1.2 The Plasma Environment of Our Earth

1.2 The Plasma Environment of Our Earth

We start our grand tour through natural plasmas in the solar system. The physics of the Sun–Earth system is governed by many plasma processes and comprises nuclear reactions in the Sun’s interior, plasma eruptions from the Sun’s surface, a steady-state solar wind, and the interaction of the solar wind with the Earth’s magnetosphere and the formation of an ionosphere.

1.2.1 The Energy Source of Stars

The most important plasma object in our space vicinity is the Sun, which provides the thermal radiation that makes the Earth habitable. Because the Sun is our nearest star, it is a well studied object and its inner mechanisms are well understood. The Sun, and the stars in general, are examples for working steady-state fusion reactors that convert protons to heavier elements and radiate the produced energy away. In starswithaboutonesolarmass,theproton-protoncycleburnshydrogenintohelium according to the main nuclear reaction chain p+p → 2D+e++νe2 D+p → 3He3 He+3He → 4He+2p In each cycle there is a resulting energy of 26.21MeV, which is available as heat while 0.51MeV escape with the neutrino. The key features of the Sun are compiled in Table 1.1. Between 1920 and 1950, the understanding of the inner structure of stars has grown in parallel with the development of plasma and nuclear physics. In the center of a star, the high densities and temperatures are sufficient to ignite nuclear fusion reactions. On the other hand, the produced energy keeps the interior hot to provide the pressure that balances the weight of the outer layers and prevents the collapse of the star. The transport of energy to the surface involves radiation and convection. Star spectra give us information about the surface temperature and the chemical composition of a stellar atmosphere, which are linked to the state of evolution of this star. The plasma physics of stellar atmospheres can be found in classical astrophysical textbooks, e.g. [15].

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Table 1.1 Characteristics of the Sun Mass, m 1.989×1030 kg Radius 6.955×108 m Pressure (center) 1.3×109 bar Temperature (center) 15×106 K (surface) 15,000 K (corona) (1–2)×106 K (prominences) (5,000–10,000) K Luminosity 3.90×1026 W Magnetic field (polar) ≈10−4 T (prominences) ≈10−3–10−2 T (sun spots) ≈0.3T Plasma density (corona) 1.7×1014 m−3 (prominences) (1016–1017)m−3

1.2.2 The Active Sun

Already in 1616, Galileo Galilei (1564–1642) had detected dark spots on the Sun. Today, we know that these spots are the footpoints of strong magnetic fields. On a smaller scale, magnetic dipolar structures appear where a plasma-filled magnetic flux tube rises above the solar surface and forms so-called coronal loops. Figure 1.2 showsthelightemissionfromcoronalloopsinthesoftX-rayregimeasobservedby theTransitionRegionandCoronalExplorer(TRACE)satellite.TRACEisamission of the Stanford-Lockheed Institute of Space Research and part of the NASA Small Explorer program. The magnetic fields are produced by a dynamo mechanism that takes its geometry and energy from the Sun’s differential rotation.

Fig. 1.2 Coronal loops filled with hot plasma that emits in the soft X-ray regime. Observed at 17.1nm wavelength by the Transition Region and Coronal Explorer (TRACE) satellite. (Courtesy NASA/TRACE)

6 1 Introduction

Our Sun is an active star. Solar prominences are huge magnetic structures that separate from the solar surface and are filled with plasma. Prominences can last for several days and demonstrate the co-existence of a plasma with a magnetic field. Explosive emission of particles and radiation occurs in solar flares, which is the process of destroying active coronal loops. Figure 1.3 shows the evolution of a flare according to the Sweet-Parker model [16, 17]. The dipolar field of a coronal loop is partially connected to the interplanetary field. The elongated field lines contain magnetic energy, which can be released by reconnection of field lines. The plasma trapped inside the magnetic field is accelerated by the contracting field lines. The largest explosive events on the Sun are coronal mass ejections (CMEs), which release on average 1.6 × 1012 kg of plasma moving at a speed between (200–2700)km s−1. The frequency of CME events varies according to the 11-year sunspotcyclewithtypicallyoneeventperdayatsolarminimumand5–6eventsduring solar maximum. As an example, the CME event of February 27, 2000 (during solar maximum conditions) is shown in Fig. 1.4. The observation was made with the Large Angle Spectrometric Coronograph (LASCO) aboard the SOHO satellite.2 The central disk blocks direct light from the sun. The diameter of the sun is indicated by the white circle. The plasma bubble released in this CME event did not propagate towards the Earth. A new pair of satellites, NASA’s Solar TErrestrial RElations Observatory (STEREO)3 was launched in 2006 to observe, in three dimensions, plasma structures that may be heading towards the Earth. When such plasma bubbles hit the Earth’s magnetosphere, magnetic storms can be triggered, which may lead to disruptions in power line grids by large induced currents and can damage communication satellites. CME’s and the associated high energy particles are a major hazard for astronauts. The CME that hit the Earth on October 30, 2003, expanded the auroral zone, which (in Europe) usually has a southern boundary in

NN S S SS NN (a) (b) (c) (d)

Fig. 1.3 Development of a solar flare in the Sweet-Parker model. (a) The dipolar field of a coronal loop connects to the interplanetary magnetic field. (b) By reconnection of antiparallel field lines the stress of the field lines is released. (c, d) The relaxing magnetic field accelerates the trapped plasma

2 see: http://lasco-www.nrl.navy.mil/ 3 see: http://www.nasa.gov/mission_pages/stereo/main/index.html

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Fig.1.4 CoronalmassejectionofFebruary27,2000asobservedbytheLASCOinstrumentaboard the SOHO satellite. (Courtesy NASA/SOHO)

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