The diamagnetic cavity is the innermost region of the magnetosphere of an active comet, from which the magnetic field is expelled by the outflowing matter. This phenomenon, first detected around ...comet 1P/Halley, was extensively studied recently by the Rosetta comet chaser mission. Rosetta observed a surprisingly large diamagnetic cavity around comet 67P/Churyumov-Gerasimenko and revealed an unforeseen structure, rich and highly dynamic. Here we present a simple (1+1)-dimensional analytic MHD model of the diamagnetic cavity, which for the first time explains the unexpected size and variability of the cavity. In this model, inward and outward moving time-dependent solutions emerge, featuring distinct differences, in accordance with observations. The plasma density is enhanced in the entire magnetized region. The density enhancement is more pronounced for weak comets, resulting in a stronger interaction and hence the larger than expected cavity. Space weather effects determine the asymptotic plasma speed, thus driving the variations of the plasma properties observed near the cavity.
Abstract
The ESA’s comet chaser Rosetta has monitored the evolution of the ionized atmosphere of comet 67P/Churyumov–Gerasimenko (67P/CG) and its interaction with the solar wind, during more than ...2 yr. Around perihelion, while the cometary outgassing rate was highest, Rosetta crossed hundreds of unmagnetized regions, but did not seem to have crossed a large-scale diamagnetic cavity as anticipated. Using in situ Rosetta observations, we characterize the structure of the unmagnetized plasma found around comet 67P/CG. Plasma density measurements from RPC-MIP are analysed in the unmagnetized regions identified with RPC-MAG. The plasma observations are discussed in the context of the cometary escaping neutral atmosphere, observed by ROSINA/COPS. The plasma density in the different diamagnetic regions crossed by Rosetta ranges from ∼100 to ∼1500 cm−3. They exhibit a remarkably systematic behaviour that essentially depends on the comet activity and the cometary ionosphere expansion. An effective total ionization frequency is obtained from in situ observations during the high outgassing activity phase of comet 67P/CG. Although several diamagnetic regions have been crossed over a large range of distances to the comet nucleus (from 50 to 400 km) and to the Sun (1.25–2.4 au), in situ observations give strong evidence for a single diamagnetic region, located close to the electron exobase. Moreover, the observations are consistent with an unstable contact surface that can locally extend up to about 10 times the electron exobase.
In this paper, some concepts of convex analysis on hyperbolic spaces are studied. We first study properties of the intrinsic distance, for instance, we present the spectral decomposition of its ...Hessian. Next, we study the concept of convex sets and the intrinsic projection onto these sets. We also study the concept of convex functions and present first- and second-order characterizations of these functions, as well as some optimization concepts related to them. An extensive study of the hyperbolically convex quadratic functions is also presented.
We study the dynamics of the interaction between the solar wind ions and a partially ionized atmosphere around a comet, at a distance of 2.88 AU from the Sun during a period of low nucleus activity. ...Comparing particle data and magnetic field data for a case study, we highlight the prime role of the solar wind electric field in the cometary ion dynamics. Cometary ion and solar wind proton flow directions evolve in a correlated manner, as expected from the theory of mass loading. We find that the main component of the accelerated cometary ion flow direction is along the antisunward direction and not along the convective electric field direction. This is interpreted as the effect of an antisunward polarization electric field adding up to the solar wind convective electric field.
Key Points
Prime role of the SW electric field in the cometary ion dynamics, through mass loading, at 2.88 AU
The cometary ion flow direction has a main antisunward component
We find an indication for an antisunward polarization electric field developing in the coma
In this paper, some concepts related to the intrinsic convexity of non-homogeneous quadratic functions on the hyperbolic space are studied. Unlike in the Euclidean space, the study of intrinsic ...convexity of non-homogeneous quadratic functions in the hyperbolic space is more elaborate than that of homogeneous quadratic functions. Several characterizations that allow the construction of many examples will be presented.
