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| Auroral-particles precipitation |
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The auroral radiation is emitted by atmospheric constituents that are excited by precipitating particles. The primary auroral particles, populations of electrons and ions with energy from ~ 10 eV up to small multiples of 100 keV, can be measured directly by the use of instrumented rockets and satellites.
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Some of these will precipitate into the atmosphere, causing atmospheric excitation and ionization. In the near Earth space such particles are found mainly above 55o magnetic latitude (Λ).
They have their sources in the plasma sheet and in the polar-cusp region. As the high energy tail (> 30 keV for electrons & 1MeV for ions) is not important for the auroral emissions, it will not be discussed further here.
The rate of precipitation of auroral particles into the polar atmosphere is schematically illustrated in Figure 5.2. The dots represent mainly > 30 keV particles and the triangles the medium-energy – 0,5 to 30 keV responsible for the visual aurora. The stars mark particles with energy < 1 keV that enter the polar ionosphere through the cusp, causing the dayside oval aurora. This figure is important for understanding the general auroral characteristics – its temporal and spatial distributions. The most energetic particles lie on a circle zone of nearly constant latitude.
The medium-energy particles lie on an oval zone tipped back away from the Sun, as would particles accelerated along the Earth’s magnetic field at polar latitudes . The low-energy particles are confined to the footprint of the midday field lines that could funnel solar-wind particles directly into the upper atmosphere with minimum acceleration. New, extensive observations show overlapping of principal zones and gradual transition from one to another. The strong asymmetry in the location of the auroras at night to the dayside gives the impression of an oval-shaped region. Precipitating ions producing auroras show a dawn-dusk asymmetry, displaced toward dusk with respect to the auroral electrons.
Figure 5.2a: The precipitation of auroral particles as function of latitude and diurnal time is shown. The dots represent mainly > 30 keV particles and the triangles the medium-energy – 0,5 to 30 keV, responsible for the visual aurora. The stars mark particles with energy < 1 keV that enter the polar ionosphere through the cusp, causing the dayside oval aurora. |
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| Fluxes of auroral particles measured by rockets at ARS. |
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In the dayside cusp-region – i. e. between 70 and 80ø Λ , the average energy is < 1 keV - similar to those of the magnetosheath.
They produce the dayside cusp auroras in a narrow region centered at ~78ø Λ.,
stretching for about +/- 2 hours around magnetic noon.
A fraction of these particles will have their mirror points in the loss region. The precipitating particles gradually lose their energy to the atmosphere – the main sink for the energetic particles. Precipitating charged particles in the ionosphere are subject to inelastic and elastic collisions with the constituents.
They lose their energy gradually by
1) ionizing and exciting the upper atmosphere
2) dissociating atmospheric molecules
3) heating the upper atmosphere, and
4) producing bremsstrahlung X-rays. The energy depositing is thus also used to produce optical emissions – i. e. auroras. |
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Experimental data show that fast electrons (e) and protons (i) produce about one ion pair (1 ion & 1 electron) per 36 eV of their initial energy. The particle ionization due to energetic e and i can be written symbolically as: |
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The two equations can be read in the following way: Energetic auroral particles collide with the atmospheric constituents – molecule or atom. The result of this reaction is a positive ion and a free electron with very low energy. e’ and i’ are marked with ‘ because the original particles have lost some energy in these collisions.
en and in have very little energy and are called a thermal electron and ion, respectively. On average about 15 eV, about 40 % goes into ionization, where as about 55 % goes into motion, while only ~ 2 percent of the energy goes into excitation. |
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