Thermal Ion Measurements Research Articles

Saturn suprathermal O2+ and mass‐28+ molecular ions: Long‐term seasonal and solar variation

Suprathermal singly charged molecular ions, O2+ (at ~32 Da/e) and the Mass‐28 ion group 28M+ (ions at ~28 Da/e, with possible contributions from C2H5+, HCNH+, N2+, and/or CO+), are present throughout Saturn's ~4–20 Rs (1 Saturn radius, Rs = 60,268 km) near‐equatorial magnetosphere from mid‐2004 until mid‐2012. These ~83–167 keV/e heavy ions measured by Cassini's CHarge‐Energy‐Mass Spectrometer have long‐term temporal profiles that differ from each other and differ relative to the dominant water group ions, W+ (O+, OH+, H2O+, and H3O+). O2+/W+, initially ~0.05, declined steadily until equinox in mid‐2009 by a factor of ~6, and 28M+/W+, initially ~0.007, declined similarly until early‐2007 by a factor of ~2. The O2+/W+ decline is consistent with Cassini's in situ ring‐ionosphere thermal ion measurements, and with proposed and modeled seasonal photolysis of Saturn's rings for thermal O2 and O2+. The water ice‐dominated main rings and Enceladus plume depositions thereon are the two most likely O2+ sources. Enceladus' dynamic plumes, though, have no known long‐term dependence. After declining, O2+/W+ and 28M+/W+ levels remained low until late‐2011 when O2+/W+ increased, but 28M+/W+ did not. The O2+/W+ increase was steady and became statistically significant by mid‐2012, indicating a clear increase after a decline, that is, a possibly delayed O2+ “seasonal” recovery. Ring insolation is driven by solar UV flux which itself varies with the sun's 11 year activity cycle. The O2+/W+ and 28M+/W+ declines are consistent with seasonal ring insolation. No O2+/W+ response to the late‐2008 solar‐cycle UV minimum and recovery is evident. However, the O2+/W+ recovery from the postequinox baseline levels in late‐2011 coincided with a strong solar UV enhancement. We suggest a scenario/framework in which the O2+ observations can be understood.

MESSENGER observations of the plasma environment near Mercury

The MESSENGER Fast Imaging Plasma Spectrometer (FIPS) measured the bulk plasma characteristics of Mercury's magnetosphere and solar wind environment during the spacecraft's first two flybys of the planet on 14 January 2008 (M1) and 6 October 2008 (M2), producing the first measurements of thermal ions in Mercury's magnetosphere. In this work, we identify major features of the Mercury magnetosphere in the FIPS proton data and describe the data analysis process used for recovery of proton density ( n p) and temperature ( T p) with a forward modeling technique, required because of limitations in measurement geometry. We focus on three regions where the magnetospheric flow speed is likely to be low and meets our criteria for the recovery process: the M1 plasma sheet and the M1 and M2 dayside and nightside boundary-layer regions. Interplanetary magnetic field (IMF) conditions were substantially different between the two flybys, with intense reconnection signatures observed by the Magnetometer during M2 versus a relatively quiet magnetosphere during M1. The recovered ion density and temperature values for the M1 quiet-time plasma sheet yielded n p∼1–10 cm −3, T p∼2×10 6 K, and plasma β∼2. The nightside boundary-layer proton densities during M1 and M2 were similar, at n p∼4–5 cm −3, but the temperature during M1 ( T p∼4–8×10 6 K) was 50% less than during M2 ( T p∼8×10 6 K), presumably due to reconnection in the tail. The dayside boundary layer observed during M1 had a density of ∼16 cm −3 and temperature of 2×10 6 K, whereas during M2 this region was less dense and hotter ( n p∼8 cm −3 and T p∼10×10 6 K), again, most likely due to magnetopause reconnection. Overall, the southward interplanetary magnetic field during M2 clearly produced higher T p in the dayside and nightside magnetosphere, as well as higher plasma β in the nightside boundary, ∼20 during M2 compared with ∼2 during M1. The proton plasma pressure accounts for only a fraction (24% for M1 and 64% for M2) of the drop in magnetic pressure upon entry into the dayside boundary layer. This result suggests that heavy ions of planetary origin, not considered in this analysis, may provide the “missing” pressure. If these planetary ions were hot due to “pickup” in the magnetosheath, the required density for pressure balance would be an ion density of ∼1 cm −3 for an ion temperature of ∼10 8 K.

