Forecasting the aurora is always done in a four-step workflow. It consists in monitoring and analyzing space weather conditions (1) at the Sun, (2) in the solar wind, (3) throughout Earth’s magnetosphere and (4) in the atmosphere of the Earth.
The Sun is where everything begins. This giant powerhouse generates both the fuel (plasma) and the energy (magnetic, kinetic, heat) necessary for the creation of aurora on Earth via the solar wind and the magnetosphere. The Sun’s own magnetic field creates structures and events that come to enhance the solar wind and give more chances to see active auroras at Earth. As its magnetic field varies in complexity every 5.5 years on average, the Sun goes through a maximum of activity, then a minimum of activity. These periods dictate what kind of noteworthy structures enhance the solar wind. The very fast structures take from several hours to a day to travel to Earth while the slower ones take up to 5 days to arrive. By monitoring the solar surface and its near environment, we are able to forecast Earth-directed structures about 1 to 5 days ahead of time.
Space-based observatories like SDO, SOHO or GEOS harbor instruments (AIA, HMI, LASCO, SUVI…) capable of imaging the Earth-facing solar disc at different wavelengths of the Extreme Ultra-Violet (EUV) spectrum. Each wavelength reveals a specific layer of the Sun’s surface and atmosphere, enabling experts to detect and monitor noteworthy structures and events.
The solar wind is a stream of charged particles (plasma) expanding from the Sun into the interplanetary space. It drags along the Sun’s own magnetic field called the interplanetary magnetic field (IMF). It serves as both the fuel and energy to feed and power up the magnetosphere (next slide). The solar wind can either be at background levels or enhanced levels where the latter corresponds to plasma-rich and/or fast and/or magnetically strong structures expelled from the Sun (scroll down). The Earth-directed solar wind is monitored by two satellites located 1.5 million km in space. The solar wind speed read at these satellites determines approximately how much time it takes for the solar wind to reach the Earth’s magnetosphere.
Two satellites called SOLAR-1 and IMAP hover in front of the Earth- some 1.5 million kilometers from our planet (called the L1 point), right in the Earth-directed stream of solar wind. They probe the latter at all times and constitute a crucial tool to monitor its fluctuations. In turn, this allows one to make predictions about how the magnetosphere will react to the incoming solar wind (geomagnetic activity) and create aurora (auroral activity) as a by-product. On average, it takes about an hour for the solar wind to propagate from L1 to the magnetosphere. However, depending on the varying speed of the solar wind, it gives us a forecast ability with a notice ranging between 15 minutes (extreme speeds) to 120 minutes (very slow speeds).
The IMF Bt is a measure in nanoTesla of the total induction (also known as magnetic force or strength) contained in the interplanetary magnetic field (IMF) at the L1 point in space, some 1.5 million km ahead of Earth on the path of the arriving solar wind. Think of it as the tension stored as potential energy in the ‘elastic’ magnetic field of the Sun that stretches into space. The higher the Bt, the more potential magnetic energy is carried with the solar wind to Earth’s magnetosphere. As the solar wind IMF interacts with the Earth’s magnetic field, a higher IMF Bt tends to lead to stronger and more frequent magnetic reconnections, which in turn promotes higher geomagnetic activity and auroral activity. Below 5 nT, we generally say that the IMF is at low levels. Above 10 nT, the Bt is said to be elevated. Elevated Bt values can participate in stronger, more frequent and more equatorward auroral shows. Additionally, abrupt or long-term changes in the Bt can also be used as a powerful indicator of transition between solar wind structures. A rise in Bt often indicates the imminent arrival of an enhancing structure like a solar storm or coronal hole high speed stream.
The IMF Bz is a measure in nanoTesla of the magnitude of the IMF induction (magnetic force or strength) in the z-axis of the GSM coordinate system at the L1 point in space, some 1.5 million km ahead of Earth on the path of the arriving solar wind. In the GSM system, the z-axis is parallel to the magnetic dipole axis of the Earth, with +z pointing ‘upwards’, towards the geomagnetic north and -z pointing ‘downward’, towards the geomagnetic south. Therefore, a positive Bz means that the a portion of the total force carried in the IMF (Bt) is pointing ‘northward’ while a negative Bz means that the a portion of the Bt is pointing ‘southward’. Despite popular belief, the IMF Bz only assesses the amount of force in the z-axis and not the precise ‘orientation’ of the IMF in this axis. For example, a Bz = -7.5 nT means that the IMF carries a force of 7.5 nT in a rough ‘southward’ direction, which precise angle remains unknown. The theta angle gives this precise orientation in the y/z axes. In the modelization of the solar wind above, both the Bz, the theta angle and Phi angle are represented. The thickness of the IMF lines represents the value of the force. The color and arrows represent the exact orientation thanks to the theta (y/z) and Phi (x/y) angles. The redder and down-pointing the lines are, the more theta is negative (southward). Conversely, the bluer and up-pointing the lines are, the more theta is positive (northward). Bz and theta are crucial factors that determine how well, how strongly, how frequently and where the solar wind connects with the magnetic field of the Earth to produce geomagnetic and auroral activity.
