Engineering topics

Benson, R. F.

Publications and source records attributed to Benson, R. F..

At least 19 records

NASA NTRS2011

Improving the Automatic Inversion of Digital ISIS-2 Ionogram Reflection Traces into Topside Vertical Electron-Density Profiles

The topside-sounders on the four satellites of the International Satellites for Ionospheric Studies (ISIS) program were designed as analog systems. The resulting ionograms were displayed on 35-mm film for analysis by visual inspection. Each of these satellites, launched between 1962 and 1971, produced data for 10 to 20 years. A number of the original telemetry tapes from this large data set have been converted directly into digital records. Software, known as the TOPside Ionogram Scalar with True-height (TOPIST) algorithm has been produced that enables the automatic inversion of ISIS-2 ionogram reflection traces into topside vertical electron-density profiles Ne(h). More than million digital Alouette/ISIS topside ionograms have been produced and over 300,000 are from ISIS 2. Many of these ISIS-2 ionograms correspond to a passive mode of operation for the detection of natural radio emissions and thus do not contain ionospheric reflection traces. TOPIST, however, is not able to produce Ne(h) profiles from all of the ISIS-2 ionograms with reflection traces because some of them did not contain frequency information. This information was missing due to difficulties encountered during the analog-to-digital conversion process in the detection of the ionogram frame-sync pulse and/or the frequency markers. Of the many digital topside ionograms that TOPIST was able to process, over 200 were found where direct comparisons could be made with Ne(h) profiles that were produced by manual scaling in the early days of the ISIS program. While many of these comparisons indicated excellent agreement (<10% average difference over the entire profile) there were also many cases with large differences (more than a factor of two). Here we will report on two approaches to improve the automatic inversion process: (1) improve the quality of the digital ionogram database by remedying the missing frequency-information problem when possible, and (2) using the above-mentioned comparisons as teaching examples of how to improve the original TOPIST software.

Benson, R. F.↗

NASA NTRS2008

Plasmaspheric Plumes Observed by the CLUSTER and IMAGE Spacecraft

Global IMAGE/EUV observations have revealed complex changes in plasmaspheric structures as the plasmasphere responds to geomagnetic activity while remaining under varying degrees of influence by co-rotation, depending on the radial distance. The complex plasmaspheric dynamics, with different scales of variability, is clearly far from being well understood. There is now renewed interest in the plasmasphere due to its apparent connections with the development of the ring current and radiation belt, and loss of ionospheric plasmas. Early in the mission, the Cluster spacecraft only crossed the plasmapause (L - 4) occasionally and made measurements of the outer plasmasphere and plasmaspheric drainage plumes. The study by Darrouzet et al. [2006] provided detailed analyses of in situ Cluster observations and IMAGE EUV observations of three plasmaspheric plumes detected in April-June, 2002. Within the next couple of years, Cluster orbit will change, causing perigee to migrate to lower altitudes, and thus providing excellent opportunities to obtain more detailed measurements of the plasmasphere. In this paper, we report our analyses of the earlier Cluster-IMAGE events by incorporating the different perspectives provided by the IMAGE Radio Plasma Imager (RPI) observations. We will discuss our new understanding of the structure and dynamics of the Cluster-IMAGE events.

Fung, S. F.↗

NASA NTRS2008

Plasmaspheric Structures Observed by the CLUSTER and IMAGE Spacecraft

Global EUV imaging observations by the IMAGE satellite have revealed spectacularly complex changes in plasmaspheric structures as the plasmaspheric plasmas respond to geomagnetic activity while remaining under varying degrees of influence by co-rotation, depending on the radial distance. This complex plasmaspheric plasma dynamics, with its numerous scales of variability, is clearly far from being well understood. There is now renewed interest in studying the plasmasphere due to its apparent connections with the development of the ring current and radiation belt and loss of ionospheric plasmas. Earlier in the mission, the Cluster spacecraft only crossed the plasmapause (L - 4) occasionally and made measurements of the cold plasma in the plasmasphere and plasmaspheric drainage plumes. The study by Darrouzet et al. [2006], for example, provided detailed analyses of in situ Cluster observations and IMAGE EUV imaging observations of three plasmaspheric plumes detected in April-June, 2002. Within the next couple of years, Cluster orbit will change, causing perigee to migrate to lower altitudes, and thus providing excellent opportunities to obtain more detailed measurements of the plasmasphere. In this paper, we report our analyses of the earlier Cluster-IMAGE events by incorporating the different perspectives provided by the IMAGE Radio Plasma Imager (RPI) observations through the plasmasphere. We will describe our new understanding of the Cluster-IMAGE events and their implications on plasmaspheric dynamics.

