On the sunspot transition region

The EUV line emission and relative line-of-sight velocity in the transition region between the chromosphere and corona of 36 sunspot regions are investigated, based on observations with the Coronal Diagnostic Spectrometer – CDS and the Solar Ultraviolet Measurements of Emitted Radiation – SUMER on the Solar and Heliospheric Observatory – SOHO. The most prominent features in the transition-region intensity maps are the sunspot plumes. In the temperature range between log T=5.2 and log T=5.6 we find that 29 of the 36 sunspots contain one or two sunspot plumes. The relative line-of-sight velocity in sunspot plumes is high and directed into the Sun in the transition region, for 19 of the sunspots the maximum velocity exceeds 25 km s^?1. The velocity increases with increasing temperature, reaches a maximum close to log T=5.5 and then decreases abruptly.

Attention is given to the properties of oscillations with a period of 3 min in the sunspot transition region, based on observations of six sunspots. Comparing loci with the same phase we find that the 3-min oscillations affect the entire umbral transition region and part of the penumbral transition region. Above the umbra the observed relation between the oscillations in peak line intensity and line-of-sight velocity is compatible with the hypothesis that the oscillations are caused by upward-propagating acoustic waves. Information about intensity oscillations in the low corona is obtained from observations of one sunspot in the 171 Å channel with the Transition Region And Coronal Explorer – TRACE. We conclude that we observe the 3-min sunspot oscillations in the chromosphere, the transition region and the low corona. The oscillations are observable over a wider temperature range than the sunspot plumes, and show a different spatial distribution than that of the plumes.

     

Recent observations of high-degree solar p-mode oscillations at the Kitt Peak National Observatory

A brief summary is given of a program which is currently being carried out with the McMath telescope of the Kitt Peak National Observatory in order to study high-degree (l ≳ 150) solar p-mode oscillations. This program uses a 244 × 248 pixel CID camera and the main spectrograph of the McMath telescope to obtain velocity-time maps of the oscillations which can be converted into two-dimensional (k _h - ω) power spectra of the oscillations. Several different regions of the solar spectrum have been used in order to study the oscillations at different elevations in the solar atmosphere. The program concentrates on eastward- and westward-propagating sectoral harmonic waves so that measurements can be made of the absolute rotational velocities of the solar photospheric and shallow sub-photospheric layers. Some preliminary results from this program are now available. First, we have been unable to confirm the existence of a radial gradient in the equatorial rotational velocity as was previously suggested. Second, we have indeed been able to confirm the presence of p-mode waves in the solar chromosphere as was first suggested by Rhodes et al. (1977). Third, we have been able to demonstrate differences in photospheric and chromospheric power spectra.

     

160-min oscillations of the Sun as the mean of study of its internal structure

The investigation of the models of the contemporary Sun with a mixed core has shown that the amplitude of some gravity modes of oscillations of the star can be mainly concentrated in the central region. This phenomenon takes place if the node of the amplitude of radial displacement coincides with the boundary of the mixed core. In this case the core can be regarded as a driving generator of the oscillations, determining their period and phase. It is suggested as the explanation of the observational properties of the 160-min oscillation.

     

On the periodic solutions and resonance of spinning satellites in near-circular orbits

Attitude motion of spinning axisymmetric satellites in presence of gravity-gradient and solar radiation pressure torques is studied analytically. The approximate closed-form solution developed for the nonlinear, nonautonomous, coupled fourth-order system proves to be an excellent tool in locating periodic solutions of the system in both circular and noncircular orbits. The variational stability of the periodic motion is examined using the Floquet theory. The resonance analysis suggests the existence of critical combinations of system parameters leading to large amplitude oscillations.

     

Quasiperiodic oscillations in the light curve of GRB 080319B

A modified spectral analysis technique is used to study the temporal structure of the emission from the γ-ray burst GRB 080319B detected in three space missions, Swift/BAT, Wind/KONUS, and Integral/SPI ACS. The energy range of the γ rays detected in these experiments covers 15-10000 keV. The time resolution for the first two of these experiments was 64 ms and for the third, 50 ms. Quasiperiodic oscillations with periods from 0.6 s to 6 s were observed. To within the time resolution, the oscillations with periods of 1.28, 0.89, and 0.64 s are the second, third, and fourth harmonics, respectively, of oscillations with a period of 2.56 s, while oscillations with a period of 0.96 s are the third harmonic of oscillations with a period of 2.94 s. The behavior of the quasiperiodic components is examined as a function of the phase of the events. Possible physical mechanisms for the quasiperiodic oscillations in the light curves of γ-ray bursts are discussed.     

