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Kumar, S.

Publications and source records attributed to Kumar, S..

At least 145 records · Page 8
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NASA NTRS1976

Mariner 10 - Mercury atmosphere

Reduction of data from the three Mariner 10 encounters to this date have allowed identification of helium and hydrogen as atmospheric constituents. Subsolar point densities are estimated at 4500 per cu cm for He and 8 per cu cm for the thermal component of H. A nonthermal component in H with a scale height of about 70 km has been observed near the limb off the subsolar point, providing a total apparent number density of 90 per cu cm. Upper limits on other atmospheric constituents have been reduced an order of magnitude. A very tentative identification of O has been obtained but the uncertainty is large and we require further data reduction for confirmation.

Broadfoot, A. L.↗
NASA NTRS1976

Mercury's atmosphere - A perspective after Mariner 10

Implications of Mariner 10 observations are discussed regarding the composition and structure of Mercury's atmosphere as well as its interaction with the solar wind. The Mariner 10 data cited indicate that Mercury's atmosphere is apparently similar to that on the moon. It is shown that helium could be supplied to the atmosphere either by the radioactive decay of uranium and thorium in the planetary crust or by accretion of a small fraction of the solar-wind flux. It is noted that the absence of argon could be explained by a potassium-deficient crust, but this conclusion is by no means firm. Upper limits for outgassing of H2O and CO2 are estimated which suggest that either the planet is deficient in volatiles or is relatively inactive in comparison with earth.

Kumar, S.↗
NASA NTRS1975

He 584 A airglow emission from Venus - Mariner 10 observations

Observations were made across the bright limb of Venus by the ultraviolet spectrometer on Mariner 10 as the spacecraft flew by this planet on February 5, 1974. The altitude variation of the brightness of 584-A emission is interpreted with respect to the helium distribution in the atmosphere. Resonance scattering of solar radiation is considered as the source of this emission. The comparison of model calculations with the data shows that the observed altitude profile is consistent with a He density of about 2 million/cu cm at 145-km altitude and with an exospheric temperature of the order of 375 K. If the major loss of helium is by solar-wind scavenging only, the present helium content on Venus can be maintained by an outgassing rate of about 200,000 per sq cm per sec from the planet crust. By using these neutral helium measurements, it is found that the He(+) ion density in the outer ionosphere would be of the order of 200/cu cm.

Kumar, S.↗
NASA NTRS1975

The ionosphere and upper atmosphere of Venus

A summary is presented of current understanding of the upper atmosphere and ionosphere of Venus and its interaction with the solar wind, based on data from the Mariner 5 and Mariner 10 fly-bys and on far UV spectra obtained in rocket experiments. The major constituent of the upper atmosphere is CO2. Minor constituents include H, He, O, C, and CO and probably N2, Cl, and S. Although the thermal escape rate is only about 10,000/sq cm/sec, the H content in the exosphere appears to be highly variable. A prominent peak in the ionosphere profile near 140 km, appearing both on the day and nightside, is identified as an F(1) layer. An E layer and possibly an F(2) layer are present at 125 and 170 km, respectively. The dayside ionosphere may be explained in terms of the absorption of solar radiation by CO2, O, and He. The transport of ions from day to nightside may be important in the formation of the nightside ionosphere; an additional source may be needed to explain the nightside E layer. There is observational evidence that the solar wind interacts directly with the Venusian atmosphere, resulting in the formation of a bow shock. This may in part be explained by a balance at the ionopause between the solar wind ram pressure and the planetary plasma pressure.

Kumar, S.↗
NASA NTRS1974

Mercury's atmosphere from Mariner 10 - Preliminary results

Analysis of data obtained by the ultraviolet experiment on Mariner 10 indicates that Mercury is surrounded by a thin atmosphere consisting in part of helium. The partial pressure of helium at the terminator is about 5 trillionths of a millibar. The total surface pressure of the atmosphere is less than about 2 billionths of a millibar. Upper limits are set for the abundance of various gases, including hydrogen, oxygen, carbon, argon, neon, and xenon. The wavelength dependence of Mercury's surface albedo is similar to that of the moon over a broad range of wavelengths from 500 to 1600 A. Strong signals were recorded by the airglow instrument as Mariner 10 passed through the cavity behind Mercury. They are as yet unexplained but may provide information on the properties of the local plasma.

