Plasma wave observations at comet giacobini-zinner

The plasma wave instrument on the International Cometary Explorer (ICE) detected bursts of strong ion acoustic waves almost continuously when the spacecraft was within 2 million kilometers of the nucleus of comet Giacobini-Zinner. Electromagnetic whistlers and low-level electron plasma oscillations were also observed in this vast region that appears to be associated with heavy ion pickup. As ICE came closer to the anticipated location of the bow shock, the electromagnetic and electrostatic wave levels increased significantly, but even in the midst of this turbulence the wave instrument detected structures with familiar bow shock characteristics that were well correlated with observations of localized electron heating phenomena. Just beyond the visible coma, broadband waves with amplitudes as high as any ever detected by the ICE plasma wave instrument were recorded. These waves may account for the significant electron heating observed in this region by the ICE plasma probe, and these observations of strong wave-particle interactions may provide answers to longstanding questions concerning ionization processes in the vicinity of the coma. Near closest approach, the plasma wave instrument detected broadband electrostatic noise and a changing pattern of weak electron plasma oscillations that yielded a density profile for the outer layers of the cold plasma tail. Near the tail axis the plasma wave instrument also detected a nonuniform flux of dust impacts, and a preliminary profile of the Giacobini-Zinner dust distribution for micrometer-sized particles is presented.     

Observations of energetic ions from comet giacobini-zinner

During the encounter with comet Giacobini-Zinner, the energetic particle anisotropy spectrometer on the International Cometary Explorer spacecraft observed large fluxes of energetic ions, believed to result principally from ionization of the cometary atmosphere followed by pickup and acceleration by the ambient flow of the solar wind. These heavy cometary ions were observed from approximately 1 day before closest approach to about 2(1/2) days afterward. Three regimes of differing ion characteristics have been identified. An outer region with a scale of approximately 10(6) kilometers contains variable fluxes of antisolar-streaming pick-up ions in the undisturbed solar wind. In the middle region, of approximately 10(5) kilometers, fluxes have less large-scale variability and broader angular and energy distributions. This region is separated from the outer zone by a sharp transition. The inner region has a scale of approximately 10(4) kilometers and is characterized by reduced fluxes and complex angular distributions.     

Observations of a comet on collision course with the sun

A brilliant new comet (1979 XI: Howard-Koomen-Michels) was discovered in data from the Naval Research Laboratory's orbiting SOLWIND coronagraph. An extensive sequence of pictures, telemetered from the P78-1 satellite, shows the coma, accompanied by a bright and well-developed tail, passing through the coronagraph's field of view at afew million kilometers from the sun. Preliminary orbital calculations based on the observed motion of the comet's head and morphology of the tail indicate that this previously unreported object is a sungrazing comet and may be one of the group of Kreutz sungrazers. It appears from the data that the perihelion distance was less than 1 solar radius, so that the cometary nucleus encountered dense regions of the sun's atmosphere, was completely vaporized, and did not reappear after the time of closest approach to the sun. After this time, however, cometary debris, scattered into the ambient solar wind, caused a brightening of the corona over one solar hemisphere and to heliocentric distances of 5 to 10 solar radii.     

Comet giacobini-zinner: in situ observations of energetic heavy ions

Conclusive evidence is presented for the existence of energetic ( approximately 535,0000 to 150,000 electron volts), heavy (>-12 atomic mass units), singly charged cometary ions within approximately 1.5 x 10(6) kilometers of comet Giacobini-Zinner. The observations were made with the University of Maryland/Max-Planck-Institut ultralow-energy charge analyzer on, the International Cometary Explorer spacecraft. The most direct evidence for establishing the mass of these ions was obtained from an analysis of the energy signals in one of the solid-state detectors; it is significant at the three-sigma level. Maximum fluxes were recorded approximately 1 hour before and approximately 1 hour after closest approach to the cometary nucleus. Transformation of the particle angular distributions observed at approximately 50,000 kilometers radial distance from the comet during the inbound pass into a rest frame in which the distributions are nearly isotropic requires a transformation velocity that is consistent with the local solar wind velocity if one assumes that these particles are primarily singly ionized with a mass of 18 +/- 6 atomic mass units. The existence of a frame of reference in which these water-group ions were isotropic implies that they underwent strong pitch angle scattering after their ionization. Particle energies in the rest frame extend to substantially higher values than would be expected if these ions were locally ionized and then picked up by the solar wind, implying that the ions were accelerated or heated. The derived ion density, approximately 0.1 per cubic centimeter, is consistent with a crude model for the production and transport of pickup ions.     

