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Auxiliary Material for
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Solar wind driven thermospheric winds over the Venus North Polar region
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Rickard Lundin, Yushifumi Futaana, Mats Holmström
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(Swedish Institute of Space Physics, Kiruna, Sweden)
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J.-A. Sauvaud, A. Ferorov
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(CESR, Toulouse, France)
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Geophysical Research Letter, 2014
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Introduction
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Data from 96 Venus Express (VEX) orbits have been analyzed from the time period July
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2011 to March 2012, with the primary objective to study the flow morphology of Energetic
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Neutral Atoms (ENAs) over the Venus polar region, and its relation to solar wind forcing of
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the Venus ionosphere and thermosphere. The VEX pericenter passes over the Venus North
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Pole, once every 24 hours, enabling a complete longitude coverage of the ionosphere and
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thermosphere in about 3.6 months.
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The ion and ENA data have been divided in two sets. In one set the flow/flux is organized
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and averaged in 1000 x 1000 x h km “cells”, where h represents a height range over the
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North Pole region (Figs 1 and 2 in Lundin et al., 2014). The other set, used for quantitative
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analysis and modeling, altitude profiles are derived for the ion and ENA flow/flux with 50
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km height resolution at dawn and dusk (Fig. fs02.pdf (b)).
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For ions, the flow/flux is derived from n·v, where n is the number density and v the flow
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velocity computed from moments of the ion distribution function. Conversely, integral (with
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respect to energy) ENA fluxes are derived from counts/s divided by the NPI conversion
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factor.
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The process studied involves three main particle species: solar wind H+ (SWH+),
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ionospheric O+, and ENAs, the latter produced when ions exchange charge with neutral
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atoms in the Venus thermosphere. Charge Exchange (CE) (Roelof, 1987 and C:son Brandt,
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et al., 2001) is a process whereby energetic ions (e.g. SWH+ and O+) are converted to ENAs,
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while thermospheric O are converted to "cold" O+.
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The generation of ENAs, and the methodology implemented in the data analysis and
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modeling, is a three-step process:
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1. Solar wind H+ exchange charge with neutral oxygen in a Venus model thermosphere
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(adapted after Nagy et al., 1990, and Gérard et al., 2009) => hydrogen ENAs (H-ENAs)
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2. Solar wind ions transfer energy and momentum to ionospheric O+ ions inside the
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ionopause (e.g. Knudsen et al, 1981) => energized ionospheric O+
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3. Energized O+ exchange charge with neutral oxygen in the model thermosphere =>
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oxygen ENAs (O-ENAs) plus ionized “cold” oxygen (O+)
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The objective of this Auxilliary material is to consider questions that may be raised
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concerning the results described in the main article, such as:
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a)
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sensor sufficiently well distinguish ENAs from charged particles and EUV?
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b) The quantitative model relation between ions and neutrals: If the ENAs measured are
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produced by the CE process in the thermosphere, can that be quantified by a model and, if
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so, does that agree with ENA fluxes obtained from NPI?
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c)
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concept of dusk-dawn flow channels for ions and neutrals, in order to derive a modified
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ENA flux versus altitude profile based on the ion kinetic energy/flow velocity?
The quality of the data: What does the measurement data really show? Can the NPI-
Conversion of NPI counts to energy-corrected ENA fluxes: Is it conceivable to use the
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In what follows, the aforementioned questions are considered in more detail. The quality of
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the data is discussed in “text01.txt”, using two examples of ion and NPI data displayed in
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“fs01.pdf”, and a table ts01.txt describing conditions for operation.
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The quantitative model describing the relation between energetic ions and ENAs is
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discussed in “text02.txt”, with a map of the ion and ENA flow “fs02.pdf” and the NPI ENA
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flux compared to model ENA flux illustrated by “fs03.pdf”.
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The method to convert NPI counts to energy–corrected ENA fluxes is discussed in
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“text03.txt”. An energy-dependent conversion table for the ENA flux versus altitude is
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derived, by applying the energy-dependent conversion factor fs04.pdf (a) on the SWH+ and
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O++ kinetic energy versus altitude, fs05.pdf (b).
