Sunward Direction Research Articles

A Model of the Early Evolution of the 2007 Outburst of Comet 17P/Holmes

We have conducted a morphological analysis of the dust features generated during the first 5 weeks after the 2007 October outburst of comet 17P/Holmes. We used a computer simulation technique to generate synthetic images of dust ejecta released from discrete sources on a rotating nucleus. We found that the main jets are caused by discrete sources located near the south pole of the nucleus, while particle ejection is otherwise produced on the whole sunlit southern hemisphere, causing the observed dust shell in the sunward direction. The rotational parameters we derived (Φ = 213° and I = 90°, or the north pole pointing to R.A. = 38.8°, decl. = +35.1°) indicate that the subsolar latitude was near –90° at the outburst time. Assuming the same dust distribution function everywhere on the comet surface, and the same particle size dependence on the velocity, the ejection velocities for particles forming the jets must be remarkably different, by a factor of ~5 slower, than the background hemispherical particle emission. Based on a comparison of synthetic images with cometary images obtained during the 1892 outburst, we show that the rotational parameters are remarkably consistent during both 1892 and 2007 apparitions.

Asymmetries in the distribution of H 2O and CO 2 in the inner coma of Comet 9P/Tempel 1 as observed by Deep Impact

Prior to the impact event, Deep Impact monitored the ambient inner coma of Comet 9P/Tempel 1 at high spatial resolution in July 2005. Gaseous H 2O and CO 2 are unambiguously detected in the infrared spectra collected with the HRI-IR spectrometer aboard Deep Impact. Detailed distribution maps of these volatiles in the inner coma, within 60 km from the nucleus, are produced from the integrated emission bands of H 2O (2.66 μm) and CO 2 (4.26 μm). Uncorrelated asymmetries are determined in the spatial distribution of both species indicating chemical heterogeneities within the nucleus. Although present at some abundance surrounding the entire nucleus, H 2O has a pronounced enhancement in abundance in the sunward direction rotational phases, evidence that the dominant process of subliming water ice from the nucleus is solar heating. In contrast, CO 2 is enhanced in the regions near the negative rotational pole of the nucleus, suggesting localized outgassing there. Both species show an increase in radiance above the limb of the nucleus toward Ecliptic North. The distribution maps also suggest that the process of dust removal from the nucleus is strongly connected to the outgassing of volatiles. Detailed study of these coma asymmetries gives insight to the relative abundances of the dominant molecular components of the inner coma, source regions of the native volatiles, anisotropic outgassing of the nucleus, and the formation and evolution of the nucleus. A quiescent water production rate for Tempel 1 on July 4, 2005, is estimated to be 4.6 × 10 27 molecules s −1 .

Hubble Space Telescope observations of Comet 9P/Tempel 1 during the Deep Impact encounter

We report on the Hubble Space Telescope program to observe periodic Comet 9P/Tempel 1 in conjunction with NASA's Deep Impact Mission. Our objectives were to study the generation and evolution of the coma resulting from the impact and to obtain wide-band images of the visual outburst generated by the impact. Two observing campaigns utilizing a total of 17 HST orbits were carried out: the first occurred on 2005 June 13–14 and fortuitously recorded the appearance of a new, short-lived fan in the sunward direction on June 14. The principal campaign began two days before impact and was followed by contiguous orbits through impact plus several hours and then snapshots one, seven, and twelve days later. All of the observations were made using the Advanced Camera for Surveys (ACS). For imaging, the ACS High Resolution Channel (HRC) provides a spatial resolution of 36 km (16 km pixel −1) at the comet at the time of impact. Baseline images of the comet, made prior to impact, photometrically resolved the comet's nucleus. The derived diameter, 6.1 km, is in excellent agreement with the 6.0 ± 0.2 km diameter derived from the spacecraft imagers. Following the impact, the HRC images illustrate the temporal and spatial evolution of the ejecta cloud and allow for a determination of its expansion velocity distribution. One day after impact the ejecta cloud had passed out of the field-of-view of the HRC.