Electron plasma oscillations associated with type III radio emissions and solar electrons

An extensive study of the IMP-6 and IMP-8 plasma and radio wave data has been performed to try to find electron plasma oscillations associated with type III radio noise bursts and low-energy solar electrons. This study shows that electron plasma oscillations are seldom observed in association with solar electron events and type III radio bursts at 1.0 AU. In nearly four years of observations only one event was found in which electron plasma oscillations are clearly associated with solar electrons. For this event the plasma oscillations appeared coincident with the development of a secondary maximum in the electron velocity distribution functions due to solar electrons streaming outwards from the Sun. Numerous cases were found in which no electron plasma oscillations with field strengths greater than 1 μV m^?1 could be detected even though electrons from the solar flare were clearly detected at the spacecraft. For the one case in which electron plasma oscillations are definitely produced by the electrons ejected by the solar flare the electric field strength is relatively small, only about 100 μV m^?1. This field strength is about a factor of ten smaller than the amplitude of electron plasma oscillations generated by electrons streaming into the solar wind from the bow shock. Electromagnetic radiation, believed to be similar to the type III radio emission, is also observed coming from the region of the more intense electron plasma oscillations upstream of the bow shock. Quantitative calculations of the rate of conversion of the plasma oscillation energy to electromagnetic radiation are presented for plasma oscillations excited by both solar electrons and electrons from the bow shock. These calculations show that neither the type III radio emissions nor the radiation from upstream of the bow shock can be adequately explained by a current theory for the coupling of electron plasma oscillations to electromagnetic radiation. Possible ways of resolving these difficulties are discussed.     

High-resolution photography of the solar chromosphere

Measurements have been made of the heights of seven bright mottles photographed beyond the limb on sequences of broad-band Hα filtergrams. They reveal no evidence of a systematic upward or downward motion persisting throughout the lifetime of a mottle, and in no case does the observed change in height exceed 1″ (725 km) over the period of observation. On the other hand, the possibility cannot be excluded that the bright mottles execute vertical oscillations with velocities comparable to those recently deduced from Hα contrast profile analyses by Grossmann-Doerth and von Uexküll (1971) and Bray (1973) provided the periods do not exceed a few tens of seconds.

     

Lunar photoelectron layer dynamics

Possible waves and oscillations in the lunar photoelectron layer (PEL) are investigated. The steady state PEL is reviewed as a basis for discussing PEL motions. Magnetic fields are neglected, so that there are four possible wave modes to consider. The propagation through the PEL of the two electromagnetic modes is discussed. Positive-ion waves, the third mode, are dismissed and plasma waves are considered at length. It is concluded that there are no propagating waves in the PEL other than electromagnetic. However, there is a type of oscillation which appears to be new and which may not be strongly damped. With these oscillations, termed flight-time oscillations, the height of the PEL fluctuates as does the electric field. These oscillations appear to be analogous to the height oscillations of the vertical jet of water in a city park water fountain. If flight-time oscillations are not much damped then it would be simplest to interpret them as plasma oscillations continually driven by the upwelling photoelectron stream. A possible laboratory investigation of these oscillations is discussed. For the surfaces of the Moon and the planet Mercury, the flight-time oscillation frequency,ω _F, is found to be respectively ç 4 × 10⁶ and ç 10⁷ rad s^?1. The PEL's of those surfaces may be in a state of continual vertical ‘quivering’ due to flight-time oscillations, or may be quiescent.     

Temporal fluctuations of the magnetic field in sunspots

The magnetic field strength in sunspots was derived from time series of two-dimensional spectra taken with the Göttingen 2D-spectrometer at the Vacuum Tower Telescope on Tenerife in August 1997. For the present measurements the magnetically sensitive line Fe?i 684.3 nm was selected. The main spot of the investigated sunspot group has a maximum magnetic field strength of 2270 G. Enhanced power of the magnetic field variations was found at the boundary between umbra and penumbra for all frequency ranges. These fluctuations are not well correlated with those of intensity variations or line shifts. Other spatial power peaks occur in a dark patch inside the centreside penumbra and at the centres of some accompanying small spots. Since no clear peaks at certain frequencies are found, the variations are not harmonic oscillations. A possible relation to Hα flares is investigated. There are several cases of published observations of magnetic field variations where flares occurred soon after the measurements, but very little before. Therefore it is not very probable that flares act as exciters of magnetic field variations.

     

Global isothermal oscillations in the solar photosphere

Observational data on solar irradiance oscillations from the VIRGO (SOHO) and DIFOS-F (CORONAS-F) experiments are used to obtain stratifications of perturbed hydrogen concentrations that produce isothermal oscillations in the solar photosphere. The study reveals the nodes and antinodes of the oscillations in the solar photosphere. A simulation of long-period isothermal oscillations from the DIFOS data shows that the nodes and antinodes of Δn/n tend to shift towards lower photosphere layers with a decrease in the oscillation frequency.     

Observations of Excitation and Damping of Transversal Oscillations in Coronal Loops by AIA/SDO

The excitation and damping of the transversal coronal loop oscillations and quantitative relation between damping time, damping property (damping time per period), oscillation amplitude, dissipation mechanism and the wake phenomena are investigated. The observed time series data with the Atmospheric Imaging Assembly (AIA) telescope on NASA’s Solar Dynamics Observatory (SDO) satellite on 2015 March 2, consisting of 400 consecutive images with 12 s cadence in the 171 \(\mathring{\mathrm{A}}\) pass band is analyzed for evidence of transversal oscillations along the coronal loops by the Lomb–Scargle periodgram. In this analysis signatures of transversal coronal loop oscillations that are damped rapidly were found with dominant oscillation periods in the range of \(\mathrm{P}=12.25\,\text{--}\,15.80\) min. Also, damping times and damping properties of the transversal coronal loop oscillations at dominant oscillation periods are estimated in the range of \({\tau_{\mathrm{d}}=11.76}\,\text{--}\,{21.46}\) min and \({\tau_{\mathrm{d}}/\mathrm{P}=0.86}\,\text{--}\,{1.49}\), respectively. The observational results of this analysis show that damping properties decrease slowly with increasing amplitude of the oscillation, but the periods of the oscillations are not sensitive functions of the amplitude of the oscillations. The order of magnitude of the damping properties and damping times are in good agreement with previous findings and the theoretical prediction for damping of kink mode oscillations by the dissipation mechanism. Furthermore, oscillations of the loop segments attenuate with time roughly as \(t^{-\alpha}\) and the magnitude values of \(\alpha\) for 30 different segments change from 0.51 to 0.75.     

The vector approach to the problem of physical libration of the Moon: the linearized problem

A flexible and informative vector approach to the problem of physical libration of the rigid Moon has been developed in which three Euler differential equations are supplemented by 12 kinematic ones. A linearized system of equations can be split into an even and odd systems with respect to the reflection in the plane of the lunar equator, and rotational oscillations of the Moon are presented by superposition of librations in longitude and latitude. The former is described by three equations and consists of unrestricted oscillations with a period of T ₁ = 2.878 Julian years (amplitude of 1.855″) and forced oscillations with periods of T ₂ = 27.201 days (15.304″), one stellar year (0.008″), half a year (0.115″), and the third of a year (0.0003″) (five harmonics altogether). A zero frequency solution has also been obtained. The effect of the Sun on these oscillations is two orders of magnitude less than that of the Earth. The libration in latitude is presented by five equations and, at pertrubations from the Earth, is described by two harmonics of unrestricted oscillations (T ₅ ≈ 74.180 Julian years, T ₆ ≈ 27.347 days) and one harmonic of forced oscillations (T ₃ = 27.212 days). The motion of the true pole is presented by the same harmonics, with the maximum deviation from the Cassini pole being 45.3″. The fifth (zero) frequency yields a stationary solution with a conic precession of the rotation axis (previously unknown). The third Cassini law has been proved. The amplitudes of unrestricted oscillations have been determined from comparison with observations. For the ratio $ \frac{{\sin I}} {{\sin \left( {I + i} \right)}} \approx 0.2311 $ \frac{{\sin I}} {{\sin \left( {I + i} \right)}} \approx 0.2311 , the theory gives 0.2319, which confirms the adequacy of the approach. Some statements of the previous theory are revised. Poinsot’s method is shown to be irrelevant in describing librations of the Moon. The Moon does not have free (Euler) oscillations; it has oscillations with a period of T ₅ ≈ 74.180 Julian years rather than T ≈ 148.167 Julian years.     