Fung, S. F.↗

NASA NTRS2006

Topside sounders as mobile ionospheric heaters

There is evidence that satellite-borne RF sounders can act as mobile ionospheric heaters in addition to performing topside sounding. The main objective of topside sounding is to use sounder-generated electromagnetic (em) waves to obtain ionospheric topside vertical electron-density (N(sub e) profiles. These profiles are obtained from mathematical inversions of the frequency vs. delay-time ionospheric reflection traces. In addition to these em reflection traces, a number of narrowband intense signals are observed starting at zero delay times after the transmitted pulses. Some of these signals, termed plasma resonances, appear at characteristic frequencies of the ambient medium such as at the electron cyclotron frequency f(sub ce), the harmonics nf(sub ce), the electron plasma frequency f(sub pe) and the upper-hybrid frequency f(sub uh), where (f(sub uh))(exp 2) = (f(sub ce))(exp 2) + (f(sub pe))(exp 2) . These signals have been attributed to the oblique echoes of sounder-generated electrostatic (es) waves. These resonances provide accurate in situ f(sub pe) and f(sub ce) values which, in turn, lead to accurate N(sub e) and [B] values where B is the ambient magnetic field. Resonances are also observed between the nf(sub ce) harmonics both above and below f(sub uh). The former, known as the Qn plasma resonances, are mainly attributed to the matching of the wave group velocity of sounder-generated (Bernstein-mode) es waves to the satellite velocity. The frequency spectrum of these waves in the magnetosphere can be used to detect non-Maxwellian electron velocity-distributions. In addition, these resonances also exhibit components that appear to be the result of plasma emissions stimulated by the sounder pulses. The plasma resonances observed between the nf(sub ce) harmonics and below f(sub uh), known as the Dn plasma resonances, are entirely attributed to such sounder-stimulated plasma emissions. There are other sounder-stimulated plasma phenomena that also fall into this category, e.g., ion affects on electron-resonant phenomena, proton-cyclotron echoes and N(sub e) field-aligned irregularities (FAI). Some of these phenomena are more pronounced when f(sub pe)/f(sub ce) approx. = n where n is an integer significantly greater than one. The observations suggest that the sounder-stimulated plasma phenomena are stimulated, or enhanced, on a time scale much less than one second. The purpose of this presentation is to review the above topics with particular emphasis on the sounder-stimulated plasma phenomena.

Benson, R. F.↗

NASA NTRS2004

Surface Penetrating Radar Simulations for Europa

The space environment above the icy surface of Europa is a source of radio noise in this frequency range from natural sources in the Jovian magnetosphere. The ionospheric and magnetospheric plasma environment of Europa affects propagation of transmitted and return signals between the spacecraft and the solid surface in a frequency-dependent manner. The ultimate resolution of the subsurface sounding measurements will be determined, in part, by a capability to mitigate these effects. We discuss an integrated multi-frequency approach to active radio sounding of the Europa ionospheric and local magnetospheric environments, based on operational experience from the Radio Plasma Imaging @PI) experiment on the IMAGE spacecraft in Earth orbit, in support of the subsurface measurement objectives.

