Subsecond Pulses Research Articles

Bright ultraviolet knots as possible sources of coherent microwave radiation

A distinctive feature of the September 6, 2012 event was that sources of narrow-band (2–4 GHz) sub-second pulses (SSP) were observed in small areas of flare loops with so-called bright ultraviolet knots with high plasma density up to 1011 10¹¹ cm⁻³. Time profiles of hard X-rays of the flare, although similar to microwave light curves, do not have structures corresponding to SSP. Analysis of microwave, X-ray, and ultraviolet data has shown that the observable pulses of microwave radiation with a narrow spectral band are coherent in nature and are generated by electrons with energies of several tens of kiloelectronvolt in bright knots at a double plasma frequency. The results of the observations suggest that the appearance of bright knots is associated with local processes of energy release due to interaction of flare loops.

Яркие ультрафиолетовые узлы как возможные источники когерентного микроволнового излучения

A distinctive feature of the September 6, 2012 event was that sources of narrow-band (2–4 GHz) sub-second pulses (SSP) were observed in small areas of flare loops with so-called bright ultraviolet knots with high plasma density up to 10¹¹ cm⁻³. Time profiles of hard X-rays of the flare, although similar to microwave light curves, do not have structures corresponding to SSP. Analysis of microwave, X-ray, and ultraviolet data has shown that the observable pulses of microwave radiation with a narrow spectral band are coherent in nature and are generated by electrons with energies of several tens of kiloelectronvolt in bright knots at a double plasma frequency. The results of the observations suggest that the appearance of bright knots is associated with local processes of energy release due to interaction of flare loops.

A search for monochromatic light towards the Galactic Centre

ABSTRACT A region 140 square degrees towards the Galactic Centre was searched for monochromatic optical light, both pulses shorter than 1 s and continuous emission. A novel instrument was constructed that obtains optical spectra of every point within 6 square deg every second, able to distinguish lasers from astrophysical sources. The system consists of a modified Schmidt telescope, a wedge prism over the 0.28-m aperture, and a fast CMOS camera with 9500 × 6300 pixels. During 2021, a total of 34 800 exposures were obtained and analysed for monochromatic sources, both subsecond pulses and continuous in time. No monochromatic light was found. A benchmark laser with a 10-m aperture and located 100 light years (ly) away would be detected if it had a power more than ∼60 megawatt (MW) during 1 s, and from 1000 ly away, 6000 MW is required. This non-detection of optical lasers adds to previous optical SETI non-detections from more than 5000 nearby stars of all masses, from the Solar gravitational lens focal points of Alpha Centauri, and from all-sky searches for broadband optical pulses. These non-detections, along with those of broadband pulses, constitute a growing SETI desert in the optical domain.

Laser communication with Proxima and Alpha Centauri using the solar gravitational lens

ABSTRACT A search was conducted for laser signals, both sub-second pulses and continuous emission, from the regions of the sky opposite Proxima and Alpha Centauri. These regions are located at the foci of the gravitational lensing caused by the Sun, ideal for amplifying transmissions between our Solar system and those two nearest stellar neighbours. The search was conducted using two objective prism telescopes operating with exposure times of 0.25 s, enabling detection of sub-second laser pulses coming from the solar gravitational foci. During 6 months in 2020 and 2021, 88 000 exposures for Proxima Cen and 47 000 exposures for Alpha Cen were obtained. No evidence was detected of light pulses or continuous laser emission in the wavelength range of 380–950 nm. We would have detected a laser having a power of just 100 W, for a benchmark 1-m laser launcher that was diffraction-limited and located at the Sun’s gravitational focus 550 au away. To be detected, that beam must intercept Earth either by intention or, by accident, or if intended for a probe near Earth that is communicating with another one at the solar gravitational lens. These non-detections augment a previous non-detection of laser light coming directly from Proxima Centauri conducted with the HARPS spectrometer on the ESO 3.6-m telescope.

