Instrumental Noise Level Research Articles

A fast radiative transfer model for SSMIS upper atmosphere sounding channels

Special Sensor Microwave Imager/Sounder (SSMIS) on board the Defense Meteorology Satellite Program (DMSP) F‐16 satellite probes the atmospheric temperature from surface to 100 km. SSMIS channels 19–22 are significantly affected by Zeeman splitting, which is dependent on the Earth’s magnetic field. Thus, in satellite data assimilation or retrieval systems, SSMIS brightness temperatures and their Jacobians (or gradient with respect to temperature) must be computed with a fast radiative transfer (RT) scheme that takes into account the Zeeman‐splitting effect. In this study, an averaged transmittance within the channel frequency passbands is parameterized and predicted with atmospheric temperature, geomagnetic field strength, and the angle between the geomagnetic field vector and the electromagnetic wave propagation direction. The coefficients of predictors are trained with a line‐by‐line (LBL) radiative transfer model that accurately computes the monochromatic transmittances at fine frequency steps within each passband. The new radiative transfer scheme is compared to the results from the line‐by‐line model for the dependent and independent data sets. It is shown that the differences between the two models are well below the instrument noise levels but the new scheme is much faster. It is also shown that the SSMIS measurements agree well with the simulations that are based on the atmospheric profiles from the sounding of the atmosphere using broadband emission radiometry (SABER) on the Thermosphere‐lonosphere‐Mesosphere Energetics and Dynamics satellite and the COSPAR international reference atmosphere (CIRA) model.

Optical properties and instrumental performance of thin gold films near the surface plasmon resonance

The optical properties of very thin gold films have been evaluated by Fresnel analysis, with optical boundary conditions pertaining to the surface plasmon resonance (SPR) at the gold–water interface. The experimental SPR characteristic was evaluated in the angular interrogation mode. Film morphology was characterized by high resolution transmission electron microscopy. The magnitude of the resonance, i.e., the SPR signal, sensitively depends on, and is affected by film thickness and morphology. A sharply defined thickness of 55 ± 5 nm is required, to achieve optimum SPR excitation conditions, and instrumental sensitivity. With decreasing film thickness, below 40 nm, the resonance angle starts to shift to larger values. A substantial increase of the intrinsic resonance broadening parameter is observed below 70 nm, associated with an increasingly asymmetric SPR line shape. A similar effect occurs in the presence of a very thin chromium adhesion layer. Surface roughness and film thickness modulations determine the experimentally observed line broadening parameter. Instrumental noise levels largely depend on accuracy and quality at which the resonance angle can be determined. Substantial improvement and instrumental sub-pixel resolution is achievable by optimum fitting routines, accounting for drastic noise reduction and improved instrumental sensitivity, up to two orders of magnitude over the inherent geometric sensor pixel resolution.

The Total Irradiance Monitor (TIM): Science Results

The solar observations from the Total Irradiance Monitor (TIM) are discussed since the SOlar Radiation and Climate Experiment (SORCE) launch in January 2003. The TIM measurements clearly show the background disk-integrated solar oscillations of generally less than 50 parts per million (ppm) amplitude over the ∼2 ppm instrument noise level. The total solar irradiance (TSI) from the TIM is about 1361 W/m2, or 4–5 W/m2 lower than that measured by other current TSI instruments. This difference is not considered an instrument or calibration error. Comparisons with other instruments show excellent agreement of solar variability on a relative scale. The TIM observed the Sun during the extreme activity period extending from late October to early November 2003. During this period, the instrument recorded both the largest short-term decrease in the 25-year TSI record and also the first definitive detection of a solar flare in TSI, from which an integrated energy of roughly (6± 3)×1032 ergs from the 28 October 2003 X17 flare is estimated. The TIM has also recorded two planets transiting the Sun, although only the Venus transit on 8 June 2004 was definitive.

