Space Weather Storm Societal Collapse


There is always space weather, just as there is always terrestrial weather. However, when the terrestrial weather is calm with clear skies, no clouds and balmy temperatures we tend not to think about the weather.

In just the same way, when the solar wind is flowing smoothly past the Earth at an average speed of 400 km/sec with a particle density of 7 protons per cubic centimetre and there are no disturbances in the Earth's magnetosphere of ionosphere we barely give thought to space weather.

However, when storms arise in the Earth's troposphere and heavy rains fall, maybe with strokes of lightning, we most definitely notice the terrestrial weather. In like fashion, when the Sun gushes forth a large cloud of matter which races through the interplanetary medium striking the Earth's magnetosphere and creating a large geomagnetic storm which produces aurora and maybe creates damage to an electrical power grid, we suddenly become very aware of space weather.

Not all space weather disturbances can be classified as severe space weather. A small solar X-ray flare may be hardly noticed by most of the Earth's population. So how do we define severe space weather? Various space weather forecast centres around the world nave developed a range of indices (just like the Beaufort wind velocity scale) to quantify the effects of space weather. However, these scales mean different things to different people. What may be significant to one group may be insignificant to another.

An overarching definition of severe space weather should probably consider space weather to be severe when it (1) involves danger to life, or (2) results in significant monetary loss to society

The rest of this note will define and discuss the various solar events that can produce severe space weather.


Severe solar events

The following paragraphs will expand upon these solar events that may cause severe solar weather in the near Earth environment.


Because of its high temperature (~6000 K) the Sun is a very broad band emitter of electromagnetic energy - from the lowest frequencies in the radio spectrum to soft X-rays - even in quiet conditions. In active conditions the intensity of these emissions increases, at least from specific regions of the Sun. The frequency range of these localised emissions can also increase and take in hard X-rays and even gamma ray emission for the most energetic transient events.

Solar electromagnetic emissions in the infrared and visible spectral regions do not show much variation over time. This is probably just as well, considering that it is these emissions that provide the heat energy to the Earth to make it habitable. Careful satellite measurements over extended periods of time have shown that the total solar electromagnetic output (>99% IR and visible) varies by an average of 0.1% over the 11 year solar cycle with some transient variations peaking to less than 1%.

Rapid (minutes to hours) transient variations in the radio and X-ray spectra can however show changes over 6 orders of magnitude. It is these transients that can cause some severe space weather.


Particles are continuously being thrown off by the Sun, ejected through the outer atmosphere (the Corona) into the interplanetary medium. The quiet solar emission is essentially due to the high solar temperature, 6000 K at the photospheric surface, rising to around two million Kelvin in the Corona. The average energy of protons in this environment is about 200 eV and the average velocity is about 200 km/s.

The Sun's escape velocity is a little over 600 km/s so a small fraction of the protons in the Corona will have velocities greater than this and will stream out from the Sun forming a constant 'solar wind'. At the Earth's orbit the average speed of this stream is around 400 km/s and the particle density is about 7 protons per cubic centimetre. As the solar plasma is essentially electrically neutral there will also be an equal number of electrons.

On occasions there will be a release of magnetic energy which will accelerate a cloud of coronal matter into the interplanetary medium - a coronal mass ejection (CME). The particle cloud is tied together by embedded coronal magnetic fields which are dragged along by the CME and eventually disconnect from their coronal source.

The total mass of plasma (mainly protons and electrons) ejected into the CME can range from under a billion tons (1012 kg) to over fifty billion tons. The size of CMEs can easily exceed a linear dimension of several million kilometres (dwarfing the solar diameter).

Another transient particulate emission from the Sun is referred to as a solar particle event or SPE. These high energy particles are not confined by coronal magnetic fields and have energies large enough to make them a dangerous source of ionising radiation to both man and matter.


Space Weather Woman
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ASA Australian Space Academy