Electron Column Density Research Articles

A large‐amplitude traveling ionospheric disturbance produced by the May 18, 1980, explosion of Mount St. Helens

A remarkable long‐lived, large‐scale traveling ionospheric disturbance (TID), excited by the May 18, 1980, explosion of Mount St. Helens, has been detected in total electron content monitor data. Oscillatory perturbations in the electron column density of the ionosphere with amplitudes about 10% of the nominal daytime content were detected at three stations whose ionospheric penetration points lie between 1610 and 1890 km from Mount St. Helens. Smaller perturbations were detected at five of six additional stations between 3760 and 4950 km away. The period of the TID increased linearly with great‐circle distance from Mount St. Helens, ranging from ≈37 min at the nearest station to ≈116 min at the most distant one. The TID persisted for at least four cycles at the three close stations and three cycles at the more distant stations and was qualitatively similar to TIDs produced by the low‐altitude thermonuclear detonations of the 1960's. The disturbance front of this TID accelerated from an average velocity of ≈350 m/s between Mt. St. Helens and the close stations to an average velocity of ≈550 m/s to the more distant ones. In analogy with the bomb‐excited disturbances, this TID is interpreted as the ionospheric fluctuations induced by a gravity wave, excited by the explosion of Mount St. Helens, propagating in the neutral atmosphere. A model based on the free wave response of an isothermal atmosphere to a point disturbance provides a good fit to the data at the three closest stations, but no such model can account for all of the data. Modeling of the long‐distance behavior of the Mount St. Helens TID in terms of upper‐atmosphere guided gravity waves is complicated by the requirement of exciting them by a ground‐level explosion. A comprehensive explanation would probably include the propagation of energy to ionospheric heights in free modes and the excitation thereby of guided modes. There was no evidence for a strong supersonic shock wave in the ionosphere. As a result, the Mount St. Helens disturbance may prove to be a cleaner test of detailed theories of the point excitation and propagation of gravity waves in a realistic atmosphere than were TIDs excited by thermonuclear weapons.

Observations of the Ca+ twilight airglow from intermediate layers of ionization

Application of complementary optical and incoherent scatter radar techniques has detected the presence of Ca+ in lower thermospheric intermediate layers over Arecibo. The altitude distribution and density of the calcium ion is inferred from the variation of twilight resonant scattering with solar depression angle while the Arecibo 430Mhz radar measured electron densities. Comparison between the Ca+ and electron column densities indicates that the composition of low‐altitude intermediate layers was 2% Ca+. This value is consistent with both rocket mass spectrometer measurements and with the relative concentration of calcium expected for meteoric debris. These results have implications regarding both possible nighttime tropical E region ionization sources and metal ion circulation in the tropical thermosphere.

On the nature of the intercloud medium

Previous analyses of Copernicus observations of interstellar absorption in the spectra of unreddened stars are reviewed in the framework of steady-state ionization models for the intercloud (low-density) component of the interstellar medium. A simple homogeneous model is described which accounts for the H, C, N, S, and Si interstellar lines in the spectrum of lambda Sco in terms of an extensive low-density medium photoionized by Lyman continuum radiation from the II Sco OB association. Quantitative constraints are placed on the maximum permissible flux of diffuse soft X-rays, and the model is generalized to interpret the observed ion, electron, and hydrogen column densities in the low-density medium. Severe constraints are found to be imposed on ionization models by the ratios Si III/Si, Si IV/Si, and S IV/S. It is concluded that the observations may be explained if 10 to 30% of a representative line of sight passes through a photoionized medium of density 0.1 to 0.2/cu cm, if H I constitutes the bulk of the gas along the line of site, and if the gas is at a similar density or is in the form of shielded clumps or clouds.

The total electron content through the entire ionosphere by the Faraday rotation technique

The polarization twist imposed on VHF radio waves by the ionosphere has been investigated by many workers using the moon as a reflector, the purpose being to determine the total electron column density. The difficulty in using only one frequency is the impossibility of an exact determination of the total electron content because the Faraday rotation is ambiguous in multiples of π. But I have shown recently that at daytime the electron content n0 should vary with , when the ionosphere is undisturbed, where ÷ means the zenith angle of the ionizing solar radiation. Plotting the Faraday rotation against is not a result of trying to find or proving a good agreement, but it is the result and a strong consequence of my new model for the F layer [Burkard, 1961].

Faraday rotation observations of the electron content of the exosphere

The polarization twist imposed on 106.1-Mc/s radio waves by the ionosphere has been investigated by using the moon as a passive reflector. This has been done with a view to learning the total electron column density even at altitudes above the known ionosphere. Because the antenna is capable of being continuously directed at the moon for 12 consecutive hours, observations are possible from the predawn ionization minimum to the noontime maximum.

Some observations of ionospheric Faraday rotation on 106.1 Mc/s

The polarization twist imposed on 106.1 Mc/s radio waves by the ionosphere has been investigated by using the moon as a passive reflector, the purpose being to determine the total electron column density even at altitudes above the known ionosphere. Because the antenna is capable of being continuously directed at the moon for 12 consecutive hours, observations are possible from the pre-dawn ionization minimum to the noon-time maximum. A true-height profile computed from vertical-incidence ionosonde data of September 16, 1957, suggests that the total electron content throughout the entire ionosphere is not a constant factor of the integrated electron content computed up to the level of maximum ionization density.