Coseismic Strain and the Transition to Surface Afterslip Recorded by Creepmeters near the 2004 Parkfield Epicenter

The Mw 6 Parkfield earthquake at 11:15 local time, 28 September 2004 has dealt a possibly fatal blow to the notion that significant surface slip precedes earthquakes. In 1966, anecdotal reports of surface fissures along the fault zone had been reported in the preceding week, and a water pipe fractured 11.6 hours before the mainshock. This offered hope that substantial surface slip may have occurred had displacement sensors been in place to measure it. In the days, hours, and minutes prior to the 2004 Parkfield mainshock, displacement sensors were in place, and nothing happened. According to data from eight creepmeters crossing the fault, the fault remained effectively locked at the detection level of each sensor (6-20 μm) (Langbein et al. , 2005, this issue). This absence of surface slip was accompanied by an absence of significant strain on the borehole dilatometer and shear-strainmeter array in the region, and by the absence of any unusual displacements on the GPS array embracing the 2004 epicenter. Although preseismic strain signals seen on some borehole strainmeters suggest that epicentral strain was not completely indifferent to the pending earthquake, the signals are close to instrumental noise levels and pose new challenges for their reliable future detection. In contrast to the absence of preseismic fault displacement, the creepmeters all recorded an abrupt coseismic dextral offset at the time of the mainshock. The step corresponds to dextral shear of the fault zone, but in the absence of surface rupture, the creepmeters during early afterslip acted as extensometers, measuring strain rather than fault creep. The serendipitous location of creepmeters at Work Ranch both across and eccentric to the fault zone has provided insight into the development of surface faulting. The u.s. Geological Survey (USGS) has maintained a creepmeter array in Parkfield since the 1966 earthquake, increasing …

Beam deconvolution in noisy CMB maps

The subject of this paper is beam deconvolution in small angular scale CMB experiments. The beam effect is reversed using the Jacobi iterative method, which was designed to solved systems of algebraic linear equations. The beam is a non circular one which moves according to the observational strategy. A certain realistic level of Gaussian instrumental noise is assumed. The method applies to small scale CMB experiments in general (cases A and B), but we have put particular attention on PLANCK mission at 100 GHz (cases C and D). In cases B and D, where noise is present, deconvolution allows to correct the main beam distortion effect and recover the initial angular power spectrum up to the end of the fifth acoustic peak. An encouraging result whose importance is analyzed in detail. More work about deconvolution in the presence of other systematics is in progress. This paper is related to the PLANCK LFI activities.

Falcon airborne gravity gradiometer survey results over the Cannington Ag-Pb-Zn deposit

In April 2000 BHP conducted six FALCON airborne gravity gradiometer (AGG) test surveys over the Cannington Ag-Pb-Zn ore body in NW Queensland, Australia. The purpose of the test surveys was to demonstrate the capabilities of the AGG instrument by comparison with known detailed ground gravity data, to investigate the effects of source distance on signal strength by flying surveys at various altitudes, and to estimate AGG instrument noise levels in survey conditions. The processed FALCON gD data compare very favorably with the upward-continued residual ground gravity data, capturing most geological features and clearly delineating the Cannington ore body. Analysis of the effects of source distance on signal strength indicates that with a nominal flying height of 120m, the FALCON AGG instrument can detect the gravity anomaly from a deposit with the size and geometry of Cannington through 130m overburden. By repeating a survey using the same acquisition parameters we can make an estimate of the RMS noise of the instrument under survey conditions. Analysis of repeat survey readings show that the FALCON AGG instrument attained a noise level of 10 Eotvos RMS in the bandwidth from 0.0 Hz to 0.125 Hz during the Cannington test flights.

