STEREO observed stealth CME
From RHESSI Wiki
(gentle editing, part I) |
|||
Line 11: | Line 11: | ||
== Introduction == | == Introduction == | ||
- | [http://en.wikipedia.org/wiki/Coronal_Mass_Ejection Coronal mass ejections (CMEs)] were discovered in the early seventies, long after flares and prominences had been observed. CMEs are much harder to observe than flares | + | [http://en.wikipedia.org/wiki/Coronal_Mass_Ejection 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 [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 mainly through the [http://en.wikipedia.org/wiki/Thomson-scattered 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 [http://spiff.rit.edu/classes/phys440/lectures/optd/optd.html optically thin] also causes confusion. | ||
+ | These complications make it hard to observe a CME unambiguously, especially when seen face-on. | ||
+ | The face-on CMEs ([http://www.spaceweather.com/glossary/halocmes.html halo CMEs]) are the most important ones, of course, because they may be headed directly at Earth. | ||
+ | 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 an alarm for [http://en.wikipedia.org/wiki/Space_weather space weather] purposes, and when not? | ||
Revision as of 14:37, 30 May 2009
Nugget | |
---|---|
Number: | 103 |
1st Author: | Eva Robbrecht |
2nd Author: | |
Published: | 8 Jun 2009 |
Next Nugget: | |
Previous Nugget: | Hard X-ray Pulsations in Flares |
List all |
Contents |
Introduction
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 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 an alarm for space weather purposes, and when not?
STEREO observation
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 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 activity could be observed in the EUVI images from STEREO B (Fig 3). 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. Further investigation of other data showed that there was no Halpha 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.
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.
References
Robbrecht, E., Patsourakos, S. and Vourlidas, A. 2009, No Trace Left Behind: STEREO Observation of a CME without low Coronal Signatures, Astrophysical Journal 701 (in press)
RHESSI Nugget Date | 8 June 2009 + |
RHESSI Nugget First Author | Eva Robbrecht + |
RHESSI Nugget Index | 103 + |