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Topic: Exobase

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	Exobase (Site not responding. Last check: 2007-10-28)
		The exobase is the level in the atmosphere above which the mean free path of particles is longer than a scale height.
		The region above the exobase is referred to as the exosphere and it is from this region that particles can easily escape from the planet’s atmosphere.
		At levels below the exobase the atmosphere is dense enough that even if a particle does have kinetic energy greater than its gravitational potential energy, it will lose that energy in collisions with other particles before it can escape.
	www.cs.trinity.edu /~mlewis/PlAtm/Exosphere.html (144 words)

	SII 45A kinetic exospheric model with a non-monotic potential for the protons: the solution with Lorentzian ...
		The exobase is the level where the mean free path of the solar particles becomes larger than the density scale height.
		Above the exobase level, exospheric models have been used to determine the plasma density distribution by assuming that the charged particles of the solar corona move without collisions in the gravitational, electric and interplanetary magnetic field.
		In early exospheric models for the equatorial solar corona the exobase is located beyond 6 solar radii, where the total potential energy is a monotonic decreasing function for the protons, and a monotonic increasing function of the radial distance for the electrons.
	www.arcetri.astro.it /~solwind/abstract/node132.html (396 words)

	Exosphere - Wikipedia, the free encyclopedia
		It is only from the exosphere that atmospheric gases, atoms, and molecules can, to any appreciable extent, escape into outer space.
		The main gases within the exosphere are the lightest gases, mainly hydrogen and helium, with some atomic oxygen near the exobase.
		Exobase, also called the critical level, the lowest altitude of the exosphere, is defined in one of two ways:
	en.wikipedia.org /wiki/Exosphere (222 words)

	A kinetic exospheric model of the solar wind with a nonmonotonic potential energy for the protons
		In solar wind kinetic exospheric models the exobase level is defined as the altitude where the mean free paths of the coronal protons and electrons become larger than the density scale height.
		For the region above this exobase, kinetic exospheric models have been developed assuming that the charged particles of the solar wind move collisionless in the gravitational, electric, and interplanetary magnetic fields, along trajectories determined by their energy and pitch angle.
		Since the density is lower in coronal holes and the exobase is at lower altitude, the solar wind bulk velocities predicted by our new exospheric model are enhanced to values comparable to those observed in the fast solar wind.
	www.agu.org /pubs/crossref/2003/2002JA009487.shtml (427 words)

	ANGEO - Abstracts (Site not responding. Last check: 2007-10-28)
		The quiet-time structure of the plasmaspheric density was investigated using observations of the Los Alamos geosynchronous satellites, and these observations were compared with theoretical predictions of the quasi-static local-time variation by a kinetic model.
		The kinetic model predicts that most of the local-time variation at geosynchronous orbit is due to the variation of the exobase parameters.
		During quiet times, when the convection electric field is dominated by the corotation field, the effects due to flux-tube convection are less prominent than those due to the exobase variation.
	www.copernicus.org /EGU/annales/21/11/2147.htm?FrameEngine=false (182 words)

	Helium in the Earth’s Atmosphere
		At or below the exobase, therefore, the velocity distribution of the molecules of a given atmospheric constituent is the Maxwellian distribution.
		Since collisions are negligible above the exobase, the molecules in this region, called the exosphere, move along ballistic trajectories under the action of the earth’s gravitational field.
		Deciding on the height of the exobase is rather difficult because there is actually a transition region.
	www.answersingenesis.org /tj/v8/i2/helium.asp?vPrint=1 (2732 words)

