Low-altitude Measurements Research Articles

On the remote sensing of plasma sheet from low-altitude spacecraft

The possibility to estimate plasma sheet parameters from low-altitude measurements looks quite attractive, but it critically depends on how isotropic the plasma pressure is in the flux tube. To evaluate the ion pressure anisotropy we compare the values of pressure in the ionospheric and equatorial parts of the field line. Ionospheric values were computed from proton measurements at NOAA low-altitude satellites, they were compared with pressure estimates computed from empirical magnetic field models as well as with average values known from direct plasma sheet measurements. Three different methods of mapping the plasma pressure from plasma sheet to low altitude have been tried; each uses the particle isotropic boundaries observed at low altitudes and/or computed from magnetospheric models. Excluding observations obtained during substorm expansion, from these comparisons we conclude that in the plasma sheet, at geocentric distances 9–20 R E, the pressure estimates in the ionospheric and equatorial parts of the plasma sheet flux tube agree very well, suggesting a good pressure isotropy and thus justifying a possibility to monitor the plasma sheet parameters based on low-altitude measurements. The results also illustrate the usefulness of isotropic boundaries as a label of tail current intensity and as reliable tool for establishing mapping between magnetosphere and ionosphere.

Enhancing the Accuracy of Low-Altitude Target Elevation Measurements by Direction Finders of Sum-and-Difference Type

Enhancing the Accuracy of Low-Altitude Target Elevation Measurements by Direction Finders of Sum-and-Difference Type

Alfvén modulation of the substorm magnetotail transport

This work explores the proposition that magnetic field resonances modulate the transport of particle and magnetic flux in the earth's magnetotail. Coordinated high‐ and low‐altitude measurements during a substorm show periodic variations of magnetic field, plasma density and flow bursts in the magnetotail with frequencies that correlate well with those of magnetic field oscillations at geosynchronous altitude and pulsations measured by ground‐based stations. The pulsations' frequency is in the Pc 5 range. The example presented here occurred during the substorm period of May 19–20, 1996, that was chosen as the first ISTP/GGS campaign for the ground‐based and theory investigations. There appears to be a resonance between modes sustained by the near‐Earth magnetic field and compressional and flapping pulsations that can be sustained by the magnetotail.

Energetic Particles as tracers of magnetospheric configuration

The idea of using the Energetic Particles (EPs) as tracers of the magnetospheric configuration is dated back to sixties although it underwent a considerable evolution. The past experience and the interpretation of EP boundaries are briefly reviewed. A novel approach based on low-altitude measurements of isotropic EP boundaries is described as a quantitative method to investigate the magnetospheric configuration as well as to select and modify magnetospheric models. Some test results and applications are presented.

Impulsive ion injections in the morning auroral region

Low altitude (less than 1000 km) measurements of ions precipitating into the morning auroral region are presented and analyzed. The ion fluxes exhibited time‐energy signatures consistent with impulsive injection onto high‐altitude field lines, followed by time‐of‐flight dispersion. The origin of these ions is investigated through the detailed examination of these signatures in conjunction with simultaneous measurements of precipitating electrons and a magnetic field model. A model is developed which indicates that the source for these particles was located in or near the magnetopause boundary layer, with the position deduced to be in the midlatitude flank region about 20–30 RE tailward of the Earth. The model explains the existence of multiple injections on a given field line as due to a quasi‐periodic source, with the periodicity being about 100–200 s at the source. Several mechanisms are examined in an attempt to explain the injections, with a mechanism related to the propagation of waves on the surface of the boundary layer found to be the most plausible. The observations and results are compared to those of similar experiments and some unifying ideas are discussed.

Relativistic electron enhancements; Simultaneous measurements from synchronous and low altitude satellites

We present here for the first time simultaneous measurements of trapped relativistic electron enhancements at synchronous altitude and precipitating electrons in the bounce loss cone at low altitudes. The measurements show that the daily variations in the precipitation flux for L > 5 correlated well with the daily variations in the total flux at high altitude, both with respect to sudden enhancements as well as flux depletions. The daily averaged precipitating flux (E > 1 MeV) at L=6.1 to 7.1 was about 0.3 percent of the daily averaged directional flux (E ≥ 1.5 MeV) observed at synchronous altitude, whereas within narrow spikes the precipitating directional fluxes were often within a factor of 10 of the daily averaged trapped fluxes. Strong depletions in the synchronous altitude ≥1.5 MeV electron intensities are shown to be associated with low altitude measurements of the equatorward movement of precipitating spikes to lower L shells.

