Observations of Powerful Type III Bursts in the Frequency Range 10 – 30 MHz

The properties of powerful (flux >10⁻¹⁹ W m⁻² Hz⁻¹) type III bursts observed in July – August 2002 by the radio telescope UTR-2 at frequencies 10 – 30 MHz are analyzed. Most bursts have been registered when the active regions associated to these bursts were located near the central meridian or at 40° – 60° to the East or West from it. All powerful type III bursts drift from high to low frequencies with frequency drift rates 1 – 2.5 MHz s⁻¹. It is important to emphasize that according to our observations the drift rate is linearly increasing with frequency. The duration of the bursts changes mainly from 6 s at frequency 30 MHz up to 12 s at 10 MHz. The instantaneous frequency bandwidth does not depend on the day of observations, i.e. on the disk location of the source active region, and is increasing with frequency.     

A Low-Frequency (30 – 110 MHz) Antenna System for Observations of Polarized Radio Emission from the Solar Corona

An interferometer antenna system to observe polarized radio emission from the solar corona at different frequencies in the range 30?–?110 MHz has been commissioned recently by the Indian Institute of Astrophysics at the Gauribidanur Radio Observatory (latitude 13°36^′12^′′N and longitude 77°27^′07^′′E), about 100 km north of Bangalore (http://www.iiap.res.in/centres_radio.htm). This paper describes the antenna system, associated analog/digital receiver setup, calibration scheme, and preliminary results.     

Solar Radiation Fluxes in the 10–30 nm Range from Studying the E-region and E–F Valley

For the 10–30 nm interval within the extreme UV region of the solar spectrum, there are no commonly accepted views on the spectral composition and absolute magnitudes of the radiation intensity due to the lack of reliable data. This region is connected with characteristics of the ionosphere heat regime, photoelectron spectrum parameters and E–F valley characteristics. For estimating the solar radiation flux by the indirect route within the spectral region from 10 to 30 nm, which is difficult for direct measurements, it is suggested to use data on the electron concentration in the E-region maximum and E–F valley. Taken from empirical models, the data on these parameters were correlated with theoretical calculations of height profiles of electron concentration in the ionosphere. Based on the proportion between electron concentration in the E-layer maximum and E–F valley minimum, the solar radiation flux within the 10–30 nm region was shown to be 2.5 times greater than that obtained in measurements on board the ‘AE–E’ and ‘AE–C’ satellites. The results are used for correcting model spectra of the extreme UV radiation.     

Modeling of Hα Eruptive Events Observed at the Solar Limb

We present spectra and slit-jaw images of limb and on-disk eruptive events observed with a high temporal resolution by the Ond?ejov Observatory optical spectrograph. Analysis of the time series of full width at half-maximum (FWHM) in Hα, Hβ, and radio and soft X-ray (SXR) fluxes indicates two phenomenologically distinct types of observations which differ significantly in the timing of FWHM and SXR/radio fluxes. We investigated one such unusual case of a limb eruptive event in more detail. Synthesis of all observed data supports the interpretation of the Hα broadening in the sense of regular macroscopic plasma motions, contrary to the traditional view (emission from warm dense plasma). The timing and observed characteristics indicate that we may have actually observed the initiation of a prominence eruption. We test this scenario via modeling of the initial phase of the flux rope eruption in a magnetohydrodynamic (MHD) simulation, calculating subsequently – under some simplifying assumptions – the modeled Hα emission and spectrum. The modeled and observed data correspond well. Nevertheless, the following question arises: To what extent is the resulting emission sensitive to the underlying model of plasma dynamics? To address this issue, we have computed a grid of kinematic models with various arbitrary plasma flow patterns and then calculated their resulting emission. Finally, we suggest a diagnostics based on the model and demonstrate that it can be used to estimate the Alfvén velocity and plasma beta in the prominence, which are otherwise hard to obtain.     

