Equatorial Plasma Drifts: Dance your Ph.D. 2011

My dissertation uses measurements of ion velocities taken on-board the Communication/Navigation Outage Forecasting System (C/NOFS) satellite to describe the movement of plasma between the northern and southern hemispheres along terrestrial magnetic field lines. These measurements were all made close to the equator during a period of extremely low solar activity, at altitudes just above that of the peak plasma density (a part of the atmosphere known as the topside ionosphere). Very few ion velocity measurements are available for these conditions, making the results useful for improving and evaluating current space weather models. I am also using these results to provide constraints on the physical processes that cause the ion motions, as measurements for these quantities are still scarce. My dance illustrates how these physical processes cause ion drift over the course of a typical day and culminates with the measurement of an ion by the satellite.

In the dance ions are represented by the dancers in red shirts and electrons by the dancers in black dresses. The ion-electron couples dance bachata when they are bound together in a neutral particle (an atom or a molecule) and tango when the ions and electrons are only tied together by an electric field, which prevents divergent currents from forming and allows us to refer to the field-aligned ion or electron velocities as plasma drifts.

The bachata dancers (neutral particles) can only convert to tango dancers (ion-electron pairs) when the electron gains energy from an outside source and the neutral particle is ionized. Most of the time this source is the sun. Solar photons are represented by freestyle dancers in yellow and gold, who travel from the sun to the Earth's atmosphere and interact with the electrons. Once an electron gains enough energy, both it and the ion are bound to move along the magnetic field lines. Just after sunrise, when there are many neutral particles present to be ionized, so many ions are created at the foot of any given field line that the pressure created by this change in the plasma density causes some of the largest plasma drifts to be formed. These drifts send the ions and electrons towards the equator and subside when the plasma is equally distributed along the field line. After sunset, when solar photons can no longer reach the atmosphere, the influence of loss processes on the plasma drift becomes more important.

Large electric fields form lower in the ionosphere and are represented by the dancers in white shirts. In this part of the choreography the magnitude and direction of the electric field is specified by the height of the dancers' hands. The lower the hands the stronger the westward electric field, the higher the hands the stronger the eastward electric field. The electric field interacts with the ions and the magnetic field to produce plasma drifts perpendicular to the magnetic field lines. Near the equator these motions are basically vertical and will only cause plasma drifts when the electric field is strong enough to move all of the plasma along one magnetic field line to the next magnetic field line. When this happens during the day the westward electric field causes the plasma to jump upwards. Since this causes the ion-electron couples to spread out the change in density allows the plasma to fall away from the equator to lower altitudes. The situation reverses at night. When other physical processes that cause drifts between the northern and southern hemispheres are acting on the plasma, however, jumping between magnetic field lines will cause the plasma to either drift slower (when moving onto a higher field line) or faster (when moving to a lower field line) from one hemisphere to the other.

The final physical process demonstrated in this choreography is the plasma drift created by the movement of neutral particles lower down in the atmosphere. The motion of neutral particles is largely driven by temperature, with hot particles flowing to cooler locations and away from the spot on the earth closest to the sun. When the direction of this flow also aligns to some degree with the magnetic field, the collisions between ions and neutral particles will cause the ions to start moving along the magnetic field line in the same direction as the neutral particles. These collisions have the most impact below the height of the plasma density peak, since there are more neutral particles there. In the dance the effects of the motion of the neutral particles on the ions in the topside ionosphere is shown, since these are the ions measured and studied for my dissertation.

Category:

Science & Technology

Tags:

• Physics
• Tango
• Bachata
• CINDI
• Ionosphere

[giantnegro]

C/NOFS was funded by the DoD Space Test Program, the Defense Meteorological Satellite Group and the United States Air Force. Funding for the Coupled Ion Neutral Investigation was provided by NASA.

[ilmareofthemaiar]

AWESOME!!!!