Does the Ionosphere really hum?? by Ciaran Beggan

Dr Ciaran Beggan works in the Geomagnetism team at our BGS Edinburgh office. He's currently looking at what happens when the Earth's naturally occurring magnetic fields interplay with the ionosphere. In time, he hopes it'll lead to a new data set for monitoring the magnetosphere, space weather and related ionospheric phenomena; but for the time being here's Ciaran explaining a bit more:

Since June 2012, BGS have been running two induction coil magnetometers at our geophysical observatory in Eskdalemuir in the Scottish Borders. The induction coils measure the very rapid changes of the magnetic field from frequencies of around once per second (1 Hz) to fifty times per second (50 Hz).  

The BGS Induction Coil webpage shows a set of daily spectrograms – one from the North-South orientated coil (Channel 1) and the other East-West orientated coil (Channel 2). Spectrograms are images of power at each time during the day for a particular frequency.

The most obvious features in the images are the Schumann resonances (fuzzy bands at 8, 15, 22 Hz), lightning storms local to the UK (occasional horizontal lines) and the continuous vertical line from the power grid at 25 Hz. The lower frequencies below 5 Hz have been damped down to stop them over-powering the much smaller signals between 6 and 50 Hz. Here’s an example from the 26th April this year.

Spectrograms for 26th April 2013, showing the Schumann resonances, lightning storm activity between 11.00 and 18.00 and the induced 25 Hz signal from the power grid

However, that does not mean there is nothing of interest at frequencies below 5 Hz – quite the opposite, in fact. I have been looking at the low frequency data (0.1 – 10 Hz) for the past few weeks, as I am putting together a poster for the National Astronomy Meeting (NAM2013) in St. Andrews in early July. There are some fascinating effects from the Earth’s magnetosphere (the part of Earth’s magnetic field which extends into space), which can produce regular ‘pulsations’ at low frequencies (around 0.1 – 1 Hz) when it interacts with Sun’s magnetic field. This is a natural response of the magnetosphere when it has been disturbed by a passing Coronal Mass Ejection (CME) for example. Energy from the CME passes into the Earth’s magnetosphere which causes it to vibrate. It’s analogous to the vibrations of a bell when struck with a hammer. The images below show some long lived pulsations recorded on the 2nd April from midnight to around midday (the bright vertical patches between 0.1 and 1 Hz). 



Spectrograms for 2nd April 2013: pulsations between 00.00 and 11.00.
The 8 Hz Schumann resonances is also visible



Other interesting effects can be seen when the magnetosphere is very quiet. At night time, a series of oscillations of the magnetic field lines passing through the ionosphere can be detected. They vibrate slowly at 0.5 – 4 Hz in the local evening time and then over the course of a few hours begin to speed up to 5 – 7 Hz. Between 4-6 am local time (depending on season), they begin to dissipate as the ionosphere starts to change due to the effect of sunlight on its conductivity. These are called spectral resonance structures (SRS) and are generally clearer in the East-West orientated coil. They can be seen in the images below as the faint slanted lines starting around 18.00 UT, particularly in Channel 2 (right panel). This is what I have called the ‘hum’ of the ionosphere!
    

Spectrograms for 7th March 2013: Spectral resonance structures between 18.00 and 24.00.
The 8 Hz Schumann resonances is also visible

There are plenty of other odd (and for me, unexplained) phenomena in this new dataset. I’ll post some of the more mysterious ones for your opinion at some point (if Lauren lets me!).

By Ciaran Beggan

[you can be assured I'll be getting Ciaran to share some of these odd and unexplained phenomena, he's just signed up to a world of blog nagging encouragement - LAUREN]