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Complex Systems
Published in Pier Luigi Gentili, Untangling Complex Systems, 2018
Complex Systems are networks that are out-of-equilibrium. They can be either open or closed or isolated (for instance, our Universe is postulated to be an isolated thermodynamic system). They are maintained out-of-equilibrium by external and/or internal gradients of intensive variables. For example, Earth is out-of-equilibrium due to three principal contributions. The first is the gravitational field generated by the sun and the moon. The second is the thermal energy released by the processes of nuclear fissions involving unstable radionuclides that are beneath the terrestrial crust. The last contribution is the electromagnetic radiation and the wind of particles and gamma rays that come from the sun. The sun is maintained out-of-equilibrium by the nuclear fusion reactions occurring in its inner core. Through the so-called proton-proton chain reactions, hydrogen converts to helium, and vast amounts of thermal energy are unleashed.10 The thermal energy produced within the core of our star migrates by irradiation and convection towards the external surface, the so-called photosphere. The photosphere of our sun has an average temperature of 5,777 K (NASA website) and emits thermal radiation whose frequencies belong to the UV, visible and near-IR regions of the electromagnetic spectrum (see Figure 12.7). The solar thermal radiation is the primary power source for our planet (Kleidon 2010). For this reason, it is essential to know its thermodynamic properties.
Incident Radiation
Published in Robert P. Bukata, John H. Jerome, Kirill Ya. Kondratyev, Dimitry V. Pozdnyakov, of Inland and Coastal Waters, 2018
Robert P. Bukata, John H. Jerome, Kirill Ya. Kondratyev, Dimitry V. Pozdnyakov
The photosphere is the apparent solar surface and also includes the lowest layer of the solar atmosphere. The outside diameter of the photosphere is taken to be the diameter of the sun, namely 1.3914 × 109 m. The photosphere contains most of the solar mass and is, effectively, an optical boundary below which the solar gas is opaque. The chromosphere is a layer (∼10,000 km thick) of transparent, glowing gas above the photosphere, and the corona or solar crown is the faint white halo in the region above the chromosphere. It is this halo that becomes visible during a total solar eclipse when the glare of the chromosphere is temporarily arrested. The “true” corona (there is also a component of the corona which is a consequence of sunlight scattered by dust particles that are at great distances from the sun and are not part of the solar atmosphere) is an irregular halo that surrounds the sun to a mean distance of approximately 1 solar radius, as well as local streamers that extend several solar diameters into space.
Introduction to heat transfer
Published in Tariq Muneer, Jorge Kubie, Thomas Grassie, Heat Transfer, 2012
Tariq Muneer, Jorge Kubie, Thomas Grassie
The temperature at the Sun’s surface, that is, the photosphere, is around 6000°C. However, moving away from the photosphere does not lower the temperature. In the 3000-km zone between the photosphere and the corona, the temperature jumps from 6000°C to over a million celsius. In apparent defiance of common sense, the further away you go from the Sun’s surface into the solar atmosphere, the warmer the gas becomes! There have been many theories postulated that try to explain the extraordinary warmth of the Sun’s corona. The more popular of these fall into three categories, namely miniature solar flares, atmospheric waves, and electrical dissipation. A detailed discussion of these theories is beyond the scope of the present text and the reader is therefore directed to the NASA (1999) website.
Helicity and winding fluxes as indicators of twisted flux emergence
Published in Geophysical & Astrophysical Fluid Dynamics, 2021
Figure 5 shows the field line helicity rate distributions for times representing key phases in the helicity evolution. Figures 5(a)–(c) reveal an evolution that is similar qualitatively to that found in non-convective simulations. As the magnetic field first reaches the photosphere (figure 5(a)), there is an approximate balance of positive and negative helicity. Later (figure 5(b)), as emergence of the tube with left-handed twist proceeds, the helicity rate distribution is dominated, naturally, by negative helicity. Later still (figure 5(c)), the twisted core of the flux tube reaches the photosphere and this feature dominates the negative helicity rate signature. Although the band of negative helicity rate density, representing the twisted core, follows the sigmoidal polarity inversion line clearly, it also shows locations of disruption, particularly near (0,0).