Ionospheric ion composition from satellite measurements made during 1970–1980: Altitude profiles

Ion mass spectrometers were carried by a number of satellites in the 1970s. The ion composition measurements from two of these missions, the Orbiting Geophysical Observatory-6 and the Atmosphere Explorer-C, have been collected into an ion composition data base to evaluate several widely used empirical and theoretical models for the species H +, He +, N +, O +, NO +, N 2 +, and O 2 +. The data base covers all latitudes and local times, and the altitude range from 150 km to 1200 km, but here we present altitude plots of the ion densities at noon and at dip latitudes of 20–40° N. The satellite data are compared with an early ion density profile by Johnson, with the Kochnlein and IRI-90 empirical models, and with the Utah State University theoretical ionosphere model. These comparisons serve to verify some aspects of the models, but they also reveal some outstanding differences. The solar activity dependence of H +, He +, N +, and O + is demonstrated, although this has not been possible for the molecular ions because low altitude measurements have not been made near solar maximum.

94-GHz propagation in the evaporation duct

One-way, low-altitude radio propagation measurements at 94 GHz and simultaneous surface meteorological measurements were made on a 40.6 km, over-horizon, over-water path along the southern California coast to assess the effects of the evaporation duct on signal propagation. More than 2000 h of RF and meteorological data were recorded in eight measurement periods from July 1986 to July 1987. On this path, the transmission loss in a standard atmosphere is nearly 280 dB; the median transmission loss measured is approximately 220 dB. This significant decrease in loss is due to the evaporation duct. A propagation model is used to predict transmission loss from observed surface meteorology; predictions derived from 10-min averages of wind speed, air temperatures, sea temperature, and humidity compare favorably to the measured values. On average, the modeling underestimates observations by only 10 dB. The propagation model is used to predict transmission loss from an independent climatology of evaporation duct heights. A comparison of the modeled distribution to the observed distribution is favorable. The accuracy of the propagation model provides a strong justification for using it to assess the propagation characteristics of millimeter-wave communication and radar systems operating in many, if not all, ocean regions. >

Electric fields in the magnetosphere-the evidence from ISEE, GEOS and Viking

Electric field measurements on the satellites GEOS-1, GEOS-2, ISEE-1, and Viking have extended the empirical knowledge of electric fields in space to include the outer regions of the magnetosphere. While the measurements confirm some of the theoretically expected properties of the electric fields, they also reveal unexpected features and a high degree of complexity and variability. The existence of a magnetospheric dawn-to-dusk electric field, as expected on the basis of extrapolation from low-altitude measurements, is confirmed in an average sense. However, the actual field exhibits large spatial and temporal variations, including strong fields of inductive origin. At the magnetopause, the average (dawn-to-dusk directed) tangential electric field component is typically obscured by irregular fluctuations of larger amplitude. In addition, data from electric-field measurements provide further support for the conclusion that a nonvanishing magnetic-field aligned electric field exists in the auroral acceleration region.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Comparison of field-aligned currents at ionospheric and magnetospheric altitudes

Using the empirical terrestrial magnetospheric magnetic field models of Tsyganenko and Usmanov (TU) and Tsyganenko (T87), we determined the average field-aligned currents (FACs) in the magnetosphere as a function of the K p index. We identify three major model FAC systems; dayside region 1 (DR1), nightside region 1 (NR1), and nightside polar cap (NPC). The models provide information about the sources of the current systems. Mapped ionospheric model FACs are compared with low-altitude measurements obtained by the MAGSAT spacecraft. We find that low-altitude data can reveal either classic region 1 2 or more highly structured FAC patterns. We therefore expect statistical results, either obtained from observations or inferred from models, to be averages over temporally and spatially shifting patterns.

