A search for high energy gamma-rays from solar active regions

The Elliot model for solar flares predicts weak -ray emission from the flare region prior to large flares. A search has been made for such -radiation of energy > 50 MeV. The experiment was performed using balloon-borne detectors flown from an equatorial station during the 1967/1968 solar maximum. A number of small flares were observed, but no associated -rays were detected. A limit of 2.3 × 10⁴ photons/cm² s was placed on the emission from an importance 1N flare. The general lack of major solar activity during the period of the balloon flights precluded a test for the Elliot model.     

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.     

New absolute measurements of the solar spectrum 310–685 nm

During 1986–1989 at the high-altitude station on the Peak Terskol, Caucasus (h = 3000 m) absolute measurements of the solar disk-centre intensity were performed. The observations were carried out with the specialized solar telescope (D = 23 cm,F = 3 m) and grating spectrometer (F = 2 m, grating 140 × 150 mm, 600 grooves mm^–1). The ribbon tungsten lamps used for absolute calibration were calibrated to the USSR standard of spectral intensity and were also compared with the irradiance standard of the PMO/WRC (Davos, Switzerland), with the lamps used in the Alma-Ata Observatory (Kazakhstan) and in Simferopol University for absolute measurements of stellar spectra. Methods and apparatus were improving step by step during 1985–1988. Special care was paid to the study of all possible sources of errors, in particular to the method of correction for atmospheric extinction, to polarization properties of optical elements of the apparatus, and to establishing the most reliable absolute calibration system. Finally, the observations performed during 1989 utilized only the refined methods and apparatus. As a result, the absolute integrals of the solar disk-centre intensity for 1-nm wide spectral bands in the range 310–685 nm are available. We estimate the total error is 2.5% at 310 nm and 2.1% at 680 nm. The absolute irradiance for 5-nm wide spectral bands is also obtained. We compare our results with results by Neckel and Labs (1984), with the irradiance filter measurements performed in PMO/WRC and calibration of the Sun's spectral irradiance to the stellar irradiance standard Vega by Lockwood (1992). Our results show a systematic difference with data by Neckel and Labs in the near-ultraviolet. The results by Neckel and Labs are probably underestimated in this spectral range by 8%.Deceased 20 January 1994.     

Magnetically closed regions in the solar wind

Interplanetary plasma and magnetic field data collected by Helios-1, Helios-2 and IMP-8 satellites over the periods December 1974–December 1976, January 1976–December 1976 and December 1974–December 1976, respectively, are analysed. From this analysis, we identified 85 about cases in which the proton temperature was very low. In 50 of these cases, the interplanetary magnetic field showed characteristic variations favorable for closed structures in the solar wind.By using the calculated radial temperature gradients as a function of the solar wind speed and the heliocentric distance we were able to identify cold protons in the neighborhood of the Sun (0.3 AU).The estimation of the distance at which regions of cold protons are formed (10R ) shows that this distance is the same whether we are using solar wind plasma data measured in fixed or in varied heliocentric distances.     

A model of the spectrum of the slowly varying component of solar radio emission from active regions

Statistical analyses of solar radio observations have shown that proton events are likely to occur when the flux at 3 cm, S₃, of the SVC is greater than 25 sfu and when S₃/S₈ is grater than 1. A theoretical explanation of this fact is attempted in this paper. I calculated the spectrum of the SVC using the gyro-radiation emission and found that the main reason for S₃ >; 25sfu is a ten-fold increase in the conductive energy flux in the active region over the quiet region, raising the height of the gyro-resonance layer for the 3 cm emission, and that the main reason for S₃/S₈ ? 1 is an increase in the coronal magnetic field gradient in the active region, causing a decrease in the optical thickness for the gyro-resonance absorption at 3 cm. It is precisely these active regions that are most favourable for the production of proton events.     

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.     

Cyclotron lines in the spectra of solar flares and solar active regions

It was shown by Zheleznyakov and Zlotnik (1980a, b) that in complex configurations of solar magnetic fields (in hot loops above the active centres, in neutral current sheets in the preflare phase, in hot X-ray kernels in the initial flare phase) a system of cyclotron lines in the spectrum of microwave radiation is likely to be formed. Such a line was obtained by Willson (1985) in the VLA observations at harmonics of the electron gyrofrequency. This communication interprets these observations on the basis of an active region model in which thermal cyclotron radiation is produced by hot plasma filling the magnetic tube in the corona above a group of spots. In this model the frequency of the recorded 1658 MHz line corresponds to the third harmonic of electron gyrofrequency, which yields the magnetic field (196 ± 4) G along the magnetic tube axis. The linewidth f/f 0.1 is determined by the 10% inhomogeneity of the magnetic field over the cross-section of the tube; the line profile indicates the kinetic temperature distribution of electrons over the tube cross-section with the maximum value 4 × 10⁶ K. Analysis shows that study of cyclotron lines can serve as an efficient tool for diagnostics of magnetic fields and plasma in the solar active regions and flares.     