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Finally, txt04txt presents conclusions from the analysis of potentially critical questions.
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text01.text
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Despite a careful design to avoid forward scattering of light, NPI is yet sensitive to light,
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especially solar UV and EUV. A restrictive operations strategy was therefore implemented
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for NPI and the other ASPERA-3 Neutral Particle Detectors, NPD. The restrictions, defined
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by the spacecraft attitude, were determined on a weekly basis as described in table ts01.txt.
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Notice that the NPI condition allows NPI to point closer (up to 40°) to the solar direction.
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However, when NPD is on, the NPD restriction applies for NPI as well, implying a more
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confined FoV. Because scattered light from the spacecraft and the planet may still affect
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measurements, a thorough quality control of NPI data is required, rejecting orbits with high
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noise background from e.g. scattered light. The result from the screening process is that
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light scattering and background noise accounts for less than 20 counts/readout, i.e. less than
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10%, of the ENA data. Background counts from charged particles, specifically ions, can be
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ruled out because the highest ion energy in the background plasma is in the few keV-range,
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the NPI electrostatic filter effectively removing all charged particles with energies less than
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≈100 keV.
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Figure fs01.pdf displays ion and NPI data from two traverses over the Venus polar region,
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fs01.pdf (a). A midnight - noon traverse, MEX moving in the sunward direction,
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fs01.pdf(b). A dusk - dawn traverse. The two top panels give energy-time spectra for H+ and
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O+, illustrating VEX approaching the planet and the ionosphere/thermosphere from the
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nightside (a), respectively along the terminator (b). The third panels display NPI counts
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versus sector angle from the 32 NPI sectors covering a 360° field-of-view. The fourth and
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fifth panels show time series data of average counts per readout, the 32 sectors now
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combined and averaged in 8 sectors. The sixth panel, displaying changes in the X (Alpha)
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and YZ (Theta) spacecraft orientation angles, exemplifies pointing changes of NPI. Figure
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fs01.pdf displays some general characteristics of the NPI time series data: Count rates
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smoothly rising and falling in “repeated” sequence depending on orbit plane and pointing
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direction; Maximum count rate detected at altitudes below 1000 km; ENAs are associated
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with/originate from both SWH+ and O+ as illustrated by the blue and red dashed boxes,
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respectively, in fs01.pdf.
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text02.txt A quantitative model of the relation between energetic ions and ENAs.
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fs02.pdf summarizes in diagrammatic form the observational results, i.e. the asymmetric
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SWH+, O+, and ENA flow over the Venus North Pole region - with higher O+ fluxes
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generated in the more extended dayside/dusk ionosphere, and higher ENA fluxes from CE
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of O+ traversing regions with a denser thermosphere (also dayside/dusk). The two figures,
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(a) and (b), display the generation of ENAs from two different vantage points. fs02.pdf (a)
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illustrates the "flow channels" used to derive the O-ENA flux altitude profiles. Blue dashed
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lines mark the 60° sector describing the distance, ∆L(h), used in the ENA-model. Circular
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altitude sectors are assumed, since most O+ (blue arrows) as well as O-ENAs (red arrows),
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are gravitationally trapped below ≈1000 km. Conversely, most H-ENAs and SWH+ (white
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arrows) traverse the Venus thermosphere along hyperbolic trajectories. Decreasing/
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increasing widths of individual arrows mark the gradual CE-conversion of ion flux to ENA
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flux, i.e. reduced O+ flux corresponds to enhanced ENA flux.
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fs02.pdf (b) describes the asymmetric reduced/enhanced flux of O+ and ENAs over the
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North Pole (XY-projection). Notice that besides a dawn-dusk asymmetry of the O+ and
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ENA fluxes, there is also a noon-midnight asymmetry as a consequence of the low
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probability for CE in the more tenuous night-side thermosphere. Dashed rectangles in
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fs02.pdf (a) mark the +Y and -Y sectors within which average ENA, SWH+, and O+ flux
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versus altitude profiles were determined. Solid black arrows mark the average circulation
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direction of the Venus atmosphere.
