Local Magnetic Field Strength Research Articles

The VLF Transmitter, Narrowband Receiver, and Tuner Investigation on the DSX Spacecraft

AbstractArtificially enhancing the pitch angle diffusion of the relativistic electrons trapped in the radiation belts with transmission of very‐low frequency (VLF) electromagnetic waves is the primary science objective of the wave‐particle interaction experiments (WPIx) on the Demonstration Science Experiments (DSX) spacecraft. The pitch angle diffusion occurs due to the interaction of stimulated whistler‐mode waves with these extremely high energy electrons. The primary payload of the DSX spacecraft is a high‐power VLF transmitter paired with a matched tuner, a two‐channel narrowband receiver, and an 81.6 m tip‐to‐tip carbon fiber longeron dipole antenna, collectively known as Transmitter, Narrowband receiver, and Tuner (TNT) instrument. TNT can execute a wide variety of programmable WPIx experiments and provide supporting in situ and remote plasma characterizations. In its main high‐power mode, the whistler stimulation on selected frequencies between 2.5 and 34 kHz is done by applying a high‐voltage signal, up to nominally 5 kV, at each transmit antenna monopole feed. Maximum power is radiated when the antenna‐plasma circuit reactance is tuned out, leading to resonance conditions at the selected frequency. In the radiation belts, the unknown and varying contributions of the ambient plasma to the total reactance have to be determined automatically and tuned out adaptively, steering the overall VLF circuit within the resonance state as DSX orbits the Earth. Auxiliary to the main WPIx mode, the low‐power relaxation sounding, remote plasma sensing, and listen‐only VLF‐HF emission spectrographic operations are used to probe the surrounding and remote plasma density and the local magnetic field strength.

Turbulent Energy Transfer at Dipolarization Fronts

AbstractDipolarization fronts (DFs), ion‐scale magnetic transients characterized by dramatic enhancement of northward magnetic field, have been documented as crucial energy transfer regions in the magnetosphere. DF‐driven energy transfer has hitherto been studied mainly in the laminar regime. Energy transfer driven by turbulent processes, however, remains unclear. Here we perform a comprehensive investigation of turbulent energy transfer (TET) developed at DFs, via using high‐cadence data from Magnetospheric Multiscale mission. We find that: (a) TET is equally governed by energy loads and generators, different from laminar energy transfer which is typically dominated by energy loads; (b) ion and electron currents play comparable roles in driving TET; (c) TET is positively correlated with local magnetic field strength and ion speed; (d) TET shows asymmetric global distributions along the dawn‐dusk direction. These features implicate that TET is primarily related to electromagnetic turbulence at electron‐ion hybrid scales. These new results, uncovering unique characteristics of DF‐driven TET, can deeply advance our understanding of energy budgets in the magnetosphere.

Effects of strong fringing magnetic fields on turbulent thermal convection

We study the influence of fringing magnetic fields on turbulent thermal convection in a horizontally extended rectangular domain. The magnetic field is created in the gap between two semi-infinite planar magnetic poles, with the convection layer located near the edge of the gap. We employ direct numerical simulations in this set-up for fixed Rayleigh and small Prandtl numbers, but vary the fringe width by controlling the gap between the magnetic poles and the convection cell. The magnetic field generated by the magnets is strong enough to cease the flow in the high magnetic flux region of the convection cell. We observe that as the local vertical magnetic field strength increases, the large-scale structures become thinner and align themselves perpendicular to the longitudinal sidewalls. We determine the local Nusselt and Reynolds numbers as functions of the local Hartmann number (based on the vertical component of the magnetic field), and estimate the global heat and momentum transport. We show that the global heat transport decreases with increasing fringe width for strong magnetic fields but increases with increasing fringe width for weak magnetic fields. In the regions of large vertical magnetic fields, the convective motion becomes confined to the vicinity of the sidewalls. The amplitudes of these wall modes show a non-monotonic dependence on the fringe width.

