A search for hard X-ray microflares near solar minimum

Near solar maximum, hard X-ray microflares with peak 20 keV fluxes of 10^–2 (cm² s keV)^–1, more than ten times smaller than for typical flares and subflares, can occur at the rate of about once every five minutes. We report here on a search for hard X-ray microflares made on a long duration balloon flight in February 1987 near solar minimum, at a time when no active regions were on the Sun. No microflares were observed over a total observing time of 16.5 hours spread over three days, implying a statistical upper limit to their rate of occurrence about a factor often lower than observed near solar maximum. Thus hard X-ray microflaring appears to be an active region phenomenon, and apparently not associated with flaring of soft X-ray bright points.     

Theoretical intensities of Fexiv in the solar EUV spectrum

Equilibrium population of Fexiv levels in coronal conditions was calculated including configurations 3s ²3p, 3s3p ², 3s ²3d, 3p ³, 3s3p3d, 3s ²4s, 3s ²4p, 3s ²4d, 3s ²4f. Relative populations of selected levels are given in Table VII. Figure 1 shows the dependence of relative intensities of the strongest lines on electron density. Certain line ratios can be used for the determination of N _e .E.g., at T=2 × 10⁶ K and with a dilution factor 0.4, the intensity ratio of λ211.3 and λ219.0 changes by a factor of 65 if N _e increases from 10⁷ to 10¹¹ (Table VIII). Cascades from the 3s3p3d and 3p ³ configurations are important in the population of some levels of 3s3p ² (Table VI). A possibility of identification of additional lines in the solar spectrum is indicated. NAS-NRC Resident Research Associate.     

Satellite lines in the solar X-ray spectrum

Observations of solar X-ray line emission using crystal spectrometers during a large chromospheric flare have provided a list of wavelengths with a precision of 0.003 Å in first order of diffraction and correspondingly better in higher orders. In addition to the resonance, intersystem (1 ¹ S ₀-2 ³ P ₁) and forbidden (1 ¹ S ₀-2 ³ S ₁) transitions of ions of the Hei isoelectronic sequence, we have recorded satellite lines arising from ions in the Lii, Bei and Bi isoelectronic sequences. These satellite features are most prominent in the iron spectrum. Apparent decreases in the ratio of forbidden and intersystem line intensities of Mgxi and Sixiii during the flare are used to derive electron densities possibly as high as 1 × 10¹³ cm^–3 in the Mgxi emitting region and 1 × 10¹⁴ cm^–3 in the Sixiii region during the event. A search for satellite lines on the long-wavelength side of the Lyman-alpha line of Hi-like ions has yielded no positive identifications.     

A study of the background corona near solar minimum

The white light coronagraph data from Skylab is used to investigate the equatorial and polarK andF coronal components during the declining phase of the solar cycle near solar minimum. Measurements of coronal brightness and polarization brightness product between 2.5 and 5.5R during the period of observation (May 1973 to February 1974) lead to the conclusions that: (1) the equatorial corona is dominated by either streamers or coronal holes seen in projections on the limb approximately 50% and 30% of the time, respectively; (2) despite the domination by streamers and holes, two periods of time were found which were free from the influences of streamers or holes (neither streamers nor holes were within 30° in longitude of the limb); (3) the derived equatorial background density model is less than 15% below the minimum equatorial models of Newkirk (1967) and Saito (1970); (4) a spherically symmetric density model for equatorial coronal holes yields densities one half those of the background density model; and (5) the inferred brightness of theF-corona is constant to within ±10% and ±5% for the equatorial and polar values, respectively, over the observation period. While theF-corona is symmetric at 2R it begins to show increasing asymmetry beyond this radius such that at 5R the equatorialF-coronal brightness is 25% greater than the polar brightness.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The relative intensities of CI lines in the solar EUV spectrum

The relative intensity of two Ci lines at 1993.6 Å and 1657.4 Å, observed in the limb spectrum of the sun, is a factor 2.6 × 10³ larger than that expected if both lines were optically thin. It is shown that the observed intensity ratio may be explained in terms of the transfer of photons from 1657.4 Å to 1993.6 Å due to a large optical depth in the line at 1657.4 Å. The observed upper limit on the relative intensity of two further lines at 1992.0 Å and 1657.0 Å has been used to show that the line at 1993.6 Å is optically thin. Hence it is shown that (1657.4 Å) = 1300, and (1993.6 Å) = 0.44. These values provide an independent evaluation of optical depths against which chromospheric models may be checked. Assuming a mean temperature of T _e = 8000 °K, and a mean scale height of 350 km, the optical depths lead to a mean hydrogen-particle density of N (H) = 1.4 × 10¹² cm^–3.     

