Disconnection Events Research Articles

The disconnection events of 1986 April 13-18 and the cessation of plasma tail activity in Comet Halley in 1986 May

The disconnection events (DEs) in the 1986 April 13-18 time period appear to be the last major ones of comet Halley's 1986 apparition. We determined the time of the first disconnection event in this sequence, April 13.9±0.1 days, from kinematic analysis of photographic images. We show here that at the time of this DE, comet Halley had just crossed a magnetic field sector boundary and was in a moderate-density solar wind region with a speed of approximately 350 km s -1 . A sequence of DEs began approximately 1 day after the April 13.9 DE and was easily visible on April 16 and 17. Based on IMP 8 data, we show that this sequence was associated with a complex magnetic structure of the solar wind, showing multiple reversals

Simulations of coronal disconnection events

The lack of evidence for magnetic disconnection of coronal mass ejections (CMEs) from the Sun has long been a puzzle, as it implies a buildup of the interplanetary magnetic field (IMF) magnitude over time. Such a buildup is ruled out by observations. Magnetic reconnection above helmet streamer configurations could provide a mechanism for maintaining the observed relative constancy of the IMF [McComas et al., 1989]; McComas et al. [1991] showed observational evidence of reconnection above a streamer. We investigate this interpretation using time‐dependent MHD simulations. We model the opening of new magnetic flux on the Sun (as might occur in a CME or other transient event) as an increase in magnetic flux at the poles of a simulated corona. We find that this perturbation can in fact cause reconnection above an equatorial helmet streamer, and the resultant density signature is similar to the observations of McComas et al. [1991].

Interplanetary magnetic field connection to the Sun during electron heat flux dropouts in the solar wind

Gosling (1975) and MacQueen (1980) pointed out that coronal mass ejections (CMEs) add significant magnetic flux which is connected to the Sun, to the interplanetary medium. Since the interplanetary magnetic field is observed to stay at a roughly constant level, some compensating removal of flux is necessary. McComas et al. (1989) suggested that magnetic reconnection in current sheets near the Sun removes flux by producing U‐shaped magnetic structures in the interplanetary medium which are disconnected from the Sun at both ends. Since solar wind halo electrons normally carry heat flux outward from the Sun's hot corona to the cool interplanetary medium, McComas et al. (1989) interpreted observations of dropouts of the solar wind electron heat flux as evidence for such disconnections. Here we use 2‐ to 8.5‐keV solar electrons observed by ISEE 3 as tracers of the magnetic topology for these heat flux dropout (HFD) events. These electrons are superior to the ∼0.1‐ to 1‐keV halo electrons as test particles since they have longer mean free paths, they are less affected by electric fields, and their transit times from the Sun to 1 AU are only ∼1–2 hours compared to 3–10 hours for the halo electrons. Furthermore, ≥2‐keV electrons are often accelerated in impulsive events, which are sometimes associated with solar flares, and they also generate type III solar radio bursts, which can be tracked from the Sun to 1 AU by the ISEE 3 radio experiment. In at least eight of the 25 HFDs we find strong streaming of the ≥2‐keV electrons outward from the Sun. In one HFD an impulsive solar electron event was observed with an associated type III radio burst which could be tracked from the Sun to ∼1 AU. We conclude that in many HFDs the interplanetary field is still connected to the Sun, and that some energy‐dependent process may produce HFDs without significantly perturbing electrons of higher energies. For two of the 25 HFDs, however, the ≥2‐keV electron observations exhibit all the characteristics of real magnetic disconnection events, including a depletion in the total ≥2‐keV fluxes.

Comet Halley remote plasma tail observations and in situ solar wind properties: VEGA- [formula omitted] IMF/plasma observations and ground-based optical observations from 1 December 1985 to 1 May 1986

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, Nature 321, 288; Gringauz et al., 1986, Nature 321, 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.

The plasma condensation region in the coma of Halley's Comet

The cometary images taken on 1986 January 8.590 and 8.638 UT (R-0.9 AU, δ ~ 1.29 AU) at Gurushikhar, Mt. Abu, India (24 °39′ N, 72 °43′ E alt: 1700 m) show distinct condensation region in the tail direction. The size of the condensation region is 4 × 103 km. The condensation region showed up strongly in the blue emission, implying the abundance of CO+. It was inferred to be moving with a velocity of 37 ± 3 km/s relative to the comet at a distance of 2.3 × 105 km from the nucleus in the tailward direction. The analysis show that the condensation was a result of rapid ionization mechanism, with a time scale of \~103 to 104 sec. The most probable mechanism for producing the ionization region was found to be the discharge of cross tail electric current passing through the neutral sheet in the near nucleus region followed by an outburst observed in IR wavelengths at 8.1 UT. It was accelerated by J × B drift at a rate of ~24 cm/sec2 to the position observed by us. This feature, most probably is the precursor of the first dramatic Disconnection Event (DE) observed in Halley's Comet at Jan.10.375 UT. This supports the conjecture that the tail features originate in the coma with a velocity of ~20–40 km/s.

