GLOVER 6/13/03 9:34 AM Page 28 Feature Tomorrow’s Space Weather Forecast ? 28 esa bulletin 114 - may 2003 www.esa.int GLOVER 6/13/03 9:34 AM Page 29 Space Weather Alexi Glover, Alain Hilgers & Eamonn Daly Electrical Engineering Department, ESA Directorate of Technical and Operational Support, ESTEC, Noordwijk, The Netherlands Richard Marsden Research and Scientific Support Department, ESA Directorate of Scientific Programmes, ESTEC, Noordwijk, The Netherlands What is Space Weather? The definition most frequently quoted is that of the US National Space Weather Programme: "conditions on the Sun and in the solar wind, magnetosphere, ionosphere, and thermo- sphere that can influence the performance and reliability of space-borne and ground-based technological systems and can endanger human life or health". This definition demonstrates that space weather encompasses a collection of physical phenomena having their origin at the The solar-terrestrial system. Space weather events take place when transient Sun and which give rise to effects that are of concern to activity on the Sun leads to consequences at and close to the Earth. Large society. The two accompanying figures illustrate the solar- clouds of magnetised plasma ejected from the Sun may lead to changes in the terrestrial system and the complexity of the phenomena that structure of the Earth’s magnetic field, and bursts of solar radiation can cause rapid changes in the composition of the upper layers of the Earth's atmosphere lead to space weather effects. Much of the science that underpins space weather is part of the larger field of solar- terrestrial physics. Here we focus on some of the main aspects of the system - the Sun, heliosphere, magnetosphere and ionosphere - and the key space weather phenomena that can affect us here on Earth. www.esa.int esa bulletin 114 - may 2003 29 GLOVER 6/13/03 9:35 AM Page 30 Feature • Radiation Damage • Hazard to Astronauts Radiation Magnetospheric and Aircrew Cosmic Perturbations Belts • Interference with Sensors Rays • Upsets in Electronics • Electrostatic Charging & Discharge Geomagnetic • Electrostatic Charging Solar Solar Storm & Discharge Events Energetic Particle Plasma • Interference with Experiments Solar Injections Injections • Radio (comms., nav.) Signal Wind Interference & Degradation • Disruption of Power Grids, Plasmasphere/ Pipelines & Surveying This schematic illustrates Plasmasheet/ the complexity of the space Ionosphere • Drag weather system. It shows • Attitude Perturbation the route by which solar and cosmic phenomena Atmosphere filter down to impact Modification technological systems and • Climate Change? the Earth's climate The Sun and Its Surroundings typical period of 11 years. This results in a The solar wind The Sun is the primary driver of space similar variation in the number of large In addition to these transient phenomena, weather phenomena. Most space weather sporadic space weather events. These charged particles are constantly streaming events have their origin in the solar corona: events are most frequently observed away from the Sun in the form of the the Sun’s ‘atmosphere’. The corona, which around the time of solar maximum. The magnetised solar wind. The magnetic field is normally visible only during a solar most popular and visible consequences of lines that are carried into space by the solar eclipse, is a hot (>106 degrees Kelvin) these large events here on Earth are the wind remain tied to the Sun at one end and tenuous plasma extending away from the spectacular aurorae or ‘northern/southern stretch into space at the other. Solar solar surface and out into the heliosphere. lights’, which may be observed at lower rotation pulls these field lines into the The dominant feature controlling the latitudes than normal during very shape of an Archimedean spiral. During corona is the Sun’s magnetic field. To the disturbed space weather conditions. solar minimum, when the level of solar naked eye, the Sun appears as a constant bright yellow disk. However, in reality, the Sun is far from unchanging. This is Aurorae are caused when electrons precipitate from the dramatically shown by the images and magnetosphere into the movies produced by the joint ESA-NASA atmosphere by spiralling around SOHO science mission. Viewed in these magnetic field lines. At altitudes other wavelengths, the Sun is seen as a of 90 to 5000 km, electrons will boiling cauldron of magnetic fields and hit atmospheric atoms and molecules. These atoms and plasma. Sudden changes in these coronal molecules will be given energy in magnetic fields can result in explosive the collision and will emit light. solar ‘flares’ and ‘Coronal Mass Ejections’ The colour of this light depends (CMEs). on the atom or molecule and the Solar activity is usually classified in energy of the electron involved in the collision. Oxygen