Asymmetries in the distribution of H 2O and CO 2 in the inner coma of Comet 9P/Tempel 1 as observed by Deep Impact

Prior to the impact event, Deep Impact monitored the ambient inner coma of Comet 9P/Tempel 1 at high spatial resolution in July 2005. Gaseous H 2O and CO 2 are unambiguously detected in the infrared spectra collected with the HRI-IR spectrometer aboard Deep Impact. Detailed distribution maps of these volatiles in the inner coma, within 60 km from the nucleus, are produced from the integrated emission bands of H 2O (2.66 μm) and CO 2 (4.26 μm). Uncorrelated asymmetries are determined in the spatial distribution of both species indicating chemical heterogeneities within the nucleus. Although present at some abundance surrounding the entire nucleus, H 2O has a pronounced enhancement in abundance in the sunward direction rotational phases, evidence that the dominant process of subliming water ice from the nucleus is solar heating. In contrast, CO 2 is enhanced in the regions near the negative rotational pole of the nucleus, suggesting localized outgassing there. Both species show an increase in radiance above the limb of the nucleus toward Ecliptic North. The distribution maps also suggest that the process of dust removal from the nucleus is strongly connected to the outgassing of volatiles. Detailed study of these coma asymmetries gives insight to the relative abundances of the dominant molecular components of the inner coma, source regions of the native volatiles, anisotropic outgassing of the nucleus, and the formation and evolution of the nucleus. A quiescent water production rate for Tempel 1 on July 4, 2005, is estimated to be 4.6 × 10 27 molecules s −1 .

Hubble Space Telescope observations of Comet 9P/Tempel 1 during the Deep Impact encounter

We report on the Hubble Space Telescope program to observe periodic Comet 9P/Tempel 1 in conjunction with NASA's Deep Impact Mission. Our objectives were to study the generation and evolution of the coma resulting from the impact and to obtain wide-band images of the visual outburst generated by the impact. Two observing campaigns utilizing a total of 17 HST orbits were carried out: the first occurred on 2005 June 13–14 and fortuitously recorded the appearance of a new, short-lived fan in the sunward direction on June 14. The principal campaign began two days before impact and was followed by contiguous orbits through impact plus several hours and then snapshots one, seven, and twelve days later. All of the observations were made using the Advanced Camera for Surveys (ACS). For imaging, the ACS High Resolution Channel (HRC) provides a spatial resolution of 36 km (16 km pixel −1) at the comet at the time of impact. Baseline images of the comet, made prior to impact, photometrically resolved the comet's nucleus. The derived diameter, 6.1 km, is in excellent agreement with the 6.0 ± 0.2 km diameter derived from the spacecraft imagers. Following the impact, the HRC images illustrate the temporal and spatial evolution of the ejecta cloud and allow for a determination of its expansion velocity distribution. One day after impact the ejecta cloud had passed out of the field-of-view of the HRC.

Spatial distribution of the neutral carboneous compounds glow in the sunward Halley comet coma

This work presents a study of the C 2, C 3, CH, and CN glow in the Halley’s coma in sunward direction. For that purpose, 1035 spectra in the near UV and visible region registered by the Three-Channel Spectrometer (TKS) on board Vega-2 station on 9 March 1986 are used. An improved method of the dust continuum extraction in the visible region is applied. The “dust regions” in the Halley’s comet spectra are re-examined. The spectral index u and a normalization coefficient of the continuum are computed for each spectrum by the least squares method on the basis of a linear regression. The index u is obtained to lay in the interval 3–3.4. For all spectra, the average u and d u values are: u av = 3.2126, d u av = 0.1197. Thus, the dust continuum evaluation is reliable, which conduces to a correct separation of the gas emissions. The radial profiles, presenting the carboneous compounds column intensities as a function of the distance p of the line of sight to the nucleus, termed projected distance or p-parameter, are examined. The obtained profiles correspond very well to the Haser’s distribution when the optical thickness in the inner coma environment is taken into account. The deviation from Haser’s model for CN and C 3 in the p < 1000 km region, obtained in previous investigations, is not seen now. Possibly, this deviation had been the result of a dust continuum in this zone, not perfectly subtracted. Some estimates of the production rates Q and scale lengths L and their dependences on the distance to the nucleus R are made. We obtain Q C 2 = 10 28 mol / s for 0.83 AU on 9 March 1986, which presupposes some changes of the used parameters. For the CH radical, best results are obtained when the dependencies L CH ∼ R 2 and Q ∼ R −8 are used. A slight discrepancy between the measurements and the theoretical curve is obtained in the case of C 3 and CH at p > 4000 km. The peculiarities of the C 2, C 3, CH, and CN glow, observed until now, are confirmed. Thanks to the TKS scanning, composed intensity distributions for each carboneous compound are constructed, covering a larger space region. The obtained distributions are alike and similar to the dust one and relatively stable in the measurement period. The intensities decrease with the increase of the distance to the nucleus. The two jets and the jet-free zone as well as the jet structure are examined.

