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Auroral hot-ion dynamo model with finite gyroradii
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Image of FIG. 1.
FIG. 1.

(Color) Differential energy flux of protons (color scale to the right; white is saturation), averaged over view angles , as a function of energy ( axis; ) and time ( axis; interval), during a Polar crossing from the northern lobe into the plasma sheet near midnight MLT. Flux near the top is earthward bursts, flux below consists of up-flowing protons. Note: The (two-spin) resolution (vertical ribbons) corresponds to satellite travel, equaling two local gyroradii of a proton at 90° pitch angle (adapted from Ref. 12).

Image of FIG. 2.
FIG. 2.

Earth’s magnetic field is assumed directed along the negative axis of a local Cartesian coordinate system, whose axis points from low (on the left) to high plasma density, while density is assumed constant along the axis. Perpendicular velocity vector and gyrocenter are shown for a proton.

Image of FIG. 3.
FIG. 3.

(Top) Schematic ion phase space density as function of azimuth angle , showing higher (darker) density of ions with gyrocenters to the right. The average of over defines the pitch-angle distribution . (Bottom) Number density (solid line), assuming that the associated gyrocenter velocity distribution function is isotropic (and isothermal) and that the scale length is a few average gyroradii . The gyrocenter spatial density is indicated by the dashed line. The magnetic field vector is pointing downward, along the negative axis, together with the field gradient. Whereas , because of antisymmetry, it follows that and .

Image of FIG. 4.
FIG. 4.

(a) Dashed straight line is the assumed density profile of proton gyrocenters in a magnetic field of (at equatorial plasma source). It spans a distance in that is twice the length of a thermal gyroradius , with . The distribution function is assumed to be isotropic (neglecting small loss cone) and have a Maxwell-Boltzmann-type dependence on at temperature . Solid curved line shows the material density . (b) Matching material density at of electrons with (; within the linewidth for electrons). (c, d) Corresponding density distributions at =280 nT, assuming that both proton and electron gyrocenters move along magnetic field lines and that the field lines have converged in by a factor of 0.04. (e) Difference between proton and electron densities at =280 nT due to reduced gyroradii (by factor ). (f) Resulting electric field if normalized density is multiplied by (from Ref. 18).

Image of FIG. 5.
FIG. 5.

(a) Earthward approach of paired charge layers. Small arrows indicate the electric field lines. (b) Corresponding equipotentials. Positive charge layer counteracts the local ambipolar field, increasing thermal escape rate of electrons from the topside ionosphere. (c) Conducting parts of the ionosphere become positively charged, attracting hot electrons that would otherwise mirror in the converging magnetic field.


Generic image for table
Table I.

First-invariant induced charge separation in a dipolar magnetic field at .

Generic image for table
Table II.

First-invariant induced charge separation in a dipolar magnetic field at .


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Auroral hot-ion dynamo model with finite gyroradii