Revealing the surface and bulk regimes of isothermal graphene nucleation and growth on Ni with in situ kinetic measurements and modeling

In situ optical diagnostics are used to reveal the isothermal nucleation and growth kinetics of graphene on Ni across a wide temperature range (560°C<T<840°C) by chemical vapor deposition from single, sub-second pulses of acetylene. An abrupt, two-orders of magnitude change in growth times (∼100–1s) is revealed at T=680°C. Above this temperature, sigmoidal kinetics are measured and attributed to autocatalytic nucleation and growth from carbon dissolved in the bulk of the Ni film. However, for T<680°C fast surface nucleation and growth occurring during the gas pulse appears responsible for the drastic alteration of the kinetics of subsequent dissolution-mediated growth. A simple and general kinetic model for isothermal graphene growth is developed that includes the nucleation phase and the effects of carbon solubility in metals, describes delayed nucleation, and allows the interpretation of the competition between surface- and bulk-nucleation and growth. The easily-implemented optical reflectivity diagnostics and the simple kinetic model described here allow a pathway to optimize the growth of graphene on metals with arbitrary carbon solubility.

The subsecond pulses during the August 10, 2011 flare by observations of RATAN-600 and the 4–8 GHz Siberian solar spectropolarimeter

We present the results of a study of the subsecond pulses (SSPs) registered in the microwaves during the C2.4 solar flare. The event occurred on August 10, 2011, in the 11236 active region near the western limb and reached a maximum at 0935 UT. Subsecond pulses were registered by RATAN-600 for the first time when this flare reached its maximum. The fact of detection of SSPs was confirmed by simultaneous observations carried out with a spectropolarimeter at the Radio Astrophysical Observatory, Institute of Solar-Terrestrial Physics. A former analysis of the emission nature of the main flare source revealed the presence of a high-temperature (T > 30 MK) source that determined both the microwave and X-ray emission parameters of this event. We have compared the temporal variations of the SSPs microwave emission with the variations of the plasma parameters (temperature and emission measure) taken from observations with the FERMI space telescope with a subsecond time resolution. The obtained results suggest that the nature of SSP generation, as well as of the flare itself, was determined by the high-temperature source.

Formation of pulses with controlled parameters in a resonance microwave compressor employing oscillation-mode transformation

Results of an experimental study of the formation of rf pulses with controlled power, duration, repetition rate, and envelope parameters in a resonance compressor employing oscillation-mode transformation are presented. The pulse parameters are varied by using tuned intermodal coupling elements. A possibility of the formation of a series of subsecond and nanosecond rf pulses in the partial extraction of energy and nanosecond pulses of various durations in single complete extraction of energy from the compressor resonator is shown.

Ultraviolet-B-mediated induction of protein–protein interactions in mammalian cells

Light-sensitive proteins are useful tools to control protein localization, activation and gene expression, but are currently limited to excitation with red or blue light. Here we report a novel optogenetic system based on the ultraviolet-B-dependent interaction of the Arabidopsis ultraviolet-B photoreceptor UVR8 with COP1 that can be performed in visible light background. We use this system to induce nuclear accumulation of cytoplasmic green fluorescent protein fused to UVR8 in cells expressing nuclear COP1, and to recruit a nucleoplasmic red fluorescent protein fused to COP1 to chromatin in cells expressing UVR8-H2B. We also show that ultraviolet-B-dependent interactions between DNA-binding and transcription activation domains result in a linear induction of gene expression. The UVR8-COP1 interactions in mammalian cells can be induced using subsecond pulses of ultraviolet-B light and last several hours. As UVR8 photoperception is based on intrinsic tryptophan residues, these interactions do not depend on the addition of an exogenous chromophore.

