Decimetric Radio Emission Research Articles

Correlation between decimetric radio emission and hard X-rays in solar flares

Aims. The emission of decimetric flare radiation, in particular na rrowband spikes and pulsations, is generally considered to originate in accelerated, non-thermal particles. On the other hand, non-thermal hard X-rays are also understood to be products of this acceleration. Do radio emission and hard X-ray signatures originate from the same acceleration process? A strong correlation between the light curves in the radio and HXR ranges may help answer this question. Methods. The delay between the radio and hard X-ray emission was determined by cross-correlation. The time profiles of X-ray and radio emission include a wide range of energies and frequencies. Thus, correlation is not simply a yes/no question, but must be systematically searched for in various ranges. The high spectral resolution of RHESSI ensured that it was possible to carefully choose the energy range, excluding thermal emission. The broad bandwidth of Phoenix-2 allowed the selection of any emission in the full decimetre range. The energy range and duration in hard X-rays, and the frequency range in radio spectrograms were chosen to optimize the correlation. The cross-correlation coeffi cient was then analyzed by a Gaussian fitting method. Results. The measured delays have a distribution of FWHM 4.9 s and 4.7 s for pulsations and spikes, respectively, evaluated from su ch a Gaussian fitting method. The mean delay for pulsations was f ound to be−1.4±0.9 seconds (minus indicates that hard X-ray emission comes first), and for narrowband spikes to −2.5± 2.5 seconds. There are broad wings in the distribution, which we interpret as chance coincidences. The delays do not depend on centre frequency, cross-correlation coeffi cient, duration of the correlating sequence, and position on the disk. However, we find an increase in the delay for the spikes with GOES magnitude (peak soft X-ray emission) of the flare and with peak hard X-ray flux. This was not the case for pul sations. Conclusions. In contrast to previous reports, the average delays for all p ulsations and all spike groups are consistent with zero. Thus, correlated decimetric pulsations and spikes are, on average, concomitant with non-thermal X-rays.

“Drifting tadpoles” in wavelet spectra of decimetric radio emission of fiber bursts

<i>Aims. <i/>The solar decimetric radio emission of fiber bursts was investigated searching for the “drifting tadpole” structures proposed by theoretical studies.<i>Methods. <i/>Characteristic periods with the tadpole pattern were searched for in the radio flux time series by wavelet analysis methods.<i>Results. <i/>For the first time, we have found drifting tadpoles in the wavelet spectra of the decimetric radio emission associated with the fiber bursts observed in July 11, 2005. These tadpoles were detected at all radio frequencies in the 1602-1780 MHz frequency range. The characteristic period of the wavelet tadpole patterns was found to be 81.4 s and the frequency drift of the tadpole heads is -6.8 MHz s<sup>-1<sup/>. These tadpoles are interpreted as a signature of the magnetoacoustic wave train moving along a dense flare waveguide and their frequency drift as a motion of the wave train modulating the radio emission produced by the plasma emission mechanism. Using the Aschwanden density model of the solar atmosphere, only low values of the Alfvén speed and the magnetic field strength in the loop guiding this wave train were derived which indicates a neutral current sheet as the guiding structure. The present analysis supports the model of fiber bursts based on whistler waves.

