Multiple wavelength observations of flaring active regions

We present observations of flaring active regions with the Very Large Array (V.L.A. at 6 cm and 20 cm wavelengths) and the Westerbork Synthesis Radio Telescope (W.S.R.T. at 6 cm wavelength). These are compared with photospheric magnetograms (Meudon) and with Hα and offband Hα photographs (Big Bear and Ottawa River Solar Observatories). The 6 cm radiation of these active regions marks the legs of dipolar loops which have their footpoints in lower-lying sunspots. The intense, million degree radiation at 6 cm lies above sunspot umbrae in coronal regions where the longitudinal magnetic field strength H_? = 600 Gauss and the height above the sunspot umbrae h = 3.5±0.5 × 10⁹ cm. Circularly polarized horseshoe structures at 6 cm ring the sunspot umbrae. The high degree of circular polarization (?_c = 95%) of the horseshoes is attributed to gyroresonant emission above sunspot^? penumbrae. The 20 cm radiation of these active regions exhibits looplike coronal structures which extend across regions of opposite magnetic polarity in the underlying photosphere. The 20 cm loops are the radio wavelength counterparts of the X-ray coronal loops. We infer semilengths L = 5 × 10⁹ cm, maximum electron temperatures T_e(max) = 3 × 10⁶ K, emission measures ∫N_e²d$\text{l}$ = 10²⁸ cm^?5, and electron densities N_e = 10⁹ cm^?3 (or pressures p = 1 dyn cm^?2) for the 20 cm bremsstrahlung. A total of eight solar bursts were observed at 6 cm or 20 cm wavelength with second-of-arc angular resolution. The regions of burst energy were all resolved with angular sizes between 5″ and 30″, brightness temperatures between 2 × 10⁷ K and 2 × 10⁸ K, and degrees of circular polarization between 10% and 90%. The impulsive phase of the radio bursts are located near the magnetic neutral lines of the active regions, and between the flaring Hα kernels which mark the footpoints of magnetic loops. In one case there was preburst heating in the coronal loop in which a burst occurred. Snapshot maps at 10 s intervals reveal interesting burst evolution including rapid changes of circular polarization and an impulsive burst which was physically separated from both the preburst radio emission and the gradual decay phase of the burst.     

Ionosphere-plasmasphere electron fluxes at middle latitudes obtained from whistlers

985 whistlers observed between 1970 and 1975 in Hungary have been processed for equatorial plasmaspheric electron density and tube electron content above 1000 km (N_T). The hourly median value of N_T exhibits a diurnal variation with an amplitude of 1×10¹³ electrons/cm²-tube. 75 per cent of the electron flux values obtained from the time variation of N_T are lower than 6×10⁸ el cm^?2s^?1, while in some cases the fluxes reach a value as high as 3×10⁹ el cm^?2s^?1. Between 17 and 04 LT the dominant flux direction is toward the ionosphere. The data also indicate that the day to day filling of the plasmasphere after magnetic disturbances continues through several days without exhibiting saturation, with higher filling rates for lower values of average K_p.     

Venus EUV measurements of hydrogen and helium from Venera 11 and Venera 12

Lyman α and 58.4 nm HeI radiations resonantly scattered were observed with EUV spectrophotometers flown on Venera 11 and Venera 12. The altitude distribution of hydrogen was derived by limb observations from 250 km (exobase level) to 50,000 km. In the inner exosphere (up to ? 2,000 km of altitude) the distribution can be described by a classical exospheric distribution with T_C = 275 ± 25 K and n = 4_?2⁺³ × 10⁴ atom. cm^?3 at 250 km. The integrated number density from 250 to 110 km (the level of CO₂ absorption) is 2.1 × 10¹² atom. cm^?2, a factor of 3 to 6 lower than that predicted by aeronomical models. This number density decreases from the morning side to the afternoon side, or alternately from equatorial to polar regions. Above 2,000 km a “hot” hydrogen population dominates, which can be simulated by T = 10³K and n = 10³ atom. cm^?3 at the exobase level.The optical thickness of helium above 141 km (the level of CO₂ absorption for 58.4 nm radiation) was determined to be τ_o = 3, corresponding to a density at 150 km of 1.6 × 10⁶ cm^?3. This is about 3 times less than what was obtained with the Bus Neutral Mass Spectrometer of Pioneer Venus, and about twice less than ONMS measurements, but is in agreement with earlier EUV measurement by Mariner 10 (2 ± 1 × 10⁶ cm^?3).     

