Engineering topics

Niedner, M. B., Jr.

Publications and source records attributed to Niedner, M. B., Jr..

At least 19 records

NASA NTRS1989

Plasma tail evolution in Comet P/Halley 1985-1986

Plasma tail phenomena in comet P/Halley from mid-November, 1985 to mid-June, 1986, are examined, using data from the International Halley Watch. The evolution of the plasma tail is discussed, focusing on the turn-on/turn-off of plasma tail activity, and observations of disconnection events. The interaction between the comet tail and the solar wind is considered. Also, predictions are made of the turn-on/turn-off distances and of the association of disconnection events with the proposed solar-wind causes.

Brandt, J. C.↗

NASA NTRS1987

Plasma structures in comets P/Halley and Giacobini-Zinner

An overview of large-scale plasma phenomena is presented based on results of spacecraft probing of comets Halley and Giacobini-Zinner and on worldwide submissions to the Large-Scale Phenomena Discipline Specialist Team of the International Halley Watch. Examples of tail phenomena and science are presented with emphasis on observed disconnection events. The archive of this material will clearly be very valuable for studying the comet/solar-wind interaction during the 1985-1986 apparition of Halley's comet.

Brandt, J. C.↗

NASA NTRS1986

International Halley Watch: Discipline specialists for large scale phenomena

The largest scale structures of comets, their tails, are extremely interesting from a physical point of view, and some of their properties are among the most spectacular displayed by comets. Because the tail(s) is an important component part of a comet, the Large-Scale Phenomena (L-SP) Discipline was created as one of eight different observational methods in which Halley data would be encouraged and collected from all around the world under the aspices of the International Halley Watch (IHW). The L-SP Discipline Specialist (DS) Team resides at NASA/Goddard Space Flight Center under the leadership of John C. Brandt, Malcolm B. Niedner, and their team of image-processing and computer specialists; Jurgan Rahe at NASA Headquarters completes the formal DS science staff. The team has adopted the study of disconnection events (DE) as its principal science target, and it is because of the rapid changes which occur in connection with DE's that such extensive global coverage was deemed necessary to assemble a complete record.

Brandt, J. C.↗

NASA NTRS1986

The structure of comet tails

Present models of the plasma tails of comets are described. The interaction of the solar wind with ions from the cometary atmosphere is discussed, and the phenomenon of magnetic reconnection observed in plasma tails is explained. The accomplishments of the ICE mission to the Comet Giacobini-Zinner are summarized, and the tasks and expected contributions from upcoming Soviet, European, and Japanese missions to Comet Halley are addressed.

Brandt, J. C.↗

NASA NTRS1985

The ICE project

The International Cometary Explorer (ICE) spacecraft passed through the plasma tail of Comet Giacobini-Zinner (G/Z) on Sept. 11, 1985, and made in situ measurements of particles, fields, and waves. The scientific results indicate that Alfven's magnetic-field capture and draping model is correct; that the comet/solar-wind interaction produces energetic ions (probably by the pick-up process), and that the bow wave, as seen on the flanks, is not a shock but an extended interaction region. It is concluded that the ICE spacecraft survived the encounter virtually unscathed and made field and particle measurements upstream of Comet Halley, actually detecting the comet via plasma wave and energetic particle measurements.

Brandt, J. C.↗

NASA NTRS1985

Tail phenomena

An overview of tail phenomena is presented based on worldwide submissions to the Large-Scale Phenomena Discipline Specialist Team of the International Halley Watch. Examples of tail phenomena and science are presented along with estimates of total expected yield from the Network. The archive of this material will clearly be very valuable for studying the solar-wind/comet interaction during the 1985-1986 apparition of Halley's Comet.

Brandt, J. C.↗

NASA NTRS1984

Magnetic reconnection in comets

Today many of the traditionally puzzling phenomena in the cometary plasma-tail environment can plausibly be linked to magnetic reconnection occurring in several regions of a comet (Niedner and Brandt, 1978 and 1980). The turn-on of these various reconnection sites appears to follow a cyclic pattern in which the plasma-tail disconnection event is the primary feature, and the periodic sector structure of the solar wind is the external driver. The purpose of this review is to discuss these different classes of cometary activity, to state the justifications for linking them to reconnection, to discuss proposed alternate (nonreconnection) models, and to suggest future tests of the hypotheses presented.