Variations of microwave emission from solar active regions

Based on observational data obtained with the RT-22 Crimean Astrophysical Observatory radio telescope at frequencies of 8.6 and 15.4 GHz, we investigate the quasi-periodic variations of microwave emission from solar active regions with periods T_p<10 min. As follows from our wavelet analysis, the oscillations with periods of 3–5 min and 10–40 s have the largest amplitudes in the dynamic power spectra, while there are virtually no oscillations with T_p<10 s. Our analysis shows that acoustic modes with T_p?1 min strongly dissipate in the lower solar corona due to thermal conduction losses. The oscillations with T_p=10–40 s are associated with Alfvén disturbances. We analyze the influence of acoustic and Alfvén oscillations on the thermal mechanisms of microwave emission in terms of the homogeneous model. We discuss the probable coronal heating sources.     

Seismology of a Sunspot Atmosphere

Two competing theories of sunspot oscillations are discussed. It is pointed out that the normal mode (eigenoscillations) theory is in contradiction with a number of observations. The reasons for this are discussed. The revised filter theory of the three-minute sunspot oscillations is outlined. It is shown that the reason for the occurrence of the multipassband filter for the slow waves is the interference that appears from the multilayer structure of the sunspot atmosphere. In contrast with Zhugzhda and Locans (Sov. Astron. Lett. 7, 25?–?27, 1981) it is shown that along with the Fabry?–?Perot chromospheric passband the cutoff frequency passband and a number of the high-frequency passbands occur. The effect of the nonlinearity of the sunspot oscillations in the upper chromosphere and the transition region is taken into account. The spectra of the distinct empirical models of the sunspot atmosphere are explored. An example of the interpretation of the sunspot oscillations based on the revised filter theory is presented. Only the filter theory can explain the complicated behavior of the oscillations across the sunspot. The observations provide evidence of the nonuniformity of the sunspot atmosphere.     

Simultaneous geomagnetic and ionospheric oscillations caused by hydromagnetic waves

During magnetically quiet or slightly disturbed nights, closely correlated oscillations of the geomagnetic field and the F-layer were observed by means of magnetometers and a vertical-icidence continuous-wave Doppler sounder at 3.57 MHz. The magnetic oscillations were mostly Pi2 pulsations with periods from 0.5 to 2 min, and an amplitude of 10^?9 T corresponding to a Doppler shift of the order of 0.3 Hz. The observations cannot be explained by a dynamo-motor hypothesis assuming that the magnetic and ionospheric oscillations are caused by alternating E-layer currents, but they agree well with the theory of downgoing hydromagnetic waves. In particular, this theory explains the observed effects due to sporadic E-layer ionization and ion-neutral collisions. The results are found to differ substantially from those of other authors.     

Oscillatory motions in sunspots

We observe vertical velocity oscillations in some sunspot umbrae with periods of about 180 s and peak to peak amplitudes up to 1 km s^–1. These oscillations are not visible in either the line depth, line width or the continuum intensity. No correlation seems to exist between the occurence of these oscillations and the presence of the chromospheric umbral flashes (Solar Phys. 7, 351, 1069). In the spot penumbra there is an indication of a long period oscillation, the period increasing from about 300 s in the inner penumbra to nearly 1000 s at the penumbra-photosphere boundary. An attempt has been made to interpret these oscillations in terms of gravity or acoustic waves, travelling along the magnetic field lines, taking into account the variation of scale height and magnetic field direction across the sunspot.     

