I. V. Kudryavtsev

Solar flares registered by the IRIS spectrometer onboard the CORONAS-F satellite: Peculiarities of the X-ray emission

The X-ray spectrometer IRIS was designed to register the fluxes of quanta with energies ranging from 2 to 250 keV in various time-resolution modes: 0.01, 1.0, and 2.5 s in 4, 64, and 12 channels, respectively. Owing to the high instrument sensitivity, individual spikes of tens of milliseconds can be distinguished in the time structure of the X-ray flare emission. The time spectral analysis carried out for the X-ray emission of a number of registered flares points to the presence of a quasiperiodic structure with characteristic times of the same order. For the flares of December 19, 2001, and August 10, 2002, the process of energy release has been considered with a 1-s time resolution, and its periodic character has been revealed. For the flare of December 19, 2001, the energy spectra of the hard X-ray emission have been simulated by thermal and nonthermal models. It has been shown that the both models can describe this emission.

Intensity and polarization of the hard X-ray radiation of solar flares at the top and footpoints of a magnetic loop

The spatial distributions of the intensity and polarization of hard X-rays (HXR) were modeled based on a numerical solution of the nonstationary kinetic Fokker-Plank equation. Two cases of nonstationary long injections (∼10 s) of nonthermal electrons at the top of a flaring loop are considered. In the first case, the injection occurs anisotropically in a cone directed along the field to one of the loop’s footpoints. In the second case, electrons are injected isotropically.In the case of the anisotropic injection, the degree of HXR polarization in the footpoints is low and does not exceed 8%. At the same time, the polarization of the radiation from the top in a range of photon energies from 30 to 60 keV reaches 43% at the beginning of injection. Significantly different spatial distributions of HXR characteristics are obtained for the isotropic injection of electrons. In this case, the calculated degree of HXR polarization from the top exhibits a strong variability: it reaches 15–18% at the beginning of injection at photon energies of 30–90 keV, decreases to zero after 5 s, and then, changing its sign, increases to 8%. In the footpoint of the loop, HXR is in fact nonpolarized. The features of the spatial distributions obtained in the simulation process can be measured with X-ray telescopes-polarimeters in future experiments and used for diagnosing the pitch angle distribution of accelerated electrons.

Spatial brightness distribution of hard X-Ray emission along flare loops

The work is devoted to the simulation of recently discovered hard X-ray and gamma radiation sources localized near the top of solar flare loops. The calculations were performed in the context of a model of a flare magnetic loop inhomogeneous with respect to the magnetic field. Two cases of injection were considered: isotropic and anisotropic along the loop axis. The distributions of electrons along the loop are found by solving the nonstationary relativistic kinetic equation in the Fokker-Planck form. Based on the calculated electron distribution functions, the spatial brightness distribution of hard X-ray and gamma radiation has been calculated. Radiation characteristics are compared for different sets of injection parameters.

Possible influence of climate factors on the reconstruction of the cosmogenic isotope 14C production rate in the earth’s atmosphere and solar activity in past epochs

The paper considers the probable influence of variations of the global temperature and carbon dioxide concentration in the Earth’s atmosphere on the results of reconstruction of the production rate of the cosmogenic isotope 14C in the terrestrial atmosphere for the period from the early 15th to the mid 19th century. This time interval covers the Spörer, Maunder, and Dalton minima of solar activity, as well as the Little Ice Age. It was shown that the climate changes that occurred during the Little Ice Age should be taken into account. In the Maunder and Spörer minima of solar activity, the 14C generation rate may be comparable to the values for the Dalton minimum, while exclusion of the climate effect yields extremely large values of the 14C production rate for these grand minima. In the solar activity reconstruction for past epochs, this circumstance should be taken into consideration via measurements of the 14C concentration on a long time scale.

Influence of climatic factors on the past atmospheric content of the C-14 isotope

Reconstructions of solar activity in the past epochs based on information on the past atmospheric content of the cosmogenic 14C isotope are nowadays actively discussed. The 14C isotope is generated in the atmosphere of the Earth under the influence of cosmic rays, and its concentration in annual tree rings carries information on the past solar activity. However, the concentration of this isotope in annual tree rings may also be influenced by climatic factors. In the present work, the possible correlation between variations in the 14C atmospheric content and in the Earth’s global temperature from the late 14th century to the middle of the 19th century is studied. It is shown that variations in global temperature may produce changes in the 14C atmospheric content and consequently have to be taken into account in reconstructions of the past solar activity.

