1,200 research outputs found
The First Ground Level Enhancement Event of Solar Cycle 24: Direct Observation of Shock Formation and Particle Release Heights
We report on the 2012 May 17 Ground Level Enhancement (GLE) event, which is
the first of its kind in Solar Cycle 24. This is the first GLE event to be
fully observed close to the surface by the Solar Terrestrial Relations
Observatory (STEREO) mission. We determine the coronal mass ejection (CME)
height at the start of the associated metric type II radio burst (i.e., shock
formation height) as 1.38 Rs (from the Sun center). The CME height at the time
of GLE particle release was directly measured from a STEREO image as 2.32 Rs,
which agrees well with the estimation from CME kinematics. These heights are
consistent with those obtained for cycle-23 GLEs using back-extrapolation. By
contrasting the 2012 May 17 GLE with six other non-GLE eruptions from
well-connected regions with similar or larger flare size and CME speed, we find
that the latitudinal distance from the ecliptic is rather large for the non-GLE
events due to a combination of non-radial CME motion and unfavorable solar B0
angle, making the connectivity to Earth poorer. We also find that the coronal
environment may play a role in deciding the shock strength.Comment: 16 pages, 4 figures, 1 tabl
Ground Level Enhancement in the 2014 January 6 Solar Energetic Particle Event
We present a study of the 2014 January 6 solar energetic particle (SEP)
event, which produced a small ground level enhancement (GLE), making it the
second GLE of this unusual solar cycle 24. This event was primarily observed by
the South Pole neutron monitors (increase of ~2.5%) whereas a few other neutron
monitors recorded smaller increases. The associated coronal mass ejection (CME)
originated behind the western limb and had the speed of 1960 km/s. The height
of the CME at the start of the associated metric type II radio burst, which
indicates the formation of a strong shock, was measured to be 1.61 Rs using a
direct image from STEREO-A/EUVI. The CME height at the time of GLE particle
release (determined using the South Pole neutron monitor data) was directly
measured as 2.96 Rs, from the STEREO-A/COR1 white-light observations. These CME
heights are consistent with those obtained for the GLE71, the only other GLE of
the current cycle as well as cycle-23 GLEs derived using back-extrapolation.
GLE72 is of special interest because it is one of the only two GLEs of cycle
24, one of the two behind-the-limb GLEs and one of the two smallest GLEs of
cycles 23 and 24
Radio-loud CMEs from the disk center lacking shocks at 1 AU
A coronal mass ejection (CME) associated with a type II burst and originating
close to the center of the solar disk typically results in a shock at Earth in
2-3 days and hence can be used to predict shock arrival at Earth. However, a
significant fraction (about 28%) of such CMEs producing type II bursts were not
associated with shocks at Earth. We examined a set of 21 type II bursts
observed by the Wind/WAVES experiment at decameter-hectometric (DH) wavelengths
that had CME sources very close to the disk center (within a central meridian
distance of 30 degrees), but did not have a shock at Earth. We find that the
near-Sun speeds of these CMEs average to ~644 km/s, only slightly higher than
the average speed of CMEs associated with radio-quiet shocks. However, the
fraction of halo CMEs is only ~30%, compared to 54% for the radio-quiet shocks
and 91% for all radio-loud shocks. We conclude that the disk-center radio-loud
CMEs with no shocks at 1 AU are generally of lower energy and they drive shocks
only close to the Sun and dissipate before arriving at Earth. There is also
evidence for other possible processes that lead to the lack of shock at 1 AU:
(i) overtaking CME shocks merge and one observes a single shock at Earth, and
(ii) deflection by nearby coronal holes can push the shocks away from the
Sun-Earth line, such that Earth misses these shocks. The probability of
observing a shock at 1 AU increases rapidly above 60% when the CME speed
exceeds 1000 km/s and when the type II bursts propagate to frequencies below 1
MHz.Comment: 33 pages, 11 figures, 2 table
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
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