Broadfoot, A. L.↗
NASA NTRS1974

An observation of O/+/ 834 A nightglow emission

A 40-A resolution spectrometer was employed to measure the extreme ultraviolet nightglow emissions. The spectrometer was flown on a Black Brant V-C rocket, which was launched at 21:08 MST on Feb. 9, 1973, from White Sands Missile Range, New Mexico. The O(+) 834-A emission can be identified in the spectrum and has a brightness of 15 plus or minus 5 R. This emission is probably excited by the conjugate electrons. The data also indicate the presence of some nightglow emission in the 950- to 1050-A region that is still unidentified.

Kumar, S.↗
NASA NTRS1974

Ultraviolet observations of Venus from Mariner 10 - Preliminary results

An objective grating spectrometer on Mariner 10 has measured airglow in the wavelength range 200 to 1700 angstroms. The data reveal the presence of significant concentrations of hydrogen, helium, carbon, and oxygen atoms in the upper atmosphere of Venus. A preliminary analysis of the hydrogen data indicates an exospheric temperature of 400 K. There is evidence for intense airglow emission at wavelengths longward of 1350 angstroms; the nature of this emission is unclear, but the radiation is spatially extensive and detectable on both day and night sides of the planet.

Broadfoot, A. L.↗
NASA NTRS1974

Extreme ultraviolet spectrometer for space research

A normal incidence spectrometer for use in the wavelength region from 200 to 1270 A has been developed. The design and calibration of the instrument are described in detail. The spectrometer can be employed to detect extreme UV radiation at a minimum flux level of 1 rayleigh with a spectral resolution of 40 A and a spatial resolution of 6 deg. Data on the extreme UV night sky spectrum between 780 and 1270 A obtained with this instrument on a recent rocket flight are presented.

Kumar, S.↗
NASA NTRS1974

Further evidence for an interstellar source of nighttime He I 584 A radiation

The intensity of He I, 584 A radiation in the night sky was measured from a sounding rocket at altitudes between 160 and 217 km. The results obtained confirm previous measurements of this radiation. The observed intensity and spatial variations can be entirely accounted for by resonance scattering of solar 584 A radiation from interstellar helium. The best fit to the observations in this case is obtained with an interstellar helium model in which the bulk velocity of the gas is 5 km/sec, the temperature is 10,000 K, and the helium number density is 0.032 per cu cm. Even if a geocoronal scattering component is present, the data can be used to set a lower limit to the temperature of the local interstellar medium of 1,500 (plus or minus 500) K, assuming a normal interstellar helium abundance.

Paresce, F.↗
NASA NTRS1974

Observations of He I 584 A nighttime radiation - Evidence for an interstellar source of neutral helium

The intensity and spatial variations of the He I 584 A radiation in the night sky were measured to an altitude of 264 km from a sounding rocket launched from Thumba, India. The flux varied in intensity with look angle from a high of 16 plus or minus 5 rayleighs to a low of less than 1 rayleigh. The data obtained are presented in the form of an all-sky map. Since the peak flux levels observed are approximately two orders of magnitude larger than that expected from geocoronal resonant scattering, we have compared our results with those expected from two interplanetary sources of neutral helium: the penetration of the local interstellar medium into the solar neighborhood and the dust deionization of solar-wind alpha particles. The dust deionization-model predictions using currently accepted input parameters appear to be inconsistent with the observations. The interstellar helium source is compatible with the data to within the uncertainties of the models.

Paresce, F.↗
NASA NTRS1974

On the distribution of He+ in the plasmasphere from observations of resonantly scattered He II 304-A radiation

The intensity of the He II 304-A radiation in the night sky was measured from a sounding rocket launched from White Sands Missile Range on June 9, 1972. The data obtained on this flight are compared with predictions of theoretical models of the He(+) altitude distribution within the plasmasphere. The observations are not consistent with an isothermal equilibrium model of the helium ion in the plasmasphere, confirming earlier results. The data are best fit by a diffussive equilibrium model of exospheric He(+) having a plasma temperature gradient of 0.5 K per km and a number density at 1600 km of 200,000 He(+) ions per cu cm. In this model the ion densities are calculated without allowance for thermal diffusion. A similar model that allows for thermal diffusion and that has a He(+) number density at 1600 km of 1500 per cu cm and a temperature gradient of 1.2 K per km yields a less satisfactory fit to the observations but is more consistent with in situ satellite observations.