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The quantitative model describing the relation between energetic ions and ENAs as
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determined by NPI depends on two factors: the conversion factor of NPI counts to ENA-
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fluxes and the conversion of incident ion fluxes to ENAs by the CE process. The NPI
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sensitivity is close to unity for ENAs at, and above ≈1 keV, i.e. the flux may be obtained
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directly from the geometric factor. However, for energies below 1 keV, the sensitivity drops
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drastically. The NPI sensitivity, calibrated using an H2O+ beam (Brinkfeldt et al., 2006),
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indicated that the NPI sensitivity versus energy curve goes approximately as the power of 2.
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Because the ENAs are expected to have energies in the 10 - 300 eV range, i.e. the same as
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for ions, a 100 eV fixed energy setting was implemented as baseline in the ENA analysis.
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To confirm that the fluxes measured by NPI were indeed ENAs, NPI fluxes were compared
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with model fluxes inferred from charge-exchange between ions and neutrals along a dusk-
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dawn flow channel (expression (2) in the main article). Fluxes modeled versus altitude were
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based on direct measurements of SWH+ and O+, and an altitude model of thermosphere
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atomic oxygen (Nagy et al., 1990, and Gérard et al., 2009).
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Figure fs03.pdf demonstrates that there is a general agreement between dusk (-Y) and dawn
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(+Y) altitude profiles of ENA fluxes measured by NPI, and ENA fluxes modeled under the
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assumption that the interaction distance (∆L(h) corresponded to a 60° sector angle of the
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Venus polar thermosphere (see fs02.pdf a). The disagreement between NPI-ENAs and
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model values below ≈1000 km is consistent with the low NPI sensitivity for low-energy
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ENAs.
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text03.txt Conversion of NPI counts to energy-corrected ENA fluxes
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The NPI sensitivity versus energy for ENA detection is described in fs04.pdf (a). Circles
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along the curve show calibration results of the NPI sensitivity for three energies (Brinkfeldt
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et al, 2006). The curve fitted to the calibration values (circles) gives the NPI sensitivity
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versus energy (E): S(E)=1.94·10-9 E2.0. The diamond along the curve at 100 eV marks the
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fixed energy setting used as baseline in the ENA analysis.
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Figure fs04.pdf (b) displays the dawn (+Y) and dusk (-Y) average kinetic energy of SWH+
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and O+ versus altitude. Under the assumptions that local ENAs are both generated in, and
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flow along the same height intervals as the H+ and O+ ions, it is reasonable to assume that
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ENAs have essentially the same kinetic energy as the ions there. Because the average O+
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flow velocities near pericenter is much below the spacecraft velocity, the spacecraft ram-
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velocity plays the major role for the low-altitude detection of ENAs. A spacecraft velocity
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of ≈8 km/s corresponds to a kinetic energy ≈6.5 eV for oxygen, this being added to the
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velocity/kinetic energy in fs04.pdf (b). Combining fs04.pdf (a) and fs04.pdf (b), the H+ and
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O+ kinetic energy versus altitude can now be used to replace the fixed energy ENA altitude
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profile with one derived from the energy-dependent sensitivity curve (a). The modified flux
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versus altitude curve, combined with a curve derived from a CE model based on IMA ion
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fluxes, is displayed in Fig. 4 of the main article (Lundin et al., 2014).