Soft x-ray emitting regions in the solar corona

The soft X-ray emission of the solar corona is investigated by comparison of the signals of several broad band photometers carried on the Solrad 9 satellite, and sensitive to the region 0.5–20 Å. Temperature from 1.5 × 10⁶ to 25 × 10⁶ K have been measured with emission measure N _e ² dV ranging between 10⁵⁰ cm^–3 to 10⁴⁷ cm^–3.By means of the observational data and assuming magnetic confinement and hydrostatic equilibrium, the model of an active region is investigated. For temperatures larger than 10⁷K the emission is due to flare activity and two sets of emission measure are observed which appear to be related to the evolution of flares.     

On polarimetry in solar active regions

The miscentering by the Doppler compensator of the Locarno polarimeter is investigated in detail. It is shown that the linear polarization is strongly falsified by this effect which also occurs at the Crimean and Izmiran polarimeters.The new design for the exit slits of the Locarno polarimeter is described. It avoids the ambiguities in the determination of the magnetic field vector that always occur when using two exit slits.A new simple electronic setup avoids most of the difficulties which are usually involved in eliminating instrumental polarization and compensating intensity fluctuations.The observational techniques for solar polarimetry at the Locarno observatory are described.To avoid mutual influences of V and U, the line centre ₀ (corresponding to V = 0 and U = max.) must coincide with the centre of slit II. Only in this case we have 234-01     

On polarimetry in solar active regions

Phase retardation originating in a telescope is measured by means of polarizer and analyzer. The amount of this retardation depends only on the declination of the telescope. The retardation axes rotate with the Coudé image.The influence of this retardation on a Zeeman triplet, is measured photoelectrically. Consequences for solar polarimetry are discussed: Strong effects originate from the miscentering of the asymmetric Zeeman triplet introduced by the Doppler compensator. This yields a non-vanishing V-Stokes parameter in the central exit slit and thus the often observed crosstalk between the U and V-Stokes parameters.     

On polarimetry in solar active regions

Measurements of the circular polarization V in different lines show that the deduced magnetic field strength and flux are systematically influenced by variations of the line absorption coefficient from photosphere to spot and faculae.Disbalances between preceding and following flux seem to be due mainly to such variations rather than to real physical conditions in active regions.The spatial distribution of the normal component of the magnetic field in an active region near the disc center have been observed during two days using the temperature insensitive line Fe 6302.5. The initial field structure seems to become more and more bipolar. The increase of the flux exceeds that of the area thus suggesting the appearance of new magnetic fields. Backward extrapolation in time leads to a date of first appearance of the magnetic field which agrees with the appearance of first H anomalies.     

Current convection in solar active regions

Current carrying magnetic fields which penetrate sunspots can be unstable to current convective modes caused by the large gradient of electrical conductivity. The linear growth rates and wavelengths of the unstable modes are found. The unstable modes produce fine-scale vortices perpendicular to the magnetic field, which overshoot well into the solar corona. The modes provide a turbulent vorticity source at the photospheric footpoints of the field. This can cause braiding and reconnection of the coronal magnetic field. The modes twist the coronal magnetic field into loops with a typical radius of 200 km, consistent with recent X-ray observations.     

On polarimetry in solar active regions

High resolved magnetograms ( 3) were obtained 3 hrs before and 1 hr after a 1b flare, respectively, the only bright flare reported for that active region. Careful comparison between both magnetograms shows that the line-of-sight component of the active region magnetic field remains constant. In particular there is no simplification of the rather complicated field structure in connection with the flare. Magnetic flux and field gradients also do not show any variation above the 3 scale. Essential changes, however, were observed after 19 hrs without flare activity. This indicates that evolutionary field changes predominate over flare related variations.     

