Unique solar flare of September 22, 2011: The suction effect

The specific features in the development of an X1 solar flare, which occurred on September 22, 2011, and was registered with the Atmospheric Imaging Assembly (AIA) device onboard the Solar Dynamics Observatory (SDO) in the UV line (λ = 304 Å, He II), are analyzed. During the flare, which lasted about 12 h, cold plasma was sucked up with an increasing velocity from a very distant region into the low-lying hot region of flare energy release along a flat helical trajectory. This phenomenon fundamentally differs from a surge ejection, when matter previously ejected from the flare region returns to the flare hot zone under the action of gravity. Suction of cold plasma “from outside” into the hot flare region is interpreted in the scope of the rope flare mechanism, according to which an extremely inhomogeneous plasma density distribution in the cross-section originates in an emerging magnetic rope. In the region with a sharply decreased density (which is the suction region), the drift velocity in the current chanel can reach the ion thermal velocity, which inevitably results in the excitation of plasma turbulence and anomalous resistance, i.e., in the development of a flare.     

Magnetic field variations accompanying the filament eruption and the flare related to coronal mass ejections

Field variations in the region of eruptive event of June 7, 2011, associated with the filament eruption (FE), flare, and coronal mass ejection are studied based on vector measurements of the photospheric magnetic field with the SDO/HMI instrument. Variations of the module (B), the radial (Br) and transverse (Bt) components of the magnetic induction, and the inclination angle (α) of field lines to the radial direction from the center of the Sun are analyzed. It is shown that the strongest changes of the field before the event were located near the base of the southeastern leg of the eruptive filament; after the beginning of the event, they were located in the CME flare region. It is suggested that the FE is associated with two episodes of strong and rapid field variations: before the beginning of the slow filament rise and before its sudden acceleration. For the first time, variations of the inclination angles of the field lines over time in different parts of the eruptive event are studied in detail. It was found that the inclination angles of the field lines decrease in the vicinity of its channel during the slow rise of the filament, and the inclination angles of the field lines increase sharply after the beginning of the flare in the flare region in the vicinity of the neutral line.     

地球磁尾中重联产生的磁流通管的运动

本文通过MHD理论研究了细磁流通管在二维静止平衡介质中的运动.用地球磁尾中的一维细丝来表示流通管,通过数值模拟可以得到细丝随时间变化的一些性质.重联产生的细丝磁场比周围磁场偶极性更强,运动时表现出了很强的地向流.结果还显示了阿尔芬波、慢激波等MHD波从磁层的赤道面传播到地球电离层上并部分地反射回来.细丝在电离层上的足点的赤道向运动滞后于赤道面上的地向运动.虽然在模拟中细丝的初始等离子体压强低于周围压强,但是当它开始迅速向地球方向运动时,它的等离子体压强很快上升到与周围压强相当,甚至有时候大于周围压强的值.     

木星磁尾重联产生的磁通量管向内运动的数值模拟

本文通过利用包含离心力的自洽、轴对称稳态木星磁层模型,以及等离子体细丝运动理论对磁尾重联产生的磁通量管向内输运进行研究.基于细丝运动理论模型,通过MHD数值模拟我们可以得到磁通量管随时间变化的许多特性.模拟结果表明,重联产生的磁通量管向内运动可到达10RJ以内,磁通量管赤道部分的速度可以达到350km·s-1左右,表现出很强的向行星方向的流动.初始磁通量管中的等离子体密度和压强均小于周围介质,随着它迅速向木星方向运动,它的等离子体密度由于体积压缩逐渐上升,等离子体压强则逐渐上升到与周围介质相当.磁通量管在电离层上足点向赤道方向的运动滞后于它在赤道面上向行星方向的运动.     