Massive Star Formation in the Tarantula Nebula

In this work, we present 299 candidate young stellar objects (YSOs) in 30 Doradus discovered using Spitzer and Herschel point-source catalogs, 276 of which are new. We study the parental giant molecular clouds in which these YSO candidates form using recently published Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 7 observations of 12CO and 13CO. The threshold for star formation in 30 Doradus inferred by the LTE-based mass surface density is 178 M ⊙ pc−2, 40% higher than the threshold for star formation in the Milky Way. This increase in star formation threshold in comparison to the Milky Way and increase in line width seen in clumps 11 pc away in comparison to clumps 45 pc away from the R136 super star cluster could be due to injected turbulent energy, increase in interstellar medium pressure, and/or local magnetic field strength. Of the 299 YSO candidates in this work, 62% are not associated with 12CO molecular gas. This large fraction can be explained by the fact that 75%–97% of the H2 gas is not traced by CO. We fit a Kroupa initial mass function to the YSO candidates and find that the total integrated stellar mass is 18,000 M ⊙ and that the region has a star formation rate (SFR) of 0.18 M ⊙ yr−1. The initial mass function determined here applies to the four 150″ × 150″ (37.5 pc × 37.5 pc) subfields and one 150″ × 75″ (37.5 pc × 18.8 pc) subfield observed with ALMA. The SFR in 30 Doradus has increased in the past few million years.

Radio detection of chemically peculiar stars with LOFAR

Context. Chemically peculiar stars are upper main sequence stars that show anomalies in their optical spectra. These anomalies suggest peculiar chemical abundances of certain elements. Some chemically peculiar stars possess strong magnetic fields. Electrons originating from the ionising stellar wind travel in the magnetosphere of the star and become the source of non-thermal radio and X-ray emission. Several chemically peculiar radio stars have been detected at GHz frequencies. Aims. We used the Low-Frequency Array (LOFAR) to search for radio emission from chemically peculiar stars to constrain their emission in the frequency band 120–168 MHz. We aimed to use LOFAR observations to test the models for radio emission of chemically peculiar stars. Methods. We performed a targeted search of known chemically peculiar stars in the fields of the LOFAR Two Metre Survey (LoTSS) Data Release 2 in Stokes I and V. We matched positions of radio sources in the LoTSS-DR2 catalogue with positions of chemically peculiar stars. Results. We report non-thermal emission at 120–168 MHz from two chemically peculiar stars in Stokes I, BP Boo, and α2 CVn. The ensuing incidence rate at these frequencies is significantly lower than for higher frequencies. This results from the turnover at low frequencies which was predicted from the theory of radio emission from chemically peculiar stars. BP Boo is detected for the first time at radio wavelengths, while α2 CVn had already been detected at higher frequencies. The upper limit of V/I indicates a level of circular polarisation significantly below 60%. We combined data obtained at different frequencies to derive the radio spectrum of α2 CVn. The spectrum is nearly flat beyond turnover at low frequencies. We modelled radio emission for a large magnetosphere and small local magnetic field strength. The amplitude of variation in radio emission with the rotational phase of the system decreases at low frequencies.

Repumping atomic media for an enhanced sensitivity atomic magnetometer.

Atomic vapour magnetometers sense the local magnetic field strength by measuring the resulting precession rate of a well-defined quantum state. An essential prerequisite for this approach is a requirement to drive the media into this quantum state, which is frequently achieved via optical pumping. In real-world alkali-metal atoms, with their multiplicity of ground states, the optical pumping process is necessarily lossy, with a large fraction of the atoms being lost to quantum states that do not contribute to the useful magnetically sensitive signal. This consequently reduces the sensitivity of all optically-pumped atomic sensors. Here we theoretically and experimentally study the population changes of the quantum ground states of 87Rb during optical pumping. We use this understanding to develop a repumping scheme that allows us to increase the number of atoms that are contributing to the useful magnetic sensing output. Unlike prior schemes, our approach delivers this improved sensitivity with significantly suppressed fictitious magnetic fields associated with the repumping, which would otherwise reduce the accuracy of the sensor. When operated at Earth's field strength (∼50µT), the repumped sensor shows a magnetic sensitivity of 200 fT/Hz, that is nearly three times higher than the non-repumped version.