Decimetric radio dot emissions

Context. We study a rare type of solar radio bursts called decimetric dot emissions. Aims. In the period 1999−2001, 20 events of decimetric dot emissions observed by the Brazilian Solar Spectroscope (BSS) in the frequency range 950−2640 MHz are investigated statistically and compared with radio fine structures of zebras and fibers. Methods. For the study of the spectral characteristics of the dot emissions we use specially developed Interactive Data Language (IDL) software called BSSView and basic statistical methods. Results. We have found that the dm dot emissions, contrary to the fine structures of the type IV bursts (i.e. zebras, fibers, lace bursts, spikes), are not superimposed on any background burst emission. In the radio spectrum, in most cases the dot emissions form chains that appear to be arranged in zebra patterns or fibers. Because some zebras and fibers, especially those observed with high time and high spectral resolutions, also show emission dots (but superimposed on the background burst emission), we compared the spectral parameters of the dot emissions with the dots being the fine structure of zebras and fibers. For both these dots, similar spectral characteristics were found. Some similarities of the dot emissions can be found also with the lace bursts and spikes. For some events the dot emissions show structural evolution from patterns resembling fibers to patterns resembling zebras and vice versa, or they evolve into fully chaotic patterns. Conclusions. For the first time, we present decimetric dot emissions that appear to be arranged in zebra patterns or fibers. We propose that these emissions are generated by the plasma emission mechanism at the locations in the solar atmosphere where the double resonance condition is fulfilled.

Observations of radio spectra at 1–2.5 GHz associated with CME start time

AbstractWe know Coronal Mass Ejections (CME) and flares are the most energetic phenomena happening on the Sun. Until now the information about origin and trigger mechanism of CMEs remains scarce. Also, there is unconclusive information about the association between them and flares although progress has been made in recent years. Multi-spectral observations suggested that the flare energy release occurs in regions from where the decimetric radio emission originates. In this case, investigations of the solar emission in this wavelength range can give us valuable information about these questions. During last solar maximum the Brazilian Solar Spectroscope (BSS) observed the solar radio spectrum (1–2.5 GHz) with high time (100–20 ms) and frequency (50–100 channels) resolutions on a daily (11–19 UT) basis. A survey during the period 1999–2002, shows that a significant fraction (20% –57 events) of CMEs recorded by LASCO has an association with the spectra of radio bursts recorded by BSS. Analysis of the radio spectrum associated to CME shows there is a dominance of continuum and/or pulsation and that the association becomes stronger when we consider the CME acceleration since its origin on the Sun. A statistics of this association between CME dynamics and the characteristics of decimetric radio bursts recorded by BSS is presented. Emphasis is given to observations of the association with CME start time.

Radio emission observed in decimetric waves associated with the onset of CMEs

Since the first observations by Skylab and SMM satellites coronal mass ejections (CME) have been more and more investigated. However, until now their origin and trigger mechanism remain an open question no matter if they are associated to flares or not. Recent observations over a broad spectrum suggest that flare energy is released in regions from where the decimetric emission is coming. Then, investigations of decimetric radio emission observed in association with CME phenomena may give clues to solve the previously mentioned questions. Using the Brazilian solar spectroscope (BSS), observations of solar bursts dynamic spectra with high time (100, 50, 20 ms) and frequency (50–100 channels) resolutions have been carried out daily (∼11–19 UT) within the range of 1000–2500 MHz. A sample of 274 CMEs were recorded by the large angle spectroscopic coronagraph (LASCO) instrument, on board the solar and heliospheric observatory (SOHO) satellite, within 11–19 UT, during the period of 1999–2002. From those, 42 CMEs are associated to BSS data and selected for analysis. It is interesting to note that in about half of the cases only one type of burst radio emission was recorded while in the remaining cases either two or more types were observed. There is a dominance of either continuum and/or pulsations. Here, we describe the association of burst radio emission with the starting time of CME phenomena.

Temporal Correlation of Hard X-Rays and Meter/Decimeter Radio Structures in Solar Flares

We investigate the relative timing between hard X-ray (HXR) peaks and structures in metric and decimetric radio emissions of solar flares using data from the RHESSI and Phoenix-2 instruments. The radio events under consideration are predominantly classified as type III bursts, decimetric pulsations and patches. The RHESSI data are demodulated using special techniques appropriate for a Phoenix-2 temporal resolution of 0.1 s. The absolute timing accuracy of the two instruments is found to be about 170 ms, and much better on the average. It is found that type III radio groups often coincide with enhanced HXR emission, but only a relatively small fraction (∼20%) of the groups show close correlation on time scales < 1 s. If structures correlate, the HXRs precede the type III emissions in a majority of cases, and by 0.69 ± 0.19 s on the average. Reversed drift type III bursts are also delayed, but high-frequency and harmonic emission is retarded less. The decimetric pulsations and patches (DCIM) have a larger scatter of delays, but do not have a statistically significant sign or an average different from zero. The time delay does not show a center-to-limb variation excluding simple propagation effects. The delay by scattering near the source region is suggested to be the most efficient process on the average for delaying type III radio emission.

