A theoretical study of the direction and polarization of solar radio millisecond spike radiation

We investigated the angular direction and polarization of the solar radio millisecond spike emission in the model in which the spike emission is due to the second harmonic instability modes driven by electron cyclotron maser of loss cone distributed electrons during the propagation of a nonlinear plasma density wave near the magnetic mirror. We found that, when the angle θ between the wave vector and the magnetic field is > 60 °, the emission is in 100% X-mode polarization; when 40 ° < θ<60 °, the emission is in 100% O-mode polarization provided the amplitude of the density wave is below a certain limit; above that limit, the polarization will fall from 100% O-mode to even the X-mode. We also found that only 0.1% of the free energy of energy carrying electrons in the source region is converted into radiation wave energy.     

Radio emission observed by Galileo in the inner Jovian magnetosphere during orbit A-34

The Galileo spacecraft encountered the inner magnetosphere of Jupiter on its way to a flyby of Amalthea on November 5, 2002. During this encounter, the spacecraft observed distinct spin modulation of plasma wave emissions. The modulations occurred in the frequency range from a few hundred hertz to a few hundred kilohertz and probably include at least two distinct wave modes. Assuming transverse EM radiation, we have used the swept-frequency receivers of the electric dipole antenna to determine the direction to the source of these emissions. Additionally, with knowledge of the magnetic field some constraints are placed on the wave mode of the emission based on a comparative analysis of the wave power versus spin phase of the different emissions. The emission appears in several bands separated by attenuation lanes. The analysis indicates that the lanes are probably due to blockage of the freely propagating emission by high density regions of the Io torus near the magnetic equator. Radio emission at lower frequencies (<40 kHz) appears to emanate from sources at high latitude and is not attenuated. Emission at is consistent with O-mode and Z-mode. Lower frequency emissions could be a mixture of O-mode, Z-mode and whistler mode. Emission for shows bands that are similar to upper hybrid resonance bands observed near the terrestrial plasmapause, and also elsewhere in Jovian magnetosphere. Based on the observations and knowledge of similar terrestrial emissions, we hypothesize that radio emission results from mode conversion near the strong density gradient of the inner radius of the cold plasma torus, similar to the generation of nKOM and continuum emission observed in the outer Jovian magnetosphere and in the terrestrial magnetosphere from source regions near the plasmapause.     

Distinguishing RBL-like Objects and XBL-Like Objects With thePeak Emission Frequency of the Overall Energy Spectrum

We investigate quantitatively how the peak emission frequency of the overall energy spectrum is at work in distinguishing RBL-like and XBL-like objects. We employ the sample of Giommi et al. (1995) to study the distribution of BL Lacertae objects with various locations of the cutoff of the overall energy spectrum. We find that the sources with the cutoff located at lower frequency are indeed sited in the RBL region of the α_ro-α_oxplane, while those with the cutoff located at higher frequency are distributed in the XBL region. For a more quantitative study, we employ the BL Lacerta esamples presented by Sambruna et al. (1996), where the peak emission frequency, ν_p, of each source is estimated by fitting the data with a parabolic function. In the plot of α_rx -log ν_p we find that, in the four different regions divided by the α_rx = 0.75 line and thelogν_p = 14.7 line, all the RBL-like objects are inside the upper left region, while most XBL-like objects are within the lower right region. A few sources are located in the lower left region. No sources are in the upper right region. This result is rather quantitative. It provides an evidence supporting what Giommi et al. (1995) suggested: RBL-like and XBL-like objects can be distinguished by the difference of the peak emission frequency of the overall energy spectrum. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Spatial Distribution of Gaseous Atomic Sodium in the Comae of Comets: Evidence for Direct Nucleus and Extended Plasma Sources