Thermal tolerance of E. faecalis to pulsed heating in the millisecond range

Our aim was to evaluate thermal damage to endodontic pathogen Enterococcus faecalis (E. faecalis) caused by sub-second laser-generated heat pulses by determining the parameters for the thermal damage survival curve (TDSC). A novel experimental method for thermal pulsing of bacteria in the millisecond range was developed. After cultivation, E. faecalis was inoculated on anodized aluminum substrate and heated with a pulsed Nd:YAG laser. Viability was assessed with both plate count and flow cytometry methods. An E. faecalis TDSC for single-pulse millisecond range heating times was derived from the Arrhenius equation. Results gained from single-pulse heating viability measurements were used to predict the bactericidal effect of multiple sequential pulses (pulse train), and compared to experimental measurements. The thermal damage model was then applied to determine the relationship between laser fluence, pulse width, and the viability decrease of E. faecalis in a simulated root canal disinfection procedure. The application of the model to calculate the required lethal laser fluence levels on dentin during endodontic laser procedures seems to indicate that for endodontic procedures, the sub-millisecond pulsed Nd:YAG lasers are more effective in comparison with continuous-mode diode lasers and will cause less undesirable bulk heating of the tooth and surrounding tissues. The results of the study can be applied to create a model for predicting the impact of sub-second temperature increase on viability of bacteria on various surfaces and calculate required fluences and pulse widths to achieve the aforementioned effects with laser pulses.

Apparent angular sizes of the sources of microwave subsecond pulses and electron-density fluctuations in the lower solar corona

Data on the visible angular sizes of sources of microwave subsecond pulses (MSPs) obtained using the Siberian Solar Radio Telescope are analyzed assuming a dominant role for scattering on small-scale electron-density inhomogeneities in the solar corona. The observed dependence of the angular sizes of MSPs on the distance from the solar-disk center confirms that the MSP sources are localized in low layers of the solar corona. Both absolute and fractional levels of small-scale electron-density fluctuations have been estimated. These estimates suggest that flicker-noise-type turbulence power spectra are formed in the lower corona, and are preserved in the solar-wind acceleration region. A composite dependence of the scattering angle of a sounding radio wave on distance from the Sun is presented.

The signatures of magnetic reconnection according to the microwave drifting bursts observations

A series of drifting microwave bursts during the 30 March 2001 flare are analyzed using the Siberian Solar radiotelescope (SSRT) images at 5.7 GHz and dynamic spectra obtained simultaneously by the spectropolarimeters of National Astronomic Observatories in China (NAOC) in the range 5.2–7.6 GHz. While observing the event with the SSRT, the burst sources were simultaneously recorded at two frequencies, which allowed their relative spatial shifts to be measured and source velocity along the flare loop (observed in soft X-ray and ultraviolet emission) to be evaluated. Estimates were made of the plasma density gradient along the source movement direction, the plasma emission being assumed to be generated at the second harmonic. Drifting burst series occur during transient hard X-ray brightenings. Burst drift rates ranged from 10 to 20 GHz/s, with a mean value of about 6 GHz/s. The shape of the drift rate distribution around the mean value is nearly symmetric. It is suggested that the mean value distribution may be related to increased plasma density in the source of subsecond pulses. In particular, the corresponding density variations may be associated with magnetic reconnection processes.

Spectral and temporal properties of narrowband dm-spikes and broadband pulses in the August 5, 2003 flare

On August 5, 2003 the Ondřejov radiospectrograph and the Brazilian Solar Spectrograph recorded simultaneously the narrowband dm-spikes superimposed on broadband pulses in the frequency ranges of 0.8–4.5 and 1.75–2.25 GHz, respectively. Using a new method of wavelet filtering broadband sub-second pulses with a frequency width of 0.48 GHz and narrowband millisecond spikes with a frequency width 0.13 GHz were recognized and analysed in detail. Filtered radio spectra showed that the spikes were clustered in stripes at different frequencies. These stripes drifted and their frequency ratios changed during short time intervals. Periods of the narrowband spikes and their stripes were ∼0.4 s and 4, 8–10, and 16 s, respectively. The main period of the broadband pulses was ∼4 s. Values of significant periods of the narrowband spikes coincided with those of the broadband pulses. We found significant peak-to-peak correlations with zero time lags among stripes of the narrowband spikes on different frequencies. The characteristics of the narrowband dm-spikes and broadband pulses indicate that mutually linked emission processes generate both fine structures.