Lightning‐induced lower‐hybrid turbulence and trapped Extremely Low Frequency (ELF) electromagnetic waves observed in deep equatorial plasma density depletions during intense magnetic storms

During the early phase of the intense magnetic storm of 7–11 November 2004, the DEMETER satellite encountered large‐scale equatorial plasma density depletions with density decreases of two or three orders of magnitude. Wave measurements carried out inside these depletions show the occurrence of broadband and localized lower‐hybrid turbulence triggered by whistlers propagating from thunderstorm lightning occurring below the orbit path. High‐sample‐rate waveforms reveal that this lower‐hybrid turbulence can evolve into localized large‐amplitude quasi‐monochromatic wave packets similar to lower‐hybrid structures that were, up to now, only observed in the auroral regions, usually on high‐latitude magnetic field lines associated with discrete aurora. These equatorial structures have typical amplitudes of up to 20 mV/m and durations of ∼20–30 ms. Simultaneous thermal ion measurements show that these bursts are often correlated with small‐scale density depletions of 5–10%. Although the lower‐hybrid structures are less intense than those observed in the auroral zone and although their energy source is different, our observations lend support to the idea that the formation of lower‐hybrid structures is an universal mechanism operating in inhomogeneous magnetized space plasmas in the presence of VLF whistler mode turbulence. Besides the lower‐hybrid turbulence, another interesting feature is the occurrence of strong narrowband electromagnetic ELF emissions with amplitudes of a few millivolts per meter at frequencies below the proton gyrofrequency. They are continuously observed throughout the entire depletion. These emissions occur not only within the depletions but also, although less intense, outside of them over a large latitudinal range. They are tentatively identified as magnetospheric line radiations (MLRs) commonly observed during magnetically disturbed periods. Similar events were observed on 15 May 2005 and on 24 August 2005 during two other intense magnetic storms.

A study of the electrical charging of the Rosetta orbiter: 1. Numerical model

Charging of spacecraft in terrestrial or planetary ionospheres and magnetospheres is a well‐known effect which may have detrimental consequences for in‐situ plasma measurements. The case of the Rosetta mission is of particular importance because the expected temperatures of cometary ions and electrons in the inner coma of comet Wirtanen are very low. For this reason, we have undertaken a numerical simulation study of the electrical equilibrium of the Rosetta spacecraft in various conditions during its operational phase in order to determine its floating potential and the structure of the surrounding plasma sheath. The results of this study demonstrate that the electric equilibrium of the spacecraft is essentially controlled by the photoelectron current emitted by its very large solar panels. Only very close to the nucleus the expected plasma density is large enough to allow the spacecraft to collect enough thermal electrons to stay within a few kTe from the plasma potential. At larger distances it will float positive and at high potentials compared with the characteristic energy of cometary particles, thus hindering reliable ion measurements. A possible way to circumvent this effect is suggested by our numerical model. It relies on disconnecting the sunlit surface of the solar panels from the spacecraft ground. With such a grounding scheme the equilibrium electric potential of the spacecraft would stay at moderate values compatible with thermal ion measurements.

Electrostatic interaction between Interball-2 and the ambient plasma. 1. Determination of the spacecraft potential from current calculations

