Francis S. Johnson

Gravity wave seeding of equatorial plasma bubbles

Some examples from the Atmosphere Explorer E data showing plasma bubble development from wavy ion density structures in the bottomside F layer are described. The wavy structures mostly had east–west wavelengths of 150–800 km; in one example it was about 3000 km. The ionization troughs in the wavy structures later broke up into either a multiple-bubble patch or a single bubble, depending upon whether, in the precursor wavy structure, shorter wavelengths were superimposed on the larger-scale wavelengths. In the multiple-bubble patches, intrabubble spacings varied from 55 km to 140 km. In a fully developed equatorial spread F case, east–west wavelengths from 690 km down to about 0.5 km were present simultaneously. The spacings between bubble patches or between bubbles in a patch appear to be determined by the wavelengths present in the precursor wave structure. In some cases, deeper bubbles developed on the western edge of a bubble patch, suggesting an east–west asymmetry. Simultaneous horizontal neutral wind measurements showed wavelike perturbations that were closely associated with perturbations in the plasma horizontal drift velocity. We argue that the wave structures observed here that served as the initial seed ion density perturbations were caused by gravity waves, strengthening the view that gravity waves seed equatorial spread F irregularities.

Eddy mixing and circulation at ionospheric levels

Abstract The rates of heat input into the mesosphere and lower thermosphere are calculated and compared with the heat losses. The worldwide average eddy diffusion coefficient required to maintain continuity in the heat budget is calculated and found to vary from about 10 7 cm 2 /sec at 120 km down to about 10 5 cm 2 /sec at 60 km. From the global asymmetry in heating at the solstice, it is concluded that a systematic pattern of vertical velocities prevails ranging from less than 1 cm/sec in the mesosphere up to 10 cm/sec near 120 km, upward over the summer polar region and downward over the winter polar region. This can be balanced by a wind system towards the winter polar region with velocities near 1 m/sec at 60 km increasing to 30 m/sec at 120 km. Such a wind system provides an explanation for the helium bulge in the upper thermosphere over the winter polar region.

The lunar atmosphere

In contrast to earth, the atmosphere of the moon is exceedingly tenuous and appears to consist mainly of noble gases. The solar wind impinges on the lunar surface, supplying detectable amounts of helium, neon and 36Ar. Influxes of solar wind protons and carbon and nitrogen ions are significant, but atmospheric gases containing these elements have not been positively identified. Radiogenic 40Ar and 222Rn produced within the moon have been detected. The present rate of effusion of argon from the moon accounts for about 0.4% of the total production of 40Ar due to decay of 40K if the average abundance of potassium in the moon is 1000 ppm. Lack of weathering processes in the regolith suggests that most of the atmospheric 40Ar originates deep in the lunar interior, perhaps in a partially molten core. If so, other gases may be vented along with the argon.

AVERAGE LATITUDINAL VARIATION IN ULTRAVIOLET RADIATION AT THE EARTH'S SURFACE

Abstract— Tabulated values are presented for ultraviolet radiation at the earths surface as a function of wavelength, latitude, and season, for clear sky and seasonally and latitudinally averaged ozone amounts. These tabulations can be combined with any biological sensitivity function in order to obtain the seasonal and latitudinal variation of the corresponding effective doses. The integrated dosages, based on the erythemal sensitivity curve and on the Robertson‐Berger sunburn‐meter sensitivity curve, have also been calculated, and these are found to vary with latitude and season in very nearly the same way as 307 and 314 nm radiation, respectively.

Morphology of equatorial plasma bubbles

Some occurrence characteristics of plasma bubbles based on data from the ion drift meter aboard the Atmosphere Explorer E satellite are presented. Of 5200 bubbles detected between 250 and 475 km altitude, fi5% had ion density depletions of less than 1 order of magnitude below the ambient ion density, and the occurrence frequency decreased with increasing depletion depth. Depletion depth decreased with local time, probably due to diffusive decay of bubbles. Bubble activity above 300 km altitude had its onset around 1900 and peaked around 2130 LT. Late, shallow bubbles appeared to be associated with frequency spread F, and early, deep bubbles with range spread F. Increase in magnetic activity inhibits bubble generation except near sunrise, at which time bubble activity has been found to increase with magnetic disturbance. Bubble activity is restricted largely to a belt within about ±20° dip latitude.