Soft X-ray variability over the present minimum of solar activity as observed by SphinX

Solar Photometer in X-rays (SphinX) is an instrument designed to observe the Sun in X-rays in the energy range 0.85–15.00 keV. SphinX is incorporated within the Russian TESIS X and EUV telescope complex aboard the CORONAS-Photon satellite which was launched on January 30, 2009 at 13:30 UT from the Plesetsk Cosmodrome, northern Russia. Since February, 2009 SphinX has been measuring solar X-ray radiation nearly continuously. The principle of SphinX operation and the content of the instrument data archives is studied. Issues related to dissemination of SphinX calibration, data, repository mirrors locations, types of data and metadata are discussed. Variability of soft X-ray solar flux is studied using data collected by SphinX over entire mission duration.     

Prediction for the amplitude of solar cycle 24 from the pattern of activity near the cycle minimum

Correlations are investigated between the pattern of solar activity described by the smoothed monthly relative sunspot numbers (Wolf numbers) near the minimum of a solar cycle and the cycle amplitude. The closest correlation is found between the amplitude of a solar cycle and the sum of the decrease in activity over two years prior to the cycle minimum and the increase in activity over two years after the minimum; the correlation coefficient between these parameters is 0.92. This parameter is used as a precursor to predict the amplitude of solar cycle 24, which is expected to reach its maximum amplitude (85 ± 12) in February 2014. Based on the correlations between the mean parameters of solar cycles, cycle 24 is expected to last for approximately 11.3 years and the minimum of the next cycle 25 is predicted for May 2020.     

On the spectrum of solar impulsive hard X-ray bursts

This article puts forward a new method for the theoretical analysis of the X-radiation spectrum of impulsive hard X-ray bursts. It points out that the electron density energy state function must obey the fundamental kinetic equation. In the case of several model source functions, the electron density energy spectra are deduced. This can serve as a basis for an analysis of the spectrum of X-radiaiton in impulsive hard X-ray bursts. The article also makes a preliminary discussion of these energy state functions which help to explain the phenomena of softening of the X-radiation spectrum.     

The spectrum of the Ap star HR 465 near the rare-earth minimum phase

Four spectra of the long-period Ap star HR 465 were taken in 1969–70 near its rare-earth minimum phase and analysed. Equivalent widths or upper limits were obtained for lines of 23 elements. Lines of Cr were very strong. The relative abundances of the iron-peak elements were derived by a differential curve-of-growth analysis using Gem as the comparison star. The Sc/Fe, Ti/Fe, V/Fe, and Ni/Fe abundance ratios were found to be abnormally low, while the Cr/Fe ratio was abnormally high (0.08). The equivalent widths in 1969–70 were compared with those measured by Aller on a spectrum taken in 1960 near the rare-earth maximum phase. Ionized lines of Sc, Ti, V, Te, Nb, Mo, and the rare-earths all vary out of phase with the Cri and Crii lines. Although Mo and Cr occupy corresponding positions in the periodic table, the Mo/Cr abundance ratio was apparently at least 180 times higher in 1960 than in 1969–70.     

Average photospheric poloidal and toroidal magnetic field components near solar minimum

Average (over longitude and time) photospheric magnetic field components are derived from 3 Stanford magnetograms made near the solar minimum of cycle 21. The average magnetograph signal is found to behave as the projection of a vector for measurements made across the disk. The poloidal field exhibits the familiar dipolar structure near the poles, with a measured signal in the line Fe i 5250 Å of 1 G. At low latitudes the poloidal field has the polarity of the poles, but is of reduced magnitude ( 0.1 G). A net photospheric toroidal field with a broad latitudinal extent is found. The polarity of the toroidal field is opposite in the nothern and southern hemispheres and has the same sense as subsurface flux tubes giving rise to active regions of solar cycle 21.These observations are used to discusse large-scale electric currents crossing the photosphere and angular momentum loss to the solar wind.Now at Kitt Peak National Observatory, Tucson, Ariz. 85726, U.S.A.     

Determination of the solar X-ray spectrum by using the atmospheric extinction

Indications about the spectral distribution of the solar radiation below 20 Å are obtained by comparing the atmospheric extinction of the solar radiation measured by three ion chambers in the satellites SOLRAD 7 and 8.The data refer to July 5, 6 and 8, 1965 and July 23, 1966 when the satellites passed from light to darkness into or out of the earth's shadow.The spectral intensity distribution is shown for the four days, and a comparison is made between active and quiet days.     

Coronal X-ray activity preceding solar flares

A comprehensive survey of Skylab S-054 soft X-ray images was performed to investigate the characteristics of coronal enhancements preceding solar flares. A search interval of 30 min before flare onset was used. A control sample was developed and tests of the statistical results performed. X-ray images with preflare enhancements were compared with high resolution H images and photospheric magnetograms.The results are as follows: preflare X-ray enhancements were found in a statistically significant number of the preflare intervals, and consisted of one to three loops, kernels or sinuous features per interval. Typically, the preflare feature was not at the flare site and did not reach flare brightness. There was no systematically observed time within the preflare interval for the preflare events to appear and no correlation of preflare event characteristics with the subsequent flare energy. Gas pressures of several preflare features were calculated to be on the order of several dyne cm^–2, typical of active region loops, not flares. These results suggest that observations with both high spatial resolution and low coronal temperature sensitivity are required to detect these small, low pressure enhancements that preceded the smaller flares typical of the Skylab epoch. H brightenings were associated with nearly all of the preflare X-ray enhancements. Changing H absorption features in the form of surges or filament activations were observed in about half of the cases. These results do not provide observational support for models which involve preheating of the flare loop, but they are consistent with some current sheet models which invoke the brightening of structures displaced from the flare site tens of min before onset.     

