Recent measurements of middle atmospheric electric fields and related parameters

In 1989, two series of rocket measurements were carried out to investigate middle atmosphere electric fields. The measurements were taken both in the Northern Hemisphere on Heiss Island (80°37′N and 58°03′E) and in the Southern Hemisphere in the Indian Ocean (40–60°S and ~45°E) on board the research vessel ‘Akademik Shirshov’. Along with the vertical electric fields, aerosol content and positive ion density were also measured. Some of the rocket launches were made during the extremely strong solar proton events (SPE) of October 1989. The experiments showed the strong variability of the electric fields in the middle atmosphere at polar and high middle latitudes. In all the measurements the maximum of the vertical electric field height profile in the lower mesosphere was observed to be more than ~ 1 V/m. The electric field strength and the field direction at maximum varied considerably among the launches. A maximum value of + 12 V/m was detected at a height of about 58 km at 58°30′S on 21 October 1989 during the SPE. The simultaneous measurements of the electric field strength, positive ion density and aerosols point out both an ion -aerosol interaction and a connection between the mesospheric electric fields and aerosol content.     

Rocket measurements of nitric oxide in the equatorial region

Measurements of nitric oxide (NO) concentrations were carried out from Thumba, India, using rocket-borne radiometers. The technique of measurement is based on the detection of day-glow emissions of the NO gamma (1,0) band. The results obtained are presented in this paper. The peak NO concentration shows a very good correlation with integrated value of the solar X-ray flux in the 0.1–0.8 nm band, thereby indicating the influence of the X-ray flux on the NO concentration. The observed variability of NO is thought to be mainly due to solar activity and partly due to different X-ray flux values on the days of the flights. Theoretical model calculations for rocket flight conditions were found to be in fairly good agreement with the observed profiles. The differences below 90 km altitude in the NO profiles are thought to be due to eddy turbulence. This model is also used to study changes in the NO concentration with solar activity and latitude.     

Results inferred from electron density measurements at Saskatoon,Canada (L=4.4) by a partial reflection technique—II. Ion production rates and nitric oxide in the D-region during post-storm periods

Electron density profiles measured, at Saskatoon (52°N, L=4.4) during and after magnetic disturbances have been examined. It is seen that there is an enhanced electron density during magnetic disturbances, the enhancement being different at different heights. This enhancement is maintained for a number of days after the end of the disturbance. An increase of ion production rate and an increase of nitric oxide density are required for the maintenance of increased electron density in the post-storm period.     

EISCAT incoherent scatter radar observations and model studies of day to twilight variations in the D-region during the PCA event of August 1989

An intense solar proton event causing enhanced ionization in the ionospheric D-region occurred on 12 August 1989. The event was partially observed during three successive nights by the EISCAT UHF incoherent scatter radar at Ramfjordmoen near Tromsa, Norway. Ion production rates calculated from GOES-7 satellite measurements of proton flux and a detailed ion chemistry model of the D-region are used together with the radar data to deduce electron concentration, negative ion to electron concentration ratio, mean ion mass and neutral temperature in the height region from 70 to 90 km, at selected times which correspond to the maximum and minimum solar elevations occurring during the radar observations. The quantitative interpretation of EISCAT data as physical parameters is discussed. The obtained temperature values are compared with nearly simultaneous temperature measurements at Andøya based on lidar technique.     

Positive and negative ion composition measurements in the D- and E-regions during the 26 February 1979 solar eclipse

