Longitudinal Magnetic Field Strength Research Articles

Simultaneous Visible and Infrared Spectropolarimetry of a Solar Internetwork Region

We present the first simultaneous infrared (IR) and visible spectropolarimetric observations of a solar internetwork region. The Fe I lines at 6301.5, 6302.5, 15648, and 15652 A were observed, with a lag of only 1 minute, using highly sensitive spectropolarimeters operated in two different telescopes (Vacuum Tower Telescope and THEMIS at the Observatorio del Teide). Some 30% of the observed region shows IR and visible Stokes V signals above noise. These polarization signals indicate the presence of kilogauss (kG) magnetic field strengths (traced by the visible lines) coexisting with sub-kG fields (traced by the infrared lines). In addition, one-quarter of the pixels with signal have visible and IR Stokes V profiles with opposite polarity. We estimate the probability density function of finding each longitudinal magnetic field strength in the region. It has a tail of kG field strengths that accounts for most of the (unsigned) magnetic flux of the region.

Analysis of FORTE data to extract ionospheric parameters

The ionospheric transfer function is derived for a spherically symmetric ionosphere with an arbitrary radial electron density profile in the limit where the radio frequencies of interest ω are much larger than the plasma frequency ωpe. An expansion of the transfer function to second order in the parameter X (= ω2pe/ω2) is carried out. In this limit the dispersive properties of the ionosphere are manifested as a frequency‐dependent time of arrival that includes quadratic, cubic, and quartic terms in 1/ω. The coefficients of these terms are related to the total electron content (TEC) along the slant path from transmitter to receiver, the product of TEC and the longitudinal magnetic field strength along the slant path, and refractive bending and higher‐order electron density profile effects, respectively. By fitting the time of arrival versus frequency of a transionospheric signal to a polynomial in 1/ω it is possible to extract the TEC, the longitudinal magnetic field strength, the peak electron density, and an effective thickness for the ionosphere. This exercise was carried out for a number of transionospheric pulses measured in the VHF by the FORTE satellite receiver and generated by the Los Alamos Portable Pulser. The results are compared with predictions derived from the International Reference Ionosphere and the United States Geological Survey geomagnetic field model.

Does the solar magnetic field increase?

We consider measurements of the general magnetic field (GMF) of the Sun as a star at four world observatories from 1968 until 1999. We show that, within the error limits, the mean strength of the photospheric magnetic field H (of its longitudinal component, in magnitude) has not changed over the last 32 years. This is in conflict with the recent conclusion by Lockwood et al. (1999) that the solar coronal magnetic field increased by 40% from 1964 until 1996 and has almost doubled in the last 100 years. The causes of discrepancies in the results are discussed. At the same time, the GMF exhibits a natural 11-year variation associated with the solar cycle. The strength of the photospheric longitudinal magnetic field (in absolute value) averaged over 32 years is 0.46 G (at an rms GMF strength of 0.57 G). The mean GMF for all years of measurements had a south polarity: \(\bar H = - 0.030 \pm 0.018 G\). The difference from zero is statistically significant at 1.7σ (90%) and may be directly related to the outstanding problem of the solar magnetic “monopole.”

Muon spin depolarization in noble gases during slowing down in a longitudinal magnetic field

The spin depolarization of the positive muon during slowing down in He, Ne and Ar has been calculated as a function of gas pressure and longitudinal magnetic field strength. The calculation is based on the cross sections for the atomic processes contributing to the slowing down of the positive muon and muonium , obtained from the corresponding cross sections for the proton and hydrogen by simple scaling schemes. The depolarization mechanism taking place in the subpicosecond time regime is discussed in the context of recent experiments to determine the muon decay parameters precisely and accurately in the search for right-handed neutrinos. It has been shown that the polarization loss in He is 1% at and 1 atm pressure, an amount which is unacceptably large for accurate (100 ppm or better) measurements of the muon decay parameters. It is also pointed out that at this level of accuracy a strong longitudinal magnetic field is a much less effective means of quenching the muon spin depolarization mechanism than previously thought. Effects of the electron spin polarization induced by a strong longitudinal magnetic field are also discussed.

