Observations of Unresolved Photospheric Magnetic Fields in Solar Flares Using Fe i and Cr i Lines

The structure of the photospheric magnetic field during solar flares is examined using echelle spectropolarimetric observations. The study is based on several Fe i and Cr i lines observed at locations corresponding to brightest Hα emission during thermal phase of flares. The analysis is performed by comparing magnetic-field values deduced from lines with different magnetic sensitivities, as well as by examining the fine structure of I±V Stokes-profiles’ splitting. It is shown that the field has at least two components, with stronger unresolved flux tubes embedded in weaker ambient field. Based on a two-component magnetic-field model, we compare observed and synthetic line profiles and show that the field strength in small-scale flux tubes is about 2?–?3 kG. Furthermore, we find that the small-scale flux tubes are associated with flare emission, which may have implications for flare phenomenology.     

Photospheric Stratification of a Central Umbral Dot Using Stokes Profiles Recorded by Hinode: A Different Single-Case Study

We aim to study the physical nature of a central umbral dot (UD) close to disk center by analyzing full-Stokes spectra of the two Fe?i lines at 630 nm recorded by the spectropolarimeter on Hinode. Thermal and magnetic properties of the UD were directly inferred from Stokes profiles. Then, we applied the inversion code SIR to retrieve a single-component magnetic model atmosphere that recovers the observed full-Stokes profiles. The inversion results and direct inferences from the iron line pair are consistent. The studied UD does not show any signatures of upflows, but tends to show downflows. At the deeper-half of the photosphere (logτ>?1.0), the UD exhibits rapid changes in temperature with respect to its surroundings. The UD has a large magnetic field strength of about 3000 G without significant reduction at its center. Magnetic field lines are more vertical and twisted in the UD area than in the magnetic field of its surroundings. To explain the observational findings, we propose that the UD perturbs the funnel magnetic field of the umbra, making a tilt-ankle-knee configuration. A new interesting inference, deduced from the blending spectral lines in the observed wavelength interval, is that the shape and surface span of the UD in normalized intensity filtergrams computed at the core of the blending lines differ from the UD area seen in continuum intensity and in the filtergrams computed at the core of the iron line pair.     

耀斑色球压缩区的传播与谱线的不对称性

色球压缩区是耀斑大气动力学过程的一个基本特征，是产生色球谱线红不对称性的基础。本文基于压缩区从大气高层向低层传播的理论公式，在二种不同情况下，计算得到了压缩区内物质运动速度随高度和时间的变化．结果表明，色球蒸发区压力增量Δｐ为常数时压缩区之寿命比压缩区波阵面后的压力ｐ２为常数时要长得多，这就大大缓解了以往谱线不对称性的延续时间的理论值比观测值小的矛盾。形成高度不同的谱线具有不同程度的不对称性这一观测现象也同色球压缩区的传播特性相一致。     

Periodicities in the Time Series of Solar Coronal Radio Emissions and Chromospheric uv Emission Lines

A spectral analysis of the time series of daily values of ten solar coronal radio emissions in the range 275–1755 MHz, the 2800 MHz radio flux, several UV emission lines in the chromosphere and in the transition region, and sunspot number, for six successive intervals of 132 days each, during June 1997–July 1999 (26 months) showed that the spectral characteristics were not the same for all intervals. Details are presented for Interval 1, where there was no 27-day oscillation, and Interval 2, where there was a strong 27-day oscillation. In every interval, periodicities were remarkably similar in most of these indices, indicating that the solar atmosphere (chromosphere and corona) rotated as one block, up to a height of 150000 km. Above this height, the periodicities became obscure. Near the solar surface, sunspots showed extra or different periodicities, some of which vanished at low altitudes. For the 27-day feature as also for the long-term rise during 1996–1998, the maximum percentage changes were for radio emissions near 1350–1620 MHz.     

An Early Diagnostics of the Geoeffectiveness of Solar Eruptions from Photospheric Magnetic Flux Observations: The Transition from SOHO to SDO