Two rockets bearing quadrupole mass spectrometers capable of measuring both positive and negative ion composition were launched from Red Lake, Canada, during the solar eclipse. Both instruments had liquid helium cryopumps and shock-attaching conical samplers. The payloads also contained two Gerdien condensers to measure total positive and negative ion concentrations and ion mobilities. Attitude control systems aligned the payloads with the velocity vector throughout ascent and descent. The first rocket was launched so that the D-region was in darkness 35 ± 8 s on the upleg and about 150 ± 15 s on the downleg for the study of ionospheric decay processes. The second rocket was fired after totality into 75% solar illumination for the study of ionospheric recovery. The positive ion composition above 105 km exhibited a strongly increasing NO⁺/O₂⁺ ratio with time after second contact due to O₂⁺ charge transfer with NO and a sharply diminished ionization rate. However, in both nights, the ionization below 105 km was created mainly by energetic particle deposition as exemplified by the increased ion concentrations and the composition signatures of a particle event: asignificant enhancement of O₂⁺ below 105 km and large amounts of H₅O₂⁺ ions in the D-region which result from the O₂⁺ clustering scheme. H₅O₂ was the major ion in the upper D-region while H₇O⁺₃, H₉O₄⁺ and H₅O₂⁺ were dominant ions at lower altitudes. Numerous minor species were also detected. The negative ion distributions in both flights exhibited a distinct shelf at 83 ± 2 km, decreasing by more than an order of magnitude by 90 km and with minima near 75 km. In the 75–90 km range, a significant percentage of the negative ions had masses exceeding 160 a.m.u. Comparisons are made with prior negative ion measurements during similar daytime auroral zone absorption (AZA) events. Two striking characteristics of the precipitating particles were apparent from these and past observations in daytime AZA events: there is a near absence of low energy electrons capable of ionizing above about 105 km and there is'a significant spatial and/or temporal variability in the electron flux. This paper is devoted principally to a presentation of the ion composition measurements and associated uncertainties.     

Results inferred from electron density measurements at Saskatoon,Canada (L = 4.4) by a partial reflection technique—I. Variations of nitric oxide in the D-region during quiet periods

Measurements of electron densities at Saskatoon (52°N, 106° W, L = 4.4) from 1976 to 1979 reveal seasonal variations which cannot be explained solely by solar zenith angle variations. These profiles have been used to infer the variations of nitric oxide density with season and solar activity for quiet-time conditions. It is found that while nitric oxide varies with season, it remains unchanged with the change in solar activity. The summer and spring profiles are much lower than the measured values of Baker et al. (1977) for heights below 85 km, while the winter estimated values show differences from the measured values in the height range 77.5–85 km. Above 85 km the values for all the seasons are close to the measured values. A dip in the nitric oxide distribution is obtained in all the cases around 80–82 km and the values of nitric oxide at the minimum are less than those measured by Baker et al. (1977) or Meira (1971).     

Rocket-borne measurements of atmospheric infrared fluxes

In the Energy Budget Campaign two rockets, one from Andøya Rocket Range, Norway, and one from Esrange, Sweden, each carrying a liquid helium cooled infrared spectrometer, were simultaneously launched as part of salvo B. The launches occurred during the recovery phase of the last of four auroral magnetic events after a Joule heating criteria was exceeded. At Andøya, zenith radiance altitude profiles were obtained of nitric oxide (NO) near 5.4μm from 70 to 185 km (rocket apogee), of ozone (O₃) near 9.6 μm from 70 to 105 km (instrument sensitivity) and of carbon dioxide (CO₂) near 15 μm from 70 to 150 km (instrument sensitivity). Measured CO₂ spectra at 72 km are shown to compare favorably to those calculated for local thermodynamic equilibrium conditions and instrument resolution. By comparing Andøya and Esrange CO₂ radiance profiles it is shown that there is evidence for spatial variation in the emission. Further, it is shown that the very disturbed conditions of salvo B prior to and during these launches appears to have significantly changed the O₃ 9.6μm radiance profiles compared to previous rocket measurements in polar disturbed and quiet atmospheres. Using the nitric oxide radiance profiles and spectrum, previous rocket results and computed models it is shown that no radiance increase could be detected from prompt auroral energy deposition. The results support the thesis that the NO density in auroral regions is significantly enhanced over mid-latitude values and that for weak auroras, the reaction NO(v = 0) + O → NO(r = 1) + O is the dominant radiation mechanism.     