Markus, T.↗

NASA NTRS2004

Radio Sounding Science at High Powers

Future space missions like the Jupiter Icy Moons Orbiter (JIMO) planned to orbit Callisto, Ganymede, and Europa can fully utilize a variable power radio sounder instrument. Radio sounding at 1 kHz to 10 MHz at medium power levels (10 W to kW) will provide long-range magnetospheric sounding (several Jovian radii) like those first pioneered by the radio plasma imager instrument on IMAGE at low power (less than l0 W) and much shorter distances (less than 5 R(sub E)). A radio sounder orbiting a Jovian icy moon would be able to globally measure time-variable electron densities in the moon ionosphere and the local magnetospheric environment. Near-spacecraft resonance and guided echoes respectively allow measurements of local field magnitude and local field line geometry, perturbed both by direct magnetospheric interactions and by induced components from subsurface oceans. JIMO would allow radio sounding transmissions at much higher powers (approx. 10 kW) making subsurface sounding of the Jovian icy moons possible at frequencies above the ionosphere peak plasma frequency. Subsurface variations in dielectric properties, can be probed for detection of dense and solid-liquid phase boundaries associated with oceans and related structures in overlying ice crusts.

Green, J. L.↗

NASA NTRS2003

Remote Radio Sounding Science for JIMO

Radio sounding of the Earth's top side ionosphere and magnetosphere is a proven technique from geospace missions such as the International Satellites for Ionospheric Studies (ISIS) and the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE). Application of this technique to the Jupiter Icy Moons Orbiter (JIMO) mission will provide unique remote sensing observations of the plasma and magnetic field environments, and the subsurface conductivities, of Europa, Ganymede, and Callisto. Spatial structures of ionospheric plasma above the moon surfaces vary in response to magnetic field perturbations from (1) magnetospheric plasma flows, (2) ionospheric currents from ionization of sputtered surface material, and (3) induced electric currents in salty subsurface oceans. Radio sounding at 3 kHz to 10 MHz can provide globally-determined electron densities necessary for the extraction of the oceanic current signals and supplements in-situ plasma and magnetic field measurements. Subsurface variations in conductivity, can be investigated by radio sounding from 10 MHz to 40 MHz allowing the determination of the presence of dense and solid-liquid phase boundaries associated with oceans and related structures in overlying ice crusts.

Green, J. L.↗

NASA NTRS2003

Surface Penetrating Radar Simulations for Jupiter's Icy Moons

The icy moons of Jupiter (Europa, Callisto, and Ganymede) are of similar overall composition but show different surface features as a result of different sub-surface processes. Furthermore, each of these moons could have a liquid ocean of water buried underneath the icy crust, but their depth can only be speculated. For Europa, estimates put the thickness of the ice shell anywhere between 2-30 km, with'a few models predicting up to 100 km. Much of the uncertainties are due to the largely unknown temperature gradients and levels of water impurities across different surface layers. One of the most important geological processes is the possible transportation of heat by ice convection. If the ice is convecting, then an upper limit of about 20 km is set for the depth of the ocean underneath. Convection leads to a sharp increase in temperature followed by a thick region of nearly constant temperature. If ice is not convecting, then an exponentially increasing temperature profile is expected. The crust is thought to be a mixture of ice and rock, and although the exact percentage of rock is not known, it is expected to be low. Additionally, the ice crust could contain salt, similar to sea ice on Earth. The exact amount of salt and how that amount changes with depth is also unknown. In preparation for the Jupiter Icy Moons Orbiter (JIMO) mission, we performed simulations for a surface-penetrating radar investigating signatures for different possible surface and sub-surface structures of these moons in order to estimate the applicability of using radar with a frequency range between 1 and 50 MHz. This includes simulations of power requirements, attenuation losses, layer resolutions for scenarios with and without the presence of a liquid ocean underneath the ice, cases of convecting and non-convecting ice, different impurities within the ice, and different surface roughnesses.

