Microwave Flux Research Articles

Solar modulation of galactic cosmic-ray intensity as seen by neutron monitors during 1990–1999 – I. Rigidity Dependence and Correlations with Solar Activity Parameters

We analyze the evolution of cosmic ray intensity detected by six neutron monitors located at high altitude from 1990 to 1999, that includes most of solar cycle 22 and the start of cycle 23. This set of neutron monitors covers a wide range of geomagnetic cutoff rigidities. We discuss the most significant characteristics of the cosmic ray modulation during the period as: the extraordinary decreases produced by the events of the first half of 1991, the significant two step evolution of the recovery phase of solar cycle 22 and the start of cycle 23. We also determine the rigidity dependence of the different phases of the modulation cycle. Cosmic ray intensity correlations with several solar activity parameters as sunspots, microwave flux at 10.7 cm and solar flares and with the intensity of the interplanetary magnetic field are studied.

An Alternative to Spinning Dust for the Microwave Emission of LPH 201.663+1.643: An Ultracompact H [CSC]ii[/CSC] Region

The microwave spectral energy distribution of the dusty, diffuse H II region LPH 201.663+1.643 has been interpreted by other authors as tentative evidence for microwave emission from spinning dust grains. We present an alternative interpretation for that particular object, specifically, that an ultracompact H II region embedded within the dust cloud would explain the available observations as well or better than spinning dust. Parameters for the size, surface brightness, and flux density of the putative ultracompact H II region, derived from the microwave observations, are within known ranges. A possible candidate for such an ultracompact H II region is IRAS 06337+1051, based on its infrared colors. However, the infrared flux of IRAS 06337+1051 appears to be too small to be consistent with the microwave flux required for this alternative model to explain the observations.

Synthesis of multi-walled carbon nanotubes by microwave plasma-enhanced chemical vapor deposition

Microwave plasma-enhanced chemical vapor deposition (MW-PECVD) has been employed to synthesize carbon nanostructures by using Fe (or Co, Ni)/γ-Al 2O 3 as catalysts and a mixture of benzene, hydrogen, and argon as precursors. By regulating the types of catalyst, the microwave incident power, the ratio and flux of the precursors, many morphologies such as ordinary geometric, helix-shaped, and planar spiral carbon nanotubes with aspect ratios of 100–1000 have been observed. Furthermore, two novel nanostructures, which are probably the missing link between onion-like carbon particles and nanotubes, have also been obtained. The striking feature of this new approach is the low synthesis temperature (<520°C) due to the non-equilibrium characteristic of microwave plasma operated at low pressure, which is crucial for some fascinating applications.

Microwave flux density variations of compact radio sources monitored by real‐time very long baseline interferometry

Compact and strong radio sources were repeatedly observed in regular geodetic very long baseline interferometry experiments under the Key Stone Project. The flux densities of these sources in the S and X bands were estimated from a set of correlated amplitudes obtained through the correlation processing of the observed data. From ∼5 years of the observed data, irregular variations in the flux densities were detected for several radio sources using the source 2134+004 as the calibrator. The results showed the monitoring of flux density variations by means of very long baseline interferometry is feasible. Since the correlation processing of the observed data are performed real‐time using a high‐speed digital communication technique, this method gives a capability to monitor flux density variations of quasars and BL‐Lac types of extragalactic radio sources.

A measurement of the quiet network contribution to solar irradiance variation

A large increase in quiet network area since the 17th century Maunder Minimum has been suggested as a mechanism for increasing solar irradiance sufficiently to drive global warming. We show that this mechanism requires essentially complete disappearance of network proceeding back in time to the beginning of the 20th century. This disappearance is ruled out by the many Ca K spectroheliograms taken since the discovery of the network in the early 1890's. Furthermore, network area measurements we have carried out on Ca K spectroheliograms digitized from the Mt. Wilson and NSO/Sacramento Peak archives, for the nine solar activity minima between 1914 and 1996, show no evidence of network area variations large enough to produce a significant long‐term component of total irradiance variation. A network brightness variation of sufficient magnitude is also unlikely, given the linear dependence of solar microwave flux on area of bright structures.More generally, recent analyses of cycle 21,22 pyrheliometry, and of broadband stellar photometry, provide little support for any long‐term irradiance component These results do not rule out a secular irradiance increase. But they suggest that high climate sensitivity to the relatively small changes in solar total and UV irradiance that have been observed, provides a more likely explanation of the global temperature‐solar activity correlation.