The numerical calculation of plasma beam propagation in a toroidal duct with magnetized electrons and unmagnetized ions

Electron-magnetized vacuum arc plasma transport in a magnetic toroidal duct is calculated numerically taking in account electron - ion collisions, electron and ion temperatures, and the high conductivity of the duct wall. The longitudinal magnetic field in the duct, the fully ionized plasma density and the electric potential distribution at the torus entrance are given, while the plasma density, electrical field and current, and macroscopic plasma velocity across the magnetic field inside the duct are calculated. Toroidal coordinates are used to describe plasma beam propagation. A Runge - Kutta routine is used for the calculations along the torus while a finite difference method is used across the torus cross section. It is found that plasma loss due to particle flux to the duct wall depends on the electron and ion temperatures and the plasma density distribution at the torus entrance cross section. With an electron temperature of , 30 000 K and 50 000 K, an ion temperature and a Gaussian distribution of plasma density at the torus entrance with a maximum value , we found that the duct efficiency was less than 10% for longitudinal magnetic field strengths of 10 mT and 20 mT. In the case where only the electrons are magnetized, filter efficiency depends only weakly on the magnetic field strength, on , and on .

A bichromatic imaging method in solar studies

The basic concept of a bichromatic imaging technique has been formulated. Specific applications of this method are analyzed in the context of the examination of solar plasma characteristics by pinpointing its merits and demerits. Specialized requirements for the spectral devices are set forth, which can be summarized as: a) the presence of two nearby identical spectral passbands, b) the mutual orthogonality for the polarization of the light transmitted through the neighboring bands, and c) the possibility of controlling the relative position of the bands. Several alternative plausible implementations of the method are considered, using Fabry-Perot interferometers (FPI) and magnetooptical filters (MOF) to measure the longitudinal magnetic field strength and the intensity field. A new MOF-based design of a filter magnetograph is proposed, in which two optical resonance cells are combined into one.

Study of magnetized plasma-filled dielectric cherenkov maser

Injection of a background plasma into a dielectric Cherenkov maser can effectively increase the beam-wave interaction efficiency and the microwave power output of the device. Here, the effect of the magnetized plasma on the interaction of the dielectric Cherenkov maser is examined by use of the linear theory. The coupled equation of E-mode with H-mode in the magnetized plasma is derived. It's shown that the E-mode and H-mode are undistinguished in the magnetized plasma and the coupling coefficient is wholly determined by the longitudinal magnetic field strength. The uncoupled condition of the E-mode with H-mode is obtained. The dispersion equation of the beam-plasma, dielectric lined slow-wave waveguide with different background plasma densities inside and outside the electron beam is derived. And by solving the dispersion equation, the effect of the magnetized plasma on the linear spatial growth rate is presented.

The effect of strong longitudinal magnetic fields on dose deposition from electron and photon beams.

The use of strong, uniform, longitudinal magnetic fields for external electron and photon beam irradiation is considered. Using the EGS4 Monte Carlo code modified to account for the presence of magnetic fields, dramatic narrowing of penumbra for photon and electron irradiations is demonstrated. In the vicinity of heterogeneities, "hot" and "cold" spots due to multiple scattering in electron beams are reduced substantially. However, in the presence of strong magnetic fields, the effect of inhomogeneities can be observed far from the location of the inhomogeneity due to reduced "washout" caused by lateral multiple scattering. The enhanced "Bragg peak," proposed or calculated by other authors, is not observed on the central axis of broad beams, owing to lateral equilibrium. It is proven that for broad parallel beams, the central axis depth-dose curve is independent of the strength of the external longitudinal magnetic field, as long as it is uniform. However, strong longitudinal magnetic fields can induce enhancements by redirection of the electron fields coming from point sources. Strong uniform longitudinal magnetic fields provide a way of controlling the spreading of electron beams due to multiple scattering, making the electron beams more "geometrical" in character, simplifying dose-deposition patterns, possibly allowing electron beams to be used in new ways for radiotherapy. Photon therapy also benefits from strong uniform longitudinal magnetic fields since the penumbra or other lateral disequilibrium effects associated with lateral electron transport can be eliminated.