Estimating stochastic gravitational wave backgrounds with the Sagnac calibration

Armstrong et al. have recently presented new ways of combining signals to precisely cancel laser frequency noise in spaceborne interferometric gravitational wave detectors such as LISA. One of these combinations, which we will call the ``symmetrized Sagnac observable,'' is much less sensitive to external signals at low frequencies than other combinations, and thus can be used to determine the instrumental noise level. We note here that this calibration of the instrumental noise permits smoothed versions of the power spectral density of stochastic gravitational wave backgrounds to be determined with considerably higher accuracy than earlier estimates, at frequencies where one type of noise strongly dominates and is not substantially correlated between the six main signals generated by the antenna. We illustrate this technique by analyzing simple estimators of gravitational wave background power, and show that the instrumental sensitivity to broad-band backgrounds at some frequencies can be improved by a significant factor of as much as $(f\ensuremath{\tau}{/2)}^{1/2}$ in spectral density ${h}_{\mathrm{rms}}^{2}$ over the standard method, where f denotes frequency and $\ensuremath{\tau}$ denotes integration time, comparable to that which would be achieved by cross-correlating two separate antennas. The applications of this approach to studies of astrophysical gravitational wave backgrounds generated after recombination and to searches for a possible primordial background are discussed. With appropriate mission design, this technique allows an estimate of the cosmological background from extragalactic white dwarf binaries and will enable LISA to reach the astrophysical confusion noise of compact binaries from about 0.1 mHz to about 20 mHz. In a smaller-baseline follow-on mission, the technique allows several orders of magnitude improvement in sensitivity to primordial backgrounds up to about 1 Hz.

Simulations of Foreground Effects for Cosmic Microwave Background Polarization

We use a simple model to investigate the effect of polarized Galactic foreground emission on the ability of planned cosmic microwave background (CMB) missions to detect and model CMB polarization. Emission from likely polarized sources (synchrotron and spinning dust) would dominate the polarization of the microwave sky at frequencies below 90 GHz if known Galactic foregrounds are at least 10% polarized at high latitude (|b| > 30°). Maps of polarization at frequencies below 90 GHz will likely require correction for foreground emission to enable statistical analysis of the individual Stokes Q or U components. The temperature-polarization cross-correlation is less affected by foreground emission, even if the foreground polarization is highly correlated with the foreground intensity. Polarized foregrounds, even if uncorrected, do not dominate the uncertainty in the temperature-polarization cross-correlation for instrument noise levels typical of the Microwave Anisotropy Probe (MAP) experiment. Methods that remove Galactic signals at the cost of signal-to-noise ratio should carefully balance the value of rejecting faint foregrounds with the cost of increased instrument noise.

Global variation in the 2.7 µm NO overtone limb‐emission from the lower thermosphere

The overtone vibration‐rotation band (Δv=−2) limb‐emission from NO around 2.7 µm observed by the Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS‐1A) at 120 km tangent altitude is shown to arise from the nascent molecule produced by the reactions of N(4S) and N(²D) atoms with O2. Measurement of the 2.7 µm emission from NO therefore permits modeling of the local rate of production of NO, a quantity important in odd nitrogen chemistry in the thermosphere. The 2.7 µm limb‐radiance in the nonauroral region is observed, above the instrument noise level, only during the day and shows about a factor of 2 variation indicating a similar variation in the rate of production of NO. In the aurorally dosed region however the observations lead to about an order of magnitude variation in this rate.

Waves and fluctuations in the jovian magnetosphere

The dynamics of the jovian magnetosphere are dominated by the mass loading of the 10 torus. Sulfur dioxide and possibly other gases such as hydrogen are sputtered from 10 and are ionized. These ions in turn are accelerated by the corotational electric field. The hot torus moves slowly outward and then ever more rapidly until reconnection releases the ions down the tail. The outward transport is far from steady. Near 10 are intense ion cyclotron waves. As observed by Galileo these waves weakened rapidly on the side of 10 closest to Jupiter. Beyond 10 10 radii (RI,) at periods less than 150s there were no discernable waves above the instrument noise level. On the side of 10 away from Jupiter the 10 cyclotron waves disappeared at about 20 Rr,, and the noise level in the torus reached values almost as low as those inside the 10 orbit. However, as the radial distance increased so did the noise levels. The noise levels from 0.01 to 0.1 Hz are about 2 orders of magnitude greater at 7.7 RJ than inside the 10 orbit at 5.5 RJ. The noise appears to be due to steps in the magnetic field strength and direction perhaps due to the interchange of tubes of flux with differing plasma content. The transverse power spectral density is greater than the compressional power in this region. The next region in which we have observations is the orbit of Europa and beyond. At frequencies of about 1 Hz the waves are more strongly compressional than transverse in the equatorial regions and more transverse than compressional off the equator. The outer edge of the dipolar region is about 24 RJ. The noise just inside this distance is quite variable from rotation to rotation. Outside this distance in the magnetodisk the field crossing the current sheet varies significantly from orbit to orbit as if the magnetodisk were globally unstable. Beyond about 40 Rr the location of the current sheet oscillates with a period of about 10 minutes. Beyond 50 RJ the plasma enters a regime of

