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Glossary on Space Weather and Geomagnetic Hazard

This is a relatively concise glossary of the main terms used in space weather, with an emphasis on effects observed at the Earth's surface and in its magnetic field.

More detailed glossaries, including definitions of many more terms used in solar-terrestrial physics can be found at NOAA's Space Weather Prediction Center and the Australian IPS Radio and Space Services.


ACE (L1 Satellite)
NASA's Advanced Composition Explorer (ACE) mission is positioned approximately 1.5 million km closer to the Sun than the Earth, at a point (called the Lagrangian L1 point) where the gravitational pull of both bodies balances. ACE measures IMF and solar wind data in near real time. These data can give advance warning of the arrival at the Earth's magnetosphere of, for example a CME, with about 20-60 minutes lead-time, depending on the solar wind speed.

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Active Region (Solar)
A region of intense magnetic fields on the visible surface of the Sun, usually related to a group of sunspots. Active regions are often source points for ejection of material from the overlying solar corona (as CMEs) into interplanetary space. The magnetic complexity of active regions determines the likelihood of flares and CMEs.

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Ap
A daily planetary index of geomagnetic activity based on deviations of the horizontal magnetic field from a baseline and derived from data at thirteen Northern and Southern mid-latitude magnetic observatories. Expressed as a daily average of the eight three-hour ap values per day. Other magnetic indices also exist, characterising magnetic activity levels at different latitudes, for example, in the polar caps and in the equatorial region.

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Corona (Solar)
The Sun's million-degree tenuous outer atmosphere, overlying the visible disk (or 'photosphere').

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Coronal Hole (Solar)
A region in the Sun's corona where the solar magnetic field lines are open to interplanetary space (rather than closed back onto the visible solar surface) and that can connect via the solar wind with the Earth's magnetosphere. Typically associated with recurring minor magnetic storms and streams of faster than average solar wind.

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Coronal Mass Ejection (CME)
A reconfiguration of the Sun's magnetic field in the corona that results in the ejection of a cloud of coronal plasma (i.e. positively charged ions and negatively charged electrons). May be associated with solar flares and magnetic fields in a solar active (sunspot) region. Typically associated with major magnetic storms when the cloud is Earth-directed, the IMF is 'southward' and the solar wind speed is elevated. High-energy particles may be accelerated to near-light velocities by the shock front ahead of a CME. (Also see Halo CME).

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Disappearing Filament
An eruption of solar plasma (i.e. ions and electrons) associated with the upward movement of solar magnetic field lines into the corona. Filaments are usually dark against the bright solar disk but can appear bright (as 'erupting prominences') on the limbs of the Sun against the darkness of space. Filaments are often associated with CMEs and active regions.

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Energetic Particle Emission (Solar)
Is often a consequence of CMEs, flares and disappearing filaments. Very fast solar particles may be accelerated by the shock front ahead of fast CMEs and may reach both ACE and the Earth some hours after the solar event. Thus they may be useful precursors of CMEs at the Earth, especially if the energetic particle stream is sustained for some days after the event.

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Flare (Solar)
Eruption of solar material accompanied by electromagnetic and particle emissions. Usually detected by white light and/or X-ray brightness. May be associated with disappearing filaments, active regions and CMEs. Flares are magnetic phenomena and result from the reconfiguration of the solar magnetic field to reduce high magnetic field stress. (Also see X-ray event.)

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Geoeffective (Space Weather)
A term used to denote space weather events, originating from solar activity that leads to travelling disturbances in the solar wind that, in turn, interacts with the magnetosphere and which drives geomagnetic storms and/or substorms.

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Geomagnetic Activity
See magnetic storm and magnetic substorm.

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Geomagnetic Field
The magnetic field of the Earth, composed of magnetic fields from various sources of which the main field, generated in the core of the Earth, is the most significant. Rapid variations in the field (seconds to minutes) can give rise to, for example, GIC and are usually due to energetic solar wind events.

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Geomagnetic Induced Current (GIC)
A near-DC current which can flow in grounded conductors as a result of surface electric fields arising from the energised currents and associated time-varying magnetic fields in the ionosphere. Ultimately, GIC are driven by magnetic storms and substorms.

