The relation between low-latitude neutral density variations near 400 km and magnetic activity indices

Neutral density data were obtained near 400km (1600 LT) from a microphone density gauge on OGO-6 from 0°G to 40°N magnetic latitude for 25 September–3 October 1969. Several geomagnetic storms occurred during this period (a_p varied from 0 to 207). Least-squares fits were made to data points on density-a_p and density-D_st scatter diagrams, where the density values selected were delayed in time behind a_p and D_st. An equation representing the least-squares fit was computed for each delay time. The equation of best fit (and the corresponding time delay between the density and the magnetic index which resulted in this best fit) was found by choosing the equation that gave the minimum standard error. For example, the best fit at 10°N geomagnetic latitude occurred for a_p at t — 3 hr, where t is the time of the density values. The implications of the time differences associated with the best fits at various latitudes and longitudes are discussed with regard to the time delays involved in geomagnetic heating of the neutral upper atmosphere.

A low-latitude density bulge has been found between 0°N and 40°N whose magnitude varies with a_p. DeVries (1972b) has independently discovered this daytime phenomenon. If the bulge is a semi-permanent feature near the equinoxes because of the enhanced geomagnetic activity, this may help explain the semi-annual effect in density, which was uncovered first in the drag data from low inclination satellites.     

Annual and semi-annual zonal wind components and corresponding temperature and density variations, 60–130 km

From rocket and radar-meteor wind observations, annual and semi-annual components of the zonal flow are derived for latitudes N at heights between 60 and 130 km. Height regions of maximum and minimum amplitude are described with reference to changes in phase. The annual components decrease with height throughout the mesosphere and, after a reversal of phase, enhance to 25 m/sec at 100 ± 5 km. The semi-annual components have maximum amplitudes of 25 m/sec over a wide range of latitude in two height regions at 90 and 120 km and in a limited range of latitude (near 50°) at 65 km.

Calculated temperatures and log densities are discussed in terms of amplitude and phase as functions of height and latitude. Below 100 km a comparison is made with temperature amplitudes derived from independent temperature data. Above 100 km the annual temperature variation maximizes at 115 km and is particularly large at high latitudes (exceeding 50°K). On the other hand, the semi-annual component increases rapidly with height between 110 and 120 km at all latitudes maximizing at the 120 km level, where amplitudes exceed 25°K at high and low latitudes and 10°K at mid-latitudes. The annual component of log density, like the temperature variation, is largest at high latitudes up to 125 km. The semi-annual variation has a minimum at 110–115 km, above which amplitudes increase with height, reaching 5–12 per cent at 130 km according to latitude. The phases at and near 130 km for the annual and semi-annual density variations are very close to those found at greater heights from satellite orbits and amplitudes could be readily extrapolated to agree with those in the satellite region.     

A study of “chorus” observed at Australian stations

The very-low-frequency emission known as “chorus” has been studied for four Australian recording stations. The phenomenon exhibits a maximum of occurrence at about geomagnetic latitude 60° and occurs only about one-tenth as frequently at geomagnetic latitude 45°. It was never definitely observed at geomagnetic latitude 35°. Marked diurnal variations are present with morning peaks for all stations and also an evening peak at Adelaide and Hobart. Analysis of the diurnal variations suggests that two different mechanisms are involved. The average strength of chorus increases with increasing value of K-index at lower latitude stations and shows a maximum at moderate values of the index for geomagnetic latitude 61°. This is interpreted as meaning that the region of maximum average-chorus-strength moves towards lower latitudes at times of magnetic disturbance.     

Electron precipitation observations from a rocket flight through the dayside auroral oval

Energy spectra of electrons between 30 eV and 18 keV were obtained with a spectrometer on a Black Brant rocket launched from Cape Parry, N.W.T. (Λ = 75.2°) on December 6, 1974 to study the dayside magnetospheric cleft. The rocket flew to an apogee of 236 km and travelled poleward to 80° invariant latitude. The cleft was observed to extend from 76.9 to 78.4° invariant latitude. Equatorward of this electrons of a few keV energy were observed with a total energy flux of up to 2 erg/cm² sec ster. Variable fluxes of electrons with a spectrum fitted by a Maxwellian distribution of 150 eV characteristic energy were observed through most of the cleft. One inverted V structure was crossed. In that region, the electron energy increased to 650 eV and a total energy flux of 8 erg/cm² sec ster was measured. The event was a temporal one and only a few km in width, as deduced from optical data. Fluxes of about 10⁻² erg/cm² sec ster were recorded poleward of the cleft.     