Sodium D-line emission (5890 and 5896 Å) has been observed in bright comets at small to moderate heliocentric distances for many years. We present here the first in depth study of a set of spatial profiles of the sodium D-line emission constructed from long-slit spectroscopic observations of Comets Bennett C/1969 Y1, Kohoutek C/1973 D1, and 1P/Halley. Preliminary analysis of these data lead to the suggestion by Combiet al.(1996,A Plasmagenic Source for Gaseous Sodium in Comets.Presented at Asteroids, Comets, Meteors) that a major fraction of the gaseous sodium was produced by an extended source in the tail and that the source was likely to be some charged species. Dissociative recombination of a molecular ion was suggested. The spatial profiles of sodium are not like typical neutral species. The inner region from the nucleus (<2 × 10⁴km) can be explained in terms of a model that accounts for collisional entrainment in the expanding coma and the heliocentric velocity dependent fluorescence rate and radiation pressure acceleration. This source comes either directly from the nucleus or has a very short-lived parent (?10³s). Away from the nucleus, down the tail and to the sides, the spatial profile slope flattens, indicating a second extended source. The striking similarity of the extended region of sodium spatial profiles with those of ions (H₂O⁺), both along and perpendicular to the tail, is highly suggestive that an ion source is responsible for the production of the extended component of gaseous sodium in the coma. The production rate of the highly variable extended source when present is four to five times that of the direct nucleus source. Observations (Schneideret al., 1991,Science253,1394–1397) and quantitative model analyses (Wilson and Schneider, 1994,Icarus111,31–34) have shown that a dissociative recombination of a sodium bearing molecular ion (NaX⁺) produces a peculiar component of the neutral sodium near Io. It displays a variable spatial morphology consistent with that of a molecular ion source “picked-up” in the plasma torus corotating with Jupiter's magnetic field. The rapid onset of the appearance of gaseous sodium in bright comets, its spatial distribution in the extended coma and near tail, and recent observations of sodium tails are all consistent with our original suggestion of this plasma source for sodium in comets.     

High energy phenomena in eta carinae

We have investigated with BeppoSAX the long term behaviour of the harder X-ray component of the supposed supermassive binary system η Car along its 5.52 year cycle. We have found that in March 1998 during egress from the last December 1997 eclipse, this component was the same as outside eclipse, but for a large (×3.5) increase of NH _h , that can be attributed to the presence or formation of opaque matter in front of the source near periastron. Unexpectedly, at that time the iron 6.7 keV emission line was 40% stronger. BeppoSAX has for the first time found ahard X-ray tail extending to at least 50 keV, that cannot be adequately fitted with an additional hotter thermal component. The 2–100 keV spectrum of η Car is instead well fitted with an absorbed powerlaw spectrum with photon index 2.53, suggesting non-thermal emission as an alternative model for the core source.     

On the variability timescales and magnetic field in OJ 287

The continuum spectrum of OJ 287, like most other BL Lac objects, is featureless- no emission or absorption lines are observed. However, OJ 287 shows variations at different timescales in flux and polarization at various wavelength bands. Using the available variability data one can estimate the sizes of the emission regions in the source from light travel time arguments. We assume the emission mechanism to be synchrotron radiation by high energy electrons with single power law energy distribution. Theoretical synchrotron spectrum in the frequency range 10¹¹–10¹⁷ H _z is compared with the observed spectral shape, obtained from new multifrequency quasi-simultaneous observations, to estimate the lower and upper cut off frequencies. These frequencies are used to obtain theoretical values of the variability timescales and magnetic field in the emission region. We obtain a value of 0.93 G for the magnetic field and 5.184×10⁴ sec for the cooling time from the quiescent continuum spectrum. The shock-in-jet model explains the spectrum where shocks accelerate the particles and amplify the magnetic field in the jet. This timescale is compared with the one obtained from observed short timescale variability (20 minutes) with proper beaming correction. The short timescale variations (200 minutes in the source frame), possibly caused by an additional, flaring, component of the source, are also used to calculate compressed magnetic field. The observed and theoretically estimated variability timescales and the shape of the spectrum suggest that there are more than one emission components in OJ 287.     

A study of non-thermal radio emission features using fine spectral BAO and high-sensitivity RATAN observations of a solar active region

Based on spectral observations of active region NOAA 8545 on 19 May 1999, we describe the processes responsible for non-thermal long-lasting radio emission and for narrow-band non-drifting bursts observed at the same time. Non-thermal long-lasting radio emission consisted of two components: short-duration (1–2 s) microbursts with fluxes about 0.001 s.f.u. and continuum emission with growing spectrum in the range of 1000–2000 MHz. Energetic electrons continuously existed in the active region for more than 2.2 hours. The nature and parameters of microbursts were discussed by Bogod, Mercier, and Yasnov (2001). Here we consider the continuum source nature. It is shown that the model, taking into account the cyclotron loss-cone instability of hot electrons and the generation of plasma waves at the upper hybrid frequency, may explain the observed continuum source parameters. For the narrow-band non-drifting bursts we consider two models: the first taking into account an excitation of weak shock waves across the magnetic field and the second with an excitation of the upper hybrid waves under the double plasma resonance. Continuum source parameters are close to the last model. Our estimations for the magnetic field strength are as follows: H=120–126 G, which is valid for the region where the electron density of background plasmas n=(1.4–1.9) ×10⁹ cm^–3; H=180–190 G for the region where n=(3.0–4.3) ×10⁹ cm^–3; H=290 G for the region where n _e=2.5×10¹⁰ cm^–3; and H=350 G for the region where n _e=3.5×10¹⁰ cm^–3. The speed of the fast electrons is about 0.10–0.14 c.     

