STEREO observed stealth CME

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Number: 103
1st Author: Eva Robbrecht
2nd Author:
Published: 8 Jun 2009
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Coronal mass ejections (CMEs) were discovered in the early seventies, long after flares and prominences had been first observed. The CMEs are much harder to observe than flares for 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 mainly through the Thomson scattering of sunlight by the electrons in the CME. Because the efficiency of Thomson scattering changes with the angle of scattering, the same CME can look different when seen from a different viewpoint. The fact that the corona is optically thin also causes confusion. These complications make it hard to observe a CME unambiguously, especially when seen face-on. The face-on CMEs (halo CMEs) are the most important ones, of course, because they may be headed directly at Earth. Luckily, the sun has other indicators that can act as "smoking guns": flares, filament eruptions, dimmings, waves, etc. These forms of solar activity can be used as warning signs 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, even though they usually have an intimate relationship. The same is true for the other activity indicators. So, how do you know when to raise an alarm for space weather purposes, and when not to?

Figure 1: Composite view from STEREO-A of a CME that erupted on June 1st 2008. From right to left, an EUV image and the two STEREO coronagraphs. The colors are false but help to distinguish the different instruments used.

STEREO observation

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 53^{\circ}. 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 Figure 1, the angle between STEREO A and STEREO B reached 53^{\circ}. 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 observe the CME motions with much less ambiguity. In Figure 2 we show a top view of the CME relative to the Sun and the STEREO 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 solar activity could be observed in the EUVI images from STEREO B (Figure 3). This kind of 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. Further investigation of other data showed that there was no Hα filament present. 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 could image this type of CME. Since this CME developed very gradually, this is the sort of event that RHESSI wouldn't have any chance at all of seeing, most likely simply as a matter of sensitivity.

Figure 3: There was nothing on the Sun's surface that warned us that a CME was about to erupt.

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. The early geophysicist Bartels called their sources "M-regions", where the "M" stood for "magnetic" but otherwise didn't offer any physical insight. 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. But this observation raises an issue: how can a CME, which manifestly reconfigures the corona magnetic field, not have perceptible consequences where the field is anchored to the Sun? This may be one of the more important problems in our physical understanding of how the Sun works.


[1] No Trace Left Behind: STEREO Observation of a CME without low Coronal Signatures

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