Spatial structure of solar wind at a time of minimum solar activity

Interplanetary scintillation measurements of the solar wind speed in 1976 show the expected trend that higher speeds are found at higher heliographic latitudes or larger angular distances from the interplanetary current sheet deduced from coronal observations. A careful examination of variations in the speed where the current sheet departs from the equator reveals that the wind speed is not symmetrically distributed about the equator, and the minimum speed occurs at the current sheet. The variation of the speed u with the angular distance from the current sheet, λ, during 1976 is

$\text{u(λ) = 800}\text{sin}{}^{\mathrm{?2\lambda }}\text{+ 350}\text{km/s}\text{,|λ| ?35° = 600}\text{km/s}\text{, |λ| > 35°}$

.     

Enhancement of the solar X-ray line intensities due to dielectronic recombination as an excitation process

Intensities of resonance lines in X-ray region for Si ix–Si xiv ions are calculated by considering total excitation, i.e., excitation due to electron impact and dielectronic recombination. It is found that the contribution of the latter is quite significant. For consistency, the electron density effect in our calculations is not only accounted for in the ionization equilibrium but also in the total rate of excitation. It is also found that the contribution of electron density effect is pronounced with the inclusion of dielectronic recombination as an excitation mechanism. The computed average line intensities are compared with the available observations and a table of line flux for various wavelengths of the above-mentioned ions at different temperatures is also given.     

Energy spectrum of the time profiles for weak solar soft X-ray bursts

We analyze the time profiles for weak solar soft X-ray bursts over the period 1995–1999 within the framework of the Interball-Tail Probe project. We have revealed a tendency for the peak of the energy spectrum of the time profiles for weak solar soft X-ray bursts to shift and their correlation with the thermal background.     

Scattering of fast flare electrons in solar atmosphere and their X-ray spectrum

The evolution of the energy distributions of fast flare electrons injected towards the chromosphere are computed by the Monte Carlo method for different depths. Using these distributions, power law bremsstrahlung spectra having spectral indices increasing with photon energies are obtained.     

Absolute flows in the solar transition region

It is the objective of the present study to establish an absolute scale for flows in the solar transition region in observations obtained with the UVSP/SMM. By use of the polar limbs as reference one finds that the downflows range between 3 and 10 km s^–1. The brighter regions show the largest downward flows.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Two-dimensional spatial spectrum of the photospheric brightness field near to the solar disc center

From high-quality direct frames taken by the Soviet Stratospheric Solar Observatory, using coherent optical methods, the two-dimensional spatial spectrum of the photospheric brightness field was obtained (Figure 1). This spectrum is isotropic and continuous. Spectral densities P(k) and A(k) = 2k P(k), where k is the radial wavenumber, were estimated for the solar disc centre, and their statistical uncertainty calculated. P(k) has a maximum near k = 10^–3 km^–1 and then it tends to fall after that to zero frequency. The k dependence of A(k) cannot be satisfactorily approximated by a power law. For the highest frequencies studied, the spectrum falls as k ^–9. The measured statistical uncertainty of the spectra of individual domains for k 125 × 10^–4 km^–1 is in agreement with that calculated for a gaussian homogeneous field. But for a higher k the uncertainty may essentially exceed that of a gaussian homogeneous field. The true rms value for 4650 is equal to about 29%.     

Determination of the solar minimum period between cycles 22 and 23 from the coronal index of solar activity

We present the coronal index of solar activity (CI) for 1997 and use the 1996 and 1997 data to examine the properties of the solar minimum between cycles 22 and 23. To compute CI, we used only the intensities of the green corona from Lomnický tít and Sacramento Peak coronal stations. Values of CI were low in the first half of 1997 with an increase from September toward the end of 1997. We determined the minimum in the green corona to be May 1996, which is in coincidence with the results from 2800 MHz radio flux, the Mg II index and the Wolf number.     

Magneto-sensitive lines in the solar spectrum

In this paper a numerical solution of the equations of transfer of the Stokes parameters in the presence of magnetic fields has been carried out. This yields the profiles of the Stokes parameters for three magneto-sensitive spectral lines (FEI λλ 6302.499, 6173.341, 5250.216). The magnetic broadening, temperature sensitivity and some other properties of these lines have also been investigated.In Unno's pioneering work, an algebraic solution of the equations of transfer of the Stokes parameters were obtained under a series of assumptions. Two of these are: 1) The source function B, varies linearly with the optical depth τ_λ; 2) The atmosphere follows the Milne-Eddington model. In this paper we have abandoned both these unrealistic assumptions. In consequence, our numerical solution agrees with observations better than the algebraic solution of Unno.