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The question about the applicability of the aforementioned height-dependence of the ion and
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ENA energy/velocity distribution, i.e. the flow channels, relates to the conservation of total
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ion flow from dusk to dawn over the polar region. This implies that the momentum flux
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gained by one species (e.g. O+) in a flow channel leads to a corresponding momentum flux
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loss by other species (e.g. SWH+) in the same flow channel. That this is the case may not be
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obvious from the flux versus altitude panels displayed the main article (Lundin et al. 2014,
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Fig. 4), neither does it reveal in what altitude range it applies. However, by converting ion
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fluxes to ion densities in phase space makes it possible to find out if, and if so, where the
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density in phase-space is constant (Liouville’s theorem). Figure fs05.pdf resembles Fig. 3 in
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the main article (Lundin et al., 2014) with the difference that the SWH+ and O+ flux in
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fs05.pdf is converted to densities in phase space (in normalized units). The dashed box in
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grey marks the altitude interval (600 – 1300 km) within which the total density in phase
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space (O+ plus SWH+) varies by about ≈30%, i.e. in fair agreement with the Liouville’s
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theorem of constant density in phase space in a flow channel. The test fails below 600 km
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and above 1300 km, in both cases indicating no E&M transfer between ions. The
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momentum flux is carried by O+ below 600 km and by SWH+ above 1300 km. The altitude
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range 600 – 1300 km therefore represents the main region of SWH+ E&M transfer to O+
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(Lundin et al., 2013)
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txt04txt Conclusions
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A detailed analysis of potential critical question regarding the detection of ENAs by the NPI
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instrument on VEX leads us to the following conclusions:
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A. The NPI sensor proved to be capable of distinguishing ENA fluxes from the ambient
Venus particle and solar EUV environment.
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B. The ENA fluxes measured by NPI are in qualitative agreement with an independent
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model computation of ENA fluxes derived from charge-exchange of IMA SWH+
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and O+ with neutral oxygen in a Venus model thermosphere.
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C. The concept of ion charge-exchange with neutral oxygen in dawn-dusk “flow
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channels”, whereby a modified (energy-dependent) ENA flux profile versus altitude
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can be derived, is consistent with the ion density in phase space being constant
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(Liouville's theorem) in the altitude range 600 - 1300 km.
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References
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C:son Brandt, P., Barabash, S., Roelof, E.C., Chase, C.J., (2001). Energetic neutral atom
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imaging at low altitudes from the Swedish microsatellite Astrid: Observations at low
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(<10 keV) energies. J. Geophys. Res. 106 (A11), 24663–24674.
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Barabash. S., J.-A. Sauvaud, H. Gunell, H. Andersson, et. al. (2007), The Analyzer of Space
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Plasmas and Energetic Atoms (ASPERA-4) for the Venus Express Mission, Planetary
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and Space Science, 55, 12, 1772-1792.
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Brinkfeld, K, H. Gunnell, P.C. Brandt, S. Barabash et al. (2006), Observations of energetic
neutral atoms on the nightside of Mars, Icarus, 182, 2, 439.
Gérard, J.-C., A. Saglam, G. Piccioni, P. Drossart, et al., (2009), Atomic oxygen
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distribution in the Venus mesosphere from observations of O2 infrared airglow
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bu VIRTIS-Venus Express, Icarus, 199, 264-271.
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Keating, G. M, j. L. Bertaux, S. W. Bougher, et al., Models of Venus neutral upper
atmosphere: Structure and composition, Adv Space Res.. 5, 11, 117-171, 1985.
Knudsen, W.C., Spenner, K., Miller, K.L, Anti-solar acceleration of ionospheric plasma
across the Venus terminator, (1981). Geophys. Res. Lett, 8, 241-244.
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Lundin et al, Barabash, S.; Futaana, Y.; Holmström, M.; Perez-De-Tejada, H.; Sauvaud, J.-
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A, (2013), A large-scale flow vortex in the Venus plasma tail and its fluid dynamic
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interpretation, Geophys. Res. Lett., 40, 7, pp. 1273-1278
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Lundin, R.; Barabash, S., Futaana, Y., Holmström, M., Sauvaud, J.-A., Fedorov, A. (2014),
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Solar wind driven thermospheric winds over the Venus North Polar region, Geophys.
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Res. Lett, submitted Feb 2014.
