Field direction in aurora chasing: The interplanetary magnetic field, also known as IMF, plays a vital role in aurora creation. The measure of its direction (Bz) is a key component in the recipe. It acts as a master switch that decides to either light up the sky with dancing lights or to make all aurora completely disappear. Everyone aspiring to know a bit more about when to see the aurora should read along!
The creation of aurora on Earth happens thanks to electromagnetism. The Sun constantly sends highly energetic plasma containing billions of particles in the outer space. Earth is in the path of these streams called solar wind. But these particles don’t glide on their own. They are trapped into the Sun’s own magnetic filed called Interplanetary Magnetic Field or IMF.
As these trapped plasma crashes into Earth’s own magnetic field, interactions called magnetic reconnections happen under certain conditions, propelling the particles above Earth’s poles. They are injected into the atmosphere with high energy, ionizing some of the air atoms and molecules and making them glow: this is aurora.
Nonetheless the magnetic reconnections- so the aurora, only happen when the IMF and the magnetic field of the Earth are arranged in a certain way. Earth’s magnetosphere has lines of force that currently open from the south pole, go around the earth and close at the north pole. We say that they are oriented or pointing north. For reconnections to occur, the two field must be pointing in opposite directions. Indeed in physics magnetism works in the same way as electricity: like-fields repel and opposite ones attract. In other words the IMF needs to be pointing south for anything to happen.
Thanks to measuring instruments on satellites, we are now able to follow the evolution of the direction of the IMF in real time and also one hour before it reaches earth. This data called Bz can be found in all the apps and websites will most likely be collected by the ACE and DSCVR satellites about an hour from Earth at background solar wind speed. The satellites’ magnetometers measure the solar wind’s field direction like magnetometers on Earth.
The direction of the IMF is a key parameter here. It really acts like an auroral master switch, deciding whether or not there will be aurora. The IMF lines of force must point southwards to be able to couple with Earth’s field lines (directed north) and create reconnections thus aurora. This parameter always varies because in actuality the IMF can get distorted by all the interplanetary ‘junk’ (pockets of faster, slower, denser, less dense winds…). Its variability also depends on the field strength.
The satellite’s magnetometers measure the IMF in all three directions x, y and z. In this case the z component is the most important because it measures how much ‘south’ or ‘north’ it points. It is not to be mistaken with Earth’s magnetometers where the x component measures N/S variations.
In theory when the z component (also called Bz) of the IMF points south (negative z), it allows reconnections thus aurora. The more negative it is, the more likely we are to see aurora. Yet it is not the only important fact. For the reconnection process to happen, the Bz needs to be negative for a long enough period of time. Usually 15 minutes to an hour of negative Bz is enough for the sky to go from nothing to all filled up with aurora at high latitudes. Lower latitudes might require a significantly longer, stronger Bz dip usually provided by bigger solar storms with strong IMF.
A solar storm can be as big as it is, it won’t create major geomagnetic disturbances or aurora if the Bz doesn’t point south long enough to trigger the reconnection process. That’s why it’s so hard to forecast aurora until it reaches the satellites! We never know if the upcoming storm will couple nicely with our shield or if it will just blow right past us without doing anything. That is why a lot of inexperienced chasers trusting click-bait headlines often get disappointed when they don’t see anything happening during a storm that was ‘promised’ to deliver…
However there is an unsuspected silver lining for high geomagnetic latitudes. In fact the y component (By) may also be relevant in some cases. It’s never shown in major apps or websites and yet the By can show the perturbation of sideways currents around the Earth that help drive the aurora. They are in the East/West direction (y). A strong By might also strengthen the currents and give birth to reconnections, mostly in the magnetotail. That’s how you can get high latitude aurora without necessarily having a deep or long southward Bz.
The effects of the IMF direction are actually quite spectacular. The sky can literally light up within seconds if the fields aligns correctly or disappear as quickly when they misalign. Sporadic aurora spreading down to quite low geomagnetic latitudes have been reported with quite poor speed, poor density but strong and long-lasting south Bz. The direction of the IMF directly governs the fluctuations of aurora during the course of the night. It is the main factor responsible for its elusive and unpredictable nature.
You can find very good sources to follow the Bz and never miss out on any aurora action here: Field direction in aurora chasing
Field direction in aurora chasing: