Dawn-dusk (y) component of the interplanetary magnetic field and the local time of the harang discontinuity

We describe a simple method for determining the time at which the meridian of a sub-auroral magnetic observatory crosses that of the Harang discontinuity—the separation of the eastward and westward electrojets which flow in the evening and morning sectors of the auroral oval. We then consider how this time, determined from examination of magnetograms from sub-auroral observatories varies with the dawn-dusk (y) component of the Interplanetary Magnetic Field. We find that the time at which the Harang discontinuity is identified in the Northern Hemisphere is earlier for B_y > 0 than the occasions when B_y < 0, and that the converse is observed in the Southern Hemisphere. Also we suggest that there is no significant seasonal variation in the relationship between the time of the discontinuity and B_y. The sense of the azimuthal shift of the auroral electrojet currents with changes in B_y is consistent with the theory of Cowley (1981). However, the magnitude of the observed shifts is approximately an order of magnitude greater than the theoretical predictions. We suggest that this difference between observation and theory arises from the use of a dipole magnetic field model at auroral zone latitudes in the theoretical estimation of azimuthal displacement.     

The effect of f(T) gravity on an interplanetary clock and its time transfer link

As an extension of the"teleparallel"equivalent of general relativity,f(T)gravity is proposed to explain some puzzling cosmological behaviors,such as accelerating expansion of the Universe.Given the fact that modified gravity also has impacts on the Solar System,we might test it during future interplanetary missions with ultrastable clocks.In this work,we investigate the effects of f(T)gravity on the dynamics of the clock and its time transfer link.Under these influences,theΛ-term and theα-term of f(T)gravity play important roles.Here,Λis the cosmological constant andαrepresents a model parameter in f(T)gravity that determines the divergence from teleparallel gravity at the first order approximation.We find that the signal of f(T)gravity in the time transfer is much more difficult to detect with the current state of development for clocks than those effects on dynamics of an interplanetary spacecraft with a bounded orbit with parameters 0.5 au≤a≤5.5 au and 0≤e≤0.1.     

Laboratory investigation of the ionospheric O2+(X2πg, v ≠ 0) reaction with NO

Laboratory measurements of the reaction of O₂⁺ with NO from thermal energy to 0.6 eV in an Ar buffered flow drift tube agree with similar measurements made earlier in the same drift tube with He buffer. Since the O₂⁺ ions are substantially vibrationally excited in Ar and not in He it follows that the reaction is not enhanced by vibrational excitation of the O₂⁺.     

The height,spectrum and mechanism of type-B red aurora and its bearing on the excitation of O(1S) in aurora

The height of the lower red border of type-B aurora has been determined by triangulation using TV cameras at two ground stations. A mean height of 91.4 ± 1.1 km was determined from a set of 12 measurements made under ideal conditions. A TV spectrograph was used simultaneously to seek possible spectral changes between 6400 and 6900 Å which would be indicative of changes in the vibrational distribution in the N₂ First Positive bands. No significant difference was found in this distribution between the spectra from 93 and 122 km. The height distribution of contributions to the OI 5577 Å emission relative to the N⁺₂ First Negative emission was modelled from 80 to 160 km. Contributions from electron impact on atomic O, O⁺₂ dissociative recombination and N₂(A)O energy transfer were included. Account was taken of recent laboratory data on O(¹S) quenching. It was concluded that these processes could explain the excitation of O(¹S) in normal aurora and the height distribution of OI 5577 Å in type-B red aurora. It was confirmed that the lifetime ofO(¹S) in type-B red auroral rapid time variations is about 0.5 s and it was found from the model that the observed time variation can be reproduced by the mechanisms considered, provided the concentration of NO in the auroral atmosphere is about 1 × 10⁹ at 95 km. Before reasonable certainty can be attained in the correctness of the interpretation it will however be necessary to have reliable simultaneous observations of neutral atmospheric composition particularly for O and NO as well as unchallengeable measurements of the yields of O(¹S) for the processes considered and for several other processes which have been suggested recently.     

measurement of noise power spectrum in photon counting PMT's and the effect of the observed (1/f) noise on astronomical photometry

In astronomical photometry, the sensitivity of observations is limited by the dark counts of the photomultiplier tube. In the present work, the effect of dark count noise in photon counting systems is investigated by theory and experimental measurements. Dark counts are considered to be originating from two sources, namely: dc fluctuations and random pulses.Experimental measurements were carried out to determine noise effects in different operating regions of noise dominance. The results provide strong evidence that: in normal operating mode, where the effect of random pulses is dominant, dark counts do not follow Poisson statistics. The observed noise shows strong (1/f) power spectrum, where the observed noise power is found to increase with time of observation.The results are important in photon counting systems operating under dark count limited mode. The conclusions drawn can be useful in obtaining more accurate error estimates and in assessing astronomical photometric observations and data reduction techniques.     

EUV resonance fluorescence of the c′4→X (0,v″) and b′→X (1,v″) transitions of N2

Extreme ultraviolet (EUV) resonance fluorescence of the (0,v″) bands of the c′₄¹Σ_u⁺→X¹Σ_g⁺ and the (1,v″) bands of the b′ ¹Σ_u⁺→X¹Σ_g⁺ transitions of N₂ has been observed by photon excitation of N₂ in the vicinity of 95.8 nm. The integrated fluorescence intensities of the c′₄→X (0,v″) emission become saturated at N₂ pressures higher than ∼0.16 mTorr. The emission features in the spectral region between 105 and 130 nm become progressively significant as the N₂ pressure is increased. The (1,v″) progression for v″ up to 11 of the b′→X transition and two progressions of the Lyman-Birge-Hopfield (LBH) system have been identified. The multiple scattering processes apparently cause significant reduction in the c′₄→X (0,0) emission rates. The present results may be useful in the explanation of the weak c′₄→X (0,0) fluorescence as well as the significant c′₄→X (0,v″) features in the dayglow of the Earth observed by the Far Ultraviolet Spectroscopic Explorer.     

Laboratory measurements of the Ka-band (7.5 mm to 9.2 mm) opacity of phosphine (PH3) and ammonia (NH3) under simulated conditions for the Cassini-Saturn encounter

Recently, a model for the centimeter-wavelength opacity of PH₃ under conditions characteristic of the outer planets was developed by Hoffman et al. (2001, PhD thesis), based on centimeter wavelength laboratory measurements. New laboratory measurements have been conducted which show that this model is also accurate at low pressures and temperatures, and at millimeter wavelengths such as will be employed in Cassini Ka-band (9.3 mm) radio occultation studies. The opacity of PH₃ in a hydrogen/helium (H₂/He) atmosphere has been measured at frequencies in the Ka-band region at 32.7 GHz (9.2 mm), 35.6 GHz (8.4 mm), 37.7 GHz (8.0 mm), and 39.9 GHz (7.5 mm) at pressures of 0.5, 1, and 2 bar and at temperatures of 295, 209, and 188 K. Additionally, new high-precision laboratory measurements of the opacity of NH₃ in an H₂/He atmosphere have been conducted under the same temperature and pressure conditions described for PH₃. These new measurements better constrain the NH₃ opacity model supporting use of a Ben-Reuven lineshape model. These measurements will also elucidate the interpretation of millimeter wavelength observations conducted with the NRAO/VLA at 43 GHz (7 mm).     

Search for and limits on plume activity on Mimas, Tethys, and Dione with the Cassini Visual Infrared Mapping Spectrometer (VIMS)

Cassini Visual Infrared Mapping Spectrometer (VIMS) observations of Mimas, Tethys, and Dione obtained during the nominal and extended missions at large solar phase angles were analyzed to search for plume activity. No forward scattered peaks in the solar phase curves of these satellites were detected. The upper limit on water vapor production for Mimas and Tethys is one order of magnitude less than the production for Enceladus. For Dione, the upper limit is two orders of magnitude less, suggesting this world is as inert as Rhea (Pitman, K.M., Buratti, B.J., Mosher, J.A., Bauer, J.M., Momary, T., Brown, R.H., Nicholson, P.D., Hedman, M.M. [2008]. Astrophys. J. Lett. 680, L65-L68). Although the plumes are best seen at ∼2.0 μm, Imaging Science Subsystem (ISS) Narrow Angle Camera images obtained at the same time as the VIMS data were also inspected for these features. None of the Cassini ISS images shows evidence for plumes. The absence of evidence for any Enceladus-like plumes on the medium-sized saturnian satellites cannot absolutely rule out current geologic activity. The activity may below our threshold of detection, or it may be occurring but not captured on the handful of observations at large solar phase angles obtained for each moon. Many VIMS and ISS images of Enceladus at large solar phase angles, for example, do not contain plumes, as the active “tiger stripes” in the south pole region are pointed away from the spacecraft at these times. The 7-year Cassini Solstice Mission is scheduled to gather additional measurements at large solar phase angles that are capable of revealing activity on the saturnian moons.     

Spectral alteration of the Meteorite Epinal (H5) induced by heavy ion irradiation: a simulation of space weathering effects on near-Earth asteroids

We present results obtained for Epinal (H5), an ordinary chondrite meteorite, irradiated with 60 keV Ar⁺⁺ ions, simulating solar wind heavy particle irradiation. Bidirectional reflectance spectra (0.3-2.67 μm) measured after irradiating Epinal samples with different ion fluences exhibit a progressive reddening that is similar to the spread of spectra observed for S-type near-Earth asteroids. The timescales for inducing the same effects in space as those obtained in laboratory are estimated to be ¹⁰⁴-¹⁰⁶ yr. These results suggest irradiation by heavy ions may be a very efficient weathering process in near-Earth space.     

The nondetection of continuum radiation from comet IRAS-Araki-Alcock (1983d) at 2- to 6-cm wavelengths and its implication on the icy-grain halo theory

Observations of Comet IRAS-Araki-Alcock have been made with the VLA (Very Large Array) at 6 and 2 cm, when the comet was at geocentric distances of 0.08 and 0.035 AU, respectively. The 3σ upper limits are 90 and 750 μJy at 6 and 2 cm, respectively. We show that the “conventional” icy-grain halo theory is not adequate to explain the data. If there is such a halo, it is either very thin, or does not contain grains with sizes larger than 10–100 μm. Comparison of our limits with a reported detection at 1.3-cm wavelengths shows that if the centimeter-wavelength radiation all arises in the halo, the halo should have an extent of the order of 300–400 km, but an effective area of 100 km². If only thermal emission from the nucleus is significant, the temperature decreases from about 200°K at the layers probed at 1.3 cm to about 50°K or less at depths probed at 2 cm (assuming unit emissivity at all wavelengths and depths). This can be due to a combination of a lower emissivity and lower physical temperature at larger depths in the comet; both effects are expected when considering theories on microwave emission from glaciers on Earth.     

Expansions of (r/a)m cos jv and (r/a)m sin jv to high eccentricities

Expansions of the functions (r/a)^cos jv and (r/a)m ^sin jv of the elliptic motion are extended to highly eccentric orbits, 0.6627 ... <e<1. The new expansions are developed in powers of (e–e*), wheree* is a fixed value of the eccentricity. The coefficients of these expansions are expressed in terms of the derivatives of Hansen's coefficients with respect to the eccentricity. The new expansions are convergent for values of the eccentricity such that |e–e*|<(e*), where the radius of convergence (e*) is the same of the extended solution of Kepler's equation. The new expansions are intrinsically related to Lagrange's series.     

Low phase angle effects in photometry of trans-neptunian objects: 20000 Varuna and 19308 (1996 TO66)

We present the results of photometric observations of trans-neptunian object 20000 Varuna, which were obtained during 7 nights in November 2004-February 2005. The analysis of new and available photometric observations of Varuna reveals a pronounced opposition surge at phase angles less than 0.1 deg with amplitude of 0.2 mag relatively to the extrapolation of the linear part of magnitude-phase dependence to zero phase angle. The opposition surge of Varuna is markedly different from that of dark asteroids while quite typical for moderate albedo Solar System bodies. We find an indication of variations of the scattering properties over Varuna's surface that could result in an increase of the lightcurve amplitude toward zero phase angle. It is shown that a similar phase effect can be responsible for lightcurve changes found for TNO 19308 (1996 TO₆₆) in 1997-1999.     