X-ray emission from comets

The discovery of x-ray emission from comet Hyakutake was surprising given that comets are known to be cold. Observations by x-ray satellites such as the R?ntgen Satellite (ROSAT) indicate that x-rays are produced by almost all comets. Theoretical and observational work has demonstrated that charge-exchange collisions of highly charged solar wind ions with cometary neutral species can explain this emission. X-ray observations of comets and other solar system objects may be used to determine the structure and dynamics of the solar wind.     

International cometary explorer encounter with giacobini-zinner: magnetic field observations

The vector helium magnetometer on the International Cometary Explorer observed the magnetic fields induced by the interaction of comet Giacobini-Zinner with the solar wind. A magnetic tail was penetrated approximately 7800 kilometers downstream from the comet and was found to be 10(4) kilometers wide. It consisted of two lobes, containing oppositely directed fields with strengths up to 60 nanoteslas, separated by a plasma sheet approximately 10(3)kilometers thick containing a thin current sheet. The magnetotail was enclosed in an extended ionosheath characterized by intense hydromagnetic turbulene and interplanetary fields draped around the comet. A distant bow wave, which may or may not have been a bow shock, was observed at both edges of the ionosheath. Weak turbulence was observed well upstream of the bow wave.     

Observations at mercury encounter by the plasma science experiment on mariner 10

A fully developed bow shock and magnetosheath were observed near Mercury, providing unambiguous evidence for a strong interaction between Mercury and the solar wind. Inside the sheath there is a distinct region analogous to the magnetosphere or magnetotail of Earth, populated by electrons with lower density and higher temperature than the electrons observed in the solar wind or magnetosheath. At the time of encounter, conditions were such that a perpendicular shock was observed on the inbound leg and a parallel shock was observed on the outbound leg of the trajectory, and energetic plasma electron events were detected upstream from the outbound shock crossing. The interaction is most likely not atmospheric, but the data clearly indicate that the obstacle to solar wind flow is magnetic, either intrinsic or induced. The particle fluxes and energy spectra showed large variations while the spacecraft was inside the magnetosphere, and these variations could be either spatial or temporal.     

Plasma diagnosis from thermal noise and limits on dust flux or mass in comet giacobini-zinner

Thermal noise spectroscopy was used to measure the density and temperature of the main (cold) electron plasma population during 2 hours (1.5x10(5) kilometers perpendicular to the tail axis) around the point of closest approach of the International Cometary Explorer (ICE) to Comet Giacobini-Zinner. The time resolution was 18 seconds (370 kilometers) in the plasma tail and 54 seconds (1100 kilometers) elsewhere. Near the tail axis, the maximum plasma density was 670 per cubic centimeter and the temperature slightly above 1 electron volt. Away from the axis, the plasma density dropped to 100 per cubic centimeter (temperature, 2x 10(4) K) over 2000 kilometers, then decreased to 10 (1.5x 10(5)K) over 15,000 kilometers; outside that region (plasma tail), the density fluctuated between 10 and 30 per cubic centimeter and the temperature between 1x 10(5) and 4 x10(5) K. The relative density of the hot population rarely exceeded a few percent. The tail was highly asymmetrical and showed much structure. On the other antenna, shot noise was recorded from the plasma particle impacts on the spacecraft body. No evidence was found of grain impacts on the antennas or spacecraft in the plasma tail. This yields an upper limit for the dust flux or particle mass, indicating either fluxes or masses in the tail smaller than implied by the models or an anomalous grain structure. This seems to support earlier suggestions that these grains are featherlike. Outside the tail, and particularly near 10(5) kilometers from its axis, impulsive noises indicating plasma turbulence were observed.     

Deimos: an obstacle to the solar wind

Two isolated solar wind disturbances about 5 minutes in duration were detected aboard the Russian spacecraft Phobos-2 upon its crossing the wake of the martian moon Deimos about 15,000 kilometers downstream from the moon on 1 February 1989. These plasma and magnetic events are interpreted as the inbound and outbound crossings of a Mach cone that is formed as a result of an effective interaction of the solar wind with Deimos. Possible mechanisms such as remanent magnetization, cometary type interaction caused by heavy ion or charged dust production, and unipolar induction resulting from the finite conductivity of the body are discussed. Although none of the present models is fully satisfactory, neutral gas emission through water loss by Deimos at a rate of about 10(23) molecules per second, combined with a charged dust coma, is favored.     