Markus, Thorsten↗

NASA NTRS1999

The Radio Plasma Imager Investigation on the IMAGE Spacecraft

Radio plasma imaging uses total reflection of electromagnetic waves from plasmas whose plasma frequencies equal the radio sounding frequency and whose electron density gradients are parallel to the wave normals. The Radio Plasma Imager (RPI) has two orthogonal 500-m long dipole antennas in the spin plane for near omni-directional transmission. The third antenna is a 20-m dipole. Echoes from the magnetopause, plasmasphere and cusp will be received with three orthogonal antennas, allowing the determination of their angle-of-arrival. Thus it will be possible to create image fragments of the reflecting density structures. The instrument can execute a large variety of programmable measuring programs operating at frequencies between 3 kHz and 3 MHz. Tuning of the transmit antennas provides optimum power transfer from the 10 W transmitter to the antennas. The instrument can operate in three active sounding modes: (1) remote sounding to probe magnetospheric boundaries, (2) local (relaxation) sounding to probe the local plasma, and (3) whistler stimulation sounding. In addition, there is a passive mode to record natural emissions, and to determine the local electron density and temperature by using a thermal noise spectroscopy technique.

Reinisch, Bodo W.↗

NASA NTRS1999

Radio Plasma Imager Simulations and Measurements

The Radio Plasma Imager (RPI) will be the first-of-its kind instrument designed to use radio wave sounding techniques to perform repetitive remote sensing measurements of electron number density (N(sub e)) structures and the dynamics of the magnetosphere and plasmasphere. RPI will fly on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission to be launched early in the year 2000. The design of the RPI is based on recent advances in radio transmitter and receiver design and modern digital processing techniques perfected for ground-based ionospheric sounding over the last two decades. Free-space electromagnetic waves transmitted by the RPI located in the low density magnetospheric cavity will be reflected at distant plasma cutoffs. The location and characteristics of the plasma at those remote reflection points can then be derived from measurements of the echo amplitude, phase, delay time, frequency, polarization, Doppler shift, and echo direction. The 500 m tip-to-tip X and Y (spin plane) antennas and 20 m boom Z axis antenna on RPI will be used to measures echoes coming from distances of several R(sub E).

Green, J. L.↗

NASA NTRS1999

The Plasmasphere as "Seen" by the IMAGE Mission

The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) is the first mission designed exclusively to remotely measure the magnetosphere. As such, it will reveal the ring current, plasmasphere, polar cusp, and magnetopause as whole extended, interacting systems. For the first time, our impressions of the global magnetosphere, synthesized through many years of whistler and in situ measurement, will be replaced by images. The overall morphology of each system of plasma and the correspondence of changes between them in response to the sun and solar wind will become available. The Extreme Ultraviolet Imager (EUV) and the Radio Plasma Imager (RPI) are the two IMAGE instruments which will remotely measure and image the plasmasphere. What we expect to "see" from these instruments and how it may be interpreted is the subject of this presentation. The EUV instrument includes three optical cameras, with an almost 90 degree field of view, transverse to the spin axis. EUV is designed to see He+ ions in resonantly scatter solar light at 30.4rim. The IMAGE spacecraft will spin with a period of about 2 minutes, with its spin axis parallel to the orbit normal. The IMAGE orbit will be highly inclined, with a high latitude apogee at a geocentric distance of 8RE and perigee of about 1.2RE. The normal observing integration time of 10 minutes will easily see to the outer edge of the plasmasphere. The RPI instrument makes use of three orthoganal dipole antennas: two in the spin plane with a tip-to-tip length of 500m and one along the spin axis with a length of 20 meters. Using coded pulse transmissions, the RPI instrument will broadcast from 3kHz to 3MHz. With one minute resolution, plasma densities from about 0.1 cm(exp -3) to 100,000 cm(exp -3), along with line-of-sight bulk velocities and locations, will be obtained from all returned radio wave signals. When transmitting from the high latitude magnetospheric cavity, RPI will measure density profiles for the major plasma structures in the magnetosphere, including the magnetopause, polar cusp, and plasmasphere. RPI should also see isolated density irregularities and possibly the plasma sheet. Observations The EUV instrument will return line-of-sight integrated images through the optically thin helium medium of the plasmasphere and magnetosphere. A variety of techniques have been suggested for the translation of the images into physically useful data, such as plasmapause location and three dimensional density distribution. The RPI instrument will return quantitative density values and line-of-sight velocity as a function of position along reflecting wave propagation paths. How they may be used individually and together to study plasmaspheric dynamics and global structure will be discussed. Attention will also be given to the data products and how access to IMAGE data will be provided by the IMAGE team and the NSSDC.