太阳耀斑电子束在日冕区产生的静电不稳定性

依据太阳耀斑爆发特征,建立了双耀斑电子束与日冕背景相作用的模型,数值结果表明,该等离子体系统将激发静电不稳定性,其时间增长率ω_i受耀斑热束密度与日冕背景密度比值（n_h/n_o）以及耀斑冷束相对论电子密度与日冕密度值（n_c/n_o）影响较大,并随它们增大而增大,其实频大小在耀斑热束等离子体频率附近。因此,此系统可激发大于1GHz的高频静电辐射,这些结果对揭示耀斑粒子在日冕空间传播行为有一定作用,并可用于探讨高频Ⅲ型射电机制。     

Influence of Whistler Turbulence on Fast Electron Distribution and Their Microwave Emissions in a Flare Loop

The influence of whistlers on the distribution and gyrosynchrotron radiation of fast electrons injected into a coronal magnetic trap is considered. The kinetic equation in the Fokker–Planck approximation with consideration of fast electron scattering, both on background plasma particles and on whistlers, is solved for an inhomogeneous trap. It is supposed that the source of whistlers is a nonstationary process of flare energy release. Having found the fast electron distribution, we can calculate their gyrosynchrotron microwave emission. The influence of nonthermal electron scattering on whistlers are compared with the effects of scattering on Coulomb collisions. It is shown that whistlers considerably modify the emission characteristics of a loop at a certain energy density; in particular, they steepen the frequency spectrum. This is useful for microwave diagnostics of plasma turbulence in the flare loop.     

Variations of the Photospheric Magnetic Field Following the Eruptive Event on June 7, 2011

Field variations in the region of the eruptive event on June 7, 2011 are studied based on vector measurements of the photospheric magnetic field by the SDO/HMI instrument. Variations of the modulus (B), the radial (B_r) and the transverse (B_t) components of the magnetic induction, and the inclination angle (α) of the field lines to the radial direction from the center of the Sun are analyzed. It is found that, in the part of the flare region near the polarity inversion line (PIL) after the onset of the flare, the magnitude and the transverse component of the magnetic induction as well as the angles α abruptly increase. During the slow rise of filament near its channel, the inclination angles of the field lines decrease. It is shown that diverging flare ribbons are above the regions of the photosphere with local maxima of the field modulus and with deep minima of the inclination angles of the field lines at all stages of their existence over their entire length with the exception of small areas. It is established that the azimuth decreases after the onset of the flare near the PIL of the photospheric magnetic field, which means an increase in the shear. On the contrary, at a distance from the PIL there is a slight decrease in the shear.

     

Modeling of the Fine Filament Structure of Quiescent Solar Prominences

Several new models of solar prominences with helical geometry of the magnetic field were suggested; the possibility of the construction of a filament model with a fine filament structure was shown. For the first time, numerical solutions of the magnetohydrostatical problem, in which a fine structure of an equilibrium solar filament is expressed not only in the vertical direction but also in the direction along the filament axis, were obtained.     

Structure and development of the spotless flare on March 16, 1981

The morphological peculiarities of the 1N (N09W22) two-ribbon spotless flare on March 16, 1981, as well as its connection with a magnetic field, have been considered. In contrast to major flares of the active region, this spotless flare is characterized by a large-scale development process, a large distance from the magnetic neutral line, and the absence of the spread of the ribbons. The development of the flare had four periods. At the beginning of each period, a sharp increase in the brightness of the flare was observed along with a simultaneous decrease in the area of the flare ribbons. The areas of the ribbons varied synchronously during all of the periods. However, the situation changed abruptly near the maximum: the area of one of the ribbons increased, whereas the area of the other ribbon decreased. In our opinion, this behavior is a manifestation of real physical processes in the flare source and was a precursor of the beginning of the flare decay. The magnetic field and its topology, as well as the cellular structure of the chromosphere, were primarily responsible for the evolution of the flare. Almost all of the mottles and bright parts of the flare were localized in the immediate vicinity of magnetic hills with field intensities from 80 to 250 G. The main structural elements of the flare have been identified. A phenomenon called the tunnel effect has been revealed: the flare progresses inside a tunnel formed by the system of dark arch structures (filaments). The results indicate that spotless flares apparently constitute a specific class of flare phenomena and the study of them is of great interest for understanding of the origin of solar flares.     

