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.     

Type III solar radio burst storms observed at low frequencies

Storms of type III solar radio bursts observed from 5.4 ot 0.2 MHz consist of a quasi-continuous production of type III events observable for half a solar rotation but persisting in some cases for well over a complete rotation (Fainberg and Stone, 1970). The observed burst drift rates are a function of the heliographic longitude of the associated active region. This apparent drift rate dependence is a consequence of the radio emission propagation time from source to observer. Based on this dependence, a least squares analysis of 2500 drift rates between frequencies in the 2.8 to 0.7 MHz range yields an average exciter speed of 0.38 c for the height range from approximately 11 to 30 R . In conjunction with the available determinations of exciter speeds of 0.33 c close to the sun, i.e. less than 3 R , and with in situ measurements of 40 keV solar electrons by space probes, the present results suggest that the exciters are electron packets which propagate with little deceleration over distances of at least 1 AU.     

Detection of langmuir solitons: implications for type III burst emission mechanisms at 2ω PE

We present the experimental verification of existing theoretical models of emission mechanisms of solar type III bursts at the second harmonic of the plasma frequency, _pe . This study is based on the detection of Langmuir and envelope solitons by the Ulysses spacecraft inside three type III burst source regions. We show that the oscillating-two-stream instability, coherent radiation by Langmuir solitons and stochastic phase mixing of the Langmuir waves in the strong turbulence regime are the appropriate emission mechanisms at 2 _pe .     

Relationship between type III-V radio and hard X-ray bursts

Type III–V radio bursts are found to be closely associated with impulsive hard X-ray bursts. Probably 0.1% to 1% of the fast electrons in the X-ray source region escape to heights > 0.1R in the corona and excite the type III–V burst.     

Radio and soft X-ray evidence for dense non-potential magnetic flux tubes in the solar corona

The source positions of solar radio bursts of spectral types I, III(U) and III(J) and V observed by the Culgoora radioheliograph are found to lie almost radially above soft X-ray loops on pictures taken by the S-056 telescope aboard Skylab. The radio source positions and the X-ray loops occur near magnetic loops on computed potential field maps. However, the magnetic induction required to explain the radio observations is much greater than the computed potential field value at that height. Dense current-carrying magnetic flux tubes emanating from active regions on the Sun and extending to 1.5R above the photosphere provide a satisfactory model for the radio bursts.     

Densities Probed by Coronal Type III Radio Burst Imaging

We present coronal density profiles derived from low-frequency (80?–?240 MHz) imaging of three Type III solar radio bursts observed at the limb by the Murchison Widefield Array (MWA). Each event is associated with a white-light streamer at larger heights and is plausibly associated with thin extreme-ultraviolet rays at lower heights. Assuming harmonic plasma emission, we find average electron densities of 1.8\(\times10^{8}\) cm^?3 down to 0.20\(\times10^{8}\) cm^?3 at heights of 1.3 to 1.9 R_⊙. These values represent approximately 2.4?–?5.4× enhancements over canonical background levels and are comparable to the highest streamer densities obtained from data at other wavelengths. Assuming fundamental emission instead would increase the densities by a factor of four. High densities inferred from Type III source heights can be explained by assuming that the exciting electron beams travel along overdense fibers or by radio propagation effects that may cause a source to appear at a larger height than the true emission site. We review the arguments for both scenarios in light of recent results. We compare the extent of the quiescent corona to model predictions to estimate the impact of propagation effects, which we conclude can only partially explain the apparent density enhancements. Finally, we use the time- and frequency-varying source positions to estimate electron beam speeds of between 0.24 and 0.60 c.     

A self-consistent model for the storm radio emission from the Sun

A self-consistent theoretical model for storm continuum and bursts is presented. We propose that the Langmuir waves are emitted spontaneously by an anisotropic loss-cone distribution of electrons trapped in the magnetic field above active regions. These high-frequency electrostatic waves are assumed to coalesce with lower-hybrid waves excited either by the trapped protons or by weak shocks, making the observed brightness temperature equal to the effective temperature of the Langmuir waves.It is shown that whenever the collisional damping ( _c ) is more than the negative damping (- _A ) due to the anisotropic distribution, there is a steady emission of Langmuir waves responsible for the storm continuum. The type I bursts are generated randomly whenever the collisional damping ( _c ) is balanced by the negative damping (- _A ) at the threshold density of the trapped particles, since it causes the effective temperature of Langmuir waves to rise steeply. The number density of the particles responsible for the storm radiation is estimated. The randomness of type I bursts, brightness temperature, bandwidth and transition from type I to type III storm are self-consistently explained.On leave from Indian Institute of Astrophysics, Bangalore 560034, India.     

