Solar Radio Bursts Research Articles

Unusual stripes in emission and absorption in solar radio bursts: Ropes of fibers in the meter wave band

Based on data from the spectrographs of IZMIRAN and Tremsdorf station (Astronomical Institute, Potsdam), we analyze the ropes of narrow-band fibers in the spectra of solar radio bursts in the meter wave band by invoking events of satellite data (SOHO/LASCO, EIT, MDI) for the analysis. We consider in detail basic properties of the ropes in four events in comparison with previously known data. The fibers in ropes are more commonly observed with an overlap in time and frequency, but occasionally (more often at the end of the ropes) they can follow with a separation in time. The fiber duration and recurrence period seldom remain stable and, in general, increase from 0.3–0.5 s at the beginning to several seconds at the end of the rope. The relative values of the instantaneous and total fiber frequency bandwidths change only slightly in different events; δ f / f ≈ 0.003–0.005 and Δf / f ≈ 0.02–0.03. Most of the ropes exhibit a low-frequency absorption. The fibers in ropes are similar to ordinary intermediate drift bursts (fiber bursts), but they drift in a narrow frequency band and have a more frequent recurrence in some events. The ropes of fibers are usually observed in the time interval when the shock front catches up with the leading edge of a coronal mass ejection. Under the condition of a unified approach to interpreting the ropes of fibers in all events, their basic properties can be explained in terms of the model of fiber bursts. The connection of fibers with the developed zebra pattern is shown within the framework of a unified approach to the formation theory of stripes in emission and absorption in the model on whistlers.

Solar Origin of the Radio Attributes of a Complex Type III Burst Observed on 11 April 2001

We report here on the solar origin of distinctive radiation characteristics observed for a decametric type III solar radio burst that was associated with a major solar flare and CME on 11 April 2001. The associated decimeter (Ondřejov) and meter (Potsdam) wavelength emissions, as well as the GOES soft X-ray lightcurve, suggest that there were two successive events of energy release and electron acceleration associated with this solar eruption. The Nancay radioheliograph images and additional evidence of plasmoid propagation suggest that the second event of electron acceleration resulted from coronal reconfigurations, likely caused by the erupting CME. These observational analyses provide new insights into the physical origin of the distinguishing characteristics of complex type III-like radio emissions that are typically observed at decameter wavelengths during major solar eruptive events.

Resonant Transition Radiation and Solar Radio Bursts

We calculate the resonant transition radiation (RTR) using the most general formulas and obtain approximate analytical expressions describing the RTR dependence on the fast-particle velocity and magnetic-field intensity. Dependence of the RTR on the angle between the wave vector and the magnetic-field direction and also on the thermal velocity of background-plasma electrons is considered on the basis of approximate expressions for RTR, in which the longitudinal dielectric permittivity of the plasma is written neglecting the wave vector of transverse electromagnetic waves. The obtained dependences are combined into unified expressions for RTR coefficients. Comparison with the earlier found RTR coefficients is performed. It is shown that the obtained coefficients correspond better to the nature of the resonant transition radiation. Finally, the obtained RTR coefficients are used for interpretation of the radio emission of the solar burst of December 24, 1991.

Particle-in-cell simulations of shocks and band splitting of type II solar radio bursts

Aims.We investigate the emission process of electromagnetic waves from proton beams reflected by the front of a fast magnetosonic shock that propagates in a non-uniform density region, by changing the propagation angle with respect to a uniform magnetic field from 90° to 45°.

Coronal scattering of radio emission under strong regular refraction

Abstract The influence of strong regular refraction (that leads to regular caustics and multipathing) in the solar corona on the structure of radio emission scattered by coronal turbulence is investigated. Distant cosmic sources and Sun's own radio sources are considered. It is shown that observations of the energy spectrum of spacecraft radio signals and of the mean profile of a pulsar pulse in the Sun's caustic shadow zone can be used for coronal turbulence diagnostics. It is also shown that strong regular refraction plays an important role in formation of solar millisecond spike bursts and of type IIId solar decameter radio bursts with echo components.