A Probable Short Decimetric Type I-like Noise Storm: Associated with Type III Bursts?

A rare Type I-like noise storm was observed with the solar radio spectrometers (1.0-2.0GHz and 2.60-3.8GHz) at National Astronomical Observatories of China (NAOC) on September 23, 1998. We concentrate on checking the Type I-like noise storm occurred in the decay phase of a Type IV radio burst. This noise storm consists of many Type I bursts and isolated Type III or Type III pair bursts. It has a bandwidth of <= 0.5GHz. The duration of each Type I burst is of the order of 100-300ms. The total duration is greater than 11 minutes. The circular polarization degree of the components of Type I and associated Type III bursts are about 40%-100% and almost 100%, respectively, which is greater than that of the background continuum (nearly the precision of our instrument). This short decimetric Type I-like storm may be another kind or the extension of the kind of metric Type I storm, and may possess the duality of metric and decimetric radio emission. It may be in favor of an earlier emission mechanism of the fundamental plasma radiation due to the coalescence of Langmuir waves with low-frequency waves.

On the Envelope Soliton – Like Fine Structure in Solar Radio Emission

Several quasi-periodic, milliseconds fine structures in the metric wave band occurring during the evolution of solar type IV bursts have been observed by Yunnan Radio Telescope, Trieste Radio Telescope and IZMIRAN dynamic spectrometer. The envelope of these quasi-period modulational fine structures have a soliton pattern, so it is called an envelope soliton-like fine structure. A modulational instability model of electromagnetic wave has been adopted here. It is found that the longitudinal modulational instability can occur only in the solar coronal region of low magnetic field and high temperature, as well as high density plasma, which will give rise to the envelope soliton-like fine structures in the solar metric and decimetric radio emission. The propagation effects of envelope soliton-like fine structure from corona to the observer on the Earth have been briefly considered.

Hard X-rays, ejecta, and their associated decimetric radio emission in solar flares

We investigate temporal and spatial correlations in solar flares of hard X-rays (HXR) and decimetric continuum emissions, ejecta, and CMEs. The focus is on three M-class flares, supported by observations from other flares. The main conclusions of our observations are that (1) major hard X-ray flares are often associated with ejecta seen in soft X-rays or EUV. (2) Those ejecta seem to start before HXR or related decimetric radio continua (DCIM emission). (3) DCIM occurring nearly simultaneously with the first HXR peak are located very close to the HXR source. Later in the flare, DCIM generally becomes stronger, drifts to lower frequency and occurs far from the HXR source. Thus the positions at high frequency are generally closer to the HXR source. DCIM emission consists of pulses that drift in frequency. The very high and sometimes positive drift rate suggests spatially extended sources or type III like beams in an inhomogeneous source. Movies of selected flares used in this study can be found on the CD-ROM accompanying this volume.