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Halo or Semi-Halo CME
In images of the Sun produced by the SOHO spacecraft, a CME may appear as a halo or semi-halo expanding over time around the Sun. The halo effect is due to the scattering of sunlight from the denser gas of a CME, compared to that of the solar corona, as it escapes from the Sun. A full halo is often an indication that the CME is Earth-directed and may give rise to a magnetic storms. A semi-halo CME usually indicates, at best, a 'glancing blow' to the magnetosphere, and it may signal a CME that will miss the magnetosphere completely. Uncertainty in the direction of propagation of CMEs occurs because of the interaction of the CME with the ambient solar wind and because of the absence of suitable monitoring satellites in space.

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Heliosphere
The heliosphere is the region within the galaxy where the Sun's magnetic field dominates over magnetic fields from other sources in the galaxy. It is analogous, in some respects, to the description of the Earth's magnetosphere in relation to the Sun.

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Hourly Standard Deviation (HSD)
The HSD index is a measure of the spectral power in the magnetic field that drives GIC and is used by BGS as an appropriate index of activity in the UK, given the time scale of geomagnetic storms and substorms.

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Interplanetary Magnetic Field (IMF)
The solar wind carries with it the Sun's coronal magnetic field as the interplanetary magnetic field. A 'southward' pointing IMF (i.e. towards the south magnetic pole) can initiate and maintain geomagnetic activity. On average the IMF is zero, but values more than about 10 nT southward will typically drive magnetic storms and substorms, depending on the duration of sustained southward field.

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Magnetic storm
A descriptive term for the changes in the global magnetic field as a result of the solar wind (in terms of energy, particles, magnetic field and pressure variations). Storms often begin with a sudden enhancement in the horizontal field strength followed over a number of hours by a decline in this field strength before recovery to normal levels over 1-3 days. Storms are often characterised by the behaviour of indices of magnetic activity such as Ap. Magnetic storm activity levels are described as 'Minor', 'Major' or 'Severe' and have specific numerical meanings in terms of Ap. Storms occur under 10% of the time (e.g. 10% of the number of days in a year). 'Quiet-Unsettled' and 'Active' conditions are therefore much more common but are not regarded as important for GIC. Major and Severe storms probably occur no more than 5% of the time in total. Storms and substorms drive ionospheric electrical currents that induce electric fields in the Earth, potentially giving rise to GIC.

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Magnetic sub-storm
A descriptive term for the changes over typically one to three hours in the local magnetic field, at high latitudes, as a result of input from the solar wind and current flows in the magnetotail. Can cause GIC at high latitudes (also see:magnetic storm).

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Magnetosphere
Region of near-Earth space where the Earth's field dominates, but is influenced by, the interplanetary magnetic field that originates in open field structures on the Sun. Contains a number of current systems, for example, in a ring surrounding the Earth, in the magnetotail (the region anti Sun-ward from the Earth) and on the magnetopause (where the magnetosphere is distinguished from the interplanetary medium, where the solar wind dominates). The magnetosphere is connected to the ionosphere by currents that flow along geomagnetic field lines and can therefore drive electrical currents into and out of the ionosphere.

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Persistence
Geomagnetic data display a tendency to persistence in that tomorrow's activity is often similar to today's activity. This is often the case when magnetic conditions are 'quiet'. However coronal hole related geomagnetic activity can also persist at more active levels for a number of days. Persistence can be used as a forecasting benchmark.

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Recurrence
The Sun rotates every 27 days as seen from the Earth. This means that any solar active region or coronal hole that causes geomagnetic activity will tend to return every 27 days (until it decays). Thus geomagnetic activity often follows a 27-day recurrence cycle.

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Shock (Solar Wind)
Abrupt changes in solar wind parameters, for example in the magnetic field strength, wind direction, speed and density, as a result of a flow speed in excess of characteristic speeds for the solar wind plasma. A solar wind shock as seen in the ACE L1 monitor may herald the arrival at the Earth of a CME or the interface between fast and slow solar wind streams. Both can give rise to magnetic storms and substorms.

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Solar Wind
The Sun's corona is too hot to be held back by gravity and expands radially into interplanetary space as the solar wind. The solar wind carries with it the Sun's open magnetic field as the interplanetary magnetic field (IMF). A 'southward' IMF can initiate and sustain geomagnetic activity in the forms of magnetic storms and substorms. Various high and low speed streams may be found in the solar wind which, when they interact, create complex shocks, turbulence and other features.

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X-Ray Event (Solar)
High and localised solar emission in X-ray wavelengths usually denotes solar magnetic activity and may indicate CME eruption and solar flaring. Astronomers use different classes for X-ray emission: the most important classes are labelled 'M' and 'X'; 'C' class rarely indicates a significant solar event.

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