Slowly-varying Component Research Articles

Quasi-Periodic Pulsations in an M-Class Solar Flare

We have studied the quasi-periodic pulsations (QPPs) of the M2.3 flare that occurred in the active region NOAA 12172 on 23 September 2014. Through the fast Fourier transform (FFT) method, we decompose the flare light curves into fast- and slowly-varying components, and the cut-off threshold is 100 s. We find that the QPPs have a period of 40 s at soft X-ray (SXR), hard X-ray (HXR), radio and ultraviolet (UV). Based on the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO), we find that the QPPs take place at the same time interval as the flare ribbon separation, and that the QPPs seem to originate from the flare ribbons. Our observations tend to support the mechanism of the periodic nonthermal electron injection during the flare eruption.

One-Minute Quasi-Periodic Pulsations during an M-Class Solar Flare

We study the Quasi-Periodic Pulsations (QPPs) of an M4.4 class solar flare, which occurred in active region NOAA 11165 on 8 March 2011. With the Fast Fourier Transform (FFT) method, we decompose the flare light curve into fast- and slowly-varying components. The 100 s (0.01 Hz) is selected as the cutoff threshold between the fast- and slowly-varying components. One-minute QPPs are found around flare peak at soft X-ray (SXR) and Extreme Ultraviolet (EUV). Using the data from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO), the intermittent jets are detected and the interesting fact is that the jets also display one-minute period. The correlationship between the fast-varying components of SXR or EUV emissions and the jets suggests that the QPPs on light curves and periodic jets could come from the same origination, e.g., the periodic magnetic reconnection in this event.

PI Control Loop–Based Frequency Smoothing of a Doubly-Fed Induction Generator

This paper proposes a frequency-smoothing scheme of a doubly-fed induction generator (DFIG) that can significantly eliminate the frequency fluctuations caused by continuously fluctuating wind speeds. To achieve this, a proportional-integral (PI) control loop depending on the frequency deviation instead of a proportional control loop used in conventional schemes is added to the maximum power point tracking loop. The P control loop in the proposed frequency-smoothing scheme eliminates the alternating component in the frequency deviation, whereas the I control loop eliminates the slowly-varying component. The gains for the P control loop and I control loop are suggested for effectively using the rotating masses of a DFIG while avoiding over-deceleration of the rotor speed. The simulation results evidently demonstrate that the proposed scheme nearly removes the frequency deviation under various wind conditions, by using the proposed PI control loop, especially in high penetration levels of wind.

IMAGING AND SPECTRAL OBSERVATIONS OF QUASI-PERIODIC PULSATIONS IN A SOLAR FLARE

We explore the Quasi-Periodic Pulsations (QPPs) in a solar flare observed by Fermi Gamma-ray Burst Monitor (GBM), Solar Dynamics Observatory (SDO), Solar Terrestrial Relations Observatory (STEREO), and Interface Region Imaging Spectrograph (IRIS) on 2014 September 10. QPPs are identified as the regular and periodic peaks on the rapidly-varying components, which are the light curves after removing the slowly-varying components. The QPPs display only three peaks at the beginning on the hard X-ray (HXR) emissions, but ten peaks on the chromospheric and coronal line emissions, and more than seven peaks (each peak is corresponding to a type III burst on the dynamic spectra) at the radio emissions. An uniform quasi-period about 4 minutes are detected among them. AIA imaging observations exhibit that the 4-min QPPs originate from the flare ribbon, and tend to appear on the ribbon front. IRIS spectral observations show that each peak of the QPPs tends to a broad line width and a red Doppler velocity at C I, O IV, Si IV, and Fe XXI lines. Our findings indicate that the QPPs are produced by the non-thermal electrons which are accelerated by the induced quasi-periodic magnetic reconnections in this flare.

Multifractality and long memory of a financial index

In this paper we will try to assess the multifractality displayed by the high-frequency returns of Madrid’s Stock Exchange Ibex35 index. A Multifractal Detrended Fluctuation Analysis shows that this index has a wide singularity spectrum which is most likely caused by its long-memory. Our findings also show that this long-memory can be considered as the superposition of a high-frequency component–related to the daily cycles of arrival of information to the market–over a slowly-varying component that reverberates for long periods of time and which shows no apparent relation with human/economic cycles. This latter component is therefore postulated to be endogenous to market’s dynamics and to be also the most probable source of some of the stylized facts commonly associated with financial time-series.