The Characteristics and Distribution of Cloud Water over the Mountains of Northern Colorado during Wintertime Storms. Part I: Temporal Variations

Abstract The spatial and temporal evolution of supercooled water fields in ten wintertime storm systems occurring over the northern Colorado Rocky Mountain region have been examined using data collected by the recently developed scanning dual-channel microwave radiometer. These data were supported by several independent datasets including vertically pointing radar data, mountaintop liquid water content measurements, low and high altitude measurements of crystal rime characteristic rawinsonde data and precipitation intensity measurements. The ten case studies discussed in this paper represent various stages in the synoptic scale evolution of storms that affect the northern Colorado Rockies. Liquid water was found to occur in nearly all stages of most of these storms. The temporal variations in the magnitude of the liquid water content were significant. Three common features concerning the evolution of the liquid water field were observed in the prefrontal cloud systems: 1) an inverse relationship between p...

On the ionic identity of the ring current particles

Tinsley (1976) and Lyons and Evans (1976) have suggested that the near-earth ring current during the recovery phase consists largely of helium ions. This report describes an indirect method of identifying the ring current ions, using the ions found at low altitude beneath the radiation belts (Moritz, 1972; Hovestadt et al., 1972). The energy spectrum of the low-altitude ions depends upon the energy spectrum and the relative abundance of the various ionic species as a function of altitude. Because the pertinent cross sections differ greatly between hydrogen and helium and the near-earth portion of the ring current is the primary source of the low-altitude ions, the low-altitude ion energy spectrum strongly depends on the ionic identity of the near-earth ring current in the energy range 10–100 keV. One set of measurements exists of low-altitude ions during storm recovery (Mizera and Blake, 1973). The energy spectra observed do indeed suggest the absence of hydrogen in the near-earth ring current late in a recovery phase as proposed by Tinsley (1976) and Lyons and Evans (1976). However, a definitive test will require extensive low-altitude measurements throughout a storm period, preferably in conjunction with ring current spectral measurements.

Radiation belt-produced energetic hydrogen in interplanetary space

Trapped protons in the outer radiation belt of the earth can be removed through charage exchange by collisions with neutral exospheric hydrogen. The neutralized energetic particles escape into interplanetary space as well as to low alitudes. At low altitudes these particles constitute a copy of the outer radiation belt and have been repeatedly observed. It is proposed that the energetic neutral hydrogen escaping into interplanetary space is responsible for the recently observed highly anisotropic hydrogen flux at energies 0.29--0.5 MeV during solar quiet time. The expected energy spectrum of the neutral hydrogen flux in interplanetary space is derived from low-altitude measurements obtained on board the Esro 4 satellite and is compared with the observations. The result supports the hypothesis. (AIP)

L-dependent energetic electron precipitation and mid-latitudeDregion ion pair production profiles

Energetic electron precipitation at middle latitudes is at times intense enough to contribute significantly to the ionization of the ionospheric D region. The observed fluxes are often latitude dependent. This L-dependent precipitation may give rise to a latitude variation in the free electron density at D region heights. Specifically, low-altitude satellite measurements show that the electron environment on L shells which connect to the atmosphere at D regio heights above the ionospheric research stations operated by the Ionosphere Research Laboratory at University Park, Pennsylvania, and the Communications Research Centre at Ottawa, Canada, may be very different. Although the present study is not very extensive and does not cover all seasons, it shows that for a number of cases, precipitating electrons were observed over Ottawa but not over University Park. The resulting difference in D region ionization rates due to the additional ionization source at Ottawa may be important in light of the apparent conflict between D region density profiles derived at the two stations. Owing to lack of relevant information, it was assumed in an earlier intercomparison of electron density profiles measured simultaneously at University Park and Ottawa that no such difference existed. Our data show that it may bemore » important to have simultaneous knowledge also of the electron precipitation versus latitude in order to resolve questions relating to the accuracy of the wave interaction and partial reflection techniques used at the University Park and Ottawa sites, respectively, for obtaining the electron density profile. (AIP)« less

An improved radiosonde for the measurement of electric potential gradient in the atmosphere

The electrical potential gradient sonde developed by Venkiteshwaran and his co-workers and used during the I.G. Y. for regular soundings at Delhi and Poona though satisfactory for low altitude measurements was not quite suitable for high altitudes where the electric field is normally of the order of a few volt per metre or less. As a result of a series of experiments to improve its performance, the improved sonde now has a much faster response time with a very stable and consistent reference zero. Provision has been made to make the transmitter and the batteries much less temperature dependant throughout the sounding and to monitor the reference zero during the flight. The sonde thus has an accuracy of ±1 volt per metre and has been continuously used during the IQSY successfully.