In our previous articles (Chertok et al. in Solar Phys. 282, 175, 2013; Chertok et al. in Solar Phys. 290, 627, 2015), we presented a preliminary tool for the early diagnostics of the geoeffectiveness of solar eruptions based on the estimate of the total unsigned line-of-sight photospheric magnetic flux in accompanying extreme ultraviolet (EUV) arcades and dimmings. This tool was based on the analysis of eruptions observed during 1996?–?2005 with the Extreme-ultraviolet Imaging Telescope (EIT) and the Michelson Doppler Imager (MDI) onboard the Solar and Heliospheric Observatory (SOHO). Empirical relationships were obtained to estimate the probable importance of upcoming space weather disturbances caused by an eruption, which just occurred, without data on the associated coronal mass ejections. In particular, it was possible to estimate the intensity of a non-recurrent geomagnetic storm (GMS) and Forbush decrease (FD), as well as their onset and peak times. After 2010?–?2011, data on solar eruptions are obtained with the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). We use relatively short intervals of overlapping EIT–AIA and MDI–HMI detailed observations, and additionally, a number of large eruptions over the next five years with the 12-hour cadence EIT images to adapt the SOHO diagnostic tool to SDO data. We show that the adopted brightness thresholds select practically the same areas of arcades and dimmings from the EIT 195 Å and AIA 193 Å image, with a cross-calibration factor of 3.6?–?5.8 (5.0?–?8.2) for the AIA exposure time of 2.0 s (2.9 s). We also find that for the same photospheric areas, the MDI line-of-sight magnetic flux systematically exceeds the HMI flux by a factor of 1.4. Based on these results, the empirical diagnostic relationships obtained from SOHO data are adjusted to SDO instruments. Examples of a post-diagnostics based on SDO data are presented. As before, the tool is applicable to non-recurrent GMSs and FDs caused by nearly central eruptions from active regions, provided that the southern component of the interplanetary magnetic field near the Earth is predominantly negative, which is not predicted by this tool.     

Ultraviolet Observations of a Dynamic Event in the Solar Corona

The Solar Ultraviolet Measurements of Emitted Radiation instrument (SUMER) observations show high Doppler shifts and temporal variations in profiles of ultraviolet lines from low temperature gas in the corona above the active region NOAA 7974. The profiles indicate 100 km s^-1 flows coming from an almost stationary source that appears bright in the lines of N III and Si III. The variations in line-of-sight velocities and intensities suggest small knots of cooling plasma emanating from a small region high in the corona. A few arc sec sunward of the region where the cool flows are seen is an elongated region of enhanced higher temperature, low velocity Ne VI and Mg VI line emission.     

Plasma and Magnetic-Field Characteristics of Magnetic Decreases in the Solar Wind at 1 AU: Cluster-C1 Observations

Magnetic decreases (MDs) are structures observed in interplanetary space with significant decreases in magnetic-field magnitude. Events with little or no change in the field direction are called linear magnetic decreases (LMDs), the others are called nonlinear MDs (NMDs). In this article we focus on LMD and NMD trains, where LMD trains are defined as at least three LMDs in a row and NMD trains as trains (≥ three MDs in a row) that are not all linear. If the temporal separation between two MDs was shorter than five minutes, they were considered as one train event. A total of 16?273 MD trains (including 897 LMD trains and 15?376 NMD trains) were identified and studied. The details of the background magnetic-field and plasma (e.g. ion-density and velocity) features were examined and compared with the average solar-wind properties. LMD trains are found to occur in regions with relatively low magnetic-field strengths, high ion-number densities, and large plasma βs (ratio of the plasma thermal pressure to the magnetic pressure). In sharp contrast, NMD trains have plasma properties similar to the average solar wind. Forty-three LMD trains are related to interplanetary coronal mass ejections (ICMEs) (including 19 events that occurred in ICME sheaths and 24 in the ICME proper), while 222 LMD trains occurred in corotating interaction regions (CIRs), and the remaining 632 events in the normal solar wind. The LMD trains that occurred in ICME sheaths are thought to be associated with the generation mechanism of the mirror-mode instability. Only 552 of the NMD trains are related to ICMEs (including 236 events in ICME sheaths and 316 in ICMEs proper), while 3889 (25 %) NMD trains occurred in CIRs, and the remaining 71 % occurred in the normal solar wind. Because the NMD trains have various plasma properties that differ from the LMD trains, we suggest that NMD trains may be generated by different mechanisms, for instance by a steepening of Alfvén waves.     

