2,597 research outputs found
Allee Effects May Slow the Spread of Parasites in a Coastal Marine Ecosystem
Allee effects are thought to mediate the dynamics of population colonization, particularly for invasive species. However, Allee effects acting on parasites have rarely been considered in the analogous process of infectious disease establishment and spread. We studied the colonization of uninfected wild juvenile Pacific salmon populations by ectoparasitic salmon lice (Lepeophtheirus salmonis) over a 4-year period. In a data set of 68,376 fish, we observed 85 occurrences of precopular pair formation among 1,259 preadult female and 613 adult male lice. The probability of pair formation was dependent on the local abundance of lice, but this mate limitation is likely offset somewhat by mate-searching dispersal of males among host fish. A mathematical model of macroparasite population dynamics that incorporates the empirical results suggests a high likelihood of a demographic Allee effect, which can cause the colonizing parasite populations to die out. These results may provide the first empirical evidence for Allee effects in a macroparasite. Furthermore, the data give a rare detailed view of Allee effects in colonization dynamics and suggest that Allee effects may dampen the spread of parasites in a coastal marine ecosystem
Energetic proton spectra in the 11 June 1991 solar flare
We have studied a subset of the 11 June 1991 solar flare Îł-ray data that we believe arise from soft proton or ion spectra. Using data from the COMPTEL instrument on the Compton Observatory we discuss the gamma-ray intensities at 2.223 MeV, 4â7 MeV, and 8â30 MeV in terms of the parent proton spectrum responsible for the emission
Interviews with the Apollo lunar surface astronauts in support of planning for EVA systems design
Focused interviews were conducted with the Apollo astronauts who landed on the moon. The purpose of these interviews was to help define extravehicular activity (EVA) system requirements for future lunar and planetary missions.RTOP 199-06-1
Differential herbivory on disk and ray flowers of gynomonoecious asters and goldenrods (Asteraceae)
The selective advantage of gynomonoecy, the sexual system wherein plants produce a mixture of female and bisexual flowers, is poorly understood. One hypothesis for the evolution of this system is that the absence of androecia from female flowers reduces herbivore damage to the gynoecia of these flowers. Here, we examined patterns of herbivore damage in 53 collections representing 25 species of asters and goldenrods from Massachusetts, USA. In these taxa flowers are crowded into compact capitula, with bisexual flowers occupying the centre and female flowers situated on the periphery. Damage to gynoecia of bisexual flowers was significantly greater than damage to gynoecia of female flowers overall, and in about half of the individual populations. We also compared damage to central and peripheral flowers in the heads of 16 collections of other Asteraceae that produce only bisexual flowers to see whether the location of flowers rather than their sex might determine the patterns of herbivory. In only one of these 16 collections did we find a significant difference in herbivory between flower positions. We conclude that herbivore damage is influenced by flower type in asters and goldenrods, a pattern consistent with a role for herbivory in the evolution and maintenance of gynomonoecy. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society
Gamma ray measurements of the 1991 November 15 solar flare
The 1991 November 15 X1.5 flare was a well observed solar event. Comprehensive data from ground-based observatories and spacecraft provide the basis for a contextual interpretation of gamma-ray spectra from the Compton Gamma Ray Observatory (CGRO). In particular, spectral, spatial, and temporal data at several energies are necessary to understand the particle dynamics and the acceleration mechanism(s) within this flare. X-ray images, radio, Ca XIX data and magnetograms provide morphological information on the acceleration region [4,5], while gamma-ray spectral data provide information on the parent ion spectrum. Furthermore, time profiles in hard X-rays and gamma-rays provide valuable information on temporal characteristics of the energetic particles. We report the results of our analysis of the evolution of this flare as a function of energy (âŒ25 keVâ2.5 MeV) and time. These results, together with other high energy data (e.g. from experiments on Yohkoh, Ulysses, and PVO) may assist in identifying and understanding the acceleration mechanism(s) taking place in this event
The Locations of Gamma-Ray Bursts Measured by COMPTEL
The COMPTEL instrument on the Compton Gamma Ray Observatory is used to
measure the locations of gamma-ray bursts through direct imaging of MeV
photons. In a comprehensive search, we have detected and localized 29 bursts
observed between 1991 April 19 and 1995 May 31. The average location accuracy
of these events is 1.25\arcdeg (1), including a systematic error of
\sim0.5\arcdeg, which is verified through comparison with Interplanetary
Network (IPN) timing annuli. The combination of COMPTEL and IPN measurements
results in locations for 26 of the bursts with an average ``error box'' area of
only 0.3 deg (1). We find that the angular distribution of
COMPTEL burst locations is consistent with large-scale isotropy and that there
is no statistically significant evidence of small-angle auto-correlations. We
conclude that there is no compelling evidence for burst repetition since no
more than two of the events (or 7% of the 29 bursts) could possibly have
