Harmonic oscillations of the X-ray emission of a solar flare

An analysis is presented for the class-M9.3 solar flare of November 6, 2004, whose decay phase displayed weakly damped harmonic oscillations of the predominantly thermal X-ray flux detected by the RHESSI spacecraft (at energies ≲25 keV). The period of these oscillations was ≈78 s, and their characteristic decay time ≈100 min. Similar quasi-periodic pulsations were observed in the decimeter-centimeter radio flux (pulsations of a type-IV radio outburst), but were less pronounced in the non-thermal hard X-ray flux (≳25 keV). The area of the quasi-stationary X-ray source, which was located primarily at the apex of a set of flare loops (≲15 keV) that were cooled primarily via thermal conduction, was found to be in anti-phase with the oscillating X-ray flux it emitted. The observed oscillations are interpreted as harmonic modulations of the radiation flux emitted by the heated thermal flare-loop plasma, due to the global, standing, sausage mode of fast magnetoacoustic waves excited in the loop.     

Spectral-temporal evolution of low-frequency pulsations in the microwave radiation of solar flares

Low-frequency pulsations of 22 and 37 GHz microwave radiation detected during solar flares are analyzed. Several microwave bursts observed at the Metsähovi Radio Observatory are studied with time resolutions of 100 and 50 ms. A fast Fourier transformation with a sliding window and the Wigner-Ville method are used to obtain frequency-time diagrams for the low-frequency pulsations, which are interpreted as natural oscillations of coronal magnetic loops; the dynamical spectra of the pulsations are synthesized for the first time. Three types of low-frequency fluctuations modulating the flare microwave radiation can be distinguished in the observations. First, there are fast and slow magneto-acoustic oscillations with periods of 0.5–0.8 s and 200–280 s, respectively. The fast magneto-acoustic oscillations appear as trains of narrow-band signals with durations of 100–200 s, a positive frequency drift dν/dt=0.25 MHz/min, and frequency splitting δν=0.01–0.05 Hz. Second, there are natural oscillations of the coronal magnetic loops as equivalent electrical circuits. These oscillations have periods of 0.5–10 s and positive or negative frequency drift rates dν/dt=8×10^?3 Hz/min or dν/dt=?1.3×10^?2 Hz/min, depending on the phase of the radio outburst. Third, there are modulations of the microwave radiation by short periodic pulses with a period of 20 s. The dynamical spectra of the low-frequency pulsations supply important information about the parameters of the magnetic loops: the ratio of the loop radius to its length r/L≈0.1, the plasma parameter β≈10^?3, the ratio of the plasma densities outside and inside the loop ρ_e/ρ_i≈10^?2, and the electrical current flowing along the loop I≈10¹² A.     

Flare-plasma diagnostics from millisecond pulsations of the solar radio emission

Two solar radio bursts exhibiting narrow-band millisecond pulsations in intensity and polarization are analyzed. There were considerable time delays between the left-and right-circularly polarized components of the radio emission. The observed oscillations of the degree of polarization are due to the different group velocities of the ordinary and extraordinary modes in their propagation from the source to the observer; the frequency dependence of the delay is in excellent agreement with the theoretically calculated group delay in a magnetoactive plasma. It unambiguously follows that the pulsed radio emission is generated near the double upper hybrid frequency by the nonlinear plasma mechanism, since the source emission has a low degree of polarization. In addition to dispersion effects, a Fourier analysis also reveals effects associated with the source inhomogeneity. We detected a frequency drift of pulsations (autodelays) with different signs for different polarization components. This drift suggests that, apart from the dispersion effects, there are also the effects related to inhomogeneity of the radio source. It is shown, in particular, that the upper hybrid modes (generating the radio emission) are unstable in regions with enhanced gradients of the plasma density and/or magnetic field.     