The nitrate content of Greenland ice and solar activity

Past solar activity is studied based on analysis of data on the nitrate content of Greenland ice in the period from 1576–1991. Hundred-year (over the entire period) and quasi-five-year (in the middle of the 18th century) variations in the nitrate content are detected. These reflect the secular solar-activity cycle and cyclicity in the flare activity of the Sun.

Reconstruction of the energy spectrum of electrons accelerated in the April 15, 2002 solar flare based on IRIS X-ray spectrometer measurements

We reconstruct the energy distribution of electrons accelerated in the April 15, 2002 solar flare on the basis of the data from the IRIS X-ray spectrometer onboard the CORONAS-F satellite. We obtain the solution to the integral equations describing the transformation of the spectrum of X-ray photons during the recording and reconstruction of the spectrum of accelerated electrons in the bremsstrahlung source using the random search method and the Tikhonov regularization method. In this event, we detected a singularity in the electron spectrum associated with the existence of a local minimum in the energy range 40–60 keV, which cannot be detected by a direct method.

Reconstructions of the heliospheric modulation potential and Wolf numbers based on the content of the 14C isotope in tree rings during the Maunder and Spörer minimums

Data on variations in the content of the 14C cosmogenic isotope in tree rings and the Earth’s atmosphere (Δ14C) make it possible to study the behavior of solar activity (SA) in previous centuries and millenniums. The latter is related to the fact that SA temporal variations result in a change in the IMF (Interplanetary Magnetic Field) parameters and, as a consequence, in the galactic cosmic ray (GCR) flux, under the action of which the 14C isotope is produced in the Earth’s atmosphere. This makes it possible to study SA history based on data on the 14C isotope content in tree rings. However, in this case we have several difficulties related to climate change. Climate changes result in carbon redistribution between natural reservoirs, which is reflected in radiocarbon data and results in solar signal distortion. The effect of variations in the global temperature and carbon dioxide concentration on the reconstruction of the heliospheric modulation potential and Wolf numbers from the late 14th century to the early 19th century is considered. It has been shown that the radiocarbon data do not make it possible to conclude that SA during the Maunder minimum was extremely low as compared to SA during the Dalton minimum.

Hard X rays of relativistic electrons accelerated in solar flares

The direction and polarization degree of hard X rays (HXRs) in solar flares are studied. The continuous injection of relativistic electrons, which is implemented in powerful flares, is considered. The stationary relativistic kinetic equation is studied by using the method of expansion in terms of the Legendre polynomial and by integrating the equations for the expansion coefficients. The HXR characteristics are calculated using the bremsstrahlung relativistic cross-section for different angular and energetic electron distributions in the acceleration region. A high linear polarization degree of HXRs (∼35%) has been obtained for narrow (∼cos6θ) beams of electrons with a soft spectrum (∼E−6); the polarization degree decreases with increasing quanta energy, whereas the directivity of a high-energy emission increases. This effect is absent for a nonrelativistic approximation. The considered model is applied to one of the most powerful flares in cycle 23, registered on October 28, 2003. The measured polarization degree values at relativistic energies (0.2–0.4 and 0.4–1 MeV) agree with the results achieved in the considered model when the electron energy spectrum index (δ = 2.5), angular distribution part (∼cos6θ), and the spectrum cutoff energy (Emax = 1.3 MeV) were specified.

Peculiarities of X-ray emission of the solar flare on 29 October 2002

[1] Time structure of the soft and hard X-ray emission of the solar flare on 29 October 2002 is studied. The flare began at 2148:49 UT and was measured by the IRIS spectrometer on board the CORONAS-F spacecraft. High sensitivity of the device made it possible to detect in the flare X-ray emission a pulse structure with timescale of the order of tens of milliseconds. Similarly, spike structure was observed on board of Compton Gamma Ray Observatory. A quasiperiodic structure with a magnitude of the order of 10 s was revealed by means of spectral time analysis. This structure can be explained by magnetohydrodynamic sausage-mode oscillations generated in the flare loop with the characteristic parameters of the plasma of the coronal region of the solar atmosphere. The evolution of the energetic spectrum of the hard X-ray emission of this flare with the resolution of 1 s during the entire event is studied.