Joseph E. Salah

Revised global model of thermosphere winds using satellite and ground-based observations

Thermospheric wind data obtained from the Atmosphere Explorer E and Dynamics Explorer 2 satellites have been combined with wind data for the lower and upper thermosphere from ground-based incoherent scatter radar and Fabry-Perot optical interferometers to generate a revision (HWM90) of the HWM87 empirical model and extend its applicability to 100 km. Comparison of the various data sets with the aid of the model shows in general remarkable agreement, particularly at mid and low latitudes. The ground-based data allow modeling of seasonal/diurnal variations, which are most distinct at mid latitudes. While solar activity variations are now included, they are found to be small and not always very clearly delineated by the current data. They are most obvious at the higher latitudes. The model describes the transition from predominately diurnal variations in the upper thermosphere to semidiurnal variations in the lower thermosphere and a transition from summer to winter flow above 140 km to winter to summer flow below. Significant altitude gradients in the wind are found to extend to 300 km at some local times and pose complications for interpretation of Fabry-Perot observations.

The Murchison Widefield Array: The Square Kilometre Array Precursor at Low Radio Frequencies

The Murchison Widefield Array (MWA) is one of three Square Kilometre Array Precursor telescopes and is located at the Murchison Radio-astronomy Observatory in the Murchison Shire of the mid-west of Western Australia, a location chosen for its extremely low levels of radio frequency interference. The MWA operates at low radio frequencies, 80–300 MHz, with a processed bandwidth of 30.72 MHz for both linear polarisations, and consists of 128 aperture arrays (known as tiles) distributed over a ~3-km diameter area. Novel hybrid hardware/software correlation and a real-time imaging and calibration systems comprise the MWA signal processing backend. In this paper, the as-built MWA is described both at a system and sub-system level, the expected performance of the array is presented, and the science goals of the instrument are summarised.

Science with the Murchison Widefield Array

Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the southern hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope will enable new advances along four key science themes, including searching for redshifted 21-cm emission from the EoR in the early Universe; Galactic and extragalactic all-sky southern hemisphere surveys; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. The Murchison Widefield Array is located in Western Australia at the site of the planned Square Kilometre Array (SKA) low-band telescope and is the only low-frequency SKA precursor facility. In this paper, we review the performance properties of the Murchison Widefield Array and describe its primary scientific objectives.

Interim standard for the ion-neutral atomic oxygen collision frequency

A growing body of evidence based on radar and optical observations of winds in the Earths upper atmosphere indicates that the commonly utilized value for the ion-neutral atomic oxygen (O+-O) collision frequency for momentum transfer is too low by a factor of 1.3–2. A revised standard for the O+-O momentum transfer collision frequency is therefore recommended based on all the available measurements, in order to facilitate comparisons with thermospheric and ionospheric models and among data sets until future accurate laboratory measurements and more reliable observations can be made. The new interim standard, representing a 70% increase from the previous value, is given by: ν o+−o = 4×10−17 Tr0.5 no, where Tr = (Ti+Tn)/2, Ti being ion temperature, Tn neutral temperature, and no atomic oxygen number density in m−3.

A study of fundamental limitations to statistical detection of redshifted H i from the epoch of reionization

In this paper, we explore for the first time the relative magnitudes of three fundamental sources of uncertainty, namely, foreground contamination, thermal noise, and sample variance, in detecting the H I power spectrum from the epoch of reionization (EoR). We derive limits on the sensitivity of a Fourier synthesis telescope to detect EoR based on its array configuration and a statistical representation of images made by the instrument. We use the Murchison Widefield Array (MWA) configuration for our studies. Using a unified framework for estimating signal and noise components in the H I power spectrum, we derive an expression for and estimate the contamination from extragalactic point-like sources in three-dimensional k -space. Sensitivity for EoR H I power spectrum detection is estimated for different observing modes with MWA. With 1000 hr of observing on a single field using the 128 tile MWA, EoR detection is feasible (S/N >1 for k ≲ 0.8 Mpc -1 ). Bandpass shaping and refinements to the EoR window are found to be effective in containing foreground contamination, which makes the instrument tolerant to imaging errors. We find that for a given observing time, observing many independent fields of view does not offer an advantage over a single field observation when thermal noise dominates over other uncertainties in the derived power spectrum.