On solar 5.7 GHz subsecond burst source sizes

Due to the narrow bandwidth and the small size of the subsecond microwave pulses, we can use them as probe sources for study of propagation effects in the low corona. More than 160 microwave bursts with subsecond pulses (SSP) have been observed with the Siberian Solar Radio Telescope at 5.7 CHz for the period 2000–2004. Working with a large dataset of homogeneous observational material (spatial resolution from 15″ to 20″, temporal resolution 14 mc), we estimated sizes of SSP and studied relation between SSP sizes and sense of the polarization and their position on the Sun. Our results are in accordance with those obtained during the 22nd solar cycle. The apparent sizes of SSP increase toward the solar limb. The obtained dependence is in agreement with Bastian‘s (Bastian, T.S. Angular scattering of solar radio emission by coronal turbulence. ApJ 426, 774, 1994.) model calculations. The center-to-limb variation of the source size is explained by scattering on plasma turbulence along the ray path in the solar corona. The most events with high polarization occur near the central meridian (±30°). The polarization sense corresponds mainly to the ordinary mode.

Pulsed Particle Injection in a Reconnection‐Driven Dynamic Trap Model in Solar Flares

The time structure of hard X-ray emission during solar flares shows subsecond pulses, which have an energy-dependent timing that is consistent with electron time-of-flight delays. The inferred time-of-flight distances imply an injection height about 50% above the soft X-ray-bright flare loops, where electrons are injected in a synchronized way. No physical injection mechanism is known that can account for the energy synchronization and duration of subsecond pulses. Here we propose a model in terms of dynamic loss cone angle evolution of newly reconnected magnetic field lines that relax from the cusp at the reconnection point into a force-free configuration, which can explain the subsecond pulse structure of injected particles as well as the observed correlation between the pulse duration and flare loop size. This quantitative model predicts that the pulse duration of hard X-ray or radio pulses scales with tw ≈ 2LB/vA (s), which is the Alfvénic transit time through the magnetic outflow region of a Petschek-type X-point with magnetic length scale LB, and thus provides a direct diagnostic of the magnetic reconnection geometry. It also demonstrates that the observed pulse durations are primarily controlled by the injection time rather than by the acceleration timescale.

Observations of microwave bursts with different types of fine structure using data with high spatial and spectral resolution

The observations of the bursts with fine structures are one of the best remote sensing probe of the primary energy release sites in solar flares. The Siberian Solar Radiotelescope (SSRT) gives us the unique possibility of direct measurements of their positions with temporal resolution up to 14 ms and spatial resolution of a few arcsec. National Astronomical Observatories of China (NAOC) spectropolarimeters (5.2–7.7 GHz) provide us with the simultaneous spectral data around the SSRT receiving frequency (5.7 GHz). An analysis is made of the subsecond pulses observations: short wide range pulses, U-type microwave bursts, the bursts with zebra pattern.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

On the wave mode of subsecond pulses in the cm-range

In this paper we determined the wave mode of subsecond pulses (SSP). We used the data on pulses with the degree of polarization over 30%, with the sources located at −60 to +60 deg from the central meridian, for the period 2000 - 2002. The superposition of SSRT radio maps and MDI magnetograms has shown that radio SSP sources are typically located near the polarity inversion line of the active region magnetic field. Such an arrangement indicates that SSP sources are located at the tops of magnetic loops. The ordinary mode of electromagnetic radiation is recorded in SSP sources located from the inversion line by no less than about 10 arcsec.