Gallagher, D. L.↗

NASA NTRS1993

Sounder stimulated D(sub n) resonances in Jupiter's Io plasma torus

On February 8, 1992, the Ulysses spacecraft passed through Jupiter's Io plasma torus, where rich spectra of narrow-band resonances were stimulated by the relaxation sounder of the Ulysses unified radio and plasma wave (URAP) instrument. Since the gyrofrequency f(sub g) is comparable to the plasma frequency f(sub p) in the Io torus, it was predicted that the general classification of stimulated ionospheric D(sub n) resonances, developed for 1 is less than or equal to f(sub p)/f(sub g) is less than or equal to 8 in the Earth's topside ionosphere, should apply in the Io torus as well as the Earth's magnetosphere (Osherovich, 1989). The URAP plasmagrams (sounder spectra) in the portions of the Io torus satisfying these plasma conditions are dominated by the D(sub n) resonances for frequencies below f(sub p). On most of these plasmagrams the f(sub p) resonance is also present, but it is seldom the dominant resonance. Neither upper hybrid nor nf(sub g) resonances have been found on these plasmagrams. The identification of D(sub n) resonances has allowed both the electron density and the magnetic field amplitude to be calculated. The derived densities on the outbound pass agree well with a Voyager model of Bagenal (1992). The derived magnetic field values are close to the Goddard Space Flight Center O(sub 6) magnetic field model.

Osherovich, V. A.↗

NASA NTRS1990

The ionosphere as a focusing lens - A case study involving simultaneous type III solar radio storm measurements from the ISIS 1 and 2 and ISEE 3 satellites

The possibility of using terrestrial ionospheric focusing to improve the directivity of electric dipoles on space missions has been investigated by comparing simultaneous observations of a solar radio storm by the ISIS 1 and ISIS 2 spacecraft, in near earth orbit, and the ISEE 3 spacecraft located beyond the magnetosphere. To this end, a three-dimensional ray tracing in a spherically stratified ionosphere has been carried out for conditions appropriate to the observations by the ISIS spacecraft of a solar radio storm in September 1983. The procedure allows Poynting flux spectral densities measured on ISIS to be converted to spectral densities well outside the ionosphere where they can be compared directly with simultaneous observations on ISEE 3. The results demonstrate good agreement over their common observing frequency range (1-2 MHz).

James, H. G.↗

NASA NTRS1985

Field-aligned electron density irregularities near 500 km Equator to polar cap topside sounder observations

In addition to spread F, evidence for field-aligned electron density irregularities is commonly observed on Alouette 2 topside sounder ionograms recorded near perigee (500 km). This evidence is provided by distinctive signal returns from sounder-generated Z mode waves. At low latitudes these waves become guided in wave ducts caused by field-aligned electron density irregularities and give rise to strong long-duration echoes. At high latitudes, extending well into the polar cap, these Z mode waves (and stimulated electrostatic waves at the plasma frequency) produce a series of vertical bars on the ionogram display as the satellite traverses discrete field-aligned density structures. The radio frequency (RF) noise environment to be expected in the 400 to 500 km altitude region from low to high latitudes was examined by analyzing perigee Alouette 2 topside sounder data. All observed noise bands were scaled on nearly 200 topside sounder ionograms recorded near perigee at low, mid, and high latitude telemetry stations. The minimum and maximum frequencies of each noise band were entered into a data base or computer analysis. The signals of primary interest in the perigee study were found to be sounder-generated.

