Wednesday, February 11, 2015

A New Model for Magnetic Substorms Shows Promise

Regions of the magnetosphere of Earth. Our planet is situated at the nexus of the two blue lobes to the left.

Space physics, alas, is not something familiar to most people who may confuse it with solar physics, or shooting rockets into space. In fact, space physics primarily involves the study of near Earth plasma particularly in the magnetosphere - which is shown in the diagram above. Basically, the magnetosphere can be considered to be the region of plasma influenced by the magnetic field of Earth.  It is the region within which all -any charged particles proximate to our planet are susceptible to that B-field.

In the diagram shown, Earth is situated at the midpoint of the two blue lobes on the left side. The immediate magnetic field is indicated by the first several outer lobes - much as you would expect to see if tracing the magnetic field lines of a bar magnet.  As can be seen, the magnetosphere is short and squat on the side of Earth that faces the Sun- the "dayside" - but features a long tail extending away on the "night side".

This "magnetotail" is shaped primarily by the powerful force of the solar wind and the energetic plasma flow within it. Here on Earth, at certain times of the year and at specific places we observe the effects of this flow in the form of the aurora, such as we photographed in Chena Hot Springs, Alaska in 2005 e.g.

During high solar activity (e.g. near sunspot maximums) a higher flux of these charged particles inundates the solar wind, and the region around the Earth.

The Earth's magnetic field traps the particles, and the highest density is around the polar regions, which we refer to as the "auroral  ovals". In these regions, very large electric currents are set up, as the charged particles start moving in unison about the magnetic field lines. These currents can easily reach a few million amperes.

As this discharge occurs- in what we call "substorms" -  one or more outer electrons is stripped from the  atoms, for example from oxygen in the atmosphere then recombines again to form new (e.g. oxygen) atoms.  With this recombination there is emission of light, for a certain part  of the visible spectrum. This  is basically what gives rise to the aurora.

Now in Eos Transactions, Vol. 96, No. 1, January  15, 2015, p. 27), it is reported that  original work published by Pritchett et al  in The Journal of Geophyical Research, it's proposed that substorms may arise from Earth-sized bubbles of low density plasma that "ride" magnetic field lines through the magnetotail toward Earth.

Using a computer model simulating how charged particles behave in magnetic fields they showed these bubbles preferentially form in parts of the magnetotail where entropy increases with distance from Earth Thus the properties of the bubbles and the "auroral streamers" they send earthward change as they move through the magnetic field . Eventually this triggers a full blown substorm.

The model is useful in that it helps to explain the link between the magnetic perturbations that seem to accompany substorms (usually covered under one of two distinct energy paradigms, the B-v (driven) and E-J ("unloading") and auroral streamers. What's encouraging is that their predictions also match observations of the structure and behavior of auroral streamers made by the THEMIS satellites (THEMIS for 'Time History of Events and Macroscale Interactions During Substorms'.)

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