Ground level enhancements (GLEs) are sudden increases in the cosmic ray intensity recorded by ground based detectors. GLEs are invariably associated with large solar flares.
A sharp increase in ground-level cosmic ray count to at least 10% above background, associated with solar protons of energies greater than 500 MeV. GLEs are relatively rare, occurring only a few times
each solar cycle. When they occur, GLEs begin a few minutes after flare maximum and last for a few tens of minutes to hours. Intense particle fluxes at lower energies can be expected to follow this initial burst of relativistic particles. GLEs are detected by neutron monitors, e.g., the monitor at Thule, Greenland.
It is a type of [load](https://n2t.net/99152/h1352) and a historical or synthetic acceleration record of the ground motion to be used as input for a shake table test or a [hybrid_simulation](https://n
2t.net/99152/h1420).
A radar, usually a pulsed system with one transmitting and one receiving antenna, operating at a frequency suitable for imaging the subsurface. In glaciology, low frequencies (2220 mhz) are suitable f
or ice thickness measurements whereas higher frequencies of several hundred mhz are suitable for snow thickness measurements, including detection of the current summer surface and older Annual layering (see radar method). Higher frequencies yield better resolution but may not allow very deep penetration; lower frequencies exhibit the reverse properties. Choice of frequency is therefore paramount. Radar imaging of the subsurface relies on accurate determination of the two-way travel time of the radar wave, which depends on the density. Reflections are caused by contrasts in the (complex) relative dielectric constant at interfaces between layers. Illustrative values of the real part of the relative dielectric constant, at frequencies used by ground-penetrating radars, are 1 for air, ~3.15 for pure ice, ~10 for bedrock and 88 for water at 0
Ground speed is the speed of an aircraft relative to the ground. It is the sum of the aircraft's true airspeed and the current wind and weather conditions; a headwind subtracts from the ground speed,
while a tailwind adds to it. Winds at other angles to the heading will have components of either headwind or tailwind as well as a crosswind component.