Woher kennen wir die Richtung eines CME und wissen, wann er an der Erde ankommt?

Wenn eine Sonneneruption oder ein Filamentausbruch auftritt, wissen wir nicht sofort, ob sie einen koronalen Massenauswurf (CME) ausgelöst hat. Selbst wenn ein koronaler Massenauswurf ins All ausbricht, stellt das keine Garantie dar, dass er in Richtung Erde unterwegs ist. In diesem Artikel zeigen wir Ihnen Methoden, mit denen Sie feststellen können, ob ein koronaler Massenauswurf von der Sonne ausgegangen ist, ob er auf die Erde gerichtet ist und wann er eintreffen könnte.

1. Verwendung des Protonenflussmonitors

Auf unserer Website finden Sie eine Wertedarstellung, der die Menge von ≥10 MeV Sonnenprotonen zeigt, die in der Nähe der Erde gemessen wurden. Bei gewaltigen Explosionen auf der Sonne kann ein Strahlungssturm ausgelöst werden. Sonnenprotonen werden in den Weltraum geblasen und können sich bei Extremereignissen mit Geschwindigkeiten nahe der Lichtgeschwindigkeit fortbewegen. Sie sind die ersten Teilchen, die auf der Erde ankommen, sodass sich nach einem großen Ereignis auf der Sonne sehr schnell ein Sonnenstrahlungssturm entwickeln kann. Diese Sonnenprotonen sind ein guter Indikator dafür, dass es ein eruptives Sonnenereignis gab, das wahrscheinlich einen koronalen Massenauswurf in den Weltraum ausgelöst hat. Obwohl wir ziemlich sicher sein können, dass ein koronaler Massenauswurf in den Weltraum geschossen wurde, ist dies nicht der ideale Weg, um festzustellen, ob der koronale Massenauswurf erdgerichtet ist, da diese Partikel dazu neigen, dem interplanetaren Magnetfeld zu folgen: der Parker-Spirale. Es ist vorgekommen, dass wir einen Sonnenstrahlungssturm auf der Erde gesehen haben, obwohl der damit verbundene koronale Massenauswurf nicht auf die Erde gerichtet war. Dies tritt häufig bei Ereignissen in der Nähe des westlichen Randes auf, aber in seltenen Fällen haben sogar Protonen von großen Ereignissen auf der anderen Seite der Sonne unseren Planeten erreicht.

Bild: Ein Beispiel für ein Diagramm der ≥10 MeV-Solarprotonen, das Sie auf unserer Website finden. Dieses Diagramm zeigt den solaren Protonenfluss von ≥10 MeV in der Nähe der Erde am 14. Juli 2000, als eine große X5,7-Sonneneruption stattfand. Wir sehen einen starken Anstieg bei den ≥10 MeV Sonnenprotonen ab 10:35 UTC, nur 32 Minuten nach Beginn der Sonneneruption. Schnell entwickelte sich ein starker Sonnenstrahlungssturm S3.

2. Verwendung von SOHO/LASCO-Koronagraph-Bildern

The best way to be sure that a coronal mass ejection is Earth-directed is with the help of images taken by the LASCO C2 and C3 coronagraph instruments which are located on the SOHO (Solar and Heliospheric Observatory) spacecraft. SOHO watches the Sun from Earth's perspective so potential Earth-directed coronal mass ejections can easily be identified. Coronal mass ejections that are Earth-directed will show up as partial or full halo coronal mass ejections as they propagate away from the Sun. Keep in mind that the images from SOHO are not always in real-time and you often have to wait several hours for new imagery and thus it can often take a while before we know for sure if a coronal mass ejection has an Earth-directed component or not.

Below we have two great examples of how a coronal mass ejection could look like on the images coming from the SOHO/LASCO instrument package. The animation on the left shows a coronal mass ejection as seen by the SOHO/LASCO C2 instrument which is heading towards the north and did not impact Earth. There is no halo outline so we can easily conclude that this plasma cloud isn't aimed at Earth. On the right however we see a full halo coronal mass ejection as seen by SOHO/LASCO C2. The outline of this coronal mass ejection forms a perfect circle engulfing LASCO's entire field of view. This means one of two things: the plasma cloud is aimed straight towards or away from us.