The Deep Space 1 Encounter with Comet 19P/Borrelly

On 22 Sept 2001 at 22h 29m 33s UTC the Deep Space 1 (DS1) spacecraft encountered the comet 19P/Borrelly at a distance of 2171 km. Measurements were undertaken by two dedicated science instruments on the spacecraft and by the spacecraft’s ion propulsion system diagnostic sensors (IDS). The images are the highest resolution ever obtained of a cometary nucleus and show topography and albedo differences on the surface at a resolution of 47m. The disk averaged geometric albedo is less than 0.03, among the lowest of any solar system object observed. The surface reflectance varies by a factor of three; the most absorbing areas having a normal reflectance of less than 0.01. Material is being emitted of from localized regions on the nucleus generally in the sunward direction. These regions are cylindrically shaped about 0.5 km in diameter and about 5 km in length. The cometary bow shock and other features associated with the cometary environs are asymmetrically offset from the sun-Borrelly axis. This suggests that the localized emission regions seen in the images may cause the coma to be asymmetrically offset from the sun-nucleus line. Such an offset has not been reported in previous spacecraft encounters with other comets.

Dust impacts at Comet P/Borrelly

The NASA DS1 spacecraft flew past comet P/Borrelly on September 22, 2001, on the sunward side of the comet. Dust impacts were detected by sharp increases (or decreases) in electric fields measured by the plasma wave dipole antennae. Electric pulses from as small as 0.014 Vm−1 to saturation (∼±0.8 Vm−1) were detected. Pulse overshoots were noted in the largest electric pulses. No simultaneous dc magnetic signatures &gt;1 nT were detected in the magnetometer data. Assuming that cometary dust grains propagating in the sunward direction were decelerated by solar radiation pressure, arguments are made that the size of the earliest dust particles detected were probably ∼0.4 μm in radius. Elaboration of the characteristics of these electric pulses and pulse overshoots and their interpretations will be reserved for a subsequent work.

Dispersion analysis of ULF waves in the foreshock using cluster data and the wave telescope technique

Cluster provides us with a unique possibility to study ULF waves. We analyze ULF wave activity in the near‐Earth upstream solar wind. Using Cluster as a wave telescope we investigate in detail wave propagation directions and wave numbers for various frequencies, obtaining, for the first time, three dimensional dispersion relations experimentally. After Doppler shift correction, we find that the dispersion relations are not linear and the waves are propagating in the sunward direction in the plasma rest frame. Comparison of the experimentally derived dispersion relation with that one for a beam plasma system shows good agreement. We suggest that the ULF waves in the foreshock are generated by a proton population backstreaming from the shock.

Hubble Space TelescopeSTIS Observations of Comet 19P/Borrelly during theDeep Space 1Encounter

In support of the NASA Deep Space 1 (DS1) mission to comet 19P/Borrelly, we obtained Hubble Space Telescope (HST) images and ultraviolet (UV) spectra of the comet near the time of the DS1 flyby in 2001 September. The HST data provide context information on 19P/Borrelly's circumnuclear dust environment, the rotational period and rotational phase of its nucleus, the H2O and CS2 production rates, the dust production rate, the dust reflectivity in the visible and mid-UV, and the time variability of these quantities around the time of the DS1 encounter. We derive average values of Q = (3.0 ± 0.6) × 1028 molecules s-1, [CS2/H2O] = (1.0 ± 0.3) × 10-3, and Qdust ≈ 240 kg s-1. The corresponding dust-to-gas mass ratio is 0.24, but this is only a rough estimate because the dust production rate is uncertain by about an order of magnitude. The dust continuum was strongly reddened between 2400 and 3200 Å, and the Afρ value of 745 ± 15 cm near 6500 Å was ∼2.5 times larger than the value near 2900 Å. The observed coma morphology consisted of a strong jet centered ∼6° from the projected solar vector, one broad fan centered ∼23° from the sunward direction, and another broad fan centered ∼18° from the antisunward direction. The light curve of the optical continuum, as measured in target acquisition images, has an amplitude of ∼40% in a square aperture that subtends 160 km at the comet; the rotational period could not be independently derived from the HST images but is consistent with the value of ∼26 hr derived from HST observations in 1994 and ground-based images in 2000. The new HST data reveal a prominent offset in the emission peak of neutral gas molecules, and therefore in the peak column densities of gas in the coma, relative to the position of the cometary nucleus, which may be related to the offset in ion densities observed in situ by the DS1 Plasma Experiment for Planetary Exploration (PEPE) plasma spectrometer.