Lifetime of the Outer Solar System Nebula From Carbonaceous Chondrites

AbstractThe evolution and lifetime of protoplanetary disks (PPDs) play a central role in the formation and architecture of planetary systems. Astronomical observations suggest that PPDs evolve in two timescales, accreting onto the star for up to several million years (Myr) followed by gas dissipation within ≲1 Myr. Because solar nebula magnetic fields are sustained by the gas of the protoplanetary disk, we can use paleomagnetic measurements to infer the lifetime of the solar nebula. Here, we use paleomagnetic measurements of meteorites to constrain this lifetime and investigate whether the solar nebula had a two‐timescale evolution. We report on paleomagnetic measurements of bulk subsamples of two CO carbonaceous chondrites: Allan Hills A77307 and Dominion Range 08006. If magnetite in these meteorites can acquire a crystallization remanent magnetization that recorded the ambient field during aqueous alteration, our measurements suggest that the local magnetic field strength at the CO parent body location was <0.9 μT at some time between 2.7 and 5.1 Myr after the formation of calcium‐aluminum‐rich inclusions. Coupled with previous paleomagnetic studies, we conclude that the dissipation of the solar nebula in the 3–7 AU region occurred <1.5 Myr after the dissipation of the nebula in the 1–3 AU region, suggesting that protoplanetary disks go through a two‐timescale evolution in their lifetime, consistent with dissipation by photoevaporation and/or magnetohydrodynamic winds. We also discuss future directions necessary to obtain robust records of solar nebula fields using bulk chondrites, including obtaining ages from meteorites and experimental work to determine how magnetite acquires magnetization during chondrite parent body alteration.

Distribution of O<sup>+</sup> and ${\text{O}}_{\text{2}}^{\text{+}}$ fluxes and their escape rates in the near-Mars magnetotail: A survey of MAVEN observations

Tailward ion outflows in the Martian-induced magnetotail are known to be one of the major channels for Martian atmospheric escape. On the basis of nearly 6.5 years of observations from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, we investigate the statistical distribution of tailward and Marsward fluxes of heavy ions (i.e., O⁺ and <inline-formula><tex-math id="Z-20220914082156">\begin{document}${\rm{O}}_2^+ $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="RA335-duhengle-F_Z-20220914082156.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="RA335-duhengle-F_Z-20220914082156.png"/></alternatives></inline-formula>) in the near-Mars magnetotail and explore their characteristic responses to the corotating interaction region (CIR), solar wind dynamic pressure, and local magnetic field intensity. Our results show that the tailward fluxes of oxygen ions and molecular oxygen ions in the magnetotail are significantly greater than their Marsward fluxes and that the tailward flux of molecular oxygen ions is generally larger than that of oxygen ions. Furthermore, the tailward ion flux distribution exhibits dependence on the CIR, solar wind dynamic pressure, and local magnetic field strength in a manner stronger than the Marsward ion flux distribution. According to the distribution of tailward ion fluxes, we calculate the corresponding escape rates of heavy ions and show that when the CIR occurs, the total escape rates of oxygen ions and molecular oxygen ions increase by a factor of ~2 and ~1.2, respectively. We also find that the escape rates of heavy ions increase with the enhancement of solar wind dynamic pressure, whereas the overall effect of the local magnetic field is relatively weak. Our study has important implications for improved understanding of the underlying mechanisms responsible for the Martian atmospheric escape and the evolution of the Martian atmospheric climate.

Transient buildup and dissipation of a compressed plasma shockwave in arc-discharge plasma beams

Electric propulsion offers the advantage of a high specific impulse through a large exhaust velocity and has seen significant progress in space flight applications. Recently, we observed a transient plasma shockwave during pulsed plasma thruster operation when the plasma beam impacted a probe surface. However, details regarding the plasma shockwave formation are still unknown. This work is an experimental investigation of the compression-induced plasma shockwave in the presence of a planar obstruction. To study the complete shockwave buildup and dissipation process, an ultra-high-speed imaging system was set up to visualize the time-resolved shockwave morphology at a sub-microsecond level. In addition, the local magnetic field and plasma density were measured using 2D magnetic coils and a triple Langmuir probe, respectively. The successive images of the shockwave give us a comprehensive understanding of the shockwave buildup process. During the 12 μs operational period of the thruster, two shockwaves were formed during the first cycle of the discharge. It is also interesting to note that there is a 1 μs dissipation period between the two shockwaves with the same cloud of plasma compressing against the probe surface. A shockwave model is also developed to predict the appearance of the two shockwaves. The implication is that the local magnetic field strength can be a key indicator for the plasma shockwave buildup and dissipation process.