	Geocorona (Site not responding. Last check: 2007-10-28)
		The exosphere's lower boundary is termed the critical level or "exobase".
		At and below this altitude, the atmosphere is sufficiently dense that collisions dominate the motion of the gas molecules and atoms; above the exobase, on the other hand, collisions are so infrequent that atoms moving with sufficient velocity have a high probablility of escaping from the Earth's gravitational field into interplanetary space.
		The density and structure of the exosphere are influenced by a number of factors: variations in the temperature and density of the atmosphere below the exobase, photoionization and ionization by impact with solar wind particles, charge exchange with the plasma of the plasmasphere, and radiation pressure exerted by solar far-ultraviolet photons.
	pluto.space.swri.edu /image/glossary/geocorona.html (446 words)

	exosphere
		The extremely tenuous, outermost layer of Earth’s atmosphere; it lies above the ionosphere and extends from the so-called exobase, at a height of about 500 km, to the edge of interplanetary space.
		At or below the exobase, the atmosphere is sufficiently dense that collisions dominate the motion of gas molecules and atoms; above the exobase, on the other hand, collisions are so infrequent that atoms moving with sufficient velocity have a high probability of escaping from Earth's gravitational field into interplanetary space.
		The density and structure of the exosphere are influenced by a number of factors, including variations in the temperature and density of the atmosphere below the exobase, photoionization and ionization by impact with solar wind particles, charge exchange with the plasma of the plasmasphere, and radiation pressure exerted by solar far-ultraviolet photons.
	www.daviddarling.info /encyclopedia/E/exosphere.html (289 words)

	[No title]
		The exobase is defined as the altitude at which the scale height is equal to the mean free path, the average distance between collisions of major component particles: EMBED Equation.DSMT4 , here (is the collision cross section between particles.
		The position of the exobase is determined when the mean free path becomes greater than the scale height.
		The outgoing flow at the exobase is set to be equal to a modified Jeans’ escape flow, in which the effect of ionization and interaction between escaping particles and solar wind is considered.
	www.gps.caltech.edu /~cdp/tian_feng.doc (6137 words)

	Astron. Astrophys. 324, 725-734 (1997)
		The interplanetary electrical potential difference between the exobase and 1 AU can be determined assuming that the core/halo parametrization reflects the existence of two electron populations predicted by some previous exospheric theories of the solar wind (Jockers 1970; Schulz and Eviatar 1972; Perkins 1973).
		Among the various hydrodynamic models of the solar wind, the one which is most comparable to our exospheric model, in terms of sophistication, is Parker's (1963) thermally driven solar wind model.
		Therefore, it is likely that in the case A the tail of the VDF at the exobase is more populated than in the case E: the value of
	aa.springer.de /papers/7324002/2300725/sc3.htm (1466 words)

	Astron. Astrophys. 324, 725-734 (1997)
		This model, initially dedicated to the study of the polar wind, had subsequently been applied to model the solar wind (Lemaire and Scherer 1971b), with the assumption of Maxwellian VDF's for the protons and electrons at the top of the collision-dominated part of the corona.
		In zero order kinetic approximations, or exospheric models, two separate regions are considered: first, the collision-dominated barosphere at low altitude, in which the particles are assumed to be in hydrostatic/hydrodynamic equilibrium, and secondly, an exosphere in which the collision rate between particles is assumed to be negligibly small.
		The exobase location will be determined, as initially proposed by Jeans (1923) for neutral particles and by Spitzer (1949) for charged particles, by the altitude where the Knudsen number is equal to unity.
	aa.springer.de /papers/7324002/2300725/sc2.htm (2131 words)

	A NON-SPHERICAL MODEL FOR THE HOT OXYGEN CORONA OF MARS (Site not responding. Last check: 2007-10-28)
		Exospheric oxygen densities are calculated by integrating ensemble of ballistic and escaping oxygen atoms from the exobase over the entire planet.
		The densities of hot oxygen atoms at the exobase are estimated from electron densities which have been measured to vary with solar zenith angle.
		Our model shows that the density difference of hot oxygen atoms between noon and terminator is about two orders of magnitude near the exobase, but reduces abruptly around altitudes of 2000 km due to lateral transport.
	www.kas.org /e-journals/Web_JKAS/data/331/331137.html (219 words)

	WSEF 2005, Schloss Seggau, Graz, Austria
		Above this exobase altitude the collision frequency is indeed too small to maintain a closely isotropic and maxwellian VDF, like that implicitly postulated in hydrodynamic PW and SW models, as well as in the Chapman-Enskog or Grad theories of non-uniform gases.
		Kinetic models were introduced to extend the controversial hydrodynamic descriptions beyond the exobase transition altitude.
		Despite the well-known and accepted limitations of these early collisionless models, these (zero-order) kinetic models have been most useful to understand the origin of the polarization electric field that accelerates the ionospheric ions to supersonic polar wind velocities at high altitudes.
	wsef05.oeaw.ac.at /viewabstract.php?ID=59 (268 words)