Brightness distribution at λ = 3 and 21 cm near the solar north pole

The brightness distribution of the Sun near the north pole at = 3 and 21 cm is obtained using the results of the partial eclipse of February 25, 1971. The solution obtained by expanding B (r) in orthogonal polynomials does not show any brightening at either frequency, but a sensible darkening. If this result is explained by the presence of spicules, the coronal emission must be negligible with respect to the emission coming from the transition region. This is possible, at = 21 cm, only if we assume the presence of a coronal hole near the pole.     

High excitation emission lines in binary systems with roundchroms

An unexpected empirical fact, a dependence of the observed luminosities inhigh excitation emission lines – 1240 NV, 1400 SiIV, 1550 CIV, 1640 HeII– on the intercomponent distance a of RS CVn type close binary systems,is revealed. It is assumed that those high excitation emission lines aregenerated most probably in a cone-like region between the Lagrangian pointL ₁ and the surface of the primary component of the system. Thebehavior of high excitation emission lines at various phases of theeclipse in the case of two binary systems, SX Cas and 22 Vul, indicatesthe possibility of existence of such a `Lagrangian cone' in the structureof common chromospheres – roundchroms – of close binary systems as amain source of generation of high excitation emission lines.     

The saturation of electron-cyclotron maser and the time profile of emitted spikes

In this paper the evolution of the hollow beam distribution of energetic electrons giving rise to ECM instability is investigated and the spatial dispersion term is included in the equation of wave energy. The instability causes the growth of wave energy, while the propagation of waves evacuates the electromagnetic energy from the source region. By analysing these two effects spike-like time profiles of waves are obtained. It is found that the saturation time t _s of ECM emission and the duration of spikes increase with the decrease of the frequency of solar radio spike emission. The approximate expressions of t _sand of the peak wave energy density are derived.Proceedings of the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986.On leave from the Department of Astronomy, Nanjing University, Nanjing, People's Republic of China.     

Meter-wavelength observations of the solar radio burst storm of August 17–22, 1968

Meter-wavelength observations are presented for the solar radio storm of August 17–22, 1968. The data comprise dynamic spectra and high-resolution brightness distributions from the 80 MHz radioheliograph.It is found that the storm consisted essentially of type III bursts at the lower frequencies and type I at the higher frequencies; the transition, usually near 60 MHz, was fairly sharp. The type I source was located over an active region associated with a large sunspot group. The type III position was displaced about 0.5 R transversely from the type I, in a region of low magnetic field.The evident close association between the two types of emission can best be explained by disturbances originating in the type I region, propagating outwards through a region of weak magnetic field, and triggering an electron acceleration process, probably at the cusp of a helmet structure. The observed frequency and spatial relationship between the type I and type III components in events of this kind follow as a natural consequence of this model.A comparison of these results with the hectometer-wavelength satellite observations of the 1968 August event makes possible a qualitative estimate of the outward path of the type III exciters through the corona, and it is apparent that below the solar wind region of the corona this path departs considerably from the radial direction.     

The necessity of fundamental emission in type III bursts

Observations of some type III radio bursts in the hectometer and kilometer wave range are compared with theoretical predictions. It is shown that the burst emission must be near the plasma frequency in the region between 10 R _⊙ and 50 R _⊙ in order to be consistent with the observed steep rise in brightness temperature for these bursts. The results of Fainberg, Malitson et al., and Haddock and Alvarez are discussed and compared with the interpretation of emission near the plasma frequency.     