If we are unsure if a coronal mass ejection detected in LASCO imagery comes from the Earth-facing solar disk perhaps due to no clear signs of an eruption, we can look at the images made by the STEREO (Solar TErrestrial RElations Observatory) mission. The STEREO mission consists of two spacecraft which are named STEREO Ahead & STEREO Behind. They are watching the farside of the Sun. The imagery of STEREO and SOHO combined will give us a 3D representation of the coronal mass ejection and tell us if the coronal mass ejection is coming towards Earth or traveling away from Earth. Imagery from both the SOHO and STEREO missions can be found on the website.

By using the all of the imagery available from the SOHO and STEREO missions, the space weather scientists can calculate the departure speed and set an Estimated Time of Arrival (ETA) for the coronal mass ejection. After they are finished with their reports you can look on our site and look at the solar wind models and the daily reports from the NOAA SWPC to see when the coronal mass ejection is expected to arrive. The SpaceWeatherLive team will also provide an analysis during major events.

3. Verwenden der SIDC Cactus-Software

The Belgian Solar Influences Data Analysis Center (SIDC) developed the CACTUS program which stands for Computer Aided CME Tracking. It automatically scans imagery from SOHO/LASCO to determine whether a coronal mass ejection is likely to hit Earth or not. It will show if a coronal mass ejection is a halo coronal mass ejection or partial halo coronal mass ejection and also determines the lift off speed of the coronal mass ejection which will make it possible to determine when the coronal mass ejection could arrive.

4. Verwenden des EPAM-Plots

EPAM stands for the Electron, Proton and Alpha Monitor and is an instrument on the ACE satellite that measures the electrons and protons that are send out with the solar wind. It's a very useful instrument to know if a coronal mass ejection is Earth directed and when it's going to arrive. We'll be making it a bit clear with some EPAM plots. Below you can find an EPAM plot as it can look like a few hours after a solar flare.

When a huge explosion occurs on the Sun, electrons and protons are hurled away from the Sun into space. The electrons and protons are pushed out with the solar wind flow. Immediately following an event that launched a coronal mass ejection, the EPAM plot will show a rise in the low-energy electrons which marks the start of the flare. The low-energy proton plot will also show a steady rise. This often indicates that a part of the coronal mass ejection is Earth-directed. If we put everything together and we know that a coronal mass ejection is on it's way to Earth... one critical question remains: when is the coronal mass ejection going to arrive?

Wann wird CME ankommen?

Once we know the speed of the coronal mass ejection, we can determine ourselves when it might arrive. With the following table you can determine how long the coronal mass ejection will take to travel from the Sun to Earth providing it does not slow down along the way. The times listed below are thus only a guide. It is common for coronal mass ejections to arrive earlier or later than the predicted arrival time with a margin of sometimes 6 hours or more!

Bestimmen Sie die Auswirkungen mithilfe des EPAM-Monitors

Once a coronal mass ejection has been launched and we determined it is Earth-directed, the only thing we can do is wait and watch the EPAM plot. Most coronal mass ejections form a shock ahead of the plasma cloud itself and this accelerates protons that we can measure with the help of the EPAM instrument on ACE. We will see how the proton plot keeps rising until the arrival of the coronal mass ejection. The first rise in the plot (just after a solar flare) is called the "onset" phase. It keeps rising slowly (ramp up phase) as the coronal mass ejection gets closer. A few hours before the actual arrival of the coronal mass ejection a new sharper rise takes place, this indicate that the coronal mass ejection is going to arrive soon. When the EPAM plot peaks, then it indicates that the coronal mass ejection has arrived at the DSCOVR satellite. The solar wind and interplanetary magnetic field data should now clearly show that the coronal mass ejection has arrived. After the coronal mass ejection arrival you will see that the proton levels will slowly decline to normal values... unless there is another coronal mass ejection on it's way to Earth of course. The image below shows an example of the EPAM plot where you can clearly see the different phases. Note that slow and weaker coronal mass ejections sometimes do not push a shock wave in front of them. These are much harder or impossible to pick out on EPAM!

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