A New Solar Burst Spectral Component Emitting Only in the Terahertz Range

Solar flare energy manifestations were believed to be the result of the same kind of particle acceleration. It is generally accepted that a population of relativistic electrons accelerated during the impulsive phase of solar flares produces microwaves by synchrotron losses in the solar magnetic field and X-rays by collisions in denser regions of the solar atmosphere. We report the discovery of a new intense solar flare spectral radiation component, peaking somewhere in the shorter submillimeter to far-infrared range, identified during the 2003 November 4 large flare. The new solar submillimeter telescope, designed to extend the frequency range to above 100 GHz, detected this new component with increasing fluxes between 212 and 405 GHz. It appears along with, but is separated from, the well-known gyrosynchrotron emission component seen at microwave frequencies. The novel emission component had three major peaks with time, originated in a compact source whose position remained remarkably steady within 15''. Intense subsecond pulses are superposed with excess fluxes also increasing with frequency and amplitude increasing with the pulse repetition rate. The origin of the terahertz emission component during the flare impulsive phase is not known. It might be representative of emission due to electrons with energies considerably larger than the energies assumed to explain emission at microwaves. This component can attain considerably larger intensities in the far-infrared, with a spectrum extending to the white-light emission observed for that flare.

The microwave subsecond pulse of September 17, 2001: The spectrum, location and size of the source

We examine simultaneous observations of microwave subsecond pulses with high temporal, spatial and spectral resolution from the Siberian Solar Radio Telescope (5.7 GHz, 14 ms resolution) and from the spectropolarimeters (5.2-7.6 GHz, 6 ms) of the National Astronomical Observatories. The September 17, 2001 flare is discussed in detail. The subsecond pulse (SSP) was observed in the initial phase of the flare, and its HWFH duration was 40 ms. The pulse was accompanied by a rapid change of the dynamic spectrum whose width was about 1%, and the degree of polarization made up 35%. The time profile at the SSRT frequency depends substantially on the SSP's spectral features. The SSP was observed by both the NS (North-South) and EW (East-West) arms (in two interference orders in the latter case). SSP sources locations in burst structures were determined. We also find that the source was not a point-like one, but its apparent size was about the beam width (15 arcsec) for the NS scanning direction. In the EW direction the SSP was less than 10 arcsec in size.

On the origin of subsecond pulses at 17 GHz

We have analyzed three flare events with subsecond structures in hard X-rays (CGRO/ BATSE) and 17 GHz data (Nobeyama radioheliograph). It was shown that microwave subsecond brightenings (SSB) were generated by directly precipitating electrons with energy of 100–200 keV from tiny regions close to footpoints. In two events, when high correlation between microwaves and X-rays was observed, the SSB can be interpreted in terms of gyrosynchrotron emission. Plasma emission seems to be a more credible explanation of the spontaneous pulses in the event when poor correlation with X-rays was observed.

The limb flare of November 2, 1992: Physical conditions and scenario

The powerful X9 class flare was investigated using interferometric data of Nobeyama Radioheliograph (17 GHz) and SSRT (5.7 GHz). According to images obtained in circular polarization, it was found that the microwave burst was generated consequently in the sites, where opposite magnetic loops were in the close contact. Steady exponential growth of the total microwave flux in the initial stage of the flare under significant reconstruction of the radio sources certifies that the development of the flare was conducted by some large-scale instability. Radiation during the impulsive phase was produced by gyrosynchrotron emission of electrons trapped within a wide dome-like volume with G and cm-3 . In the decay phase, the emission at higher frequencies was mainly produced by bremsstrahlung from high loops emitting soft X-rays. There is evidence of the energy release long after the impulsive phase: an intensive, rather compact microwave source in the locus of contact of two oppositely polarized regions in these loops and sub-bursts during 2 hours after the peak at low heights. For the first time, a compact source of the sub-second pulses at 17 GHz was found to be located as high as km. They were due to gyrosynchrotron emission.