Numerical Simulation of a Solar Active Region. I: Bastille Day Flare

We present three-dimensional unsteady modeling and numerical simulations of a coronal active region, carried out within the compressible single-fluid MHD approximation. We focus on AR 9077 on 14 July 2000, and the triggering of the X5.7 GOES X-ray class “Bastille Day” flare. We simulate only the lower corona, although we include a virtual photosphere and chromosphere below. The boundary conditions at the base of this layer are set using temperature maps from line intensities and line-of-sight magnetograms (SOHO/MDI). From the latter, we generate vector magnetograms using the force-free approximation; these vector magnetograms are then used to produce the boundary condition on the velocity field using a minimum energy principle (Longcope, Astrophys. J. 612, 1181, 2004). The reconnection process is modeled through a dynamical hyper-resistivity which is activated when the current exceeds a critical value (Klimas et al., J. Geophys. Res. 109, 2218, 2004). Comparing the time series of X-ray fluxes recorded by GOES with modeled time series of various mean physical variables such as current density, Poynting energy flux, or radiative loss inside the active region, we can demonstrate that the model properly captures the evolution of an active region over a day and, in particular, is able to explain the initiation of the flare at the observed time.     

Profile of an Average Magnetic Cloud at 1 au for the Quiet Solar Phase: Wind Observations

Using WIND magnetic field (MFI) and plasma (SWE) data, an `average' profile of an interplanetary magnetic cloud was developed in terms of five physical (scalar) quantities based on appropriately selected individual clouds. The period of study was from early 1995 to late in 1998, primarily during the quiet part of a solar cycle. The physical quantities are: magnetic field magnitude, proton density, solar wind bulk speed, proton thermal speed, and proton plasma beta. Selection of the clouds was based on two considerations: (1) their `quality', determined objectively from the application of a static magnetic field model of cloud field structure, had to be good, and (2) distant spacecraft approaches from the cloud axes were not accepted. Nineteen clouds resulted out of 35 original cases. A superposed epoch analysis was performed on the 5 parameters generating summary profiles of a generic magnetic cloud at 1 AU. The density within the generic magnetic cloud reached a distinct minimum near the center and peaked in the trailing part (closest to Sun) after a slow rise. The individual clouds fall into two classes, those that have such an enhanced density feature (about of them) and those that have an overall nearly flat density profile. For the first 85% of the generic magnetic cloud the bulk speed decreased almost uniformly by 45 km s^–1 indicating marked expansion over 1 AU. The field intensity peaked very near the cloud's center but was noticeably asymmetric. Proton thermal speed was quite symmetric with local maxima at the front, center, and rear. Proton plasma beta was low throughout the cloud (0.12 on average), but had a broad minimum at its center. The relative degree of fluctuation level for the parameters ranged from the most quiet for both speed and field magnitude, to the most `noisy' for proton plasma beta, with fluctuations in density and thermal speed at intermediate levels, all being below 0.2, based on a sample-scale of frac1100 of the cloud duration. These profiles may be useful in constraining future structural and thermodynamic models of clouds with regard to their solar birth conditions and interplanetary evolution.     

太阳活动区纵向电流密度的计算

本文利用太阳活动区光球横向磁场观测资料推算纵向电流密度分布,论述了具体的计算方法和取得的结果,并简要讨论了太阳活动区电流计算在太阳物理研究中的应用。     

The 26 December 2001 Solar Event Responsible for GLE63. I. Observations of a Major Long-Duration Flare with the Siberian Solar Radio Telescope

Ground level enhancements (GLEs) of cosmic-ray intensity occur, on average, once a year. Because they are rare, studying the solar sources of GLEs is especially important to approach understanding their origin. The SOL2001-12-26 eruptive-flare event responsible for GLE63 seems to be challenging in some aspects. Deficient observations limited our understanding of it. Analysis of additional observations found for this event provided new results that shed light on the flare configuration and evolution. This article addresses the observations of this flare with the Siberian Solar Radio Telescope (SSRT). Taking advantage of its instrumental characteristics, we analyze the detailed SSRT observations of a major long-duration flare at 5.7 GHz without cleaning the images. The analysis confirms that the source of GLE63 was associated with an event in active region 9742 that comprised two flares. The first flare (04:30?–?05:03 UT) reached a GOES importance of about M1.6. Two microwave sources were observed, whose brightness temperatures at 5.7 GHz exceeded 10 MK. The main flare, up to an importance of M7.1, started at 05:04 UT and occurred in strong magnetic fields. The observed microwave sources reached a brightness temperature of about 250 MK. They were not static. After appearing on the weaker-field periphery of the active region, the microwave sources moved toward each other nearly along the magnetic neutral line, approaching the stronger-field core of the active region, and then moved away from the neutral line like expanding ribbons. These motions rule out an association of the non-thermal microwave sources with a single flaring loop.     

