Electrical conductivities,ion densities and mobilities in the middle atmosphere over India—balloon measurements

Polar conductivities, ion densities and mobilities measured in three balloon experiments are reported here. Of these, two were made with self-aspirated and one with pumped Gerdien condensers. These measurements were carried out over the period 1985–1986. Both positive and negative ion data were obtained only in the first flight. Positive ions were not collected in the other two flights. The conductivity profiles obtained from the self-aspirated condensers showed fluctuations around the tropopause. These were not seen when the pumped condenser was used. Reduced mobilities of both positive and negative ions agree well with those reported by other experimenters. The ion density profile from the pumped condenser measurement showed a broad maximum around 15 km, as expected from theory. The values were less than what are obtained using available ion pair production rates and recombination coefficients.     

Topside and intelhemispheric ion flows in the mid-latitude plasmasphere

A modelling study has been carried out of field-aligned ion flows in the topside ionospheres of conjugate hemispheres under solstice conditions at mid to low latitudes. In the model calculations coupled time-dependent O⁺, H⁺ and electron continuity, momentum and heat balance equations are solved along dipole magnetic field lines at L = 1.5 and 3.0 Sunspot medium and sunspot minimum atmospheric conditions are considered.It has been found that thermal coupling between conjugate hemispheres gives rise to strong flows of O⁺ in the topside ionosphere of the summer hemisphere that are directed upwards at conjugate sunrise and directed downwards at conjugate sunset. At conjugate sunrise in the winter hemisphere there is a small upward-directed signature in the O⁺ field-aligned flux; there is no observable signature in the O⁺ field-aligned flux in the winter hemisphere at conjugate sunset. There are strong upward and downward flows of O⁺ at local sunrise and local sunset, respectively, in both the summer and winter hemispheres.At both L = 1.5 and 3.0 the 24 h time-integrated interhemispheric H⁺ flux is in the direction summer hemisphere to winter hemisphere. At L = 1.5 its magnitude is in good agreement with the magnitude of the 24 h time-integrated plasma (O⁺ + H⁺) field-aligned flux at 1000 km altitude; there are no such agreements at L = 3.0.A study of the roles played by the individual terms of the O⁺ momentum equation has demonstrated the complex structure of momentum balance. Certain of the terms may be orders of magnitude greater than the combined total of the individual terms, i.e. the O⁺ field-aligned flux.     

Positive and negative ion densities and mobilities in the middle atmosphere over India—rocket measurements

This paper discusses the results from four rocket experiments conducted from Thumba, India, during the Indian Middle Atmosphere programme (IMAP). These rockets carried instrumented Gerdien Condenser payloads to measure ion densities and their mobilities. In the first two flights only positive ion measurements were attempted while the other two measured both positive and negative ion values. The results show that the positive ion density profiles go through a minimum around 62 km, as expected from the ion production models for this region. The ion density distribution is a function of solar zenith angle. An asymmetry with respect to noon is seen in these measurements, which is not expected theoretically. The positive ion mobilities indicate the ions to be water clusters, of the type H⁺ (H₂O)_n with n = 2 or 3, similar to the earlier reported ones. The negative ion density profile exhibits a maximum around 85 km, which is not predicted by the currently available ion density models and theories of D-region ionisation processes. The negative ion mobility measurements show the ions to have a mass range of 30–60 amu, which is within the range of mass spectrometric measurements.     

Summertime stratospheric wind measurements above the South Pole

Observations of the mean wind flow and wave motions in the stratosphere at the South Pole are presented. The atmospheric motions are determined from the tracking of a high altitude, zero-pressure balloon launched from Amundsen-Scott Station during the austral summer of 1985–1986. The balloon position was precisely monitored by an optical theodolite for a large portion of the flight so that small scale motions could be resolved. The mean flow above the pole was approximately 3ms⁻¹. Atmospheric motions characteristic of internal gravity waves were observed with an intrinsic period of approximately 4.5 h and vertical and horizontal wavelengths of approximately 2.5km and 125km, respectively. The horizontal perturbation velocity of the observed waves was large compared to the mean horizontal flow velocity. The implication is that wave motions play a dominant role in the transport of stratospheric constituents in regions where the mean winds are light, such as over the South Pole during austral summer.     

Fine scale structure and turbulence parameters in the equatorial middle atmosphere

Two rockets carrying identical spherical probe payloads were launched from Thumba to measure positive ion density of the mesosphere and lower thermosphere over an equatorial location. Data obtained show the presence of strong irregularities in the ion density. From the measured positive ion current, the spectra of the spatial density fluctuation, turbulent velocity, energy dissipation rate and eddy diffusion coefficients have been derived in the altitude range of 70–100 km. The results are found to be different from those at middle and high latitudes.     