Traveling convection vortices induced by solar wind tangential discontinuities

Two typical magnetic impulse events (MIEs) accompanied by traveling convection vortices (TCVs) are investigated. The analysis of their conjugate equivalent convection patterns is performed using magnetic field data obtained from high‐latitude ground magnetometer networks in the Northern and Southern Hemispheres. A three‐dimensional analysis of solar wind structures is also performed using solar wind data obtained from multiple International Solar‐Terrestrial Physics satellites. In the first event observed at ∼1310 UT on 22 May 1996, a westward moving TCV appeared simultaneously in the noon‐to‐dawn sector in the Northern and Southern Hemispheres. The solar wind source of this TCV is found to be a tangential discontinuity (TD), which causes a rapid northward turning of the interplanetary magnetic field (IMF) and abrupt dynamic pressure changes. In the second event observed at ∼1610 UT on 27 May 1998, an eastward moving TCV appeared in the noon sector in the Northern and Southern Hemispheres, with a timing delay of 2 to 3 min in the Southern Hemisphere. The solar wind source of this TCV is found again to be a TD, which causes a rapid IMF By negative turning and an abrupt enhancement of dynamic pressure. Analyses show that the TDs driving these events have their motional electric fields pointing toward the TDs and their normal vectors with large cone angles from the sunward direction. These TDs satisfy the conditions for the formation of a hot flow anomaly (HFA) at the bow shock. The sweeping motion across the magnetosphere of the intersection of the TD and the bow shock is found to be consistent with the observed TCV motion in each event. Magnetopause deformations due to HFAs can explain all the observed morphological features and the triggering process of these two MIEs. It is suggested, however, that bursty merging and/or pressure pulses would reinforce the processes produced by the HFAs, since the TDs are usually accompanied by both abrupt IMF changes and pressure enhancements. Consequently, it seems reasonable to conclude that the integrated processes of HFA, bursty magnetic field merging, and pressure pulses produce the evolution of these MIEs and TCVs.

Strong sunward propagating flow bursts in the night sector during quiet solar wind conditions: SuperDARN and satellite observations

Abstract. High-time resolution data from the two Iceland SuperDARN HF radars show very strong nightside convection activity during a prolonged period of low geomagnetic activity and northward interplanetary magnetic field (IMF). Flows bursts with velocities ranging from 0.8 to 1.7 km/s are observed to propagate in the sunward direction with phase velocities up to 1.5 km/s. These bursts occur over several hours of MLT in the 20:00–01:00 MLT sector, in the evening-side sunward convection. Data from a simultaneous DMSP pass and POLAR UVI images show a very contracted polar cap and extended regions of auroral particle precipitation from the magnetospheric boundaries. A DMSP pass over the Iceland-West field-of-view while one of these sporadic bursts of enhanced flow is observed, indicates that the flow bursts appear within the plasma sheet and at its outward edge, which excludes Kelvin-Helmholtz instabilities at the magnetopause boundary as the generation mechanism. In the nightside region, the precipitation is more spot-like and the convection organizes itself as clockwise U-shaped structures. We interpret these flow bursts as the convective transport following plasma injection events from the tail into the night-side ionosphere. We show that during this period, where the IMF clock angle is around 70°, the dayside magnetosphere is not completely closed.Key words. Ionosphere (Auroral ionosphere; Ionospheremagnetosphere interactions; Particle precipitation)

High‐latitude magnetic reconnection in sub‐Alfvénic flow: Interball Tail observations on May 29, 1996

The Interball Tail spacecraft crossed the high‐latitude magnetopause near the cusp region under northward interplanetary magnetic field (IMF) conditions on May 29, 1996, with magnetic local time and magnetic latitude of ∼73 hours and ∼65.4°, respectively. Under these IMF conditions the Interball Tail spacecraft observed quasi‐steady reconnection in progress and evidence for a relatively stable reconnection site at high latitudes. Sunward plasma flow observed by Interball Tail and a determination of the tangential stress balance indicated that reconnection was occurring poleward of the Earth's magnetic cusp, above the spacecraft's location. At these high latitudes the gasdynamic model of the solar wind/magnetosphere interaction indicates that the magnetosheath flow should be super‐Alfvénic, and therefore that the reconnection site should have propagated tailward. However, the spacecraft observed sub‐Alfvénic flow in the magnetosheath region adjacent to the magnetopause current layer near the reconnection site indicating that the reconnection site may have moved in the sunward direction. These observations suggest that the region of sub‐Alfvénic flow and stable, quasi‐steady reconnection extend to very high latitudes under northward IMF conditions. It is shown that the thickness of the magnetopause current layer for this event (estimated as ∼1600 km) is consistent with that found for reconnection at the dayside magnetopause.