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 .     

Frequency and time splitting of decameter solar radio bursts

The paper deals with the observations of the fine structure of type III bursts in the 12.5–25 MHz band using the UTR-2 (IRE AN UkSSR, Kharkov) radio telescope. A fine structure arises in the form of chains of short-lived narrow-band bursts. The chains have a frequency drift analogous to type III bursts. Observations allow two different-type chains to be singled out. Ordinary stria-bursts, split-pairs and triplets belong to the first type chains. They may also involve the echo-type phenomena The second type chains (IIId) involve diffusive stria-bursts, diffusive split-pairs and triplets. The analysis of a harmonic structure of chains incidates that the first type chains are generated at the frequencies close to the local plasma electron frequency _pe . The second type chains and, consequently, diffusive stria-bursts correspond to the second harmonic of the plasma frequency 2 _pe . Experimental data evidence that the type III bursts with a fine structure are excited by the faster particle streams than the ordinary type III bursts with a diffusive character both of the fundamental and the second harmonic.     

Numerical simulation of type III solar radio bursts caused by high-density electron beam

Numerical simulation for the type III solar radio bursts in meter wavelengths was made with the electron beam of a high number density enough to emit fundamental radio waves comparable in intensity with the second harmonic.This requirement is fulfilled if the optical thickness ₁ for the negative absorption (amplification) becomes -23 to -25. Since ₁ is roughly proportional to the time-integral of the electron flux of the beam, the intensity of the fundamental waves depends strongly on the parameters which determine the electron flux. Therefore, it is most unlikely that the harmonic pairs of type III bursts of the first and the second harmonics occur frequently with comparable intensities in a wide frequency range, say 200 MHz to 20 MHz, if we take the working hypothesis that the fundamental waves are caused by the scattering of electron plasma waves by thermal ions and amplified during the propagation along the beam.However, we cannot rule out the possibility that single type III bursts with short durations or group of such bursts are the fundamental waves emitted by the above mechanism, but only if the observed large size of the radio source can be attributed to the radio scattering alone.     

Correlated variations of planetary albedos and coincident solar-interplanetary variations

To locate two-dimensional positions of the solar decametric radio bursts a heliograph was developed on the basis of the UTR-2 radiotelescope (Khar'kov) operated in the range 10–26 MHz. The beamwidth of the heliograph rapid-scanning pencil-beam is 25 arc min at 25 MHz, and its field of view is about 3.5° (E-W) × 2.0° (N-S). The instrument yields rapidly forty records of the radio brightness of all (8 × 5) elementary parts (each 25 arc min in diameter) of the investigated sky area during every period of 1/4 s. Both coordinates of a burst center are measured with an accuracy 5 arc min. The bandwidth of the receiving system is 10 kHz. The heliograph operates in conjunction with a radiospectrograph connected to the output of a N-S arm of the UTR-2 array. The data observations with the UTR-2 correspond only to one linear polarized component.The ionospheric distortion of the test records of the radio source Cassiopeia-A that occurred sometimes is illustrated.First results of 25 MHz observations of the solar radio storm in August, 1976 with the heliograph are presented here. This storm is accompanied by the compact sunspot group travelling all over the optical disk. The type III and stria bursts were predominant during the storm. On the given day the scattering regions of their apparent centers were overlapped and the sizes of these regions were usually not more than 5 arc min. On some days there occurred additional burst sources displaced in position from the persistent storm region. It was found out that, as a rule, 25 MHz stria-bursts from the type IIIb chain coincided in position with the following type III burst at the same frequency. The difference of the daily averaged coordinates of both stria and type III bursts was considerably smaller than the mean diameter of their sources.The radial distance of the 25 MHz storm region from the solar center was calculated by using the three methods. The storm height was estimated as 1.8R from the rotation rate close to the central meridian of the storm center. Definite association of the spots with the storm near the limb allowed to determine the average value 2.1R for the height. The limb measurements give the mean height of 2.3R .The center-to-limb variation of the storm source height is a known fact in the meter-wavelength range. This is the evidence of the propagation effects in the solar corona being essential to interpret the results of the radio source location.     