Some features of the vertical ozone distribution from millimeter wave measurements at Pushchino and Onsala observatories

A number of features of the stratospheric ozone distribution were revealed by joint millimeterwave observations of ozone emission lines at 142,175 and 110,836 GHz carried out during the winter periods of 1988–1989 and 1989–1990 at the Radioastronomical Observatory of the P.N. Lebedev Physical Institute of the Russian Academy of Sciences and at the Onsala Space Observatory of Chalmers University of Technology, Sweden. It is shown that vertical ozone variations observed at the two observatories were connected with large scale dynamical processes that occurred in the stratosphere. When the stratosphere was relatively undisturbed the ozone profiles obtained at both observatories were close to the ozone reference model given by Keating and Pitts. There were periods during a stratospheric warming when the ozone content measured at the two observatories in the 25–40 km altitude range was higher by a factor ~ 1.5 than the model values. Dynamical processes in the stratosphere also gave rise to rapid (4 h duration) and large deviations from the model ozone profile. An ozone layer depletion was observed in the 27–55 km altitude range. The observed ozone variations illustrate the sensitivity of the ozone distribution to stratospheric disturbances including stratospheric warmings.     

Sudden sodium layers in polar latitudes

We discuss in this paper sudden sodium layers (SSLs), which we observe with a sodium lidar instrument at Andenes, Norway (69°N). We speak of a SSL if, in a narrow altitude range (typically less than 2km), the Na density increases over the normal Na density by a factor of at least 2 within 5 min. Between December 1985 and November 1987, we have observed 42 such layers in 378 h of lidar measurements. This number increases to 75 if we only require an increase of a factor of 1.5 within 8 min. At our observation site, SSLs have the following properties: (a) they develop between 90 and 110 km altitude, (b) they develop between 20 and 02 LT, (c) their appearance shows a strong, positive correlation with that of ƒ-type E_s layers, and (d) their appearance does not show a strong correlation with either riometer absorption or meteor showers. We discuss a number of potential processes for SSL formation. SSLs above 100km can be formed in ƒ-type E_s layers by the conversion of Na⁺ ions into neutral Na. The development of SSLs below 95 km requires the presence of an additional reservoir of Na, such as Na-bearing molecules, ions, and/or ‘smoke’ particles. We also evaluate the proposal that SSLs are the outcome of single meteoroids entering the upper atmosphere, a proposition for which we find little observational support.     

Tropical behavior of mesospheric ozone as observed by SMM

The seasonal behavior of low latitude mesospheric ozone, as observed by the SMM satellite solar occultation experiment, is detailed for the 1985–1989 period. Annual as well as semi-annual waves are observed in the 50–70 km altitude region. In the latitude range of ±30 the ozone phase and amplitude are functions of temperature and seasonal changes in solar flux. Temperature is the controlling factor for the equatorial region and seasonal changes in solar flux become more dominant at latitudes outside the equatorial zone (greater than ±15). There is a hemispheric asymmetry in the ozone annual wave in the 20 30 region, with northern hemispheric ozone having a larger amplitude than southern hemispheric ozone. In this region temperature is nearly in phase with ozone in both hemispheres and is reduced in amplitude in the northern hemisphere. The equatorial region is characterized by a strong semi-annual wave in addition to the annual variation, and temperature is nearly out of phase with ozone. At all latitudes there is a larger ozone concentration at sunrise than at sunset. The sunrise sunset difference increases with increasing altitude     

Case study of modelled and measured D-rmregion plasma densities

Two of the many sounding rockets launched as part of the MAC/EPSILON campaign are particularly suited to test the validity or quality of D-region models under night-time conditions. The measurements covered atomic oxygen, neutral temperatures and fluxes of charged particles. With these parameters as inputs, one is in the position to assess their relevance for the plasma parameters ion composition, transition heights of clusters and negative ions, and total plasma density. The two IOMAS payloads are believed to be the only night-time flights with associated measurements of atomic oxygen extending below the ledge at about 80 km. The relevance of atomic oxygen for the ion chemistry of the lower D-region could be experimentally demonstrated.     

Parachute measurements of positive ion density of the middle atmosphere over the dip equator by spherical probe

A parachute-borne gridded spherical probe has been used to measure the total positive ion density. Two launches were made, using Soviet M-100 rockets, on 22 and 29 April 1987, at 1200 UT, from an equatorial station, Thumba (8°N, 76°E) India. Data were obtained for the altitude region 10 to 80 km. A broad maximum around 15 km and a broad minimum around 60 km have been noticed in the ion current profiles obtained in both flights. The theory of the operation of the probe has been given. A detailed discussion of the results obtained has also been included.     