The SoHO CELIAS/SEM data base

The CELIAS/SEM spectrometer aboard SoHO continuously monitors the full disk solar EUV and XUV irradiance. The instrumental noise level of the SEM is less than 0.5% of the signal in all channels, hence the short term signal variations larger than this in the SEM data base represent true solar variations in the EUV/XUV flux. Thrree sounding rocket calibration underflights since the SoHO launch have provided absolute EUV/soft X-ray solar flux values with which the long term stability of the SoHO SEM can be determined. In each of those flights the SEM “clone” has been calibrated before and after launch at the SURF II and SURF III synchrotron radiation facility of NIST in Gaithersburg, Maryland. The relative uncertainty of the SEM clone over a three year period is only ± 5.5% (1σ) and yields an absolute uncertainty of only 8.5% in transfering the absolute flux calibration to the SoHO CELIAS/SEM instrument.

A search for the minimum number of stations needed for seismic networking on Mars

The efficiency of a seismic network in providing information on the rate of seismicity, and on the inner structure of Mars, is estimated through a statistical analysis which takes into account the possible existence of a liquid core, the expected low rate of seismicity of Mars when compared to the Earths, and the attenuating properties of the mantle. The tests are performed for two frequency ranges (0.1–1.0 Hz and 0.5–2.5 Hz), for three instrumental noise amplitude densities ranging from 5 to 500×10−10 m s−2 Hz−1/2, and for three network configurations consisting of 4, 12 and 16 stations. Travel time tables are computed for P, S, PcP, ScS, and PKP phases using a simplified three layer model. Present-day estimates of liquid core radius induce a 25° wide shadow zone beginning at epicentral distances larger than 110°. Consequently, the best detection efficiency which can be expected from any network is of the order of 60% for mantle body waves. The detection efficiency is primarily controlled by the instrumental noise level. Since the amplitude of mantle body waves rapidly decreases with epicentral distance, high noise level instruments can only detect local events. Therefore, the detection score attained by 4 highly sensitive stations can be up to 30 and 7 times better than the score attained by 12 high noise level sensors, for mantle P and S waves, respectively. If crustal scattering is negligible, the record of mantle P waves on a network consisting of four low noise level instruments would permit to sample Mars mantle down to the core-mantle boundary. Conversely, the deepest penetration of rays recorded by a network of 12 high noise level sensors would hardly reach 300 km. In fact, strong crustal scattering might be the most important difficulty to be encountered in a seismic exploration of Mars. A possibility to deal with this problem would be to associate each of the four low noise instruments with three medium noise level sensors. This network strategy might permit to sample P and S mantle waves travelling down to 400–600 km, even if a lot of seismic energy is lost through crustal scattering.

Current channelling and three-dimensional effects detected from magnetotelluric data from a sedimentary basin in Sierras Pampeanas, Argentina

Aquifer-bearing intermontane sedimentary basins of the Sierras Pampeanas in the northwest of Argentina are in general very deep and narrow and contain economically important deposits of Tertiary sediments. This paper presents the results of a study to characterize the sedimentary basin bounded to the west by Sierra de Famatina and to the east by Sierra de Velasco, where an electromagnetic sensing technique, the magnetotelluric (MT) method, was applied. 12 MT sites were deployed along a 30 km E-W transect. Some of the data collected were used to derive a 2-D resistivity model of the basin. The model shows a subsurface trough with a thick (approximately 8 km) sedimentary sequence above it. Anomalous behaviour of the E-W electric-field component (Ey) was detected in the period range 1–100s, where the amplitude of this component was below the instrumental noise level. The cause of this anomaly is not known, but it might be due to the presence of an embedded conducting body between 8 and 10 km, which would give rise to N-S current channelling.