	Solar System Physics Group: Research Projects (Site not responding. Last check: 2007-10-28)
		In this case we have to assume that the fragments were ejected isotropically over a hemispheric region of Vesta, instead of being concentrated near the surface of a 90$^\circ$ aperture cone, as suggested by laboratory impact experiments with planar targets.
		This method is based on a detailed comparison between the observed spectra and synthetic spectra numerically simulated with a model derived from the Smyth and Combi (1988a) formalism.
		We have applied the polienergetic model spectra approach to analyse a sequence of 59 observations of the sodium cloud spanning a period of four years from February 1990 to April 1994.
	www.pd.infn.it /astro/re.html (492 words)

	exobase - OneLook Dictionary Search (Site not responding. Last check: 2007-10-28)
		We found 2 dictionaries with English definitions that include the word exobase:
		Tip: Click on the first link on a line below to go directly to a page where "exobase" is defined.
		Exobase : Eric Weisstein's World of Astronomy [home, info]
	www.onelook.com /?w=exobase (77 words)

	Blowing old-earth belief away
		Collisions between atoms slow them down, but above a critical height (the exobase) of about 500 kilometres (300 miles) above the earth, collisions are very rare.
		Thus we can calculate how many atoms would cross the exobase fast enough to escape into space.
		But even if we assume a temperature of 1500 K (1227°C or 2241°F), higher than the average, the most common speed of helium atoms is only 2.5 kilometres per second (5625 mph), or less than a quarter of the escape velocity.
	www.answersingenesis.org /creation/v20/i3/old_earth.asp (1581 words)

	[CS] Reminder and Room Change: Mats Holmstrom, Swedish Institute of Space Physics - today! (Site not responding. Last check: 2007-10-28)
		Those are the upstream solar wind conditions (velocity and density), the position of the bow shock, the position of the "magnetopause" and the proton penetration of the "magnetopause".
		The neutral atmospheric models include two parameters for each gas (exobase density and the scale height).This work is the first step in developing inversion techniques for ENA imaging of non-magnetized planets.
		Those are the upstream solar wind conditions (velocity and density), the position of the bow shock, the position of the andquot;magnetopauseandquot; and the proton penetration of the andquot;magnetopauseandquot;.
	mailman.cs.uchicago.edu /pipermail/cs/2000-May/000130.html (668 words)

	[No title]
		Indeed, more recent studies of the Mars and Venus solar wind interactions indicate that while at Venus at solar maximum the exobase is usually shielded by the fields of currents induced in the ionosphere, the Martian ionosphere is often too weak to provide a similar "ionopause" barrier (e.g.
		Some of the scattered particles that are heated but do not gain escape velocities contribute to the upper atmosphere reservoir for ion scavenging and the production of a new generation of sputtering ions.
		In the case of a large (~500 km) scale height component of the upper atmosphere like Mars' nonthermal oxygen exosphere, the field would have to deflect the solar wind several planetary radii above Mars' surface to be an effective shield.
	sprg.ssl.berkeley.edu /~jgluhman/dpsmag.html (2195 words)

	1 Introduction
		In these models the solar atmosphere is assumed to change from fully collisional to collisionless at a sharply defined level called the exobase.
		For non pathological distribution functions this means that the plasma bulk velocity at the exobase is of the order of the proton thermal velocity, i.e.
		Since the typical velocity of a proton at the exobase is by construction of the order of the radial bulk velocity, it follows that the exobase is located at a heliocentric distance comparable to the distance
	www.arcetri.astro.it /~slandi/wind/node1.html (1157 words)

