Malcolm B. Niedner

Ultraviolet spectrophotometry of comet Giacobini-Zinner during the ICE encounter

Abstract A program of ultraviolet spectrophotometry using the International Ultraviolet Explorer (IUE) Observatory was carried out in support of the International Cometary Explorer (ICE) mission. The H2O production rate was monitored from 1985 June to October. Between 1985 September 9 and 12, the spatial and temporal variation and abundance (or upper limits) of the remotely detectable species, C, CO, CO+, CO2+, CS, H, Mg+, O, OH, and S, were obtained. These observations included the time of the ICE encounter (1985 September 11.46) when the H2O production rate was 3 × 1028 sec−1 ± 50%. This rate is consistent with a number of gas production rates derived indirectly from the ICE experiments. The comet was in a nearly steady state around the time of encounter showing no evidence of short-term temporal fluctuations in brightness greater than 6%. A sunward-tailward asymmetry of the OH brightness was observed at 10,000 km from the nucleus. The absence of detected Mg+ rules out this species as a possible ion of M Q = 24 which was detected by the Ion Composition Instrument, part of the ICE complement of instruments. Comparison of the abundance of CO2+ ions with total electron density measured by the plasma electron and radio science experiments on ICE indicates a deficiency of ions relative to electrons. To satisfy charge balance criteria, a major population of ions not detected by remote sensing must be present.

Comet Halley remote plasma tail observations and in situ solar wind properties: Vega-1/2 IMF/plasma observations and ground-based optical observations from 1 December 1985 to 1 May 1986

Abstract Properties of the interplanetary magnetic field and solar wind plasma are analysed from continuous observations from 1 December 1985 to 1 May 1986 made by the Vega spacecraft (Riedler et al., 1986, Nature321, 288; Gringauz et al., 1986, Nature321, 282). Several high speed streams and flare-related shocks are found; the magnetic field sector boundaries are also seen. Many Earth-based observations of disconnection events in the plasma tail of comet Halley are now available and several mechanisms [changes in the solar wind (Jockers, 1985, Astron. Astrophys. Suppl. Ser. 62, 791; 1986, Asteroids, Comets and Meteors II, Reprocentralen HSC, Uppsala), interplanetary magnetic field features, e.g. day-side reconnection (Niedner and Brandt, 1978, Astrophys. J.223, 665) or night-side reconnection (Russell et al., 1986, J. geophys. Res.91, 1417)] have been proposed to explain the occurrence of such events. We investigated the correlation between the IMF sector boundaries measured by Vega and the observed disconnection events; such a correlation would support the day-side reconnection explanation. It is found that only in 50% of the considered events does a correlation between these two phenomena exist. For the other cases, a sector boundary of the IMF sweeping over the comet cannot explain the occurrence of disruptions of the main plasma tail. An indication is also seen that density enhancements (e.g. as they occur for high speed streams) in the solar wind are connected with large tail events.

Plasma-tail activity at the time of the Vega encounters

Physical associations are sought between plasma-tail activity seen in ground-based imagery and near-comet, interplanetary magnetic field (IMF) measurements obtained by the Vega spacecraft. Emphasis is placed on Disconnection Events (DE’s) and on testing the sector boundary/frontside magnetic reconnection model (Niedner and Brandt, 1978) of their origin. Strong support for the model comes from the spectacular DE whose onset, on 1986 March 7–8, is strongly correlated with a reversal of the comet’s magnetic barrier observed by Vega-1 and Vega-2 at the encounters on March 6.3 and 9.3 UT, respectively, and with an IMF sector boundary observed by Vega-1 on March 7.9 UT. Additional work is needed to determine the possible role of solarwind plasma effects in producing the DE. Relationships between other occurrences of plasma-tail activity and Halley Armada spacecraft measurements remain to be explored.

Dynamics of cometary plasma tails

Abstract As a result of the visibility imparted by fluorescing ions (mostly CO+ and H2O+), cometary plasma tails are the one variety of magnetotails whose large-scale structure and multi-point temporal variations can be studied from a single observation, or pair of observations, respectively; the type of data is wide-field (≥ 5°) imagery. When concurrent solar-wind and interplanetary magnetic field (IMF) data are available (near-Earth, interplanetary, or-- preferably--near-comet), the opportunity exists for studying the large-scale comet/solar-wind interaction. The purposes of the present paper are to review, briefly, some of the principal results which existed before the present apparition of Halleys Comet, but mostly to present initial findings based on the Halley imaging data returned by the Large-Scale Phenomena Network of the International Halley Watch. Most of the latter discussion will concentrate on Disconnection Events (DEs) and the overall interaction of bright comets with the IMF. The importance of correlative studies utilizing remote imaging and the near-comet solar wind/IMF data returned by the Halley Armada spacecraft is particularly stressed. An exciting initial result is that the cometary magnetic barriers observed by VEGA-1 and VEGA-2 had opposite magnetic polarities, in agreement with the frontside magnetic reconnection model of DEs /1/ and with the fact that a major DE took place on 8 March 1986, between the VEGA-1 and VEGA-2 encounters.