Wavefront analysis with high-accuracy inversion interferometer

A recently introduced wavefront analyzer, based on a Fresnel-lens array, is improved by incorporating phase-shifting procedures. The resulting compact and stable interferometer can measure wavefront distortions with extremely high accuracy. In our experiments, with no special efforts at environmental isolation, the instrumental noise level was under 0.02?.

Confusion noise level due to galactic and extragalactic binaries

We have revised our earlier rough estimate of the combined galactic and extragalactic binary confusion noise level curve for gravitational waves. This was done to correct some numerical errors and to allow for roughly three frequency bins worth of information about weaker sources being lost for each galactic binary signal that is removed from the data. The results are still based on the spectral amplitude estimates for different types of galactic binaries reported by Hils et al in 1990, and assume that the gravitational wave power spectral densities for other galaxies are proportional to the optical luminosities. The estimated confusion noise level drops to the LISA instrumental noise level at between roughly 3 and 8 mHz.

Magnetic measurements of Greenland and Himalayan ice-core samples

This paper summarizes Isothermal Remanence (IRM300mT) measurements from Holocene ice-core samples from two contrasting glaciological regimes: (i) the Greenland ice sheet and (ii) a Himalayan glacier. Cores had been previously characterized in terms of chronologies, geochemistry and, for the Himalayan core, microparticle concentrations. Six samples from the Greenland 20-D ice core from the time period AD 1876-1983 did not yield magnetic measurements above instrumental noise-level. Significant Isothermal Remanence measurements (IRM300mT) were obtained from some levels in the Himalayan ice core. Scanning electron microscopy and EDAX analyses indicate weathered crustal material. The techniques used require ice-core samples with relatively high dust loadings and/or larger volume ice samples.

Plasma waves and field‐aligned currents in the Venus plasma mantle

Plasma waves, observed above the dayside Venus ionosphere by the Pioneer Venus Orbiter (PVO) plasma wave instrument, have been attributed to whistler mode waves, lower hybrid waves, or ion acoustic waves. In order to clarify the nature of the waves, we have performed both case study and statistical analyses of the plasma wave and magnetic field data. We find that the plasma wave data are well ordered by altitude with respect to the location where the PVO Langmuir probe, or orbiter electron temperature probe (OETP), measures a density of 100 cm−3, known as the OETP ionopause. The dominant signature in the wave data appears to be a change in the instrument noise level because of changes in the plasma Debye length. However, there is a burst of wave activity near the OETP ionopause. Also, we find that there is a rotation in the magnetic field at or near this location. By casting the magnetic field data into a coordinate system ordered by the presumed magnetosheath flow, we find that the rotation of the field tends to orient the field in a more flow‐aligned direction at lower altitudes. We attribute this to mass loading at lower altitudes. We further suggest that the field‐aligned current associated with the field rotation corresponds to a shear Alfvén wave standing in the magnetosheath flow. The field‐aligned currents are present because of boundary conditions imposed on the flow, and it is not clear if the waves are actually associated with the field‐aligned currents or are simply coincidental. Since the waves are observed at the OETP ionopause, further progress in understanding these waves will be made though determining what underlying plasma structure, if any, is related to the OETP ionopause, which is defined by a specific instrument threshold. Nevertheless, our study confirms that the wave activity, field‐aligned currents, and OETP ionopause all occur within the plasma mantle above the ionosphere. As such, the plasma waves are not an energy source for the dayside ionosphere.