On the Solar Origin of Complex Type III–like Radio Bursts Observed at and below 1 MHz

By simultaneously analyzing decimetric, metric, and hectometric radio observations, we provide evidence that a class of hectometric type III-like radio events are associated with electrons accelerated during the primary flare energy release process. We do this by demonstrating that there is a good temporal correspondence between the hectometric and decimetric radio emissions that are believed to involve the acceleration of electrons in deep layers of the solar atmosphere at heights estimated to be from 10,000 to 30,000 km (0.014 to 0.043 R☉). This class of hectometric type III-like events may have simple or complex intensity-time profiles. When they have complex profiles of long duration, the decimeter emissions also have complex long-duration profiles. In this latter case they are also often associated with metric type II radio bursts and coronal mass ejections (CMEs). However, we argue on the basis of the observations that the hectometric radio emissions are not necessarily associated with the metric phenomena. Specifically, they are not likely generated by a secondary acceleration process associated with the shock generating a metric type II burst or with a shock driven ahead of the CME despite the fact that the hectometric emission time profiles would (erroneously) be morphologically classified as shock-accelerated (SA) events.

VERY LARGE ARRAY OBSERVATIONS OF EVOLVING NOISE STORM SOURCES ON THE SUN

The presence of coronal magnetic fields connecting active regions is inferred from decimetric observations of solar noise storms with the Very Large Array (VLA) and from soft X-ray images taken by Yohkoh. Temporal changes in the noise storms appear to be correlated with some soft X-ray bursts detected by both Yohkoh and the GOES satellite. Combined analysis of the radio and X-ray data suggests a re-arrangement of the coronal magnetic field during the onset of impulsive noise storm burst emission. On one day during the combined VLA–Yohkoh–GOES observations, two widely-separated active regions appear to be connected by a faint trans-equatorial 91 cm source as well as two distinct soft X-ray loops. The two active regions show anti-correlated fluctuations in decimetric radio emission. On another day of combined VLA–Yohkoh observations, a series of 91 cm noise storm bursts are observed along the major axis of the associated noise storm continuum. Time sequences of Yohkoh soft X-ray images show a contraction of coronal loops prior to the onset of this series of bursts and a corresponding increase in the X-ray flux in the apparent footpoint of the overarching loop containing the noise storm. These observations imply that energy from a realignment of the magnetic field is being transferred, possibly by accelerated particles, along loops connecting separated active regions on the Sun.

Jovian magnetospheric and auroral effects of the SL9 impacts

The collisions of comet Shoemaker-Levy 9 with Jupiter have produced many surprising auroral and magnetospheric phenomena. The energy released during the passage of the cometary dust comas through the jovian magnetosphere and at atmospheric explosion could lead to impulsive particle acceleration, enhanced radial diffusive transport, and the establishment of field-aligned current systems connecting the comet impact sites to their respective magnetic conjugate points. Some of the observed effects such as the abrupt increase of decimetric radio emission, the excitation of infraredemissions and mid-latitude auroral emission in the ultraviolet, could be interpreted within the framework of these mechanisms. Several auroral features like the X-ray outbursts and short-term variations in the UV emissions are more puzzling and require further observation of jovian auroral dynamics in these wavelength ranges in coordination with the Galileo mission.

Chromospheric evaporation and decimetric radio emission in solar flares

We have discovered decimetric signatures of the chromospheric evaporation process. Evidence for the radio detection of chromospheric evaporation is based on the radio-inferred values of (1) the electron density, (2) the propagation speed, and (3) the timing, which are found to be in good agreement with statistical values inferred from the blueshifted Ca xix soft X-ray line. The physical basis of our model is that free-free absorption of plasma emission is strongly modified by the steep density gradient and the large temperature increase in the upflowing flare plasma. The steplike density increase at the chromospheric evaporation front causes a local discontinuity in the plasma frequency, manifested as almost infinite drift rate in decimetric type III bursts. The large temperature increase of the upflowing plasma considerably reduces the local free-free opacity (due to the T-(exp -3/2) dependence) and thus enhances the brightness of radio bursts emitted at the local plasma frequency near the chromospheric evaporation front, while a high-frequency cutoff is expected in the high-density regions behind the front, which can be used to infer the velocity of the upflowing plasma. From model calculations we find strong evidence that decimetric bursts with a slowly drifting high-frequency cutoff are produced by fundamental plasma emission, contrary to the widespread belief that decimetric bursts are preferentially emitted at the harmonic plasma level. We analyzed 21 flare episodes from 1991-1993 for which broadband (100-3000 MHz) radio dynamic spectra from Phoenix, hard X-ray data from BATSE/CGRO, and soft X-ray data from GOES were available. We detected slowly drifting high-frequency cutoffs between 1.1 and 3.0 GHz, with drift rates of -41 +/- 32 MHz/s, extending over time intervals of 24 +/- 23 s. Developing a density model for type III-emitting flare loops based on the statistically observed drift rate of type III bursts by Alvarez & Haddock, we infer velocities of up to 360 km/s for the upflowing plasma, with an average of v(sub CE) = 236 +/- 130 km /s for episodes with 5-15 s duration. The mean electron density of the upflowing plasma is n(sub e) = 5.2(+/-3.1) x 10(exp 10) /cu cm when it is first detected in radio, at coronal altitudes of h(sub 0) = 9.2 +/- 2.3 Mm.

