OVRO Research Articles

A multisource limb flare observed at multiple radio wavelengths

A flare with several radio sources occurred on the solar limb at 2155 UT on 1989 June 20. It was observed by the Very Large Array (VLA) and the Owens Valley Radio Observatory (OVRO). The VLA data consisted of images at 1.4 GHz, while OVRO provided spectral and spatial information over the range 1-15 GHz. We develop a new gyrosynchrotron model to analyze the sources observed at flare peak. This model differs from many previous ones in that it contains spatial variations of both the magnetic field and accelerated particle density. It uses a new gyrosynchrotron approximation which is valid at very low harmonics of the gyrofrequency. For the first time we find that the cause of the change of microwave source size with frequency in the event studied was due primarily to the spatial variation of the accelerated electron density. This is contrary to the common assumption that the variation in size is due to a nonuniform magnetic field. We also investigate a polarized source which brightened later in the flare, finding that it could have been due to plasma radiation or gyrosynchrotron emission.

Spectral evolution of microwaves and hard X-rays in the 1989 March 18 flare and its interpretation

We analyze the time variation of microwave spectra and hard X-ray spectra of 1989 March 18, which are obtained from the Solar Array at the Owens Valley Radio Observatory (OVRO) and the Hard X-Ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM), respectively. From this observation, it is noted that the hard X-ray spectra gradually soften over 50–200 keV on-and-after the maximum phase while the microwaves at 1–15 GHz show neither a change in spectral shape nor as rapid a decay as hard X-rays. This leads to decoupling of hard X-rays from the microwaves in the decay phase away from their good correlation seen in the initial rise phase. To interpret this observation, we adopt a view that microwave-emitting particles and hard X-ray particles are physically separated in an inhomogeneous magnetic loop, but linked via interactions with the Whistler waves generated during flares. From this viewpoint, it is argued that the observed decoupling of microwaves from hard X-rays may be due to the different ability of each source region to maintain high energy electrons in response to the Whistler waves passing through the entire loop. To demonstrate this possibility, we solve a Fokker-Planck equation that describes evolution of electrons interacting with the Whistler waves, taking into account the variation of Fokker-Planck coefficients with physical quantities of the background medium. The numerical Fokker-Planck solutions are then used to calculate microwave spectra and hard X-ray spectra for agreement with observations. Our model results are as follows: in a stronger field region, the energy loss by electron escape due to scattering by the waves is greatly enhanced resulting in steep particle distributions that reproduce the observed hard X-ray spectra. In a region with weaker fields and lower density, this loss term is reduced allowing high energy electrons to survive longer so that microwaves can be emitted there in excess of hard X-rays during the decay phase of the flare. Our results based on spectral fitting of a flare event are discussed in comparison with previous studies of microwaves and hard X-rays based on either temporal or spatial information.

Imaging spectroscopy of solar microwave radiation. 1: Flaring emission

We present observations of an impulsive microwave burst on the Sun with both high spatial and spectral resolution, made with the Solar Array at the Owens Valley Radio Observatory (OVRO). We used the measured brightness temperature spectrum to infer the emission process responsible for each microwave source, and to derive physical conditions in the source region. We confimed our predictions using soft X-ray measurements from Geostationary Operational Environmental Satellite (GOES), soft X-ray images from Yohkoh, and H-alpha flare images together with sunspots and magnetogram images from the Big Bear Solar Observatory.

High spectral resolution, high sensitivity microwave and associated hard X-ray bursts

We have carried out mm-wavelength (18 – 23) GHz observations of solar bursts in June 1989, in Brazil. Nine of the bursts were observed simultaneously with the Hard X-ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM). The Owens Valley Radio Observatory (OVRO) observed five of these events from 1 to 18 GHz. To date, we have analyzed six of these events and the preliminary results are: (i) The turnover frequency of all of these bursts were in the frequency range of 7 – 11 GHz and the radio spectral index ranged from −0.3 to −5.3; (ii) In three bursts, dominant features of high energy (≥ 200 keV) X-rays coincided in time with mm-wavelength peaks, suggesting that the high energy electrons are responsible for the mm-wavelength emission.