Average high latitude magnetic field: Variation with interplanetary sector and with season—II : Comparison of disturbance levels and discussion of ionospheric currents

Average high latitude magnetic field data from northern observatories are examined for three ranges of magnetic disturbance level, Kp = 1− to 1+,2− to 3+ and ≥ 4−. Except for 0–8^h MLT, 55–78° invariant latitude, during away interplanetary magnetic field sectors, the variations between season and sector have the same characteristics at all Kp ranges. Because the amplitude of sector differences is much larger at sunlit local times than in the midnight sector, it is concluded that the current system of Svalgaard (1973) is not adequate to describe the sector variations in magnetic disturbance. Other current systems are discussed briefly. The disturbance morphology and seasonal variation at all Kp levels confirms the results of previous studies which indicate that latitudinally broad current systems, like S_q^p and nonionospheric sources are present in addition to latitudinally narrow electrojet currents. Comparison of data between Kp levels indicates that the Harang discontinuity shifts toward earlier MLT with increasing Kp level.     

Limits on the TeV flux of diffuse gamma rays as measured with the HEGRA air shower array

Using data from the HEGRA air shower array, taken in the period from April 1998 to March 2000, upper limits on the ratio I_γ/I_CR of the diffuse photon flux I_γ to the hadronic cosmic ray flux I_CR are determined for the energy region 20–100 TeV. The analysis uses a gamma–hadron discrimination which is based on differences in the development of photon- and hadron-induced air showers after the shower maximum. A method which is sensitive only to the non-isotropic component of the diffuse photon flux yields an upper limit of I_γ/I_CR (at 54 TeV) <2.0×10⁻³ (at the 90% confidence level) for a sky region near the inner galaxy (20°< galactic longitude <60° and |galactic latitude |<5°). A method which is sensitive to both the isotropic and the non-isotropic component yields global upper limits of I_γ/I_CR (at 31 TeV) <1.2×10⁻² and I_γ/I_CR (at 53 TeV) <1.4×10⁻² (at the 90% confidence level).     

The establishment of a continuous declination system with photoelectric astrolabe Mark I—An international cooperation between Lintong and Irkutsk

We propose a cooperative observational project of using the photo-electric astrolabe Mark I of Shaanxi Observatory in Irkutsk. As the instrument will be equipped with a Z = 45° angle prism, it will have no blind zone when used at latitude (φ = 52°. Important contribution can be expected to the establishing of fundamental reference system and the maintenance of the Hipparcos Catalogue system.     

Methods to determine the angular resolution of the HEGRA extended air shower scintillator array

A precise knowledge of the angular resolution of scintillator arrays used to observe extended air showers (EAS) is of key importance in the search for VHE/UHE γ point sources. Four independent methods have been used to determine the mean resolution for which a value of ΔΘ₆₃ of 0.8°(1.0°) at a proton threshold of 50 (40) TeV has been found for the HEGRA EAS-array.     

Median ionospheric height variations over a sunspot cycle in the Australian-Japanese longitudinal sector

The monthly median virtual height (h′F) of the F-region was studied for a period of 6 years (1980–1985) from sunspot maximum to minimum, using data from 11 ionosonde stations in the Japanese-Australian longitudinal sector, in an invariant latitude range: 37°N to 54°S. The night-time maximum in the median height progressively decreases equatorwards, particularly in the local winter and spring, while a reverse weak tendency is observed in summer. The median height reaches peak in both hemispheres from 1 to 2 years after sunspot maximum then decreases towards sunspot minimum. A second diurnal maximum in h′F, preceded by a well-defined minimum, was consistently observed over the solar cycle close to the sunrise time at the F-region, mainly at low invariant latitudes (9–20°). The second maximum has a distinct seasonal variation, being most pronounced in winter and diminishing in summer. It is envisaged that the second peak in h′F is associated with the wave disturbance generated by the supersonic motion of the sunrise terminator. Possible effects of the background height variations on the propagation of the magnetic storm-induced travelling ionospheric disturbances are discussed.     

Stereoscopic observations of gamma rays at the Whipple observatory

Observations of photons at E ≥ 550 GeV from the Crab Nebula are presented and used to assess the potential of multi-telescope systems for γ-ray astronomy.

The Whipple observatory 10 m and 8 m imaging atmospheric erenkov telescopes have been used to provide a stereoscopic view of air showers to make a more complete measurement of air shower parameters. Here we present a measurement of the spread in the arrival direction of primary γ-rays originating from a point source. The data show that the shower arrival direction can be reconstructed with an accuracy of σ = 0.°14.     

On the heliographic latitude dependence of the solar wind velocity

Plasma data from Pioneers 6–7 and from a variety of satellites operating near the Earth are used to investigate the heliographic latitude dependence of the solar wind bulk speed near the sunspot maximum. No evidence is found for a latitude effect: the latitudinal gradient, if any, turns out to be 2 km (sec degree)^–1, to be compared with the gradient of 10 km (sec degree)^–1 observed in periods of low or moderate solar activity.     

