119306237
Half a Century of Kinetic Solar Wind Models
I outline the development of four generations of kinetic models, starting with Chamberlain’s solar breeze exospheric model. It is shown why this first kinetic model did not give apposite supersonic evaporation velocities, like early hydrodynamic models of the solar wind. When a self‐consistent polarization electric potential distribution is used in the coronal plasma, instead of the Pannekoek‐Rosseland’s one, supersonic bulk velocities are readily obtained in the second generation of kinetic models. It is outlined how the third and fourth generations of these models have improved the agreement with observations of slow and fast speed solar wind streams.
119298597
Convective Instability Of The Solar Corona: Why The Solar Wind Blows
Chapman’s conductive model of the solar corona is characterized by a temperature varying as r−2/7 with heliocentric distance r. The density distribution in this non‐isothermal hydrostatic model has a minimum value at 123 RS, and increases with r above that altitude. It is shown that this hydrostatic model becomes convectively unstable above r = 35 RS, where the temperature lapse rate becomes superadiabatic. Beyond this radial distance heat conduction fails to be efficient enough to keep the temperature gradient smaller than the adiabatic lapse rate. We report the results obtained by Lemaire who showed that an additional mechanism is then required to transport the energy flux away from the Sun into interplanetary space. He pointed out that this additional mechanism is advection: i.e. the stationary hydrodynamic expansion of the corona. In other words the corona is unable to stay in hydrostatic equilibrium. The hydrodynamic solar wind expansion is thus a physical consequence of the too steep (superadiabatic...
116897797
What Happens When the Geomagnetic Field Reverses
During geomagnetic field reversals the radiation belt high-energy proton populations become depleted. Their energy spectra become softer, with the trapped particles of highest energies being lost first, and eventually recovering after a field reversal. The radiation belts rebuild in a dynamical way with the energy spectra flattening on the average during the course of many millennia, but without ever reaching complete steady state equilibrium between successive geomagnetic storm events determined by southward turnings of the IMF orientation. Considering that the entry of galactic cosmic rays and the solar energetic particles with energies above a given threshold are strongly controlled by the intensity of the northward component of the interplanetary magnetic field, we speculate that at earlier epochs when the geomagnetic dipole was reversed, the entry of these energetic particles into the geomagnetic field was facilitated when the interplanetary magnetic field was directed northward. Unlike in other complementary work where intensive numerical simulations have been used, our demonstration is based on a simple analytical extension of Stormer's theory. The access of GCR and SEP beyond geomagnetic cut-off latitudes is enhanced during epochs when the Earth's magnetic dipole is reduced, as already demonstrated earlier.
118184102
Determination of coronal temperatures from electron density profiles
The most popular method for determining coronal temperatures is the scale-height-method (shm). It is based on electron density profiles inferred from White Light (WL) brightness measurements of the corona during solar eclipses. This method has been applied to several published coronal electron density models. The calculated temperature distributions reach a maximum at r > 1.3 RS, and therefore do not satisfy one of the conditions for applying the shm method. Another method is the hydrostatic equilibrium method (hst), which enables coronal temperature distributions to be determined, providing solutions to the hydrostatic equilibrium equation. The temperature maximas using the hst method are almost equal to those obtained using the shm method, but the temperature peak is always at significantly lower altitude when the hst-method is used than when the shm-method is used. A third and more recently developed method, dyn, can be used for the same published electron density profiles. The temperature distributions obtained using the dyn method are regular solutions of the hydrodynamic equations. They depend on the expansion velocity of the coronal plasma considered as a free input parameter in the calculations. At the base of the solar corona, where the coronal bulk velocity is small (subsonic), the dyn and hst methods give similar temperature values. Larger differences are found at higher altitudes where the expansion velocity approaches and exceeds the velocity of sound. This paper shows also the effects of the flow tube geometry and ion temperatures on the electron temperature distributions. The dyn method is a generalization of the hst method, where the electron temperature distribution tends to zero at infinite radial distances. It is a new diagnostic tool for determining from WL coronal observations the range of radial distances where the coronal heating rate is at its maximum.