	PS_obj.html
		The concept of exobase, the lower boundary of the exosphere, is introduced as a height at which the atmospheric particle mean free path is equal to the density scale height (Chamberlain and Hunten, 1987).
		For example, in the Earth’s upper atmosphere, the exobase is at a height of about 500 km, and the exosphere here is populated mainly by atomic oxygen with small admixtures of hydrogen and helium.
		The heavier carbon, nitrogen, and oxygen atoms can escape from the atmospheres of the terrestrial planets only through the collisional processes that determine such nonthermal escape mechanisms as photodissociation, charge exchange and sputtering by magnetospheric plasma, and ion capture by the solar wind (Chamberlain and Hunten, 1987; Johnson, 1990; Hunten, 2002; Johnson, 2002).
	www.inasan.rssi.ru /~shematov/HotCoronas.html (1562 words)

	[18.20] Modeling of Saturn's Inner Plasmasphere: Predictions for Cassini (Site not responding. Last check: 2007-10-28)
		2004) are extended above the plasma exobase using the formalisms of Lemaire (1976) and Pierrard and Lemaire (1996), which evaluate the balance of gravitational, centrifugal and electric forces on the plasma.
		The parameter space of low energy ionospheric contributions to Saturn’s plasmasphere is explored by comparing results that span the observed extremes of plasma temperature, 420
		For the most likely combination of plasma exobase parameters, the inner plasmasphere is predicted to be between 1-10 electrons cm
	www.aas.org /publications/baas/v36n4/dps2004/233.htm (225 words)

	sa42a in sm99
		The ion exobase marks the transition between collisional below and non-collisional above.
		An ion "exobase" is obtained by determining the altitude at which the mean free path of the ion equals the scale height of the scattering species.
		The exobase heights here obtained become the lower boundary conditions for our kinetic model of the plasmasphere.
	www.agu.org /cgi-bin/SFgate/SFgate?&listenv=table&multiple=1&range=1&directget=1&application=sm99&database=/data/epubs/wais/indexes/sm99/sm99&maxhits=200&="SA42A" (3001 words)

	ENA IMAGING FROM THE SWEDISH MICRO SATELLITE ASTRID DURING THE MAGNETIC STORM OF 8 FEBRUARY, 1995
		The pitch angle distribution is assumed to be isotropic with an empty loss cone consistent with the observations of isotropic nightside ion precipitation studied by Seergev et al.
		The significantly thin layer of ENA emissions could not be reproduced with an exospheric model without oxygen suggesting that the thin layer is an effect of the much smaller scale height and, near exobase altitudes (300-400 km), higher density of oxygen.
		We suggest that the most intense ENA emissions are produced by the precipitating/mirroring ions in the dusk region on auroral/subauroral field lines charge exchanging with the oxygen exosphere near the exobase (300-400 km).
	www-ssc.igpp.ucla.edu /IASTP/103 (3152 words)

	About Munin satellite project (Site not responding. Last check: 2007-10-28)
		However, in the auroral region where the ring current / radiation belt particles plunge into the dense upper atmosphere / exosphere, the charge - exchange process is much more effective and ENAs emissions are much more intense.
		Theoretically, ENAs from low altitudes are emitted from a very thin area near the exobase and the ENA generation region is, essentially, two dimensional.
		The ENA camera PIPPI on board the microsatellite Astrid has performed ENA measurements from the 1000 km polar orbit and demonstrated potentials of low altitude ENA imaging.
	munin.irf.se /frames/science_main_high.html (526 words)

	Abstract AAS 97-634 (Site not responding. Last check: 2007-10-28)
		NEUTRAL AND ION DRAG EFFECTS NEAR THE EXOBASE: MSX SATELLITE MEASUREMENTS OF He AND O+ Uy, R.C. Benson and R.E. Erlandson - The Johns Hopkins University Applied Physics Laboratory; G.M. Keating and J.C. Leary - The George Washington University
		The winter helium bulge near the exobase was first discovered from measurements in the helium regime by a series of spherical drag satellites, Explorers 9, 19, 24, and 39 (Keating and Prior, 1967).
		The DOD's Midcourse Space Experiment (MSX) satellite, launched in 1996 during solar minimum, flies in a near polar circular orbit at approximately 900 km in the helium regime of the terrestrial upper atmosphere.
	www.space-flight.org /AAS_meetings/1997_astro/abstracts/97-634.html (204 words)

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