Benson, R. F.↗

NASA NTRS1985

Auroral kilometric radiation - Wave modes, harmonics, and source region electron density structures

A change from extraordinary (X) mode to ordinary (O) mode dominance is observed in the auroral kilometric radiation (AKR) detected on ISIS 1 topside sounder ionograms as the source region plasma to gyrofrequency ratio fN/fH varies from 0.1 to 1.3. The X and O mode AKR, Z (the slow branch of the X mode) and whistler (W) mode are also observed. The Z mode is typically slightly less intense than the O-mode. The W-mode is confined to frequencies less than fH/2, suggesting that it is the result of field aligned ducted signals reaching the satellite from a source at lower altitudes. Harmonic AKR bands are commonly observed and the 2nd harmonic appears to be due to propagating signals. The deduced (fN/fH) at the bottom of the AKR source region is always less than 0.4 and is typically less than 0.2 during the generation of X-mode AKR, but approaches 0.9 for O-mode AKR. No large density enhancements were observed within AKR source region density cavities. It is suggested that the observed intense AKR is cyclotron X-mode radiation rather than plasma frequency O-mode radiation.

Benson, R. F.↗

NASA NTRS1984

Auroral kilometric radiation: Wave modes, harmonic and source region electron density structures

A change from extraordinary (X) mode to ordinary (0) mode dominance is observed in the auroral kilometric radiation (AKR) detected on ISIS 1 topside sounder ionograms as the source region plasma to gyrofrequency ratio fN/fH varies from 0.1 to 1.3. The X and 0 mode AKR, Z (the slow branch of the X mode) and whistler (W) mode are also observed. The Z mode is typically slightly less intense than the 0-mode. Thw W-mode is confined to frequencies less than fH/2, suggesting that it is the result of field aligned ducted signals reaching the satellite from a source at lower altitudes. Harmonic AKR bands are commonly observed and the 2nd harmonic appears to be due to propagating signals. The deduced (fN/fH) at the bottom of the AKR source region is always less than 0.4 and is typically less than 0.2 during the generation of X-mode AKR, but approaches 0.9 for 0-mode AKR. No large density enhancements were observed within AKR source region density cavities. It is suggested that the observed INTENSE AKR IS cyclotron X-mode radiation rather than plasma frequency 0-mode radiation.

Benson, R. F.↗

NASA NTRS1984

Field-aligned electron density irregularities near 500 km. Equator to polar cap topside sounder Z mode observations

In addition to spread F, evidence for field-aligned electron density irregularities is commonly observed on Alouette 2 topside sounder ionograms recorded near perigee (500 km). This evidence is provided by distinctive signal returns from sounder-generated Z mode waves. At low latitudes these waves become guided in wave ducts caused by field-aligned electron density irregularities and give rise to strong long-duration echoes. At high latitudes, extending well into the polar cap, these Z mode waves (and stimulated electrostatic waves at the plasma frequency) produce a series of vertical bars on the ionogram display as the satellite traverses discrete field-aligned density structures. The radio frequency (RF) noise environment to be expected in the 400 to 500 km altitude region from low to high latitudes was examined by analyzing perigee Alouette 2 topside sounder data. All observed noise bands were scaled on nearly 200 topside sounder ionograms recorded near perigee at low, mid, and high latitude telemetry stations. The minimum and maximum frequencies of each noise band were entered into a data base or compuer analysis. The signals of primary interest in the perigee study were found to be sounder-generated.

Benson, R. F.↗

NASA NTRS1984

Auroral kilometric radiation/aurora correlation

Auroral kilometric radiation (AKR) observations from the ISIS 1 topside sounder receiver were compared with visual auroral observations from the network of Alaskan all-sky camera stations. The goal was to relate AKR source region encounters to specific auroral forms on the same magnetic field line. Thirty-eight simultaneous data sets were identified and analyzed. In general, intense AKR was associated with bright auroral arcs and conditions of weak or no AKR corresponded to times when either no aurora or only a faint arc or weak diffuse aurora were observed. Five cases, when both intense AKR and bright visual aurora were present, were analyzed in detail. Complete electron density N sub e contours, from the satellite altitude down to the F region ionization peak, were obtained along N-S traversals of the AKR source region. In addition, the ISIS 1 orbital tracks were projected down the magnetic field lines to the auroral altitude and compared to auroral features on a map derived from the all sky camera images. Density cavities (regions where N sub e 100/cu cm) were encountered on each of these passes. Previously announced in STAR as N83-27516

Benson, R. F.↗