How do we know if a CME is Earth-directed and when it's going to arrive?

When a solar flare or filament eruption occurs then we do not know immediately if the event launched a coronal mass ejection or CME for short. Not every filament eruption reaches escape velocity and not every solar flare is eruptive. So what do we look our for to determine if there was a coronal mass ejection associated with an event? How do we know if an eruption is aimed at Earth and how do we determine when it might arrive?

1. Using solar imagery

The very first tool we have available to us are of course the near real-time imagery of the Sun taking by satellites like NASA's Solar Dynamics Observatory and GOES-16. These satellites regularly take detailed images of our Sun in different wavelengths and are often available to us just minutes after they were taken. With the help of all of the different wavelength available to us we can pinpoint the location of the solar flare or filament eruption and take a first educated guess if there is a chance of an Earth-directed eruption. There are two things to look out for here. First of the location where the solar event took place: an eruptive event near the center of the solar disk is much more likely to impact our planet than an eruption near the solar limbs. Secondly we look at how the eruption evolved over time. For filament eruptions: does it look large and fast enough that it flung material into space or did it collapse? For solar flares difference imagery can be a great tool: did the solar flare disturb the corona? Can we see a coronal wave, coronal dimming? Studying incoming real-time solar imagery is a fast way to make a first educated guess to determine if an eruption launched a coronal mass ejection and to determine its trajectory but to confirm our suspicions we need more tools.

Solar radiation stormSolar radiation storm

Left image: An example of a strong eruptive solar flare at the west limb as seen by GOES-16 in extreme ultraviolet. Note the major disturbance of the solar corona, a great sign that solar plasma might have been launched into space.

Right image: An example of a strong eruptive solar flare near the center of the Earth-facing solar disk as seen by SDO using difference imagery. Difference images are created by subtracting one image from the foregoing picture. This shows what has changed from one frame to the other and are commonly used when analyzing solar events. Note the shock wave traveling trough the Sun's corona and the darkening of the solar corona where the eruption occurred. This phenomenon is called coronal dimming. It is caused by the ejection of plasma that forms part of a coronal mass ejection.

2. Solar radio emissions

Energetic solar events often produce characteristic radio emissions. These emissions are generated by solar material plunging through the solar corona and are another great sign that an eruptive event might have taken place on the Sun. These radio emissions are often referred to as radio bursts or sweeps. What you have to understand is that the Sun actually gets very noisy during a solar eruption at radio frequencies ranging from a few MHz to a few GHZ. This radio noise gives us some vital clues and are along with the solar imagery discussed earlier a tool that we can use to decide if a solar event was eruptive or not. There are many different types of radio bursts but for our purposes we are keeping an eye out for Type II and Type IV radio bursts as these bursts are caused by solar material forming a shock wave while propagating outward in the corona into interplanetary space. The NOAA SWPC provides alerts for these kind of radio bursts which we then display on our website and trough our alert system. Do note that significant uncertainties exist in determining exact coronal mass ejection velocities with this technique so any stated, observed velocities should be confirmed with coronagraph imagery. We will talk about coronagraph imagery later in this article. While strong solar eruptions are almost always accompanied by Type II and/or Type IV radio bursts, this is not always the case with weaker, more slower coronal mass ejections. Slower coronal mass ejections do not always drive shock waves and will thus not cause any radio noise.

3. Using the ≥10 MeV solar proton flux monitor

On our website you'll find a data plot showing the amount of ≥10 MeV solar protons measured near Earth. During huge explosions on the Sun a solar radiation storm can be triggered. Solar protons get expelled into interplanetary space space and can travel at speeds near the speed of light in extreme events. They are the first particles to arrive at Earth so a solar radiation storm can develop very quickly following a major event on the Sun. These solar protons are another solid indicator that there was an eruptive solar event that likely launched a coronal mass ejection into space. While we can be pretty confident at this stage that a coronal mass ejection was launched into space, this is not the ideal way to determine if the coronal mass ejection is Earth-directed as these particles tend to follow the interplanetary magnetic field: the Parker spiral. It has happened that we experienced a solar radiation storm at Earth even though the associated coronal mass ejection did not have an Earth-directed component. This often occurs during events near the western limb but in rare cases, even protons from major far side solar events have reached our planet. However, like with solar radio emissions: not every solar eruption causes solar protons to be launched into space. Usually only the more stronger eruptions cause solar radiation storms.

Solar radiation storm

Image: An example of the ≥10 MeV solar protons plot that you can find on our website. This plot shows the ≥10 MeV solar proton flux near Earth on 14 July 2000 when A major X5.7 solar flare took place. We see a sharp increase in the amount of ≥10 MeV solar protons starting at 10:35 UTC, only 32 minutes after the solar flare started. A strong S3 solar radiation storm quickly developed.