Abstract. The Interball-2 spacecraft travels at altitudes extending up to 20 000 km, and becomes positively charged due to the low-plasma densities encountered and the photoemission on its sunlit surface. Therefore, a knowledge of the spacecraft potential Fs is required for correcting accurately thermal ion measurements on Interball-2. The determination of Fs is based on the balance of currents between escaping photoelectrons and incoming plasma electrons. A three-dimensional model of the potential structure surrounding Interball-2, including a realistic geometry and neglecting the space-charge densities, is used to find, through particle simulations, current-voltage relations of impacting plasma electrons Ie (Fs ) and escaping photoelectrons Iph (Fs ). The inferred relations are compared to analytic relationships in order to quantify the effects of the spacecraft geometry, the ambient magnetic field B0 and the electron temperature Te . We found that the complex geometry has a weak effect on the inferred currents, while the presence of B0 tends to decrease their values. Providing that the photoemission saturation current density Jph0 is known, a relation between Fs and the plasma density Ne can be derived by using the current balance. Since Jph0 is critical to this process, simultaneous measurements of Ne from Z-mode observations in the plasmapause, and data on the potential difference Fs - Fp between the spacecraft and an electric probe (p) are used in order to reverse the process. A value Jph0 ~ = 32 µAm-2 is estimated, close to laboratory tests, but less than typical measurements in space. Using this value, Ne and Fs can be derived systematically from electric field measurements without any additional calculation. These values are needed for correcting the distributions of low-energy ions measured by the Hyperboloid experiment on Interball-2. The effects of the potential structure on ion trajectories reaching Hyperboloid are discussed quantitatively in a companion paper.Key words. Space plasma physics (charged particle motion and acceleration; numerical simulation studies; spacecraft sheaths, wakes, charging)

Thermal ion measurements on board Interball Auroral Probe by the Hyperboloid experiment

Abstract. Hyperboloid is a multi-directional mass spectrometer measuring ion distribution functions in the auroral and polar magnetosphere of the Earth in the thermal and suprathermal energy range. The instrument encompasses two analyzers containing a total of 26 entrance windows, and viewing in two almost mutually perpendicular half-planes. The nominal angular resolution is defined by the field of view of individual windows ≈13° × 12.5°. Energy analysis is performed using spherical electrostatic analyzers providing differential measurements between 1 and 80 eV. An ion beam emitter (RON experiment) and/or a potential bias applied to Hyperboloid entrance surface are used to counteract adverse effects of spacecraft potential and thus enable ion measurements down to very low energies. A magnetic analyzer focuses ions on one of four micro-channel plate (MCP) detectors, depending on their mass/charge ratio. Normal modes of operation enable to measure H+, He+, O++, and O+ simultaneously. An automatic MCP gain control software is used to adapt the instrument to the great flux dynamics encountered between spacecraft perigee (700 km) and apogee (20 000 km). Distribution functions in the main analyzer half-plane are obtained after a complete scan of windows and energies with temporal resolution between one and a few seconds. Three-dimensional (3D) distributions are measured in one spacecraft spin period (120 s). The secondary analyzer has a much smaller geometrical factor, but offers partial access to the 3D dependence of the distributions with a few seconds temporal resolution. Preliminary results are presented. Simultaneous, local heating of both H+ and O+ ions resulting in conical distributions below 80 eV is observed up to 3 Earth's radii altitudes. The thermal ion signatures associated with large-scale nightside magnetospheric boundaries are investigated and a new ion outflow feature is identified associated to the polar edge of the auroral oval. Detailed distribution functions of injected magnetosheath ions and ouflowing cleft fountain ions are measured down to a few eVs in the dayside.Key words. Ionosphere (auroral ionosphere; particle acceleration; ionosphere-magnetosphere interactions)

An experiment to study and control the Langmuir sheath around INTERBALL-2

Abstract. The satellite INTERBALL-2 has an orbit with high inclination (62.8°), covering the altitude range between a few hundred and about 20000 km. The ambient plasma conditions along this orbit are highly variable, and the interactions of this plasma with the spacecraft body as well as the photo-electron sheath around it are considered to be interesting topics for detailed studies. The electric potential of the spacecraft with respect to the ambient plasma that develops as a result of the current equilibrium reacts sensitively to variations of the boundary conditions. The measurement and eventual control of this potential is a prerequisite for accurate measurements of the thermal plasma. We describe the purpose and technical implementation of an ion emitter instrument on-board INTERBALL-2 utilising ion beams at energies of several thousand electron volts in order to reduce and stabilise the positive spacecraft potential. First results of the active ion beam experiments, and other measures taken on INTERBALL-2 to reduce charging are presented. Furthermore, the approach and initial steps of modelling efforts of the sheath in the vicinity of the INTERBALL-2 spacecraft are described together with some estimates on the resulting spacecraft potential, and effects on thermal ion measurements. It is concluded that even moderate spacecraft potentials as are commonly observed on-board INTERBALL-2 can significantly distort the measurements of ion distribution functions, especially in the presence of strongly anisotropic distributions.Key words. Space plasma physics (active perturbation experiments; spacecraft sheaths · wakes · charging; instruments and techniques).