Electron Temperature in the Solar Wind From a Kinetic Collisionless Model: Application to High-Latitude Ulysses Observations

We use a kinetic collisionless model of the solar wind to calculate the radial variation of the electron temperature and obtain analytical expressions at large radial distances. In order to be compared with Ulysses observations, the model, which initially assumed a radial magnetic field, has been generalized to a spiral magnetic field. We present a preliminary comparison with Ulysses observations in the fast solar wind at high heliospheric latitudes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Determination of the Topology Skeleton of Magnetic Fields in a Solar Active Region

Magnetic topology has been a key to the understanding of magnetic energy re-lease mechanism. Based on observed vector magnetograms, we have determined the three-dimensional (3D) topology skeleton of the magnetic fields in the active region NOAA 10720.The skeleton consists of six 3D magnetic nulls and a network of corresponding spines, fans,and null-null lines. For the first time, we have identified a spiral magnetic null in Sun's corona.The magnetic lines of force twisted around the spine of the null, forming a 'magnetic wreath'with excess of free magnetic energy and resembling observed brightening structures at extra-ultraviolet (EUV) wavebands. We found clear evidence of topology eruptions which are re-ferred to as catastrophic changes of topology skeleton associated with a coronal mass ejection(CME) and an explosive X-ray flare. These results shed new lights on the structural complex-ity and its role in explosive magnetic activity. The concept of flux rope has been widely used in modelling explosive magnetic activity, although their observational identity is rather ob-scure or, at least, lacking of necessary details up to date. We suggest that the magnetic wreath associated with the 3D spiral null is likely an important class of the physical entity of flux ropes.     

Study of Corotating Interaction Regions in the Ascending Phase of the Solar Cycle: Multi-spacecraft Observations

We combined simultaneous solar wind observations from five different spacecraft: Helios 1, Helios 2, IMP-8, Voyager 1 and Voyager 2, from November 1977 to February 1978 (Carrington rotations 1661?–?1664, ascending phase of Solar Cycle 21). The concurrence of the five trajectories makes this interval unique for the purpose of studying solar wind dynamics during this phase of the cycle. We analyzed the observations identifying five corotating interaction regions (CIRs) and produced maps of interplanetary large-scale features, unifying and summarizing the data. The maps show the compressive events and the magnetic sectors associated with the solar wind streams causing the CIRs. We analyzed the relative position of the stream interfaces immersed within the CIRs. About 70 % of the stream interfaces in this study were located closer to the forward edge of the CIR. From the analysis of the geometry of the stream interfaces, we found that all the CIRs presented latitudinal tilts, having their fronts pointing towards the ecliptic plane and their tails northwards or southwards. These results are in agreement with the origin of the fast streams coming from mid-latitude coronal holes and the predominance of forward shocks over reverse shocks bounding the CIRs, which characterize this phase of the cycle. From the analysis of the ratio of dynamic pressures between fast and slow solar wind streams associated with the CIRs, we found that in about 60 % of the cases the fast stream was transferring momentum to the slow one ahead, but in the rest of the cases the momentum was flowing sunward. This result indicates significant inhomogeneities in the solar wind streams during the ascending phase of the cycle that affect the local form and evolution of CIR events. We did a limited comparison between a global magneto-hydrodynamic (MHD) model of SW flows and the orientation of the SI from in-situ observations, we found, in general, a qualitative agreement between the pressure profiles at 1 AU predicted by the model and the inclinations of the stream interfaces deduced from the data analysis.     

On Mechanisms of Proton Perpendicular Heating in the Solar Wind:Test Results Based on Wind Observations

The solar wind protons undergo significant perpendicular heating when they propagate in the interplanetary space.Stochastic heating and cyclotron resonance heat...     

On the Transits of Solar System Objects in the Forthcoming Planck Mission: Data Flagging and Coeval Multi-Frequency Observations

In the context of current and future microwave surveys mainly dedicated to the accurate mapping of Cosmic Microwave Background (CMB), mm and sub-mm emissions from Solar System will represent a potential source of contamination as well as an opportunity for new Solar System studies. In particular, the forthcoming ESA Planck mission will be able to observe the point-like thermal emission from planets and some large asteroids as well as the diffused Zodiacal Light Emission (ZLE). After a brief introduction to the field, we focus on the identification of Solar System discrete objects in the Planck time ordered data.     

Oxygen-Rich Cool Stars in the Cepheus Region. New Spectroscopic Observations of Selected Objects from the KP2001 Catalog

Astrophysics - Spectra taken with the 2.6-m telescope at the Byurakan Astrophysical Observatory (BAO) in the λλ4000-7250Å range for eight M giants from the KP2001 catalog confirm...