A numerical study of local stellar motions

The orbits of over 10000 stars are integrated in a steady-state model of the Galaxy for a time 6.0×10⁸ yr. Initially, the stars are placed randomly inside spheres of 500 pc and 50 pc radius and are given random velocities, such that the sample has a Maxwellian or a spheroidal velocity distribution. The spheres are placed at the Sun's distance from the galactic centre (10 kpc) and are given a circular velocity of 250 km s^?1. The mean velocities and dispersions of stars within 1 kpc of an ‘observer’ moving at the circular velocity are calculated as functions of time. The quantities show a strong time-dependence with oscillations of period 10⁸ yr. The oscillations are independent of the mass model and occur also in an inverse square force field. A vertex deviation of the velocity ellipsoid, an asymmetric drift and aK-effect occur as natural consequences of the oscillations. Attempts to apply the Oort method for density determinations in the galactic plane are also influenced by the oscillations. Spiral density waves appear to have a small effect on the motions of the test stars.     

The relationship of electron plasma oscillations to type III radio emissions and low-energy solar electrons

An extensive study of the IMP-6 and IMP-8 plasma and radio wave data has been performed to try to find electron plasma oscillations associated with type III radio noise bursts and low energy solar electrons. This study shows that electron plasma oscillations are seldom observed in association with solar electron events and type III radio bursts at 1.0 AU. In nearly four years of observations only one event was found in which electron plasma oscillations are clearly associated with solar electrons. Numerous cases were found in which no electron plasma oscillations with field strengths greater than 1 V/m could be detected even though electrons from the solar flare were clearly detected at the spacecraft.For the one case in which electron plasma oscillations are definitely produced by the electrons ejected by the solar flare, the electric field strength is very small, only about 100 V/m. This field strength is about a factor of ten smaller than the amplitude of electron plasma oscillations generated by electrons streaming into the solar wind from the bow shock. Electromagnetic radiation, believed to be similar to the type III radio emission, is also observed coming from the region of more intense electron plasma oscillations upstream of the bow shock. Quantitative calculations of the rate of conversion of the plasma oscillation energy to electromagnetic radiation are presented for plasma oscillations excited by both solar electrons and electrons from the bow shock. These calculations show that neither the type III radio emissions nor the radiation from upstream of the bow shock can be adequately explained by a current theory for the coupling of electron plasma oscillations to electromagnetic radiation. Possible ways of resolving these difficulties are discussed.     

Oscillations in nuclear reaction systems inside stars

Oscillations related to the particle number density in the nuclear reaction systems inside stars are investigated, which include the oscillations in the helium nuclear reaction and in the P-PI branch of proton-proton reaction under the (g - T) effect and the gas pressure effect. The investigation shows that there is a supercritical oscillating bifurcation with time in the helium nuclear reaction system. If the oscillations are excited by the pressure effect, the nuclear reaction must occur in the convecting zone with a radially reducing convective velocity; if the oscillations are excited by the (g - T) effect, the nuclear reaction may occur in the convecting zone or no-convection zone. The supercritical oscillating bifurcation with time also exists in the P-PI reaction system, which is excited only by the (g - T) effect, while the oscillation excited by the pressure effect is decaying.     

Hale Cyclicity of Solar Activity and Its Relation to Climate Variability

The periodicity of climatic processes along the Russian Arctic Ocean coast has been studied by analyzing the tree-ring chronologies for the regions close to the northern timberline. The wavelet analysis of annual series of conifer tree rings for the period 1458–1975 has revealed climatic oscillations with periods of 20–25 years. The amplitudes and periods of climatic oscillations in the region of Russian Arctic Ocean proved to exhibit appreciable changes. Especially strong climatic variations in comparison with the recent ones were found to occur during the Maunder minimum epoch when the period of oscillations increased from 22–23 years to 24–29 years, and oscillations with periods of 15 years appeared. After the Maunder minimum, the periods of oscillations and their amplitudes again decreased, and the 15–16-year maximum disappeared. Analysis of solar activity based on of radiocarbon (¹⁴C) concentration in annual tree rings has revealed a similar pattern in changes of periodicity before, during, and after the Maunder minimum. This suggests that quasi-bidecadal climatic oscillations and variations in solar activity can be connected with each other. A possible solar forcing of periodic climatic processes and its nonlinear influence on the atmosphere-ocean-continental system are discussed. The intense quasi-bidecadal climatic oscillations can be, in all probability, interpreted as resulting from amplification of a weak solar signal in the atmosphere-ocean system that has its own noises whose frequencies are close to the 22–23-year solar cycles.