Galactic satellite systems: radial distribution and environment dependence of galaxy morphology

We have studied the radial distribution of the early (E/S0) and late (S/Irr) types of satellites around bright host galaxies. We made a volume-limited sample of 4986 satellites brighter than   M _r=−18.0  associated with 2254 hosts brighter than   M _r=−19.0  from the Sloan Digital Sky Survey Data Release 5 sample. The morphology of satellites is determined by an automated morphology classifier, but the host galaxies are visually classified. We found segregation of satellite morphology as a function of the projected distance from the host galaxy. The amplitude and shape of the early-type satellite fraction profile are found to depend on the host luminosity. This is the morphology–radius/density relation at the galactic scale. There is a strong tendency for morphology conformity between the host galaxy and its satellites. The early-type fraction of satellites hosted by early-type galaxies is systematically larger than that of late-type hosts, and is a strong function of the distance from the host galaxies. Fainter satellites are more vulnerable to the morphology transformation effects of hosts. Dependence of satellite morphology on the large-scale background density was detected. The fraction of early-type satellites increases in high-density regions for both early- and late-type hosts. It is argued that the conformity in morphology of galactic satellite system is mainly originated by the hydrodynamical and radiative effects of hosts on satellites.     

Latitudinal variations in Titan’s methane and haze from Cassini VIMS observations

We analyze observations taken with Cassini’s Visual and Infrared Mapping Spectrometer (VIMS), to determine the current methane and haze latitudinal distribution between 60°S and 40°N. The methane variation was measured primarily from its absorption band at 0.61 μm, which is optically thin enough to be sensitive to the methane abundance at 20-50 km altitude. Haze characteristics were determined from Titan’s 0.4-1.6 μm spectra, which sample Titan’s atmosphere from the surface to 200 km altitude. Radiative transfer models based on the haze properties and methane absorption profiles at the Huygens site reproduced the observed VIMS spectra and allowed us to retrieve latitude variations in the methane abundance and haze. We find the haze variations can be reproduced by varying only the density and single scattering albedo above 80 km altitude. There is an ambiguity between methane abundance and haze optical depth, because higher haze optical depth causes shallower methane bands; thus a family of solutions is allowed by the data. We find that haze variations alone, with a constant methane abundance, can reproduce the spatial variation in the methane bands if the haze density increases by 60% between 20°S and 10°S (roughly the sub-solar latitude) and single scattering absorption increases by 20% between 60°S and 40°N. On the other hand, a higher abundance of methane between 20 and 50 km in the summer hemisphere, as much as two times that of the winter hemisphere, is also possible, if the haze variations are minimized. The range of possible methane variations between 27°S and 19°N is consistent with condensation as a result of temperature variations of 0-1.5 K at 20-30 km. Our analysis indicates that the latitudinal variations in Titan’s visible to near-IR albedo, the north/south asymmetry (NSA), result primarily from variations in the thickness of the darker haze layer, detected by Huygens DISR, above 80 km altitude. If we assume little to no latitudinal methane variations we can reproduce the NSA wavelength signatures with the derived haze characteristics. We calculate the solar heating rate as a function of latitude and derive variations of ∼10-15% near the sub-solar latitude resulting from the NSA. Most of the latitudinal variations in the heating rate stem from changes in solar zenith angle rather than compositional variations.     

Variations in the solar wind speed along the earth's orbit

Comparison of solar wind speed data obtained from the Pioneer 6 and 7 and Vela 3, 4, and 5 satellites from January 1969 through July 1970 has been undertaken. The distribution of measured speeds is similar for all satellites, despite wide separations along the Earth's orbit. For satellite separations (along the Earth's orbit) of 0.5 AU or less, the speeds measured by different satellites are closely correlated, i.e., it is usually possible to predict (to within ± 100 km sec^–1) the arrival of a particular solar wind speed at one satellite on the basis of earlier measurements at another. For separations larger than 1.0 AU it is usually not possible to make accurate predictions in this manner. This appears to be evidence that: (1) the boundary conditions on the coronal expansion at the base of the corona are a sensitive function of latitude and/or (2) the boundary conditions at any one point on the Sun evolve sufficiently in 4 days to alter significantly the speed of the wind at 1 AU.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Energetic electrons in the cusp and in the high latitude plasma sheet: Evidence for source regions

HEOS-2 has observed energetic electrons (> 40 keV) in the high latitude magnetosphere appearing as one or more peaks outside and often well separated from the trapping boundary. Most of the observations are between 70° and 80° invariant latitudes both in the day and nightside. The peaks are located in the dayside adjacent to the polar cusp and coincide in the nightside with the edge of the plasma sheet. The electron peak intensity on the nightside shows a clear correlation with AE. The electron peak intensities on the dayside exceed those on the nightside and are generally higher in the pre-noon than in the afternoon sector. Observations on the dayside in the distant cusp region and in the adjacent magnetosheath show high and fluctuating intensities of energetic electronswith an energy spectrum much harder than in the outermost trapping region.