Charge exchange-induced X-ray emission from comet C/1999 S4 (LINEAR)

Using soft x-ray observations of the bright new comet C/1999 S4 (LINEAR) with the Chandra x-ray observatory, we have detected x-ray line emission created by charge exchange between highly ionized solar wind minor ions and neutral gases in the comet's coma. The emission morphology was symmetrically crescent shaped and extended out to 300,000 kilometers from the nucleus. The emission spectrum contains 6 lines at 320, 400, 490, 560, 600, and 670 electron volts, attributable to electron capture and radiative deexcitation by the solar wind species C(+5), C(+6), N(+7), O(+7), and O(+8). A contemporaneous 7-day soft x-ray light curve obtained using the Extreme Ultraviolet Explorer demonstrates a large increase in the comet's emission coincident with a strong solar flare on 14 and 15 July 2000.     

Ion composition results during the international cometary explorer encounter with giacobini-zinner

The International Cometary Explorer spacecraft passed through the coma of comet Giacobini-Zinner about 7800 kilometers antisunward of the nucleus on 11 September 1985. The ion composition instrument was sensitive to ambient ions with mass-to-charge ratios in the ranges 1.4 to 3 atomic mass units per electron charge (amu e(-1)) and 14 to 33 amu e(-1). Initial interpretation of the measurements indicates the presence of H(2)O(+), H(3)O(+), probably CO(+) and HCO(+), and ions in the mass range 23 to 24; possible candidates are Na(+) and Mg(+). In addition to these heavy ions, measured over the velocity range 80 to 223 kilometers per second, the instrument measured He(2+) of solar wind origin over the range 237 to 463 kilometers per second. The heavy ions have a velocity distribution which indicates that they have been picked up by the motional electric field, whereas the light ions are steadily decelerated as the comet tail axis is approached. These results are in agreement with the picture of a comet primarily consisting of water ice, together with other material, that sublimes, streams away from the nucleus, becomes ionized, and interacts with the solar wind. K. W. Ogilvie, NASA/Goddard Space Flight Center, Code 692, Greenbelt, MD 20771.     

Ultraviolet spectroscopy and the composition of cometary ice

Our current knowledge of the composition of the cormetary nucleus is largely inferred from observations of the gas and dust comae that are produced by sublimation of cometary ice when a comet is near the sun. During the past decade, far-ultraviolet spectroscopy from above the terrestrial atmosphere has shed new light on the physics and chemistry of the gaseous component of the coma. The advent of interplanetary missions to Halley's comet in 1986 and the development of a new generation of earth-orbiting observatories promise further insights into the nature of these frozen remnants of the primordial solar system.     

Chondrulelike objects in short-period comet 81P/Wild 2

The Stardust spacecraft returned cometary samples that contain crystalline material, but the origin of the material is not yet well understood. We found four crystalline particles from comet 81P/Wild 2 that were apparently formed by flash-melting at a high temperature and are texturally, mineralogically, and compositionally similar to chondrules. Chondrules are submillimeter particles that dominate chondrites and are believed to have formed in the inner solar nebula. The comet particles show oxygen isotope compositions similar to chondrules in carbonaceous chondrites that compose the middle-to-outer asteroid belt. The presence of the chondrulelike objects in the comet suggests that chondrules have been transported out to the cold outer solar nebula and spread widely over the early solar system.     

Preliminary results on the plasma environment of saturn from the pioneer 11 plasma analyzer experiment

The Ames Research Center Pioneer 11 plasma analyzer experiment provided measurements of the solar wind interaction with Saturn and the character of the plasma environment within Saturn's magnetosphere. It is shown that Saturn has a detached bow shock wave and magnetopause quite similar to those at Earth and Jupiter. The scale size of the interaction region for Saturn is roughly one-third that at Jupiter, but Saturn's magnetosphere is equally responsive to changes in the solar wind dynamic pressure. Saturn's outer magnetosphere is inflated, as evidenced by the observation of large fluxes of corotating plasma. It is postulated that Saturn's magnetosphere may undergo a large expansion when the solar wind pressure is greatly diminished by the presence of Jupiter's extended magnetospheric tail when the two planets are approximately aligned along the same solar radial vector.     