The Rotation Rates of Solar Magnetic Fields During Solar Cycles 21 – 23

Employing the synoptic maps of the photospheric magnetic fields from the beginning of solar cycle 21 to the end of 23, we first build up a time – longitude stackplot at each latitude between ±35°. On each stackplot there are many tilted magnetic structures clearly reflecting the rotation rates, and we adopt a cross-correlation technique to explore the rotation rates from these tilted structures. Our new method avoids artificially choosing magnetic tracers, and it is convenient for investigating the rotation rates of the positive and negative fields by omitting one kind of field on the stackplots. We have obtained the following results. i) The rotation rates of the positive and negative fields (or the leader and follower polarities, depending on the hemispheres and solar cycles) between latitudes ±35° during solar cycles 21–23 are derived. The reversal times of the leader and follower polarities are usually not consistent with the years of the solar minimum, nevertheless, at latitudes ±16°, the reversal times are almost simultaneous with them. ii) The rotation rates of the three solar cycles averaged over each cycle are calculated separately for the positive, negative and total fields. The latitude profiles of rotation of the positive and negative fields exhibit equatorial symmetries with each other, and those of the total fields lie between them. iii) The differences in rotation rates between the leader and follower polarities are obtained. They are very small near the equator, and increase as latitude increases. In the latitude range of 5° – 20°, these differences reach 0.05 deg day⁻¹, and the mean difference for solar cycle 22 is somewhat smaller than cycles 21 and 23 in these latitude regions. Then, the differences reduce again at latitudes higher than 20°.     

Imaging Observations of X-Ray Quasi-periodic Oscillations at 3 – 6 keV in the 26 December 2002 Solar Flare

Quasi-periodic oscillations in soft X-rays (SXR) are not well known due to the instrument limitations, especially the absence of imaging observations of SXR oscillations. We explore the quasi-periodic oscillations of SXR at 3?–?6 keV in a solar flare observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) on 26 December 2002. This was a B8.1 class event and showed three X-ray sources (S₁, S₂, and S₃) at 3?–?6 keV and two sources (S₁ and S₂) at 12?–?25 keV. The light curves of the total fluxes display a two-minute oscillation at 3?–?6 keV, but not in the energy bands above 8 keV. To investigate imaging observations of the oscillations, we prepared CLEAN images at seven energy bands between 3 keV and 20 keV with an eight-second integration. The light curves of three sources were analyzed after integrating the flux of each source region. We used the Fourier method to decompose each source light curve into rapidly varying and slowly varying components. The rapidly varying components show seven individual peaks which are well fitted with a sine function. Then we used the wavelet method to analyze the periods in the rapidly varying component of each source. The results show that three sources display damped quasi-periodic oscillations with a similar two-minute period. The damped oscillations timescale varies between 2.5 to 6 minutes. Source S₁ oscillates with the same phase as S₃, but is almost in anti-phase with S₂. Analyzing the flaring images in more detail, we found that these oscillation peaks are well consistent with the appearance of S₃, which seems to split from or merge with S₂ with a period of two minutes. The flare images with a high cadence of one second at 3?–?6 keV show that source S₃ appears with a rapid period of 25 seconds. The two-minute oscillation shows the highest spectral power. Source S₃ seems to shift its position along the flare loop with a mean speed of 130 km?s^?1, which is of the same order as the local sound speed. This connection between the oscillation peaks and emission enhancement appears to be an observational constraint on the emission mechanism at 3?–?6 keV.     

Some Observed Results of Solar Radio Spectrometer at 4.5—7.5GHz

A new instrument of broadband solar radio spectrometer working at waveband 4.5-7.5GHz was developed at Purple Mountain Observatory for Solar Maximum 23. Some new results of spectral observation have been obtained since August 1999.Two typical type Ⅲμbursts with rich fine structures are presented and some interesting features discussed.     

Prominence hydrogen lines at 10–20 microns

An analysis of prominence hydrogen lines is presented in the spectral band 10–20 . The results are consistent with earlier work.Operated by the Association of Universities for Research in Astronomy, Inc., under contrast AST 84-18716 with the National Science Foundation.     

