Discuss The Effect Of The Solar Wind On The Earth

Cosmic Radiation Field in LEO

Planets and moons of our Solar System are exposed to a complex radiation field of galactic and solar origin (Fig. 11.1). Galactic cosmic radiation (GCR) originates outside of our Solar System in previous cataclysmic events, such as supernovae explosions. When it enters our Solar System, its energies must be high enough to overcome deflection by the magnetic fields of the Solar System. Solar cosmic radiation (SCR) consists of two components, the low-energy solar wind particles that flow constantly from the Sun and the highly energetic solar particle events (SPE) that are emitted from magnetically disturbed regions of the Sun in sporadic bursts.

The surface of the Earth is largely spared from these cosmic radiations because of the deflecting effect of the Earth's magnetic field and the huge shield of 1 000 g m-2 provided by the atmosphere. The terrestrial average annual effective dose equivalent from cosmic rays amounts to 0.30 mSv, which is about 100 times lower than that experienced in LEO.

Sievert (Sv) is a measure of the dose equivalent, i.e., the biologically effective dose of radiation. It is the product of a quality factor specifying the biological effectiveness of a certain radiation quality and the absorbed dose. The absorbed dose is

Space Electron Protons Heavy Ions
Fig. 11.1 Space radiation sources of our Solar System.

measured in Gray (Gy), with 1 Gy being equal to the net absorption of 1J in 1 kg of material (water). In relation to the previously used unit rad, 1 Gy is equal to 100 rad.

11.1.1.1 Galactic Cosmic Radiation

Detected particles of GCR consist of 98% baryons and 2% electrons. Taking the baryonic component as 100%, then it is composed of 85% protons (hydrogen nuclei), with the remainder being alpha particles (helium nuclei) (14%) and heavier nuclei (about 1%). The latter component comprises the so-called HZE particles (particles ofhigh charge Z and high energy), which are defined as cosmic ray nuclei of charges Z >2 and of energies high enough to penetrate at least 1 mm of spacecraft or of spacesuit shielding. Though they contribute to only roughly 1 % of the flux of GCR, they are considered a potential major concern to living beings in space, especially for long-term missions at high altitudes or in high-inclination orbits or for missions beyond the Earth's magnetosphere. Reasons for this concern are based, on the one hand, on the inefficiency of adequate shielding and, on the other hand, on the special nature of HZE particle-produced lesions (explained in Section 11.4.3.3). The fluence of GCR is isotropic and energies up to 1020 eV may be present. When GCR enters our Solar System, it must overcome the magnetic fields carried along with the outward-flowing solar wind, the intensity of which varies according to the about 11-year cycle of solar activity (Fig. 11.2). With increasing solar activity, the interplanetary magnetic field increases, resulting in a decrease in the intensity ofGCR oflow energies. This modulation is effective for particles below some GeV per nucleon. Hence the GCR fluxes vary with the solar cycle and differ by a factor ofapproximately five between solar minimum and solar maximum, with a peak level during minimum solar activity and the lowest level during maximal solar activity.

The fluxes of GCR are further modified by the Earth's magnetic field. Only particles of very high energy have access to low-inclination orbits. At the poles,

Fig. 11.2 Eleven-year cycle of solar activity; the solar proton fluxes are superimposed to the sunspot numbers.

particles of all energies can impinge in the direction of the magnetic field axes. Because of this inclination-dependent shielding, the number of particles increases in LEO from lower to higher inclinations.

11.1.1.2 Solar Cosmic Radiation

Our sun emits two types of radiation: (1) low-energy solar wind particles (mainly protons) that flow constantly from the sun and (2) the so-called solar particles events (SPEs) that originate from magnetically disturbed regions of the sun, which sporadically emit bursts of charged particles with high energies (Figs 11.1 and 11.2). These latter events are composed primarily of protons, with a minor component (5-10%) being helium nuclei (alpha particles) and an even smaller part (1 %) heavy ions and electrons. SPEs develop rapidly and generally last for no more than a few hours; however, some proton events observed near Earth continued over several days. The emitted particles can reach very high energies, up to several GeV. In a worst-case scenario, doses as high as 10 Gy could be received within a short time, which is dangerous to both the spacecraft electronics and the astronauts. Such strong events are very rare, typically occurring about one time during the 11-year solar cycle. Although SPEs frequently occur during solar maximum (Fig. 11.2), they have rarely been observed at other intervals of the solar cycle. The next solar cycle (cycle 24) will peak around the year 2010. Spacecraft in LEO are largely protected from SPEs because the Earth's magnetic field provides a latitude-dependent shielding against SPEs. Only in high-inclination orbits do SPEs create a hazard to humans in space, especially during extravehicular activities. The mechanism is the same as for GCR: particles of all energies can impinge

Fig. 11.3 Effect of solar wind on the Earth's magnetosphere.
Solar Wind Affect Earth

Fig. 11.4 Polar horns and South Atlantic Anomaly, regions of elevated radiation doses in low Earth orbit at 28°, 57° and 90° inclination; numbers give doses at an altitude of 500 km behind 0.2 g cm-2 aluminium shield (100 is equal to approximately 860 mGy perday) (source: http://parts.jpl.nasa.gov/mmic/10.pdf).

Fig. 11.4 Polar horns and South Atlantic Anomaly, regions of elevated radiation doses in low Earth orbit at 28°, 57° and 90° inclination; numbers give doses at an altitude of 500 km behind 0.2 g cm-2 aluminium shield (100 is equal to approximately 860 mGy perday) (source: http://parts.jpl.nasa.gov/mmic/10.pdf).

in the direction of the magnetic field axes. Hence, equatorial orbits provide the highest radiation protection.

11.1.1.3 Radiation Belts

In LEO, in addition to GCR and solar cosmic radiation (SCR), the radiation field comprises a third source of radiation: the van Allen Belts, which result from the interaction of GCR and SCR with the Earth's magnetic field and with the atmosphere. Above all, electrons and protons and some heavier ions are trapped by the geomagnetic field in closed orbits around the Earth (Fig. 11.3). These particles form two belts of radiation, an inner and an outer one, which differ in their type of formation. The main production process for the inner belt particles is the decay of neutrons produced in cosmic particle interactions with the atmosphere. The outer belt consists mainly of trapped solar particles. In each zone, the charged particles spiral around the geomagnetic field lines and are reflected back between the magnetic poles, acting as mirrors. Electrons reach energies of up to 7 MeV and protons up to about 200 MeV. The energy of trapped heavy ions is less than 50 MeV. The trapped radiation is also modulated by the solar cycle: proton intensity decreases with high solar activity, while electron intensity increases, and vice versa.

For space missions in LEO, depending on the orbit parameters and flight data, radiation doses in the range of 20 mSv per month may be received. Of special importance is the so-called South Atlantic Anomaly (SAA), where the fringes of the inner proton radiation belt reach down to altitudes of 400 km (Fig. 11.4). This behavior reflects the displacement of the axis of the geomagnetic (dipole) field by about 450 km with respect to the axis of the geoid (rotation axis), with a corresponding distortion of the magnetic field. This SAA region accounts for the dominant fraction - up to 90% - of the total radiation exposure of spacecraft in LEO.

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