Engineering topics

Elphic, R. C.

Publications and source records attributed to Elphic, R. C..

123 records · Page 7

NASA NTRS1980

Solar wind absorption by Venus

The portion of solar wind interacting with the dayside ionosphere and atmosphere of Venus was determined based on magnetic field fluctuations in the ionosheath and the interaction with the upper neutral atmosphere above the ionopause. Fluctuations with the ratio of the number of particles intersecting the daytide ionopause to the total number of particles of 0.3 suggest that about 0.3% of solar wind may be absorbed. Most of fast H atoms resulting from the charge exchange interactions with the atmosphere escape; some of the energy deposition processes produce observable signatures (such as a narrow Lyman alpha emission region), but penetrating solar wind particles do not control the physical and/or chemical structure of the daytime Venus ionosphere.

Gombosi, T. I.↗

NASA NTRS1980

Limits on the possible intrinsic magnetic field of Venus

Magnetic field measurements obtained by the Pioneer Venus orbiter at low altitudes in the solar wind wake region are examined for possible surface-correlated features and any possible intrinsic magnetic moment. The field variations observed in the wake do not resemble those expected for a solar wind interaction with even a weak intrinsic magnetic field. Little orbit-to-orbit persistence of features is found in the magnetic records. The magnetic field measurements in the wake are averaged in 10 deg x 10 deg bins to minimize the effects of external field sources. In these 37 bins, the average fields appear to be randomly oriented and consistent with zero mean in the region mapped. Using these 37 averaged vector fields, a maximum intrinsic magnetic dipole moment is obtained of 4.3 + or - 2.0 x 10 to the 21st G cu cm, approximately an order of magnitude less than previous estimates. It is noted that a more conservative estimate of the probable error of the mean is 5.5 x 10 to the 21st G cu cm. The Pioneer Venus measurements are thus consistent with zero planetary moment. The present measurements are found to be far below estimates made on the basis of angular momentum, the so-called magnetic Bode's law, and far below the dynamo scaling law of Busse.

Russell, C. T.↗

NASA NTRS1980

Observations of large scale steady magnetic fields in the dayside Venus ionosphere

Although the dayside ionosphere of Venus is often field-free except for fine-scale features, large-scale steady ionospheric magnetic fields with magnitudes sometimes exceeding 100 gammas are occasionally observed by the Pioneer Venus Orbiter magnetometer. These fields are mainly horizontal and can assume any angle in the horizontal plane. The orientation of the field may change along the spacecraft trajectory. The field magnitude in the upper ionosphere usually shows a distinct minimum near approximately 200 km altitude, but the altitude profile is otherwise arbitrary. With few exceptions, the observations of these large scale fields occur when periapsis is at solar zenith angles less than 50 deg. The occurrence of large-scale fields is often coincident with the observation of high solar wind dynamic pressures by the Pioneer Venus Orbiter plasma analyzer closely following the ionosphere encounter. However, the detection of this phenomenon even during some orbits for which the dynamic pressure is not extraordinarily high suggests that other factors, such as hysteresis effects, must also play a role in determining the occurrence frequency of large-scale magnetic fields in the dayside Venus ionosphere.

Luhmann, J. G.↗

NASA NTRS1980

The location of the dayside ionopause of Venus - Pioneer Venus Orbiter magnetometer observations

The location of the dayside Venus ionopause, as observed by the Pioneer Venus Orbiter, is shown to depend on the magnetic pressure in the shocked, highly compressed solar wind plasma just outside the ionopause. Assuming a balance exclusively between this external magnetic pressure and internal ionospheric thermal pressure, invariance of ionospheric conditions, and an isothermal ionosphere, it is possible to determine pressure scale heights for various solar zenith angle intervals. These scale heights yield ionospheric temperatures which agree with direct measurements obtained independently. Not surprisingly, the average ionopause altitude is higher near the terminator, where the average external magnetic pressure is lower. The near-terminator ionopause has much greater positional variability than that at lower solar zenith angles; this appears to be due principally to concomitant variations in the external magnetic pressure, presumably related to solar wind pressure changes.