On Solar Intermediate Drift Radio Bursts at Decimeter and Meter Wavelength

Fiber – or intermediate drift – bursts are a continuum fine structure in some complex solar radio events. We present the analysis of such bursts in the X17 flare on 28 Oct. 2003. Based on the whistler wave model of fiber bursts we derive the 3D magnetic field structures that carry the radio sources in different stages of the event and obtain insight into the energy release evolution in the main flare phase, the related paths of nonthermal particle propagation in the corona, and the involved magnetic field structures. Additionally, we test the whistler wave model of fiber bursts for the meter and the decimeter wave range. Radio spectral data (Astrophysikalisches Institut Potsdam, Astronomical Observatory Ondřejov) show a continuum with fibers for ≈ 6 min during the main flare phase. Radio imaging data (Nancay Radio Heliograph) yield source centroid positions of the fibers at three frequencies in the spectrometer band. We compare the radio positions with the potential coronal magnetic field extrapolated from SOHO/MDI data. Given the detected source site configuration and evolution, and the change of the fiber burst frequency range with time, we can also extract those coronal flux tubes where the high-frequency fiber bursts are situated even without decimeter imaging data. To this aim we use a kinetic simulation of whistler wave growth in sample flux tubes modeled by selected potential field lines and a barometric density model. The whistler wave model of fiber bursts accurately explains the observations on 28 Oct. 2003. A laterally extended system of low coronal loops is found to guide the whistler waves. It connects several neighboring active regions including the flaring AR 10486. For varying source sites the fiber bursts are emitted at the fundamental mode of the plasma frequency over the whole range (1200 – 300 MHz). The present event can be understood without assuming two different generation mechanisms for meter and decimeter wave fiber bursts. It gives new insight into particle acceleration and propagation in the low flare and post-CME corona.

Magnetic Field Strength in the Solar Corona from Type II Band Splitting

The phenomenon of band splitting in type II bursts can be a unique diagnostic for the magnetic field in the corona, which is, however, inevitably sensitive to the ambient density. We apply this diagnostic to the CME-flare event on 2004 August 18, for which we are able to locate the propagation of the type II burst and determine the ambient coronal electron density by other means. We measure the width of the band splitting on a dynamic spectrum of the bursts observed with the Green Bank Solar Radio Burst Spectrometer (GBSRBS), and convert it to the Alfven Mach number under the Rankine-Hugoniot relation. We then determine the Alfven speed and magnetic field strength using the coronal background density and shock speed measured with the MLSO/MK4 coronameter. In this way we find that the shock compression ratio is in the range of 1.5-1.6, the Alfvenic Mach number is 1.4-1.5, the Alfven speed is 550-400 km s-1, and finally the magnetic field strength decreases from 1.3 to 0.4 G while the shock passes from 1.6 to 2.1 R☉. The magnetic field strength derived from the type II spectrum is finally compared with the potential field source surface (PFSS) model for further evaluation of this diagnostic.

Shock drift electron acceleration and generation of waves

An analytically derived distribution function of reflected and accelerated electrons at a nearly perpendicular shock is presented. Then this distribution in a simplified form is introduced into a 1.5-D relativistic electromagnetic particle-in-cell (PIC) model and a generation of waves is studied. Numerical modeling shows not only a generation of Langmuir and high-frequency electromagnetic waves as expected, but also an efficient generation of whistler waves. Their role in emission processes of type II solar radio bursts is discussed.

Monitoring the Sun: A Global Space Weather Enterprise

Monitoring the Sun: A Global Space Weather Enterprise

Extraordinary-Mode Radiation Produced by Linear-Mode Conversion of Langmuir Waves

Linear-mode conversion (LMC) of Langmuir waves to radiation near the plasma frequency at density gradients is important for space and astrophysical phenomena. We study LMC in warm magnetized plasmas using numerical electron fluid simulations when the density gradient is parallel to the ambient magnetic field (B0). We demonstrate that LMC can produce extraordinary- (x-) as well as ordinary- (o-) mode radiation from Langmuir waves, contrary to earlier expectations of o mode only. Equal amounts of o- and x-mode radiation are produced in the unmagnetized limit. The x-mode efficiency decreases as B0 increases, while the o-mode efficiency oscillates due to interference between incoming and reflected Langmuir waves. Both x and o modes should be produced for typical coronal and interplanetary parameters, alleviating the depolarization problem for type III solar radio bursts.

Radio Frequency Interference Excision Using Spectral‐Domain Statistics

ABSTRACTA radio frequency interference (RFI) excision algorithm based on spectral kurtosis, a spectral variant of time‐domain kurtosis, is proposed and implemented in software. The algorithm works by providing a robust estimator for Gaussian noise that, when violated, indicates the presence of non‐Gaussian RFI. A theoretical formalism is used that unifies the well‐known time‐domain kurtosis estimator with past work related to spectral kurtosis, and leads naturally to a single expression encompassing both. The algorithm accumulates the first two powers of M power spectral density (PSD) estimates, obtained via Fourier transform, to form a spectral kurtosis (SK) estimator whose expected statistical variance is used to define an RFI detection threshold. The performance of the algorithm is theoretically evaluated for different time‐domain RFI characteristics and signal‐to‐noise ratios η. The theoretical performance of the algorithm for intermittent RFI (RFI present in R out of M PSD estimates) is evaluated and shown to depend greatly on the duty cycle, d = R/M. The algorithm is most effective for d = 1/(4 + η), but cannot distinguish RFI from Gaussian noise at any η when d = 0.5. The expected efficiency and robustness of the algorithm are tested using data from the newly designed FASR Subsystem Testbed radio interferometer operating at the Owens Valley Solar Array. The ability of the algorithm to discriminate RFI against the temporally and spectrally complex radio emission produced during solar radio bursts is demonstrated.

