Northeast Radio Observatory Corporation

Analysis of very long baseline interferometry data indicates that systematic errors in prior estimates of baseline length, of order 5 cm for ∼ 8000‐km baselines, were due primarily to mismodeling of the electrical path length of the troposphere and mesosphere (“atmospheric delay”). Here we discuss observational evidence for the existence of such errors in the previously used models for the atmospheric delay and develop a new “mapping” function for the elevation angle dependence of this delay. The delay predicted by this new mapping function differs from ray trace results by less than ∼ 5 mm, at all elevations down to 5° elevation, and introduces errors into the estimates of baseline length of ≲ 1 cm, for the multistation intercontinental experiment analyzed here.

The new Horizontal Wind Model (HWM07) provides a statistical representation of the horizontal wind fields of the Earth's atmosphere from the ground to the exosphere (0–500 km). It represents over 50 years of satellite, rocket, and ground‐based wind measurements via a compact Fortran 90 subroutine. The computer model is a function of geographic location, altitude, day of the year, solar local time, and geomagnetic activity. It includes representations of the zonal mean circulation, stationary planetary waves, migrating tides, and the seasonal modulation thereof. HWM07 is composed of two components, a quiet time component for the background state described in this paper and a geomagnetic storm time component (DWM07) described in a companion paper.

The interplanetary shock/electric field event of 5–6 November 2001 is analyzed using ACE interplanetary data. The consequential ionospheric effects are studied using GPS receiver data from the CHAMP and SAC‐C satellites and altimeter data from the TOPEX/Poseidon satellite. Data from ∼100 ground‐based GPS receivers as well as Brazilian Digisonde and Pacific sector magnetometer data are also used. The dawn‐to‐dusk interplanetary electric field was initially ∼33 mV/m just after the forward shock (IMF B Z = −48 nT) and later reached a peak value of ∼54 mV/m 1 hour and 40 min later (B Z = −78 nT). The electric field was ∼45 mV/m (B Z = −65 nT) 2 hours after the shock. This electric field generated a magnetic storm of intensity D ST = −275 nT. The dayside satellite GPS receiver data plus ground‐based GPS data indicate that the entire equatorial and midlatitude (up to ±50° magnetic latitude (MLAT)) dayside ionosphere was uplifted, significantly increasing the electron content (and densities) at altitudes greater than 430 km (CHAMP orbital altitude). This uplift peaked ∼2 1/2 hours after the shock passage. The effect of the uplift on the ionospheric total electron content (TEC) lasted for 4 to 5 hours. Our hypothesis is that the interplanetary electric field “promptly penetrated” to the ionosphere, and the dayside plasma was convected (by E × B ) to higher altitudes. Plasma upward transport/convergence led to a ∼55–60% increase in equatorial ionospheric TEC to values above ∼430 km (at 1930 LT). This transport/convergence plus photoionization of atmospheric neutrals at lower altitudes caused a 21% TEC increase in equatorial ionospheric TEC at ∼1400 LT (from ground‐based measurements). During the intense electric field interval, there was a sharp plasma “shoulder” detected at midlatitudes by the GPS receiver and altimeter satellites. This shoulder moves equatorward from −54° to −37° MLAT during the development of the main phase of the magnetic storm. We presume this to be an ionospheric signature of the plasmapause and its motion. The total TEC increase of this shoulder is ∼80%. Part of this increase may be due to a “superfountain effect.” The dayside ionospheric TEC above ∼430 km decreased to values ∼45% lower than quiet day values 7 to 9 hours after the beginning of the electric field event. The total equatorial ionospheric TEC decrease was ∼16%. This decrease occurred both at midlatitudes and at the equator. We presume that thermospheric winds and neutral composition changes produced by the storm‐time Joule heating, disturbance dynamo electric fields, and electric fields at auroral and subauroral latitudes are responsible for these decreases.