At the frontside magnetosphere where the Earth’s magnetic field naturally encounters the IMF, the former is always oriented in a dominant ‘northward’ manner.
• A positive IMF Bz and theta should create a ‘northward-northward’ confrontation where the two fields repel each other just as magnets do with like polarities. If this situation is sustained over extended periods of time, it stalls the coupling process up front and forces a more idle magnetosphere regime in which the auroral activity tends to dim down and relocate to very high geomagnetic latitudes on Earth. A sustained and highly positive Bz is generally speaking less favorable for aurora watchers located below ~70° MLAT.
• Conversely, a negative IMF Bz and theta should generate a ‘southward-northward’ confrontation where the IMF and Earth’s field attract each other just like opposite polarities in a magnet do. This facilitates the coupling of the two fields up front by way of more or less efficient and frequent magnetic reconnections. In turn, this regime leads to higher geomagnetic and auroral activity. Sustained, strong negative Bz and theta angle periods are often associated with the strongest and most equatorward auroral activity on Earth! Therefore, negative Bz periods tend to be much more favorable for aurora watchers.
The IMF Bz is likely the most complex value to interpret and use in aurora chasing, as its effects can intervene at multiple levels in the aurora creation process. The Bz value, the Bz sign, the sustained or periodic nature of the Bz, as well as the precise theta angle- and to some significant degree the Phi angle, the Bx and By, all have an important role to play in determining the strength, frequency, location and dynamics of auroral shows. It is never recommended to automatically assume that ‘negative Bz is all good while positive Bz is all bad’. For example, not all sustained negative Bz periods favor the production of bright auroras. In the same way, short positive Bz periods typically do not inhibit bright auroras. For a more precise reading of the solar wind, we encourage you to open the full solar wind graph where you can see more details about the IMF to make more precise interpretations.
The solar wind carries electrically charged particles expelled out of the solar corona with a certain speed. Near the Sun, solar wind speeds are typically very high. Near the Earth though, we often encounter speeds in the range of 150 to 450 kilometers per second (km/s), which are considered ‘background’ or ‘slow’ speeds. The solar wind speed can increase from these background values inside enhancing structures like solar storms (CMEs) or coronal hole high speed steams (CHHSS). In the field, we commonly say that anything above 500 km/s is ‘fast’ solar wind. The high speed of solar particles is considered an amplification factor of geomagnetic and auroral activity. However, it is not a necessary factor and low speeds can still yield high aurora activity, especially at high geomagnetic latitudes and provided some other solar wind metrics are favorable. In the scientific literature, it has been demonstrated that the solar wind speed plays a significant role in modulating the rate of magnetotail substorms (bursts of brighter and more dynamic auroras on the night side of the Earth).
The solar wind carries electrically charged particles expelled from the solar corona with a certain density. The density of the near-Earth solar wind is an L1 measure of the number of protons per unit volume. It is expressed in protons per centimeter cubed (p/cm3). The solar wind carries many different species of charged particles like protons, electrons and ions. Protons have a big mass compared to electrons and represent a big part of these entities, hence their assessment. The density of the solar wind depends on the nature of the structure that sweeps through the L1 measuring satellites. For example, coronal streamers feature a low speed and a proton density that is actually mildly elevated (5 to 10 p/cm3). Conversely, events like coronal hole high speed streams tend to have an elevated speed but a lower density, often below 5 p/cm3. Extreme compression events like CIRs, CME sheaths or closed structures like CME flux ropes often contain elevated density levels above 10 p/cm3. Like the solar wind speed, the density is an amplifying factor of the geomagnetic and auroral activity but not a necessary one. Despite popular belief, most of the visible aurora created on Earth comes from the precipitation of energetic electrons coming from the magnetosphere, not protons from the solar wind. Protons in the solar wind have a huge mass compared to electrons, and so they will participate in the amount of dynamic pressure applied to the magnetosphere, making the aurora move in latitude. Naturally, the solar wind also carries electrons that can produce visible aurora but it is often not a very direct process, especially for nightside auroras. A non-negligible part of the aurora-producing electrons do not originate from the solar wind itself, and the ones that do often do not produce nightside auroras directly. In other words, higher solar wind densities enable more fuel for the aurora in the long haul but are not a requirement to create very active and vivid auroras. Just like the Bt, the solar wind density can serve as a powerful indicator of the arrival of an enhancing event in the solar wind like a CHHSS or a CME. It’s always good to keep an eye out of abrupt or slow increases in the density past the 10 p/cm3 line.