Parametric excitation of acoustic oscillations in closed coronal magnetic loops

Quasi-periodic modulations of the microwave emission from solar outbursts at 37 GHz are studied based on 17 events observed in 1989–2000 at the Metsähovi Observatory. Low-frequency modulations with periods of ～5 min were found in approximately 90% of the observed microwave outbursts. The most likely origin of this modulation is modulation of the current flowing along a closed coronal magnetic loop due to the five-minute oscillations of the photospheric-convection velocity. In approximately 70% of the cases, oscillations with periods ～10 min were observed simultaneously with the five-minute oscillations in the same events. In 30% of the cases, simultaneous modulation of the microwave emission by three low-frequency signals with periods of 3, 5, and 10 min was observed. One possible origin of these “double” and “triple” modulations is parametric excitation of acoustic oscillations with periods of 10 and 3 min in a closed coronal magnetic loop as a result of coupling with the five-minute photospheric oscillations. This can occur when the period of the natural acoustic oscillations of the closed magnetic loop is about 10 min (the resonance condition). Since the ten-minute oscillations are excited more easily than the three-minute oscillations at the parametric instability, the latter are observed less frequently. For the same reason, the observed linewidth of the ten-minute oscillations is considerably greater than that of the three-minute oscillations.     

Pulsations of microwave flaring emission at low and high frequencies

A wavelet analysis of the flare-intensity variations has been carried out for a solar flare observed with the Nobeyama Radioheliograph at 17 and 34 GHz and the spectrometer of Purple Mountain Observatory at 4.5–7.5 GHz. The signals contain a well-pronounced periodicity with a period of P = 14–17 s and stable, coherent behavior at frequencies higher and lower than the peak frequency. We simulated the modulation of the gyrosynchrotron emission by fast sausage magnetoacoustic oscillations for the cases of low and high plasma density in the radio source. The synchronism of the pulsations at high and low frequencies can be realized only in the case of high plasma density, when the low-frequency turnover of the microwave spectrum is due to the Razin effect, not self-absorption. Published in Russian in Astronomicheskiĭ Zhurnal, 2007, Vol. 84, No. 7, pp. 655–664. The article was translated by the authors.     

H<Subscript>2</Subscript>O maser emission in the direction of Sagittarius B2: Results of monitoring for 1982–1992

The results of monitoring the water-vapor maser at λ=1.35 cm in Sgr B2 are presented. The observations were carried out on the 22-m radio telescope of the Pushchino Radio Astronomy Observatory (Russia) in 1982–1992. A strong flare of the maser radiation associated with Sgr B2(N) was detected in this period. The absolute strength of this flare is comparable to the megamaser emission observed in Orion in 1979–1987. The flare is probably due to a strengthening of the flow of material from the rotating accretion disk, in which are embedded the three ultracompact HII regions K1, K2, and K3. A subsequent excitation of emission features at increasingly higher radial velocities was observed, associated with a gradient of V_LSR along the direction of the outflow. The large width of the lines (>0.86 km/s) could reflect a complex structure for the maser spots, such as a chain or filamentary structure, as has been observed in Orion and S140.     

Characteristics of Pi2 pulsations at an equatorial station and its occurrence association with the phase of the Moon

Diurnal variations of occurrence hours and the period of Pi2 pulsations at Choutuppal (India) for nearly half a solar cycle are presented. Maximum occurrence is noticed to be around local midnight in all the seasons. Shorter periods are observed in the late afternoon hours in E-and J-seasons. Lunar influence on the occurrence of Pi2 pulsations at this station is inferred with the occurrence due to this influence peaking in the vicinity of the lunar phase4.     

Analysis of the fine temporal structure of optical flares on AD Leo on February 4, 2003

The fine temporal structure of two flares observed on the red-dwarf flare star AD Leo on February 4, 2003 with the 1.25-m telescope of the Crimean Astrophysical Observatory in a rapidphotometry mode is studied. One flare lasted approximately 5 min and another was longer than 8 min. The amplitudes in the U band were 1.65 ^m and 1.76 ^m . A detailed color analysis shows that the flare parameters at the maximum brightnesses corresponded to blackbody radiation with temperatures of approximately 14 000 and 13 000 K, enabling the monitoring of temperature — the rapid cooling of flare plasma near the flare maxima—for the first time. During 1.5 and 3.5 min at the maxima, the flares radiated as blackbodies, but these behaved as optically thick plasmas in the Balmer continuum on the second half of the descending branches. At the end of the first flare, the plasma became optically thin in the Balmer continuum; the final stage of the second flare was not observed. The flare areas at the luminosity maxima were 2.1 × 10¹⁸ and 3.0×10¹⁸ cm² in a blackbody approximation, or 0.07%and 0.11%of the visible stellar disk. The occurrence of quasi-periodic brightness pulsations during red-dwarf flares on time scales of approximately ten seconds is confirmed.     