The Interaction of Cometary Plasma with Interplanetary Medium - A Post-Halley View

The strong interaction of cometary plasma with the interplanetary medium results the disconnection events and other features observed in plasma tail and the coma of comets. The understanding of these features provide direct probe of the heliospheric magnetic field. However the interpretation of these events is not without ambiguity, as revealed in the recent work with comet Halley. In this paper a review of the “Post-Halley era” developments of the cometary plasma physics is presented.

MHD Reconnection Model for Optical Jets, H-H Objects and GGD Objects

Many objects have been observed that are excited by shocks, but nevertheless do not form a shell shape in a variety of astrophysical environments such as in protostellar and circumstellar medium or as seen in optical jets, H-H objects, and GGD objects. The models for these objects, which have been proposed for explaining observed spectrum, can be divided to two types. First one is the cloudlets interaction model (e.g., Schwartz 1978; Norman and Silk 1979; Raga, Böhm, and Solf 1986). Second one is the radiative jet model in which the knotty structure associated in some H-H objects can be suggested as the shock of Mach disks in “underexpanded” jets (e.g., Mundt 1985; Hartigan 1989). However, the wiggles and the irregular knots have been observed in some H-H objects (e.g., knots; HH33, HH40, and wiggles; HH12, HH7-11), which could not be explained well by the two models above. On the other hand, from the observations for maser emissions in star forming regions, the strong magnetic field is inferred(e.g., Fiebig and Gusten 1989). The magnetic field seems to be important for the dynamical motion in star forming regions. Then, a magnetohydrodynamical reconnection model for these phenomena is investigated. We consider the interaction between dense magnetized cloudlet and wind from the central star, through an 2-D MHD code with the MacCormack-Doner cell hybrid scheme to well represent shock phenomena as well as advective phenomena. The density contours and magnetic field lines of numerical results for γ=5/3 and 1.1 are shown in figure (a) and (b), respectively. We present two case for γ=5/3 and 1.1. Results show that the compression effects related to the adiabatic indexes γ are important in the shocked region. The interaction between the cloudlet and wind induces the bow shock around the cloudlet and elongated magnetic structure in the downstream direction, which is similar between two cases. However, the downstream flow patterns behind the cloudlet are different for each other. For γ=1.1, we have a jet-like structure in downstream region. This is induced by magnetic reconnection with strong compressed effects which produces secondary plasma acceleration. This magnetic reconnection is fast (Patscheck) type, which have been well studied for the model of the earth magne-tospheric substorm, related to the slow shock formation and the strong plasma jet generation (e.g. Sato and Hayashi 1979). Furthermore, these behavior is similar to the disconnection events of comets (Alfvén 1957, Niedner 1980), which suggests us the knots in HH objects can be explained as some separated reconnection points in which the magnetic energy is released explosively. For real interstellar medium, γ=5/3, however, we must consider the effect of radiative cooling (Spitzer 1978). This cooling seems to be important for outflows in the star forming regions (Hanami and Sakashita, 1987) Then, we can interpret the compression effect with small γ as that of the cooling, approximately. This compression mechanism related to the cooling effect in interstellar and magnetic reconnections are important for interpreting the formation, and the structure, and the dynamics of optical jets, H-H objects, and GGD objects.

A determination of the aberration angle of the plasma tail of Comet Halley

A disconnection event (DE) occurred in the plasma tail of Comet Halley on 1986 March 8. The aberration angle of the new main tail differed from that of the old one. In this paper, the old and new apparent aberration angles on our large scale photographs are determined using a photometricmethod and a transformation formula is given. With this formula the actual aberration angles are found. Finally, the results obtained are discussed.