Detectability of Electrostatic Decay Products in [ITAL]Ulysses[/ITAL] and [ITAL]Galileo[/ITAL] Observations of Type III Solar Radio Sources

Recent in situ Ulysses and Galileo observations of the source regions of type 3 solar radio bursts appear to show an absence of ion acoustic waves S produced by nonlinear Langmuir wave processes such as the electrostatic (ES) decay, in contradiction with earlier ISEE 3 observations and analytic theory. This letter resolves these apparent contradictions. Refined analyses of the maximum S-wave electric fields produced by ES decay and of the characteristics of the Ulysses Wave Form Analyzer (WFA) instrument show that the bursty S waves observed by the ISEE 3 should be essentially undetectable by the Ulysses WFA. It is also shown that the maximum S-wave levels predicted for the Galileo event are approximately less than the instrumental noise level, thereby confirming an earlier suggestion. Thus, no contradictions exist between the ISEE 3 and Ulysses/Galileo observation, and no evidence exists against ES decay in the published Ulysses and Galileo data. All available data are consistent with, or at worst not inconsistent with, the ES decay proceeding and being the dominant nonlinear process in type 3 bursts.

Twilight rocket measurements of high-latitude atomic oxygen density during the DYANA campaign

During the DYANA ( DYnamics Adapted Network for the Atmosphere) campaign, a rocket containing a resonance fluorescence experiment for measurement of atomic oxygen concentrations was launched at twilight-dawn from ESRANGE, Kiruna, Sweden in March 1990. The measured atomic oxygen concentration rose very sharply near 80 km to about 10 11 atoms cm −3, achieved a peak value of 3 × 10 11 atoms cm −3 between 90 to 105 km, and decreased with increasing rocket altitude to about 155 km where the instrument's noise level was reached. In addition, ground-based, near-infrared radiometric and spectral measurements of the mesospheric and lower thermospheric hydroxyl (OH) airglow emissions were also obtained during the night up to the time of rocket launch. We have used least-squares spectral fitting procedures to obtain the OH Meinel (3-1) band intensities and accurate ( ± 2.5K) rotational temperatures from individual scans. The band intensities show considerable structure throughout the night, dropping sharply by about a factor of 3 during sunrise when the rocket was launched. The rotational temperatures were consistently around 225 K throughout the night and during the rocket flight. During the MAP/WINE campaign in February 1984, similar measurements using identical rocket-borne and ground-based techniques were made at night from ESRANGE. In this paper, the DYANA and MAP/WINE measurements are inter-compared and discussed and further compared with model calculations.

Secular variations of the integrated pulse profile of, and torque on, PSR 2217+47: Precession of the neutron star?

view Abstract Citations (17) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Secular Variations of the Integrated Pulse Profile of, and Torque on, PSR 2217+47: Precession of the Neutron Star? Suleymanova, Svetlana A. ; Shitov, Yurii P. Abstract A new component of the integrated pulse profile of PSR 2217+47 has been found at 102 MHz during observations at the Pushchino Observatory in 1991. Subsequent examination of archival observations going back to 1978 has revealed that this weak component was not visible above the instrumental noise level until 1983. During the intervening decade its relative intensity changed quasi-periodically between 1% and 3% with a timescale of 6-8 yr. Moreover, we found that this new 'postcursor' component 'drifted' monotonically toward the main pulse at a rate of 2.8 deg/yr. Timing measurements show that the first derivative of the pulsar period also suffered rather significant change, so that the second derivative is equal to -(6.59 +/- 0.57) x 10-27 sec/sq sec over the last decade. Precession of the pulsar spin axis is suggested as the common cause for the observed variations of both pulse shape and arrival times. Publication: The Astrophysical Journal Pub Date: February 1994 DOI: 10.1086/187201 Bibcode: 1994ApJ...422L..17S Keywords: Integral Transformations; Pulsar Magnetospheres; Pulsars; Pulse Amplitude; Pulse Modulation; Faraday Effect; Integrals; Radio Bursts; Unsteady Flow; Astrophysics; STARS: PULSARS: GENERAL; STARS: PULSARS: INDIVIDUAL ALPHANUMERIC: PSR 2217; 47 full text sources ADS | data products SIMBAD (3)