The symbiotic system Z Andromedae during the flare of 2000–2002

Optical and infrared photometric observations of the classical symbiotic binary Z And obtained in quiescent and active phases in 1999–2002 are analyzed. RI photometric data from the literature are used to estimate the parameters of the binary system’s components. The data as a whole suggest that the hot compact component expands and the emission measure of the surrounding nebula grows during the flare. Possible interpretations of the flare based on several mechanisms are considered.     

Evolution of the H<Subscript>2</Subscript>O and OH Maser Emission in W75 N

The results of a study of H₂O and OH maser emission in the complex region of active star formation W75 N are presented. Observations were obtained using the 22-m radio telescope of the Pushchino Radio Astronomy Observatory (Russia) and the Nan3ay radio telescope (France). Flaring H₂O maser features may be identified with maser spots associated with the sources VLA 1 and VLA 2. Themain H₂O flares occurred in VLA 1. The flare emission was associated with either maser clusters having closely spaced radial velocities and sizes up to ~2 AU or individual features. The maser emission is generated in a medium where turbulence on various scales is present. Analysis of the line shapes during flare maxima does not indicate the presence of the simplest structures—homogeneous maser condensations. Strong variability of the OH maser emission was observed. Zeeman splitting of the 1665-MHz line was detected for several features of the same cluster at a radial velocity of +5.5 km/s. The mean line-of-sight magnetic field in this cluster is ~0.5 mG, directed away from the observer. Flares of the OH masers may be due to gas compression at a shock or MHD wave front.     

Studies of the atmospheric structure of the solar white-light flare of August 9, 2011

Semi-empirical models for three kernels emitting in the continuum during the pre-impulsive and impulsive phases of the white-light flare of August 9, 2011 have been calculated, based on observations of the continuum brightness near 6579 Å, Hα profiles, and photospheric iron lines. These computations show that, in order to achieve agreement between the computed and observed profiles and the contrast of the continuum emission of the impulsive kernels of the white-light flare, the temperature must be increased in both the lower chromosphere and the upper photosphere. The most efficient heating is located deeper in the photosphere in the pre-impulsive than in the impulsive phase, and chromospheric heating is negligible in the pre-impulsive phase. Spectral data and the results of model computations indicate that it is difficult to explain the emission of the white-light flare kernels as the effect of heating by energy transported from the corona into lower-lying, deep layers of the atmosphere by canonical transport mechanisms.     

Common features in the development of powerful long-duration solar X-ray flares

We have begun an investigation of the possible origins of considerable of powerful solar flares. This effect is manifest, first and foremost, in the existence of high-temperature plasma in flare loops over many hours. Analysis of the soft X-ray emission in two energy bands detected by the GOES satellites for about 20 powerful solar flares reveals long time intervals during the decay phase when the source temperature decreases, in general, exponentially. The characteristic time t _i for a decrease in the temperature by a factor of ten is 3–10 hours for most powerful events. In addition, another interval of very slow decrease with a characteristic time t _i of tens of hours can be identified in some cases. We found a gradual change in the dependence of the temperature on the square root of the emission measure for the source as a whole, which characterizes the transition from purely coronal processes to powerful flares with a prolonged inflow of plasma from the chromosphere. Modeling the energy balance in a loop can yield the requirements for the source of plasma heating in a long-lived arch system. A necessary condition for the development of prolonged flares seems to be a powerful coronal mass ejection, which initiates the formation of a source of plasma heating at coronal heights. Our analysis shows that a considerable fraction of the energy is often released in the region of the cusp, and that systems of giant coronal arches rising to heights of about 100 000 km above the limb are formed in most prolonged events (called dynamical flares in the terminology of Svestka).     

