Solar Sources of Severe Space Weather

Severe space weather is characterized by intense particle radiation from the Sun and severe geomagnetic storm caused by magnetized solar plasma arriving at Earth. Intense particle radiation is almost always caused by coronal mass ejections (CMEs) traveling from the Sun at super-Alfvenic speeds leading to fast-mode MHD shocks and particle acceleration by the shocks. When a CME arrives at Earth, it can interact with Earth's magnetopause resulting in solar plasma entry into the magnetosphere and a geomagnetic storm depending on the magnetic structure of the CME. Particle radiation starts affecting geospace as soon as the CMEs leave the Sun and the geospace may be immersed in the radiation for several days. On the other hand, the geomagnetic storm happens only upon arrival of the CME at Earth. The requirements for the production of particles and magnetic storms by CMEs are different in a number of respects: solar source location, CME magnetic structure, conditions in the ambient solar wind, and shock-driving ability of CMEs. Occasionally, intense geomagnetic storms are caused by corotating interaction regions (CIRs) that form in the interplanetary space when the fast solar wind from coronal holes overtakes the slow wind from the quiet regions. CIRs also accelerate particles, but when they reach several AU from the Sun, so their impact on Earth's space environment is not significant. In addition to these plasma effects, solar flares that accompany CMEs also produce excess ionization in the ionosphere causing sudden ionospheric disturbances. This paper highlights these space weather effects using space weather events observed by space and ground based instruments during of solar cycles 23 and 24

An electro-chemo-mechanical analysis of solid oxide fuel cell considering evolution of microstructure in porous electrode using phase-field method

For the numerical simulation of oxygen potential distributions in Solid Oxide Fuel Cell (SOFC), the time-evolution of the anode microstructure is reflected in the macroscopic electrical conductivities and the amount of triple-phase boundaries. Once the oxygen potential distributions are determined, the time-variation of the reduction-induced strains due to nonstoichiometry of oxide materials is calculated along with the thermal strains. These strains cause the macroscopic stresses in mutually constrained components. Thus, the capability of the proposed method is demonstrated in characterizing the aging degradation of the macroscopic electro-chemo-mechanical behavior of SOFC that is caused by the Nisintering in cermet microstructures during long-period control

Three-dimensional Evolution of Solar Wind during Solar Cycles 22-24

This paper presents the analysis of 3D evolution of solar wind density turbulence and speed at various levels of solar activity between solar cycles 22 and 24. The solar wind data has been obtained from interplanetary scintillation (IPS) measurements made at the Ooty Radio Telescope. Results show that (i) on the average, there was a downward trend in density turbulence from the maximum of cycle 22 to the deep minimum phase of cycle 23; (2) the scattering diameter of the corona around the Sun shrunk steadily towards the Sun, starting from 2003 to the smallest size at the deepest minimum, and it corresponded to a reduction of ~50% in density turbulence between maximum and minimum phases of cycle 23; (3) The latitudinal distribution of solar wind speed was significantly different between minima of cycles 22 and 23. At the minimum phase of solar cycle 22, when the underlying solar magnetic field was simple and nearly dipole in nature, the high-speed streams were observed from poles to ~30 deg. latitudes in both hemispheres. In contrast, in the long-decay phase of cycle 23, the sources of high-speed wind at both poles, in accordance with the weak polar fields, occupied narrow latitude belts from poles to ~60 deg. latitudes. Moreover, in agreement with the large amplitude of heliospheric current sheet, the low-speed wind prevailed the low- and mid-latitude regions of the heliosphere. (4) At the transition phase between cycles 23 and 24, the high levels of density and density turbulence were observed close to the heliospheric equator and the low-speed speed wind extended from equatorial- to mid-latitude regions. Results are consistent with the onset of the current cycle 24, from middle of 2009 and it has almost reached near to the maximum phase at the northern hemisphere of the Sun, but activity not yet developed in the southern hemisphere.Comment: 14 pages, 9 figures, Accepted for Publication in The Astrophysical Journa

Influence of Anode Functional Layers on Electrochemical Performance and Mechanical Strength in Microtubular Solid Oxide Fuel Cells Fabricated by Gel-Casting

Anode-supported microtubular solid oxide fuel cells (mT-SOFCs) using samaria-doped ceria (SDC) as electrolyte were fabricated, varying the composition and number of anode functional layers (AFLs), by combining the aqueous gel-casting and spray-coating techniques. Suitable aqueous slurry formulation of NiO-SDC was prepared using agarose as a gelling agent for gel casting of tubular supports. Afterward, 40:60 and 50:50 NiO:SDC (wt %) as AFLs and SDC electrolyte were deposited by spray-coating and subsequently co-sintered. Finally, mT-SOFCs with 2.5 mm outer diameter and thicknesses of 380 pm support; 0, 12, and 24 mu m AFLs; 15 mu m electrolyte; and 30 mu m cathode were obtained. The influence of AFLs on the performance and mechanical integrity was investigated for the three cells. For this purpose, electrochemical and mechanical tests at both macroscopic and micro-/nanometric scales (at the AFLs region) were determined by flexural strength and nanoindentation techniques, respectively. The results evidence that the use of AFLs with an adequate composition and microstructure in the mT-SOFCs is required to improve the performance and mechanical strength of the cell. The cell with a single-layer AFL of 50:50 NiO:SDC (wt %) and 12 pm thickness exhibited the best performance (0.52 W.cm(-2)) at 650 degrees C using hydrogen as fuel and air as oxidant