The solar wind carries electrically charged particles expelled from the Sun with a certain density and speed. This combined effect exerts a force on the magnetosphere called dynamic pressure, measured in nanoPascals. The solar wind dynamic pressure bears major consequences on the magnetosphere. It not only helps to activate the geomagnetic activity but it also affects the shape of the magnetosphere in a crucial way. As the dynamic pressure in the solar wind increases, the shape of the magnetosphere gets compressed. Just like a puppeteer pulls on the strings to move the marionette, the solar wind pulls on the Earth’s magnetic field strings that produce the auroral oval. In turn, this opens the auroral oval like the pupil of an eye and the aurora expands towards the equator, making it visible to people outside the auroral zone. The dynamic pressure is not the only metric that can control the latitude of the aurora. For example, the IMF strength and polarity also have a great effect in that regard. Nonetheless, higher dynamic pressure readings often mean higher chances to see aurora at lower latitudes than normal!
The solar wind carries electrically charged particles expelled from the Sun with a certain temperature measured in degrees Kelvin. The solar wind temperature of these particles do not have a direct effect on the nighttime aurora. Rather, it serves as an indicator. As different solar wind structures pass the L1 satellites, higher or lower temperatures arise and can help identify the type of structure the Earth will pass through. For example, coronal hole high speed streams tend to harbor higher temperatures while the core of coronal mass ejections inherit much lower ones.
The IMF Phi angle is the direct measure of the orientation of the IMF in the x/y plane in a GSM reference frame. It forms an angle that can be measured in degrees and determines whether the IMF points towards or away from the Sun. Between 0 and 180 degrees, the IMF is oriented away from the Sun. Conversely, between 180 and 360 degrees, the IMF is oriented towards the Sun. In some way, the Phi angle follows the Bx because they are related. The Phi angle is mainly used in certain cases by experienced chasers to determine how a certain radial solar wind polarity may affect the Earth’s magnetosphere or whether certain structures arise at L1.
The IMF Theta angle (also known as the IMF clock angle) is the direct measure of the orientation of the IMF in the y/z plane in a GSM reference frame. It forms an angle that can be measured in degrees and determines whether the IMF points northward or southward with respect to the Sun. Between 0 and -90°, the IMF is oriented southward. Conversely, between 0 and +90°, the IMF is oriented northward. In some way, the Theta angle follows the Bz closely because they are related. However, they are not the same. The Bz measures the amplitude of a magnetic force applied through the IMF in the north / south direction. The clock angle measures how north or south the IMF is pointing towards. The IMF may carry the same amount of force in the southward direction but with a more or less steep angle. Realistically speaking, the clock angle measures how ‘effective’ an IMF Bz value can be. For example, a solar wind sector carrying a Bz = -10.2nT and a Theta = -35° will likely be less effective to create geomagnetic activity than a similar Bz value with a Theta = -81°. Not many people look at the clock angle but it is quite an important value to monitor in order to anticipate the potential intensity of the auroral activity.
The magnetosphere corresponds to the magnetic field of the Earth and its surroundings. It is a very dynamic, buoyant and layered structure that can be seen as both a motor and a reservoir. The incoming solar wind (previous slide) interacts with the magnetosphere by altering its shape, filling up its tank with fuel and powering up its activity. Its shape controls the latitude of the auroral oval while its activity (called geomagnetic activity) is responsible in part for the auroral activity. The activity of some parts of the magnetosphere can be assessed by ground-based observatories with instruments called magnetometers. A magnetogram- the written result of a magnetometer, serves as a base to assess the geomagnetic activity and by proxy the auroral activity 1 to 5 minutes ahead of time.
As magnetograms are quite hard to read and interpret, we built two easy-to-use geomagnetic indices that assess the current activity in the magnetosphere at both high (H-Lys index) and mid geomagnetic latitudes (M-Lys index). Both indices were computed based on magnetometer data from their respective latitudes. They will enable you to see at a glance whether the magnetosphere mapping to high or low geomagnetic latitudes is more or less active. Such indices are often used as a proxy to give us a rough idea about the level of auroral activity at each respective latitude. Please note that although they are useful, such indices often fail at giving a very clear and accurate picture of the aurora in terms of location, behavior, shape, color… Information on how to use the graphs can be found in their respective information button.
The aurora can be seen as a beautiful and mobile exhaust gas coming out of the giant magnetospheric motor. All the visual aurora occurs in the atmosphere as a belt-shape structure centered around the geomagnetic poles called the auroral ovals. Our innovative model aims to show you the real-time latitude and intensity of the northern auroral oval over Europe, Iceland, Greenland and north America. This interactive map is based on magnetometer data and calculates the probability of position / activity of the aurora with rather good temporal and spatial accuracy. The model is updated every minute.
About the model