# Presentation: 007184

## The Magnetosphere

*Mauricio Peredo *					 *August 11, 1999*

## Mauricio Peredo 					 August 11, 199

*The Sun is dynamic****We live in the atmosphere of the Sun******Earth responds to the changing Sun - that response is known as Space Weather******Scientists study the Sun-Earth Connection using data from spacecraft, ground stations, supercomputers, and rocket experiments (i.e.the ISTP Program)***

***Fundamental Concepts about the Sun-Earth Connection***

## The Magnetosphere

- Region around an object where the dominant force on a moving charged particle is the magnetic force from the magnetic field of the object

*The term “Magnetosphere” was proposed in 1959 by Thomas Gold of Cornell University to refer to the Earth’s magnetic environment*

## Magnetospheres of Different Objects

- The universe is divided by magnetic bubbles within bubbles ranging from the planetary, stellar, and perhaps galactic and local group scale.

## The Bow Shock

- Because the solar wind flow is supersonic, a bow wave is formed in front of the Earth’s magnetosphere.
- Much like the bow wave in front of a fast boat.

## Earth’s Magnetosphere

- The Earth is a huge magnet, and its magnetic influence extends far into space.
- The solar wind stretches the “magnetosphere” creating a long tail.

*The N-S pointing of a compass needle was discovered *
*in China around the year 1000; in 1600 William Gilbert*
* **in London showed it could be explained by the Earth being a huge magnet.*

## The “Terella”

- Gilbert used a spherical magnet, which he called "terrella",  or little Earth.
- He moved a small compass over the surface of the terrella and demonstrated that it always pointed towards its magnetic poles.

*Kristian Birkeland and his Terella in the late 1800’s*

## Our Shield in Space

- The magnetosphere is thus a cavity where we are shielded from the solar wind.
- Electric and magnetic forces - tied to electric currents - control the behavior of charged particles.

*In the 1830s a world-wide network of magnetic observatories was established and showed that, occasionally, magnetic disturbances occurred on a global scale. Von Humboldt named them magnetic storms.*

## Connection to the Sun

- By the end of the 19th century some relations between solar events and magnetic variations on Earth were noticed.

*In 1930, Sidney Chapman and Vincent Ferraro proposed the Sun sent out  huge clouds of electrically neutral plasma, and that  magnetic storms arose when those clouds enveloped the Earth*

## The Magnetopause

- The Chapman-Ferraro current is created by the solar wind impacting on the Earth's magnetic field.  It creates the boundary of the magnetosphere.

## Earth’s Magnetic Field

- Field lines in Earth’s magnetosphere resemble those of a bar magnet - they converge where the magnetic force is strong, and spread out where it is weak.

*Michael Faraday introduced the concept of field lines in 1846. He called them lines of force and believed they had “physical” meaning..*

## Ion & Electron Motion

- In space, charged particles tend to become attached to the field lines on which they reside, spiraling around them while sliding along them, like beads on a wire.

## How can you tell if you are in a magnetosphere?

- A charged particle shot in the same direction as the magnetic field and generally towards the body will hit the body.

*Typically invisible to the naked eye, motion of charged particles along magnetic field lines was captured during chemical release experiments.*

## Particle Motion: Trapped Particles

- As they spiral into a region of strong field, charged particles are reflected back along the magnetic field line.
- Such particles are thus “trapped” and bounce back and forth between opposite hemispheres.

*Birkeland’s terella experiments showed particle focusing into the poles. Poincare solved the mathematical motion near isolated magnetic pole.*

## Particle Motion: Drifting Particles

- Under the influence of magnetic forces, ions and electrons drift around the Earth in opposite directions.
- This motion creates an electric current - the Ring Current - that flows clockwise around the earth when viewed from above.

## The Radiation Belts

- The Earth actually has two radiation belts of different origins.
- The inner belt, discovered by Van Allen, is a by-product of cosmic radiation.

*Populated by protons of 10-100 MeV energy it can, on prolonged exposure, pose a hazard to instruments and astronauts.*

## Outer Radiation Belt

- The outer radiation belt is nowadays seen as part of the plasma trapped in the magnetosphere.
- It corresponds to the more energetic (~1MeV) particles of the ring current.

*The outer belt is far more variable and its plasma is replenished from the tail during magnetic storms and substorms.*

## Radiation Belt Discovery

- In response to the Soviet Sputnik, the US launched Explorer 1 in Jan 31, 1958, equipped with a Geiger counter.
- Explorer 3, launched in Mar 26, 1959 further documented the “saturation” of the counter as the spacecraft traversed the radiation belt.

## Project Argus

- In Oct 1957, Nicholas Christofilos proposed to detonate small atomic bombs in space and create an “artificial” radiation belt.
- Project Argus was carried out above the South Atlantic in Aug-Sep 1958 and studied by Van Allen with Explorer 4.
- The artificial belts decayed over a few weeks.

## “Starfish” and Nature’s own Surprises

- Following 3 Soviet tests with large bomb explosions, the US “Starfish” H-bomb test was conducted in July 1962 west of Hawaii.
- The intense artificial belt from the blast disabled 3 s/c and some of its particles persisted over a year.

*In Mar 1991,nature created its own extra belt following a strong compression of the magnetosphere. It remained when CRRES failed at the end of 1991.*

## The Magnetotail

- Away from the Sun, the magnetosphere stretches over 200 Re, eventually connecting to the solar wind.
- This region of the magnetosphere is quite dynamic; ions and electrons are often energized here.

*Here too, electric currents play a key role in particle dynamics. The cross-tail current joins the magnetopause currents to form a “theta” shape..*

## Magnetic Substorms

- Solar wind plasma that manages to enter the magnetosphere slowly falls toward the plasma sheet.
- Energy accumulates until it can’t be contained, and particles are explosively released toward Earth along field lines.

## Ground Signature of Substorms: Aurora

- Traveling along magnetic field lines, the accelerated particles reach the Earth along an oval region about the magnetic pole.
- The aurora is seen to expand and contract as the magnetotail lobes swell and shrink.

*POLAR is the first spacecraft capable of imaging the aurora simultaneously in visible, ultraviolet and X-ray wavelenghts.*

## Electric Currents from Space

- Reporting in 1903 on his expeditions to the auroral zone, Kristian Birkeland proposed that magnetic disturbances were caused by large electric currents flowing down the length of auroral formations.
- Later these were called auroral electrojets.

## Electric Currents from Space

- An artist's conception of magnetic-field-aligned current systems (Birkeland currents).
- To this day, the path taken by this currents far from Earth remains unknown.
- The total current flowing into/out of the ionosphere is approximately 2 Million Amperes.

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