A sudden ionospheric disturbance or SID is a natural phenomenon in which the Earth's ionosphere is temporarily disturbed by a burst of extraterrestrial high-energy photons. The most common source of SIDs is The Sun, especially X-ray photons from solar flares. SIDs can also be created by X-ray bright gamma ray bursts or other luminous X-ray sources. In a SID, the ionosphere temporarily expands in size, and the change can be detected from the ground by variations in the measured strength of distant, low-frequency radio transmissions.
What are they?
The ionosphere of the Earth is a layer in the atmosphere in which nearly all of the atoms are ionized by solar ultraviolet radiation. The ionosphere is the top of the atmosphere -- beyond it lies the exosphere of the Earth, which extends out into interplanetary space. It has long been known that the ionosphere can be used in low-frequency radio communication to broadcast information
well beyond the horizon of the transmitter. This is because the ionosphere acts like a mirror to low-frequency radio waves; FM and television frequencies pass right through the ionosphere out into space, but low-frequency transmissions (like shortwave, AM, VLF and ELF) are reflected by the ions in the ionosphere.
The size of the ionosphere is determined in part by the flux of ultraviolet and higher-energy photons at the Earth, which is
largely determined by the current activity state of the Sun. Like any gas or plasma, it will react to an input of energy and will expand and contract depending upon the incoming ionizing flux. More flux, the ionosphere expands; less flux, it contracts. There are variations in
the incident ionizing flux over time caused by the diurnal rotation of the Earth, the 25-day rotation of the Sun, and the solar activity cycle, so the ionosphere undergoes mild fluctuations on timescales of hours to years. However, solar flares and astrophysical high-energy events can develop in a matter of seconds and the ionosphere can respond on the same timescales. This is why they're called sudden ionospheric disturbances.
What causes them?
The earliest reference to SIDs in the literature is from a paper entitled "Ionospheric effects of solar flares" by Mervyn A. Ellison in the Publications of the Royal Observatory, Edinburgh.(1) Radio Astronomers at the Cavendish laboratory measured the intensity variations in the Royal Navy's Rugby transmitter at 16 KHz throughout 1949, and noted a correlation between these variations and Hα flares on the Sun. They realized that the variations occur because the height of the ionospheric D-layer were changing, and that these changes were caused by heating of this layer by high-energy radiation generated by solar flares. When the flares ended, the ionosphere cooled and the layer contracted back to its equilibrium height. They detected several other ionospheric changes as well, such as changes in natural radio activity and shortwave radio propagation, and geomagnetic activity.
There are lots of astrophysical phenomena that can generate ionizing photons in the UV and X-ray energy regimes. Foremost of these in our local universe is our own
Sun. Magnetic storms on or near the surface of the Sun can generate explosions of X-rays, gamma rays, and
high-energy particles. These are solar flares, and we've been studying them for
well over a century now. These flares temporarily bathe our solar system in a sea of high energy particles that impact the Earth's ionosphere at the same time that we receive the visual light generated by these flares. You can later correlate the SIDS with flare observations or other indicators of solar activity.
We've also discovered that other astrophysical phenomena can also cause SIDs. We've known since the early 1960's that X-rays can be generated in stars other than our Sun, and often with much greater intensity. Gamma ray bursts were discovered in the 1970s, and are sometimes accompanied by X-rays that are better able to affect the ionosphere(2). More recently, we've discovered that some X-ray bright sources can even briefly outshine our own Sun. The first of these(3) was noted on August 27, 1998, when the magnetar known as SGR 1900+14 emitted an incredible burst of X-rays that was detected here on Earth.
A large SID was one of the many effects of this event felt in the Earth's near-space environment.
If you have a bit of electronics expertise (or know someone who does), you can build a SID detector for about $30 and a coupld of trips to a hardware store and a Radio Shack. This is because rather than observing the ionosphere itself, you're really just observing the effect that a sudden ionospheric disturbance has upon it by seeing how the strength of a distant, low-frequency radio
signal changes with time.
Basically, all you need is a receiver and an appropriate antenna designed to pick up a VLF station (like NAA at 24 kHz), along with some means of measuring the strength of the signal. If the ionosphere is disturbed in some way (as by a solar flare), the propagation of the signal is affected, resulting in a change in signal strength. You can then correlate the SID event times with other indicators of solar or cosmic activities like gamma ray burst alerts or solar flare announcements.
Schematics for SID detectors have been published in a few places, and I know of some schools and universities that built them for educational purposes. There's also a thriving community of amateur astronomers and others who observe SIDs for fun. You may find descriptions of a few designs at
(1) see a summary at http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1950MNRAS.110..626E
(2) Fishman, G.J. et al., 2002, GCN 1394: GCN GRB Observation Report: XRF 020427 Sudden Ionospheric Disturbance, http://gcn.gsfc.nasa.gov/gcn3/1394.gcn3