Plasma tail evolution in comet P/Halley 1985–1986

Plasma tail phenomena in P/Halley existed from a sporadic turn-on in mid-November 1985 to at least mid-June 1986 and were extensively documented by the Large-Scale Phenomena Network of the International Halley Watch (IHW). Although plasma tail activity clearly turned-off at approximately mid-June 1986, observations currently available do not show the turn-off sequence. Disconnection Events (DEs) were plentiful during this apparition; approximately 30 DEs were observed of which perhaps 20 were considered prominent. The evolution of the plasma tail is described in detail with emphasis on: (1) the turn-on/turn-off of plasma tail activity and (2) the extensive observations of disconnection events. Comparisons to theory are made where appropriate and include predictions of the turn-on/turn-off distances and associations of the disconnection events with the proposed solar-wind causes, i.e., sector-boundary crossing and high-speed solar-wind streams. The status of understanding the evolution of the plasma tail in comet Halley during 1985–1986 is summarized.

Was the eclipse comet of 1893 a disconnected coronal mass ejection?

A re-evaluation of observations of the 16 April, 1893 solar eclipse suggests that the ‘comet’ photographed during totality was, in fact, a disconnected coronal mass ejection. Like the disconnection event in 1980 reported by Illing and Hundhausen, the outward speed of the convex (toward the Sun) surface for the 1893 event was relatively low (∼90 km s−1). Candidate disconnection events were also observed during solar eclipses in 1860 and 1980.

Observations on near nucleus activity in comet Halley during January 1986

A large data base of near nucleus images of Comet Halley in white light, Kodak wratten filters and IHW filters was built up using an image intensifier camera and a 35 cm aperture f/11 telescope at Gurushikhar, Mt.Abu., India (24°39′N, 72°43′E, Alt.1700 m) during 1985–1986. The images taken on January 8.590 and 8.632 UT (R∼0.9 AU, Δ ∼ 1.29 AU) show distinct condensation region in the tail direction. From the relative position of the condensation region in two frames, its velocity was inferred as ∼ 37±3 km/s in the anti-sunward direction. The wide field photographic study shows this feature to be the precursor of the disconnection event observed on 10th Jan 1986. Based on the Near Nucleus photographs obtained by us and wide field photographs from other sources, it is shown that there is a strong correlation of nuclear activity with the tail disturbances observed on 31st Dec 1985, 8th and 10th Jan 1986.

P/Halley: The Disconnection Event of 1986 April 11

This work was carried out by an expedition organized by the S.A.F. in 1986 April to La Réunion. The aim was to observe and photograph P/Halley, within the framework of IHW, as part of the Island Network in the southern hemisphere. To be more precise, our work consisted of studying large-scale phenomena: the structure, dynamics and possible disconnection events in the plasma tail. We were lucky enough to observe one of the latter on the night of April 11/12, and describe it here.For the Island Network, IHW had a number of Schmidt telescopes (Celestron 8). One was lent to the S.A.F. and this is what we used. This telescope has a focal ratio of 1.5, with a 200-mm (8-inch) objective and 300-mm focal length. We used only Kodak TP2415 film, hypersensitized in forming gas (24h at 60°C).

A Study of Disconnection Velocities in the Plasma Tail of P/Halley

During a photographic campaign on Comet P/Halley in 1986 April, a considerable number of photographs were obtained, especially on April 8, of a disconnection event in the plasma tail. Initial details were presented at the Symposium in Heidelberg at the end of 1986.The photographs were obtained with a camera five examples of which were specially constructed (by Carron), and known as the K600. The objective is a 100-mm OG, focal length 600 mm and the image is corrected with a field lens. The camera body is a Hasselblad, taking 120 roll film, which was Agfa 1000.

Disconnection events in comet Halley during the ICE‐Halley Radial Period

The combination of International Cometary Explorer (ICE) data and International Halley Watch (IHW) photographs of Comet Halley have provided strong evidence that an interplanetary magnetic field polarity reversal alone is sufficient to bring about a disconnection event (DE), in which the entire visible plasma (ion) tail separates from and flows downstream of the comet head. The possibility that a compression region in the solar wind can also bring about a DE is not precluded; however, neither of the two DEs in the study reported here could be clearly attributed to a compression region alone.

The nature of comets

The vast scientific campaign associated with the 1986 return of Halley’s Comet has greatly improved and expanded our knowledge of comets. An overview of the first results is presented here with emphasis on the large-scale structure, the chemistry, and the nucleus. Biermann and Alfven’s basic large-scale picture involving the interaction with the solar wind was confirmed. The interaction extends over very large distances and involves the draping of magnetic field lines from the solar wind around the head region. The near-nuclear region is essentially free of magnetic field. The cometary environment is a rich plasma physics laboratory as well as the site of spectacular disconnection events. As Whipple proposed, the chemical composition of the nucleus is largely water, and the breakup of the water molecule produces the large hydrogen-cloud surrounding the comet. Minor constituents with high molecular mass have been observed in the comet. The composition of the dust generally resembles carbonaceous chondrites enriched in the elements H, C, N and O. The interest in the cometary chemistry stems from the belief that cometary material is probably the best remnant of the solar nebula’s original composition. The nucleus is monolithic, as predicted by Whipple’s icy-conglomerate model. Far from spherical, the nucleus is irregular and peanut- or potato-shaped. The surface is very dark, and the emission of gas and dust occurs in jets on the sunward side. Irregular erosion of the surface, which is covered by a dust crust, could lead to many interesting possibilities for outbursts or splitting. Even with our current enhancement of knowledge, comets will continue to excite scientific curiosity. Future research on comets should be very fruitful.