Dual‐spacecraft observations of standing waves in the magnetosheath

An unambiguous determination of the wave modes observed in plasma turbulence requires the determination of the dispersion characteristics of the wave. The method adopted for the determination of the wave dispersion relation is based upon the study of the phase difference of the waves at two closely separated spacecraft [Balikhin and Gedalin, 1993]. It is applied to observations of waves in the magnetosheath magnetic field made by AMPTE UKS and AMPTE IRM. We show that the observed waves are propagating in the sunward direction but are convected toward the magnetopause by the plasma flow. As a result, the observed waves are quasi‐standing in the flow. These waves have been previously identified as either slow or mirror modes depending the procedure adopted for their analysis. They may also be an experimental observation of the mirror and slow (MIAOW) waves identified near the magnetopause boundary in the hybrid simulations of Omidi and Winske [1995]. The observed waves possess short wavelengths (λ) such that RBi/λ ≈ 1 (where RBi is the ion Larmor radius). Thus they may only be studied analytically within the framework of the kinetic approximation. Both the nonlinear amplitudes and possibly the strong underlying inhomogeneity are responsible for the significant differences between the properties of the observed modes and those resulting from linear homogeneous kinetic theory.

Cluster PEACE observations of electrons of spacecraft origin

Abstract. The two PEACE (Plasma Electron And Current Experiment) sensors on board each Cluster spacecraft sample the electron velocity distribution across the full 4&lt;pi&gt; solid angle and the energy range 0.7 eV to 26 keV with a time resolution of 4 s. We present high energy and angular resolution 3D observations of electrons of spacecraft origin in the various environments encountered by the Cluster constellation, including a lunar eclipse interval where the spacecraft potential was reduced but remained positive, and periods of ASPOC (Active Spacecraft POtential Control) operation which reduced the spacecraft potential. We demonstrate how the spacecraft potential may be found from a gradient change in the PEACE low energy spectrum, and show how the observed spacecraft electrons are confined by the spacecraft potential. We identify an intense component of the spacecraft electrons with energies equivalent to the spacecraft potential, the arrival direction of which is seen to change when ASPOC is switched on. Another spacecraft electron component, observed in the sunward direction, is reduced in the eclipse but unaffected by ASPOC, and we believe this component is produced in the analyser by solar UV. We find that PEACE anodes with a look direction along the spacecraft surfaces are more susceptible to spacecraft electron contamination than those which look perpendicular to the surface, which justifies the decision to mount PEACE with its field-of-view radially outward rather than tangentially.Key words. Magnetosheric physics (general or miscellaneous) Space plasma physics (spacecraft sheaths, wakes, charging)

Polar/Toroidal Imaging Mass‐Angle Spectrograph survey of earthward field‐aligned proton flows from the near‐midnight tail

Data from the Toroidal Imaging Mass‐Angle Spectrograph (TIMAS) ion mass spectrometer on the Polar satellite, covering 15 eV e−1 to 33 keV e−1 in energy and essentially 4 π in view angles, are used to investigate the properties of earthward (sunward) field‐aligned flows of ions, especially protons, in the plasma sheet‐lobe transition region near local midnight. A total of 142 crossings of this region are analyzed at 12‐s time resolution, all in the Northern Hemisphere, at R(SM) ∼4–7 RE, and most (106) in the poleward (sunward) direction. Earthward proton flows are prominent in this transition region (&gt;50% of the time), typically appearing as sudden “blasts” with the most energetic protons (∼33 keV) arriving first with weak flux, followed by protons of decreasing energy and increasing flux until either (1) a new “blast” appears, (2) the flux ends at a sharp boundary, or (3) the flux fades away within a few minutes as the mean energy drops to a few keV. Frequent step‐like changes (&lt;12 s) of the flux suggest that perpendicular gradients on the scale of proton gyroradii are common. Peak flux is similar to central plasma sheet proton flux (105–106 (cm2 sr s keV/e)−1) and usually occurs at E ∼ 4–12 keV. Only the initial phase of each “blast” (∼1 min) displays pronounced field alignment of the proton velocity distribution, consistent with the time‐of‐flight separation of an otherwise more or less isotropic distribution (at Polar altitude) with dƒ/dv &lt; 0. The temporal dispersive signatures are often consistent with a source at R(SM) ≤ 30 RE. No systematic latitudinal velocity dispersion is found, implying that the equatorial plasma source is itself convecting. In short, the proton “blasts” appear as sudden local expansions of central plasma sheet particles along reconfigured (“dipolarized”) magnetic field lines.

