking gun": flares, filament eruptions, dimmings, waves, etc. This solar activity can be used as warning sign that a CME is on its way. But, it's not as easy as it seems. It's possible to observe flares without CMEs and CMEs without flares. The same is true for the other activity indicators. So, how do you know when to raise alarm for [http://en.wikipedia.org/wiki/Space_weather space weather] and when not? |
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== STEREO observation == | == STEREO observation == | ||
[[Image:Where_is_STEREO-1Jun2008.png|300px|thumb|right|'''Figure 2''': Schematic view of the CME that erupted on June 1st 2008, projected on the ecliptic plane (top view). The separation angle between the A and B spacecraft was <math>53^{\circ}</math>. The CME traveled towards STEREO B and was clearly visible in coronagraph images from STEREO A.]] | [[Image:Where_is_STEREO-1Jun2008.png|300px|thumb|right|'''Figure 2''': Schematic view of the CME that erupted on June 1st 2008, projected on the ecliptic plane (top view). The separation angle between the A and B spacecraft was <math>53^{\circ}</math>. The CME traveled towards STEREO B and was clearly visible in coronagraph images from STEREO A.]] | ||
| - | The twin [http://stereo.gsfc.nasa.gov/ STEREO] probes are two identical spacecraft, one flying ahead (A) of the Earth in its orbit and one flying behind it (B). They were launched on Wednesday, October 25th, 2006 from Cape Canaveral, Florida. The separation angle between the two spacecraft increases as time progresses. This double observation allows us to view CMEs and other solar activity from two different vantage points. On June 1st 2008, the date of the CME shown in Fig 1, the angle between STEREO A and STEREO B reached <math>53^{\circ}</math>. Because the corona is an optically thin medium, it is hard usually to determine the true direction in which a CME travels. But since we have a double STEREO view on this event, we can better estimate the CME direction. In Fig 2 we show a top view of the CME with respect to the sun | + | The twin [http://stereo.gsfc.nasa.gov/ STEREO] probes are two identical spacecraft, one flying ahead (A) of the Earth in its orbit and one flying behind it (B). They were launched on Wednesday, October 25th, 2006 from Cape Canaveral, Florida. The separation angle between the two spacecraft increases as time progresses. This double observation allows us to view CMEs and other solar activity from two different vantage points. On June 1st 2008, the date of the CME shown in Fig 1, the angle between STEREO A and STEREO B reached <math>53^{\circ}</math>. Because the corona is an optically thin medium, it is hard usually to determine the true direction in which a CME travels. But since we have a double STEREO view on this event, we can better estimate the CME direction. In Fig 2 we show a top view of the CME with respect to the sun and the A and B spacecraft. As can be seen, the CME travels in the direction of STEREO B. |
==The 'quiet' sun == | ==The 'quiet' sun == | ||
| - | As we went through the STEREO B images during May 31st, 2008 - June 2nd, 2008, we hardly found any trace of this event. This was very surprising, since STEREO B had full view on the eruption. Only small scale activity | + | As we went through the STEREO B images during May 31st, 2008 - June 2nd, 2008, we hardly found any trace of this event. This was very surprising, since STEREO B had full view on the eruption. Only small scale activity could be observed in the EUVI images from STEREO B. This activity can be found anywhere in the quiet sun at all times, and is thus not CME-specific. In coronagraphic images from COR2B only a very very faint halo CME was observed. Without the STEREO A images, we definitely would have missed this CME! This observation confirms earlier suspicions that CMEs exist that have no clear surface signatures. The question is whether instruments can be developed that can image this type of CME. |
[[Image:STEREO-B_171.jpg|300px|thumb|center|'''Figure 3''': There was nothing on the Sun's surface that warned us that a CME was about to erupt.]] | [[Image:STEREO-B_171.jpg|300px|thumb|center|'''Figure 3''': There was nothing on the Sun's surface that warned us that a CME was about to erupt.]] | ||
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== Problem storms and stealth CMEs== | == Problem storms and stealth CMEs== | ||