A Possible Cause of the Diminished Solar Wind During the Solar Cycle 23 – 24 Minimum

Interplanetary magnetic field and solar wind plasma density observed at 1 AU during Solar Cycle 23?–?24 (SC-23/24) minimum were significantly smaller than those during its previous solar cycle (SC-22/23) minimum. Because the Earth’s orbit is embedded in the slow wind during solar minimum, changes in the geometry and/or content of the slow wind region (SWR) can have a direct influence on the solar wind parameters near the Earth. In this study, we analyze solar wind plasma and magnetic field data of hourly values acquired by Ulysses. It is found that the solar wind, when averaging over the first (1995.6?–?1995.8) and third (2006.9?–?2008.2) Ulysses’ perihelion (\({\sim}\,1.4~\mbox{AU}\)) crossings, was about the same speed, but significantly less dense (\({\sim}\,34~\%\)) and cooler (\({\sim}\,20~\%\)), and the total magnetic field was \({\sim}\,30~\%\) weaker during the third compared to the first crossing. It is also found that the SWR was \({\sim}\,50~\%\) wider in the third (\({\sim}\,68.5^{\circ}\) in heliographic latitude) than in the first (\({\sim}\,44.8^{\circ}\)) solar orbit. The observed latitudinal increase in the SWR is sufficient to explain the excessive decline in the near-Earth solar wind density during the recent solar minimum without speculating that the total solar output may have been decreasing. The observed SWR inflation is also consistent with a cooler solar wind in the SC-23/24 than in the SC-22/23 minimum. Furthermore, the ratio of the high-to-low latitude photospheric magnetic field (or equatorward magnetic pressure force), as observed by the Mountain Wilson Observatory, is smaller during the third than the first Ulysses’ perihelion orbit. These findings suggest that the smaller equatorward magnetic pressure at the Sun may have led to the latitudinally-wider SRW observed by Ulysses in SC-23/24 minimum.     

Optical Thickness of the 2–4.5 GHz Solar Plasma Emission

For radio emission at the frequency corresponding to the second harmonic of the local plasma frequency, the optical thickness in the solar atmosphere is calculated. Three types of models are assumed: the model with radio emission from the narrow transition region, and models with radio emission from a cool and dense plasma filament embedded in hotter plasma at the transition region and in the corona. The optical thickness is computed by integration of the collisional (free–free) absorption along a radio-ray path radial in the solar atmosphere. In all models considered the optical thickness can be sufficiently low for appropriate parameters. For example, in the narrow (<100 km) transition region where the density scale height is much less than that of the pressure one, the optical thickness can be lower than 1. Furthermore, the optical thickness can be decreased if the radio emission is generated in the cool and dense plasma filament surrounded by hotter and thinner plasma. But the models differ in density scale heights and thus in distances between plasma emission levels. This difference is essential for the interpretation of high-frequency type III radio bursts.     

Quasi-Biennial Oscillations in the North – South Asymmetry of Solar Activity

The north – south (N – S) asymmetry of solar activity is investigated by using the data on coronal green-line brightness and total number and total area of sunspots over the period of 1939  –  2001. Typical time variations of the N – S asymmetry are found to be consonant in these indices. Quasi-biennial oscillations (QBO) of solar activity are well recognizable in the N – S asymmetry of the examined indices. Moreover, the QBO are much better manifested in the N – S asymmetry of the individual indices than in the original (N plus S) indices. The time variations of relative QBO power are synchronous for the N – S asymmetry of various solar activity indices whereas such a synchronization is weaker for the indices themselves. It is revealed that the relative QBO power found in the N – S asymmetry of the studied indices has a negative correlation with the value of the N – S asymmetry itself. The findings indicate that the N – S asymmetry should be regarded as a fundamental phenomenon of solar activity similarly manifested in different activity indices. These findings should be taken into account when any dynamo theory of solar activity is constructed.     

Measurements of the Solar Radius in Antalya between 2001–2003

The results of the solar radius measurements from February 2001 to November 2003 with the solar astrolabe at the TUBITAK National Observatory are presented. The mean semi-diameter for the period, corrected for systematic effects such as the Fried parameter and the zenith distance, is found to be 959.29 ± 0.01 arc sec. A comparison of the monthly averages of the solar radius with the monthly means of sunspot numbers shows that the semi-diameter of the Sun increases with an amplitude of 0.017 arc sec per year in opposite phase with solar cycle 23.     