Microwave Emission from a Sunspot III. Implications for the Force Balance in a Static Sunspot

AbstractWe present our earlier measurement of the coronal fields above a sunspot in the active region, NOAA #4741, which was obtained using microwave spectroscopy from the Owens Valley Radio Observatory (OVRO) in a form that allows comparison with theoretical models. Our empirical results for the coronal fields are compared with several theoretical models in order to understand the magnetic structure of a static sunspot. It is found that the presence of transverse pressure gradients should be considered to get a satisfactory fit.

Radio observations of the M8.1 solar flare of 23 June, 1988: Evidence for energy transport by thermal processes

The Very Large Array (VLA) and the frequency agile interferometer at the Owens Valley Radio Observatory (OVRO) were used to observe the M8.1 flare of 23 June, 1988. The VLA obtained images prior to and during the flare at 333 MHz, and at 1.5 and 4.7 GHz. The frequency agile interferometer at Owens Valley obtained interferometer amplitude and total power spectra of the flare at 45 frequencies between 1 and 18 GHz. The observations were supplemented by radiometer measurements made by the USAF RSTN network site at Palehua, HI, by GOES soft X-ray observations, by USAF SOON Hα filtergrams, and by a KPNO photospheric magnetogram. The radio data reveal a wide variety of phenomena, including: (i) a multiply impulsive microwave burst that is essentially thermal in character; (ii) stationary discrete components at 1.5 GHz, associated temporally and spatially with distant brightenings in Ha; (iii) a dynamical component at 1.5 GHz associated with hot plasma moving subsonically into the corona; (iv) the appearance of intense, short-lived, decimetric burst activity near the lead sunspot in the active region at 1.5 GHz, indicative of a high degree of inhomogeneity in the source. The unusually complete radio coverage allows us to investigate the transport of energy from the initial site to sites of distant Hα brightenings. The transport of energy appears to be most consistent with slow, thermal processes, rather than rapid transport by nonthermal electron beams.

An Upper Limit to the Intrinsic Anisotropy of the Microwave Background Radiation on a Scale of 7 Arc Minutes

For the last four years we have been engaged on a program to look for intrinsic variations in the Microwave Background Radiation (MBR) at the Owens Valley Radio Observatory (OVRO). We summarize here the results of this continuing search.

VLBI Structure of NGC 1275 at 3 Millimeters

We have made λ 3mm VLBI observations of 3C84 at 6 epochs between 1981 and 1987, using 6m antennas at the Hat Creek Radio Astronomy Observatory (HCRK), 10m antennas at the Owens Valley Radio Observatory (OVRO), the 12m Kitt Peak telescope (KTPK), and the 14m telescope at the Five College Radio Astronomy Observatory (QBBN). Model fitting suggests a core-halo structure, which we associate with the active nucleus of NGC 1275. The unresolved core, which is smaller than 0.1 pc (0.2 mas; z=0.018; H =50 km s−1 Mpc−1) shows a slow decay corresponding to the decrease in total flux density since the flare in 1980. In 1985 we resolved the core component on a transcontinental baseline (109λ) into a compact nucleus, < 0.1 mas, and a “jet” extended at a position angle 205° and unresolved in width, < 0.1 mas. There is also evidence for a compact emission region located 10 mas from the nucleus at a position angle 170°. This structure corresponds closely with that observed at the same epoch in VLBI maps at 22 GHz.

Molecular abundances in OMC-1 - The chemical composition of interstellar molecular clouds and the influence of massive star formation