4. Using coronagraph imagery

The best way to confirm the existence, the speed and the trajectory of a coronal mass ejection is with the help of coronagraph imagery. A coronagraph creates an artificial solar eclipse by covering the Sun with a small disk in front of the camera. Coronal mass ejections are very faint and can not be observed otherwise. The SOHO (Solar and Heliospheric Observatory) and STEREO (Solar TErrestrial RElations Observatory) missions have white-light coronagraphs on board to detect coronal mass ejections. Coronagraph images are by far the most important tool in our toolbox but not its a tool that is not always available in near real-time. Sometimes it could take a couple of hours before these images are available.

The images taken by the LASCO C2 and C3 coronagraph instruments which are located on the SOHO spacecraft are the most important images at our disposal. SOHO is in an orbit around the Sun-Earth L1 point and watches the Sun from Earth's perspective so potential Earth-directed coronal mass ejections can easily be identified. Coronal mass ejections that are directly aimed at Earth will show up as a full halo coronal mass ejections as they propagate away from the Sun. What we mean with a full halo is that the coronal mass ejection will engulf the entire image as the cloud propagates out into interplanetary space. Sometimes you will also come across the terms "symmetrical full halo" and "asymmetrical full halo". A symmetrical full halo coronal mass ejection means that the ejecta shows up over an equal amount of the coronagraph's 360 degrees field of view. If the eruption comes from the front side we can conclude we are getting a head on impact. An asymmetrical full halo coronal mass ejection still covers the entire field of view of the coronagraph but it could be that the bulk of the coronal mass ejection is aimed away from Earth. An impact is still very likely but we might not be hit the most dense portion of the plasma cloud.

Coronal mass ejections that are not perfectly aimed straight towards Earth but can still hit us are often identified as partial halo coronal mass ejections. In contrast to full halo coronal mass ejections, partial halo coronal mass ejections can for example cover 180 degrees of the coronagraph's field of view which means the plasma cloud will likely still hit us but it might perhaps be more of a glancing blow and not a head-on collision. Coronal mass ejections that cover for example only 90 degrees of SOHO's field of view are likely aimed away from Earth.

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.

A CME as seen by SOHO/LASCOFull halo CME

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 far side 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 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. We won't go into great detail how to estimate a coronal mass ejection's speed but there are some tools available that can be used to estimate the speed of a coronal mass ejection like the SECCHI Time-Elongation Plots, the Solar Eruptive Event Detection System and the Computer Aided CME Tracking (CACtus) tool. After the space weather experts are finished with their reports you can look on our site, 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.

However, if you want to give it a go yourself continue reading here! Once we know the speed of the coronal mass ejection, we can try to determine when the plasma cloud might arrive at Earth. 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! The speed and density of the ambient solar wind between the Sun and our planet is a key factor to take into account when determining how much a coronal mass ejection will slow down as it travels from the Sun to Earth. This is however a difficult factor to accurately predict as we only have the satellites at the Sun-Earth L1 point to provide us with accurate information about the solar wind conditions. We have no idea what the solar wind is like between the Sun and the relatively close-by Earth, Sun-Earth L1 point which makes it very difficult to accurately predict an arrival time.

CME speed (km/s) Travel time (hours) Days Hours
300 138,88 5 18.88
400 104,16 4 8,16
500 83.33 3 11.33
600 69.44 2 21.44
700 59.52 2 11.52
800 52.08 2 4.08
900 46.30 1 22.30
1000 41.67 1 17.67
1100 37.88 1 13.88
1200 34.72 1 10.72
1300 32.05 1 8.05
1400 29.76 1 5.76
1500 27.78 1 3.78
1600 26.04 1 2.04
1700 24.51 1 0.51
1800 23.15 0 23.15
1900 21.93 0 21.93
2000 20.83 0 20.83
2100 19.84 0 19.84
2200 18.94 0 18.94

5. Using the EPAM Plot

We have now covered the most important tools we need to know about but there is one more tool we should take a look at: the EPAM plot. 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. Its a very useful instrument which we can use to somewhat track a strong coronal mass ejection from the Sun to Earth.

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 eruption. The low-energy proton plot will also show a steady rise. This often indicates that at least a part of the coronal mass ejection is Earth-directed. The EPAM plot below shows how an EPAM plot could look like just a few hours after a strong eruptive solar flare. Note the sudden onset at around 7:30.

EPAM plot just after a solar flare

Once a coronal mass ejection has been launched and we determined that it is aimed at our planet, there is only one thing we can do before the plasma cloud arrives at the satellites at the Sun-Earth L1 point and that is to watch the EPAM plot. Most of the more stronger coronal mass ejections drive a shock wave 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 which we looked at earlier in this article) is called the "onset" phase. The plot keeps rising slowly (ramp up phase) as the coronal mass ejection gets closer. Usually a few hours before the actual arrival of the coronal mass ejection a new sharper rise takes place. This indicates that the coronal mass ejection is going to arrive soon. When the coronal mass ejection arrives at the ACE satellite, the plot peaks. 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 its 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!

EPAM plot

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