Dr Colin Forsyth, UCL Mullard Space Science Laboratory: talk to Newbury Astronomical Society, 29th March 2019
Matter has four phases, three of which: solid, liquid and gas, are familiar to us. The fourth phase is plasma, which is essentially ionized gas, and is complicated. Although it is electrically neutral or nearly so, it is affected by, and can exert, electric and magnetic forces. Magnetic fields bring some order to a plasma, because charged particles move along the field easily, but across it with difficulty.
At least 99.99 percent of the visible universe is plasma, and space in the solar system is full of plasma ejected by the Sun. In visible light the Sun is “boring”, but in ultraviolet wavelengths it is highly active, and the emissions reveal arches of plasma near sunspots where the magnetic field emerges. The activity changes on an 11-year cycle, which has been tracked since the time of Galileo’s first telescopic observations. The sunspot records show the so-called “butterfly diagram”, with the spots in a new cycle starting at high latitudes and moving towards the equator.
At times the Sun is quiet, but at others there are huge eruptions called CMEs or coronal mass ejections. CMEs have a mass similar to Mt Everest and can have a volume many times greater than the Earth; a large CME can produce effects over much of the solar surface. The plasma is very hot, but not very dense by the time it reaches Earth. As well as CMEs the Sun “leaks” a continuous stream of plasma which we see as the solar wind, travelling at around 450 km/second. This flow, and the CMEs, are visible in images from satellites such as the Solar Dynamics Observatory (SDO) and the Solar and Heliospheric Observatory (SOHO). The plasma is trapped in the Sun’s magnetic field, and also pulls the field along with it.
When the solar wind encounters Earth, the trapped field interacts with our magnetic field to form the magnetosphere, which protects us; however, the Earth’s field can get broken under some circumstances. The magnetosphere has several components: the “plasmasphere” at around 58,000 °C; a plasma sheath at 12 million °C; a “ring current” with temperatures up to 230 million °C and the van Allen belts (VAB) in which the temperature can reach 12,000 million °C. The VAB overlap with geosynchronous orbit. The particle density is typically around 0.1 per cm3, so the heating effect is minimal, but the energetic particles can penetrate spacecraft in those orbits and cause problems. The VAB are very dynamic and variable, and their behaviour needs to be understood to keep spacecraft safe and operating correctly.
The magnetosphere is also “leaky” so that some plasma moves down field lines towards Earth and causes aurorae in a ring around each magnetic pole. The Earth’s field inside the auroral ring is directly linked to the Sun’s magnetic field. All the variations in the magnetosphere are collectively known as “space weather”, and they affect modern technology and present various risks. Examples of space weather events include loss of contact and control of satellites leading to so-called “zombiesats”, such as the IMAGE satellite in 2005, and induced currents in the ground which can damage power equipment, which led to the Quebec blackout of March 1989. The Carrington event in 1859 which affected telegraphic communications was caused by a large CME. GPS is affected because changes in the ionosphere affect the signal timings which are used to calculate positions. There are now around 400 satellites in geosynchronous orbit, which can be damaged by the radiation, or use more fuel in maintaining their positions. Auroral currents can block radio signals, which forces pilots to avoid trans-polar routes where they would be out of communication.
The Met Office Space Weather Operations Centre (MOSWOC) studies the effects of space weather and issues predictions. One key question is: what events on the Sun give rise to plasma conditions that affect the Earth? The Solar Orbiter probe due to be launched by ESA in February 2020 will study the Sun from inside the orbit of Mercury. It will view the regions of the Sun where the plasma originates, and correlating the data with plasma observations will help to answer this question. MSSL is building the Electron Analyser System which will fly on the probe, which is being built by Airbus in St. Evenage (aka Stevenage!). A Chinese/ESA collaborative mission called SMILE, due for launch in 2024, will use X-ray imaging to study the Earth’s exosphere, especially the region where the solar wind interacts with the magnetosphere. ESA is not part of the EU, so regardless of what happens with Brexit, these collaborations will continue provided the UK pays its ESA subscription.
Asked “What happens during field reversals?” the speaker replied “We don’t know!” He added that the field could either flip fairly rapidly, or fade out and then reappear. Some understanding may come from observing Uranus, whose magnetic field axis is currently at right angles to its spin axis, or Venus, which has no magnetic field but does have an atmosphere.
Notes and summary by Chris Hooker.