Hard x-ray and Microwave Flux Spectra of the 2 November 1991 Solar Flare

We analysed the hard X-ray and microwave flux spectra of the solar flare (BATSE No. 1791) on 2 November 1991, which started at 16:11:03 UT and ended at 16:56:10 UT. This flare is particularly interesting because of its deep cyclic intensity modulation. Data are available simultaneously from the 16-channel BATSE/LAD hard X-ray and 45-frequency OVRO microwave database. We quantitatively compare the time variations in profiles of the hard X-ray spectral photon index, the 50 keV X-ray flux, and microwave spectral indices (at both high and low frequency ends). As expected, the X-ray photon spectral index decreases as the hard X-ray flux increases. This pattern appears in all the sub-peaks. This is consistent with previous observations that hard X-ray emission hardens at the emission peak. However, the behaviour of the high-frequency microwave index is unexpected. We observe an anti-correlation between the high-frequency microwave index and the hard X-ray photon index during the course of the flare. Finally, we study the arrival time of microwave flux peaks as a function of frequency and find that the microwave peak at a higher frequency comes earlier than that at a lower frequency. A maximum delay of 72 s is found among the main peaks at different frequencies. Shorter delays are found for the other five sub-peaks.

A Microwave Study of Coronal Ejecta

Using Nobeyama 17 GHz data, we have studied the radio properties of 19 coronal jets identified in Yohkoh soft X-ray imaging telescope (SXT) X-ray observations. The radio data provide information on the physical conditions in the jets, which complements the data from the X-ray surveys. Microwave emission was associated with the majority of the X-ray jets in our sample. The radio emission typically came from the base or the base and lower part of the jets. We detected radio emission from almost all jets that showed flarelike activity at their bases. The jets that were not associated with radio emission did not show any significant increase in X-ray emission at their bases. The strongest radio emission came from two of the largest jets in our sample. Our data show a general correlation between the X-ray jet fluxes and the associated radio fluxes. The 17 GHz time profiles were gradual and unpolarized, implying that the emission was thermal. In a two-sided-loop jet (1992 July 22 event) and one anemone-type jet (1993 February 9 event), the observed microwave fluxes from the lower part of the jets were well above the fluxes calculated from the computed physical parameters of the soft X-ray-emitting material on the basis of thermal free-free emission. We interpret the large discrepancies in terms of the presence of lower temperature material, which cannot be detected by the SXT (the SXT is most sensitive to hot plasma above 2 × 106 K), but which produces strong microwave free-free emission. This is the first time that such material has been observed in two-sided-loop-type jets. We also observed motion of a jet-associated microwave source with a velocity of 55 km s-1. The microwave motion occurred after the appearance of the X-ray jet. There is clear evidence that the microwave emission of that source was associated with the jet and not with the associated small flare.

The limb flare of November 2, 1992: Physical conditions and scenario

The powerful X9 class flare was investigated using interferometric data of Nobeyama Radioheliograph (17 GHz) and SSRT (5.7 GHz). According to images obtained in circular polarization, it was found that the microwave burst was generated consequently in the sites, where opposite magnetic loops were in the close contact. Steady exponential growth of the total microwave flux in the initial stage of the flare under significant reconstruction of the radio sources certifies that the development of the flare was conducted by some large-scale instability. Radiation during the impulsive phase was produced by gyrosynchrotron emission of electrons trapped within a wide dome-like volume with G and cm-3 . In the decay phase, the emission at higher frequencies was mainly produced by bremsstrahlung from high loops emitting soft X-rays. There is evidence of the energy release long after the impulsive phase: an intensive, rather compact microwave source in the locus of contact of two oppositely polarized regions in these loops and sub-bursts during 2 hours after the peak at low heights. For the first time, a compact source of the sub-second pulses at 17 GHz was found to be located as high as km. They were due to gyrosynchrotron emission.