Horizontal electrojet associated with the sun-aligned arcs

Analysis of two strong sun-aligned arcs over the Canadian Eureka Observatory (89° CGM) near the north magnetic pole and accompanying related ground magnetometer measurements has identified an electrojet current of ∼1×10 4 A flowing within the arcs in a sunward direction. The electrojet current was carried by low energy electrons created by impacting precipitation drifting at E× B/B 2 velocity within the arcs, where E is the dawn-to-dusk electric field. One of the arcs moved rapidly in a dawn to dusk direction. The measured arc velocity was 365 m/s. This agrees well with the velocity of 380 m/s inferred from the magnetic field signature of the electrojet current. This study suggests that such an electrojet is present whenever a polar arc is set up. However, a few conditions are required to observe clear ground magnetic signatures of the electrojet: (1) quiet local magnetic conditions; (2) a single sun aligned arc near or moving across zenith; and (3) arc excitation by ≥1 keV electrons.

On the generation mechanism of Pi 2 pulsations in the magnetosphere

We present a theoretical study on how Pi 2 pulsations are excited in the magnetosphere. When impulsive disturbances associated with the substorm onset are assumed at the tailward region, their propagation toward the sunward direction is investigated with a wave equation. In order to examine the effect of the plasmapause on the initial disturbances, we analytically solve the wave equation based on the model of a reasonable Alfven speed profile. The exact solution shows that virtual resonant states exist in the plasmasphere and plasmapause. We obtain the result that these unique modes strongly persist for arbitrary incoming impulses from the source in the magnetotail, which corresponds to the signature of Pi 2 pulsations.

Activity and the Rotation Period of Comet Hyakutake (1996 B2)

Narrowband photometry of Comet Hyakutake (1996 B2) revealed periodic variability in the production of dust and gas during the comet's close approach to Earth in March 1996. The photometry alone was insufficient to unambiguously discriminate among several possible periods, but a unique period determination was possible by utilizing repeating morphological features in CCD images obtained simultaneously with the photometry on the nights of March 23–25. In particular, a large puff or blob of material was seen to be released in the sunward direction every 6.23 ± 0.03 h. This periodic release of material, presumably from a single dominant active region on the surface, was the source of the observed lightcurve variations. A second, much smaller morphological feature also repeats each cycle, confirming the value of 6.23 h for Hyakutake's synodic rotational period.

Characteristic boundary transitions in energetic particle data (60– ≥ 260 keV) recorded at comets P/Grigg-Skjellerup and P/Halley by the EPONA instrument on Giotto

An overview of published magnetic field, low energy electron and low to intermediate energy ion data recorded on board Giotto during an encounter with P/Grigg-Skjellerup (G-S) on 10 July, 1992 is presented and these measurements compared with particle data secured by the onboard energetic particles experiment EPONA ( E = 60 to ≥260 keV). It is shown that, in general, the same cometary boundaries (Bow Wave, Bow Shock, Mystery Boundary Transitions (MBTs)) were sensed in energetic particles as by the other experiments. Elevated fluxes of accelerated particles, interpreted to be ions of species M = 16–18, were identified in the G-S cometosheath. It is inferred that heavy cometary ions of species M = 28–33 were present in the innermost region of the comet (as viewed in the sunward direction). Energetic particles recorded by EPONA on Giotto at P/Halley are considered relative to other particles and fields measurements obtained onboard. Evidence based on pitch angle data indicates that, inbound, the MBTs at G-S and at Halley, were spatially associated with changes from trapped to “flowing” distributions of water group ions, suggesting that mirroring of these particles was taking place during encounter in the magnetic field piled up around each comet. Downstream of the MBTs, outbound, flowing distributions were, in each case, identified. Evidence of MBTs in energetic particle data are, herein, reported for the first time. The roles of various mechanisms in accelerating cometary ions in association with Bow Wave/Bow Shock boundaries at G-S and at Halley are inferred from comparisons between experimental and theoretical spectra pertinent to each comet. Similarities and differences between the signatures of characteristic boundaries recorded in each case are discussed in the light of basic differences between the comets themselves, and between the ambient interplanetary circumstances pertaining during each cometary encounter.