Big broadcast: Solar radio bursts put a new wrinkle in space weather

Big broadcast: Solar radio bursts put a new wrinkle in space weather

Statistical study of radio drifting pulsation structures with associated CMEs and other observations

Solar radio burst, especially the fine structures (FSs) and the drifting pulsation structures (DPSs), may be used as an important diagnostics tool to draw the evolution map of the flare loop in the initial phase of solar flares. In this work, 52 radio events were found accompanying with DPSs. They were all observed with the Solar Radio Spectrometers (0.625–7.6 GHz) of China during 1998–2004. Combining the radio observations with LASCO-C2, Goes-8 SXR, H α, EUV and Trace observations, we analyzed all these events and obtained some statistic conclusions: First, 88% DPSs take place at the initial phase of the radio burst, and their rich spectrum characteristics are helpful to understand the events further. Second, 83% DPSs are associated with CMEs or ejection events, and all the events are accompanied by Goes SXR flare. Third, for CMEs and DPSs, which take the first step, there is no significant predominance of either of them. The relationship between the DPSs and CMEs is still not clear in this study because of the lack of spatial resolution in the centimeter–decimeter band. However, the EIT or Trace ejection happened during the onset/end time of DPSs. They are signatures of the initial phase of CMEs. Two events will be illustrated to explain this.

Resonant transition radiation of solar radio bursts

We calculate the resonant transition radiation (RTR) using the most general formulas and obtain approximate analytical expressions describing the RTR dependence on the fast-particle velocity and magnetic-field intensity. Dependence of the RTR on the angle between the wave vector and the magnetic-field direction and also on the thermal velocity of background-plasma electrons is considered on the basis of approximate expressions for RTR, in which the longitudinal dielectric permittivity of the plasma is written neglecting the wave vector of transverse electromagnetic waves. The obtained dependences are combined into unified expressions for RTR coefficients. Comparison with the earlier found RTR coefficients is performed. It is shown that the obtained coefficients correspond better to the nature of the resonant transition radiation. Finally, the obtained RTR coefficients are used for interpretation of the radio emission of the solar burst of December 24, 1991.

Predictions of Intense Solar Activity Needed

Many of the sophisticated modeling efforts of the Sun-to-Earth system currently underway do not address some very important impacts of the Sun on terrestrial technologies. Thus, there is a serious need for more detailed models and improved predictions of intense solar activity. This gap in current capabilities of predicting the intensity of solar activity–especially the intensities of solar X-ray and radio bursts–was spectacularly illustrated in December 2006 during the passage of solar active region 0930 across the Sun. The solar radio burst at about 1900 UT on 6 December lasted for more than an hour, is reported to have been one of the largest (if not the largest) bursts ever recorded at gigahertz frequencies, and caused havoc for GPS measurements on Earth. Accompanying the solar burst was one of the largest increases in solar electron fluxes observed at the L1 ACE spacecraft in the past decade or more. If the active region at the time of the solar burst was more centered in the direction of the Earth, a coronal mass ejection with a resulting huge magnetic disturbance with resulting technology impacts might have been expected at Earth. As it was, the technologies largely affected were GPS receivers and other receiving systems such as radars oriented slightly towards the Sun (including possibly some cellular phone cell sites, a situation still under investigation by researchers). None of these effects are addressed by most current modeling of the Sun-Earth system. Just because the effects from this event were limited to certain technologies and did not generate a large magnetic storm (which is what most space weather modeling is currently directed toward) does not make this solar event less important for space weather researchers, modelers, and system operators. Rather, it clearly points out that considerably more research and understanding are needed to predict the types of solar activity that can produce so unexpectedly the intense electromagnetic emissions observed on 6 December. Like large magnetic storms, we now know first hand that these types of events can also affect important navigation and communications technologies at essentially the same moment (just the light travel time) when the solar event is first observed at Earth. Louis J. Lanzerotti is editor of Space Weather, a distinguished research professor at the New Jersey Institute of Technology, and a consultant at Alcatel-Lucent Technologies’ Bell Laboratories.