atoms terms of the number of sunspots seen on will emit green and red light, the solar disk, and the frequency of whereas nitrogen atoms will emit occurrence of flares and CMEs. This violet and blue (Image courtesy activity shows cyclical behaviour, with a of J. Manninen) 30 esa bulletin 114 - may 2003 www.esa.int GLOVER 6/13/03 9:35 AM Page 31 Space Weather activity is low, the Sun's magnetic field effects occurring on an almost regular time to block out light from the bright solar disk resembles that of a bar magnet, and field scale. Coronal holes are long-lasting (indicated by the central circle in the lines open out into space at the northern features and may endure for many weeks, accompanying images). These particular and southern polar regions. Coronal rotating with the Sun every 27 days. This CMEs were not directed towards the Earth, material escapes in high-speed streams leads to recurrent space weather activity on but illustrate the spectacular nature of such from these regions, which are known as a similar time scale. This pattern can be eruptions in the corona. ‘polar coronal holes’. The boundaries of seen most clearly during solar minimum. Most CMEs that lead to Space Weather these polar coronal holes can extend to During solar-active years, the changing effects at the Earth are called ‘halo’ CMEs. quite low latitudes in places, even crossing magnetic field at the Sun disrupts this The Earth-directed plasma is observed as a the solar equator on occasion. pattern, and the coronal holes become bright halo completely surrounding the Compression regions form at the smaller and move to lower latitudes rather occulting disk and expanding radially boundaries between these high-speed than remaining close to the poles. outward in all directions from the Sun. In order to be able to predict space weather effects, we need to be able to predict not only what will happen, but also when. CMEs can take several days to reach the Earth after we see them leave the Sun. Predicting whether a halo CME will lead to strong effects at the Earth, and when these effects will begin, remains a major challenge in space weather research. Solar flares Solar flares are sudden and rapid releases of magnetic energy in the solar atmosphere. They are most commonly observed from sunspot regions where magnetic loops interact, with explosive consequences. Flares can last for a few minutes to several hours, releasing energies of the order of 1025 joules – fifty billion times the energy released by a 50-megaton H-bomb! The radiation from these events covers a wide range of wavelengths, from radio waves to gamma rays. This radiation reaches the Earth in a matter of minutes, making advance warning extremely difficult. Solar Energetic Particles (SEPs) CMEs as seen by SOHO/LASCO. CMEs are huge clouds of Coronal Mass Ejections (CMEs) Another key aspect of space weather is the magnetised plasma that erupt from the solar corona with speeds CMEs are huge clouds of magnetised arrival at Earth of sudden bursts of highly of up to 2000 km/s. This figure shows two CMEs expanding plasma that erupt from the solar corona at energetic particles originating from the simultaneously in the coronagraph image. One expands to the speeds of up to 2000 km/s. These clouds Sun. The largest such events can enhance northwest and the other to the southeast. Note how the size of these clouds appears to dwarf the solar disk. This image of the carry a mass of the order of a billion the radiation intensity in near-Earth space solar disk was taken by the SOHO EIT telescope and shows solar tonnes – more than the mass of Mount by several orders of magnitude. features at temperatures of about 80 000 K Everest! – and are superimposed onto the Solar energetic particles can either expanding background solar wind. originate from a solar flare or from The Large Angle Spectroscopic Corona- acceleration at interplanetary shocks streams and the slower solar wind ahead. graph (LASCO) onboard the SOHO driven by fast CMEs. High-energy SEPs Here charged particles can be accelerated spacecraft allows us to see CMEs more can arrive at the Earth within tens of to higher energies. The interaction between often and in far greater detail than ever minutes of the flare onset, and SEP events these compression regions and the Earth’s before. Like the Moon during a solar can last for several days. magnetic field can lead to space weather eclipse, a so-called ‘occulting disk’ is used www.esa.int esa bulletin 114 - may 2003 31 GLOVER 6/13/03 9:35 AM Page 32 Feature A solar-particle event seen by the NOAA GOES spacecraft on 14 July 2000. This particle event followed the solar flare shown below by about 30 minutes. Note the speed with which the particles arrive and the steep increase in particle flux. Because they carry an electric charge, SEPs can only propagate along magnetic field lines.