A galaxy that produces huge amounts of energy at its center, which cannot be attributed to normal processes from stars, interstellar medium, and their interactions. There are several types of active g
alaxies: Seyfert galaxies, quasars, and blazars. All of these objects show brightness variations, some as short as 3 hours. These fluctuations indicate a relatively very small size for the central object, because an object cannot vary in brightness faster than light can travel across it. For example, an object that is one light-year in diameter cannot vary significantly in brightness over a period of less than one year.
An Ice Wedge is termed 'active' if it is growing as a result of repeated (but not necessarily annual) winter cracking in Ground Ice containing Permafrost. Active Ice Wedges developed in mineral soil o
ccur primarily in areas of Continuous Permafrost.
The Active Layer is a layer of ground that is subject to annual thawing and freezing in areas underlain by Permafrost. In the zone of Continuous Permafrost the Active Layer generally reaches the Perma
frost Table; in the zone of Discontinuous Permafrost it often does not. The Active Layer includes the uppermost part of the Permafrost wherever either the salinity or clay content of the Permafrost allows it to thaw and refreeze annually, even though the material remains cryotic (T < 0C). The Active Layer depth may vary considerably from one year to another depending on a number of variables (cf. Active Layer Thickness). If the ground thermal regime is no longer in long-term climatic equilibrium, Active Layers tend to show trends of increasing thickness.
That part of the soil included with the suprapermafrost layer (i.e., existing above permafrost) that usually freezes in winter and thaws in summer. Its bottom surface is the frost table, beneath which
may lie permafrost or talik. The depth of the active layer varies anywhere from a few inches to several feet. (Also called frost zone, mollisol.)
A general term referring to several forms of slope failures or failure mechanisms commonly occuring in the Active Layer overlying Permafrost. Such failures are often triggered by loss of shear strengt
h due to high pore water pressures which result because the underlying Permafrost is quasi-impermeable.
The thickness of the layer of the ground that is subject to annual thawing and freezing in areas underlain by Permafrost. The thickness of the Active Layer depends on factors such as the ambient air t
emperature, vegetation, drainage, soil or rock type (thermal properties of soil or rock), total water content, Snow Cover, and degree and orientation of slope. The thickness of the Active Layer can vary from year to year, primarily due to variations in the mean annual air temperature, distribution of soil moisture, and Snow Cover. The thickness of the Active Layer includes the uppermost part of the Permafrost wherever either the salinity or clay content of the Permafrost allows it to thaw and refreeze annually, even though the material remains cryotic (T < 0C).
A range of heliographic longitudes in either the northern or southern solar hemisphere (seldom both at the same time) containing one or more large and complex active regions formed by the frequent, lo
calized emergence of new magnetic flux. Individual sunspot groups within the complex can have relatively short lifetimes (a week or two); the complex may persist for several solar rotations because additional spot groups form as earlier ones decay.