Abstract NRLMSIS® 2.0 is an empirical atmospheric model that extends from the ground to the exobase and describes the average observed behavior of temperature, eight species densities, and mass density via a parametric analytic formulation. The model inputs are location, day of year, time of day, solar activity, and geomagnetic activity. NRLMSIS 2.0 is a major, reformulated upgrade of the previous version, NRLMSISE‐00. The model now couples thermospheric species densities to the entire column, via an effective mass profile that transitions each species from the fully mixed region below ~70 km altitude to the diffusively separated region above ~200 km. Other changes include the extension of atomic oxygen down to 50 km and the use of geopotential height as the internal vertical coordinate. We assimilated extensive new lower and middle atmosphere temperature, O, and H data, along with global average thermospheric mass density derived from satellite orbits, and we validated the model against independent samples of these data. In the mesosphere and below, residual biases and standard deviations are considerably lower than NRLMSISE‐00. The new model is warmer in the upper troposphere and cooler in the stratosphere and mesosphere. In the thermosphere, N 2 and O densities are lower in NRLMSIS 2.0; otherwise, the NRLMSISE‐00 thermosphere is largely retained. Future advances in thermospheric specification will likely require new in situ mass spectrometer measurements, new techniques for species density measurement between 100 and 200 km, and the reconciliation of systematic biases among thermospheric temperature and composition data sets, including biases attributable to long‐term changes.

Approximately 10% of active galactic nuclei exhibit relativistic jets, which are powered by the accretion of matter onto supermassive black holes. Although the measured width profiles of such jets on large scales agree with theories of magnetic collimation, the predicted structure on accretion disk scales at the jet launch point has not been detected. We report radio interferometry observations, at a wavelength of 1.3 millimeters, of the elliptical galaxy M87 that spatially resolve the base of the jet in this source. The derived size of 5.5 ± 0.4 Schwarzschild radii is significantly smaller than the innermost edge of a retrograde accretion disk, suggesting that the M87 jet is powered by an accretion disk in a prograde orbit around a spinning black hole.

Data from the Millstone Hill incoherent scatter radar taken over two solar cycles (1979–2000) are examined to determine the average characteristics of the disturbance convection electric field in the midlatitude ionosphere. Radar azimuth scans provide a regular database of ionospheric plasma convection observations spanning auroral and subauroral latitudes, and these scans have been examined for all local times and activity conditions.We examine the occurrence and characteristics of a persistent secondary westward convection peak which lies equatorward of the auroral two‐cell convection. Individual scans and average patterns of plasma flow identify and characterize this latitudinally broad and persistent subauroral polarization stream (SAPS), which spans the nightside from dusk to the early morning sector for all Kp greater than 4. Premidnight, the SAPS westward convection lies equatorward of L = 4 (60° invariant latitude, Λ), spans 3°–5° of latitude, and has an average peak amplitude of >900 m/s. In the predawn sector, SAPS is seen as a region of antisunward convection equatorward of L = 3 (55° Λ), spanning ∼3° of latitude, with an average peak amplitude of 400 m/s.

The Murchison Widefield Array is a dipole-based aperture array synthesis telescope designed to operate in the 80-300 MHz frequency range. It is capable of a wide range of science investigations but is initially focused on three key science projects: detection and characterization of three-dimensional brightness temperature fluctuations in the 21 cm line of neutral hydrogen during the epoch of reionization (EoR) at redshifts from six to ten; solar imaging and remote sensing of the inner heliosphere via propagation effects on signals from distant background sources; and high-sensitivity exploration of the variable radio sky. The array design features 8192 dual-polarization broadband active dipoles, arranged into 512 ldquotilesrdquo comprising 16 dipoles each. The tiles are quasi-randomly distributed over an aperture 1.5 km in diameter, with a small number of outliers extending to 3 km. All tile-tile baselines are correlated in custom field-programmable gate array based hardware, yielding a Nyquist-sampled instantaneous monochromatic <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">uv</i> coverage and unprecedented point spread function quality. The correlated data are calibrated in real time using novel position-dependent self-calibration algorithms. The array is located in the Murchison region of outback Western Australia. This region is characterized by extremely low population density and a superbly radio-quiet environment, allowing full exploitation of the instrumental capabilities.

