Identification of a Thunderstorm as a Source of Short Period Gravity Waves in the Upper Atmospheric Nightglow Emissions

EISCAT: early history and the first ten years of operation

Stably stratified fluids, such as stellar and planetary atmospheres, can support and propagate gravity waves. On Earth these waves, which can transport energy and momentum over large distances and can trigger convection, contribute to the formation of our weather and global climate. Gravity waves also play a pivotal role in planetary sciences and modern stellar physics. They have also been proposed as an agent for the heating of stellar atmospheres and coronae, the exact mechanism behind which is one of the outstanding puzzles in solar and stellar physics. Using a combination of high-quality observations and 3D numerical simulations we have the first unambiguous detection of propagating gravity waves in the Sun's (and hence a stellar) atmosphere. Moreover, we are able to determine the height dependence of their energy flux and find that at the base of the Sun's chromosphere it is around 5 kW m−2. This amount of energy is comparable to the radiative losses of the entire chromosphere and points to internal gravity waves as a key mediator of energy into the solar atmosphere.

https://iopscience.iop.org/article/10.1086/590495/pdf

The energy flux of internal gravity waves in the lower solar atmosphere

Geomagnetic pulsations associated with the polar cusp have been detected at Cape Parry, Northwest Territories, Canada, using flux gate and induction magnetometers. Data from this station show enhanced power almost every day near local noon. It is this enhancement which is taken as the signature of the polar cusp. Maximum signal levels are observed prior to local noon, although the signals are present for several hours on each side of local noon. The spectrum of pulsations near the cusp is broadband, reaching from a few millihertz to above 1 Hz. On average the power spectrum decreases with frequency as f−2.6. Within this general decrease there are often found regions of enhanced power, principally near 5 mHz and near 40 mHz. We have identified the 5-mHz band with fluctuations in ionospheric currents associated with the cusp boundary and its motions. The 40-mHz band is presumed to be due to direct penetration of the hydromagnetic radiation in the cusp to the ground. From the measured spectrum we infer an rms fluctuation amplitude of the cusp currents of approximately 3×10³ A, and from this we estimate the total power dissipated by these fluctuations of approximately 4×107 W.

ULF signatures of the polar cusp

The generation and propagation of atmospheric gravity waves observed during the Worldwide Atmospheric Gravity-wave Study (WAGS)

Atmospheric gravity waves play an important role in transporting energy from one region of the atmosphere to another. But the sources of these waves and their propagation characteristics are not fully understood. Here we report on measurements made during the Worldwide Atmospheric Gravity Wave Study (WAGS) in the auroral source region showing that enhanced Joule heating and Lorentz forcing lead to the generation of large-scale thermospheric waves. Both of these source mechanisms depend on the auroral electric field and the intrinsic periodicity of that field determines the periodicity of waves observed an hour later propagating southwards over the UK. These observations provide the first confirmation of a link between a periodic auroural source and a large-scale gravity wave.

Observations of the source and propagation of atmospheric gravity waves

Geomagnetic micropulsations are fluctuations of the Earth's magnetic field on time-scales ranging from a few seconds to several minutes and with amplitudes varying between a few and several hundred nanotesla. In the inner magnetosphere, where the Earth's magnetic field lines are closed, the oscillations can adopt a standing wave pattern with a scale size comparable to the dimensions of the magnetospheric cavity. These hydromagnetic waves constitute one of the primary mechanisms for the transfer of energy from the outer regions of the magnetosphere to the high-latitude ionosphere during geomagnetic substorms. In the ionosphere, Joule heating is thought to be the dominant damping process for the pulsations and the rate of energy dissipation during a typical event1 is of the order of 109 W. By means of the EISCAT (European Incoherent Scatter) radar facility it has been possible to measure directly the ionospheric ion-temperature and ion-velocity fluctuations during a micropulsation. The relationship predicted by the theory of Joule heating for these two parameters is confirmed in detail by the high time-resolution results reported here.

G. Crowley

Papers

Observations of the source and propagation of atmospheric gravity waves

High time-resolution observations of periodic frictional heating associated with a Pc5 micropulsation