Time variations of the Jovian synchrotron radiation following the collisional impacts of comet Shoemaker-Levy 9—2. Flux enhancement induced by neutral atmospheric turbulence

The observed increase of the decimetric radio emission from Jupiter after the atmospheric impacts of comet Shoemaker-Levy 9 may be the result of enhanced radial diffusion driven by the neutral wind dynamo. A simple model is considered here which invokes an increase of the radial diffusion coefficient by a factor of about two in the magnetic flux shell within 5° of the impact site latitude. The explosion-related neutral wind flow should be of a transient nature such that a continuous increase in the synchrotron radiation could only be sustained as long as there was energy release in the Jovian atmosphere. After the comet impacts, the decimetric radio emission will return to its normal level within 40 days.

Radio astronomical studies of solar activity and its effects on the Solar Corona and the heliosphere

The radio astronomical studies of solar activity are powerful complementary tools to the studies in the other parts of the electromagnetic spectrum and of the corpuscular emissions. We can either observe the Solar radio emission directly or resort to the occultation techniques. The solar radio emission is monitored world wide in all the radio bands, both as the quiet Sun and the bursty solar radio events which can reach levels up to about one million times over the quiet Sun radio emission level. The solar cycle is very much more pronounced in the bursty than it is in the quiet solar emission. The four decades of solar coronal scattering observations have given us extensive information about the varying conditions in the solar outer corona as well as in the remote regions of the heliosphere (by means of the interplanetary scintillation technique). In addition the radio observations have the important advantage which permits us to study the Jovian magnetosphere at the decimetre wavelength all the time from the ground based radio observatories, where it was first discovered more than a decade before the first independent Pioneer and later by the Voyager spacecraft confirmations. Due to the effect of the 22 year solar magnetic cycle which reverses the magnetic polarity of the Sun an unexpected decline has been observed in the late 60-ties and early 70-ties of the scattering of radio waves in the Solar Corona. During the same time there was also a marked decline of the radio emission from the Jovian magnetosphere. This 30% fall of the Jovian decimetric radio emission recovered within several years.

The effect of comet Shoemaker‐Levy 9 on Jupiter's synchrotron radiation

Solid material from comet Shoemaker‐Levy 9 is expected to modify Jupiter's decimetric radio emission. Electrons passing through dust grains suffer a degradation in their energy, while larger sized material absorbs all electrons impinging on it. If there is enough dust and/or larger sized material in the magnetosphere, the effect on the energetic electrons can be observed via their synchrotron radiation, emitted at microwave frequencies. The planet's radio emission is expected to decrease, and the presence of dust induces a hardening in the radio spectrum. We expect the intensity to decrease immediately after cometary material enters the radiation belts; the radio emission continues to drop for many months as electrons diffuse inwards through a dusty magnetosphere. Radio observations of the planet during and after the cometary impact thus yield information on the amount of cometary material (optical depth of the dust and macroscopic material), impacts between cometary material and Jupiter's ring, and the diffusion process of energetic electrons through the magnetosphere.