It is well known that the interplanetary electric field can penetrate to the low‐latitude ionosphere. It is generally believed that the penetration of electric fields can last only for ∼30 min because of the shielding effect in the ring current. In this paper we present the observations of the dayside ionospheric electric field enhancements at middle and low latitudes in association with reorientations of the interplanetary magnetic field (IMF). In six cases, the eastward electric field in the dayside equatorial ionosphere, measured by the Jicamarca incoherent scatter radar, was enhanced for 2–3 hours after the IMF turned southward and remained continuously southward. In one case the eastward electric field in the dayside midlatitude ionosphere, measured by the Millstone Hill incoherent scatter radar, was continuously enhanced for ∼10 hours during southward IMF. Since Millstone Hill is close to the equatorward boundary of the auroral zone during magnetic storms, the penetration electric field there may be different from that at the equatorial ionosphere. The most striking feature of the measurements is that the enhancements of the ionospheric electric field can last for many hours without significant decay. The electric field enhancements in the middle‐ and low‐latitude ionosphere are closely related to magnetic activity and occur during the main phase of magnetic storms. The observations show that the interplanetary electric field can continuously penetrate to the low‐latitude ionosphere without shielding for many hours as long as the strengthening of the magnetic activity is going on under storm conditions.

Electron densities retrieved from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation (RO) measurements are compared with those measured by incoherent scatter radars (ISR) and ionosondes in this paper. These results show that electron density profiles retrieved from COSMIC RO data are in agreement with the ISR and ionosonde measurements. The ionospheric characteristics ( N m F 2 and h m F 2 ) derived from the COSMIC satellites are also compared with those calculated by the latest International Reference Ionosphere model (IRI‐2001) and the National Center for Atmospheric Research Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (NCAR‐TIEGCM). The comparison of the magnitude of the COSMIC N m F 2 data with those calculated by the IRI model and the TIEGCM is good. However, features such as the north‐south asymmetry and longitudinal variation of the equatorial anomaly that are seen in the COSMIC data and the TIEGCM simulations are not fully present in the IRI model. On the other hand, the TIEGCM produces a stronger winter anomaly than that seen in either the COSMIC data or the IRI model.

The coupling of the ionosphere to processes from below remains an elusive and difficult problem, as rapidly changing external drivers from above mask variations related to lower atmospheric sources. Here we use superposition of unique circumstances, current deep solar minimum and a record‐breaking stratospheric warming event, to gain new insights into causes of ionospheric perturbations. We show large (50–150%) persistent variations in the low‐latitude ionosphere (200–1000 km) that occur several days after a sudden warming event in the high‐latitude winter stratosphere (∼30 km). We rule out solar irradiance and geomagnetic activity as explanations of the observed variation. Using a general circulation model, we interpret these observations in terms of large changes in atmospheric tides from their nonlinear interaction with planetary waves that are strengthened during sudden warmings. We anticipate that further understanding of the coupling processes with planetary waves, accentuated during the stratospheric sudden warming events, has the potential of enabling the forecast of low‐latitude ionospheric weather up to several days in advance.