118396623
A 3D view of the outflow in the Orion Molecular Cloud 1 (OMC-1)
Context. Stars whose mass is an order of magnitude greater than the Sun play a prominent role in the evolution of galaxies, exploding as supernovae, triggering bursts of star formation and spreading heavy elements about their host galaxies. A fundamental aspect of star formation is the creation of an outflow. The fast outflow emerging from a region associated with massive star formation in the Orion Molecular Cloud 1 (OMC-1), located behind the Orion Nebula, appears to have been set in motion by an explosive event. Aims. We study the structure and dynamics of outflows in OMC-1. We combine radial velocity and proper motion data for near-IR emission of molecular hydrogen to obtain the first 3-dimensional (3D) structure of the OMC-1 outflow. Our work illustrates a new diagnostic tool for studies of star formation that will be exploited in the near future with the advent of high spatial resolution spectro-imaging in particular with data from the Atacama Large Millimeter Array (ALMA). Methods. We used published radial and proper motion velocities obtained from the shock-excited vibrational emission in the H2 v = 1−0 S(1) line at 2.122 μm obtained with the GriF instrument on the Canada-France-Hawaii Telescope, the Apache Point Observatory, the Anglo-Australian Observatory, and the Subaru Telescope. Results. These data give the 3D velocity of ejecta yielding a 3D reconstruction of the outflows. This allows one to view the material from different vantage points in space giving considerable insight into the geometry. Our analysis indicates that the ejection occurred <720 years ago from a distorted ring-like structure of ∼15 �� (6000 AU) in diameter centered on the proposed point of close encounter of the stars BN, source I and maybe also source n. We propose a simple model involving curvature of shock trajectories in magnetic fields through which the origin of the explosion and the center defined by extrapolated proper motions of BN, I and n may be brought into spatial coincidence.
12220966
Studies on halo formation in a long magnetic quadrupole FODO channel. First experimental results
A study is progress to prepare an experiment on the transport of an intense proton beam, with high optical qualities, through a periodic magnetic quadrupole FODO channel (29 periods). It is intended to provide a better understanding of how halo develops and can be controlled as a crucial issue for the design of high-intensity linear accelerators. Simulations were done showing that this experiment can be performed with a high-brightness proton beam produced by a duoplasmatron source, with an energy of 500 keV and a current of tens of mA. Beam pulse length will be less than 1 ms and normalized emittance better than 1./spl pi/ mm.mrad (90% of total beam). Thus, transverse tune can be sufficiently depressed in the transport channel to allow instabilities to grow and halo to develop or stay under control. The overall experiment is divided in two phases. Phase 1, the subject of this paper, is dedicated to precise measurements on initial beam conditions, such as emittance, for the beam entering the FODO channel. Phase 2, concerning the halo formation phenomena will be carried on latter, if results of phase 1 are satisfactory.
44206795
A new diagnostic for the measurement of the ionic emission generated by an electron-photon conversion target
A beam diagnostic made of a double sheet sensor is designed measure ions emitted by an electron-photon conversion target under the impact of an intense relativistic electron beam. This device is tested on the PIVAIR prototype accelerator. Signals attributed to ions emitted from the target are observed for different target elements: Ta, C, Ti, Au and light species are identified by their time of flight. The mean velocities of C/sup +/ and H/sup +/ ions are found to be respectively equal to 1.2 /spl times/10/sup 8/ cm/s and 5 /spl times/10/sup 8/ cm/s for C and Ta targets. The ionic appearance and rise time signals are closed to the expected values. Besides, the current signals are characterized by a plateau to which level corresponds to the Child-Langmuir current (25 A for H/sup +/ ions). Furthermore, the fall-off of this signal is found to be in synchronism with the electron beam signal. This behaviour represents the deconfinement of the ion beam under the effect of its own space charge. These experimental results confirm the preliminary calculations obtained with the M2V Maxwell-Vlasov PIC code which has been coupled with an ionic emission model.
122689831
On emittance evolution in the extraction system of high-current electron and ion sources
A simple model has been built to study the evolution of high-current electron or ion beams in axially symmetric extraction systems (electron guns or ion sources). Both electric and magnetic field components induced by a HV diode and a focusing solenoid respectively are computed using analytical formulas. The space-charge forces are computed using a ring method. In a first step, the individual effects of the 3 nonlinear forces (electric, magnetic and space charge) on the emittance evolution are presented. We then discuss how the knowledge acquired in the first step can be used to understand and optimize the emittance evolution in systems combining these effects.
119413417
VLT/NACO near-infrared imaging and spectroscopy of N159A in the LMC HII complex N159
We present high-resolution near-infrared imaging of the compact HII region N159-5 and its immediate environment in the giantstar forming region N159 in the LMC. N159-5 was observed at high spatial resolution ∼0. 11−0. �� 25 in the K-band using the ESO Very Large Telescope UT4 (VLT), equipped with the NAOS adaptive optics system. Our data reveal that N159-5 has a complex morphology formed mainly by two wings and probably a single central bright star, embedded in diffuse emission of ∼4. �� 5 diameter. A remarkable embedded tight cluster of approximatively the same size, containing at least 38 faint stars coinciding with N159-5, is also detected. Such clusters can be found in galactic HII regions like the star-forming regions SH2 269 or M42. At the location of the radio peak, especially in the bright western wing, this cluster is rich in stars. Spectroscopic observations reveal that the diffuse region is constituted mainly of dust continuum and that the bright star #2-55 could be of type O8 V. A comparison with the radio observation flux of N159-5 published in the literature seems to show that the bright star #2-55 is not the only ionization source of N159-5. Towards N159-5 molecular H2 emission is detected. A model of the region is proposed.