Polarization Observation Experiments for Radio Flares of Stars at the 6 cm Band

We have performed radio polarization observation experiments of the stars V772 Her and β Per with the Urumqi 25 m radio telescope at the 6cm waveband, and obtained light curves of the stars after data processing and calibration. A radio flare from the star V772 Her was detected on 2011 April 13. The degree of linear polarization of this flare is about 30%, and the polarization angle is about 4°. We have detected also the slowly-varying component of the radio radiation from β Per, as well as a short flare superposed on it, which has a very weak linear polarization.

10.1007/s11444-008-3001-y

Photometry of the nucleus of the Seyfert 1 galaxy Ark 120 acquired in 1996–2005 with telescopes of the Crimean Astrophysical Observatory and the Sternberg Astronomical Institute’s Crimean Laboratory is used to construct and analyze the object’s U BV RI variability. The U BV RI variations have different timescales, and the variability amplitude grows towards shorter wavelengths. Changes in the color indices of the variable source appear to be due to variations in the opacity of gas at temperatures T > 15 000 K. The spectral energy distribution (SED) of the rapidly varying component is bluer than the SED of the slowly-varying component. The rapid variations are due to instabilities in the inner part of the accretion disk where, according to current ideas, the continuum is formed. Based on the peak of the cross-correlation function, the V RI variations lag the B variations by 1.4 to 5.6 days, with the lag increasing systematically from V to I. The lag calculated from the centroid of the cross-correlation function is from 2.8 days for V to 10.5 days for I. We demonstrate that the wavelength dependence of the lag is well approximated by the function τ ∝ λ4/3, characteristic of optically thick accretion disks.

The Seyfert 1 galaxy Ark 120 in 1996–2005: U BV RI photometry

Photometry of the nucleus of the Seyfert 1 galaxy Ark 120 acquired in 1996–2005 with telescopes of the Crimean Astrophysical Observatory and the Sternberg Astronomical Institute’s Crimean Laboratory is used to construct and analyze the object’s U BV RI variability. The U BV RI variations have different timescales, and the variability amplitude grows towards shorter wavelengths. Changes in the color indices of the variable source appear to be due to variations in the opacity of gas at temperatures T > 15 000 K. The spectral energy distribution (SED) of the rapidly varying component is bluer than the SED of the slowly-varying component. The rapid variations are due to instabilities in the inner part of the accretion disk where, according to current ideas, the continuum is formed. Based on the peak of the cross-correlation function, the V RI variations lag the B variations by 1.4 to 5.6 days, with the lag increasing systematically from V to I. The lag calculated from the centroid of the cross-correlation function is from 2.8 days for V to 10.5 days for I. We demonstrate that the wavelength dependence of the lag is well approximated by the function τ ∝ λ4/3, characteristic of optically thick accretion disks.

Correlation and asymmetry between solar flare hard X-ray footpoints: a statistical study

Aims. We study the correlation and asymmetry between solar flare hard X-ray footpoint time profiles that can be used to constrain the energy release and particle acceleration processes in solar flares. Methods. We perform a statistical analysis of correlations and asymmetries of hard X-ray footpoints for 29 solar flares that were observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in 2002–2005. Using the CLEAN method, we derive the hard X-ray images and then extract light curves for each footpoint. We decompose the hard X-ray light curves into slowlyvarying components and fast-varying components using the Fourier method and check their correlations separately. Results. Our analysis indicates that the asymmetry between hard X-ray footpoints is a ubiquitous feature in solar flares. However, in most asymmetric cases, the correlations between the light curves of the two footpoints remain high. In particular, the correlation coefficients between the slowly-varying components are significantly higher than those between the fast-varying components. We discuss the causes and implications of the asymmetries and correlations between hard X-ray footpoints.

S-COMPONENT SOURCES AT 21 cm WAVELENGTH IN THE RISING PHASE OF CYCLE 23

The Synthesis Radio Telescope at the Dominion Radio Astrophysical Observatory is being used to make a long-term study of the relationship between sources of the slowly-varying component at 21-cm wavelength and the active regions with which they are associated. Although the arc-minute angular resolution of the radio telescope is insufficient to map individual active regions, the unique capability of this radio telescope to map the whole solar disc in a single operation makes it useful for identifying and measuring sources of the slowly-varying component, which at this wavelength are largely due to free–free thermal emission from plasma concentrations trapped in active region magnetic fields. The brightness temperatures of the emission are higher in large, complex active regions, suggesting the plasma slab producing the emission is thicker and/or denser in such regions. We find also that the relationship between the brightness temperature and the size and complexity of the host region is a function of the phase of the 10–13-year solar activity cycle.