A representative sample of 12 extended quasars from the 3CR catalog has been imaged at 4.9 GHz using the VLA. These full synthesis observations typically achieve an rms noise of 20 µJy per beam, at a resolution (FWHM) of 0.34" to 0.38". Jets are detected on at least one side of every source. The jets are well collimated compared with those in less powerful sources, but spreading is detected in most of them. The opening angles of several jets are not constant, but show recollimation after an initial regime of rapid spreading. Many of the jets contain quasiperiodic strings of knots, of which the knot closest to the central feature is usually the brightest (until the jet nears its hot spot). The degrees of linear polarization at the jet knots range from &lt; 5% to ~ 50%, but show no common trend with distance along the jets. In knots that are elongated in directions close to that of the jet, the E vectors tend to be orthogonal to the jet axis. The exceptions - misaligned knots with misaligned polarizations-tend to be bright features near large bends in the jets. Many of the jets are initially straight to within a few degrees, but bend more in the outer part of the source. The prominence of the inner, straighter jet segments relative to the extended lobes correlates significantly with the prominence of the milliarcsecond-scale central features, but the prominence of the more bent jet segments does not. Candidates for counterjet emission are detected in seven sources, but there is no unambiguous, continuous counterjet in any of them. Estimates of the flux density ratios between the straighter jet segments and the counterjets based on these tentative detections range from 1.2:1 to &gt; 175:1. There is no evidence in this sample that counterjet detectability correlates with such putative inclination indicators as central feature prominence or projected linear size. There is also no evidence that the prominence of the counterjets anticorrelates with that of the jets as predicted by simple relativistic-beaming models for the jet/counterjet asymmetry. There is, however, strong evidence that large bends in the main jet favor counterjet detection, and there are no counterjet candidates opposite long, uninterrupted straight segments of the main jets. The detectability of the counterjets in these quasars may therefore be strongly influenced by interactions between the underlying beams and inhomogeneities in the surrounding material. We offer a new empirical definition of the term "hot spot" that is intended to improve the distinction between such features and "jet knots." Both the compactness of hot spots and their position in the lobe are affected by whether they are fed by a detectable jet. When the hot spots differ significantly in compactness, the more compact one is always on the jetted side. Jetted hot spots are also more likely to be recessed deeply from the outer edge of their lobes than are their counterjetted counterparts. The jetted hot spot is less prominent relative to other extended emission if the jet bends through a large angle, particularly if a large bend occurs abruptly. The counterjetted hot spot is also less well defined if the jet is more bent. The lobes of several sources show considerable inhomogeneity, including filamentation. There is little difference in the inhomogeneity of the jetted and counterjetted lobes if the hot spots are excluded. The lobes have a common linear polarization pattern, with low polarization at the center and high polarization (often reaching 40% to 60%) near the edges. This pattern matches the expectations of models in which the magnetic field in the lobes is provided by passively expanding the field in the jets. The lobes in sources with promising counterjet candidates are often S-symmetric relative to the jet axis and their hot spots are more misaligned than in sources without such candidates. Counterjets may therefore be easier to detect if the jets change orientation during the lifetime of the source. We outline the implications of our results for various models of the prominence and asymmetries of central features, jets, counterjets, and hot spots. The correlations between the prominence and sidedness of the large-scale straight jet segments and of the small-scale central features favor models in which the kiloparsec-scale jets initially have bulk relativistic velocities. The slope of the prominence correlation is less than expected if the larger-scale jets have characteristic Lorentz factors as high as those in the milliarcsecond-scale features, however. This result is fragile within our small sample, but other aspects of our data also suggest that another phenomenon, closely coupled to jet bending, helps to determine the prominence of features far from the central region. Overall, our data favor "tired jet" models in which the average jet velocity decreases with increasing distance from the central object. This makes it harder for the simplest relativistic-jet models to account for the systematic differences in compactness and placement of jetted and counterjetted hot spots. The models may need refinement to include a range of Lorentz factors in the jets at any given distance from the quasar.

A precipitation index is described which quantifies the intensity and spatial extent of high‐latitude particle precipitation based on observations made along individual satellite passes. By sorting plasma convection data according to this index, average patterns of the ionospheric convection electric field have been derived from a data set consisting of five years' observations by the Millstone Hill radar. Reference to the instantaneous precipitation index, and the average patterns keyed to it, provides a means of characterizing the global precipitation and convection patterns throughout an event.

We present strong evidence that during the January 2008 minor sudden stratospheric warming (SSW) event, the equatorial vertical E × B drifts exhibit a unique and distinctive daytime pattern. We do not think one event causes the other, however both events might be related through the global effects of planetary waves. The drifts were measured by the Jicamarca Incoherent scatter radar located under the magnetic equator. We have observed an anomalous temporal variation of the vertical E × B drifts during the minor SSW event, showing a semidiurnal variation with very large amplitudes lasting for several days. Large differences in the E × B drifts were observed during a period of large increase of temperature and a large decrease of mean zonal wind, in the high latitude stratosphere (60°–90°N). This high correlation is an unexpected finding which might shed new light on sources and mechanisms of quiet‐time ionospheric variability.