Concentrations of H2O and NO in the mesosphere and the lower thermosphere at high latitudes

Water vapour and nitric oxide concentrations in the mesosphere and lower thermosphere were derived from infrared emission and positive ion composition measurements above northern Europe during the Energy Budget Campaign 1980. The experiments were performed at different levels of geomagnetic disturbance. Both water vapour and nitric oxide are highly variable. Water vapour mixing ratios between 0.2 ppm and 10 ppm were observed. The nitric oxide peak densities varied by more than a factor of ten. Maximum values of 2 × 10⁹cm⁻³ were obtained.     

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.     

D-region observations of polar cap absorption events during the EISCAT operation in 1981–1989

During the years 1981–1989, 71 solar proton events altogether were observed. Dividing the events into strong, p.f.u. > 1000 (p.f.u.—proton flux measured at geosynchronous satellite orbit in units of (cm² s sr)⁻¹), medium, 100 < p.f.u. < 1000 and weak events, p.f.u. < 100, only the strong and medium events have a considerable effect on the lower ionosphere. The mean daily absorption at 30 MHz (A), measured in the auroral zone, is >2 dB during strong events, <2 dB during medium events and < l dB during weak events. The most active year during the EISCAT operation was 1989 when 23 solar proton events were observed including six strong events. Diurnal variation of the electron density in the D-region during PCA is a function of the solar zenith angle. However, south of L = 5 a minimum in absorption is observed during the noon hours. During sunrise the absorption increases simultaneously with solar elevation angle, but during sunset there is about 2 h delay between the decrease of absorption and the solar elevation angle.     

The quiet midlatitude D-region: a comparison between modeling efforts and experimental measurements

Two long-standing problems in the atmospheric sciences have been the correct modeling of the ion chemistry in the earth's atmosphere and the proper determination of the ion species and densities through in situ measurements. Comparison between experimental data and simulations of the data by computer modeling of atmospheric chemistry is a means of validating the model as well as indicating which processes are in need of further study. The DAIRCHEM computer code is used here to simulate data taken in the midlatitude D-region during quiet conditions. On the one hand, comparison between the total positive ion density profile derived from rocket measurements and the one computed by the code shows very good agreement in the 30–90 km range, with the exception that the simulated ion profile is somewhat smaller than the experimental one in the 60–75 km region. Such discrepancy is only partially explained by the inherent uncertainties in the NO density profile or the total ionization rate profile. On the other hand, comparison between the measured and the computed electron density profiles shows that the measured profile is consistently smaller than the computed profile in the 65–85 km range. We interpret this discrepancy as a deficiency in the modeling of the negative ion chemistry. Also, this deficiency is probably the main cause of the disparity between the total positive ion density profiles in the corresponding altitude range. It is felt that the positive ion chemistry of the D-region is reasonably well understood. However, the negative ion chemistry is in need of further study. Specifically, alternate electron attachment/detachment processes should be considered, as well as an as yet undetermined, possibly very massive, negative species which may affect the ion recombination rates.     

Atomic oxygen determination from a nitric oxide point release in the equatorial lower thermosphere

Atomic oxygen density values in the 80–105 km altitude equatorial region have been obtained by analyzing the chemiluminescence of nitric oxide point releases from three CENTAURE II-C rockets. The light emission produced by the NO—O chemiluminous recombination was sufficiently high to render the artificial clouds observable only by ground-based instruments. The difficulties associated with these kind of experiments have been greatly avoided by a new technique ejecting the NO gas into the backward direction of the flight. It has been found that below 90 km the derived atomic oxygen densities are in relatively good agreement with those reported by other workers. At approximately 105 km the measured value is about two times higher than the n(O) density obtained by averaging a set of data from a great number of other nights but coincides rather well with the measurements of Dickinsonal. (1980).     

