High Telemetry Rate Research Articles

Introduction to special section: The Freja Mission

Freja was a joint Swedish and German scientific satellite project to study the interaction between hot magnetospheric plasma and the topside atmosphere/ionosphere. Freja was launched on October 6, 1992, and it operated successfully during 4 years until October 1996 when the command system ceased to work. Freja enabled high temporal/spatial resolution measurements of auroral plasma characteristics. With a high telemetry rate (520 kbit/s) and ∼15 Mbyte distributed onboard memories Freja could resolve mesoscale and microscale phenomena in the 100 m range for particles and the 1–10 m range for electric and magnetic fields. Novel plasma instruments enabled Freja to increase the spatial/temporal resolution orders of magnitudes above that achieved by its predecessors. The main scientific objective of Freja was to study the interaction between the hot magnetospheric plasma with the topside atmosphere/ionosphere. This interaction leads to a strong energization of magnetospheric and ionospheric plasma and an associated erosion, and loss, of matter from the Terrestrial exosphere. Freja orbited with an altitude of ∼600–1750 km, thus covering the lower part of the auroral acceleration region. This altitude range hosts processes that heat and energize the ionospheric plasma above the auroral zone, leading to the escape of ionospheric plasma and the formation of large density cavities.

PCA observations of Cyg X-1 from RXTE using fast timing and high telemetry rates

The Rossi X-ray Timing Explorer (RXTE) spacecraft contains a very large area, high resolution timing experiment provided by the NASA's Goddard Space Flight Center. This Proportional Counter Array (PCA) has an area of 7000 cm 2 and a time resolution down to one micro second. A 450 Mbit memory on board the spacecraft allows science data acquisition at rates up to 512 kbps but a more typical high rate of ∼256 kbps can be continuously supported for up to 30 minutes. This performance far surpasses previous rocket and satellite timing experiments. Since the launch of RXTE several observations have been made of Cyg X-1 in this special high telemetry rate mode to specifically search for, and attempt to confirm, the existence of millisecond bursts. Such features were reported in several observations of Cyg X-1 in the mid 1970's. This paper briefly summarizes the PCA performance compared to earlier large timing experiments and then presents early results from the analysis of these data sets. Individual millisecond bursts do not seem to exist in the data examined so far.

The Freja science mission

Freja*, a joint Swedish and German scientific satellite launched on October 6 1992, is designed to give high temporal/spatial resolution measurements of auroral plasma characteristics. A high telemetry rate (520 kbits s-1) and ≈15 Mbyte distributed on board memories that give on the average 2 Mbits s-1 for one minute enables Freja to resolve meso and micro scale phenomena in the 100 m range for particles and 1–10 m range for electric and magnetic fields. The on-board UV imager resolve auroral structures of kilometer size with a time resolution of one image per 6 s. Novel plasma instruments give Freja the capability to increase the spatial/temporal resolution orders of magnitudes above that achieved on satellites before. The scientific objective of Freja is to study the interaction between the hot magnetospheric plasma with the topside atmosphere/ionosphere. This interaction leads to a strong energization of magnetospheric and ionospheric plasma and an associated erosion, and loss, of matter from the Terrestrial exosphere. Freja orbits with an altitude of ≈600–1750 km, thus covering the lower part of the auroral acceleration region. This altitude range hosts processes that heat and energize the ionospheric plasma above the auroral zone, leading to the escape of ionospheric plasma and the formation of large density cavities.