Probing spin fluctuations in NaOsO3 by muon spin rotation and NMR spectroscopy

We have used muon spin rotation and relaxation (μSR) and 23Na nuclear magnetic resonance (NMR) spectroscopic methods in the NaOsO3 antiferromagnetic phase to determine the temperature evolution of the magnetic order parameter and the role of the magnetic fluctuations at the Néel temperature. Additionally, we performed muon spin relaxation measurements in the vicinity of T A = 30 K, where the appearance of an anomaly in the electrical resistivity was suggested to be due to a progressive reduction of the Os magnetic moment associated with spin fluctuation. Our measurements suggest the absence of prominent change in the spin fluctuations frequency at T A, within the muon probing time scale and the absence of a reduction of the localized Os magnetic moment reflected by the stability within few permille of the local magnetic field strength sensed by the muons below 50 K.

Rapid modification of neutron star surface magnetic field: a proposed mechanism for explaining radio emission state changes in pulsars

ABSTRACT The radio emission in many pulsars shows sudden changes, usually within a period, that cannot be related to the steady state processes within the inner acceleration region (IAR) above the polar cap. These changes are often quasi-periodic in nature, where regular transitions between two or more stable emission states are seen. The durations of these states show a wide variety ranging from several seconds to hours at a time. There are strong, small-scale magnetic field structures and huge temperature gradients present at the polar cap surface. We have considered several processes that can cause temporal modifications of the local magnetic field structure and strength at the surface of the polar cap. Using different magnetic field strengths and scales, and also assuming realistic scales of the temperature gradients, the evolutionary time-scales of different phenomena affecting the surface magnetic field were estimated. We find that the Hall drift results in faster changes in comparison to both Ohmic decay and thermoelectric effects. A mechanism based on the partially screened gap (PSG) model of the IAR has been proposed, where the Hall and thermoelectric oscillations perturb the polar cap magnetic field to alter the sparking process in the PSG. This is likely to affect the observed radio emission resulting in the observed state changes.

PROPAGATION OF LARGE-SCALE SOLAR WIND EVENTS IN THE OUTER HELIOSPHERE FROM A NUMERICAL MHD SIMULATION

Voyager 1 occasionally detected sudden jumps of the local interstellar magnetic field strength since its heliopause crossing in August 2012. These events were believed to be associated with outward propagating solar wind shocks originating in the inner heliosphere. Here we investigate the correlation between interstellar shocks and large-scale solar wind events by means of numerical MHD simulation. The solar wind is simplified as a symmetric flow near the equatorial plane, and the interstellar neutrals are treated as a constant flow with a fixed density distribution along the upwind direction of the local interstellar medium. The charge exchanges between the solar wind plasma and the interstellar neutrals are taken into account. At a heliocentric distance of 1 AU, the solar wind data from OMNI, STEREO A and B during the period between 2010 and 2017 are used as the inner boundary conditions to drive the simulation. The simulation results showed that the solar wind gradually merges into large-scale structures as the radial distance increases, consistent with observations by New Horizons. After propagating into the inner heliosheath, the shocks are fully developed and the corresponding pressure pulses roughly agree with the observations by Voyager 2 in the inner heliosheath. The arrival of the shocks beyond the heliopause is estimated and found to be consistent with the observed signatures of interstellar shocks by Voyager 1. The possible origins of interstellar shocks in the inner heliosheath are discussed based on the simulation.

Observation of High‐Energy Electrons Precipitated by NWC Transmitter From PROBA‐V Low‐Earth Orbit Satellite

AbstractThe very low‐frequency transmitter in the Northwest Cape of Australia (NWC) has previously been observed to pitch‐angle scatter electrons with energies from 30–400 keV, creating enhanced fluxes measured by low‐Earth orbiting (LEO) satellites. Here we use observations from the Energetic Particle Telescope on PROBA‐V. We compare the measured flux, as a function of local magnetic field strength, when the NWC transmitter is “on” versus “off,” and find enhanced fluxes only when NWC is “on” and located on the nightside. The enhanced fluxes occur in the population gradually transitioning from “permanently trapped” to “quasi‐trapped.” We show that electrons up to 800 keV, substantially higher energy than previously studied, are scattered by resonant interactions with NWC to produce enhanced fluxes. The enhanced fluxes appear at multiple L‐shells for each energy channel, consistent with resonance conditions at distinct wave normal angles, that indicate ducted interactions at L &lt; 1.55 and unducted interactions at L &gt; 1.65.

