Real-Time Auroral Images
The second-generation Space Environment Monitor (SEM-2) onboard the NOAA Polar Orbiting Environmental Satellite (POES) orbits the Earth in a high-inclination (polar), sun-synchronous orbit at about 800 km altitude. In addition to the Total Energy Detector (TED) that provides the data used to determine auroral activity, it contains a Medium Energy Proton and Electron Detector (MEPED) that monitors the intensities of charged particle radiation at higher energies extending up to cosmic rays. These higher energy particles can produce ionization deep within the Earth's atmosphere that can degrade radio communications (occasionally making short wave radio communication impossible in the polar regions), can occasionally disrupt the proper operation of satellite systems, and, when intensities are high, increase the radiation dose to astronauts in space.
The Space Environment Center processes the particle counts received from the POES MEPED sensors into color-coded plots showing the intensity of recently measured counts for the current day and also for the three previous days, in comparison to a one-year average of similar counts from the same sensor taken at the same geographic location. This type of plot provides an instant estimate of whether the current particle environment is more, or less, intense than usual.
D Layer Absorbtion
Long-range communications using high frequency (HF) radio waves (3 - 30 MHz) depend on reflection of the signals in the ionosphere. Radio waves are typically reflected near the peak of the F2 layer (~300 km altitude), but along the path to the F2 peak the radio wave signal suffers attenuation due to absorption by the intervening ionosphere.
Absorption is the process by which the energy of radio waves are converted into heat and electromagnetic (EM) noise through interactions between the radio wave, ionospheric electrons, and the neutral atmosphere (For a more extensive description of the absorption process see Davies (1990)). Most of the absorption occurs in the ionospheric D region (50 - 90 km altitude) where the product of the electron density and the electron/neutral collision frequency attains a maximum. Within this region the neutral density is relatively constant over time, so variations in the local electron density drive the total amount of absorption. The electron density is a function of many parameters and normally varies with local time, latitude, season, and over the solar cycle. These "natural" changes are predictable, and affect absorption only moderately at the lowest HF frequencies. Much more significant changes to the electron density, and therefore the absorption strength, are seen as a result of solar x-ray flares (the classic short wave fade).
Solar x-ray flares have significant emission in the 0.1-0.8 nm [1-8 Å] wavelength range. This is important because these wavelengths ionize the D region, dramatically increasing local electron density, and hence the total EM absorption. The flares, which can last from a few minutes to several hours, are rated C, M, or X according to the 0.1-0.8 nm flux as measured by instruments on the GOES satellites. To qualify as a C-class flare the flux, F, must fall within the range 10-6 <= F < 10-5 W m-2, for M-class 10-5 <= F < 10-4, and X-class 10-4 <= F. In standard notation the letters act as multipliers, for example C3.2 equates to a flux of 3.2 x 10-6 W m-2.
The C, M, and X classification is based on the full-disk x-ray emission from the sun. During periods of high solar activity, such as solar maximum, the background flux may increase to C-class levels for days at a time, even without flare activity. The D region electron density is directly driven by the total x-ray flux regardless of the source, so these periods of high background flux are equally important to radio absorption.
Due to geometric effects, D region ionization is greatest at the sub-solar point, where the sun is directly overhead. The amount of ionization and absorption falls with distance away from the sub-solar point, reaching zero at the day/night terminator. The night-side of the Earth is unaffected.
The D Region Absorption Product