Long‐term changes in Jovian synchrotron radio emission: Intrinsic variations or effects of viewing geometry?

A long‐term decadal scale variation of Jovian decimetric radio emission has been reported that could indicate intrinsic time variability in the Jovian radiation belts and/or magnetic field. However, the strong beaming of the radiation by trapped relativistic electrons in the complex Jovian magnetic field can also result in apparent decimetric radiation changes due to the evolving viewing geometry of Jupiter during its 11.9‐year orbit. To investigate this problem, radial diffusion models of Jovian trapped electron fluxes at relevant energies are first constructed to simulate possible causes of intrinsic variations. Results indicate several inconsistencies with observations. For example, an increase in diffusion rate with other parameters held constant results in an inward displacement of the peak emission radial distance that is not observed. Second, effects of viewing geometry changes are examined. The possible importance of such effects is suggested by a correlation between the total decimetric radio flux and the Jovigraphic latitude of the Earth, DE, which varies between −3.3° and +3.3° during one Jovian orbital period. Because the Jovian central meridian longitudes where the magnetic latitude passes through zero during a given Jovian rotation change substantially with DE and since significant longitudinal asymmetries exist in both the volume emissivity and the latitudinal profile of the beam, the total intensity should be at least a partial function of DE. Thus variations of viewing geometry associated with Jupiter's orbital motion should be more fully accounted for if long‐term intrinsic variability of Jovian synchrotron radiation is to be unambiguously detected.

Observation and analysis of solar decimetric spike radiation

From January, 1988 to June, 1989 the solar radio synchronous observational system with high temporal resolution of the Yunnan Observatory was used to carry out observations at a decimetric wavelength (1420 MHz), and the data are analyzed in this paper. The result shows a relation between the millisecond spike radiation of the solar decimetric radio emission and the radio bursts recorded with a time constant of one second. It also reveals the coexistence of decimetric radio millisecond spike radiations and Hα flares, type III radio bursts as well as soft X-ray bursts. A solar activity event with abundant radio millisecond spike radiations is investigated preliminarily and the relation between the duration and flux of individual spikes is discussed.

Nonlinear parametric instability and millisecond solar radio spikes

A model of nonlinear parametric instability (NLPI) modulation is suggested to explain the millisecond spikes in solar decimetric radio emission. The radio emission energy will periodically transfer to two other waves, and the intensity of the radio emission will be modified by NLPI processes in the corona, when the matching conditions of three-wave coupling are satisfied. This model can simultaneously explain the duration of spikes, why the millisecond spikes have not been observed in other solar radio emission, and the relation between the duration of the spikes and the intensity and the frequency of the radio emission as well.

Correlation studies between solar wind parameters and the decimetric radio emission from Jupiter

Results of a study comparing long‐term time variations (years) in Jupiter's synchrotron radio emission with a variety of solar wind parameters and the 10.7‐cm solar flux are reported. Data from 1963 through 1985 were analyzed, and the results suggest that many solar wind parameters are correlated with the intensity of the synchrotron emission produced by the relativistic electrons in the Jovian Van Allen radiation belts. Significant nonzero correlation coefficients appear to be associated with solar wind ion density, ram pressure, thermal pressure, flow velocity, momentum, and ion temperature. The highest correlation coefficients are obtained for solar wind ram pressure (NV²) and thermal pressure (NT). The correlation analysis suggests that the delay time between fluctuations in the solar wind and changes in the Jovian synchrotron emission is typically about 2 years. The delay time of the correlation places important constraints on the theoretical models describing the radiation belts. The implication of these results, if the correlations are real, is that the solar wind is influencing the supply and/or loss of electrons to Jupiter's inner magnetosphere. We note that the data for this work spans only about two periods of the solar activity cycle, and because of the long time scales of the observed variations, it is important to confirm these results with additional observations.