Positron spin relaxation in naphthalene

The remaining spin polarizationP of positrons was measured in naphthalene against longitudinal magnetic-field strength. The measuring method was based on the relative yield of Ps in triplet and singlet state. In a zero fieldP is reduced to 50% of the initial polarization and approaches 100% upon increasing the applied field to larger values. This effect, which in teflon is quite absent, appears to be determined by the specific behaviour of positrons in naphthalene.

Formation of local space-charge densities in gas breakdown in magnetic fields

Experimental and theoretical data on the dependence of the delay time of breakdown on the longitudinal and transverse magnetic field strength are given. The theoretical data correlate with the analogous experimental dependence with weak irradiation of the discharge interval. Equivalent-pressure theory gives satisfactory agreement with experiment in the breakdown of atmospheric-pressure gases in a transverse magnetic field.

Millisecond radio spikes from the dwarf M flare star AD Leonis

Arecibo radio observations of millisec bursts of radio signals at 1415 MHz from AD Leonis are reported. The observed burst had an ellipticity of 0.95, 50-100 percent circular polarization, and a flux density maximum of 30 mJy. The 50 sec burst featured five quasi-periodic oscillations with a mean periodicity of about 3.2 sec. A second, less intense burst that occurred 20 sec later was 100 percent circularly polarized. The area emitting the bursts covered an estimated 0.005 of the radius of AD Leonis and had an electron density of 6 billion/cu cm and a longitudinal magnetic field strength of 250 gauss, if the source was an electron-cyclotron maser. A coherent plasma source would require, for the first harmonic, an electron density of 20 billion/cu cm and a magnetic field much less than 500 gauss. A second harmonic of the plasma frequency would require an electron density of 6 billion/cu cm and a field strength much less than 250 gauss. The possibility that the source was periodic oscillations in coronal loops is discussed.

Radio Wavelength Observations: Structure, Strength and Variability of Magnetic Fields in the Transition Region and the Solar Corona

The 6 cm radiation of solar active regions marks the legs of dipolar loops which have their footpoints in lower-lying sunspots. The temperatures T ≈ 106K and longitudinal magnetic field strengths Hℓ ≈ 600 Gauss at heights h ≈ 4 × 109 cm above sunspot umbrae. The circularly polarized emission at 6 cm delineates the magnetic structure above sunspot penumbrae. The 20 cm radiation of solar active regions delineates the ubiquitous coronal loops previously detected at X-ray wavelengths. We infer semilengths L ≈ 5 × 109cm, maximum electron temperatures Te(max) ≈ 3 × 106K, emission measures and electron densities Ne ≈ 109cm−3 for the 20 cm bremsstrahlung. Future V.L.A. observations at 20 cm may be used to determine the magnetic field strength of coronal loops. Changes in temperature and magnetic structure before and during solar bursts are briefly discussed.

Active region coronal evolution

Scaling relations between coronal base pressure and longitudinal photospheric magnetic field strength are tested for the case of a single active region observed for five solar rotations from Skylab. The evolution of measureable quantities, such as coronal thermal energy content, total longitudinal photospheric magnetic flux, region scale size, and peak energy density, is traced throughout the five rotations observed. The theoretically derived scaling law of Golub et al. (1980) is found to provide an acceptable fit to the data throughout the entire evolutionary history of the region from an age of about 3 days to the fully evolved state in which the mature active region merges into the general large-scale structure of the quiet corona. An alternative scaling law obtained by including the results of Galeev et al. (1981), however, is found to provide a somewhat better fit to the data. The study is seen as providing additional justification for the belief that magnetic field-related heating is the operative mechanism in the solar corona.