On the origin of type III fundamental

A two-component scheme for the generation of type III fundamental radiation is proposed. The first component of the fundamental arises at a plasma level _{L t} because of the Rayleigh scattering of the plasma waves into electromagnetic radiation. The other component arises at _{L t} /2 because of the decay of the first component into plasma waves and the subsequent rescattering of the plasma waves into electromagnetic radiation _t 2( _t /2). By its properties (location, directivity, polarization) the second component is essentially the same as the second harmonic radiation produced by a stream of fast electrons at _L ( _t /2). This scheme is used to solve the main problems (localization and directivity of the source, polarization of type III fundamental) of the harmonic theory of type III solar bursts.     

Interpretation of distinct type IVmA- and IVμ-bursts on the basis of micro-instabilities and of resonant nonlinear interaction of waves

The generation of space charge waves by micro instabilities of the Harris type and their conversion into electromagnetic waves is discussed in the framework of the dispersion curves of the extraordinary wave mode in the warm plasma. Acceleration of electrons as also nonlinear interactions of waves are taken into account. A survey of the parameter regions of the Harris instabilities is given. Distinct values _p / _c and _p / _c result, enabling the instability as well as the conversion. The moving type IVmA bursts, and on the other side the impulsive cm-bursts and the first phase of type IV bursts are correlated to different values _p / _c and corresponding heights in the corona. The space charge waves can produce hydromagnetic waves by parametric excitation, too (type II bursts). The proposed mechanism is discussed with respect to the energy balance and to the magnetic configurations derived from observations with the Culgoora radioheliograph.     

Location of type I radio continuum and bursts on Yohkoh soft X-ray maps

A solar type I noise storm was observed on 30 July, 1992 with the radio spectrometer Phoenix of ETH Zürich, the Very Large Array (VLA) and the soft X-ray (SXR) telescope on board theYohkoh satellite. The spectrogram was used to identify the type I noise storm. In the VLA images at 333 MHz a fully left circular polarized (100% LCP) continuum source and several highly polarized (70% to 100% LCP) burst sources have been located. The continuum and the bursts are spatially separated by about 100 and apparently lie on different loops as outlined by the SXR. Continuum and bursts are separated in the perpendicular direction to the magnetic field configuration. Between the periods of strong burst activities, burst-like emissions are also superimposed on the continuum source. There is no obvious correlation between the flux density of the continuum and the bursts. The burst sources have no systematic motion, whereas the the continuum source shows a small drift of 0.2 min^–1 along the X-ray loop in the long-time evolution. The VLA maps at higher frequency (1446 MHz) show no source corresponding to the type I event. The soft X-ray emission measure and temperature were calculated. The type I continuum source is located (in projection) in a region with enhanced SXR emission, a loop having a mean density of n _e = (1.5 ± 0.4) × 10⁹ cm^–3 and a temperature ofT = (2.1 ± 0.1) × 10⁶ K. The centroid positions of the left and right circularly polarized components of the burst sources are separated by 15–50 and seem to be on different loops. These observations contradict the predictions of existing type I theories.Presented at the CESRA-Workshop on Coronal Magnetic Energy Release at Caputh near Potsdam in May 1994.     

Supernova models with slow energy pumping and galactic supernova remnants

The study of supernova (SN) models with slow energy pumping is continued. At maximum luminosity the main characteristics of a SN are shown to be independent of the initial structure of the model (Table I, Figure 1). However, they depend on the massM _e of the envelope, and on the intensity of energy pumpingL , with an increase ofM _e leading qualitatively to the same changes in the SN parameters as a decrease inL (Table I, Figures 2 and 3). A simple relationship connecting the important SN parameters is obtained (Equation (6)). From the inflection of the color indexB-V curve, the possibility of deriving the characteristic time of energy pumping with intensityL 10⁴⁴ erg s^–1 is pointed out. The comparison of the extragalactic type I SN observations with the results of calculations leads to the estimate ofM _e 0.3–0.7M.An investigation of the galactic type I SN remnants is carried out (Table III). The estimate ofM _e 0.2–0.3M is obtained for the remnants of supernovae SN 1006, SN 1572, and SN 1604. It completely fits the results for the extragalactic type I SNs. The total initial mass of SN 1604 presupernova was shown to be at least about 7M .It was established that the Crab nebula resulted from the outburst of a peculiar SN. The unique properties of such SNs, including SN 1054, are due to the low intensity of energy pumping (L 10⁴² erg s^–1). The mass of the envelope of the Crab nebula is evaluated to beM _e 0.7M . SN 1054 was shown to have m _max ^v =–4 _m ^. 0 at maximum luminosity.     