Time – Frequency Analysis of GALLEX and GNO Solar Neutrino Data

Time – frequency analysis of data from the GALLEX and GNO solar neutrino experiments shows that some features in power-spectrum analyses of those datasets are due to aliasing (a result of the fact that run durations tend to be small multiples of one week). Displays formed from the published GALLEX data show a sharp discontinuity that we attribute to some systematic effect. We therefore normalize data for each of the four experiments in the GALLEX series and concatenate the resulting normalized data. This step effectively removes the presumed systematic effect. To help understand the effect of aliasing, we form time – frequency displays of the two principal modulations found in the data, at 11.87 year⁻¹ and at 13.63 year⁻¹. We also form time – frequency displays of datasets formed by subtracting these modulations from the actual (normalized) data. The results suggest that the true principal modulation is that at 11.87 year⁻¹. Comparison with helioseismology data suggests that modulation may be occurring in the core, perhaps resulting from inhomogeneities and fluctuations in the nuclear-burning process, and that the sidereal rotation rate of the core is 12.87 year⁻¹, or 408 nHz.     

Numerical Simulations of Magnetoacoustic–Gravity Waves in the Solar Atmosphere

We investigate the excitation of magnetoacoustic–gravity waves generated from localized pulses in the gas pressure as well as in the vertical component of velocity. These pulses are initially launched at the top of the solar photosphere, which is permeated by a weak magnetic field. We investigate three different configurations of the background magnetic field lines: horizontal, vertical, and oblique to the gravitational force. We numerically model magnetoacoustic–gravity waves by implementing a realistic (VAL-C) model of the solar temperature. We solve the two-dimensional ideal magnetohydrodynamic equations numerically with the use of the FLASH code to simulate the dynamics of the lower solar atmosphere. The initial pulses result in shocks at higher altitudes. Our numerical simulations reveal that a small-amplitude initial pulse can produce magnetoacoustic–gravity waves, which are later reflected from the transition region due to the large-temperature gradient. The cavities in the lower solar atmosphere are found to have the best conditions to act as a resonator for various oscillations, including their trapping and leakage into the higher atmosphere. Our numerical simulations successfully model the excitation of such wave modes, their reflection and trapping, as well as the associated plasma dynamics.     

Solar Magnetism and the Activity Telescope at HSOS

A new solar telescope system is described, which has been operating at Huairou Solar Observing Station (HSOS), National Astronomical Observatories, Chinese Academy of Sciences (CAS), since the end of 2005. This instrument, the Solar Magnetism and Activity Telescope (SMAT), comprises two telescopes which respectively make measurements of full solar disk vector magnetic field and Hα observation. The core of the full solar disk video vector magnetograph is a birefringent filter with 0.1  bandpass, installed in the tele-centric optical system of the telescope. We present some preliminary observational results of the full solar disk vector magnetograms and Hα filtergrams obtained with this telescope system.     

Physical State of the Photosphere at the Onset Phase of a Two-Ribbon Solar Flare

We study the physical state of the photosphere at about 30 minutes before and at the onset of a 2N/M2 two-ribbon solar flare. Semiempirical photospheric models are obtained for two Hα-kernels with the help of the SIR inversion code described by Ruiz Cobo and del Toro Iniesta (Astrophys. J. 398, 375, 1992). The models derived from the inversion reproduce spectral observations in seven Fraunhofer lines. The inferred models show variations in all photospheric parameters both before and at the onset of the flare relative to the quiet-Sun model. The temperature enhancement in the upper photospheric layers is found in the atmospheres in both kernels. The dynamical structure in the models reveals the variations at the onset of the flare relative to the preflaring ones. The inferred atmospheres show some difference in the thermodynamical parameters of two kernels.     

Solar rotation, 1966–1978

Photospheric and chromospheric spectroscopic Doppler rotation rates for the full solar disk are analyzed for the period July, 1966 to July, 1978. An approximately linear secular increase of the equatorial rate of 3.7% for these 12 years is found (in confirmation of Howard, 1976). The high latitude rates above 65 ° appear to vary with a peak-to-peak amplitude of 8%, or more, phased to the sunspot cycle such that the most rapid rotation occurs at, or following, solar maximum. The chromosphere, as indicated by H, has continued to rotate on the average 3% faster than the photosphere agreeing with past observations. Sources of error are discussed and evaluated.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.