The International Cometary Explorer

The International Cometary Explorer (ICE) spacecraft passed through the plasma tail of comet Giacobini-Zinner (G/Z) on September 11, 1985 and made in situ measurements of particles, fields, and waves. The results establish comets and their interaction with the solar wind as a rich source of plasma physics phenomena. Time of closest approach to the comet was approximately 11:02 UT and occurred at a tailward distance of 7,800 km. The spacecraft was in the strong interaction region for roughly 3½ h, and spent about 10 min in the central tail region.

The cause of two plasma-tail disconnection events in comet P /Halley during the ICE-Halley radial period

We have examined ICE magnetometer and electron plasma data for possible causes of the plasma tail disconnection events (DE’s) which were observed in Halley’s comet on 1986 March 20–22 and April 11–12. We attribute the DE of March 20–22 to an interplanetary magnetic field polarity reversal, and the DE of April 11–12 to either a compression region in the solar wind, an interplanetary magnetic field polarity reversal, or a combination of the two. The measured average speeds of tail recession determined from pairs of successive photographs for both events vary between about 40 and 90 km s− 1, and the average acceleration determined from successive pairs of average velocity measurements is quite variable. The calculated times of disconnection for the two events are, in UT decimal dates, March 19.55 ± 0.16 and April 10.91 ± 0.36. Assuming that the two disconnection events are due to frontside reconnection after an interplanetary magnetic field reversal (the most likely explanation for the March 20–22 event, and one possible explanation for the April 11–12 event), we estimate that the time period between the onset of reconnection and the final disconnection of the tail is 0.1 ≦τ rec ≦ 0.6 day. This suggests that the average speed at which reconnection proceeds through the cometary magnetic field pileup region is 1 ≦ υ rec ≦ 6 km s −1, or several tenths the local Alfven speed.

The ICE project

The International Cometary Explorer (ICE) spacecraft passed through the plasma tail of comet Giacobini-Zinner (G/Z) on September 11, 1985 and made in situ measurements of particles, fields, and waves. The preliminary results appear to establish comets and their interaction with the solar wind as a rich source of plasma physics phenomena. Time of closest approach to the comet was approximately 11:02 UT and occurred at a tailward distance of 7,800 km. The spacecraft was in the strong interaction region for roughly 3-12 hours, and spent about 10 minutes in the central tail region. The scientific results indicate: 1. that the magnetic field capture and draping model, originated by Alfven, is correct. Oppositely-polarized magnetic tail lobes, and a current sheet separating them, were observed. 2. that the comet/solar-wind interaction produces energetic ions, probably by the “pick up” process 3. the presence of intense plasma wave activity 4. that the central plasma tail is dense and cold 5. that the principal ions are in the H2O+ - H3O+ group 6. that the bow wave, as seen on the flanks, is not a shock but an extended interaction region 7. that impacts of micron-sized dust particles were detected. The ICE spacecraft survived the encounter with comet G/Z relatively unscathed and made fields and particles measurements upstream of Halleys Comet, actually detecting the comet via plasma wave and energetic particle measurements over the nuclear distance range 28,000,000–35,000,000 km.

Generation of the IHW Comet Halley CD-ROM Archive

Abstract The digital Archive of the IHW is a collection of 26 CD-ROMs containing the totality of data submitted by over 1500 observers of Comet Halley during its most recent apparition. The collection includes remote (mostly ground based) observations as well as measurements made in situ by the spacecraft that encountered the comet in March 1986. Only the set of 24 CD-ROMs that comprise the remote observations which were released in 1993 are discussed. The observations of Halley are collected chronologically on 23 CD-ROMs; an added volume (24) includes data from comets Giacobini-Zinner and Crommelin. The data were collected into nine disciplines of astronomical observations: Astrometry, Infrared Studies, Large-Scale Phenomena, Near Nucleus Studies, Photometry and Polarimetry, Radio Studies, Spectroscopy and Spectrophotometry, Amateur Observations, and Meteor Studies. Observations cover the time period from December 1981 through April 1989. There were over 60,000 individual observations recorded. The design and production of this collection is discussed as well as statistics giving the full scope of the data. Initially, the CD-ROMs are being distributed to all of the contributors who requested them from the IHW and to major astronomical facilities around the world. Additional copies of the set of 24 CD-ROMs can be obtained through the NASA Planetary Data System Small Bodies Node at the University of Maryland and/or NASAs National Space Sciences Data Center.

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.

Model-based thermal system design optimization for the James Webb Space Telescope

Abstract. Spacecraft thermal model validation is normally performed by comparing model predictions with thermal test data and reducing their discrepancies to meet the mission requirements. Based on thermal engineering expertise, the model input parameters are adjusted to tune the model output response to the test data. The end result is not guaranteed to be the best solution in terms of reduced discrepancy and the process requires months to complete. A model-based methodology was developed to perform the validation process in a fully automated fashion and provide mathematical bases to the search for the optimal parameter set that minimizes the discrepancies between model and data. The methodology was successfully applied to several thermal subsystems of the James Webb Space Telescope (JWST). Global or quasiglobal optimal solutions were found and the total execution time of the model validation process was reduced to about two weeks. The model sensitivities to the parameters, which are required to solve the optimization problem, can be calculated automatically before the test begins and provide a library for sensitivity studies. This methodology represents a crucial commodity when testing complex, large-scale systems under time and budget constraints. Here, results for the JWST Core thermal system will be presented in detail.