Moreton wave, “EIT wave”, and type II radio burst as manifestations of a single wave front

We show that a Moreton wave, an “EIT wave,” and a type II radio burst observed during a solar flare of July 13, 2004, might have been a manifestation of a single front of a decelerating shock wave, which appeared in an active region (AR) during a filament eruption. We propose describing a quasi-spheroidal wave propagating upward and along the solar surface by using relations known from a theory of a point-like explosion in a gas whose density changes along the radius according to a power law. By applying this law to fit the drop in density of the coronal plasma enveloping the solar active region, we first managed to bring the measured positions and velocities of surface Moreton wave and “EIT wave” into correspondence with the observed frequency drift rate of the meter type II radio burst. The exponent of the vertical coronal density falloff is selected by fitting the power law to the Newkirk and Saito empirical distributions in the height range of interest. Formal use of such a dependence in the horizontal direction with a different exponent appears to be reasonable up to distances of less than 200 Mm around the eruption center. It is possible to assume that the near-surface shock wave weakens when leaving this radius and finally the active region, entering the region of the quiet Sun where the coronal plasma density and the fast-mode speed are almost constant along the horizontal.     

Solar flare model: Comparison of the results of numerical simulations and observations

The electrodynamic flare model is based on numerical 3D simulations with the real magnetic field of an active region. An energy of ∼10³² erg necessary for a solar flare is shown to accumulate in the magnetic field of a coronal current sheet. The thermal X-ray source in the corona results from plasma heating in the current sheet upon reconnection. The hard X-ray sources are located on the solar surface at the loop foot-points. They are produced by the precipitation of electron beams accelerated in field-aligned currents. Solar cosmic rays appear upon acceleration in the electric field along a singular magnetic X-type line. The generation mechanism of the delayed cosmic-ray component is also discussed.     

Magnetic fields in a limb flare on July 19, 2012

The magnetic field intensity in the limb solar flare of July 19, 2012, was measured by the Zeeman bisector splitting in the H _α line. An average magnetic field is established to attain 200 G at a measurement error of ±100 G in the top of a radiant flare loop at a height of about 40 Mm. The confinement of a strong magnetic field in the high coronal arcade at a relatively weak external field of the corona (about 1–2 G) is considered using a model of a force-free magnetic flux rope with a fine magnetic structure at a scale of about 300 km.     

Microwave radiation of solar active regions before X flares according to the RATAN-600 observations in 2011

The evolution of the microwave radiation from four active regions, where strong X-ray flares (X-class, GOES) occurred in 2011, has been studied. Daily multiwavelength RATAN-600 radio observations of the Sun in the 1.6–8.0 cm range have been used. It has been indicated that the radiosource above the photospheric magnetic field neutral line (above the region with the maximal convergence of the fields opposite in sign) becomes predominant in the structure of the active region microwave radiation one to two days before a powerful flare as in the eruptive events previously studied with RATAN-600. The appearance of such a radiosource possibly reflects the current sheet formation in the corona above the active region. The energy necessary for a flare is stored in the magnetic field of active region, which can be considered as a factor for predicting a powerful flare.     

Modeling the responses of the middle latitude ionosphere to solar flares

In this paper, we investigate the solar flare effects of the ionosphere at middle latitude with a one-dimensional ionosphere theoretical model. The measurements of solar irradiance from the SOHO/Solar EUV Monitor (SEM) and GOES satellites have been used to construct a simple time-dependent solar flare spectrum model, which serves as the irradiance spectrum during solar flares. The model calculations show that the ionospheric responses to solar flares are largely related to the solar zenith angle. During the daytime most of the relative increases in electron density occur at an altitude lower than 300 km, with a peak at about 115 km, whereas around sunrise and sunset the strongest ionospheric responses occur at much higher altitudes (e.g. 210 km for a summer flare). The ionospheric responses to flares in equinox and winter show an obvious asymmetry to local midday with a relative increase in total electron content (TEC) in the morning larger than that in the afternoon. The flare-induced TEC enhancement increases slowly around sunrise and reaches a peak at about 60 min after the flare onset.     