Diagnostic of a Solar Flare via Analyses of Emission in Spectral Lines of Highly Ionized Iron

An analysis of the dynamics of the electron temperature of the solar atmosphere in regions where solar flares appear is presented. The temperatures are estimated from the emission in spectral lines of ions with various degrees of ionization. The emission of ionized helium and highly ionized iron was used. Images of preflare states and of flares from the archive of the American SDO spacecraft are analyzed. A solar flare is usually preceded by the registration of a bright glowing structure above the action region, with a temperature exceeding that of the corona. This preflare structure (~10¹⁰ cm) is identified with the development of a system of currents, which, according to numerical simulations, is responsible for the accumulation of energy above the active region before the flare. After several tens of hours of a slow increase in the brightness of the preflare glow in the 94 Å iron (FeXVIII) line, the emission in the 193 Å line of FeXXIV increases sharply, indicating a flare-like growth of the temperature up to at least 20 MK. This growth of the emission coincides with the onset of the solar flare. The observed dynamics of the emission in spectral lines of highly ionized ions is consistent with an electrodynamic model of a solar flare based on the accumulation of magnetic energy in a current sheet above the active region and the explosive release of the stored energy. Studies of mechanisms for solar flares are of special importance in connection with the discovery of solar cosmic rays. Information from the worldwide network of neutron monitors and from the GOES spacecraft has made it possible to firmly state that the source of solar rays is solar flares, not shocks generated by such flares. It cannot be ruled out that a similar mechanism, not shocks, is also responsible for the acceleration of cosmic rays in the Galaxy.     

Regions of primary energy release of solar flares associated with singularities in the magnetic field

The locations of sites of primary energy release of solar flares are studied. Magnetic singularities revealed earlier—self-intersections (reconnections) of F = 0 surfaces, where F is a differential factor determining the structural singularity in a potential magnetic field—are considered as possible sites of energy release. Six flare events demonstrating paired sources of non-thermal hard X-rays emission observed on March 17, 2002, July 17, 2002, April 6, 2004, November 4, 2004, November 6, 2004, and December 1, 2004 are analyzed for probable singularities. In each event analyzed, each source of non-thermal hard X-rays emission can be associated with an individual magnetic singularity; in other words, there is a magnetic-field line passing near the singularity and ending near (i.e. within about 10″) the source located on the photosphere (in the chromosphere). For the homologous flares observed on November 4 and 6, 2004, the same magnetic singularity is responsible for the source of non-thermal hard X-rays emission observed in the eastern sector of the flare region on November 4 and the source observed in the western part on November 6. A proposed interpretation associates these observations with a reversal of the electric field generated in the magnetic singularity on November 6, compared with the electric field generated on November 4, attributed to corresponding changes occurring in the photospheric magnetic field.     

Variability of Hell and hydrogen line profiles in the spectrum of HD 93521

We have studied the variability of the Hell λλ 4686 and 5411 Å Hβ, and Hα lines in the spectrum of the pulsating star HD 93521. All these line profiles display the same variability pattern relative to the average profiles: a sinusoidal wave that moves systematically from the short-to the long-wavelength wing of the profile. This variability is due to non-radial pulsations. To study the pulsation movements and stratification of the radial velocity in the atmosphere of HD 93521, we analyzed the variability of the radial velocities measured individually for the blue and red halves of the absorption profile at the half-level of the line intensity. The periods and amplitudes of this radial-velocity variability are different for different lines and are well correlated with their central depths. In the transition from weak to strong lines (i.e., from lower to upper layers of the atmosphere), the period of the radial-velocity variations measured using both halves of the absorption profile increases, while its amplitude decreases. When the morphology and variability of photospheric and wind-driven lines are compared, it is clear that the variability of their absorption components is due to the same process—non-radial pulsations. In this way, the non-radial pulsations partly affect the variability of the stellar wind. The effect of the stellar wind on the profiles of strong lines is observed as a variable absorption feature that moves along the profiles synchronous with the axial rotation of the star.     