Atomic oxygen concentrations in the lower auroral thermosphere

Atomic oxygen concentrations derived from the nightglow components of O₂(b¹Σ_g⁺ ? X³Σ_g^?) (0-0) Atmospheric Band emission profiles obtained during two rocket-borne photometer measurements of aurora are presented. The peak atomic oxygen concentrations of 2–3 × 10¹¹ cm^?3 are in better agreement with those of the MSIS-83 model than the CIRA 1972 mean reference atmosphere, although the shapes of the two derived profiles differ considerably from that of MSIS-83. The derived concentrations do not contradict the suggestion made by some investigators that the atomic oxygen concentrations in the auroral zone are depleted relative to their mid-latitude values.     

Impact of a novel hybrid accelerometer on satellite gravimetry performance

The state-of-the-art electrostatic accelerometers (EA) used for the retrieval of non-gravitational forces acting on a satellite constitute a core component of every dedicated gravity field mission. However, due to their difficult-to-control thermal drift in the low observation frequencies, they are also one of the most limiting factors of the achievable performance of gravity recovery. Recently, a hybrid accelerometer consisting of a regular EA and a novel cold atom interferometer (CAI) that features a time-invariant observation stability and constantly recalibrates the EA has been developed in order to remedy this major drawback. In this paper we aim to assess the value of the hybrid accelerometer for gravity field retrieval in the context of GRACE-type and Bender-type missions by means of numerical closed-loop simulations where possible noise specifications of the novel instrument are considered and different components of the Earth’s gravity field signal are added subsequently. It is shown that the quality of the gravity field solutions is mainly dependent on the CAI’s measurement accuracy. While a low CAI performance (10^?8 to 10^?9?m/s²/Hz^1/2) does not lead to any gains compared to a stand-alone EA, a sufficiently high one (10^?11?m/s²/Hz^1/2) may improve the retrieval performance by over one order of magnitude. We also show that improvements which are limited to low-frequency observations may even propagate into high spherical harmonic degrees. Further, the accelerometer performance seems to play a less prominent role if the overall observation geometry is improved as it is the case for a Bender-type mission. The impact of the accelerometer measurements diminishes further when temporal variations of the gravity field are introduced, pointing out the need for proper de-aliasing techniques. An additional study reveals that the hybrid accelerometer is – contrary to a stand-alone EA – widely unaffected by scale factor instabilities.     

Thermal and thermosolutal convective effects in bridgman solidification: Effect of orientation relative to gravity

A concentrated binary system (Ge-Si) and a dilute one (Ge-Ga) have been solidified at 1 g, in a new vertical Bridgman furnace where radial thermal gradients have been minimized. Very different solutal boundary layer extents (δ_Ge-Si = 3 cm, δ_Ge-Ga = 2.5 mm) obtained for both systems solidified in the same thermal conditions are explained by simple analytical hydrodynamic models. It is demonstrated that :1. The convective transport of Ga in the Ge-Ga system is due to the unavoidable residual horizontal thermal gradients associated with discontinuity of thermal properties at the solid-liquid interface.2. The larger boundary layer extent observed for Ge-Si and the corresponding pure diffusive transport is the result of the stabilizing effect of the longitudinal solutal gradient in this system.Thus, in the case of dilute systems, convective effects can only be cancelled through a reduction of the gravity level. On the opposite, pure diffusive solute transport can be achieved in the vertical configuration a 1 g in the case of concentrated systems where a stabilizing solutal effect is operating.     