Shock waves and type II radiobursts in the interplanetary medium

The planetary radio astronomy experiment on the Voyager spacecraft observed several type II solar radiobursts at frequencies below 1.3 MHz; these correspond to shock waves at distances between 20R and 1 AU from the Sun. We study the characteristics of these bursts and discuss the information that they give on shock waves in the interplanetary medium and on the origin of the high energy electrons which give rise to the radioemission. The relatively frequent occurence of type II bursts at large distances from the Sun favors the hypothesis of the emission by a longitudinal shockwave. The observed spectral characteristics reveal that the source of emission is restricted to only a small portion of the shock. From the relation between type II bursts, type III bursts and optical flares, we suggest that some of the type II bursts could be excited by type III burst fast electrons which catch up the shock and are then trapped.     

Flat Galaxies of the RFGC Catalog Detected in the HIPASS Survey

Data from the H I Parkes All-Sky Survey (HIPASS) of the southern sky in the neutral hydrogen line are used to determine the radial velocities and widths of the H I line for flat spiral galaxies of the Revised Flat-Galaxy Catalog (RFGC) seen edge-on. The sample of 103 flat galaxies detected in HIPASS is characterized by a median radial velocity of +2037 km/sec and a median width of the H I line at the level of 50% of maximum of 242 km/sec. For RFGC galaxies the 50% detection level in HIPASS corresponds to an apparent magnitude B _t = 14 ^m .5 or an angular diameter a = 2.9. The relative number of detected galaxies increases from 2% for the morphological types Sbc and Sc to 41% for the type Sm. The median value of the ratio of hydrogen mass to total mass for RFGC galaxies is 0.079. With allowance for the average internal extinction for edge-on galaxies, <B _t< = 0 ^m .75, the median ratio of hydrogen mass to luminosity, M _{H I}/L _B = 0.74 M /L , is typical for late-type spirals. Because of its small depth, HIPASS reveals only a few RFGC galaxies with previously unknown velocities and line widths.     

19–20 May 1969, an example of type III emission during the impulsive phase of flares

I examined two multiple impulsive events in May 1969 and compared their impulsive Hex, hard X-ray and microwave components to the observed type III emission. No good correlation was observed between meter intensity and the degree of simultaneous H outflow although almost every type III burst had some associated H activity. Correspondingly, an inverse relationship existed between X-ray and type III emission, i.e., a strong meter burst accompanied weak hard X-radiation. This, along with the fact that sufficient energetic electrons were normally present to produce the X-rays, implies that the trajectory, rather than electron spectrum or supply, determines whether type III noise is observed during an impulsive event.     

Air drag effect on the motion of an artificial Earth satellite

Two methods have been used to compute and compare the perturbations in perigee distance for an artificial Earth satellite. The two methods have used different air density models. The first (Helali, 1987) used the TD model, formulated by Sehnel (1986a), which contains terms that describe all the principal changes of the thermospheric density due to solar activity, geomagnetic activity, and the height. The second method (Davis, 1963) used a model of the density which takes into account the rotation of the atmosphere, the bulging atmosphere and the height. For different values of eccentricities from 0.001 to 0.05 we computed the perturbations P _r in the perigee distance at different heights from 200 to 350 km for both methods. The results show a good agreement for the computed values of P _r for different values of e (0 < e 0.02) in both methods at perigee heights from 250 to 350 km. Meanwhile, for perigee heights smaller than about 250 km we found a maximum difference in P _r amounting to 20 metres/revolution for e = 0.005 and 0.01.     

Electron beams and associated rapid fluctuations in solar flares

Recent observations show that the rapid fluctuations in radio, hard X-ray, and H emissions are closely associated with type III and microwave (or decimetric) bursts during the impulsive and/or preimpulsive phases of solar flares.In order to clarify the physical processes of these observed phenomena, this paper proposes a tentative model of two acceleration regions A (source of type III bursts) and B (source of microwave or decimetric bursts) formed in the neutral sheet and at the top of a flaring loop, respectively; and also suggests that the electron beams streaming from region A and/or region B downward to the chromosphere are responsible for the rapid fluctuations in the different emissions mentioned above during the impulsive and/or pre-impulsive phases of solar flares.