Thawing of Foods in a Microwave Oven: I. Effect of Power Levels and Power Cycling

Microwave thawing is faster than other methods, but it can produce significant non-uniformity of heating. The objective of this study was to perform comprehensive experimentation and heat transfer modeling to relate the time to thaw and the non-uniformity of thawing to power cycling, power level and the surface heat transfer coefficient. The governing energy equation was formulated with an exponential decay of the microwave flux from the surface. Surface microwave flux was obtained from the measured temperature rise using inverse heat transfer analysis. Gradual phase change was formulated as an apparent specific heat, and was obtained for the experimental material tylose from differential scanning calorimetry (DSC) measurements. The temperatures were measured immediately following heating with a fast response thermocouple. Dielectric properties were measured above freezing. Results show that the microwave flux at the surface and its decay are affected by the changes in the power level. Power cycling has an almost identical effect as continuous power at the reduced level of the average cycled power. As power level increases, the surface flux increases by the same fraction. At higher power levels, however, the outside thaws relatively faster. A “shield” develops due to a much reduced microwave penetration depth at the surface. This thawing time at higher power levels is reduced considerably. Temperature increases initially are non-uniform since the surface is heated at afaster rate than the interior. In keeping with the assumption that once the temperature reaches 100°C, all energy absorbed goes into evaporation, and subsequent temperature is maintained at 100°C. Thus, eventually, non-uniformity starts to decrease.

Simultaneous Observations ofCompton Gamma Ray Observatory–BATSE Gamma‐Ray Bursts with theCOBEDMR

Data acquired with the COBE Differential Microwave Radiometer (DMR) provide a unique opportunity to observe simultaneous emission from cosmic gamma-ray bursts in the previously unexplored microwave region of the spectrum. We have searched the COBE DMR time-ordered data sets for instances when one of the DMR horns (FWHM ~ 7°) was pointing in the direction of a gamma-ray burst at the time of burst occurrence. During the overlap period 1991 April-December corresponding to the first public release of COBE data, 210 Compton Gamma Ray Observatory (CGRO)/BATSE gamma-ray bursts listed in the Third BATSE (3B) Catalog were viewable by the COBE DMR. For five of these events the DMR was pointing within 7° of the burst positions at the exact moment of burst occurrence. For another four events the DMR was pointed within 2° of the BATSE positions within 10 s of the burst trigger time. No obvious microwave emission (at 31.5, 53, or 90 GHz), with upper limits in the 10-100 kJy range, can be associated with any of these events. The COBE DMR has a relatively low sensitivity for the detection of point sources within its field of view. A positive detection of a gamma-ray burst by the COBE DMR would imply that the integrated microwave flux must be of the same order as the energy observed in gamma rays. By extending an acceptance window in time of up to 20 minutes before and after a gamma-ray burst another 60 bursts are sampled by the DMR, whose signals are analyzed statistically. We conclude that the average gamma-ray burst produces less than about 7-42 kJy in simultaneous microwave radiation.

Characterization of Spatial Non-Uniformity in Microwave Reheating of High Loss Foods

In order to characterize the non-uniformity of microwave heating when power deposition can be approximated as simple exponential decay, a correlation was developed between temperature non-uniformity and the significant parameters, such as surface microwave flux, penetration depth and the duration of heating. An index of temperature non-uniformity was developed as the difference between the average temperatures of the hottest 5% of the volume and the coldest 5% of the volume. Temperatures calculated using a numerical solution to the energy equation were used to develop the correlation. Experimental verifications were carried out using both a fiberoptic temperature measurement system and an infrared thermal imaging system. Results are provided in terms of a simple expression that can be used to estimate the expected non-uniformity of heating in product and process development, by knowing the dielectric and the thermal properties of a food material.