Positive ion composition and electron density in a combined auroral and NLC event

The positive ion composition and electron density were measured in the lower ionosphere above Kiruna in salvo A of CAMP (Cold Arctic Mesopause Project). The CAMP/P (S37/P) payload carrying a magnetic ion spectrometer, positive ion and electron probes, and propagation experiments was launched on 3 August 1982 2332 UT during extended Noctilucent Clouds (NLC) and auroral activities over Kiruna. The measured electron density was 5×10³cm^?3 at 80 km and 2.5×10⁵cm^?3 at 90 km. The increase of ion and electron densities in the D- and E-region during twilight was caused by precipitating auroral particles. The height distribution of the positive ions measured by the mass spectrometer in the mass range 19–280 amu is different from a winter flight with similar auroral conditions. Below 85.5 km proton hydrates H⁺(H₂O)₃ ? H⁺(H₂O)₈ were the dominant ions. The heaviest proton hydrates H⁺(H₂O)₇ and H⁺(H₂O)₈ were most abundant at 82–85.5 km, the altitude of visible NLC. Above 85.5 km O₂⁺ and NO⁺ became dominant. A small metal ion layer was observed between 90.5–93 km with a maximum ion density of 10% of the total positive ion density at 91 km altitude. The metal ion density disappeared within about a km below 90.5 km.     

Mass transport in the near vicinity of solidification fronts under conditions of microgravity

Cellular solidification has been known and discussed since a long time. The appearance of cellular and dendritic microstructure closely resembles Benard cells known from fluid physics. Similar generation mechanismus may possibly be assumed. Both g-dependent and g-independent convective phenomena may probably be linked to the occurrence of instabilities at solidification fronts. It is to be expected that normal freezing of model alloys (advantageously such with no volume change at the freezing point) in a defined temperature gradient (gradient furnace) and quenching them may help to quantity g-influence on solidification.     

On the physical state in the narrow-line region of the classical Seyfert galaxy NGC 7469

Based on the spectrophotometric data, by decomposition of the observed continuum, the power - law continuum characteristics of the central source are obtained. The behaviour of both electron temperature T_e and density n_e in the Narrow - Line Region (NLR) of NGC 7469 is discussed on the thermal and ionization equilibria calculations. T_e in NLR of the Sy1 galaxies are higher than the Sy2 ones, and the possible explanation is the lack of the dense (n_e 10¹⁰ cm⁻³) zone close to the central source in the Sy2 galaxies.     

Boundary-layer analysis for the convection/diffusion transition in dendritic growth

Earlier ground-based studies have shown that a sharp transition occurs in the dominant mode of heat transport during dendritic solidifcation. Specifically, convective heat transport dominates below the transition supercooling and diffusive heat transport dominates above. Thus the influence of gravitational acceleration on dendritic crystallization depends critically on the overall thermodynamic driving force. Since many practical solidification processes occur at relatively small supercoolings, understanding the nature of the transition in transport mode is essential for both terrestrial and space processing. A boundary-layer analysis has been carried out wherein the diffusion boundary-layer thickness (Stefan length) is compared with the thermal boundary layer thickness determined from the fluid mechanics of natural convection. Our analysis shows that the Stefan length δ_S decreases rapidly with supercooling (ΔT^?2.5) whereas the thermal boundary layer thickness δ_T decreases slowly (ΔT^?.25). The transition supercooling at which the transport mode switches from convective to diffusive occurs where δ_T = δ_S. The influence of gravitational acceleration, and the thermo-physical properties of the molten phase will be discussed, along with avenues for conducting future research.     