Sources of the Slowly-Varying Component of Solar Microwave Emission and their Relationship with their Host Active Regions

Daily surveys of the solar disc made at 2.8 cm wavelength over the period 1–13 November 1981, complemented by magnetograms and Hα filtergrams, are used to examine the relationship between sources of the slowly varying component of solar radio emission and properties of their host regions. Two classes of source are noted: diffuse and compact. Sources are designated compact when smaller than 40′′. The diffuse sources may be explained in terms of free – free thermal emission from trapped plasma in loops overlying the active region. The great majority of compact sources can be accounted for in terms of gyroresonance from thermal electrons in the strong magnetic fields overlying sunspots. A small minority are less amenable to this explanation. They are associated with magnetic complexity and dynamism, lie close to magnetic polarity reversals, and could be non-thermal. Microwave sources are an evolutionary feature common to all but the smallest active regions.

Identifying Slowly Varying and Turbulent Wind Features for Flight Loads Analyses

Abstract : A methodology developed to determine the spectral boundary between the slowly-varying components and the more rapidly varying, or turbulent components, of measured wind profiles is presented. Pairs of measured wind velocity versus altitude profiles from the Eastern and Western launch ranges of the United States were used to establish the vertical wavelengths which could no longer be considered slowly varying over discrete time intervals. Analyses were performed for wind pairs that were 30, 60, 90, and 120 minutes apart. The wavelength boundary between slowly-varying and turbulent wind features as a function of time interval is presented. The results of this work make it now possible to identify and extract the slowly-varying and/or turbulent wind features, as needed for a specific day-of-launch flight loads analysis, for these two launch facilities.

Imaging the Sun at 21 cm: Budgetting the S-component

On fourteen days in July and August 1992 and June 1993, we used the 7-element synthesis radio telescope at the Dominion Radio Astrophysical Observatory to make full-disc, arc-min resolution images of the Sun at 21 cm, with the objective of budgetting the contributions to the slowly-varying component of solar radio emission. This instrument has the advantage that the mapping field at this wavelength is about 2.5° wide. However, it has also the severe disadvantage that with only 12 hours to record each image, the brightness distribution is severely undersampled. This difficulty, along with solar rotation and declination motion during each observation, required development of special image correction procedures.

A model of SVC radiation in magnetic dipole field and radiation varying with magnetic field. I - Active region model and SVC radiation source model

In our model, the latest EUV data on the chromosphere-corona transition region havebeen used. We have assumed that the plasma conditions above an active region are varyingcontinuously from the centre of the region to the adjacent quiet region. We have taken thethree physical parameters of the plasma (electron temperature and density, and magneticfield) to be functions of both the height above the photosphere and the distance from thecentral axis of the active region. According to the combined mechanism including both the gyroresonance radiation and thebremsstrahlung, using a dipole magnetic field and above-mentioned model of the electrontemperature and density, we have calculated the radio radiation of the slowly varying compo-nent (SVC) and emphatically studied the radiative characteristics of the SVC radiation (suchas the spectra of flux density, polarization degree, and brightness temperaturc, and thebrightness distribution) and the geometrical features of the SVC radiation source (such asheight and radius), and their variations with the magnetic field. Some very interesting resultshave been obtained and they are considerably in agreement with the observations.

Solar Radio Observations at 843 MHz

AbstractThe Molonglo Observatory Synthesis Telescope (MOST) has been used to observe the Sun with total-power fan-beams having a one-dimensional resolution of 41 arcsec at 843 MHz. The scans reveal clearly the rotation and evolution of the slowly-varying component as well as some burst activity. Low radio brightness features have also been identified, but the exact relationship between these features and coronal holes is, as yet, unclear. Several partial synthesis observations have been used to generate two-dimensional radioheliograms.