Velocity oscillations in solar plage regions

The properties of wave propagation in a perfectly electrically conducting, plane-stratified, inviscid, compressible atmosphere premeated by a horizontal magnetic field which varies with height are investigated. It is shown that a diagnostic diagram can be constructed through a generalization of the propagation equation to account for the presence of a magnetic field.The effect of the magnetic field on the oscillations in solar plages around the temperature minimum is studied and compared with the non-magnetic case based on the Bilderberg Continuum Atmosphere. Due to the joint action of ionization and the magnetic field, a potential well for internal gravity waves is formed. The properties of the trapped waves are in qualitative agreement with the observations.The Brunt-Väisälä frequencies in the presence (N) and absence (N ₀) of the magnetic field around the temperature minimum are shown to obey the relation N/N ₀ (1 + ²)^–1/2, where is the ratio between the Alfvén and the sonic speeds. The modified Brunt-Väisälä frequency (N) is decreased as the strength of the magnetic field increases. The magnetic effect makes the potential well shallower, thus shrinking the domain for the trapped gravity modes.     

On polarimetry in solar active regions

A new modulation procedure for Zeeman polarimeters is described and tested. The azimuth rotation by means of two steady /4-plates, combined with the common EOLM, has several advantages as compared to two-EOLM-polarimeters. The new polarimeter operates with two /4-plates which are alternately passed through the beam in front of the EOLM by means of an electro-mechanical chopper. The exact time of the /4-plate change is monitored by a photoelectric sensor. The obtained signals drive a number of relays by use of an intervening bistable electronic device. These relays allow to cut-off the erroneous Doppler signal mode and they furthermore distribute the U and Q signals into the corresponding lock-in amplifiers. As a first application of the new polarimeter, the linear polarization is measured in a sunspot penumbra. The telescope was first compensated for instrumental linear polarization down to 5 × 10^-4 by means of a tilted glass plate and well as for phase retardation down to 1° by means of a Bowen compensator.     

On polarimetry in solar active regions

The usefulness of magnetically sensitive iron lines Fe 5250.2, 6173.3 and 6302.3 for solar polarimetry is investigated. The line-to-continuum absorption coefficient ₀ for Fe 5250.2 depends strongly on temperature variations. Thus a photospheric calibration of polarimeter signals cannot be used for the different parts of an active region. This is also true for the Doppler calibration of longitudinal magnetographs.Fe 6302.5 is shown to be useful for polarimetry in active regions. A Milne-Eddington approximation is possible so that Unno's formulae are sufficient for the interpretation of polarimetric data.The ambiguity of the azimuth of the linear polarization prevents the determination of the true field structure with respect to the solar surface. The magnetic flux cannot be determined outside the disc centre without making assumptions for the unknown field structure. This determination is possible only for single stable sunspots; for irregular active regions the field configuration remains ambiguous even at the disc centre.     

24MgH+ in the solar photospheric spectrum

The²⁴MgH⁺ (A ¹⁺ –X ¹⁺) molecular lines have been identified in the photospheric spectrum. The rotational excitation temperature determined from the analysis of molecular line intensities of²⁴MgH⁺ is found to be of the order of 4850 K which corresponds to the photospheric temperature of the Sun. The CNDO/2 dipole moments of²⁴MgH⁺ for internuclear distance range: (1.3–2.1) Å in theX ¹⁺ state can be approximated byM(R)=4.92+1.33R. Estimations for the spontaneous emission Einstein coefficients (A _{v v} ) and the absorption oscillator strengths (f _{v v} ) for the (1, 0), (2, 0), and (2, 1) transitions in theX ¹⁺ state of the²⁴MgH⁺ ion are also made.Work partially supported by the CNPq, Brasilia under contract number 30.4076/77.     

The spectrum of Nixix in the solar corona

The wavelengths and intensities of the stronger transitions in the spectrum of Ni xix are reduced from measurements of the X-ray spectra of three coronal active regions. The new measured wavelengths are consistent with prediction by isoelectronic extrapolation from the wavelengths of well established transitions but are about 0.01 Å longer than previously accepted laboratory measurements. This difference appears to be crucial to the correct assignment of features in the coronal spectrum to Ni xix. The relative intensities of our new assignments to Ni xix are in broad agreement with the Loulergue-Nussbaumer calculations.     

Forbidden sulphur I lines in the solar spectrum

The [SI] lines are due to transitions within the 3s²3p⁴ ground configuration of neutral sulphur. The results are presented from a search for the [Si] lines in the Fraunhofer spectrum. Two identifications are proposed with faint features in the Fraunhofer spectrum: ¹D₂-¹S₀ at = 7725.02 Å and ³P₂-¹D₂ at = 10821.23 Å. Their measured equivalent widths are shown to confirm the value for the solar abundance of sulphur, logN _S = 7.21 (in the scale logN _H = 12.00), which is derived from the permitted high-excitation lines. These lines give the first convincing identification of [Si] lines in an astrophysical source.