Thermal net flux measurements on the pioneer Venus entry probes

Corrected thermal net radiation measurements from the four Pioneer Venus entry probes at latitudes of 60°N, 31°S, 27°S, and 4°N are presented. Three main conclusions can be drawn from comparisons of the corrected fluxes with radiative transfer calculations: (1) sounder probe net fluxes are consistent with the number density of large cloud particles (mode 3) measured on the same probe, but the IR measurements as a whole are most consistent with a significantly reduced mode 3 contribution to the cloud opacity; (2) at all probe sites, the fluxes imply that the upper cloud contains a yet undetected source of IR opacity; and (3) beneath the clouds the fluxes at a given altitude increase with latitude, suggesting greater IR cooling below the clouds at high latitudes and water vapor mixing ratios of about 2–5×10^?5 near 60°, 2–5×10^?4 near 30°, and >5×10^?4 near the equator.     

Measurement of minor species (H2, Cl,O3, NO) in the earth's atmosphere by the occultation technique

The stellar occultation technique is a clean and powerful means of detecting and quantifying minor gases in the earth's atmosphere. The results obtained are totally insensitive to knowledge of the absolute flux of the star, and are not influenced by instrument calibration problems. Pioneering observations of nocturnal mesospheric ozone and thermospheric molecular oxygen by the stellar occultation technique were made in 1970 and 1971 with the Wisconsin stellar photometers on board the Orbiting Astronomical Observatory-2. A limb crossing geometry was used. The high resolution Princeton ultraviolet spectrometer aboard Copernicus was used in the summers of 1975, 1976 and 1977 to measure altitude profiles of molecular hydrogen, atomic chlorine and nitric oxide in addition to ozone and molecular oxygen. A limb grazing geometry was employed. The ozone densities show wide variation from orbit to orbit and particularly betewen the OAO-2 and Copernicus observations. A H₂ density of 1×10⁸ cm^?3 at 95 km, and a NO density less than 10⁶ cm^?3 for altitudes greater than 85 km were measured.     

The X-ray properties of normal galaxies

An extensive program to study nearby normal galaxies was carried out by various observers using the imaging instruments on the $\text{Einstein}$ Observatory; more than 50 such galaxies were detected with 0.5 – 3.0 keV luminosities ranging from 2 × 10³⁸ ergs s^?1 to 3 × 10⁴¹ergs s^?1. The X-ray luminosity of normal galaxies is ~2 × 10^?4 of the optical luminosity and shows no strong correlation with morphological type. For the nearest galaxies, (the Large and Small Magellanic Clouds, M31 and M33,) studies, performed with the Observatory, were comparable to the $\text{Uhuru}$ survey of the Galaxy. Approximately 30 new SNR were recognized in the Magellanic Clouds as a result. Over 90 sources were detected in M31 of which at least 20 are identified with globular cluster. The numbers of luminous (>10³⁷ ergs s^?1) sources detected in the nearest galaxies per unit mass are similar to that found in our own galaxy. Individual X-ray sources in the arms of nearby spirals can be very luminous; seven with luminosities in excess of 10³⁹ergs s^?1 have been discovered. The nuclei of some, but not all, normal galaxies are luminous X-ray sources; X-ray activity is not presently predictable from the radio or optical properties of the nucleus.     

Particle and Joule heating of the neutral polar thermosphere in cusp region using Atmosphere Explorer-C satellite measurements

Simultaneous measurements taken by instruments on the Atmosphere Explorer - C satellite were used to compare electron and proton particle energy deposition, Joule heating, and neutral density perturbations in the region of the cusp.Altitude profiles of Joule heating, electron energy deposition, and electron density are derived using measurements taken by the satellite as input to a computer model. Electric fields are calculated using ion drift measurements. Figures are presented for a representative orbital pass.A peak Joule heating rate of 0.059 Wm^?2 occurred in the cusp region with a peak of 0.025 Wm^?2 in the evening auroral electrojet. Peak volume heating rates corresponding to these regions were 1.4 × 10^?6Wm^?3 and 7.10^?7 Wm^?3, both occurring at an altitude of 115 km. Particle energy deposition was about an order of magnitude less than Joule heating. Large neutral density perturbations are related to regions of heating.     

