February 1, 2006McMurry University Earth’s Invisible Shield Dr. Wayne R. Keith Angelo State University The Magnetosphere:

February 1, 2006McMurry University Introduction The Earth’s magnetosphere may be invisible, but it plays an important role in making life on Earth possible by protecting us from the solar wind and harmful high- energy cosmic rays. The lack of a global magnetic field on Venus and Mars has contributed to their inhospitality to life.

February 1, 2006McMurry University Outline History –A brief account of how our understanding of the magnetosphere and its importance has evolved over time. Anatomy –A description of the various regions. Applications/Science –how the magnetosphere affects life on Earth and the ways that we study it.

February 1, 2006McMurry University History Knowledge of some components of the magnetosphere date back centuries, but mostly this is a very young scientific field in which some of the original pioneers are still active. I will touch on only a few of the highlights, but of course important contributions have been made by many others to bring us to our current understanding.

February 1, 2006McMurry University History – Magnetic Field It has been known since the 1600’s that the Earth itself is a giant magnet. Convective motions and rotation of the iron/nickel core generate a “dipolar” field similar to a bar magnet.

February 1, 2006McMurry University History – Comets Also in the 1600’s, Johannes Kepler noted that comet dust tails always point away from the sun, and guessed it was due to sunlight. A comet’s second tail remained unexplained until the twentieth century, when the idea of a flow of ionized particles from the sun, a solar wind, gained acceptance. Comet Hale-Bopp

February 1, 2006McMurry University History – Solar Wind Kristian Birkeland was the first to predict, in 1913, that the Sun gives off a steady “wind” of ionized gas, or plasma. He used a Terrella (magnetized ball) to show that the plasma is directed to the poles, causing aurora.

February 1, 2006McMurry University History – Particle Trapping Carl Stormer used Birkland’s results and showed mathematically that particles following dipolar field lines can become trapped in a sort of “magnetic bottle”.

February 1, 2006McMurry University History – Magnetic Bubble The modern concept of a magnetosphere forming by the interaction of a neutral solar wind and the geomagnetic field was postulated by Chapman and Ferraro in Their paper correctly predicted that the Earth’s magnetic field would deflect the ionized solar wind, forming a cavity in the stream.

February 1, 2006McMurry University History – Radiation Belts The U.S. entered the Space Age with Explorer 1 in January James Van Allen’s Geiger counter saturates at two altitudes. He has discovered what will become known as the Van Allen Radiation Belts.

February 1, 2006McMurry University History – Today Dozens of spacecraft from many different countries have been launched to study the particles and fields that surround the Earth. As each new level of complexity is understood, new questions are raised to inspire the next generation of missions. IMAGE Spacecraft: EUV Instrument

February 1, 2006McMurry University Anatomy Magnetospheric physics involves a large number of regions and sub-regions, all with their own special terminology. Each region will be described separately, and then we will put the entire picture together.

February 1, 2006McMurry University Anatomy – Bow shock The solar wind is supersonic, and the Earth is a magnetic obstacle to the flow, so a shockwave is formed between the Sun and the Earth. Similar to the shockwave of a supersonic jet.

February 1, 2006McMurry University Anatomy – Magnetosheath Inside the bow shock, the solar wind has been heated and decelerated. Most of the plasma from the sun is deflected and does not enter the region dominated by Earth’s magnetic field.

February 1, 2006McMurry University Anatomy – Magnetopause The boundary between the sheath and the magnetosphere (the region dominated by Earth’s field) is called the magnetopause. Most of the solar wind is kept out… but some still gets in.

February 1, 2006McMurry University Anatomy – Cusps At the point where the magnetic field lines switch from closing on the dayside to being swept back into the tail, there is a pair of weak field regions called the cusps. Acts as a sort of plasma funnel, letting in some of the solar wind particles.

February 1, 2006McMurry University Anatomy – Reconnection Depending on the orientation of the magnetic field carried with the solar wind, the fields can also interconnect, allowing plasma to pass through the magnetopause.

February 1, 2006McMurry University Anatomy – Van Allen Belts Some of the plasma inside the magnetosphere becomes trapped in the Earth’s magnetic field, forming stable regions of high- energy particles called radiation belts.

February 1, 2006McMurry University Anatomy – Magnetotail The force of the solar wind sweeps the magnetosphere into an extended teardrop shape, forming a tail that extends past the orbit of the moon. Particles in the equatorial “plasma sheet” can be very energetic, and periodically rain down along the field lines towards the poles, exciting the atmosphere with colorful displays.

February 1, 2006McMurry University Anatomy – Magnetosphere

February 1, 2006McMurry University Applications/Science Understanding the magnetosphere is nice, but how does all this affect me here in Abilene, Texas? To see what the Earth might be like without a magnetosphere, we need look no further than our neighboring planets Venus and Mars.

February 1, 2006McMurry University Applications/Science – Venus Venus rotates too slowly to generate an internal magnetic field, so the solar wind interacts directly with the ionosphere. Hydrogen is stripped away, and over time, there is very little left to form water.

February 1, 2006McMurry University Applications/Science – Mars Mars also lacks a global magnetic field, although in this case due to the absence of a liquid core layer. Most of the Martian atmosphere has been lost to the solar wind, leaving it a dead world.

February 1, 2006McMurry University Applications/Science – CME’s In addition to the constant stream of solar wind, the Sun periodically has violent storms, and can eject huge blobs of plasma called CME’s (Coronal Mass Ejections). SOHO Spacecraft: LASCO and EIT Instruments

February 1, 2006McMurry University Applications/Science – Space Weather The magnetopause bears the brunt of these Solar assaults, but a lot of energy still gets through, which can cause power blackouts and satellite damage, not to mention some very spectacular auroral displays.

February 1, 2006McMurry University Applications/Science – Cosmic Rays Another hazard from space is cosmic rays. These super-high- energy particles come from outside the solar system at incredible velocities. Many are deflected, however, some still make it to the atmosphere.

February 1, 2006McMurry University Applications/Science – Spacecraft The fleet of scientific spacecraft dedicated to studying the magnetosphere are helping us learn exactly how our invisible shield works, and to predict when and how “space weather” will affect us.

February 1, 2006McMurry University Applications/Science – Cusps A major area of study is the cusps, important “input” regions where much of the mass and energy transfer from the solar wind takes place. The Cluster and DMSP missions study the cusps up near the magnetopause, and down close to the Earth.

February 1, 2006McMurry University 226 IL vs Emax C1,C3,F13,F14 C1 C3 F13 F14 CLUSTER Spacecraft: CIS Instrument DMSP Spacecraft SSJ4 Instrument These Data show how the two altitude regions are being compared to learn more about the entry processes.

February 1, 2006McMurry University Conclusions The Earth’s magnetosphere is a shield that protects us from dangerous charged particles from the Sun and elsewhere in the cosmos. Understanding this region of space is important for anticipating and protecting against harmful effects of space storms caused by ejections from the Sun.