Magnetohydrodynamic simulation of a solar flare: 1. Current sheet in the corona

The results of modeling the preflare situation in the solar corona, obtained using a numerical solution for a complete set of three-dimensional MHD equations, are reviewed. Any assumptions concerning the flare development character or the active region’s behavior before a flare are not introduced. The initial and boundary conditions on the photosphere are specified from magnetic field measurements before a flare. The photospheric field sources are approximated by magnetic dipoles. The usage of the PERESVET program indicated that a current sheet is formed in the vicinity of a singular magnetic field line in the corona. The sheet is formed due to disturbances coming from the photosphere. The energy necessary for a flare is stored in the current sheet magnetic field during 2–3 days. The main construction principles of the PERESVET program, which makes it possible to use the maps of a measured photospheric field as boundary conditions, are presented.     

Geospheric effects of the solar flare of December 13, 2006

Disturbances in the magnetic field and magnetospheric and ionospheric plasma registered on December 14–16, 2006, during a strong magnetic storm caused by a solar flare of 4B/X3.4 class are studied. It is shown that in the north of Yakutia, interactions between the Earth’s magnetosphere and the region of high dynamic pressure of the solar wind led to the formation of sporadic layers in the ionospheric E and F regions, depletion of the critical frequency of the F2 layer, and total absorption. At the end of the magnetic storm’s main phase, anomalously high values of foF2 exceeding the quiet level by a factor of 1.5–1.7 were detected. It was found that the disturbances detected by ground-based observatories had developed on the background of changes in the temperature, density, and the pitch-angle distribution of particles at the geostationary orbit manifesting radial shifts of magnetospheric structures (magnetopause, cusp/cleft, and plasma sheet) relative to the observation points. A conclusion is drawn that in this case, changes in the near-Earth plasma and magnetic field manifest the dynamics of the physical conditions at the magnetospheric boundary and diurnal rotation of the Earth.     

Observations of subterahertz radiation of solar flares with an RT-7.5 radiotelescope

We analyzed unique observations of two flare events at frequencies of 93 and 140 GHz. The observations were carried out with an RT-7.5 radiotelescope at the Bauman State Technical University (Moscow) using the method of continuous active region tracking with spatial resolutions of 2.5 (at a frequency of 93 GHz) and 1.5 arc-minutes (at 140 GHz). The light curves of the bursts were analyzed and compared with the time profiles of soft and hard X-ray emission obtained by the GOES and RHESSI spacecraft. The radio delete this word flux density spectra were plotted. It was found that the radiation flux at a frequency of 140 GHz exceeded the flux at 93 GHz. This constitutes a new independent confirmation of the presence of a subterahertz flare component, the appearance of which may be associated with the thermal radiation of the hot plasma at the base of flare loops.     

Model of quiescent prominence with the helical magnetic field

A new exact analytical solution of the magnetohydrostatic problem describes the equilibrium of a solitary, dense-cool solar filament maintained against the gravity by magnetic force in hot solar corona at heights up to 20–40 Mm. The filament is assumed to be uniform along the axis (the translation symmetry). The magnetic field of the filament has the helical structure (magnetic flux rope) with a typical strength of a few Gauss in the region of minimal temperature (about 4000 K). The model can be applied to the quiescent prominence of both normal and inverse magnetic polarity.

     

Probing the magnetic topology of coronal mass ejections by means of Ulysses/HI-SCALE energetic particle observations

In this work, solar flare energetic particle fluxes (E_e 42 keV) observed by the HI-SCALE instrument onboard Ulysses, a spacecraft that is probing the heliosphere in 3-D, are utilized as diagnostics of the large-scale structure and topology of the interplanetary magnetic field (IMF) embedded within two well-identified interplanetary coronal mass ejection (ICME) structures. On the basis of the energetic solar flare particle observations firm conclusions are drawn on whether the detected ICMEs have been detached from the solar corona or are still magnetically anchored to it when they arrive at 2.5 AU. From the development of the angular distributions of the particle intensities, we have inferred that portions of the ICMEs studied consisted of both open and closed magnetic field lines. Both ICMEs present a filamentary structure comprising magnetic filaments with distinct electron anisotropy characteristics. Subsequently, we studied the evolution of the anisotropies of the energetic electrons along the magnetic field loop-like structure of one ICME and computed the characteristic decay time of the anisotropy which is a measure of the amount of scattering that the trapped electron population underwent after injection at the Sun.