Recent infrared observations of comets with UKIRT and IRAS

A preliminary analysis of infrared observations of comets P/Crommelin and P/Tempel 1 is presented. Comet P/Crommelin was observed from UKIRT over the range 1–20 micron, using standard filters. From the shape of the thermal emission spectrum, the temperature of the dust grains is estimated (T = 314 ± ₃₃⁴⁴K) and also the dust production rate (1.3 × 10⁵gs^?1). Comet P/Tempel 1 was observed with the Infrared Astronomical Satellite (IRAS). The emission is found to be considerably extended and there is also evidence for temperature variation of the dust grains as indicated by the 12 to 25 micron flux ratio.     

Latitudinal influences on the quiet daytime D-region

Four versions of a steady-state quiet D-region model are presented. They differ from each other as a result of latitudinal differences in total neutral particle concentrations, nitric oxide concentrations and cosmic ray ionization rates. The total ion concentration profiles of all four versions have minima near 70 km which range from about 10⁸ m^?3 at high latitudes to 3.5 × 10⁷ m^?3 at equatorial latitudes for a solar zenith angle of 60°. Neutral density differences among the four cases result in important vertical shifts for the respective D-region profiles relative to one another. A “C-layer” is evident for the high and mild-latitude models at large solar zenith angles. The altitude where the negative ion/electron concentrations ratio is unity varies from about 63 to 67 km. The computed results are compared briefly with the extensive data base in the literature.     

Charges on dust particles

We discuss the potential (charge) on dust particles in various environments. We first consider the classical case of a single isolated dust particle. In conditions which apply to planetary dust rings, the exact value of the dust potential depends critically on several effects (e.g. secondary electron emission, photoelectric efficiency) which are not well known for small dust particles of relevant material and surface conditions. In dust clouds of high dust densities the classical approach fails to give the correct value of the dust potential due to the neglect of collective effects. In terms of an ordering parameter P = a_μN_d0/n₀ (dust radius in microns × cloud dust density/exterior plasma density) the collective effects on the dust potential become apparent at P ～ 10^?6. For increasing values of P the collective effects increase, whence the dust potentials decrease and eventually approach zero.     

Simultaneous dual wavelength observations of an impulsive microwave burst using the V.L.A.

Simultaneous observations of a microwave burst at 2 and 6 cm wavelengths were carried out with the Very Large Array (VLA). The 6 cm burst source is located close to a magnetic neutral line, presumably near the top of a flaring loop, while the 2 cm emission originates from the footpoints of the loop. It is concluded that the 6 cm emission is dominated by gyrosynchrotron radiation of the thermal electrons in the bulk heated plasma at a temperature of ～ 4 × 10⁷ K, while the 2 cm emission is due to nonthermal particles released and accelerated during the flare process. From the observed low degree of polarization and the lack of the 2 cm source cospatiality with the 6 cm source a magnetic field of 200–350 G and δ ? 4 are estimated in the flare energy release site. A DC electric field flare model is invoked to explain the long delay between the peaks at the two wavelengths. From the delay, the strength of the electric field is estimated to be 0.2–4 μ statvolt cm^?1 in the flaring region.     

Vegetation and the atmospheric cycling of mercury

Beyond their role in the cycling of the major elements, carbon, nitrogen, oxygen, etc., little is known about geobiological interactions involving plants with the elements of lower abundance. Plants influence the distribution of volcanic mercury by uptake from the atmosphere and soil fluids, transport to the shoot and re-release as Hg° into the atmosphere. Release rates as high as 2.5 × 10^?4 g.h^?1 per kg are known but even at more typical rates, fluxes far greater than the Environmental Protection Agency calculated U.S. average annual degassing rate of 130 × 10^?6 g.m^?2 should exist. On a global basis, the presence or absence of vegetative cover may be a significant regulator of Hg° transport between land surfaces and the atmosphere.