|Fig. 1: Computer model study showing the effects of a geomagnetic storm equivalent to the May 1921 event. Red lines show regions susceptible to system collapse. The network of extra high voltage, long-distance transmission lines is shown with thin black lines. The circles indicate magnitudes of geomagnetically induced current flow at each transformer in the network.  (Courtesy of National Academies Press)|
Solar storms are eruptions on the Sun known as flares and coronal mass ejections (CMEs). Space weather storms occur near Earth as a result of solar storms. There are 3 types of space weather storms: geomagnetic storms, radiation storms, and radio blackouts. Geomagnetic storms are temporary disturbances of the geomagnetic field caused by solar wind gusts. Radiation storms are caused by high levels of solar radiation that occur when the numbers of energetic particles increase. Radio blackouts are disturbances in the ionosphere caused by x-ray emission from the sun.
When a CME arrives at Earth, it does not always have the same effect. If the north-south (or z) component of the CME field is positive (northward field), the CME will have little or no effect on the Earth. However, if the north-south component of the field is negative, it opposes the direction of the Earth's magnetic field. In this case the two fields interact, allowing energy from the solar wind to enter the Earth's magnetosphere. Such events can cause magnetic storms, periods when the Earth's magnetic field is highly disturbed. The basic physical procedures that cause these magnetic disturbances are briefly described below.
At a height of 3-5 RE (RE =6378 km) almost all atmospheric particles are fully ionized. At this height, the motion of charged energetic particles may be approximated by the superposition of three types of motion: gyration about the main field, bounce along field lines and azimuthal drift in rings around the Earth. The drift of charged particles in the Earth's inner magnetic field produces the so-called ring-current. This current fills a doughnut-shaped volume around the equator and produces a magnetic field at the Earth's surface which reduces the strength of the surface field. In quiet conditions, the effect of ring current on the magnetic field is negligible but during magnetic storms the intensity of the ring current increases and produces significant disturbances.
The conductivity in the ionosphere is proportional to electron density which means that particle precipitation in the ionosphere leads to high conductivity. The plasma convection in the presence of Earth's magnetic field causes an electric field. Both the enhanced conductivity and the electric field cause large currents at altitudes between 90 and 150 km. Some of the consequences of these currents are joule heating of the ionospheric plasma and changes of the geomagnetic field on the ground. These changes of the geomagnetic field create the geomagnetically induced currents in the Earth and in conductors.
The three phases of a magnetic storm are
The initial phase is caused by the compression of the magnetosphere on the arrival of a burst of solar plasma. The main phase is due to a large increase in the ring current energy. The decay of this current will restore the magnetic field in its initial condition.
Until the eighteenth century, geomagnetic storms have little or no effect on technology. There are several examples though, of severe geomagnetic storms, in the eighteenth and nineteenth centuries, that made navigation with compasses unworkable. Compass disturbances up to 8° reported at Fort Reliance between 1833 and 1835.  The first telegraph outages reported on November 17, 1848 when the clicker of the telegraph connecting Florence and Pisa behaved in an unexpected manner during a brilliant aurora. Since then there are numerous examples of magnetic storms that disrupted telegraph services and commerce was suspended at some economic cost. During some exceptional storms in 1859, 1882 and 1921 telegraph systems were seen to produce sparks and set offices on fire. /p>
The most famous power outage, caused by a geomagnetic storm, is probably the one that occurred in Quebec on March 13, 1989. A severe geomagnetic storm caused a transformer failure on one of the main power transmission lines in the Hydro Quebec system which led, in less than 90 seconds, to the collapse of the entire power grid. Six million people lost electrical power for 9 or more hours at an eventual cost of over $2 billion.  The geomagnetic storm was the result of a CME on March 9, 1989. The Hydro Quebec system was more vulnerable to the geomagnetic storm because most of Quebec sits on a large rock shield which prevented current flowing through the earth, finding a less resistant path along the 735 kV power lines. The same magnetic storm burned up a $36 million transformer in Salem Nuclear plant in New Jersey. These transformers have typical manufacture lead times at least 12 months. Fortunately, a spare transformer from a canceled nuclear plant in Washington State was available, and the Salem plant was able to reopen 40 days later.
The great geomagnetic storm of 1921 was an extreme though rare event, which is likely to occur again in the future. The total economic impact of such a storm today could reach $2 trillion in the first year alone, 20 times greater than the cost of Hurricane Katrina.  More than 350 extra high voltage transformers are at risk of permanent damage, with the possibility of leaving 130 million people without power. (See Fig. 1) The loss of power would quickly ripple across the social infrastructure causing cascading failures. Currently there is no manufacturing capability in the US for 500 kV and 765 KV transformers and manufacturers of extra high voltage transformers have a backlog of nearly 3 years. Full recovery from such an event would take 4-10 years. 