Mechanisms for vertical separation of ions in sporadic-E layers

A model of the ionospheric E-region between 90 and 130 km altitude is constructed with normal molecular ions and two species of metal ion with different masses. This paper investigates whether the vertical structure observed in sporadic-E layers can be accounted for by separation of different ion species according to their mass. The result of the investigation is substantially negative. Another mechanism for range spread sporadic-E has signatures that may be sought in observational data.     

Spectral characteristics of magnetic storm-induced F-region scintillations extending into daytime

Magnetic storm-induced F-region scintillations extending into daytime were recorded over Bombay, situated near the anomaly crest region in India, on 12 November 1991. The scintillations at 244 MHz using the radio beacon onboard FLEETSAT (73°E), lasted till 1312 h IST (77.5°E). Observations at Trivandrum, situated close to the magnetic equator also show strong daytime scintillations lasting till 1030 h. The scintillation event followed a sudden commencement at 1748 h UT (2318 h IST) on 11 November 1991 and the ionosonde observations, both over Ahmedabad in the anomaly crest region and Kodaikanal near the magnetic equator, show large upward drift of about 50 m s⁻¹ around 0300 h IST. The scintillation index S₄, autocorrelation function and power spectra have been computed from the digital data recorded at Bombay. The time variation of S₄ shows large fluctuations with a periodicity of about half an hour. The 50% decorrelation time of the signal fluctuations is of the order of seconds. The spectral index n, of the temporal power spectra, where P(F)αF⁻ⁿ, varies between 1.5 and 5.0, with a mean value of 3.0, and shows a dependence on the S₄ index. These features are similar to those reported for night time scintillations recorded over Ahmedabad.     

Neutral air density and temperature measurements by the TOTAL instrument aboard the ROSE payloads

Four ROSE payloads, launched from November 1988 to February 1989 from northern Scandinavia, carried ionization gauges (‘TOTAL’ instruments) for neutral air density measurements in the altitude range 90–105 km. Temperature profiles are derived by integrating the number density profiles. Density and temperature data are presented. The limitations of the measurement technique as well as instrumental errors are discussed. In one of the flights (F1) a significant temperature enhancement was observed at an altitude where plasma instabilities were detected by independent measurements.     

Incoherent scatter spectral measurements of the summertime high-latitude D-region with the EISCAT UHF radar

Measurements of incoherent scatter spectra from the auroral D-region were obtained during the summer of 1985 using a sophisticated pulse-to-pulse correlation technique with the EISCAT UHF radar. The spectral width variations with altitude are interpreted in terms of ion-neutral collision frequency, neutral temperature, mean positive ion mass and negative ion number density. Close agreement with predictions of currently available atmospheric models is obtained, except for a narrow layer around 86 km altitude. This layer showed evidence of increased positive ion mass for most of the experiment, and for short intervals indicated a mean ion mass close to 200 a.m.u. It is suggested that the layer is composed of proton hydrates in the vicinity of a structured noctilucent cloud, and that the index of hydration is occasionally large.     

High latitude quiet summer ion composition profiles derived from a combined ion line/plasma line incoherent scatter experiment

High latitude quiet summer ion composition values in the altitude range from 200 to 245 km have been derived from a combined ion line/plasma line experiment in a full five-parameter fit. The EISCAT UHF radar was used with a 5 × 14 μs multipulse scheme for the ion line measurements, giving a range resolution of 3 km. Plasma line signals from the same altitudes were measured with a 70 μs pulse using a spectrum analyzer. Significant deviations from the standard EISCAT composition model were found, mainly at the upper altitudes. The O⁺ content was generally lower than predicted by the model. For the largest composition deviations, significant effects were seen in the temperatures, particularly in the electron temperature. The electron temperatures derived by a standard ion line fit applying the model were underestimated by up to 15%.     

Electrodynamic response of the middle atmosphere to auroral pulsations

On the nights of 21 and 28 October 1987, two Nike Orion payloads (NASA 31.066 and 31.067) were launched from Andøya, Norway, as part of the MAC/EPSILON campaign, to study the effect of auroral energetics on the middle atmosphere. Each payload carried detectors to measure relativistic electrons from 0.1 to 1.0MeV in 12 differential energy channels, and bremsstrahlung X-rays from >5 to >80keV in 5 integral channels. In addition, instrumentation to measure bulk ion properties and electric fields was also carried by these and/or near simultaneous flights. Flight 31.066 was launched during the recovery phase of a moderate magnetic substorm, during relatively stable auroral conditions. Flight 31.067 was launched during highly active post-break-up conditions during which Pc 5 pulsations (> 150s period) were in progress. The energetic radiation of the first event was composed almost entirely of relativistic electrons below 200 keV with negligible contributions from bremsstrahlung X-rays, while the radiation of the second event was dominated by much softer electrons ( < 100 kcV), which produced high X-ray fluxes that exceeded the cosmic ray background as an ionizing source down to altitudes below 30 km. Simultaneous conductivity measurements during both events show consistency with the ionizing radiations, with the pulsation event producing free electrons down to 55 km. far below their expected altitude range during night-time. These comparisons are discussed to evaluate the impact of such events on the middle atmosphere.     