Polarized horseshoes around sunspots at 6 centimeter wavelength

Horseshoe-shaped structures associated with sunspot penumbrae have been detected in 6 cm synthesis maps of circular polarization taken with the Westerbork synthesis radio telescope. The high degree of circular polarization (rho/sub c/ = 95%) of the horeshoes requires gyroresonant emission. In fact, the horeshoe structures were predicted from the theory of the gyroresonance emission of individual sunspots. The absence of polarized emission above the sunspot umbrae is also explained by gyroresonance theory. In sharp contrast to the polarized emission, the total intensity of the 6 cm radiation is enhanced above the sunspot umbrae and exhibits a remarkable correlation with the longitudinal magnetic field of the underlying photosphere. Brightness temperatures of T/sub B/roughly-equal10/sup 6/ K are found above sunspot umbrae in coronal regions where the longitudinal magnetic field strength Hiota = 600--900 gauss. Temporal instability precludes the detection of cool material above sunspot umbrae in synthesis maps which are only sensitive to the much more stable and long-lived hot material.

Multiple wavelength observations of flaring active regions

We present observations of flaring active regions with the Very Large Array (V.L.A. at 6 cm and 20 cm wavelengths) and the Westerbork Synthesis Radio Telescope (W.S.R.T. at 6 cm wavelength). These are compared with photospheric magnetograms (Meudon) and with Hα and offband Hα photographs (Big Bear and Ottawa River Solar Observatories). The 6 cm radiation of these active regions marks the legs of dipolar loops which have their footpoints in lower-lying sunspots. The intense, million degree radiation at 6 cm lies above sunspot umbrae in coronal regions where the longitudinal magnetic field strength H ℓ = 600 Gauss and the height above the sunspot umbrae h = 3.5±0.5 × 10 9 cm. Circularly polarized horseshoe structures at 6 cm ring the sunspot umbrae. The high degree of circular polarization ( ϱ c = 95%) of the horseshoes is attributed to gyroresonant emission above sunspot − penumbrae. The 20 cm radiation of these active regions exhibits looplike coronal structures which extend across regions of opposite magnetic polarity in the underlying photosphere. The 20 cm loops are the radio wavelength counterparts of the X-ray coronal loops. We infer semilengths L = 5 × 10 9 cm, maximum electron temperatures T e(max) = 3 × 10 6 K, emission measures ∫N e 2d l = 10 28 cm −5, and electron densities N e = 10 9 cm −3 (or pressures p = 1 dyn cm −2) for the 20 cm bremsstrahlung. A total of eight solar bursts were observed at 6 cm or 20 cm wavelength with second-of-arc angular resolution. The regions of burst energy were all resolved with angular sizes between 5″ and 30″, brightness temperatures between 2 × 10 7 K and 2 × 10 8 K, and degrees of circular polarization between 10% and 90%. The impulsive phase of the radio bursts are located near the magnetic neutral lines of the active regions, and between the flaring Hα kernels which mark the footpoints of magnetic loops. In one case there was preburst heating in the coronal loop in which a burst occurred. Snapshot maps at 10 s intervals reveal interesting burst evolution including rapid changes of circular polarization and an impulsive burst which was physically separated from both the preburst radio emission and the gradual decay phase of the burst.