Solar storms can have significant impact on satellites as well. Some of the problems include increased drag, surface charging, solar panel degradation, single event upsets, contamination of the image systems, and orientation problems. Joseph Allen and Daniel Wilkinson at NOAA's Space Environment Center kept a file of reported satellite anomalies which included more than 9000 incidents up until the 1990s. In January 1994, for example, a period of enhanced energetic electron fluxes caused the outage of two Canadian telecommunications satellites, Anik E1 and E2, disrupting communications services nationwide. The first satellite recovered in a few hours but the recovery of the second satellite took 6 months and cost $50 -$70 million.  From 1994-1999, $500 million in satellite insurance claims were attributed to space weather.  The Department of Defence estimated in 2000 that disruptions to government satellites due to space weather effects cost about $100 million a year. 
The effects of space weather are not restricted to electrical power grids and satellites. Geomagnetically induced currents may contribute to the corrosion of gas and oil pipelines. [6,7] Solar storms cause degradation or blackouts of high-frequency radio communications affecting maritime and aviation systems.  During severe events, polar flights need to be diverted to lower latitudes with additional stop-offs. Space weather effects caused, for example, the diversion of 26 United Airlines flights in January 2005 at an additional cost up to $100,000 per diverted flight.  Radiation storm levels are often high enough and create concerns for passengers and crew of airlines as well as for astronauts. [3,8] These concerns are much greater for manned missions to the Moon and Mars. The billion dollar robotic arm and workstation on the International Space Station are also very sensitive to radiation events. Air Force One en route to China experienced a high-frequency communication outage during a solar event in 1984.  Navigation aided by the global positioning system (GPS) is also affected by space weather.  A 1% gain in continuity and availability of GPS is estimated to be worth $180 million/year.  Last, the impact of geomagnetic storms may even extend to the daily stock market returns. 
One of the most important functions of a space weather forecast agency is to provide reliable and long-term forecasts, with minimal false alarms, in order to minimize the economic impact of a space storm. The Space Weather Prediction Center (SWPC) and the Air Force's Weather Agency (AFWA) are the US official sources for space weather alerts and warnings. The SWPC relies both on space- and ground-based data sources to provide forecasts and operational space weather products to civilian and commercial users. The AFWA focuses on the impact of space weather on Department of Defence missions, using NOAA data combined with data from its own assets. Currently, SWPC can predict the occurrence of a geomagnetic storm or a strong (X-class) flare 1 to 3 days in advance with moderate probability. However, the forecast of ionospheric disturbances, which is important for GPS users, is less reliable. Forecasts, for example, posted by SWPC five days before the strongest flare of the previous solar cycle were 50%, 40%, 35%, 75%, 75%. On the day of the flare, November 4, 2003, the probability remained at 75% and the following days dropped to 10%, 1%, 1%. The SWPC serves more than 400,000 unique customers every month in over 120 countries.
© Stathis Ilonidis. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 J. Lovering, The American Almanac and Repository of Useful Knowledge, (Crosby, Nichols, and Co., 1857), p. 67.
 S.M. Silverman and E.W. Cliver, "Low-Latitude Auroras: The Magnetic Storm of 14-15 May 1921," J. Atmospheric and Solar-Terrestrial Physics 63, 523 (2001).
 Severe Space Weather Events -Understanding Societal and Economic Impacts: A Workshop Report (National Academies Press, 2008).
 J. Goldman, "Solar Storms Cause Significant Economic and Other Impacts on Earth," NOAA Magazine, 5 Apr 04.
 D.J. Rodgers, L.M. Murphy, and C.S. Dyer, "Benefits of a European Space Weather Programme," European Space Weather Program, ESWPS-DER-TN-0001, 19 Dec 00.
 R. Gummow, "GIC Effects on Pipeline Corrosion and Corrosion Control Systems," J. Atmos. Solar-Terrestrial Phys. 64, 1755 (2002).
 A. Osella, A. Favetto and E. López, "Currents Induced by Geomagnetic Storms on Buried Pipelines as a Cause of Corrosion," J. Appl.Geophys. 38, 219 (1998).
 Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop (National Academies Press, 2006).
 S. Skone, R. Yousuf and A. Coster, "Performance Evaluation of the Wide Area Augmentation System for Ionospheric Storm Events," J. Global Positioning Systems, 3, 251 (2004).
 A. Krivelyova and C. Robotti, "Playing the Field: Geomagnetic Storms and the Stock Market," Federal Reserve Bank of Atlanta Working Paper 2003-5, February, 2003.