We present here an investigation of the chemical composition of the various regions in the core of the Orion molecular cloud (OMC-1) based on results from the Caltech Owens Valley Radio Observatory (OVRO) millimeter-wave spectral line survey (Sutton et al.; Blake et al.). This survey covered a 55 GHz interval in the 1.3 mm (230 GHz) atmospheric window and contained emission from over 800 resolved spectral features. Of the 29 identified species 14 have a sufficient number of detected transitions to be investigated with an LTE rotation diagram technique, in which large numbers of lines are used to estimate both the rotational excitation and the overall abundance. The rotational temperatures and column densities resulting from these fits have then been used to model the emission from those remaining species which either have too few lines or which are too weak to be so analyzed. When different kinematic sources of emission are blended to produce a single feature, Gaussian fits have been used to derive the individual contributions to the total line profile. The uniformly calibrated data in the unique and extensive Caltech spectral line survey lead to accurate estimates of the chemical and physical parameters of the Orion molecular cloud, and place significant constraints on models of interstellar chemistry. A global analysis of the observed abundances shows that the markedly different chemical compositions of the kinematically and spatially distinct Orion subsources may be interpreted in the framework of an evolving, initially quiescent, gas-phase chemistry influenced by the process of massive star formation. The chemical composition of the extended Orion cloud complex is similar to that found in a number of other objects, but the central regions of OMC-1 have had their chemistry selectively altered by the radiation and high-velocity outflow from the young stars embedded deep within the interior of the molecular cloud. Specifically, the extended ridge clouds are inferred to have a low (subsolar) gas-phase oxygen content from the prevalence of reactive carbon-rich species like CN, CCH, and C_3H_2 also found in more truly quiescent objects such as TMC-1. The similar abundances of these and other simple species in clouds like OMC-1, Sgr B2, and TMC-1 lend support to gas-phase ion-molecule models of interstellar chemistry, but grain processes may also play a significant role in maintaining the overall chemical balance in such regions through selective depletion mechanisms and grain mantle processing. In contrast, the chemical compositions of the more turbulent plateau and hot core components of OMC-1 are dominated by high-temperature, shock-induced gas and grain surface neutral-neutral reaction processes. The high silicon/sulfur oxide and water content of the plateau gas is best modeled by fast shock disruption of smaller grain cores to release the more refractory elements followed by a predominantly neutral chemistry in the cooling postshock regions, while a more passive release of grain mantle products driven toward kinetic equilibrium most naturally explains the prominence of fully hydrogenated N-containing species like HCN, NH_3 , CH_3CN, and C_2H_5CN in the hot core. The clumpy nature of the outflow is illustrated by the high-velocity emission observed from easily decomposed molecules such as H_2CO. Areas immediately adjacent to the shocked core in which the cooler, ion-rich gas of the surrounding molecular cloud is mixed with water/oxygen rich gas from the plateau source are proposed to give rise to the enhanced abundances of complex internal rotors such as CH_30H, HCOOCH_3 , and CH_30CH_3 whose line widths are similar to carbon-rich species such as CN and CCH found in the extended ridge, but whose rotational temperatures are somewhat higher and whose spatial extents are much more compact.

Water-vapour maser emission from galactic nuclei

We report here the progress of a survey of galactic nuclei for water-vapour maser emission at 22.235 GHz. We observed 29 late-type galaxies in November 1982, January 1983, and September 1983 using the 40-m radio telescope of the Owens Valley Radio Observatory (OVRO) equipped with a travelling-wave maser receiver1. We have detected maser emission from four nuclei: NGC4258 (M106), NGC1068 (M77), NGC3034 (M82), and NGC6946. The masers in NGC4258 and NGC1068 are extremely luminous; NGC1068 has the most luminous water-vapour maser yet reported—350 L⊙ assuming isotropic emission. We suggest that the extremely bright nuclear masers indicate an ongoing burst of star formation, and therefore provide a useful probe of the physical conditions of the starburst phenomenon.

VLBI Measurements of Radio Source Positions at Three U.S. Stations

AbstractResults of VLBI measurement of 14 radio source positions at three U.S. stations during the MERIT short campaign is presented. Comparisons with other solutions are given, together with the comparisons between several radio source catalogues.During the MERIT short campaign, several observatories cooperated in VLBI measurements organized jointly by the U.S. National Geodetic Survey (NGS) and the National Aeronautics and Space Administration (NASA). There were two 7-day observations arranged from Sept. 26 to Oct. 2 and from Oct. 16 to Oct. 22, with time span nearly 23 hours per day. Details of the observation and data reduction have been published elsewhere (1). This paper deals with radio source positions determined by three U.S. stations, the Haystack Observatory, the Harvard Radio Astronomy Station (HRAS) and the Owens Valley Radio Observatory (OVRO).

21 cm and 18 cm Observations of the Cygnus X-3 Radio Outburst

WE have observed the outburst of Cyg X-3 using the Owens Valley Radio Observatory (OVRO) interferometer at 21 cm and (after technical difficulties halted interferometer observations) with the 130-foot antenna at 18 cm. For much of the outburst we have observations with both systems.