Understanding the phenomena of solar storms
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Charged particles in the form of electrons and protons, as well as some heavy ions, are continuously emitted from the upper atmosphere of the sun in all directions. This emanation forms the solar wind, which from the earth’s point of view, is the fundamental component of space weather.
The average velocity of the “slow” solar wind is about 400 km/sec, and the pressure of the wind gives the earth’s magnetic field its typical shape by pushing it towards our planet on the day side and extending it on the night side to form a magnetic tail. This solar wind does not represent a steady state, however, because the sun, while stable enough to support life on earth, is subject to small variations; these create the dynamic environment constituting space weather. There are areas on the sun where the magnetic field lines are open to outer space, and solar winds can escape from these “coronal holes” at much higher velocities, up to 800 km/sec, than that of the regular solar wind. Since these high-speed winds can change the charged-particle, ie electron and proton, population within the earth’s magnetosphere, they are one source of the geomagnetic storms that can affect the earth. The coronal holes and fast solar wind streams are regular phenomena occurring especially during solar minima.
Sunspots trigger solar events
The source of the most impactful solar events is the active regions we know as sunspots on the sun’s surface. These non-static phenomena are caused by magnetic field anomalies that are so strong that they impair the convection of hot matter from the core of the sun to its surface and form areas cooler (and hence appearing darker) than the rest of the solar corona. The local anomalies can be simple, but can become extremely complex, with the poles of the involved fields becoming entangled like rubber bands twisted together. As a result, the magnetic fields sometimes reconnect, and immense amounts of energy stored in them are released in the form of solar flares emitting electromagnetic radiation covering all wavelengths, including visible light and UV radiation, gamma rays, X-rays and radio frequencies. Travelling at the speed of light, these emissions reach the earth in eight minutes. They do not have a big impact on power grids, but they change the ionisation of the earth’s upper atmosphere and affect radio wave propagation, so that GPS systems for example, can be impaired for several minutes or, in extreme cases, hours.
Coronal mass ejections (CMEs) are the source of geomagnetic storms in the atmosphere
Sometimes not only radiation is released, but an enormous quantity of plasma, ie energetic particles of solar matter itself, is blasted into space and accelerated by the fields related to the solar flare. Such “coronal mass ejections” (CMEs) are at times shot out in the direction of earth. They are not directly dangerous to life on the planet, since they are mostly absorbed by the upper atmosphere. Nevertheless, they constitute a risk to astronauts, can damage the electronic systems of spacecraft and can have several adverse effects, depending on conditions at the time of the impact. Furthermore they increase the size of the night-side magnetic tial. CMEs have an embedded magnetic field. When this collides with the earth’s magnetic field, nothing dramatic happens if the two fields are parallel, as they will repel each other. However, if the orientation of the magnetic field in the plasma cloud is opposite to that of the earth’s magnetic field, the two interact. Charged particles from the CME enter the earth’s field and in its enlarged tail the mechanism of magnetic reconnection may take place, generating large electrojets (magnetospheric/ionospheric currents) and strong geomagnetic storms. The magnitude of the disturbance can be measured by means of space-borne and ground-based devices. The readings from the two sources correlated very precisely for the Halloween solar storm of 2003, and they exactly reflected the time of the Malmö blackout.
The underestimated impact of CMEs
When a CME arrives and magnetic field interaction occurs, the situation can be dramatic. The geomagnetic storm changes the radiation environment in space, posing a health risk to astronauts and affecting spacecraft electronics and operations, possibly to the extent of modifying their orbits. Through its interaction with the upper atmosphere, the CME can cause a degradation of satellite communication links and change GPS signal propagation, which can result in navigation errors or outage of GPS systems. These effects were experienced in various parts of the world during the 2003 Halloween storms. At that time, there were also widespread HF radio outages in Africa, Asia and Australia; polar flight restrictions had to be imposed owing to impacts on radio communication, and effects on survey instruments were observed globally. Notable impacts on power grids were the blackout in Malmö and transformer damage in South Africa. Further striking, though harmless, effects were impressive aurorae borealis and australis.
Risks to railway circuits and other large-scale conductors
The difficulties and damage caused to power grids by severe solar storms is attributable to the generation of geomagnetically induced currents (GIC). The process occurs through direct and indirect inductive coupling between the electrojets a conductor on the earth’s surface, in this case an electric power transmission network. Essentially, GIC incurs saturation of transformers, which can lead to increased reactive power demands, creation of harmonics, relay tripping, hot spots in the transformers and even permanent damage, as well as system collapse. New transformers can be designed to withstand GIC to a given level, but replacement of transformers is an expensive process, and highly damaging CMEs have not been frequent. Apart from affecting power grids, GIC can also flow in and damage oil and gas pipelines (by causing corrosion), telecommunication cables, railway circuits and other large-scale conductors. There is evidence from Sweden and Russia that railway signal systems failed to operate correctly during solar storms, but the mechanisms are not yet fully understood.
The last big CME in 2012 barely missed the earth
Infrequent as major CME impacts are, they can incur tremendous costs when they do happen. The direct costs of the 1989 Quebec blackout were estimated to be CAD 13.2 million to the power company, but the overall indirect costs were calculated to be CAD 6 billion. While geomagnetic storms are more frequent and intense at higher latitudes, vulnerability to GIC depends on a number of geo-physical and technical factors, and mid-to-low latitudes are not spared. Unlike regional disasters such as floods and earthquakes, a big CME could affect a large part of our planet, causing enormous damage and staggering costs. On 23 July 2012, NASA’s Stereo-A spacecraft monitored a huge CME that blasted through the earth’s orbit, fortunately bypassing the planet by a few days only. Scientists think that if it had collided, it would have wreaked havoc lasting for years.Summary by Jeff Barnes. The article is based on the "Expert Hearing on Solar Storms" which took place on 14 March 2016 at the Swiss Re Centre for Global Dialogue.