In Armagh, aurorae were seen over five nights on 28th and 29th August, and 2nd, 3rd, and 4th September 1859. Armagh Observatory’s meteorological records show that the first of the aurorae was ‘faint’, with them becoming more ‘bright’ and ‘evidently strong’ on 2nd and 3rd September.
Delicate instruments recording fluctuations in Earth’s magnetic field shot off the scale, and electric currents surged in telegraph wires, disrupting communications. According to the New York Times (Green et al. 2006):
The auroral currents from east to west were so regular that the operators on the Eastern lines were able to hold communication and transmit messages over the line between this city [Boston] and Portland, the usual batteries being now disconnected from the wire.
The extremely high currents resulted in many telegraph wires short-circuiting, causing some telegraph poles to catch alight.
These effects were dubbed the ‘Solar Storm of 1859’ or the ‘Carrington Event’, while the solar flare that caused them is usually called the ‘Carrington Flare’. Contemporary scientists realised that the auroral effects and telegraph disruption was related to the increased solar activity, and that this solar activity had affected Earth’s magnetic field, since magnetic field recorders had been so disturbed. The Scientific American of October 15 1859 said ‘a connection between the northern lights and forces of electricity and magnetism is now fully established’.
Armagh Observatory meteorological records showing aurora was observed on the nights of 28th and 29th August 1859.
Armagh Observatory meteorological records showing aurora was observed on the nights of 2nd, 3rd, and 4th September 1859.
The bright-light eruption from the Sun’s surface observed by Carrington was a solar flare, believed to be the first ever recorded. It was simultaneously observed by Richard Hodgson, who reported (Hodgson 1859):
While observing a group of solar spots on the 1st September, I was suddenly surprised at the appearance of a very brilliant star of light, much brighter than the Sun’s surface, most dazzling to the protected eye, illuminating the upper edges of the adjacent spots and streaks, not unlike in effect the edging of the clouds at sunset; the rays extended in all directions; and the centre might be compared to the dazzling brilliancy of the bright star alpha-Lyrae when seen in a large telescope of low power. It lasted for some five minutes, and disappeared instantly about 11.25am. The phenomenon was of too short duration to admit of a micrometrical drawing, but an eye-sketch was taken […]. The magnetic instruments at Kew were simultaneously disturbed to a great extent.
Plasma exploded from the Sun’s corona, reaching temperatures of tens of million Kelvin and accelerating the plasma particles close to the speed of light. As a result of the extremely high temperatures, electromagnetic waves were emitted across the spectrum.
Carrington observed that these flares occurred in a region of sunspots; this was not coincidental. In 1955, Waldmeier showed that a statistical relationship exists between flare frequency and sunspot number. By examining the nature of sunspots, one can understand why the solar flares occur, when they tend to occur, and when they might lead to coronal mass ejections.
Sunspots, Solar Flares, And Coronal Mass Ejections
Sunspots are observed as dark areas of the solar photosphere, associated with areas of very intense magnetic fields on the Sun’s surface. Although the exact process by which the magnetic fields become so intense is not entirely understood, it is known that a strong magnetic field can inhibit the convection currents that distribute heat up towards the surface. This happens because plasma is composed of ionised particles and its movement is particularly affected by electromagnetic fields. As a result, sunspots are cooler than other regions on the Sun’s surface. Typically, the temperature of a sunspot is more than 1000K lower than the undisturbed surface (Golub & Pasachoff 2001).
Solar flares are also caused by this intense magnetic activity, which explains why flares and spots occur in the same regions of the Sun . The strong magnetic fields link the corona to the solar interior by penetrating through the photosphere, providing a ‘pathway’ that allows energy to be transferred outwards to the corona. The release of magnetic energy associated with a solar flare occurs extremely quickly, giving the appearance of an explosion, which throws mass and energy out into the solar corona.
However, the Sun’s global magnetic field is not constant, and so the frequency and magnitude of the largest solar flares also varies. The Sun’s polarity reverses approximately every 22 years, a phenomenon known as the Hale cycle, due to a dynamo process that is still not fully understood. This magnetic variation leads to corresponding periods of intense solar activity, with increased occurrences of sunspots and solar flares, and ‘quiet’ periods, which occur in an 11-year solar activity cycle. The Sun’s magnetic activity is also modulated in other ways as well, such as the Mauder and Dalton Minima (e.g. Wilson 1998).
Coronal mass ejections (CMEs) are ejections of plasma from the corona into space. Typically, a CME contains 20 billion tonnes of plasma material, which represents approximately one-tenth of the overall coronal mass (Golub & Pasachoff 2001). Of critical importance to the effect of the CME on Earth, the ejection carries with it a magnetic field from the corona.
The stream of charged particles and the associated magnetic field which forms a CME can account for many of the more severe effects of the Carrington flare on Earth. In particular, the magnificently bright and geographically wide-ranging aurorae described in detail above can be well explained by the CME. As this huge bubble of charged particles and associated magnetic field travels towards Earth, it interacts with Earth’s magnetosphere. At the magnetosphere’s strongest point (closest to the equator) almost all these charged particles are deflected. However, towards the magnetic poles, where Earth’s magnetic field weakens, fewer electrons are deflected and so more interact with Earth’s atmosphere, where they collide with atoms in the atmosphere (mainly nitrogen and oxygen), exciting them. As these atoms de-excite, photons of light are released. This light forms the aurorae.
In the case of the Carrington flare, the effects were particularly bright due to the size of the CME: the number of particles interacting with the atmosphere was large. The size of the CME also explains why the aurorae could be seen at near-equatorial latitudes: the sheer number of particles ejected meant that even though the majority were deflected by the magnetosphere at these latitudes, the tiny proportion of particles that were not deflected were still numerous enough to cause aurorae at these latitudes. Furthermore, Earth’s magnetic field must have been aligned in such a way that its interaction with the ejected plasma was particularly noticeable. Although CMEs were not known at the time of the Carrington Event and no contemporary scientific equipment would have been technologically advanced enough to record the size of such an event, it is because of the auroral observations as far south as Cuba and Hawaii that we can deduce that the aurorae were caused by a CME.
The Carrington flare, and the geomagnetic storm which followed it, was important in many respects. Not only was it the first flare ever recorded, leading to new questions about the nature of the Sun, but it was also independently observed by two scientists, Richard Carrington and Richard Hodgson (Hodgson 1859), which meant that the evidence about the flare’s brightness and duration was more reliable. Furthermore, it suggested to scientists for the first time that there was a link between solar activity and magnetism as well as between auroral activity and magnetism. It is remarkable, 150 years from the event, to think how much we have learnt about the Sun and its relationship with Earth in this period. The observations of the Carrington flare and its effects brought new ideas into circulation and renewed interest in the science of the Sun. It intrigued people in 1859, and today humans are still fascinated by aurorae produced by the CMEs and the continuing debate concerning whether they are largely caused by CMEs or massive flares.