The EoR sensitivity of the murchison widefield array

Using the final 128 antenna locations of the MurchisonWidefield Array (MWA), we calculate its sensitivity to the epoch of reionization (EoR) power spectrum of redshifted 21 cm emission for a fiducial model and provide the tools to calculate the sensitivity for any model. Our calculation takes into account synthesis rotation, chromatic and asymmetrical baseline effects, and excludes modes that will be contaminated by foreground subtraction. For the fiducial model, the MWA will be capable of a 14 σ detection of the EoR signal with one full season of observation on two fields (900 and 700 h).

Field Deployment of Prototype Antenna Tiles for the Mileura Widefield Array Low Frequency Demonstrator

Experiments were performed with prototype antenna tiles for the Mileura Widefield Array Low Frequency Demonstrator (MWA LFD) to better understand the wide-field, wide-band properties of their design and to characterize the radio-frequency interference (RFI) between 80 and 300 MHz at the site in Western Australia. Observations acquired during the 6 month deployment confirmed the predicted sensitivity of the antennas, sky-noise-dominated systemtemperatures,andphase-coherentinterferometricmeasurements.Theradiospectrumisremarkablyfreeofstrong terrestrial signals, with the exception of two narrow frequency bands allocated to satellite downlinks, and rare bursts duetoground-based transmissionsbeingscatteredfrom aircraft andmeteortrails. Resultsindicatethepotential ofthe MWA LFD to make significant achievements in its three key science objectives: epoch of reionization science, heliospheric science, and radio transient detection.

E-region temperature measurements at Millstone Hill

Abstract An incoherent scatter radar experiment involving the transmission of pairs of short pulses at various spacings was employed at Millstone Hill to study the daytime thermal structure of the lower thermosphere. An altitude range from 110 to 130 km was covered with a height resolution of 6 km. The daily temperature variation exhibited oscillations having dominant periods in the range 6–14 hr and amplitudes as large as 28 per cent of the mean temperature. These waves possessed a downward phase velocity indicating an energy source at lower heights. A spectral analysis carried out on measurements gathered on successive days provided sufficient frequency resolution to resolve the different tidal frequencies and indicated the 12-hr period as the main component. Interpreting the measured oscillations as being primarily a manifestation of the solar semidiurnal tide, the observed phase and vertical wavelength are in agreement with theoretical predictions for the thermal (2, 4) tidal mode propagating from the lower atmosphere. On average, the semidiurnal wave amplitude is about 40 K with a time of maximum near 0700 hr LT at 115 km and is damped above that height. Large variability in amplitude and phase are found from one observing period to another. The height gradient of the average daytime temperature near 120 km is about 15 K/km. This value is larger than given by some theoretical treatments but is consistent when the energy carried into the region by the semidiurnal tide is allowed for.

Fast Holographic Deconvolution: a new technique for precision radio interferometry

We introduce the Fast Holographic Deconvolution method for analyzing interferometric radio data. Our new method is an extension of A-projection/software-holography/forward modeling analysis techniques and shares their precision deconvolution and wide-field polarimetry, while being significantly faster than current implementations that use full direction-dependent antenna gains. Using data from the MWA 32 antenna prototype, we demonstrate the effectiveness and precision of our new algorithm. Fast Holographic Deconvolution may be particularly important for upcoming 21 cm cosmology observations of the Epoch of Reionization and Dark Energy where foreground subtraction is intimately related to the precision of the data reduction.

Climatology of neutral winds in the lower thermosphere over Millstone Hill (42.6°N) observed from ground and from space

Neutral winds in the lower thermosphere at altitudes of 94-130 km were measured in 1987-2000 by the ground-based incoherent scatter radar (ISR) at Millstone Hill (42.6°N, 288.5°E) and in 1992-1997 by the space-based Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite (UARS). The data from the ISR and WINDII have been separately grouped into four seasons. The daytime wind measurements from both instruments are found to be in good agreement, with similar structures and similar magnitudes. Winds from both instruments show an annual variation with a winter minimum. Winds in spring and summer are generally about a factor of two larger than those in fall and winter. The semidiurnal winds are dominant, but diurnal tides, mainly the in situ thermospheric diurnal tide, are comparable. The tidal characteristics and the background winds (the diurnal mean) derived from WINDII data show that the amplitude of the semidiurnal tide is generally 10 ms -1 larger than that of the in situ thermospheric diurnal tide, and the zonal and meridional background winds are less than 40 ms -1 and 15 ms -1 in all seasons, respectively. The TIME-GCM simulations of wind fields at 42.5°N for the March equinox and the December solstice are in reasonable agreement with the observations, but the comparisons for the June solstice are less satisfactory. More research needs to be done to understand the cause of disagreement at the June solstice and the September equinox and to properly model the tidal effects in the lower thermosphere during those periods.