The X-ray timing explorer

The X-ray Timing Explorer (XTE), to be launched in 1995, will address current scientific problems from galactic neutron star systems to active galactic nuclei with a powerful and complementary set of instruments. It features a low-background continuous 2 – 200 keV response will large apertures (a 0.63-m 2 proportional counter array and a 0.16-m 2 dual rocking NaI/CsI scintillator array). Rapid response (in hours) to temporal phenomena, e.g. transients, is obtained by virtue of a scanning all-sky monitor and rapid maneuverability. XTE will carry out detailed energy-resolved studies of phenomena close to neutron stars (e.g., QPO's) with its sub-millisecond timing (to 10 μs), its high telemetry rates (to 256 kbs), and the high throughout of its data system (to ⩾ 2 × 10 5 c s −1). In current planning, the XTE will be launched by the shuttle into a low earth orbit (∼450 km) where it will be interchanged with the Extreme Ultraviolet Explorer (EUVE) on the NASA Explorer Platform. The observing time will be completely open to competitive proposals from the community.

The X-ray timing explorer

The capabilities of the X-ray Timing Explorer (XTE) are described with particular attention paid to current scientific problems it will address from galactic neutron star systems to active galactic nuclei. It features a low-background continuous 2 – 200 keV response with large apertures (a 0.63-m 2 proportional counter array and a 0.16-m 2 dual rocking NaI/CsI scintillation array). Rapid response (in hours) to temporal phenomena, e.g. transients, is obtained by virtue of a scanning all-sky monitor and rapid maneuverability. XTE will carry out detailed energy-resolved studies of phenomena close to neutron stars (e.g. QPO's) because of its sub-milliscond timing (to 10 μs), its high telemetry rates (to 256 kbs), and the high throughput of its data system (to ≳ 2 × 10 5 c s −1).

The X-Ray Timing Explorer

AbstractThe capabilities of the X-ray Timing Explorer (XTE) are described with particular attention paid to current scientific problems it will address from galactic neutron star systems to active galactic nuclei. It features a low-background continuous 2-200 keV response with large apertures (a 0.63-m2 proportional counter array and a 0.16-m2 dual rocking NaI/CsI scintillation array). Rapid response (in hours) to temporal phenomena, e.g. transients, is obtained by virtue of a scanning all-sky monitor and rapid maneuverability. XTE will carry out detailed energy-resolved studies of phenomena close to neutron stars (e.g. QPO’s) because of its sub-millisecond timing (to 10 μs), its high telemetry rates (to 256 kb/s), and the high throughput of its data system (to ≳ 2 × 105 c s−1).

Simultaneous observations of field‐aligned currents and plasma drift velocities by Atmosphere Explorer C

A three‐axis flux gate magnetometer was included in the Atmosphere Explorer C (AE‐C) payload for engineering functions. The instrument was activated during the last half of AE‐C's life, and its output appears to be useful only during approximately one‐half of the data gathering time when the vehicle was spinning. Still, the addition of even a partial magnetic field data set to the already broad spectrum of AE‐C measurements is an exciting prospect. The high telemetry rate (128 bits/s per axis) could produce a spatial resolution in B of ∼½ km, but the 8‐bit word size yields an amplitude resolution of only 562 nT. A large modulation (15,000 nT) of the output from the spin axis (Z) magnetometer occurs during sunlit orbit segments. Application of a spin‐averaging technique to the modulated output signal has enabled us to improve the amplitude resolution of the signal to ∼40 nT (rms), although a corresponding loss in spatial resolution from ∼½ to ∼15 km results from the averaging procedure. By taking the difference between the spin‐averaged signal and a model magnetic field in a manner similar to that used by McDiarmid et al. (1978) we deduce a resultant magnetic perturbation ΔBZ. Attributing ΔBZ to field‐aligned current (FAC) sheets, we arrive at typical values of ∼1 µA/m² for sheet current densities. The amplitudes and distributions of these FAC’s are observed to be consistent with those measured by Triad (Iijima and Potemra, 1976a, b). Simultaneously measured Z components of the plasma drift velocity (Vz) show correlations with magnetometer measurements indicating that the positions of FAC's correspond to the gradients in the component of the electric field along the satellite track (X component). By using the current continuity equation for the ionosphere (▽ · I⊥=J∥), values of height‐integrated Pedersen conductivity Σp from 2.5 to 17.5 mhos are calculated.