A search for local sources of the S-component at decameter wavelengths

In an effort to find local sources of the Slowly Varying Component (SVC), an analysis is made of the episodical observations carried out since 1972 during periods of low solar activity at 20 and 25 MHz. In contrast to other writers who reported on successful observations of such sources (Kundu et al., 1977; Sastry et al., 1981, 1983), we have not found local sources, though we used the UTR-2 radio telescope to observe from several to several tens brightness distributions of the quiet Sun per day. The multiple daily measurements allowed tracing the dynamics of the burst development. As has been found, bursts of high intensity can give rise to nonthermal radiation from the region of generation, thus producing a considerable increase in the maximum brightness temperature of scans across the Sun. The relaxation time of this process is 20 to 30 min. It is not improbable that this is the effect responsible for large variations in the maximum brightness temperatures of the scans that Sastry et al. (1981) connected with the SVC. 25 MHz radioheliograms of April 29 and 30, 1976, are presented which supplement the data of observations at Clark Lake (Kundu et al., 1977). It is shown that the sources observed there on April 27 and April 29, 1976, were most likely of nonthermal nature. We conclude that at present, in spite of reports of some workers, there is no convincing evidence for the existence of local SVC sources at decameter wavelengths. Their reality could be confirmed or denied by more observations with better radio telescopes and a better account of the specific features of the decameter band.

Relaxation of the pulsed opto-galvanic current signal in a helium positive column

Transient behaviour of an opto-galvanic (OG) current signal induced by pulsed dye laser excitation has been investigated on the He I 388.9-nm (2 3S-3 3P) transition in a He positive column. The OG current signal is composed of a sharp spike at the early time stage followed by a slowly-varying component. The decay time of the sharp OG current signal has been found to be almost independent of the dc current substaining the discharge and to depend sensitivity on a product of pressure and column-radius. The experimental results show that the dominant decay process of the OG current consists in an ion current to the walls, as has been previously referred to in a mmodel of a cw OG effect in a He positive column. A model for the pulse OG effect in the early time stage is presented on the basis of the experimental results. The decay times of the OG current signals calculated with the model are in agreement with the measurements within an accuracy of 30%.

Observations on the slowly varying component of solar radio emission at decameter wavelengths

We have made two-dimensional maps of the slowly varying component (SVC) of solar radio emission at a frequency of 34.5 MHz with half power beam width of 26′/40′. It is found that a majority of SVC sources have brightness temperatures of the order of 0.5 × 106 K and half power widths of about 4 R⊙. The shifts in the positions of the centroids of the SVC sources from the center of the Sun were in the range 1.5 to 2 R⊙. These observations can be explained in terms of thermal emission from coronal regions of enhanced density and temperature located at distances of 1.5 to 2 R⊙ from the center of the Sun.

Exploration of the solar corona by high resolution solar decametric observations

In this review, current state of knowledge of high resolution observations at decameter wavelengths of the quiet Sun, the slowly varying component (SVC), type I to V bursts and noise storms is summarized. These observations have been interpreted to yield important physical parameters of the solar corona and the dynamical processes around 2R⊙ from the photosphere where transition from closed to open field lines takes places and the solar wind builds up. The decametric noise bursts have been classified into (i) ‘BF’ type bursts which show variation of intensity with frequency and time and (ii) decametric type III bursts. The angular sizes of noise storm sources taking into account refraction and scattering effects are discussed. An attempt has been made to give phenomenology of all the known varieties of decametric bursts in this review. Available polarization information of decametric continuum and bursts has been summarized. Recent simultaneous satellite and ground-based observations of decametric solar bursts show that their intensities are deeply modulated by scintillations in the Earth's ionosphere. Salient features of various models and theories of the metric and decametric noise storms proposed so far are examined and a more satisfactory model is suggested which explains the ‘BF’ type bursts as well as conventional noise storm bursts at decametric wavelengths invoking induced scattering process for 1 → t conversion. Some suggestions for further solar decametric studies from the ground-based and satellite-borne experiments have been made.

Measurements of the oscillatory and slowly-varying components of the solar velocity field

Spectroheliograms with high spatial resolution are presented to illustrate the decomposition of the solar velocity field into its oscillatory and slowly-varying components. An analysis of data obtained in the lines Feiλ 5434 and Feiiλ 4924 yield essentially the same principal results: (1) Spectroheliograms of the oscillatory component have a mottled appearance of rising and falling elements ranging from 2000 km to 3000 km in size. These elements oscillate vertically with a period in the range 275–300 s and an amplitude of 0.5 km/s. Although most oscillations last two cycles some have been observed for as many as four cycles. (2) Spectroheliograms of the slowly-varying component show a velocity granulation pattern whose spatial properties correspond closely to those of the photospheric granulation visible on direct photographs of the Sun. The velocity granules are approximately 1000 km in diameter and rise relative to their intergranular spaces with speeds that are typically 0.6 km/s, but which may occasionally be as large as 0.9 km/s. Most velocity granules seem to live for at least 10 min with many lasting 10–30 min, and a few of the biggest and fastest moving lasting 30 min to 1 hr.