This observational evidence suggests different source regions for these energetic electrons: one in the distant geomagnetic tail and another one around the dayside cusp indentation.     

Direction finding measurements of type III bursts in both elevation and azimuth

Direction finding measurements with the plasma wave experiments on the HAWKEYE 1 and IMP-8 satellites are used to find the source locations of type III solar radio bursts in elevation (geocentric solar ecliptic latitude) and azimuth (geocentric solar ecliptic longitude) in a frequency range from 31.1 kHz to 500 kHz. IMP-8 has its spin axis perpendicular to the ecliptic plane, hence by analyzing the spin modulation of the signals the location of the type III burst projected into the ecliptic can be found. HAWKEYE 1 has its spin axis nearly parallel to the ecliptic plane, hence the elevation of the source may also be determined. The trajectory of the electrons generating the burst, projected onto the ecliptic plane, follows an Archimedean spiral. Out of the ecliptic plane the trajectory is at a nearly constant heliographic latitude. The electrons originate from a region near a solar flare. With direction measurements of elevation and azimuth along with the modulation factor it is possible to determine the source size. Typical half angle source sizes range from 60° at 500 kHz to 40° at 56.2 kHz as viewed from the sun.     

The spectrum of electron content fluctuations in the ionosphere

Continuous records of the electron content of the ionosphere, from 1965 to 1970, are used to obtain power spectra covering periods from 30 sec to 2 yr at latitudes of 34°S and 42°S. At periods up to 5 min, amplitudes were less than 0.2 per cent of the total electron content. Variations produced by gravity waves were very common in the range 20–80 min, with no preferred periods. The r.m.s. amplitude per octave A₀ was about 10¹⁵ electrons/m², or 0.6 per cent of the mean electron content. The amplitude increased during the day, particularly in winter when periodic components predominated. The cut-off at about 17 min was sharply defined, giving a mean scale height for the neutral atmosphere (at 300 km) of about 43 km in summer, 47 km on winter days and 42 km on winter nights.

From 12 hr to 1 month A₀ was about 12 per cent of the mean electron content in both summer and winter at 34°S, and 10 per cent at 42°S. The 24 hr and 27 day peaks were largest just before sunspot maximum, and almost disappeared near sunspot minimum. Variations between 1 and 27 days reflect the random occurrence of ionospheric storms and show no consistent peaks. Day to day and night to night variations were both about 10 per cent of the background content for periods from 2 days to 2 yr, apart from a slight decrease between 1 and 6 months.     

Tianjin photographic zenith tube observations

In this paper we analyse the observational data obtained by the Chinese-made PZT in the two periods 1979 Feb – 1980 May and 1981 Dec – 1983 March. The internal accuracy of single star is found to be m_u = ±13.0 ms, m_φ = ±0. “144 for the first period, and m_u = ±14.6 ms, m_φ = ±0.” 152 for the second. Correction of star position is found by the chain method. The systematic error caused by the sealed window of the evacuated chamber and the temperature effect of the plate scale are investigated. Monthly means of time and latitude are given.     

N2 emission in the twilight

About a year's observations of the N₂⁺ band (3914 Å) at Kitt Peak (latitude 32°) are reported. Morning intensities are the same throughout the year, but there is a strong winter maximum in the evening. It is suggested that the additional ionization is produced by photoelectrons from the magnetic conjugate point. Heights are estimated by the zenith-horizon method, which gives 235 km for the constant component and 350 km during the evening enhancement. The intensity variation through twilight is therefore entirely due to changes of the N₂⁺ concentration; each ion scatters light at a constant rate. The rotational distribution resembles that for a temperature of 1600°K, much higher than the temperature of the atmosphere. It is suggested that part of the ions may be produced by charge transfer from metastable O⁺(²D). N₂⁺ concentrations resulting from photoionization are calculated; they give a fair account of the observed horizon intensities, but not the zenith. Non-local electrons from higher in the atmosphere are suggested as a possible extra source; alternatively, the zenith measurements may be perturbed by scattered horizon light. The band intensity in the nightglow cannot be measured; the upper limit is 1 R.     

Rare opportunity for information on the upper atmosphere of Saturn

We report on taking, successfully, the rare opportunity to monitor photoelectrically the eclipse of Saturn's largest satellite (Titan) and present a light curve. Comparing this light curve with similar ones obtained for Jovian satellites we deduce the Saturnian stratosphere to be relatively clear, at least at the latitude (25° S) probed.