Paresce, F.↗
NASA NTRS1973

Polarization of the 584- and 304-Angstrom emissions of helium in the geocorona and interplanetary medium

The polarization of the 584- and 304-A emissions of helium that have been resonantly scattered in the earth's atmosphere and in the interplanetary medium is discussed. It is shown that in the geocorona the simultaneous measurement of altitude profiles of the 584-A He dayglow intensity and polarization will test the hypothesis that the 584-A dayglow is excited by resonance scattering of sunlight from geocoronal helium. If this is indeed the major source of the 584-A dayglow emission, a simultaneous measurement of the polarization and the altitude profile of the intensity in the exosphere will uniquely determine the helium number density distribution as a function of altitude, and in the thermosphere it will alow a study of nondiffusive equilibrium processes. An attractive feature of this method is that the interpretation does not require a knowledge of the solar 584-A line center flux or line profile.

Kumar, S.↗
NASA NTRS1973

Observations of the Helium II 304-A and Helium I 584-A atmospheric dayglow radiation.

Two photometers with bandpasses of 170 to 500 and 170 to 800 A were employed to observe dayglow emissions in the extreme ultraviolet (EUV) over an altitude range of 90 to 186 km. The emissions observed with these photometers are identified as resonantly scattered He I 584-A and He II 304-A radiations. At 186 km, 209 plus or minus 70 rayleighs of 584-A and 9.3 plus or minus 3.1 rayleighs of 304-A radiation were measured. These observations are compared with theoretical calculations of resonance scattering of solar emissions from geocoronal He and He(+). Using the Jacchia (1971) atmospheric model for He, it is found that the observed brightness of the 584-A emission requires that the solar 584-A line width be 0.014 plus or minus 0.004 A. In this model the maximum overhead brightness of 584-A dayglow would occur at 900 km, and its magnitude would be 1.9 plus or minus 0.6 kR. The authors' observation of 304-A brightness requires that the overhead column density of the He(+) ions be 4.2 x 10 to the 11th ions/sq cm column. This value is consistent with a constant-density plasmasphere model with a He(+) ion density of 320 ions/cu cm in the plasmasphere.

Kumar, S.↗
NASA NTRS1973

Observations of the He II 304-A radiation in the night sky.

The intensity and spatial variations of the He II 304-A radiation in the night sky were measured to an altitude of 264 km from a sounding rocket launched from Thumba, India, on Mar. 10, 1970. The data obtained are presented in the form of an all-sky map and are compared with theoretical predictions. The data are not consistent either with diffusive equilibrium models of exospheric helium or with plausible models of helium in the interplanetary medium. The data can be fit with a constant-density plasmasphere model bounded at the magnetic shell L = 4 in combination with a tenuous gas of helium ions in the plasma sheet. Numerical results obtained with this model are compared with in situ satellite observations.

Paresce, F.↗
NASA NTRS1972

High-intensity source of extreme ultraviolet

High intensity ultraviolet radiation source was developed which is suitable for emission below 500 A. Source, useful for 100 to 1000 A range, is simple and inexpensive to construct, easy to operate, and very stable. Because of sufficiently intense output spectrum, source can be used with monochromator at wavelengths as low as 160 A.

Paresce, E.↗
NASA NTRS1972

Zenith angle dependence of the geocoronal Lyman-alpha glow.

Review of the observations made on the zenith angle dependence and intensity of the geocoronal hydrogen Lyman-alpha glow by means of one of four extreme ultraviolet photometers flown to an altitude of 264 km on a Nike Tomahawk rocket launched from Thumba, India, in March 1970. The results obtained are compared with Meier and Mange's (1970) theoretical predictions. The possible causes for the discrepancies found are discussed.

Paresce, F.↗
NASA NTRS1972

Origin of strong magnetic fields.

A possible mechanism by which extremely strong magnetic fields in neutron stars and white dwarfs could originate involves the existence of thermodynamic equilibrium states (LOFER states) of an electron gas. The essence of the stability theory of a LOFER state is discussed. The extended electronic system considered is in contact with a thermal bath in the presence of an external magnetic field. It is found that it is not necessary to require absolute stability in order to realize a LOFER state in nature.

Canuto, V.↗