Storm Sudden Commencement

This morning a few minutes after UT 04.00 a very clear jump in the solar wind speed as well as the interplanetary magnetic field strength was seen by the ACE satellite which is located upstream in the solar wind, see the figure below.

6 hours of solar wind data from ACE on March 17. 2015
6 hours of solar wind data from ACE on March 17. 2015

This is the typical the signature of an interplanetary shock and in this case the passage of the front of a coronal mass ejection (CME). The particular shock of today was, according to the Solar Influences Data Center in Belgium, associated with an earlier than predicted arrival of a CME that left the Sun on March 15.

Now, from a geomagnetic point of view, what happens when Earth is hit by such an interplanetary shock? The geomagnetic signature is very clear and global, and termed a sudden impulse (SI), or depending on what happens afterwards a Storm Sudden Commencement (SSC). Today’s event is a school example of that, the shock hit at about UT 04.45, see the figure below:

Sudden impulse observed on March 17.
Sudden impulse observed on March 17.

Roughly speaking, the SI is the superposition of two effects, seen as a very short lived oscillation and a positive step in the magnetic field. The former is the signature of the reorganization of the magnetosphere as the shock front propagates along the magnetopause towards the magnetotail, and the current system established to resist the change. Depending on local time and latitude, the oscillation will start with a negative or positive excursion. The latter is caused by the magnetopause current (Chapman-Ferraro current) increasing and being pushed closer to Earth.

A very good review article about such events has been written by Araki (1994).

So, what will happen now? As mentioned above, we may also call the event a Storm Sudden Commencement, will we get a geomagnetic storm? We have certainly been hit by an interplanetary shock associated with a CME…

What determines if we get a storm or not, is basically the Interplanetary Magnetic Field direction , if its z-component (the one which is parallel to Earth’s magnetic axis, seen as the red curve in the top figure) becomes negative for extended periods of time during the next 24 hours, we have a geoeffective situation. Then we will certainly get storm conditions. So far, during moment of writing, it has been negative most of the time, and conditions are indeed active on a global scale. How it develops, we just have to wait and see…