Sub-terahertz emission from solar flares: The plasma mechanism of chromospheric emission

We consider the plasma mechanism of sub-terahertz emission from solar flares and determine the conditions for its realization in the solar atmosphere. The source is assumed to be localized at the chromospheric footpoints of coronal magnetic loops, where the electron density should reach n ≈ 10¹⁵ cm^?3. This requires chromospheric heating at heights h ? 500 km to coronal temperatures, which provides a high degree of ionization needed for Langmuir frequencies ν _p ≈ 200–400 GHz and reduces the bremsstrahlung absorption of the sub-THz emission as it escapes from the source. The plasma wave excitation threshold for electron-ion collisions imposes a constraint on the lower density limit for energetic electrons in the source, n ₁ > 4 × 10⁹ cm^?3. The generation of emission at the plasma frequency harmonic ν ≈ 2ν _p rather than the fundamental tone turns out to be preferred. We show that the electron acceleration and plasma heating in the sub-THz emission source can be realized when the ballooning mode of the flute instability develops at the chromospheric footpoints of a flare loop. The flute instability leads to the penetration of external chromospheric plasma into the loop and causes the generation of an inductive electric field that efficiently accelerates the electrons and heats the chromosphere in situ. We show that the ultraviolet radiation from the heated chromosphere emerging in this case does not exceed the level observed during flares.     

On the complex spatial structure of a gradual microwave burst

We present a discussion of the gradual burst event on May 13,1985 which is based on observations of the RATAN-600 telescope at ten fixed frequencies in the range between 37.5 and 0.95 GHz (0.8 and 31.6 cm wavelength) and on time profiles of patrol observations of the Observatory for Solar Radio Astronomy at Tremsdorf near Potsdam. This up to now most complete data set allows new conclusions on the extended spectral/spatial structure of the source region.There is strong evidence that only less than 24% of the microwave flux from this event is emitted by an excessive small-scale burst source while the bulk of the burst emission comes from a larger region consisting of two major components covered by the source area of the S-component radiation. The different components of the burst and S-component radiation are analyzed.     

GMRT and VLA observations at 49 cm and 20 cm of the HII region near <Emphasis Type="Italic">l</Emphasis> = 24.8°, <Emphasis Type="Italic">b</Emphasis> = 0.1°

We report multi-frequency radio continuum and hydrogen radio recombination line observations of HII regions near l = 24.8°, b = 0.1° using the Giant Metrewave Radio Telescope (GMRT) at 1.28 GHz (n = 172), 0.61 GHz (n = 220) and the Very Large Array (VLA) at 1.42 GHz (n = 166). The region consists of a large number of resolved HII regions and a few compact HII regions as seen in our continuum maps, many of which have associated infrared (IR) point sources. The largest HII region at l = 24.83° and b = 0.1° is a few arcmins in size and has a shell-type morphology. It is a massive HII region enclosing ∼550 M_⊙ with a linear size of 7 pc and an rms electron density of ∼110 cm⁻³ at a kinematic distance of 6 kpc. The required ionization can be provided by a single star of spectral type O5.5. We also report detection of hydrogen recombination lines from the HII region at l = 24.83° and b = 0.1° at all observed frequencies near V _lsr = 100 km s⁻¹. We model the observed integrated line flux density as arising in the diffuse HII region and find that the best fitting model has an electron density comparable to that derived from the continuum. We also report detection of hydrogen recombination lines from two other HII regions in the field.     

On the development of magnetic fields in active regions

Transverse and longitudinal magnetic field scans together with K₂₃₂ spectroheliograms that cover the early phases of active region formation reveal the following:

1. The new active region forms near the periphery of an old magnetic region. There is evidence that the new region forms an interrelated system with the old magnetic structures on the sun.
2. Noticeable changes in the background magnetic field are seen nearly 3 days prior to the appearance of the sunspot. Magnetic hills of the longitudinal component appear along with bright localized K₂₃₂ emission. Subsequently the K₂₃₂ emission spreads along the boundary of one or two adjacent supergranules and at the time of sunspot formation occupies the whole supergranular cell.
3. Transverse fields with strengths of 100–150 gauss form closed regions in the area of the longitudinal component hills, in the very early phases of the region. These fields stretch and link up the two areas later, at which time the peak transverse fields with values near 250 gauss coincide with the zero line of the longitudinal field. When subsequently the spots appear in the new region, the transverse fields are located about the hills of the longitudinal field. The total field vectors just prior to sunspot formation are pressed to the surface. These are inclined about 45° to the surface after the spot appears. The findings indicate that the magnetic field of a new region emerges from the sub-photospheric layers. It is highly likely that the dynamics of a supergranule influences only the emergence of the magnetic field into the upper layers of the solar atmosphere.