A simple model of gravity-wave momentum and energy fluxes transferred through the middle atmosphere to the upper atmosphere

Vertical fluxes of momentum and energy through the middle atmosphere are calculated by using a simple semi-empirical model of quasi-monochromatic internal gravity waves with dominant vertical wavenumbers. In this model those dominant gravity waves are assumed to saturate and break at each observational altitude by an effective critical-layer mechanism. The dominant value of the vertical wave-number is expressed by an exponential function of altitude, decreasing upward with a scale height of 34 km. This expression gives the momentum and energy flux densities decreasing upward with scale heights of 12 and 18 km, respectively, and typical values at 100 km altitude are estimated as 4 × 10⁻⁵ Pa and 4 × 10⁻³ W/m². A heat flux induced by wavebreaking turbulence also has an order of magnitude similar to that of the wave energy flux. Variabilities around these values and comparisons with other momentum and heat inputs to the upper atmosphere are only briefly discussed.     

A narrow auroral arc observed with EISCAT

On the evening of 13 January 1983 we made simultaneous observations of optical and radar aurora using low light television cameras together with the EISCAT radar system. At 19 h 16 m 06 s UT an extremely bright auroral arc moved rapidly (about 2 km s⁻¹) through the EISCAT radar beam. The associated rapid rise and fall in the E-region electron density indicates that there was an intense narrow electron beam associated with the optical arc. We estimate that the ionisation rate in the E-region increased at least 20-fold (from 1 × 10¹⁰ m⁻³ s⁻¹ to >2 x 10¹¹ m⁻³ s⁻¹) for 1 or 2 s as the arc passed by. In addition, there was a brief (<4 s) increase of 130% in the signal returned from 250 km altitude which coincided with the arc crossing the radar beam at that height. In view of this coincidence, we find that a possible explanation is that the increase arose from short-lived molecular ions, for example vibrationally excited N⁺₂ ions, produced in the F-region by soft precipitation associated with the arc.     

The effect of vertical drift on the equatorial F-region stability

Owing to the high conductivity along magnetic field lines, the stability of the night-time equatorial F-region is determined by magnetic field line integrated quantities. However, slow vertical diffusion near the magnetic equator plus the rapid increase in ion chemistry rates at lower altitude combine to give a very small positive scale height for the electron concentration on the bottomside of the region. As a result, the field line averaged quantities are reasonably approximated by their equatorial values, provided that the E-region does not contribute significantly. The time-dependent behavior of the growth rate for the Rayleigh-Taylor gravitational instability on the F-region bottomside is examined here as a function of the vertical E × B drift velocity using reasonable chemistry to obtain approximate equatorial vertical profiles of ionospheric parameters. It is found that the growth rate exceeds the chemical recombination rate over most of the bottomside F-layer even without vertical drift, but that a realistic E × B drift can result, after about 1 h, in an increase of this growth rate by an order of magnitude. The absolute growth rate is so small (< 10⁻³ s⁻¹) with zero vertical drift that a seeding mechanism would probably be required for the formation of bubbles. The rapid appearance of bubbles shortly after sunset appears likely only after a period of upward drift, as is observed.     

Tidal wind observations with incoherent scatter radar and meteorological rockets during MAP/WINE

Measurements of winds in the mesosphere and lower thermosphere were carried out during the main phase of the MAP/WINE project in January and February 1984 with the EISCAT UHF incoherent scatter radar near Tromsö, Norway, and with meteorological rockets launched from the Andøya Rocket Range, Norway. The radar measurements yield wind profiles between the altitudes of about 80 km and 105 km and the rockets between about 60 km and 90 km. Results from both techniques are combined to yield mean profiles which are particularly evaluated in terms of tidal variations. It is found that the semidiurnal tide constitutes an essential wind contribution between 85 km and 105 km. Whereas the tidal amplitudes are below 5 m s⁻¹ at about 80 km, they increase to 20–30 m s⁻¹ at 100 km. The average vertical wavelength of 35 km points to the S₄² mode, but coupling and superposition of different modes cannot be excluded.