Fine Structure of Solar Radio Bursts Observed at Decametric and Hectometric Waves

The analysis of WIND/WAVES RAD2 spectra with fine structure in the form of different fibers in 14 events covering 1997 - 2005 is carried out. A splitting of broad bands of the interplanetary (IP) type II bursts into narrow band fibers of different duration is observed. The instantaneous-frequency bandwidth of fibers is stable: 200 - 300 kHz for slow-drifting fibers in type II bursts, and 700 - 1000 kHz for fast-drifting fibers in type II + IV (contin- uum). Intermediate drift bursts (IDB or fiber bursts) and zebra patterns with variable fre- quency drift of stripes, typical for the metric range, were not found. Comparison of spectra with the Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph (SOHO/LASCO C2) images shows a connection of the generation of the fiber structures with the passage of shock fronts through narrow jets in the wake of Coronal Mass Ejec- tions (CME). Therefore the most probable emission mechanism of fibers in IP type II bursts appears to be resonance transition radiation (RTR) of fast particles at the boundary of two media with different refractive indices. The same mechanism is also valid for striae in the type III bursts. Taking into account a high-density contrast in the CME wake and the actu- ally observed small-scale inhomogeneities, the effectiveness of the RTR mechanism in IP space must be considerably higher than in the meter or decimeter wavelengths. For the most part the fibers in the type IV continuum at frequencies of 14 - 8 MHz were seen as the direct expansion of similar fine structure (as fibers or "herringbone" structure) in the decametric range observed with the Nancay and IZMIRAN spectrographs.

Micro–Type III Radio Bursts

We present a detailed description of the features of solar radio bursts, which are elements of the so-called type III storms, using long-term observations made by the Geotail and Akebono satellites. Micro-type III bursts are characterized by short-lived, continuous, and weak emission. Their average power is estimated to be well below that of the largest type III bursts, by 6 orders of magnitude. When they occur, these bursts have a distribution of emitted power flux that is different from that of ordinary type III bursts, indicating that they are not just weaker versions of the ordinary bursts. Micro-type III burst activity is not accompanied by significant solar soft X-ray activity. We identify the active regions responsible for micro-type III bursts by examining the concurrence of their development and decay with the bursts. It is found that both micro and ordinary type III bursts can emanate from the same active region without interference, indicating the coexistence of independent electron acceleration processes. It is suggested that the active regions responsible for micro-type III bursts generally border on coronal holes.

Loss-Cone Instability and Formation of Zebra Patterns in Type IV Solar Radio Bursts

The loss-cone instability of energetic electrons at double plasma resonance is considered. Conditions required for the formation of a zebra pattern in type IV solar radio bursts are determined. It is shown that electrons with a power-law energetic spectrum can effectively excite upper-hybrid waves at double plasma resonance. Stripes of a zebra pattern become more pronounced with an increase of the loss-cone opening angle and the power-law spectral index. The growth rate at the resonance frequencies decreases with an increase of the cyclotron harmonic number. Interpretation of observations and diagnostics of plasma for the April 21, 2002, event are performed. Conclusions about the impulsive mode of injection of energetic electrons into a coronal arc are made.

Solar Radio Burst and Solar Wind Associations with Inferred Near‐Relativistic Electron Injections

The solar injections of near-relativistic (NR) electron events observed at 1 AU appear to be systematically delayed by ~10 minutes from the associated flare impulsive phases. We compare inferred injection times of 80 electron events observed by the 3DP electron detector on the Wind spacecraft with 40-800 MHz solar observations by the AIP radio telescope in Potsdam-Tremsdorf, Germany. Other than preceding type III bursts, we find no single radio signature characteristic of the inferred electron injection times. The injection delays from the preceding type III bursts do not correlate with the 1 AU solar wind βp or B, but do correlate with densities ne and inversely with speeds VSW, consistent with propagation effects. About half of the events are associated with metric or decametric-hectometric (dh) type II bursts, but most injections occur before or after those bursts. Electron events with long (≥2 hr) beaming times at 1 AU are preferentially associated with type II bursts, which supports the possibility of a class of shock-accelerated NR electron events.

Energy of solar noise-storm radio bursts

Solar noise storms (NS) are analyzed by an algorithm which separates a random signal into pulses. The burst duration distribution is shown to be inversely proportional to the squared duration of bursts. The distribution ordinates are proportional to the average pulse repetition frequency, and the distribution maximum corresponds to the limiting pulse duration equal to 0.4–0.6 s. The aggregate lifetime of all short-lasting bursts is approximately equal to the aggregate lifetime of bursts of any other duration. The energy of short-lasting bursts with a duration of 0.2–0.4 s is five times smaller than the energy of longer bursts, and it constitutes only 2–5 percent of the energy of the NS burst component. The power of bursts increases as their duration changes from 0.2 to 1.2 s until it reaches some limit at a duration of 1.2–1.4 s. The power of longer bursts remains almost unchanged up to the end of the investigated duration interval (up to durations of 300 s). Solar burst chains can be some superposition of short-lasting bursts on one longer burst. Thus, the burst energy measurements do not support the widespread point of view that solar noise storms consist of short-lasting type I bursts.