Radiative cooling within illuminated layers of dust on (pre)-planetary surfaces and its effect on dust ejection

Temperature gradients in dust beds embedded in a low pressure gaseous environment induce a lift of particles under certain conditions. This effect can erode planetesimals and enables entrainment of dust into the martian atmosphere. Here, we consider a numerical model to calculate the temperature profile in a dust bed which is subject to illumination. We consider the situation when the illumination is switched on and heats the dust bed’s surface and when it is switched off again after a certain time. The calculations focus on the heat transfer by infrared radiation within the dust layer. We find that radiative transfer within the dust bed modifies the absolute temperatures and temperature gradients significantly. This is important for effects which are sensitive to absolute temperatures, i.e. ice sublimation or melting of solids. For low thermal conductivity dust beds of 0.001 W m⁻¹ K⁻¹ it determines the temperature structure of the dust. For higher thermal conductivities the modifications are moderate with respect to dust eruptions as the order of magnitude of temperature gradients stays the same.     

Polarimetric Study of Comets C/1995 O1 (Hale–Bopp) and C/2000 Wm1 (Linear)

We made polarimetric observations of comet Hale–Bopp covering awide phase angle range, from 18.8 to 47°. At certain phase angles the heliocentricdistance of the comet was less than 1 AU during its pre and post perihelion passages. Oneof the important findings, based on the data in the visual bands, is the higher polarizationwith stronger wavelength dependence compared to comet Halley, indicating the presenceof much finer grains in comet Hale–Bopp. It may also be noted that comet Hale–Bopphas shown highest degree of polarization known so far for any comet and hence fallsin the class of high polarization comets. Polarimetric observations were made of cometC/2000 WM1 (LINEAR)using narrow band (IHW) filters 4845 Å and 7000 Åand broad bands filters BVR during November 23–26, 2001 when the phase angle rangedfrom 15 to 22°. Some of the results based on these observations are presented anddiscussed.     

Interactive effects of hypobaria, low temperature, and CO2 atmospheres inhibit the growth of mesophilic Bacillus spp. under simulated martian conditions

Robotic spacecraft are launched with finite levels of terrestrial microorganisms that are similar to the microbial communities within facilities in which spacecraft are assembled. In particular, spores of mesophilic aerobic Bacillus species are common spacecraft contaminants considered most likely to survive interplanetary transfer to Mars. During the cruise phase to Mars, and then again during surface operations, microbial bioloads are exposed to a diversity of biocidal factors that are likely to render the microbial species either dead or significantly inhibited from active metabolic activity and replication. We report here, for the first time, that interactive effects of low pressure, low temperature, and high CO₂ atmospheres approaching conditions likely to be encountered on the martian surface strongly inhibit the growth and replication of seven common Bacillus spp. isolated from spacecraft. Tests were conducted within a small glass bell-jar system maintained in a low-temperature microbial incubator. Atmospheric pressures were controlled at 1013 (Earth-normal), 100, 50, 35, 25, or 15 mb, and temperatures were maintained at 30, 20, 15, 10, or 5 °C. Experiments were carried out for 48 h or 7 days under either Earth-normal O₂/N₂ or pure CO₂ atmospheres. Results indicated that low pressure, low temperature, and high CO₂ atmospheres, applied separately or in combination, were capable of inhibiting the growth and replication of B. pumilus SAFR-032, B. pumilus FO-36B, B. subtilis HA-101, B. subtilis 42HS-1, B. megaterium KL-197, B. licheniformis KL-196, and B. nealsonii FO-092 under simulated martian conditions. Endospores of all seven Bacillus spp. strains failed to germinate and grow at 25 mb at 30 °C. Although, vegetative cells of these strains exhibited a slightly greater ability to replicate at lower pressures than did endospores, vegetative cells of these species failed to grow at pressures below 25 mb. Interactive effects of these environmental parameters acted to generally increase the inhibitory nature of the low-pressure conditions on growth and replication of the seven Bacillus spp. tested.     

Condensed species in Titan's stratosphere: Confirmation of crystalline cyanoacetylene (HC3N) and evidence for crystalline acetylene (C2H2) on Titan

Infrared spectra of crystalline HC₃N and C₂H₂ were investigated at several temperatures between 15 and 150 K. The characteristics of the 505 and 753 cm⁻¹ bands of HC₃N are in complete agreement with the emission spectral data on Titan obtained by the Voyager IRIS instrument, thus confirming the identification of crystalline HC₃N on Titan. A composite spectrum in the 720-800 cm⁻¹ region, with contributions from HC₃N and C₂H₂ in crystalline phases, reproduces the Voyager emission data in that region, thus providing a suggestion for the identification of crystalline C₂H₂ on Titan.