| - | + | CMEs that travel towards Earth can cause [http://en.wikipedia.org/wiki/Geomagnetic_storm geomagnetic storms]. In the past decade, a number of geomagnetic storms have been captured at Earth for which no clear solar source could be found. Up till now these "problem storms", as they were called, were quite a mystery. This STEREO observation proves that there exist indeed "stealth" CMEs - eruptions that are invisible when viewed face-on. The term "stealth" was chosen by a reporter who wrote an article about this event in the [http://www.newscientist.com/article/dn17148-stealth-storm-erupts-from-the-sun.html NewScientist]. It refers to [http://en.wikipedia.org/wiki/Stealth_technology stealth technology], used by the military to make aircraft, ships etc. less visible to radar, infrared and other detection methods. A similar CME but with southward Bz could have caused a mild geomagnetic storm at Earth, an unpredictable one. | |
Revision as of 17:25, 27 May 2009
Contents |
Introduction
Coronal mass ejections (CMEs) are discovered in the early seventies, long after flares and prominences had been observed. CMEs are much harder to observe than flares because of several reasons. The classical way of observing a CME is through a white light coronagraph. This instrument shows the faint corona in visible light, by mimicking an eclipse. In white light CMEs are seen through the process of Thomson scattering. Because the Thomson scattering's efficiency changes with direction, the same CME can appear differently from different viewpoints. This makes it hard to observe CMEs, especially when they are seen face-on. Luckily, the sun lets loose some other indicators that act as "smoking gun": flares, filament eruptions, dimmings, waves, etc. This solar activity can be used as warning sign that a CME is on its way. But, it's not as easy as it seems. It's possible to observe flares without CMEs and CMEs without flares. The same is true for the other activity indicators. So, how do you know when to raise alarm for space weather and when not?
STEREO observation
. The CME traveled towards STEREO B and was clearly visible in coronagraph images from STEREO A.The twin STEREO probes are two identical spacecraft, one flying ahead (A) of the Earth in its orbit and one flying behind it (B). They were launched on Wednesday, October 25th, 2006 from Cape Canaveral, Florida. The separation angle between the two spacecraft increases as time progresses. This double observation allows us to view CMEs and other solar activity from two different vantage points. On June 1st 2008, the date of the CME shown in Fig 1, the angle between STEREO A and STEREO B reached . Because the corona is an optically thin medium, it is hard usually to determine the true direction in which a CME travels. But since we have a double STEREO view on this event, we can better estimate the CME direction. In Fig 2 we show a top view of the CME with respect to the sun and the A and B spacecraft. As can be seen, the CME travels in the direction of STEREO B.
The 'quiet' sun
As we went through the STEREO B images during May 31st, 2008 - June 2nd, 2008, we hardly found any trace of this event. This was very surprising, since STEREO B had full view on the eruption. Only small scale activity could be observed in the EUVI images from STEREO B. This activity can be found anywhere in the quiet sun at all times, and is thus not CME-specific. In coronagraphic images from COR2B only a very very faint halo CME was observed. Without the STEREO A images, we definitely would have missed this CME! This observation confirms earlier suspicions that CMEs exist that have no clear surface signatures. The question is whether instruments can be developed that can image this type of CME.
Problem storms and stealth CMEs
CMEs that travel towards Earth can cause geomagnetic storms. In the past decade, a number of geomagnetic storms have been captured at Earth for which no clear solar source could be found. Up till now these "problem storms", as they were called, were quite a mystery. This STEREO observation proves that there exist indeed "stealth" CMEs - eruptions that are invisible when viewed face-on. The term "stealth" was chosen by a reporter who wrote an article about this event in the NewScientist. It refers to stealth technology, used by the military to make aircraft, ships etc. less visible to radar, infrared and other detection methods. A similar CME but with southward Bz could have caused a mild geomagnetic storm at Earth, an unpredictable one.