Thermal ion measurements on board Interball Auroral Probe by the Hyperboloid experiment

Hyperboloid is a multi-directional mass spectrometer measuring ion distribution functions in the auroral and polar magnetosphere of the Earth in the thermal and suprathermal energy range. The instrument encompasses two analyzers containing a total of 26 entrance windows, and viewing in two almost mutually perpendicular half-planes. The nominal angular resolution is defined by the field of view of individual windows ≈13° × 12.5°. Energy analysis is performed using spherical electrostatic analyzers providing differential measurements between 1 and 80 eV. An ion beam emitter (RON experiment) and/or a potential bias applied to Hyperboloid entrance surface are used to counteract adverse effects of spacecraft potential and thus enable ion measurements down to very low energies. A magnetic analyzer focuses ions on one of four micro-channel plate (MCP) detectors, depending on their mass/charge ratio. Normal modes of operation enable to measure H+, He+, O++, and O+ simultaneously. An automatic MCP gain control software is used to adapt the instrument to the great flux dynamics encountered between spacecraft perigee (700 km) and apogee (20 000 km). Distribution functions in the main analyzer half-plane are obtained after a complete scan of windows and energies with temporal resolution between one and a few seconds. Three-dimensional (3D) distributions are measured in one spacecraft spin period (120 s). The secondary analyzer has a much smaller geometrical factor, but offers partial access to the 3D dependence of the distributions with a few seconds temporal resolution. Preliminary results are presented. Simultaneous, local heating of both H+ and O+ ions resulting in conical distributions below 80 eV is observed up to 3 Earth's radii altitudes. The thermal ion signatures associated with large-scale nightside magnetospheric boundaries are investigated and a new ion outflow feature is identified associated to the polar edge of the auroral oval. Detailed distribution functions of injected magnetosheath ions and ouflowing cleft fountain ions are measured down to a few eVs in the dayside.Key words. Ionosphere (auroral ionosphere; particle acceleration; ionosphere-magnetosphere interactions)

An experiment to study and control the Langmuir sheath around INTERBALL-2

The satellite INTERBALL-2 has an orbit with high inclination (62.8°), covering the altitude range between a few hundred and about 20000 km. The ambient plasma conditions along this orbit are highly variable, and the interactions of this plasma with the spacecraft body as well as the photo-electron sheath around it are considered to be interesting topics for detailed studies. The electric potential of the spacecraft with respect to the ambient plasma that develops as a result of the current equilibrium reacts sensitively to variations of the boundary conditions. The measurement and eventual control of this potential is a prerequisite for accurate measurements of the thermal plasma. We describe the purpose and technical implementation of an ion emitter instrument on-board INTERBALL-2 utilising ion beams at energies of several thousand electron volts in order to reduce and stabilise the positive spacecraft potential. First results of the active ion beam experiments, and other measures taken on INTERBALL-2 to reduce charging are presented. Furthermore, the approach and initial steps of modelling efforts of the sheath in the vicinity of the INTERBALL-2 spacecraft are described together with some estimates on the resulting spacecraft potential, and effects on thermal ion measurements. It is concluded that even moderate spacecraft potentials as are commonly observed on-board INTERBALL-2 can significantly distort the measurements of ion distribution functions, especially in the presence of strongly anisotropic distributions.Key words. Space plasma physics (active perturbation experiments; spacecraft sheaths · wakes · charging; instruments and techniques).