come from the same source. We also find that there is no significant
correlation between the burst locations and either Abell clusters of galaxies
or radio-quiet quasars. Agreement between individual COMPTEL locations and IPN
annuli places a lower limit of 100~AU (95% confidence) on the distance to
the stronger bursts.Comment: Accepted for publication in the Astrophysical Journal, 1998 Jan. 1,
Vol. 492. 33 pages, 9 figures, 5 table
COMPTEL gamma-ray observations of the C4 solar flare on 20 January 2000
The âPre-SMMâ (Vestrand and Miller 1998) picture of gamma-ray line (GRL) flares was that they are relatively rare events. This picture was quickly put in question with the launch of the Solar Maximum Mission (SMM). Over 100 GRL flares were seen with sizes ranging from very large GOES class events (X12) down to moderately small events (M2). It was argued by some (Bai 1986) that this was still consistent with the idea that GRL events are rare. Others, however, argued the opposite (Vestrand 1988; Cliver, Crosby and Dennis 1994), stating that the lower end of this distribution was just a function of SMMâs sensitivity. They stated that the launch of the Compton Gamma-ray Observatory (CGRO) would in fact continue this distribution to show even smaller GRL flares. In response to a BACODINE cosmic gamma-ray burst alert, COMPtonTELescope on the CGRO recorded gamma rays above 1 MeV from the C4 flare at 0221 UT 20 January 2000. This event, though at the limits of COMPTELâs sensitivity, clearly shows a nuclear line excess above the continuum. Using new spectroscopy techniques we were able to resolve individual lines. This has allowed us to make a basic comparison of this event with the GRL flare distribution from SMM and also compare this flare with a well-observed large GRL flare seen by OSSE
X- and gamma-ray observations of the 15 November 1991 Solar Flare
This work expands the current understanding of the 15 November 1991 Solar Flare. The flare was a well observed event in radio to gamma-rays and is the first flare to be extensively studied with the benefit of detailed soft and hard X-ray images. In this work, we add data from all four instruments on the Compton Gamma Ray Observatory. Using these data we determined that the accelerated electron spectrum above 170 keV is best fit with a power law with a spectral index of â4.6, while the accelerated proton spectrum above 0.6 MeV is fit with a power law of spectral index â4.5. From this we computed lower limits for the energy content of these particles ofâŒ1023âergs (electrons) and âŒ1027âergs (ions above 0.6 MeV). These particles do not have enough energy to produce the white-light emission observed from this event. We computed a time constant of 26+20â15âs for the 2.223 MeV neutron capture line, which is consistent at the 2Ï level with the lowest values of âŒ70 s found for other flares. The mechanism for this short capture time may be better understood after analyses of high energy EGRET data that show potential evidence for pion emission near âŒ100 MeV
Characteristics of long-duration inhibitory postsynaptic potentials in rat neocortical neurons in vitro
1. The characteristics of long-duration inhibitory postsynaptic potentials (l-IPSPs) which are evoked in rat frontal neocortical neurons by local electrical stimulation were investigated with intracellular recordings from anin vitro slice preparation.
2. Stimulation with suprathreshold intensities evoked l-IPSPs with typical durations of 600â900 msec at resting membrane potential. Conductance increases of 15â60% were measured at the peak amplitude of l-IPSPs (150â250 msec poststimulus).
3. The duration of the conductance increases during l-IPSPs displayed a significant voltage dependence, decreasing as the membrance potential was depolarized and increasing with hyperpolarization.
4. The reversal potential of l-IPSPs is significantly altered by reductions in the extracellular potassium concentration. Therefore it is concluded that l-IPSPs in rat neocortical neurons are generated by the activation of a potassium conductance.
5. l-IPSPs exhibit stimulation fatigue. Stimulation with a frequency of 1 Hz produces a complete fatigue of the conductance increases during l-IPSPs after approximately 20 consecutive stimuli. Recovery from this fatigue requires minutes.
6. l-IPSPs are not blocked by bicuculline but are blocked by baclofen
First results of the BATSE/COMPTEL/NMSU rapid burst response campaign
The Imaging Compton Telescope (COMPTEL) on board the Compton Gamma Ray Observatory regularly observes gammaâray bursts which occur inside the instrumentâs âŒ1 sr fieldâofâview. COMPTEL images bursts in the 0.75â30 MeV energy range with a typical location accuracy of 1â3 degrees, depending on burst strength, position, duration, and spectrum. COMPTELâs imaging capability has been exploited in order to search for fading gammaâray burst counterparts at other wavelengths through the establishment of a BATSE/COMPTEL/NMSU rapid burst response campaign. This campaign utilizes near realâtime identification and preliminary burst location by BATSE, accelerated COMPTEL imaging, and a worldâwide network of observers to search COMPTEL error boxes as quickly as possible. Timely, deep searches for lingering counterpart emission of several bursts per year are the realized goal of this campaign. During its first year of operation, the rapid response program has been successfully applied to two strong bursts: GRB 930131 and GRB 930309. These bursts were imaged in record time only hours after their occurrence. Subsequently, several observations were made at radio and optical observatories worldâwide
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