Atomic oxygen transport in the thermosphere

Abstract The photodissociation of oxygen in the lower thermosphere is evaluated to obtain its global average value and the hemispheric imbalance. The observed concentrations of atomic oxygen do not reflect this imbalance in production due to the effect of seasonal wind patterns redistributing the atomic oxygen. The wind system necessary to compensate for the imbalance in solar thermal input into the lower thermosphere is found to transport an amount of atomic oxygen sufficient to compensate for the hemispheric imbalance in production. Ionospheric data indicate a winter enhancement in atomic oxygen concentration; to produce this, a higher degree of oxygen dissociation than that normally accepted (i.e. higher than an atomic to molecular oxygen ratio of unity at 120 km) is needed. The concept that the concentrations of atomic oxygen observed over the winter polar region are maintained by transport from lower latitudes requires that eddy diffusion coefficients derived from vertical transport at low latitudes (ignoring horizontal transport) be reduced by about 25 per cent.

Lower midlatitude ionospheric disturbances and the Perkins instability

Abstract This paper presents a preliminary study of ionospheric disturbances at dip latitudes less than 40° as seen by the ion drift meter and the retarding potential analyzer on board Atmosphere Explorer E during a period of low solar activity. The altitude of observation was relatively low, mostly below 300 km. The emphasis is on the midlatitude region, where some features resemble equatorial bubbles; no clear demarcation in latitude could be recognized between the bubbles and other midlatitude disturbances. Excellent evidence was found that the Perkins instability is responsible for very structured disturbances which were frequently observed. In most cases where regions of inward and outward E × B drift were encountered, diffusive motion up or down the field line partially cancelled the vertical component of the cross-field drift. In some cases cancellation was almost perfect, but in others it appeared not to occur at all (probably cases involving rapid changes). Within the larger structures caused by the Perkins instability there were places where the secondary gradient drift instability was also active.

Singular plasma disturbances in the low‐latitude F region

We describe here a new phenomenon characterized by unusual patterns of ion drifts inside ion density depletion regions observed by the AE-E satellite in the low-latitude F region. In about 30 depletions, vertical ion drift relative to the background was upward on the western sides, downward on the eastern sides, and zero near the middle where the density depletion was greatest. These drift characteristics are distinct from those observed in plasma bubble depletions. The structures reported here were observed on circular orbits below 300 km altitude and had density depletions of up to 2 orders of magnitude or more below the ambient ion density. The upward and downward drift excursions were up to 200 m/s relative to the background. Almost all these structures were observed over oceans or near coasts and largely between +/- 10 deg and +/- 30 deg clip latitude. The structures were observed mostly as isolated, single depletion regions with the majority of them about 250 km wide in the east-west direction. They occurred during quiet magnetic conditions with near-equal occurrence frequencies in the premidnight and postmidnight periods. The characteristic density and drift signatures indicate westward propagating disturbances in which the bottomside F layer is first lifted and then returned back to its original position, leaving the ionosphere undisturbed after the disturbance passes by. The estimated speed of these disturbances is of the order of 200 m/s. These unique solitary plasma disturbances, which we designate as singular plasma disturbances, are associated with a propagating source of E x B drift, not driven by neutral perturbations at the altitude of observation.

Energy input to the lower thermosphere

Abstract The asymmetry in solar heat input to the upper atmosphere at the solstice, and the asymmetry in atomic oxygen production, are largely compensated by a large scale wind system towards the winter polar region. At magnetically disturbed times, atmospheric composition at high winter latitudes changes in such a way as to indicate that polar region heating by magnetic variations, energetic particle inputs, and current systems is more intense than solar heating at low latitudes, thus leading to a reversal of the normal pattern of upper atmospheric circulation. Uncertainties in the intensity of solar radiation responsible for upper atmospheric heating and oxygen dissociation, and uncertainties in the degree of oxygen dissociation in the upper atmosphere, are such that the average rates of eddy mixing may be significantly lower than frequently assumed for the lower thermosphere.

Ozone and SSTs

Abstract The possibility exists that exhaust products from a large fleet of supersonic transports (SSTs) might seriously reduce the atmospheric ozone layer that shields the Earths surface from solar ultra-violet germicidal radiation. Both water vapour and nitrogen oxides, important constituents of SST exhaust, act to destroy ozone. Estimates of the effect suggest that it will be at least moderately serious. While confidence in the magnitude of the predicted effect is very low, the possibility exists that it might be disastrous, and some such predictions have been made. Research, including investigations of ambient conditions at flight altitude, of nitrogen oxide concentrations in jet exhausts, of reaction rates, and of stratospheric transport processes, is needed to improve the accuracy of prediction.