Thursday 25th October 2018 was a return visit by Dr Sue Bowler from the School of Physics and Astronomy at the University of Leeds.
“Lights in the sky” was the title. It was an explanation, with some stunning images of the formation of Aurora.
Most auroras occur in a band known as the “auroral zone”, which is typically 3° to 6° wide in latitude and between 10° and 20° from the at all local times (or longitudes), most clearly seen at night against a dark sky. A region that currently displays an aurora is called the “auroral oval”, a band displaced towards the night side of the Earth. Early evidence for a geomagnetic connection comes from the statistics of auroral observations.
Dr Bowler stated that the first massive coronal mass ejection was noted if not fully understood at the time by Richard Carrington in 1859. It has since been called the Carrington event. It induced one of the largest geomagnetic storms on record, in early September 1859. The associated “white light flare” in the solar photosphere was observed and recorded by British astronomers Richard C. Carrington (1826–1875) and Richard Hodgson (1804–1872).
These geomagnetic storms causes the auroral ovals (north and south) to expand, and bring the aurora to lower latitudes. The instantaneous distribution of auroras (“auroral oval”) is slightly different, being centered about 3–5° nightward of the magnetic pole, so that auroral arcs reach furthest toward the equator when the magnetic pole in question is in between the observer and the Sun. The aurora can be seen best at this time, which is called magnetic midnight.
The Earth is constantly immersed in the solar wind, a rarefied flow of hot plasma (a gas of free electrons and positive ions) emitted by the Sun in all directions, a result of the two-million-degree temperature of the Sun’s outermost layer, thecorona.. The solar wind reaches Earth with a velocity typically around 400 km/s, a density of around 5 ions/cm3 and a magnetic field intensity of around 2–5 nT (for comparison, Earth’s surface field is typically 30,000–50,000 nT). During magnetic storms, in particular, flows can be several times faster; the interplanitarymagnetic field (IMF) may also be much stronger.
Joan Feynman deduced in the 1970s that the long-term averages of solar wind speed correlated with geomagnetic activity. Her work resulted from data collected by the Explorer 33 spacecraft. The solar wind and magnetosphere consist of plasma (ionized gas), which conducts electricity. It is well known (since Michael Faraday’s work around 1830) that when an electrical conductor is placed within a magnetic field while relative motion occurs in a direction that the conductor cuts across (or is cut by), rather than along, the lines of the magnetic field, an electric current is induced within the conductor.
The strength of the current depends on a) the rate of relative motion, b) the strength of the magnetic field, c) the number of conductors ganged together and d) the distance between the conductor and the magnetic field, while the direction of flow is dependent upon the direction of relative motion. Dynamos make use of this basic process, any and all conductors, solid or otherwise are so affected, including plasmas and other fluids. The IMF originates on the Sun, linked to the sunspots, and its field lines are dragged out by the solar wind. That alone would tend to line them up in the Sun-Earth direction, but the rotation of the Sun angles them at Earth by about 45 degrees forming a spiral in the ecliptic plane), known as the Parker spiral. The field lines passing Earth are therefore usually linked to those near the western edge (“limb”) of the visible Sun at any time.
The solar wind and the magnetosphere, being two electrically conducting fluids in relative motion, should be able in principle to generate electric currents by dynamo action and impart energy from the flow of the solar wind. However, this process is hampered by the fact that plasmas conduct readily along magnetic field lines, but less readily perpendicular to them. Energy is more effectively transferred by temporary magnetic connection between the field lines of the solar wind and those of the magnetosphere. Unsurprisingly this process is known as magnetic reconnection. As already mentioned, it happens most readily when the interplanetary field is directed southward, in a similar direction to the geomagnetic field in the inner regions of both the north and south magnetic poles.