Penetration Electric Fields

The convection electric field, set up in the magnetosphere by the interaction of the solar wind plasma flowing around the Earth's magnetic field, maps to ionospheric heights along mostly equipotential magnetic field lines as a two-cell convection pattern. This high-latitude electric field is shielded from the low-latitude ionosphere by the action of the region-2 field-aligned currents under geomagnetically quiet or steady conditions. As a result, the ionospheric electric field during quiet times is mostly due to the dynamo action of the neutral atmosphere. During active times, however, the ionosphere is subjected to magnetospheric electric fields.

Penetration electric fields are the electric fields of solar wind/magnetospheric origin observed equatorward of the shielding layer. The shielding layer acts to shield the region of equatorward (earthward) of it from the magnetospheric convection electric field and is located in the inner edge of the plasma sheet/ring current. Pressure gradients in the inner edge of the plasma sheet and ring current produce the region-2 field-aligned currents that close through the conducting ionosphere, leading to a dusk-to-dawn electric field across the inner magnetosphere that opposes the dawn-to-dusk convection electric field. This electric field, under the condition of steady magnetospheric convection, will mostly cancel the driving magnetospheric convection electric field. Overshielding is a process that results in a generally dusk-to-dawn electric field equatorward (earthward) of the shielding layer. During southward IMF, a dusk-to-dawn shielding electric field occurs in the inner magnetosphere. When the IMF abruptly turns northward, the magnetospheric convection electric field decreases very quickly. However the shielding electric field persists for a while and produces a net dusk-to-dawn electric field within the inner magnetosphere and low-latitude ionosphere. This electric field is said to be caused by overshielding. The efficiency of the shielding process by the region-2 field-aligned currents appears to be very limited during the main phase of geomagnetic storms with continuously southward IMF for prolonged periods (many hours). The magnitude of penetration electric field depends on the strength of the magnetospheric convection driven by the solar wind/IMF.

Penetration electric fields have very important effects on the global plasmasphere and ionosphere during magnetic storms. An eastward penetration electric field in the dayside ionosphere will move the plasma particles to high altitudes at middle and low latitudes, resulting in the formation of high electron density and positive phase of ionospheric storms at middle latitudes. The penetration electric field will also enhance the equatorial fountain effect and cause electron density decreases over the equator and density increases in the anomaly region.

Both penetration and neutral disturbance dynamo electric fields occur at low latitudes during magnetic storms. Timing is a critical factor to separate the different sources. For the first a few (2-3) hours, penetration electric field can cause ionospheric disturbances simultaneously at all latitudes and dominate the dayside ionospheric evolution. In contrast, large-scale atmospheric gravity waves take 2-3 hours to travel from the auroral zone to the equatorial ionosphere, and a significant propagation delay can be identified at different latitudes. After 2-3 hours from storm commencement, neutral disturbance dynamo electric field will become important at low latitudes. Neutral disturbances play a critical role in the generation of negative ionospheric storms. It is widely accepted that negative phases of ionospheric storms are caused by neutral composition changes. However, negative phases of ionospheric storms occur many hours after the storm sudden commencement, on the second day, and there is no negative storm phase in the dayside ionosphere within the first several hours of magnetic storms.

Workshop on Penetration Electric Fields
and Their Effects in
the Inner Magnetosphere and Ionosphere
MIT Haystack Observatory
November 7-9, 2005.

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