Context. The Rosetta magnetometer RPC-MAG has been exploring the plasma environment of comet 67P/Churyumov-Gerasimenko since August 2014. The first months were dominated by low-frequency waves which ...evolved into more complex features. However, at the end of July 2015, close to perihelion, the magnetometer detected a region that did not contain any magnetic field at all. Aims. These signatures match the appearance of a diamagnetic cavity as was observed at comet 1P/Halley in 1986. The cavity here is more extended than previously predicted by models and features unusual magnetic field configurations, which need to be explained. Methods. The onboard magnetometer data were analyzed in detail and used to estimate the outgassing rate. A minimum variance analysis was used to determine boundary normals. Results. Our analysis of the data acquired by the Rosetta Plasma Consortium instrumentation confirms the existence of a diamagnetic cavity. The size is larger than predicted by simulations, however. One possible explanation are instabilities that are propagating along the cavity boundary and possibly a low magnetic pressure in the solar wind. This conclusion is supported by a change in sign of the Sun-pointing component of the magnetic field. Evidence also indicates that the cavity boundary is moving with variable velocities ranging from 230−500 m/s.
Particle number size distributions in a mobility diameter range of 6–1000 nm and size-resolved number concentrations were determined with a time resolution of 10 min for a near-city background, city ...centre, street canyon and road tunnel environments in Budapest. Median N6–100 concentrations for the sites listed were 3.1 × 103, 9.3 × 103, 19.4 × 103 and 123 × 103 cm−3, respectively. Contributions of the ultrafine (UF) particles (<100 nm) to the total particle number for all locations were rather large (up to 86%), and do not seem to vary substantially in time. Diurnal variations of the mean N6–100 concentrations had different patterns for both the various urban sites, and for workdays and weekends. Nucleation strength factor (NSF) was introduced for the first time to quantify the relative importance of new particle formation with respect to all sources of UF particles. During the daytime in summer, nucleation in the near-city background was a major production process of UF particles with a daily mean relative contribution of 42%. In the city centre and street canyon, the daily mean relative contributions of nucleation to the UF particles were 30% and 23%, respectively. Median particle diameters for the background, city centre, street canyon and road tunnel environments were 61, 42, 35 and 42 nm, respectively, so they were jointly influenced with the anthropogenic impact and aerosol ageing. Monthly mean frequency of new particle formation and growth events in the background seems somewhat larger, while it appears smaller for the street canyon in comparison to the city centre.
•Median particle number concentrations varied by a factor of 40 within a city.•Diurnal N6–100 patterns were different for various urban sites.•Median diameter of number size distributions decreased with anthropogenic impact.•Nucleation strength factor was introduced to quantify the contribution of nucleation.
Objectives
Guidance set out by Scottish Intercollegiate Guidelines Network (SIGN) sought to restrict the frequency of elective tonsillectomy in reaction to the recognised and substantial risks of the ...procedure, namely pain and bleeding. With stricter criteria in place before a patient can undergo tonsillectomy, patients must endure more episodes of tonsillitis than was previously the case. The cost of managing tonsillitis and its complications as an inpatient is substantial to the NHS and also to the economy as a whole in missed work days. The authors sought to establish if the reduced rate of tonsillectomy performed had any effect on the rate of acute hospital admission for tonsillitis or the associated abscesses—peritonsillar, parapharyngeal and retropharyngeal.
Methods
A retrospective multi-centre study reviewed admission data across a 4-year period. The rate of tonsillectomies performed for recurrent tonsillitis across four trusts in Mid-Essex was compared with the number of admissions for tonsillitis, peritonsillar, parapharyngeal and retropharyngeal abscesses. National hospital episode statistics data over a 5-year period was also analysed.
Results
For the regional data in 2015, 979 tonsillectomies were performed across the four centres, reducing to 875 in 2018. There was no trend in the rates of acute tonsillitis requiring admission but the rates of peritonsillar abscess increased from 156 to 192 cases per year in the same period. This correlation was found not to be statistically significant in the measured sample size. The National hospital episode statistics data showed a significant correlation between tonsillectomy rates and admissions from complications of tonsillitis as well as the associated abscesses.
Conclusion
This study shows that the reduced tonsillectomy rate was correlated with an increased number of admissions with peritonsillar abscess regionally. Nationally reduced tonsillectomy rate is significantly associated with increased admissions with tonsillitis and all its complications. A decreased rate of tonsillectomy may be increasing the rate of serious tonsillitis. This has an impact on patient morbidity, an increasing financial burden on the NHS and the UK economy.
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