Thermal ion complexities observed within the spacelab 2 bay

Examples of prominent thermal ion composition variations characteristic of the Shuttle environment as observed from within the open cargo bay on the Spacelab 2 mission are discussed. Although the prominent ionization source is the inflow of the ambient plasma (in particular O +), water ions of Shuttle origin were present throughout the mission, and there was evidence for a local source of molecular ions NO +and/or O + 2 which also exist in the ionosphere. Bursts in the fluxes of these contaminant species were frequently observed coincident with thruster firings while O + depletions or enhancements could occur depending on the scattering geometry. These effects bring into question whether reliable ambient thermal ion measurements can be made from the vicinity of such vehicles. The presence of significant inflows of contaminant ions into the bay within the Shuttle wake also indicates that Shuttle emissions play a significant role in the evolution of its wake structure.

Aperture plane potential control for thermal ion measurements

Thermal plasma measurements from the Dynamics Explorer 1 spacecraft using an aperture bias in the outer plasmasphere and over the polar cap show the existence of cold plasma (T ⩽1 eV) which may otherwise be hidden from the particle detector by positive spacecraft potentials. Operations of the retarding ion mass spectrometer instrument with the external aperture plane biased up to −8 V with respect to the spacecraft reveal the existence of low‐energy (few eV) field‐aligned flows over the polar cap. Electrostatic models of the spacecraft indicate that if the spacecraft potential is more than a few volts positive, there can be a saddle point, or an electrostatic barrier, inhibiting the effectiveness of the aperture bias. Comparison of the aperture bias experiments with the electrostatic models shows that the aperture bias is successful in allowing the measurement of those field‐aligned flows with kinetic energy greater than the predicted barrier height but is less effective in aiding the measurement of the isotropic background. The addition of electron measurements from the high‐altitude plasma instrument allows the determination of the spacecraft potential when it is greater than +5 V. This additional information allows us to determine that the dayside polar wind (reported by Sojka et al., 1983) is supersonic, up to Mach 5.

Conical ion distributions at one earth radius — observations from the acceleration region?

Thermal ion measurements from the Retarding Ion Mass Spectrometer (RIMS) on Dynamics Explorer 1 (DE 1) in the night side auroral region were surveyed for evidence of ion acceleration. The RIMS measurements showed evidence for ion acceleration in the 2–10,000 km altitude range, with ion distributions peaked near 90°, and with temperatures of 1 to 10 eV. Two illustrations of the RIMS data for such observations are given here. The conical distributions are found at the low latitude edge of the auroral region, just outside the plasmapause. In the first example, the three major ion species (H+, He+, and O+) show evidence of acceleration. The angular distributions are peaked at different pitch angles, indicating that the different species have been accelerated at different altitudes. The H+ flux is higher than the O+ flux in this first example, in the RIMS energy range (0–50 eV). This is apparently typical of the RIMS observations on the night side. In the second example, only O+ is transversely accelerated.

Initial observations of thermal plasma composition and energetics from Dynamics Explorer‐1

Initial measurements of thermal ions (0 to 50 eV) at mid‐latitudes by the Retarding Ion Mass Spectrometer (RIMS) on DE‐1 are presented. These measurements show the plasmapause signature in H+, He+, and m/q = 2 ions, an enhancement or torus of O+ and O++ in the outer plasmasphere and the suggestion of a disconnection between the ionosphere and the bulge region of the plasmasphere.

Observations of ionospheric ion flow and related convective electric fields in and near an auroral arc

Measurements of the ionospheric ion distribution function and energetic electron and ion precipitation during an auroral substorm near local midnight are presented. Thermal ion measurements are used to determine the ion bulk flow velocity, temperature, and density. It is shown that the ion flow velocity is highly correlated with energetic electron precipitation. The convective component (perpendicular to the local magnetic field B) was found to be large (equivalent electric field E⊥ ∼ 100–200 mV/m) poleward of a series of poleward expanding arcs. Inside the arc the convective flow dropped to a low value (E⊥ ∼ 10–30 mV/m). High-velocity (∼2 km/s) ion flows parallel to B were also observed and found to be correlated with electron precipitation. Poleward of the arc the flow was directed away from the ionosphere, whereas inside the arc the flow switched to earthward. Parallel electric fields of ∼0.1 mV/m required to produce these vertical flows are shown to be consistent with energetic electron pitch angle distributions. Convective electric field measurements reported here are compared with previous observations.