Elphic, R. C.↗

NASA NTRS1980

On the search for an intrinsic magnetic field at Venus

Magnetic field observations obtained by the Pioneer Venus orbiter at low altitude are now available for two sets of orbits in the Venus wake. Data from these 130 orbits are examined for possible surface correlated features or any intrinsic magnetic moment. No surface correlated magnetic fields are observed, but the threshold for the detectability of such fields at Venus is about an order of magnitude greater than at the moon. A surface feature of 10 deg extent would have to create an anomaly of at least 5 gammas at 200 km to be detected in the Pioneer Venus data. Using measurements averaged in 72 10 x 10 deg bins, a planetary magnetic dipole moment of 0.87 + or - 3.00 x 10 to the 21st gauss-cu cm is obtained. Thus the upper limit of the present day Venus moment is less than 4 x 10 to the -5th of the terrestrial moment.

Russell, C. T.↗

NASA NTRS1979

Plasma acceleration at the earth's magnetopause - Evidence for reconnection

Observations of high-speed plasma at the magnetopause in agreement with theoretical predictions of magnetic field reconnection are reported. Plasma ion and electron distributions measured by the quadrispherical analyzers on board the ISEE 1 and 2 spacecraft were obtained during the outbound traversal of the subsolar magnetopause. Plasma flow speeds of up to 450 km/sec were observed in the magnetopause layer, in contrast to speeds of 50 to 100 km/sec in the adjacent magnetosheath. The observations agree with the predictions of the reconnection model of the dayside magnetopause, in which the magnetopause is described as a rotational discontinuity, or a large-amplitude Alfven wave. It is noted that the lack of observations of plasma acceleration in most other cases of favorable magnetic field orientation could be a product of the rarity of magnetic recombination, or its small scale and nonstationarity.

Paschmann, G.↗

NASA NTRS1979

Position and shape of the Venus bow shock - Pioneer Venus Orbiter observations

Magnetometer data from the Pioneer Venus Orbiter is used to examine the position and shape of this planet's bow shock. Utilizing crossings identified on 86 occasions during the first 65 orbits a mean shock surface is defined for sun-Venus-satellite angles of 60-110 deg. Both the shock shape and variance in location are found to be very similar to the terrestrial case for the range in SVS angle considered. However, while the spread in shock positions at the earth is due predominantly to the magnetopause location varying in response to solar wind dynamic pressure, ionopause altitude variations can have little effect on total obstacle radius. Thus, the Cytherean shock is sometimes observed much closer to or farther from the planet than previously predicted by gasdynamic theory applied to the deflection of flow about a blunt body which acts neither as source nor sink for any portion of the flow.

Slavin, J. A.↗

NASA NTRS1979

A comparison of Pioneer Venus and Venera bow shock observations - Evidence for a solar cycle variation

Observations by the Venera 9 and 10 orbiters in 1975-76 have been used in previous studies to determine the mean location and shape of the Cytherean bow shock. In addition it has also been reported that the shock is found to be more distant from the planet above regions of the ionosheath where draped IMF field lines are oriented perpendicular to the flow as opposed to parallel. An examination of the dependence of shock altitude in the terminator plane on upstream IMF direction using 86 Pioneer Venus orbiter bow shock crossings in 1978-79 sets an upper limit on this asymmetry of 12% or approximately half that derived earlier from the Venera data. More significantly, the mean distance to the bow shock observed by Pioneer Venus Orbiter is 35% greater than was the case in 1975-76 near solar minimum. As the growth in effective obstacle radius is an order of magnitude larger than can be accounted for in terms of varying ionopause altitude due to all causes, these results strongly suggest that Venus can absorb significantly more of the incident solar wind plasma during solar minimum when EUV flux is low than during the current epoch in which maximum is approaching.

Slavin, J. A.↗

NASA NTRS1979

The solar wind interaction with Venus

The Pioneer Venus orbiter reveals that Venus has a well developed bow shock like the Earth's but on that is significantly weaker than the Earth's shock. The location of the bow shock is highly variable, more so than would have been expected for an obstacle of essentially fixed size. The altitude of the ionopause is also highly variable in response to changes in the solar wind. In the ionosphere, the field is often low. However, on some orbits, very large fields are seen as low as 150 km, and on most dayside orbits, thin magnetic structures of flux ropes are observed. At night, large fields are often observed which vary from orbit to orbit. Venus has a much smaller intrinsic magnetic moment than expected from scaling the terrestrial moment.