Electron temperatures and densities in the venus ionosphere: pioneer venus orbiter electron temperature probe results

Altitude profiles of electron temperature and density in the ionosphere of Venus have been obtained by the Pioneer Venus orbiter electron temperatutre probe. Elevated temperatutres observed at times of low solar wind flux exhibit height profiles that are consistent with a model in which less than 5 percent of the solar wind energy is deposited at the ionopause and is conducted downward through an unmagnetized ionosphere to the region below 200 kilomneters where electron cooling to the neutral atmosphere proceeds rapidly. When solar wind fluxes are higher, the electron temperatures and densities are highly structured and the ionopause moves to lower altitudes. The ionopause height in the late afternoon sector observed thus far varies so widely from day to (day that any height variation with solar zenith angle is not apparent in the observations. In the neighborhood of the ionopause, measuremnents of plasma temperatures and densities and magnetic field strength indicate that an induced magnetic barrier plays an important role in the pressure transfer between the solar wind and the ionosphere. The bow, shock is marked by a distinct increase in electron current collected by the instrument, a featutre that provides a convenient identification of the bow shock location.     

The spectrum of comet austin from 910 to 1180 a

A spectrum of comet Austin (1988 c(1)) has been obtained from 910 to 1180 A. Three bright emission lines were detected, including a forbidden oxygen line (1128 A), which are attributable to radiative pumping of neutral oxygen by solar Lyman beta. The relative strengths of the observed features should prove to be a useful diagnostic of the physical conditions and radiation fields in cometary comae. In addition, the absence of strong spectral features from highly volatile species such as He, Ar, or N(2) can be used to place constraints on the thermal environment under which the comet was formed and has been processed.     

Destruction of Sun-grazing comet C/2011 N3 (SOHO) within the low solar corona

Observations of comets in Sun-grazing orbits that survive solar insolation long enough to penetrate into the Sun's inner corona provide information on the solar atmosphere and magnetic field as well as on the makeup of the comet. On 6 July 2011, the Solar Dynamics Observatory (SDO) observed the demise of comet C/2011 N3 (SOHO) within the low solar corona in five wavelength bands in the extreme ultraviolet (EUV). The comet penetrated to within 0.146 solar radius (~100,000 kilometers) of the solar surface before its EUV signal disappeared. Before that, material released into the coma--at first seen in absorption--formed a variable EUV-bright tail. During the final 10 minutes of observation by SDO's Atmospheric Imaging Assembly, ~6 × 10(8) to 6 × 10(10) grams of total mass was lost (corresponding to an effective nucleus diameter of ~10 to 50 meters), as estimated from the tail's deceleration due to interaction with the surrounding coronal material; the EUV absorption by the comet and the brightness of the tail suggest that the mass was at the high end of this range. These observations provide evidence that the nucleus had broken up into a family of fragments, resulting in accelerated sublimation in the Sun's intense radiation field.     

Deep Impact observations by OSIRIS onboard the Rosetta spacecraft

The OSIRIS cameras (optical, spectroscopic, and infrared remote imaging system) onboard the European Space Agency's Rosetta spacecraft observed comet 9P/Tempel 1 for 17 days continuously around the time of NASA's Deep Impact mission. The cyanide-to-water production ratio was slightly enhanced in the impact cloud, compared with that of normal comet activity. Dust particles were flowing outward in the coma at >160 meters per second, accelerated by the gas. The slope of the brightness increase showed a dip about 200 seconds after the impact. Dust Afrho values before and long after the impact confirm the slight decrease of cometary activity. The dust-to-water mass ratio was much larger than 1.     

Magnetic Field and Plasma Observations at Mars: Initial Results of the Mars Global Surveyor Mission

The magnetometer and electron reflectometer investigation (MAG/ER) on the Mars Global Surveyor spacecraft has obtained magnetic field and plasma observations throughout the near-Mars environment, from beyond the influence of Mars to just above the surface (at an altitude of approximately 100 kilometers). The solar wind interaction with Mars is in many ways similar to that at Venus and at an active comet, that is, primarily an ionospheric-atmospheric interaction. No significant planetary magnetic field of global scale has been detected to date (<2 x 10(21) Gauss-cubic centimeter), but here the discovery of multiple magnetic anomalies of small spatial scale in the crust of Mars is reported.     

Laboratory simulation of charge exchange-produced X-ray emission from comets

In laboratory experiments using the engineering spare microcalorimeter detector from the ASTRO-E satellite mission, we recorded the x-ray emission of highly charged ions of carbon, nitrogen, and oxygen, which simulates charge exchange reactions between heavy ions in the solar wind and neutral gases in cometary comae. The spectra are complex and do not readily match predictions. We developed a charge exchange emission model that successfully reproduces the soft x-ray spectrum of comet Linear C/1999 S4, observed with the Chandra X-ray Observatory.