STEREO observed stealth CME

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Introduction

The relationship of coronal mass ejections (CMEs) to other forms of solar activity like flares and dimmings have been studied since CMEs are known. 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, CMEs appear differently from different viewpoints. This makes it hard to observe CMEs, because for this reason, some CMEs show up bright whereas others are very faint. The CME shown in Fig 1 shows a CME as seen from the 'first row'.

Before CMEs were known, it was believed that flares and prominence eruptions were the direct cause of geomagnetic storms. However, for several storms, no warning was given from the Sun. While recurrent geomagnetic storms are due to the passage of high speed solar wind coming from coronal holes, non-recurrent storms must be linked to a disturbance at the Sun.

Figure 1: Composite view from STEREO-A of the CME. From right to left, the instruments used are: EUVI 171, COR1 and COR2



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 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 solar activity from two different vantage points. On June 1st 2008, the angle between STEREO A and STEREO B reached 53^{\circ}. On this day, a coronal mass ejection (CME) was observed by the STEREO A coronagraphs (Fig 1). 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 ant the A and B spacecraft. As can be seen, the CME travels in the direction of STEREO B.

The 'quiet' sun

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

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 can be observed in the EUVI images from STEREO B. This is activity that can be found anywhere in the quiet sun. The quiet sun consists of areas on the sun outside active regions and coronal holes, as can be seen in Fig 3. coronagraphic images from COR2B a very faint halo CME was observed.

This observation confirms earlier suspicions that CMEs exist that have no clear surface signatures.


Problem storms and stealth CMEs

For the majority of CMEs, especially fast ones, it is generally easy to identify a number of associated low coronal signatures. But there exist several examples of halo CMEs with no such associations. In the past decade indeed, a number of mild 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.

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