3,348 research outputs found
The aurora as a source of planetary-scale waves in the middle atmosphere
Photographs of global-scale auroral forms taken by scanning radiometers onboard U.S. Air Force weather satellites in 1972 show that auroral bands exhibit well-organized wave motion with typical zonal wave number of 5 or so. The scale size of these waves is in agreement with that of well-organized neutral wind fields measured by the 1967-50B satellite in the 150- to 220-km region during the geomagnetic storm of May 27, 1967. The horizontal scale size revealed by these observations is in agreement with that of high-altitude traveling ionospheric disturbances. It is conjectured that the geomagnetic storm is a source of planetary and synoptic scale neutral atmospheric waves in the middle atmosphere. The possible existence of a source of waves of the proper scale size to trigger instabilities in middle atmospheric circulation systems may be significant in the study of lower atmospheric response to geomagnetic activity
Extremals of Functions on Graphs with Applications to Graphs and Hypergraphs
AbstractThe method used in an article by T. S. Matzkin and E. G. Straus [Canad. J. Math. 17 (1965), 533–540] is generalized by attaching nonnegative weights to t-tuples of vertices in a hypergraph subject to a suitable normalization condition. The edges of the hypergraph are given weights which are functions of the weights of its t-tuples and the graph is given the sum of the weights of its edges. The extremal values and the extremal points of these functions are determined. The results can be applied to various extremal problems on graphs and hypergraphs which are analogous to P. Turán's Theorem [Colloq. Math. 3 (1954), 19–30: (Hungarian) Mat. Fiz. Lapok 48 (1941), 436–452]
HIV-1 Directly Kills CD4+ T Cells by a Fas-independent Mechanism
The mechanism by which HIV-1 induces CD4+ T cell death is not known. A fundamental issue is whether HIV-1 primarily induces direct killing of infected cells or indirectly causes death of uninfected bystander cells. This question was studied using a reporter virus system in which infected cells are marked with the cell surface protein placental alkaline phosphatase (PLAP). Infection by HIV-PLAP of peripheral blood mononuclear cells (PBMCs) and T cell lines leads to rapid depletion of CD4+ T cells and induction of apoptosis. The great majority of HIV-induced T cell death in vitro involves direct loss of infected cells rather than indirect effects on uninfected bystander cells. Because of its proposed role in HIV-induced cell death, we also examined the Fas (CD95/Apo1) pathway in killing of T cells by HIV-1. Infected PBMCs or CEM cells display no increase in surface Fas relative to uninfected cells. In addition, HIV-1 kills CEM and Jurkat T cells in the presence of a caspase inhibitor that completely blocks Fas-mediated apoptosis. HIV-1 also depletes CD4+ T cells in PBMCs from patients who have a genetically defective Fas pathway. These results suggest that HIV-1 induces direct apoptosis of infected cells and kills T cells by a Fas-independent mechanism
Crohn's Disease and Urinary Bladder Mass
The presence of a bladder mass in a patient with inflammatory bowel disease poses a diagnostic
dilemma. We present the case of a 26-year-old male with a bladder mass who had not previously
been diagnosed with Crohn's disease. Initial biopsies of the bladder mass were consistent with inflammatory
changes, but superficial transitional cell carcinoma could not be reliably excluded.
Subsequent evaluation confirmed the presence of Crohn's disease with bladder involvement, and the
patient underwent bowel resection and partial cystectomy. Pathologic evaluation demonstrated
Crohn’s disease and no evidence of malignancy. Accurate differentiation of benign and malignant
bladder masses in patients with inflammatory bowel disease may be difficult and requires cooperation
between pathologists and clinicians
On the Formation Height of the SDO/HMI Fe 6173 Doppler Signal
The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO) is designed to study oscillations and the mag- netic field in
the solar photosphere. It observes the full solar disk in the Fe I absorption
line at 6173\AA . We use the output of a high-resolution 3D, time- dependent,
radiation-hydrodynamic simulation based on the CO5BOLD code to calculate
profiles F({\lambda},x,y,t) for the Fe I 6173{\AA} line. The emerging profiles
F({\lambda},x,y,t) are multiplied by a representative set of HMI filter
transmission profiles R_i({\lambda},1 \leq i \leq 6) and filtergrams
I_i(x,y,t;1 \leq i \leq 6) are constructed for six wavelengths. Doppler
velocities V_HMI(x,y,t) are determined from these filtergrams using a
simplified version of the HMI pipeline. The Doppler velocities are correlated
with the original velocities in the simulated atmosphere. The cross-
correlation peaks near 100 km, suggesting that the HMI Doppler velocity signal
is formed rather low in the solar atmosphere. The same analysis is performed
for the SOHO/MDI Ni I line at 6768\AA . The MDI Doppler signal is formed
slightly higher at around 125 km. Taking into account the limited spatial
resolution of the instruments, the apparent formation height of both the HMI
and MDI Doppler signal increases by 40 to 50 km. We also study how
uncertainties in the HMI filter-transmission profiles affect the calculated
velocities.Comment: 15 pages, 11 Figure
Probing the Solar Atmosphere Using Oscillations of Infrared CO Spectral Lines
Oscillations were observed across the whole solar disk using the Doppler
shift and line depth of spectral lines from the CO molecule near 4666~nm with
the National Solar Observatory's McMath/Pierce solar telescope. Power,
coherence, and phase spectra were examined, and diagnostic diagrams reveal
power ridges at the solar global mode frequencies to show that these
oscillations are solar p-modes. The phase was used to determine the height of
formation of the CO lines by comparison with the IR continuum intensity phase
shifts as measured in Kopp et al., 1992; we find the CO line formation height
varies from 425 < z < 560 km as we move from disk center towards the solar limb
1.0 > mu > 0.5. The velocity power spectra show that while the sum of the
background and p-mode power increases with height in the solar atmosphere as
seen in previous work, the power in the p-modes only (background subtracted)
decreases with height, consistent with evanescent waves. The CO line depth
weakens in regions of stronger magnetic fields, as does the p-mode oscillation
power. Across most of the solar surface the phase shift is larger than the
expected value of 90 degrees for an adiabatic atmosphere. We fit the phase
spectra at different disk positions with a simple atmospheric model to
determine that the acoustic cutoff frequency is about 4.5 mHz with only small
variations, but that the thermal relaxation frequency drops significantly from
2.7 to 0 mHz at these heights in the solar atmosphere
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