In addition to regular solar cycle, seasonal, and diurnal variations, the upper atmosphere experiences "weather" events which are driven by:
Weather from the lower atmosphere
Transient intrusions by, for example, meteoroids and man-made missiles or rockets
Space weather largely due to solar disturbances
A 9 minute video "Solar Storms: Why We Care"
Space Weather Basics webcast
Space Weather Prediction Center's A Primer on Space Weather
Windows to the Universe
NASA's Space Weather Media Viewer
NASA's Space Weather Action Center
NASA's Cosmicopia page on Space Weather
Ionospheric Prediction Service (IPS) Radio & Space Services
2012 Space Weather Enterprise Forum
Recent article in the San Diego Union-Tribune
Solar Proton Events since 1976
Link page to more space weather resources
Space Weather Advisories
NOAA Space Weather Scales
List of Affected Systems
The upper atmosphere is a host to a variety of "weather events" or disturbances which are
most readily apparent by their effects on radio communication, by unusual auroral displays
or by fluctuations in the magnetic field at the surface of the Earth.
Since these weather events were first identified by such indicators,
different names may be used for events that are closely associated but
one must also keep in mind that closely associated events may have
quite different causes. The details of any weather event in the upper atmosphere are
just as unique as for any storm in the lower atmosphere. Below is a listing of upper
atmospheric disturbances and irregularities with a few comments and/or links.
- Magnetospheric Storm which causes a (Geo)magnetic Storm on Earth's surface is the result of significant changes is the speed of the solar wind (i.e. gusts) due to a Coronal Mass Ejection (CME) or Coronal Hole, for instance. CME's produce the most intense storms while coronal holes are responsible for 27-day recurrent storms. Descriptions of the April 6, 2000 Geomagnetic Storm and the
May 1998 Magnetic Storms
- Magnetospheric Substorm which is evident as an Auroral Substorm or Magnetic Substorm was identified in 1963 by Sydney Chapman and Syun-Ichi Akusofu and is triggered by a solar wind disturbance when the Interplanetary Magnetic Field (IMF) has a southward polarity.
The link of magnetic activity to a southward IMF was established by Don Fairfield in 1966.
- Sudden Ionospheric Disturbance (SID) produces Shortwave Fadeout (first identified by Dellinger in 1937) on the dayside of the Earth as the result of enhanced X-rays from a solar flare
- F-region Ionospheric Storm usually lasts a day or so and is the result of altered circulation globally caused by enhanced flux of energetic electrons (electron precipitation) into Earth's ionosphere. Usually (but not always) associated with magnetospheric storms
- D-region Ionospheric Storm (aka Post-Storm Effect) of enhanced electron density lasts a week or so after the F-region ionospheric storm
- Polar Cap Absorption (PCA) Event, first observed on Feb 23, 1956, usually lasts several days over the polar regions and is the result of a solar proton flare
- Travelling Ionospheric Disturbance (TID) is a ripple or wave in the electron density structure that propagates horizontally which affects the refraction of radiowaves and so degrades directional finders. There are two main types:
- large-scale TID with 1000 km or more between wave crests usually moving from pole to equator and generated by auroral activity
- medium-scale TID with hundreds of km wavelengths thought to be generated by thunderstorm activity
- Scintillation is the random amplitude and
phase shifts of radio waves due to propagation through small-scale inhomogeneities
in electron densities. Spread F in an ionogram is indicative of ionospheric inhomogeneities.
The twinkling of stars is the same effect on visible light due to inhomgeneities in air density.
Several features contribute to scintillation including:
- Ionospheric Blob which is a "blob of air" near the pole that has
broken from the dayside F-region (with high electron density) and migrated
to the nightside region (where the electron density is otherwise low).
- Ionospheric Bubble which is a "bubble" of air (with low electron density)
that rises into the F-region (where electron densities are otherwise high). These bubbles most frequently form
in equatorial regions just after sunset. The effect on radio communication was called the "London Lunchtime Effect".
- Localized electron precipitation that also produces auroral displays.
- Sporadic E layer
(Es layer) is a thin layer (usually less than 1 mile thick) with limited horizontal extent (tens of miles)
in the E-region having enhanced electron density similar to that of the
F-region and hence causes reflection at lower altitude of radio waves normally reflected by the F-region. One source of sporadic E is the ionized trail of a meteor.
Sporadic E layers also have differing characteristics and causes depending on latitude:
- equatorial: occur at day with no seasonal preference; cause thought to be turbulence induced by atmospheric tides, gravity waves and the equatorial electrojet
- mid-latitude: relatively rare occurring around noon in summers; cause thought to be wind shear
- polar: occur late p.m./early a.m. with no seasonal preference; cause thought to be electric fields associated with auroral activity
- D-region Bite-Out is a daytime depletion of electron density as the result of electrons attaching to polar mesopheric cloud ice particles
- D-region Winter Anomaly is a period of enhanced radio wave absorption usually occurring in late winter and thought to be associated with a
sudden stratospheric warming