Space Weather | Talks | Dr. James D. Borderick

Space Weather

The Sun not only outputs heat and light in the form of electromagnetic radiation but constantly streams out plasma in the form of the solar wind. The solar wind plasma is composed of protons and alpha particles, together with approximately equal numbers of electrons such that the solar wind is overall electrically neutral. The solar wind outflow carries the Sun's magnetic field with it and, therefore, the solar magnetic field is said to be fixed or "frozen-in" to the solar wind plasma. The embedded magnetic field is called the Interplanetary Magnetic Field (IMF). The solar wind outflow is purely radial. The foot of each field line is fixed to the solar surface, which is rotating with a period of roughly 27 days. The rotational period is, in fact, faster at the equator and decreases with latitude. Since the surface is rotating, a spiral pattern is produced: the plasma element at the Sun's surface begins to flow radially outwards at the solar wind speed. At a later time, the magnetic field line is still linked to the plasma and the same location on the solar surface.

Solar-terrestrial relations involves the coupling of two principal components. These are the terrestrial magnetic field and the IMF. The terrestrial magnetic field is generated by currents flowing in the core of the Earth. Outside the Earth, the magnetic field is roughly dipolar and is tilted to the axis of rotation by about 11 degrees. The solar wind plasma confines the terrestrial magnetic field into the protective "bubble" known as the magnetosphere. The geometry of the magnetosphere clearly affects the level of protection from the solar wind.

The processes acting at the magnetopause are the ones finally responsible for determining how much energy the magnetosphere receives from the solar wind. To a first approximation, the magnetosphere contains two magnetically neutral points called "cusps," which are located at approximately 77 degrees magnetic latitude where solar wind particles gain direct access to lower altitudes within the magnetosphere and may propagate into the ionosphere. However, a more complete picture of solar-terrestrial coupling was described by the Dungey cycle, which will be outlined shortly. Clearly, the magnetosphere is not a closed system, with no entry points, but is in fact a highly dynamic region of flowing plasma guided by the terrestrial magnetic field, which at times connects to the IMF allowing an entry point for plasma, energy and momentum.

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