Exploiting Zeeman effect symmetries to measure particle velocities in magnetized plasmas

The task of obtaining flow velocities in a magnetized plasma using laser induced fluorescence (LIF) is often complicated by the need to deconvolve the velocity distribution function (VDF) from effects due to magnetic fields, for example, the Zeeman effect. Hence, precise knowledge of the local magnetic field strength and the resulting line structure is often required. This work demonstrates that in fact the Zeeman effect has no impact on the first moment of the VDF, so long as the integration covers all and components. In this way, can be obtained directly by calculating the first moment of the LIF signal, without deconvolution nor independent knowledge of the magnetic field strength. This is shown to be equivalent to finding the midpoint between the and groups of the LIF signal in the case of symmetric velocity distribution functions, such as a Maxwellian distribution. This fact was experimentally verified by employing LIF to pump the transition in singly ionized argon in a linear helicon plasma. By polarizing the incident laser light to excite the and transitions independently and measuring the fluorescence response, the Doppler shift is measured by taking the midpoint between the and groupings. Ion velocities measured using this new method are compared with those obtained by fitting a convolved profile to the LIF response and agreement between both methods is within 10%, but with the new method having an uncertainty that is a factor of two less and requiring less computation. This comparison also demonstrates that the Zeeman splitting of the degenerate energy states broadens the LIF response and can yield an overestimate of the ion temperature, Ti, if the LIF signal is fit with a pure Gaussian curve. We show the overestimate is on the order of 10%.

Emission in the ion cyclotron range of frequencies (ICE) on NSTX and NSTX-U

We report here on observations of magnetic fluctuations in the ion-cyclotron frequency range on NSTX and NSTX-U. In many respects, the fluctuations appear similar to the ion cyclotron emission (ICE) seen in conventional tokamaks. However, a significant difference between previous observations of ICE and the ICE on NSTX is that the frequency of ICE in conventional tokamaks is typically near the ion cyclotron frequency of the energetic fast ions at the plasma edge. In NSTX and NSTX-U, the magnetic fluctuation frequency corresponds to the ion cyclotron frequency deeper in the plasma, near the location of an internal transport barrier. As on conventional tokamaks, higher harmonics of the deuterium cyclotron frequency, as high as the seventh, are seen with the strongest signal sometimes from higher harmonics. The emission usually appears as an irregular sequence of short bursts typically ≤100 μs in duration although nearly continuous emission for several ms has also been seen under some conditions. Measurements of the emission with a toroidal array of fast probes show that the emission is a long wavelength, spatially coherent mode. The emission frequency does not follow an Alfvénic scaling with density, as seen for compressional Alfvén eigenmodes, but does show a linear scaling with local magnetic field strength. The measured emission shows a compressional polarization consistent with a compressional Alfvén wave. No correlation between the neutron rate and the ICE amplitude is seen. Three-wave coupling between the instability responsible for the ICE and lower frequency modes has also been observed.

Magnetic field and ISM in the local Galactic disc

Correlation analysis is obtained among Faraday rotation measure, HI column density, thermal and synchrotron radio brightness using archival all-sky maps of the Galaxy. A method is presented to calculate the magnetic strength and its line-of-sight (LOS) component, volume gas densities, effective LOS depth, effective scale height of the disk) from these data in a hybrid way. Applying the method to archival data, all-sky maps of the local magnetic field strength and its parallel component are obtained, which reveal details of local field orientation.

Correlations between acute atrial fibrillation and local earth magnetic field strength

