Low-altitude Polar-orbiting Satellite Research Articles

Solar particle cutoffs as observed at low altitudes

Differential energy measurements were made on board the low-altitude (∼500 km) polar-orbiting satellite OV1-18 of protons (1.4–46 Mev) and α particles (7–22 Mev) during several solar particle events in 1969 and 1970. The measured 1/e cutoffs for ∼2-Mev protons show the same general variation with local time as those calculated by Smart et al. (1969) but are displaced to somewhat lower latitudes, even at times of low magnetic activity. The rate of change of cutoff latitude with rigidity is generally less on the nightside than on the dayside, with the α particle 1/e cutoffs being consistent with the cutoffs for protons of the same rigidity. At a given particle energy the curves of flux versus latitude are nearly always steeper on the nightside than on the dayside, with large variations from one pass to the next sometimes occurring in the slopes of the flux profiles near cutoff. The profiles of counting rate versus latitude are often smooth near the boundary, but sometimes pronounced flux and spectral variations with time and/or latitude are observed. At low latitudes on the dayside, protons with pitch angle distributions peaking near 90° appear even at positions where the calculated B,L traces of the observation points show that the particles could have drifted westward, at a constant μ, only a few degrees in longitude. On the dayside, the fluxes of trapped protons that could have been injected on the nightside often extend to lower latitudes than those that could not have drifted very far in longitude, thus suggesting that these protons were indeed injected on the nightside and that they underwent diffusion as they drifted in longitude.

High-resolution studies of electrons 25 kev to 2.7 Mev precipitating at high latitudes

At high latitudes quasi-trapped electrons (with minimum altitudes below sea level) have been measured with high resolution in time and energy from a low-altitude polar-orbiting satellite. The data were taken during a period of low geomagnetic activity with several low-energy (>25 kev) detectors and a plastic scintillation spectrometer spanning the interval from 100 kev to 2.7 Mev with high sensitivity. Significant fluxes of quasi-trapped electrons were continually observed on several passes through the outer belt during this period. The intensities as a function of observation time were often rather steady but occasionally underwent appreciable changes on time scales as short as 50 msec (or distances of 400 meters). The quasi-trapped electrons were never observed to display a pronounced monoenergetic spectrum, nor a sharp spectral structure.

Gravity waves observed by ionospheric temperature measurements in theFregion

Ion temperature measurements below the F peak have revealed the presence of a fine structure in the ionospheric temperature. The measurements were made with an Ion Energy Analyzer carried aboard a low-altitude polar-orbiting satellite. The temporal resolution of the Ion Energy Analyzer was 3 seconds corresponding to a horizontal spatial resolution of approximately 23 km. At the altitudes of these measurements the ions and neutral species of the atmosphere have a common temperature; thus the temperature structure present in the ion temperature must also be present in the neutral atmosphere as well. The temperature structure is observed to have a wave-like character with wavelength of the order of 200 km. The fractional temperature variation is typically 13%. Much of the observed wave structure is interpreted as internal atmospheric gravity waves, and the results obtained here are compared with the existing theory. A large portion of the wave structure observed cannot be explained on the basis of the simple linearized gravity wave theory. This wave structure is either the result of some as yet unspecified phenomenon or is the result of gravity waves in a real atmosphere where the idealized conditions and restriction from linearization are removed.

Electron precipitation in the radiation belts

Fluxes and energy spectra are presented for electrons above an energy of ∼0.4 Mev on a worldwide basis over the range of L values 1.6–7, as measured in the fall of 1963 on a low-altitude polar-orbiting satellite. The data have been subjected to a systematic analysis in terms of the longitude variations in intensity and spectral shape. In contrast to the behavior at L ≲ 1.7, significant fluxes of electrons are observed in the outer radiation belt at virtually all longitudes, and the energy spectra of the precipitating electrons are comparable to those of electrons trapped at minimum altitudes of 200–350 km. For the precipitating electrons, the data have been analyzed in terms of the interval of longitude drift between the measurement location and the point where the B, L trace of the observation point was last below 100 km in altitude. The rates of flux buildup with longitude drift are rather slow in the inner belt, whereas in the outer belt regions, the intensities rapidly build up to near equilibrium values. Comparisons have been made of the rates of flux buildup with calculations of pitch-angle diffusion, using the Fokker-Planck formalism. Over a rather wide range of magnetic latitudes covering the outer radiation belt and slot regions, where the low-altitude precipitations vary enormously in magnitude, the diffusion coefficient varies by only a factor of 4 and is consistent with the dominant loss mechanism for trapped electrons being pitch-angle diffusion.

Simultaneous measurements of electron content and charged-particle precipitation in a region of auroral activity

Simultaneous measurements of charged-particle precipitation and ionospheric electron content were made as part of a coordinated auroral-zone experiment utilizing a low-altitude polar-orbiting satellite, two ground stations, and an instrumented B-17 aircraft. The satellite was equipped with particle and optical detectors and a three-frequency radio beacon, and electron contents were obtained by means of differential Doppler techniques. For the case described, the coordinated data were obtained in an aurorally active region throughout which the electron content exhibited several large peaks. Enhancements in the electron content were observed to correlate well with regions of increased particle fluxes, indicating, for this case, to what extent variations in the nighttime electron content below 200 km could be attributed to corpuscular radiation. Heights of regions of ionization deduced from the radio data agree well with those calculated on the basis of the particle flux parameters.

Ion temperature and relative ion composition measurements from a low-altitude polar-orbiting satellite

An ion energy analyzer has been flown on a recent (November 9-11, 1965) low-altitude polar-orbiting satellite. The data were collected, partially analyzed while in flight by the instrument, and stored on tape to be read out at convenient satellite tracking stations. The data sampling rate was once every three seconds, which permitted a very detailed measure of the latitude variation of ion temperature, concentration, and relative ion composition. Large temperature fluctuations were observed in the nighttime northern high-latitude regions. Mid-latitude daytime ion temperatures generally varied between 400 and 1000°K. High temperatures were observed in both the daytime southern auroral zone and the nighttime northern auroral zone. The midlatitude trough in ion concentration was observed a L = 4.5, while other sharp troughs were located at about L = 1.5.