If there was only node creation going on, propagation would be more E and less Es. There are several mechanisms that break up the higher electron density nodes. One mechanism is the Earth’s magnetic field disturbance.
First, I need to set out what the normal, stable, Earth’s magnetic field is and how it contributes to node creation.
The stable state
Movement in the Earth’s core generates an electric current. This in turn sets up a magnetic field (by Maxwell’s right-hand rule). This field can be thought of as a bar magnet aligned roughly north-south with the Earth’s poles. Lines of flux extend beyond the Earth’s surface and out into space. Those lines of flux interact with fields from other planets and specifically from our Sun.
Close to the Earth, and all other things being equal, we’d get something like the figure below. The normal solar wind of charged particles causes squashed field lines on the sunny side and elongated field lines in the shadow side. I described a similar state when discussing the magnetosphere role in radio auroras.
This state would persist – enabling routine densified nodes of ionised gas to be created by atmospheric wind shear. The nodes would be created during the day and would descend slowly. The result would be daily E region openings, particularly in the afternoon in the summer months. And there would be nothing sporadic about them!
This state forms the basis of a more complete model.
Now I need to introduce disruption of the stable state.
Extraordinary Sun eruptions
The Earth’s magnetic field as I’ve described assumes a stable magnetosphere. The magnetosphere incorporates all fields impinging on the Earth from all sources.
But, as I describe elsewhere in discussing radio auroras, things are not stable.
We can detect extraordinary amounts of plasma with high velocity ejected towards Earth from time-to-time. This follows eruptions from the Sun’s surface.
These occasional high-energy plasma streams have dramatic effects. I describe in the page on auroras how this extraordinary rush of plasma ‘twangs’ the Earth’s field lines. It’s like plucking a guitar string. The disturbance this produces is measured momentarily by the factor Dst – the disturbance storm-time. Dst is measured in nanoteslas (nT). A Dst of over 100 indicates a moderate storm or twang. A Dst of 500 is a big disturbance and a major storm – perhaps indicating exciting times for hams chasing radio auroras. But this would be a disaster for those interested in Es.
Kp, a slower acting index averaged from many observatories, is also useful in assessing the overall storm magnitude. Kp has values 0 to 9 with low values indicating calm. A value of 5 indicates a moderate storm. Kp and Dst can be used together.
Earth’s magnetic field disturbance
So the Earth’s magnetic field disturbance has a key bearing on Es.
This notion of a twang of the magnetosphere filed lines can be detected with a magnetometer. I’ve included a typical magnetometer trace below to illustrate the ‘twang’.
Let me just remind you – the densified nodes contain charged particles essential to signal refraction. A stormy magnetosphere breaks up those densified E region nodes .
So, just when you were looking forward to a great day working Es, along comes a Sun-induced storm that wrecks everything! Therefore, I conclude: the higher the value of Dst or Kp, the more disturbed the magnetosphere, and hence the less likely an E opening.
This gives one reason when a run of days with good E openings can be ended abruptly. It’s one reason how E becomes Es.