A strong flare of the H<Subscript>2</Subscript>O maser in Sagittarius B2(M)

Results of a study of a strong flare of H₂O maser emission in the star-forming region Sgr B2(M) in 2004 are reported. The observations were carried out on the 22-m radio telescope of the Pushchino Radio Astronomy Observatory. The main emission, with its flux density reaching 3800 Jy, was concentrated in a narrow radial-velocity interval (about 3 km/s) and was most likely associated with the compact group r, while the emission at V_LSR > 64 km/s came from group q. After 1994, the variations of the H₂O maser emission in Sgr B2(M) became cyclic with a mean period of 3 years.     

A model of a solar flare: Comparisons with observations of high-energy processes

New data for the energy and location of the hard-emission centers of a solar flare agree with an electrodynamic model of a solar flare based on the idea of the accumulation of free magnetic energy in the field of a current sheet. Three-dimensional MHD simulations are used to show that the energy stored in the preflare magnetic field of the current sheet is sufficient for the development of a flare and a coronal mass ejection. The flare and coronal mass ejection result from the explosive decay of the current sheet. The position of the brightness-temperature maximum of the radio emission during the flare coincides with the maximum of the current in the current sheet. The exponential spectrum of relativistic protons generated during the flare is consistent with acceleration by the electric field during the current-sheet decay.     

Superflares of H<Subscript>2</Subscript>O Maser Emission Toward the Protostellar Object G25.65+1.05 (IRAS 18316−0602)

The results of a study of the maser source IRAS 18316?0602 in the H₂O line at λ = 1.35 cm are reported. The observations were carried out on the 22-m radio telescope of the Pushchino Radio Astronomy Observatory (Russia) from June 2002 until March 2017. Three superflares were detected, in 2002, 2010, and 2016, with peak flux densities of >3400, 19 000, and 46 000 Jy, respectively. An analysis of these superflares is presented. The flares took place during periods of high maser activity in a narrow interval of radial velocities (40.5–42.5 km/s), and could be associated with the passage of a strong shock. The emission of three groups of features at radial velocities of about 41, 42, and 43 km/s dominated during themonitoring. The flare in 2016 was accompanied by a considerable increase in the flux densities of several features with velocities of 35–56 km/s.     

Observation of the powerful solar flare of October 27, 2002 on the far side of the sun

Observations of the hard X-ray and radio event of October 27, 2002 are analyzed. This flare was observed from near-Martian orbit by the HEND instrument developed at the Space Research Institute of the Russian Academy of Sciences and installed on the Mars Odyssey satellite. Although this powerful flare was observed far over the eastern solar limb, the extended source associated with the flare was detected by RHESSI at energies up to about 60 keV. The eruptive event was observed in the radio at the Nobeyama Radio Observatory. The properties of the X-ray radiation are used to calculate the spectrum of the accelerated electrons responsible for the observed radiation, assuming that the target is thick for a Martian observer and thin for a terrestrial observer. The results are compared with the results of radio observations. The conditions for electron propagation in the corona are discussed.     

Nonradial pulsations of the hot component of the symbiotic star CH Cyg during its active phase

We observed the symbiotic star CH Cyg with the 70 cm telescope of the Crimean Astrophysical Observatory in 1982, during its active phase, when it was brighter than 6^m in the V band. We simultaneously detected the continuum brightness at wavelengths of 3737, 5092, and 5500 Å with a time resolution of 20 s. We present light curves for these wavelengths for four nights between July 13 and August 22, demonstrating the character of the star’s rapid variations. Various frequency-analysis methods were applied to the data series at 3737 and 5092 Å, where the light from the hot component made an appreciable contribution. For both series, we find some 20 similar frequencies corresponding to periods ranging from 150 to 6000 s. Our estimates of the significance of the identified frequencies indicate that the significance level exceeds 3 σ in all cases, and is even higher in most cases. For many of the oscillations, the amplitudes varied on time scales shorter than one day. Multiperiodicity and variable oscillation amplitudes are characteristic of nonradial pulsations. We conclude that nonradial oscillations typical of a white dwarf’s g modes were excited during the symbiotic star’s state of highest activity.