A type of air mass with characteristics developed over high latitudes, especially within the subpolar highs. Continental polar air (cP) has low surface temperature, low moisture content, and, especial
ly in its source regions, great stability in the lower layers. It is shallow in comparison with arctic air. Maritime polar air (mP) initially possesses similar properties to those of continental polar air, but in passing over warmer water it becomes unstable with a higher moisture content.
In climate modeling studies, the tendency for simulated temperature changes to be larger at high latitudes, as in the case of the warming induced by increased greenhouse gases.
The angle between the position or measuredvector and the k-axis of the coordinate system. Mathematically defined asarctan([SQRT(i^2+j^2)]/k). This term could be also applied to angles betweenthe vecto
r and other components, for example the IMF cone angle defined asarccos(Bx/Bt).
The areas of the globe surrounding the polesand consisting of the region north of 60 degrees north latitude an theregion south of 60 degrees south latitude.
An anomalous condition of the polar ionosphere where HF and VHF (3-300 MHz) radiowaves are absorbed, and LF and VLF (3-300 kHz) radiowaves are reflected at lower altitudes than normal. PCAs generally
originate with major solar flares, beginning within a few hours of the event and maximizing within a day or two of onset. As measured by a riometer, the PCA event threshold is 2 dB of absorption at 30MIlz for daytime and 0.5 dB at night. In practice, the absorption is inferred from the proton flux at energies greater than 10 MeV, so that PCAs and proton events are simultaneous. However, the transpolar radio paths may be disturbed for days, up to weeks, following the end of a proton event.
Effect on radio waves in high latitudes, caused by a sudden increase in the electron concentration in the D layer of the ionosphere. The effect is produced by the influx of high energy solar protons;
it begins a few hours after a solar flare is observed and may in exceptional cases persist for a few days.
Episodes of intense absorption of HF and VHF radio waves over the polar caps caused by excess ionization resulting from an influx of solar energetic particles into the upper atmosphere. PCA events hav
e a pronounced diurnal variation, being much more intense during daytime than at night, and typically last for two to three days. (Abbreviated PCA events.)