Abstract
In the Earth’s inner magnetosphere, there exist regions like plasmasphere, ring current, and radiation belts, where the population of charged particles trapped along the magnetic field lines is more. These particles keep performing gyration, bounce and drift motions until they enter the loss cone and get precipitated to the neutral atmosphere. Theoretically, the mirror point latitude of a particle performing bounce motion is decided only by its equatorial pitch angle. This theoretical manifestation is based on the conservation of the first adiabatic invariant, which assumes that the magnetic field varies slowly relative to the gyro-period and gyro-radius. However, the effects of gyro-motion cannot be neglected when gyro-period and gyro-radius are large. In such a scenario, the theoretically estimated mirror point latitudes of electrons are likely to be in agreement with the actual trajectories due to their small gyro-radius. Nevertheless, for protons and other heavier charged particles like oxygen, the gyro-radius is relatively large, and the actual latitude of the mirror point may not be the same as estimated from the theory. In this context, we have carried out test particle simulations and found that the L-shell, energy, and gyro-phase of the particles do affect their mirror points. Our simulations demonstrate that the existing theoretical expression sometimes overestimates or underestimates the magnetic mirror point latitude depending on the value of L-shell, energy and gyro-phase due to underlying guiding centre approximation. For heavier particles like proton and oxygen, the location of the mirror point obtained from the simulation deviates considerably (∼ 10°–16°) from their theoretical values when energy and L-shell of the particle are higher. Furthermore, the simulations show that the particles with lower equatorial pitch angles have their mirror points inside the high or mid-latitude ionosphere.
Highlights
Some of the solar wind charged particles enter the Earth’s magnetosphere and get trapped along the magnetic field lines
∼ eV range particles are seen in the plasmasphere, ∼ 1 − 100 keV in the ring current region, and the most energetic particles, > 100 keV are dominated in the radiation belt regions (Ebihara and Miyoshi 2011; Millan and Baker 2012)
In order to investigate the effects of L-shell, energy, and gyro-phase of the charged particles on their magnetic mirror point latitudes, we have performed the simulation for electrons, protons and oxygen ions with energy 5 keV to 5 MeV at L = 3 to 6 and ψ = 0◦ to 360◦, and tracked their trajectories
Summary
Some of the solar wind charged particles enter the Earth’s magnetosphere and get trapped along the magnetic field lines. Plasmasphere, ring current, radiation belts and so on. ∼ eV range particles are seen in the plasmasphere, ∼ 1 − 100 keV in the ring current region, and the most energetic particles, > 100 keV are dominated in the radiation belt regions (Ebihara and Miyoshi 2011; Millan and Baker 2012). It was shown that in the Earth’s inner magnetosphere, where the magnetic field is assumed to be nearly dipolar, the charged particles undergo three quasi-periodic motions. These are, gyro-motion around its guiding centre, bounce motion along the magnetic field lines between the conjugate mirror points in the northern and southern hemispheres, and longitudinal
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