I found the following data pertaining to Earth's magnetic field for the previous 7 days. You can access this data from the NOAA ACE satellite page which shows real-time data from the satellite.

enter image description here

Being entirely honest, I'm not sure what the numbers or graphs mean, but the pattern recognition part of my brain noticed the yellow's increase on the 27th corresponded to a bunch of other wild variations on the other graphs for that time period.

Could anyone explain exactly what these graphs mean, and, taking it a step further, take a guess at what happened on the 27th (if anything)?


While the answer of Michael Stachowsky and the comments under that provide the answer to what you're seeing, I think some more context can be helpful.

The plots shows are a product from the NOAA Space Weather Prediction Center, which, as the name implies, is a weather center for space weather. This is a conceptual term to describe the time-varying conditions in the solar system, in particular around the Earth. Two important components of space weather are the particles emitted by the Sun (solar wind) and the interaction of the Earth's magnetic field with the interplanetary magnetic field.

The reason people are interested in space weather is because of the impact space weather has on us: small weather events upset satellite communications, larger events can cause serious damage on Earth (see solar storm of 1859).

The Advanced Composition Explorer (ACE) satellite has been monitoring space weather since 1998. Since 2016 the Deep Space Climate Observatory (DISCOVR) is the primary provider for space weather data, although ACE is still operational and used as back-up. The DISCOVR real-time solar wind parameters can be found here.

Both are at the L1 Lagrange point and can provide us with an early warning of space weather upsets about an hour in advance.

Now to explain wat we're seeing:

  • $B_t$ is the total field strength of the interplanetary magnetic field (IMF). $B_z$ is the component perpendicular to the ecliptic plane (i.e. the component that more-or-less aligns with the Earth's north-south axis and thus its magnetic field. The Earth magnetic field has its north pole in positive $B_z$ direction, so if the IMF points in negative $B_z$ direction, the field lines will connect and allow charged solar particles to flow into the Earths magnetosphere.

Earth's magnetic field in the XZ-plane)

Earth's magnetic field in the XZ-plane with the Sun on the left. The Earth's magnetic field protects us from the solar wind (source).

Connection of IMF with Earth's magnetic field

When the IMF points south, the field can connect with Earth's magnetic field (source).

  • $\phi$ is the angle that the IMF makes in the XY plane. This is due to the rotation of the Sun which tangles up its magnetic field like a spiral (Parker spiral). In addition, every now and then (11 years or so), the Sun's magnetic north and south poles flip.

Impression of the IMF
(source: nasa.gov)

Impression of the IMF (Parker spiral) (source).

  • The last three graphs indicate the "intensity" of the solar wind: density is the number of particles (originating from the Sun, e.g. during a coronal mass ejection (CME)) per unit volume, cubic centimeter in this case. Speed, in kilometers second, is the speed of these particles and Temp is the temperature in Kelvin.

The whole interaction between IMF and the Earth's magnetosphere is pretty complicated and probably Astronomy.SE can give much better explanations than what I can give here.

Earth's magnetosphere


Popular websites like Spaceweather.com and Spaceweatherlive.com focus on the possibility of seeing aurora's ("Northern lights"), which can occur if the IMF is strong ($B_t > 30$ nT), pointing southwards to allow the connection of the field lines ($B_z < -10$ nT) and a lot of hot, fast, charged particles (high density, speed and temperature), which are conditions that were showing in the graphs on the 27th ("aurora's not guaranteed"). Often, you'll see these numbers aggregated in a single number called the $K_p$ index, which is a measure for the intensity of space weather and its impact on Earth. Often, $K_p > 5$ is a sign to watch out for aurora's.

However, as you can see on this page of NOAA SWPC, there is a lot more to worry about than pretty lights.

  • $\begingroup$ Very informative answer. $\endgroup$ Aug 29 '19 at 19:10
  • $\begingroup$ Thank you for the well referenced answer, the context was indeed extremely helpful! Especially the parts about the IMF and the diagrams explaining the interplanetary-field interactions, I didn't know about any of that, including the Parker Spiral! Makes a lot of sense though. $\endgroup$ Aug 29 '19 at 20:54

What you are looking at is the Real Time Solar Wind (hence RTSW). This shows particle density, speed, and temperature (along with the first two graphs, which I'm afraid I don't know). Because the future is awesome, we have a website called "spaceweather.com", which tells us that the increase on August 27th was due to two "solar wind streams" approaching Earth source here, which is exactly what we see - increased particle activity, including increased density, speed, and therefore temperature.

  • $\begingroup$ Awesome! So my pattern recognition wasn't too far off then, and my guesses for yellow, orange and green were correct as well! Now I just need to make sense of what the red, white and blue are. Thanks for the link to space weather. Didn't know that existed. $\endgroup$ Aug 28 '19 at 17:24
  • 2
    $\begingroup$ See more here: srl.caltech.edu/ACE/ASC/level2/mag_l2desc.html the bt and bz are magnetic field magnitudes in the t (tangential?) and z directions, with z being defined in the "gsm" coordinate system (hence bz gsm). Phi...I don't know :-P $\endgroup$ Aug 28 '19 at 17:31
  • 1
    $\begingroup$ Awesome, thanks for the supplemental readings! $\endgroup$ Aug 28 '19 at 17:32
  • 1
    $\begingroup$ neat. It looks like phi, whatever it is, comes from another instrument and not the magnetometer. It's driving me nuts, so I'm going to see if I can find it $\endgroup$ Aug 28 '19 at 17:36
  • 1
    $\begingroup$ helios.gsfc.nasa.gov/qa_sun.html#magphi $\endgroup$
    – Ludo
    Aug 28 '19 at 19:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.