Niedner, M. B., Jr.↗

NASA NTRS1983

Interaction of the plasma tail of comet Bradfield 1979L on 1980 February 6 with a possibly flare-generated solar-wind disturbance

Solar wind plasma data from the ISEE-3 and Helios 2 spacecraft were examined to explain a uniquely rapid 10 deg turning of the plasma tail of comet Bradfield 1979L on 1980 February 6. It was suggested that the tail position angle change occurred in response to a solar wind velocity shear across which the polar component changed by approx. 50 km s-1. The present activity was caused by noncorotating, disturbed plasma flows probably associated with an Importance 1B solar flare.

Niedner, M. B., Jr.↗

NASA NTRS1983

The effect of MHD instabilities on the flaring of cometary plasma tails

The hypersonic pressure balance model of flaring in cometary plasma tails of Ershkovich et al. (1982) has been modified to include the effects of magnetohydrodynamic (MHD) instabilities occurring along the ionopause in the outer-tail regions. The effect of instability is to mix the solar-wind and comet-tail plasmas, increasing the tail magnetic field strength above that calculated from magnetic flux conservation. The earlier model assumed the ionopause to be a tangential discontinuity surface (flux conserving) at all distances, with the result that the magnetic field approached zero in the outer regions of strongly flaring tails. The present model is more realistic and is in better agreement with measurements of cometary plasma tail widths and flaring angles. This agreement leads to an important conclusion that the magnetic flux is not conserved in distant comet tails.

Niedner, M. B., Jr.↗

NASA NTRS1983

Interaction of the plasma tail of comet Bradfield 1979L on 1980 February 6 with a possibly flare-generated solar-wind disturbance

Solar-wind plasma data from the ISEE-3 and Helios 2 spacecraft were examined in order to explain a uniquely rapid 10 deg turning of the plasma tail of comet Bradfield 1979l on 1980 February 6. An earlier study conducted before the availability of in situ solar-wind data (Brandt et al., 1980) suggested that the tail position angle change occurred in response to a solar-wind velocity shear across the polar component changed by approximately 50 km/s. The present contribution confirms this result and further suggests that the comet-tail activity was caused by non-corotating, disturbed plasma flows probably associated with an Importance 1B solar flare.

Niedner, M. B., Jr.↗

NASA NTRS1982

On the flaring of cometary plasma tails

Assuming that hypersonic pressure balance with the solar wind governs the shape of plasma tails, it is found that the gas pressure of tail ions and the magnetic field strength at the flanks of the ionopause control the flaring state. The gas pressure exhibits the larger effect: for constant pressures above a certain critical value, the tail flares essentially without limit, while for smaller values the tail flares only near the head (becoming cylindrical at greater distances). The influence of the magnetic field is that the tail flares to larger distances the higher the field strength at the flanks of the ionopause. The observed variability in flaring (and the implied differences in gas pressure and magnetic field) are throught to be the result of changes in the position and shape of the sunward cometary ionopause. Insertion of reasonable comet and solar wind parameters into the pressure balance equations is found to give good agreement with the observations.

Ershkovich, A. I.↗

NASA NTRS1982

Interplanetary gas. XXVIII - A study of the three-dimensional properties of interplanetary sector boundaries using disconnection events in cometary plasma tails

Studies of the solar wind on the basis of cometary plasma tail observations are considered. Niedner and Brandt (1978, 1979) have concluded that the plasma tail frequently disconnects from the cometary head, and that these disconnection events (DEs) are produced by magnetic reconnection at sector boundary passages. They proposed that the disconnections are a natural combination of Alfven's model and the solar-wind sector structure first discovered by Wilcox and Ness (1965). The DEs can be utilized as probes of interplanetary sector structure. Correlations between DEs and sector boundaries observed at earth are considered, and sector boundary properties deduced from DEs are discussed. Attention is given to a review of the warped sheet model, the latitude extent of sector structure, the sector boundary tilt, and specific sources of error in the tilt angles derived from DEs.

Niedner, M. B., Jr.↗

NASA NTRS1982

A worldwide photographic network for wide-field observations of Halley's Comet in 1985-1986

A global network of ground-based observatories for the study of Halley's Comet in 1985/1986 is discussed. Recommendations are made with respect to improving coordination between reporting observatories, in order to ensure detailed imaging of such fast-generating cometary phenomena as plasma-tail knots, helices, disconnected tails, rays and condensations. A method for calibrating telescopes is considered by which well-studied objects will be photographed to provide references for images of Halley's Comet. This procedure is expected to reduce errors to approximately 0.05 mag. A coordinated study of Halley's Comet will provide important data on the physical properties of the Comet. Examples of the topics of study related to the plasma physics of the Comet's tail include: magnetic reconnection, rippling and tearing modes, kink instability, Kelvin-Helmholtz instability, and the flute instability.