We have made the first VLBI synthesis images of the H2O maser emission associated with the central engine of the Seyfert galaxy NGC 1068. Emission extends about +/-300 km/s from the systemic velocity. Images with submilliarcsecond angular resolution show that the red-shifted emission lies along an arc to the northwest of the systemic emission. (The blue-shifted emission has not yet been imaged with VLBI.) Based on the maser velocities and the relative orientation of the known radio jet, we propose that the maser emission arises on the surface of a nearly edge-on torus, where physical conditions are conducive to maser action. The visible part of the torus is axially thick, with comparable height and radius. The velocity field indicates sub-Keplerian differential rotation around a central mass of about 1e7 Msun that lies within a cylindrical radius of about 0.65 pc. The estimated luminosity of the central engine is about 0.5 of the Eddington limit. There is no detectable compact radio continuum emission near the proposed center of the torus (T_B&lt; 5e6 K on size scales of about 0.1 pc), so that the observed flat-spectrum core cannot be direct self-absorbed synchrotron radiation.

Nighttime and daytime medium‐scale traveling ionospheric disturbances (MSTIDs) are detected with dense and wide detrended total electron content (TEC) maps over North America using multiple GPS receiver networks. The TEC maps cover a wide region of 60–130°W and 24–54°N (30–65°N in geomagnetic latitude), and have a spatial resolution of 1.05° × 1.05° in latitude and longitude (0.15° × 0.15° with 7 × 7 pixel smoothing) and a temporal resolution of 30 seconds. The TEC maps reveal, for the first time, that the nighttime MSTIDs propagate southwestward with 200–500 km wavelengths over North America and have wavefronts longer than ∼2,000 km. We also observe that daytime MSTIDs with 300–1,000 km wavelengths propagate southeastward until mid‐afternoon and southwestward in the late afternoon. In the mid‐to‐late afternoon, these MSTIDs propagating in the different directions are superimposed. The TEC maps can be a new powerful tool to investigate the MSTIDs.

We investigate the ionospheric response to several stratospheric sudden warming events which occurred in Northern Hemisphere winters of 2008 and 2009 during solar minimum conditions. We use GPS total electron content data in a broad latitudinal region at ±40° geographic latitude and a single longitude, 75°W. In all cases, we find a strong daytime ionospheric response to stratospheric sudden warmings. This response is characterized by a semidiurnal character, large amplitude, and persistence of perturbations for up to 3 weeks after the peak in high‐latitude stratospheric temperatures. The ionospheric perturbations at the lower latitudes usually begin a few days after the peak in stratospheric temperature and are observed as an enhancement of the equatorial ionization anomaly (EIA) in the morning sector and a suppression of the EIA in the afternoon sector. There is also evidence of a secondary enhancement in the postsunset hours. Once observed in the low latitudes, the phase of semidiurnal perturbations progressively shifts to later local times in subsequent days. This progressive shift occurs at a different rate for different stratospheric warming events. The large magnitude and persistence of ionospheric perturbations, together with the predictability of stratospheric sudden warmings several days in advance, present an opportunity to investigate these phenomena in a systematic manner which may eventually lead to a multiday forecast of low‐latitude ionosphere conditions.