Multi-wavelength Diagnostics of the Precursor and Main phases of an M1.8 Flare on 2011 April 22

We study the temporal, spatial and spectral evolution of the M1.8 flare, which occurred in NOAA AR 11195 (S17E31) on 22 April 2011, and explore the underlying physical processes during the precursors and their relation to the main phase. The study of the source morphology using the composite images in 131 {\deg}A wavelength observed by the SDO/AIA and 6-14 keV revealed a multiloop system that destabilized systematically during the precursor and main phases. In contrast, HXR emission (20-50 keV) was absent during the precursor phase, appearing only from the onset of the impulsive phase in the form of foot-points of emitting loop/s. This study has also revealed the heated loop-top prior to the loop emission, although no accompanying foot-point sources were observed during the precursor phase. We estimate the flare plasma parameters viz. T, EM, power-law index, and photon turn-over energy by forward fitting RHESSI spectral observations. The energy released in the precursor phase was thermal and constituted ~1 per cent of the total energy released during the flare. The study of morphological evolution of the filament in conjunction with synthesized T and EM maps has been carried out which reveals (a) Partial filament eruption prior to the onset of the precursor emission, (b) Heated dense plasma over the polarity inversion line and in the vicinity of the slowly rising filament during the precursor phase. Based on the implications from multi-wavelength observations, we propose a scheme to unify the energy release during the precursor and main phase emissions in which, the precursor phase emission has been originated via conduction front formed due to the partial filament eruption. Next, the heated leftover S-shaped filament has undergone slow rise and heating due to magnetic reconnection and finally erupted to produce emission during the impulsive and gradual phases.Comment: 16 Pages, 11 Figures, Accepted for Publication in MNRAS Main Journa

Chromospheric Signatures of the Subdued Cycle 23/24 Solar Minimum in Microwaves

Coronal holes appear brighter than the quiet Sun in microwave images, with a brightness enhancement of 500 to 2000 K. The brightness enhancement corresponds to the upper chromosphere, where the plasma temperature is about 10000 K. We constructed a microwave butterfly diagram using the synoptic images obtained by the Nobeyama radio-heliograph (NoRH) showing the evolution of the polar and low latitude brightness temperature. While the polar brightness reveals the chromospheric conditions, the low latitude brightness is attributed to active regions in the corona. When we compared the microwave butterfly diagram with the magnetic butterfly diagram, we found a good correlation between the microwave brightness enhancement and the polar field strength. The microwave butterfly diagram covers part of solar cycle 22, whole of cycle 23, and part of cycle 24, thus enabling comparison between the cycle 23/24 and cycle 22/23 minima. The microwave brightness during the cycle 23/24 minimum was found to be lower than that during the cycle 22/23 minimum by approx.250 K. The reduced brightness temperature is consistent with the reduced polar field strength during the cycle 23/24 minimum seen in the magnetic butterfly diagram. We suggest that the microwave brightness at the solar poles is a good indicator of the speed of the solar wind sampled by Ulysses at high latitudes

Magnetic Field Strength in the Upper Solar Corona Using White-light Shock Structures Surrounding Coronal Mass Ejections

To measure the magnetic field strength in the solar corona, we examined 10 fast (> 1000 km/s) limb CMEs which show clear shock structures in SOHO/LASCO images. By applying piston-shock relationship to the observed CME's standoff distance and electron density compression ratio, we estimated the Mach number, Alfven speed, and magnetic field strength in the height range 3 to 15 solar radii (Rs). Main results from this study are: (1) the standoff distance observed in solar corona is consistent with those from a magnetohydrodynamic (MHD) model and near-Earth observations; (2) the Mach number as a shock strength is in the range 1.49 to 3.43 from the standoff distance ratio, but when we use the density compression ratio, the Mach number is in the range 1.47 to 1.90, implying that the measured density compression ratio is likely to be underestimated due to observational limits; (3) the Alfven speed ranges from 259 to 982 km/s and the magnetic field strength is in the range 6 to 105 mG when the standoff distance is used; (4) if we multiply the density compression ratio by a factor of 2, the Alfven speeds and the magnetic field strengths are consistent in both methods; (5) the magnetic field strengths derived from the shock parameters are similar to those of empirical models and previous estimates.Comment: Accepted for publication in ApJ, 11 Figures, 1 Tabl