Russell, C. T.↗

NASA NTRS1979

Initial Pioneer Venus magnetic field results - Nightside observations

Initial observations by the Pioneer Venus magnetometer on the nightside of Venus frequently reveal moderately strong fields from 20 to 30 nanoteslas. However, there is little evidence that these fields arise from an internal dynamo, since they are mainly horizontal and vary from orbit to orbit. Determining a precise upper limit to the intrinsic moment awaits further processing. This limit is expected to be much less than 10 to the twenty-second gauss-cubic second.

Russell, C. T.↗

NASA NTRS1979

Initial Pioneer Venus magnetic field results - Dayside observations

Pioneer Venus magnetometer observations in the sunlit ionosphere indicate that the ionosphere is dynamic and very responsive to external solar wind conditions. Bow shock location, ionosphere location, the strength of the magnetic field just outside the ionopause, and the field strength in the ionosphere are found to be variable, and the properties of flux ropes in the ionospheric magnetic field are considered. Data on magnetic energy density and on magnetic field strength are presented.

Russell, C. T.↗

NASA NTRS1979

ISEE observations of flux transfer events at the dayside magnetopause

Magnetic field measurements from the ISEE 1 and 2 spacecraft are examined in the vicinity of the magnetopause near local noon on a typical pass when the magnetosheath field is southward. The data clearly show evidence for patchy impulsive reconnection. The flux transfer rate for these events is at least of the order of 1-2 times 10 to the 12th Maxwells per second, and possibly greater. This rate is similar to rates deduced for magnetopause erosion events. Not only are these observations relevant to the substorm process, but the impulsive nature of the flux transfer events leads to boundary oscillations that could also be the source of long period magnetic pulsations in the outer magnetosphere.

Russell, C. T.↗

NASA NTRS1979

Initial Pioneer Venus magnetometer observations

Initial Pioneer Venus magnetometer observations reveal a highly dynamic interaction between the solar wind and the ionosphere and a very weak and possibly absent intrinsic magnetic field. The bow shock position and the altitude of the ionopause vary markedly from day to day. The magnetic pressure in the magnetosheath just outside the ionopause is in near balance with the thermal ionospheric pressure inside. Although the ionospheric magnetic field strength is generally low, occasional enhancements are observed with field strengths exceeding that in the magnetosheath. These bundles of magnetic flux, or flux ropes, may be convected to the night side ionosphere in which large field strengths (compared to the dayside) are common. The magnetic field magnitude and direction in this region are quite variable, suggesting that the field is not due to an intrinsic planetary source, but rather due to induced ionospheric currents. The magnetic moment is probably much less than 10 to the 22nd Gauss-cu cm.

Russell, C. T.↗

NASA NTRS1978

Initial ISEE magnetometer results - Magnetopause observations

Magnetic-field profiles across the magnetopause are examined which were obtained by the ISEE 1 and 2 spacecraft on four passes when the spacecraft separation was only a few hundred kilometers. The velocity of the magnetopause is found to be highly irregular, ranging from 4 to over 40 km/s and varying on a shorter time scale than required for a spacecraft to cross the boundary. The thickness of the magnetopause is estimated to range from 500 to over 1000 km. Clear evidence for field-line reconnection is discerned in data obtained when the magnetosheath field was southward. This evidence consists of flux-transfer events in which reconnection begins and ends on a time scale of minutes or less, leading to oscillations in the position of the magnetopause.

Russell, C. T.↗

NASA NTRS1978

On the apparent source depth of planetary magnetic fields

Two simple assumptions regarding the ratios of the strengths of the field contributions of the multipole moments of the terrestrial magnetic field at its effective source depth are used to examine the consistency between the apparent source depths for the magnetic fields of Mercury and Jupiter and the present understanding of their interior structure. Both fields are consistent with the present understanding. However, the comparison would be facilitated by further measurements of the magnetic fields at both planets, especially at Mercury.

Elphic, R. C.↗