Studies of high latitude mesospheric turbulence by radar and rocket 1: energy deposition and wave structure

During early spring, 1985, the MAE-3 (Middle Atmospheric Electrodynamics) Program was conducted at Poker Flat Research Range, Alaska to study the origin of wintertime mesospheric echoes observed with the Poker Flat MST radar there, by probing the mesosphere with in situ rocket measurements when such echoes occurred. Pre-launch criteria required the appearance of echoes exhibiting some wave structure on the MST radar display; these could be met even under weak precipitation conditions with riometer absorption near or above 1.0 dB. Two morning rockets were launched under such conditions, the first (31.048) on 29 March 1985, at 1703 UT and the second (31.047) on 1 April 1985, at 1657 UT. Both payloads were deployed on a high altitude parachute near a 95 km apogee to provide a stable platform for data acquisition within the mesosphere (below 80 km). Each payload carried a solid state detector to measure energetic electrons between 0.1 and 1.0 MeV and an NaI crystal detector to measure x-rays from >5 to >80 keV. Payload 31.048 also carried a positive ion ‘turbulence’ probe which measured ion density changes (ΔN_i/N_i) during payload descent, whereas 31.047 carried a nose tip ‘turbulence’ probe designed to measure electron density changes (ΔN_e/N_e) during upleg ram conditions plus a Gerdien condenser for the measurement of bulk ion properties during downleg. The energy deposition curves for each event exhibited peak deposition rates between 75 and 80 km with a half width of 16–18 km, almost exclusively induced by precipitating relativistic electrons. They also showed a maximum bottomside gradient between 65 and 75 km. Radar echoes and atmospheric turbulence were observed in the same altitude domain, consistent with the anticipated need for adequate free thermal electron gradients to make such phenomena visible on the radar. The vertical wave structure from radar echoes was found to be consistent with that observed in horizontal wind and temperature profiles measured by Datasondes flown shortly after each large rocket. An analysis of the wave structure from radar data has shown that although large scale waves (λ_z ~ 7 km) were found to be present, a higher frequency shorter wavelength (∼ 1–3 km) component probably played a more significant role in modulating the signal-to-noise structure of the radar echoes.     

EISCAT results during the ROSE campaign and comparison with STARE measurements

EISCAT measurements were performed during the four ROSE rocket launches. The results are presented. It is shown that the upper altitude limit of instabilities observed by in-situ measurements agrees with calculations using EISCAT results of drift and ion sound speed and assuming the two-stream-instability mechanism. The EISCAT results together with the STARE observations were used to calculate the ion velocity and the ψ-values from the dispersion relation of two-stream-instabilities. A comparison of EISCAT, STARE and in-situ measurements is discussed.     

Daytime mesospheric temperatures over the low-latitude station Thumba derived from rocket-borne solar Lyman-α absorption measurements

Molecular oxygen concentrations obtained from the rocket measurements of the absorption profile of solar Lyman-α radiation, in the upper atmosphere over Thumba (8°33′N, 76°52′E), have been used to derive gas temperatures in the 70–90 km altitude region. A mean reference temperature profile for the daytime mesosphere over Thumba obtained from these measurements shows (1) temperatures lower than those given by the CIRA model and (2) a mesopause around 83 km with a temperature of 175 K.     

The E-region electron density diurnal asymmetry at Saint-Santin: observations and role of nitric oxide

Measurements of the E-region electron density were made with the Saint-Santin incoherent scatter radar during consecutive days in June 1978, March 1979 and December 1980. On the basis of a statistical study, the observations show the presence of a diurnal asymmetry of the electron density, with morning values usually exceeding the afternoon densities by 3–20%. Two possible causes of the dissymmetry are examined: the asymmetry in the diurnal variation of the neutral composition and the effect of nitric oxide. The presence of NO partly converts O₂⁺ into NO⁺ ions and increases the effective recombination rate of the electrons in the afternoon. Numerical simulations assessing the relative importance of the two factors are, in general, in good agreement with the measurements.     