Niedner, M. B., Jr.↗

NASA NTRS1981

Observational data needs for plasma phenomena

Bright comets display a rich variety of interesting plasma phenomena which occur over an enormous range of spatial scales, and which require different observational techniques to be studied effectively. Wide-angle photography of high time resolution is probably the best method of studying the phenomenon of largest known scale: the plasma tail disconnection event (DE), which has been attributed to magnetic reconnection at interplanetary sector boundary crossings. These structures usually accelerate as they recede from the head region and observed velocities are typically in the range 50 V km/s. They are often visible for several days following the time of disconnection, and are sometimes seen out past 0.2 AU from the cometary head. The following areas pertaining to plasma phenomena in the ionoshere are addressed: the existence, size, and heliocentric distance variations of the contact surface, and the observational signatures of magnetic reconnection at sector boundary crossings.

Niedner, M. B., Jr.↗

NASA NTRS1981

Interplanetary gas. XXVII - A catalog of disconnection events in cometary plasma tails

A catalog of 72 disconnection event (DES) in cometary plasma tails is presented as a basic data source for cometary and solar wind analysis. The events span the period 1892-1976, and are characterized on the basis of photographic observations. From the observed kinematics of rejected tails, methods are derived to calculate an estimated time of tail disconnection for each DE, either from the average law of motion for DEs lacking velocity measurements, or from the observed velocities. The geometric circumstances of each DE at the times of separation and observation are also tabulated, and a brief description of each event is presented.

Niedner, M. B., Jr.↗

NASA NTRS1981

Interplanetary gas. XXVI - On the reconnection of magnetic fields in cometary ionospheres at interplanetary sector boundary crossings

The reconnection process in the cometary ionosphere believed responsible for the disconnecting plasma tails phenomenon is studied through the basic equations of reconnection theory and current sheet instability criteria. It is proposed that reconnection occurs when the interplanetary magnetic fields incident on a comet that has gone just past a sector boundary are pressed into the fields captured from the previous sector. The fields are of opposite polarity, and the previously captured fields constitute the 'roots' of the plasma tail. An estimated duration of reconnection during a disconnection event (DE) of 0.75 days is used along with estimates of other cometary parameters to construct fusion region dimensions and resistivity with the adopted time scale.

Niedner, M. B., Jr.↗

NASA NTRS1980

Interplanetary gas. XXV - A solar wind and interplanetary magnetic field interpretation of cometary light outbursts

Possible relationships of cometary brightness outbursts with the solar wind and interplanetary magnetic field are examined. Two types of outburst are distinguished: those which involve a significant brightening of both the head and the tail in a comet with a conspicuous plasma tail (Class I), and those involving the brightening of the central condensation of a previously faint comet with no detectable plasma tail (Class II). Class I bursts, as exemplified by Comet Morehouse 1908c, are attributed to the generation in the head of enhanced amounts of ions and their injection into the tail shortly before it disconnects, with ionization provided by sector boundary crossings. Class II events, as exhibited by Comet P/Tuttle-Giacobini-Kresak 1973b, are interpreted as the result of the bombardment of the nucleus by disturbed solar wind near corotated high-speed streams and sector boundaries, leading to highly exothermic chemical reactions.

Niedner, M. B., Jr.↗

NASA NTRS1980

A very rapid turning of the plasma-tail axis of comet Bradfield 19791 on 1980 February 6

Schmidt camera photographs of comet Bradfield 19791 obtained at the Joint Observatory for Cometary Research (JOCR) indicate that a rapid change took place in the comet's plasma tail on 1980 February 6. On that date, a sequence of photographs spanning 27.5 minutes shows a 10 deg shift occurring in the plasma-tail axis between the first and last exposures. The speed of this tail-turning event greatly exceeds that of any other known event and even exceeds turning rates for individual tail streamers. An interpretation based on the windsock theory of plasma tails is that the comet entered a region of rapidly changing solar-wind flow direction. While the search for an associated solar-wind event from near-earth spacecraft observations is a future activity, the present analysis shows that a 50 km/s change in the polar component of the solar-wind velocity, from about 30 km/s northward to about 20 km/s southward, would have produced the 10 deg shift in the tail axis.

Brandt, J. C.↗