Beam‐generated Plasma Turbulence during Solar Flares

We develop a simple theoretical model to analyze the harmonic plasma radiation produced by an electron beam injected in a flaring loop and traveling along its magnetic field lines. For a given atmospheric model, we consistently consider collisional effects and the generation of Langmuir waves as a function of the atmospheric depth. Langmuir wave generation is assumed to saturate by quasilinear relaxation, which in turn causes the electron distribution function to develop a plateau at its low-energy end. We formulate an iterative procedure to integrate the coupled equations for the electron distribution function and the wave energy density, taking advantage of the fact that quasilinear relaxation occurs on times much shorter than the collisional timescale. From the wave energy density as a function of depth, we computed the microwave flux generated by second harmonic radiation, taking into account the optical thickness due to the reverse process. Previous studies yield microwave fluxes much larger than those derived from observations. The smaller levels of turbulence obtained from this model, and the lower emissivity due to the relaxation of the head-on approximation, contribute to reduce significantly the predicted microwave emission. We suggest that the simplicity of the present model makes it suitable for the quantitative analysis of spatially resolved radio observations in the GHz range.

X-Band VLA observations of comet Hyakutake (C/1996 B2) and implications for nuclear properties

The results of our observations during the close approach of comet C/1996 B2 (Hyakutake) using Very Large Array (VLA) at a wavelength of 3.55 cm (X-band) are presented. The scientific goals were to observe the thermal microwave continuum from the nucleus and constrain the comet's nuclear properties. The 12 h track allows 3-σ upper limits to be placed on the emission that are about a factor of four lower than any previous VLA observation of a comet's continuum. With the caveat that some assumptions about the microwave emissivity, the temperature, and the thermal conductivity must be made, the effective radius of Hyakutake's nucleus is constrained to be less than 3.0 km, which provides an addition to the sparse body of knowledge on long-period cometary nuclei. Since the observations overlap several of the ROSAT observations of this comet, the strength of the microwave flux is estimated from the X-ray emitting region of the coma to place constraints on possible emission mechanisms.

Nonthermal Radio Emission from Solar Soft X‐Ray Transient Brightenings

We compare microwave total power spectral data from the Owens Valley Radio Observatory Solar Array with soft X-ray transient brightenings observed with the Yohkoh soft X-ray telescope. We find that the transient brightenings are clearly detected in microwaves in 12 of 34 events (35%), possibly detected in another 17 of 34 events (50%), and only five of 34 events (15%) had no apparent microwave counterpart. Comparing the radio and soft X-ray characteristics, we find that (1) the soft X-ray peak is delayed relative to the microwave peak in 16 of 20 events, (2) the microwave flux is correlated with the flux seen in soft X-rays, (3) when radio fluence is used instead of radio flux (24 events) the correlation increases substantially, (4) the microwave spectra in the range 1-18 GHz vary greatly from event to event, (5) the microwave spectra often peak in the range 5-10 GHz (13 of 16 events), and (6) the microwave spectra of some events show narrowband spectra with a steep low-frequency slope.

The microwave enhanced bolometer

The microwave enhanced bolometer is a proposed cryogenic detector with an amplification mechanism. It consists of a crystal covered with a superconducting film exposed to a microwave flux. Incident particles will create nonthermal phonons in the crystal which then produce excess quasiparticles in the film. Those quasiparticles lead to microwave absorption in the film and a subsequent heating of the entire bolometer. In this way the energy deposited by a particle in the crystal could be amplified linearly by gains of up to 10 4 . This would make possible bolometers of several kilograms with an excellent energy resolution which could be used for dark matter search or double beta decay experiments.

Microwave emission from solar flares and comparison with observations

We have studied the evolution of electron energy and angular distributions using Monte Carlo technique for electron beams directed vertically downwards towards chromosphere for incident energies 30 keV, and 300 keV at different incidence angles. Using these distributions we have calculated microwave flux for different frequencies at a fixed column density as well as for a fixed frequency at different column densities. We have also calculated the total microwave flux coming out of solar atmosphere and have compared it with observations. Our results agree well with observational results and produce the observed nature of flux.