A disturbance of the ion tail of comet Halley and the heliospheric structure as observed by Sakigake

In order to study the interaction between the solar wind measured by Sakigake and ion tail disturbances of comet Halley, more than 500 photographs of the comet taken on the ground during this apparition are surveyed. The focus of the present study is the December 31, 1985, event, when various types of disturbances occurred, including an outstanding disconnection event (DE)‐ like knot. Analysis of the Sakigake/IMF data reveals that comet Halley did not encounter the heliospheric neutral sheet on that day, demanding a new explanation for the DE‐like event, different from the Niedner‐Brandt model. During this event the comet encountered a high‐speed solar wind stream from a coronal hole tongue of the sun. The event can be explained by a dynamic pressure model, according to which the DE‐like plasmoid was caused by a sudden increase in the dynamic pressure of the solar wind. A result of the simulation work by Ogino is found to support this interpretation.

Interaction between comet Halley and the interplanetary magnetic field observed by Sakigake

During 10–12 March 1986, the magnetometer carried by the Japanese spacecraft Sakigake detected clear multiple crossings of the nearly horizontal heliospheric neutral sheet. As there was no apparent disconnection event (DE) in comet Halley's ion tail during 11–14 March, an interplanetary magnetic field (IMF)–comet interaction model with a quasi-parallel neutral sheet is proposed to explain why a DE did not occur. Enhancements of hydromagnetic (HM) waves were observed near the time of closest approach to comet Halley. Neutral particles of cometary origin are proposed to yield, at ∼7 × 106 km upstream of the comet, the ionized O+ (or H2O+) particles which excite the HM waves.

Near‐tail reconnection as the cause of cometary tail disconnections

In a cometary tail disconnection event the plasma tail appears to separate from the coma and to accelerate away from it. As this occurs a new tail begins to form. We propose that these disconnections arise in a manner analogous to geomagnetic substorms, i.e., by the formation of a strongly reconnecting region in the near tail that forms a magnetic island in the coma and ejects the plasma tail by strengthening the magnetic “slingshot” within the tail. This reconnection process may be triggered by several different processes, such as interplanetary shocks or variations in the Alfven Mach number.

Dynamics of cometary plasma tails

As a result of the visibility imparted by fluorescing ions (mostly CO + and H 2O +), 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 Halley's 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 (DE's) 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 DE's /1/ and with the fact that a major DE took place on 8 March 1986, between the VEGA-1 and VEGA-2 encounters.

Optical observations of ion tail structures and velocity flows in comet Halley during the period of the five spacecraft encounters

Observations of the distribution and evolution of a number of the major constituents of the neutral coma (CN, C 2, CH, O, H, Na) of Comet Halley were made during two observing periods, each of 3 weeks duration, from the Table Mountain Observatory, California. The first period was pre-perihelion, in late November/December 1985. The second period, from Feb 28 to March 22 1986, covered the five close spacecraft encounters with Halley, and when ICE flew some 20 M Km upstream of Halley. Sodium emission was recorded in early Dec 1985 from the near-nuclear region at a heliocentric distance of 1.4 AU, an observation confirmed with the UCL Doppler Imaging system. The CN coma could be detected to an outer diameter of more than 4M Km in Dec 1985, and 5 – 6M Km in early March 1986, allowing the production of heavy cometary pick-up ions to be estimated. Observations of the cometary ion coma (H 2O + and CO + ions) showed considerable variability from day to day, particularly during the period of the spacecraft encounters. These observations have been used, in conjuction with the neutral coma data, to map the flow field of cometary ions. In early Dec. 1985, Halley developed a traditional “type I” ion tail, which persisted until late April 1986. It has also been possible to evaluate the ion flow fields within the narrow core of the ion tail, and in the surrounding diffuse, low density, regions populated by pick-up and extracted cometary ions, and by slowed solar wind ions. Tail disconnection events were observed on several occasions, particularly between the VEGA 2 and GIOTTO encounters, and with a highly spectacular event on March 19 1986.