     

Type III radio bursts in the outer corona

Type III solar radio bursts observed from 3.0 to 0.45 MHz with the ATS-II satellite over the period April–October 1967 have been analyzed to derive two alternative models of active region streamers in the outer solar corona. Assuming that the bursts correspond to radiation near the electron plasma frequency, pressure equilibrium arguments lead to streamer Model I in which the streamer electron temperature derived from collision damping time falls off much more rapidly than in the average corona and the electron density is as much as 25 times the average coronal density at heights of 10 to 50 solar radii (R ). In Model II the streamer electron temperature is assumed to equal the average coronal temperature, giving a density enhancement which decreases from a factor of 10 close to the Sun to less than a factor of two at large distances (> 1/4 AU). When the burst frequency drift is interpreted as resulting from the outward motion of a disturbance that stimulates the radio emission, Model I gives a constant velocity of about 0.35c for the exciting disturbance as it moves to large distances, while with Model II, there is a decrease in the velocity to less than 0.2c beyond 10 R .     

Observations of horizontal transport effects on high latitude metastable O(1D), N(2D) auroral emissions

An analysis is presented of photometric measurements of the NI (λ = 520nm),OI(λ = 630nm)and other emissions made at Nord, where the invariant latitude is Λ = 80°4. The time variations of the intensities are interpreted in the following way by comparison with simultaneous ground based or satellite measurements.The N(²D) atoms formed in the dayside cleft are carried by the neutral wind in a plume across the polar cap, so that the ratio of λ(630 nm) to λ(520 nm) intensities decreases along the plume with increasing distance from the source region.In the polar cap, but outside the plume region, 630 nm emission is produced by electron impact of polar rain and by substorms that reach high latitudes. Ionization produced at the same time, especially by the substorms, will produce further 630 nm emission through dissociative recombination. In any case, the region outside the plume may be regarded as a source region, with a high value of the ratio $\text{I(630)}\text{I(520)}$. This explains in part the diurnal variations, since this ratio is depressed as Nord crosses the dayside plume.The electron energy along the oval increases progressively from the dayside to the nightside. The intensity ratio increases with increasing electron energy because N(²D) is quenched more rapidly than O(¹D). Thus the ratio rises progressively from noon to midnight.An effect of the interplanetary magnetic field is superimposed on this pattern : as its North-South component B_z increases, the oval contracts so that Nord becomes nearer from the cleft source and the intensity ratio increases on the dayside. The inverse effect is also observed. On the nightside, negative B_z is associated with substorms that produce poleward expansions of the poleward oval boundary, that brings more energetic precipitation to Nord. This causes the intensity ratio to increase with decreasing B_z in a way that is opposite to that for the dayside.     

Radio emission of the NGC 1499 nebula at decametric wavelengths

Observations of the ionized hydrogen region NGC 1499 have been carried out with the radio telescope UTR-2 at frequencies 12.6, 14.7, 16.7, 20 and 25 MHz. The half-power resolution of the instrument to zenith is 28×34 at 25 MHz. The average volume density of the non-thermal radio emission between the Sun and the nebula (1.75×10^–40 W m^–3 Hz^–1 ster^–1 at 25 MHz), the electron temperature of the HII nebula (T _e =4400 K), the measure of emission (ME=1500 cm^–6 pc) and other parameters have been obtained. Maps of brightness distribution over the source are presented for each observation frequency. The results are compared with previously obtained data.     

X-ray Emission Mechanisms of LINERs

We present here an analysis of the X-ray properties of a sample of LINER galaxies observed with the ROSAT PSPC and HRI instruments. A spatial analysis shows that the bulk of the X-ray emission is consistent with arising from a point source; some extended emission appears at weak emission levels. The X-ray spectra are formally best described by a power law with photon indexΓ_x ≈ −2 or thermal emission from a Raymond-Smith plasma with highly subsolar abundances (Z ≤ 0.1).Several emission mechanisms that might contribute to the observed X-ray spectra are discussed. In particular, we take the very subsolar abundances derived from Raymond-Smith fits as an indication of a more complex emission mechanism, like the presence of a second hard component or plasma out of equilibrium. This revised version was published online in July 2006 with corrections to the Cover Date.