The E-region Rocket/Radar Instability Study (ERRRIS): scientific objectives and campaign overview

The E-region Rocket/Radar Instability Study (Project ERRRIS) investigated in detail the plasma instabilities in the low altitude (E-region) auroral ionosphere and the sources of free energy that drive these waves. Three independent sets of experiments were launched on NASA sounding rockets from Esrange, Sweden, in 1988 and 1989, attaining apogees of 124, 129 and 176km. The lower apogee rockets were flown into the unstable auroral electrojet and encountered intense two-stream waves driven by d.c. electric fields that ranged from 35 to 115 mV/m. The higher apogee rocket returned fields and particle data from an active auroral arc, yet observed a remarkably quiescent electrojet region as the weak d.c. electric fields (~ 10–15 mV/m) there were below the threshold required to excite two-stream waves. The rocket instrumentation included electric field instruments (d.c. and wave), plasma density fluctuation (δn/n) receivers, d.c. fluxgate magnetometers, energetic particle detectors (ions and electrons), ion drift meters, and swept Langmuir probes to determine absolute plasma density and temperature. The wave experiments included spatially separated sensors to provide wave vector and phase velocity information. All three rockets were flown in conjunction with radar backscatter measurements taken by the 50MHz CUPRI system, which was the primary tool used to determine the launch conditions. Two of the rockets were flown in conjunction with plasma drift, density, and temperature measurements taken by the EISCAT incoherent scattar radar. The STARE radar also made measurements during this campaign. This paper describes the scientific objectives of these rocket/radar experiments, provides a summary of the geophysical conditions during each launch, and gives an overview of the principal rocket and radar observations.     

Auroral radio absorption as an indicator of magnetospheric electrons and of conditions in the disturbed auroral D-region

In a previous paper we demonstrated a method by which the auroral radio absorption measured with a riometer can be predicted from energetic electron measurements at geosynchronous orbit. The present paper enquires to what extent the process can be inverted: what levels of magnetospheric electron flux, and of D-region production rate, electron density and incremental absorption, are predicted by a given measurement of radio absorption and what reliance can be placed on such predictions?Using data from 45 precipitation features recorded with riometers in Scandinavia and at geosynchronous orbit with GEOS-2, it is shown that electron fluxes in the ranges 20–40,40–80 and 80–160 keV increase with increasing absorption and can be predicted to better than 50% for absorption events of 2 dB or greater. Electrons above 160 keV show little or no correlation with absorption. D-region production rates and electron densities can be predicted to within factors of 2 and √2, respectively.It is more difficult to specify the height of the absorbing region because of uncertainly in the profile of the effective recombination coefficient. Having regard to other data, an α_eff profile is proposed which satisfies rocket and incoherent scatter data as well as the present calculations. It is shown that any day-night variation in auroral absorption is associated with a change of spectrum rather than a change of recombination coefficient.     

An observational study of the D-region winter anomaly in lower latitudes during sudden stratopsheric warmings

An observational study of the D-region winter anomaly of HF radio wave absorption in lower latitudes has been made during the period of a sudden stratospheric warming of the 1967/1968 winter. By means of large-scale isopleth analysis of the absorption index, ƒ_min, and of meridional winds near 70 km height along 60°N, it is found that there exists a winter anomaly in lower latitudes which is comparable in order to that in middle latitudes, resulting from a nitric oxide (NO) increase due to southward transport from higher latitudes by well-developed planetary wave winds. From the daily changes of absorption in the equatorial region, it is found that the enhanced absorption reveals an oscillation with a period of about 2 weeks and has its maximum in the region south of 20°N. The period is similar to that of planetary wave amplitudes in the winter stratosphere and mesosphere, suggesting that an effect of planetary waves could contribute to the equatorial anomaly of the absorption in the D-region.     

Positive ion composition in the lower ionosphere at high latitudes during MAP/WINE

Positive ion composition, total ion and electron density and ion production by energetic electrons were measured by rocket-borne experiments above Andøya (69.3°N, 16.0°E) in northern Norway. Observed altitudes of transition from molecular ions to proton hydrates and from electrons to negative ions are compared to results from an ion-chemical model. Nitric oxide and water vapour densities are inferred from the ion composition.