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.
- Corona (Solar)
- The Sun's million-degree tenuous outer atmosphere, overlying
the visible disk (or 'photosphere').
- 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.
- 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).
- 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.
- 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.
- 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.)
- 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.
- Geomagnetic Activity
- See magnetic storm and magnetic
substorm.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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).
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Global Geomagnetic Models
- Space Weather and Geomagnetic Hazard
- High-frequency magnetometers
- Schumann Resonances
- Geoelectric field monitoring
- Space Weather Impact on Ground-based Systems (SWIGS)
- SWIMMR Activities in Ground Effects (SAGE)
- Geomagnetic Virtual Observatories
- Quantum magnetometers for space weather
- Publications List