We report 18 cm VLBI continuum imaging observations of Arp 220, the prototype luminous infrared galaxy (log Lfir = 12.11 L☉). In previous work, we showed that Arp 220 has compact, high-Tb nuclear radio emission that might be interpreted as a dust-enshrouded active galactic nucleus (AGN) radio core, or, alternately, as multiple, very luminous radio supernovae from a very active nuclear starburst. In this work, we present a new 18 cm VLBI image, with 3×8 mas angular resolution, showing approximately a dozen unresolved sources, S18 cm = 0.2-1.2 mJy, within a 02 × 04 (75×150 pc) region centered on the NW nucleus of this merging system. At least two additional sources are detected in the SE nucleus. These point sources account for about 3% of the total 18 cm radio emission associated with Arp 220 and for all the estimated radio flux density with Tb > 106 K. No other 18 cm emission is detected on scales from 3 to 100 mas (1-30 pc). We interpret these compact radio sources as luminous radio supernovae of the class in which RSN 1986J is a prototype. This interpretation is consistent with a simple starburst model for the infrared luminosity of Arp 220 that has a star formation rate of 50-100 M☉ yr-1 and a luminous supernova rate, νsn = 1.75-3.5 yr-1. In this model prescription, virtually all supernova explosions in Arp 220 must result in luminous RSNe, comparable to the most luminous RSNe observed. We discuss possible mechanisms for the origin of very luminous RSNe in luminous infrared galaxies and suggest that it is likely due to the dense, compact starburst environment. Although our observations do not rule out the presence of an AGN that may contribute to the infrared luminosity in Arp 220, it is not necessary to appeal to AGN activity to account for the overall radio/infrared characteristics of Arp 220.

Sudden stratospheric warming (SSW) is a large‐scale meteorological process in the winter hemisphere lasting several days or weeks. The Incoherent Scatter World Day campaign conducted on January 17–February 1, 2008 was arranged during a minor SSW event and focuses on studies of thermospheric and ionospheric response to stratospheric changes. We analyze ion temperature observations at 100–300 km height obtained by the Millstone Hill incoherent scatter radar (42.6°N, 288.5°E). Alternating regions of warming in the lower thermosphere and cooling above 150km altitude were observed by the radar. We use National Center for Environmental Prediction (NCEP) temperature data at 10hPa (∼30km) level and the F10.7 and Ap indices to identify any cause‐effect relationship between observed variations in the temperature and stratospheric warming event. We conclude that the seasonal trend, solar flux and geomagnetic activity cannot account for the observed warming and cooling temperature variation and suggest that this variation is associated with stratospheric warming. This study demonstrates a link between the lower atmosphere and the ionosphere which has not been considered before and indicates that ionospheric variability as part of space weather should be considered in conjunction with stratospheric changes.

Near a black hole, differential rotation of a magnetized accretion disk is thought to produce an instability that amplifies weak magnetic fields, driving accretion and outflow. These magnetic fields would naturally give rise to the observed synchrotron emission in galaxy cores and to the formation of relativistic jets, but no observations to date have been able to resolve the expected horizon-scale magnetic-field structure. We report interferometric observations at 1.3-millimeter wavelength that spatially resolve the linearly polarized emission from the Galactic Center supermassive black hole, Sagittarius A*. We have found evidence for partially ordered magnetic fields near the event horizon, on scales of ~6 Schwarzschild radii, and we have detected and localized the intrahour variability associated with these fields.

We have used plasma drift and magnetic field measurements during the 2001–2009 December solstices to study, for the first time, the longitudinal dependence of equatorial ionospheric electrodynamic perturbations during sudden stratospheric warmings. Jicamarca radar measurements during these events show large dayside downward drift (westward electric field) perturbations followed by large morning upward and afternoon downward drifts that systematically shift to later local times. Ground‐based magnetometer measurements in the American, Indian, and Pacific equatorial regions show strongly enhanced electrojet currents in the morning sector and large reversed currents (i.e., counterelectrojets) in the afternoon sector with onsets near new and full moons during northern winter warming periods. CHAMP satellite and ground‐based magnetic field observations indicate that the onset of these equatorial afternoon counterelectrojets is longitude dependent. Our results indicate that these large electrodynamic perturbations during stratospheric warming periods are due to strongly enhanced semidiurnal lunar wave effects. The results of our study can be used for forecasting the occurrence and evolution of these electrodynamic perturbations during arctic winter warmings.