Observations of OH and ammonia in the direction of T Tauri stars

Line emission from OH and NH/sub 3/ has been detected in the direction of small groups of T Tauri stars and the space between them. The ratios of the main and satellite line intensities are consistent, within the uncertainties of measurement, with those expected under conditions of local thermodynamic equilibrium. The molecular emission is confined to dark dust clouds in which the T Tauri stars appear to be immersed, and the linear extents of the molecular clouds are on the order of 1 pc. Excitation temperatures approx.4 K and optical depths approx.1 are inferred for both the OH and NH/sub 3/ transitions. Kinetic temperatures of approx.20 K are inferred from the NH/sub 3/ emission lines, and similar kinetic temperatures are inferred from observations of carbon monoxide emission lines in the same directions. Column densities of approx.10/sup 14/--10/sup 15/ cm/sup -2/ are inferred for OH and NH/sub 3/. Considerations of the excitation of the NH/sub 3/ transitions provide a lower limit to the molecular hydrogen density of n/sub H/2> or approx. =4 x 10/sup 3/ cm/sup -3/ and a lower limit to the total cloud mass of M> or approx. =100M/sub sun/. Most of the mass of the molecular clouds hasmore » been left behind after the fragmentation and gravitational collapse which led to the formation of the T Tauri stars. The available data are not inconsistent with the hypothesis that the molecular clouds are now in a state of hydrostatic equilibrium. Attempts to detect the Zeeman splitting of the OH emission lines led to upper limits of approx.0.1 milligauss for the longitudinal magnetic field strength over regions of approx.0.1 pc in size in three clouds. The color excesses of the T Tauri stars, and the similar angular distribution and radial velocities for the molecular clouds and the T Tauri stars, suggest that small groups of T Tauri stars are embedded within and kinematically associated with the molecular clouds.« less

The Occurrence of High-Energy Releases during Flares in Localized Areas of an Active Center and Time-Associated Changes in SPOT Umbrae

view Abstract Citations (2) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Occurrence of High-Energy Releases during Flares in Localized Areas of an Active Center and Time-Associated Changes in SPOT Umbrae McKenna-Lawlor, Susan M. P. Abstract Although many flares occurred in active center HAO-59Q during its fifth passage across the solar disk, 1959 July 7-21, it is shown that only two localized areas within this region were associated with the commencement of significant energy releases. The energy importance of individual optical brightenings has been gauged by the magnitude of time-associated single-frequency radio bursts. In certain cases, localized high-energy flaring appears to be followed by extensive low-energy flaring, and such events are here called "primary flares" and "secondary flares," respectively. The positions of persistent flare-sensitive regions, calcium plages, filaments, and spot umbrae in HAO-59Q have been compared with the daily magnetograms of this center of activity published by the Crimean Astrophysical Observatory for 1959 July 14-17 The flare-sensitive regions were located near filaments and adjacent to the two sides of the loci of zero longitudinal magnetic field strengths. The magnetograms closely reflect local daily changes in the sunspots; hence, any role played by flares of high energy in producing changes in the magnetic field cannot be deduced unambiguously from these records. It is shown that the parts of the chromosphere associated with high-energy flares were directly over areas of the photosphere which contained transient umbrae. Flares with high energy occurred in intervals within which such umbrae disappeared or diminished in area. A close association between localized, highenergy flaring and an underlying changing spot umbra may be a necessary condition for the generation of high-energy particles. Publication: The Astrophysical Journal Pub Date: January 1970 DOI: 10.1086/150289 Bibcode: 1970ApJ...159...51M full text sources ADS |

Heat transfer and resistance to movement in a rotationally advancing flow of conducting liquid in the presence of a magnetic field

The functional relationships thus obtained for the resistance and heat-transfer coefficients in a rotationally-advancing flow of a conducting liquid in a pipe with a constant internal axial magnetic field and small Reynolds magnetic numbers (Rem≪1) contains only a single longitudinal magnetic field strength component equal in the above case to the external magnetic field strength. It follows from the above that the formulas thus obtained hold both in the case when the flowing liquid does not perform any useful work, i.e., when electrical energy is not tapped off (or injected) in the form of a current, and for the case when it is tapped off (or injected). The difference consists only in the fact that in the second instance the velocity of the liquid drops along the pipe according to the amount of the work thus obtained (and, obviously, to the work in overcoming the resistance forces). In the first instance the reduction in the velocity is due only to the overcoming of the resistance forces. It is appropriate to note that formulas (17), (18), (20), and (21) for Ho=0 are converted into corresponding formulas for a rotationally-advancing liquid flow in a pipe without a magnetic field. The latter formulas are adequately confirmed by practical experience (see [2]). This provides grounds for assuming that the formulas will also be fully confirmed in practice.