Turbulent and Chaotic Dynamics Underlying Solar Magnetic Variability

view Abstract Citations (47) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Turbulent and Chaotic Dynamics Underlying Solar Magnetic Variability Lawrence, J. K. ; Cadavid, A. C. ; Ruzmaikin, A. A. Abstract We examine the temporal scaling properties of solar magnetic activity on timescales from days to decades. Because of more than 63,000 usable data points, we concentrate on the daily International Sunspot Number. Some results have been checked with other data sets, primarily the 10.7 cm microwave flux with about 16,000 data points. Such time series provide a measure whose scaling and intermittency properties are analyzed. By means of correlation analysis and both Fourier and wavelet spectral analysis, we distinguish two regimes of temporal behavior of the magnetic variability. The scaling of the time series is analyzed in terms of multiplicative cascade processes which prove to be invariant over more than two decades of scale from about 2 yr down to about 2 days or less. We interpret this result to indicate generic turbulent structuring of the magnetic fields as they rise through the convection zone. We find that a low-dimensional, chaotic behavior in the sunspot number operates entirely at timescales longer than a transition threshold scale of about 8 yr. Magnetic variability on timescales between 2 yr and 8 yr apparently requires handling by direct simulation. Publication: The Astrophysical Journal Pub Date: December 1995 DOI: 10.1086/176583 Bibcode: 1995ApJ...455..366L Keywords: SUN: ACTIVITY; SUN: MAGNETIC FIELDS; SUN: SUNSPOTS; TURBULENCE full text sources ADS |

Microwave proxies for sunspot blocking and total irradiance

The microwave flux of the Sun is responsive to the same conditions that produce magnetically structured radiation at visible and X ray wavelengths, and so the solar flux at high radio frequencies such as 2800 MHz (10.7 cm) has been used as a proxy for solar optical variations. We have previously found that the the microwave flux time series show spectral variations that provide useful proxy information for total irradiance, and we have extended our analysis of the daily solar fluxes from Toyokawa Observatory at 1000, 2000, 3750, and 9400 MHz, in addition to the Ottawa 2800‐MHz flux, for the years 1980‐1989. An essential ingredient in our analysis is the extraction of the rotationally‐modulated microwave component, which differs from the “S component” as recently defined in the literature. The rotationally‐modulated fraction of the emission contains a significant, often dominant, contribution from gyroresonance emission, whereas the S component, as defined by the excess above the cycle minimum level, usually does not. This allows us to distinguish plage‐associated emission from spot‐associated emission in the time series of microwave flux. We show that in combination, the microwave fluxes for 1000–9400 MHz, which span the spectral peak of spot‐associated emission, provide a very good proxy for both the active cavity radiometer irradiance monitor (ACRIM) total irradiance and the sunspot‐blocked component of the irradiance, even without optical sunspot observations. Over the 1984–1989 period, this proxy has a weighted correlation with ACRIM of 95%, and an RMS deviation from the total irradiance of 0.27 w/m², slightly better than the deviation (0.35 w/m²) found using optical data.

Ignition of MHD shocks associated with solar flares

We have selected single frequency recordings of 28 ‘high-frequency’ type II bursts characterized by a starting frequency greater than 237 MHz to estimate as accurately as possible the ‘launch-time’ of the flare-associated MHD shocks. We established the time associations between metric type II burst onsets and the time characteristics of the microwave and X-ray fluxes of the associated flares. The associated flares were impulsive events with rise times most often about 1 min in the hard X-ray range and 1–2 min in the microwave wavelength range. The majority of the type II bursts from our sample started about 1 min after the maximum of the microwave burst. Launch times of MHD shocks producing type II bursts were obtained using the 10 × Saito coronal model and shock velocities estimated from burst characteristics at different frequencies. Back-extrapolations of type II recordings indicate that MHD shocks are launched in the time interval prior to the maximum of the first peak in the associated microwave burst, most probably at the beginning of the rapid increase of the microwave burst.

Mechanism of modulated microwave absorption and flux distribution in granular YBa2Cu3Ox under high field sweep

The mechanism of modulated microwave absorption (MMA) is suggested from the modulation (H m ) dependence in granular YBa2Cu3Ox superconductor under high-field sweep. It is proposed that the MMA signalS be decomposed into three shape factors to interpret the experimental result. The VN factor arises from the difference in the number of vortices between the add (A) mode and the subtract (S) mode corresponding to the alternating field. The CT factor arises from the appearance of the transition region of the shielding current in the Smode where the absorption is less. The SS factor arises from the difference in the surface slope of the flux distribution between the two modes. With increasingH m , the positive VN increases linearly. The positive CT increases initially and then decreases. The slope for the Amode is very gentle, so that SS is negative, and it initially increases rapidly and then saturates. These factors can reproduce the complex behavior of theH m dependence ofS fairly well.