Objectives: Atrial fibrillation is the most frequent cardiac arrhythmia affecting over 3 percent and appears to be increasing in general population. In addition to widely discussed such risk factor as obesity, arterial hypertension, electrolytes disbalances and dysfunction of thyroid, there is more and more evidence of human heath interactions with environment parameters such as humidity, temperature, the lunar and the solar activity. Atrial fibrillation, a disorder of heart conductive system, in several studies have been indicated as affected by local Earth magnetic field changes. The study was aimed to analyse possible correlations between the power in the local Earth time-varying magnetic field and admission due to atrial fibrillation. Methods: Two-hundred-fifty-one patients diagnosed with acute atrial fibrillation and treated in Department of Cardiology of Hospital of Lithuanian University of Health Sciences Kaunas Clinics during year of 2016 were retrospectively included into the study. Weekly prevalence of acute atrial fibrillation was compared with weekly summarised changes in the local Earth magnetic field strength. One-year was divided into two time periods according to week number: the first period included weeks from 1 to 26 and the second period included weeks from 27 to 52. Results: Analyses have shown from weak to moderate significant correlations. Tendencies towards higher power magnetic field in low frequently ranges to be associated with higher admission rates were noticeable throughout all analysed periods. Atrial fibrillation concomitant with arterial hypertension was indicated as combination increasing severity of correlation coefficient. Conclusion: Significant correlations between acute atrial fibrillation and the local Earth time varying magnetic field changes were found. Increased magnetic field in low frequency ranges are associated with episodes of acute atrial fibrillation. Arterial hypertension is significantly associated with higher admission due to atrial fibrillation rate under low frequency local Earth magnetic field range.

The Initial Magnetic Susceptibility of Dense Aggregated Dipolar Fluids

Abstract To correlate the dipole moment and density dependence of the initial magnetic susceptibility on the basis of the former related theories and the probability analysis of chain formation, physically based analytical correlation equation was derived. After the local magnetic field strength and the chaining probability between two particle have been determined the chain and particle distributions came from the geometric distribution. The initial magnetic susceptibility was resulted from the summation of Langevin initial susceptibility of k-length chains. Two particles were considered in a chain if the interaction energy between them was below a certain limit. By varying slightly this energy limit around 70–75 % good agreement has been obtained between the simulation and theoretical data. Monte Carlo simulations were used to calculate the initial magnetic susceptibility of dipolar hard sphere system at different dipole moments and densities.

Optically Pumped Magnetometers for Magneto-Myography to Study the Innervation of the Hand.

The central nervous system exerts control over the activation of muscles via a dense network of nerve fibers targeting each individual muscle. There are numerous clinical situations where a detailed assessment of the nerve-innervation pattern is required for diagnosis and treatment. Especially, deep muscles are hard to examine and are as yet only accessible by uncomfortable and painful needle EMG techniques. Just recently, a new and flexible method and device became available to measure the small magnetic fields generated by the contraction of the muscles: optically pumped magnetometers (OPMs). OPMs are small devices that measure the zero-field level crossing resonance of spin-polarized rubidium atoms. The resonance is dependent on the local magnetic field strength, and therefore, these devices are able to measure small magnetic fields in the range of a few hundred femtoteslas. In this paper, we demonstrate as a proof of principle that OPMs can be used to measure the low magnetic fields generated by small hand muscles after electric stimulation of the ulnar or median nerve. We show that using this technique, we are able to record differential innervation pattern of small palmar hand muscles and are capable of distinguishing between areas innervated by the median or ulnar nerve. We expect that the new approach will have an important impact on the diagnosis of nerve entrapment syndromes, spinal cord lesions, and neuromuscular diseases.

Size of a plasma cloud matters

Context. The cometary ionosphere is immersed in fast flowing solar wind. A polarisation electric field may arise for comets much smaller than the gyroradius of pickup ions because ions and electrons respond differently to the solar wind electric field.Aims. A situation similar to that found at a low activity comet has been modelled for barium releases in the Earth’s ionosphere. We aim to use such a model and apply it to the case of comet 67P Churyumov-Gerasimenko, the target of the Rosetta mission. We aim to explain the significant tailward acceleration of cometary ions through the modelled electric field.Methods. We obtained analytical solutions for the polarisation electric field of the comet ionosphere using a simplified geometry. This geometry is applicable to the comet in the inner part of the coma as the plasma density integrated along the magnetic field line remains rather constant. We studied the range of parameters for which a significant tailward electric field is obtained and compare this with the parameter range observed.Results. Observations of the local plasma density and magnetic field strength show that the parameter range of the observations agree very well with a significant polarisation electric field shielding the inner part of the coma from the solar wind electric field.Conclusions. The same process gives rise to a tailward directed electric field with a strength of the order of 10% of the solar wind electric field. Using a simple cloud model we have shown that the polarisation electric field, which arises because of the small size of the comet ionosphere as compared to the pick up ion gyroradius, can explain the observed significant tailward acceleration of cometary ions and is consistent with the observed lack of influence of the solar wind electric field in the inner coma.