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Nagy, A. F., K. Jhoon, and T. E. Cravens, (1990), Hot hydrogen and oxygen atoms in the
upper atmospheres of Venus and Mars, Annales Geophysicae. 8, 251-256.
Roelof, E. C. (1987), Energetic neutral atom image of a storm-time ring current, Geophys.
Res. Lett., 14, 652–655.
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Figure captions
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1. fs01.pdf Ion and NPI data from two traverses over the Venus polar region. (a) A
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midnight - noon traverse, MEX moving in the sunward direction. (b) A dusk - dawn
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traverse. Dashed blue and red boxes indicate the primary origin of the ENAs measured, blue
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from SWH+, red (low altitude) from O+.
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1.1 Panel “E(eV), ion H+” SWH+ color-coded energy-time spectra
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1.2 Panel “E(eV), ion O+” O+ color-coded energy-time spectra
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1.3 Panel “NPI Phi”, color-coded counts from the 32 NPI sectors distributed over 360°.
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1.4 Panel “NPI counts ch 1-16”, time series plots of NPI counts from 8 binned and averaged
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consecutive sectors. Sectors marked by * indicate data corrupted by spacecraft shadowing
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1.5 Panel “NPI counts ch 17-31”, time series plots of NPI counts from 8 binned and
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averaged consecutive sectors. Sectors marked by * indicate data corrupted by spacecraft
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shadowing
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1.6 Panel “VEX orientation”, VEX spacecraft orientation with respect to the Sun
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1.7 Panel “Altitude km”, altitude and VSO coordinates of VEX
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2. fs02.pdf. Diagrammatic representation of the ion and ENA flow through the Venus polar
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thermosphere.
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2.1 (a) Tail (YZ VSO) view of the chain of events along the "flow channels": SWH+ E&M
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transfer => O+ flow => O+ CE with O => O-ENA flow, the flux gradually enhancing
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towards dawn. Dashed blue line marks the 60° sector used to model ENA fluxes from the
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SWH+ and O+ charge-exchange with thermospheric oxygen atoms.
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2.2 (b) A polar (XY) view of the ion and ENA flow. Dashed rectangles constitute the +Y
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and -Y sectors within which average ion and ENA fluxes are determined and presented as
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flux versus altitude profiles. Black arrows give the direction of the atmospheric circulation.
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3. fs03.pdf. ENA fluxes versus altitude at dusk (-Y) and dawn (+Y) as measured by NPI
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and a model of charge-exchange between SWH+, O+ and thermospheric atomic oxygen. In
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both cases (NPI and model), fixed energy sensitivity (at 100 eV) is used, converting NPI-
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counts to flux, and ion flux to ENA flux via charge exchange.
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4. fs04.pdf NPI sensititity curve (a) and relative (to the VEX spacecraft velocity) ion kinetic
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energies (b)
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4.1 fs04.pdf (a), the NPI ENA-sensitivity versus energy curve for H2O+ ions. Circles
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represent calibration results, diamond fixed-energy setting for NPI.
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4.2 fs04.pdf (b), the relative (to the spacecraft motion at pericenter) kinetic energy of the
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flow of neutrals over the Venus polar region as anticipated from CE of ions to neutrals
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(wind + ENAs).
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5. fs05.pdf. Normalized ion (SWH+ and O+) densities in phase space versus altitude.
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Dashed gray box illustrates that the sum of ion densities in phase space is constant. A
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consequence of the SWH+ E&M transfer to O+ at dusk is therefore a decrease of SWH+
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flow, and a corresponding increase of the O+ flow at dawn. The latter is consistent with
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Liouville’s theorem, and a conservation of E&M between both ion species in the flow
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channel.
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ts01.txt, Conditions for operating NPI and NPD
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NPI is OFF if FoV-Sun angle is less than 30°
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NPD is OFF if FoV-Sun angle is less than 60°
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NPD is OFF if FoV-Venus limb angle is less than 5°
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1. fs01.pdf
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2. fs02.pdf
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3. fs03.pdf.
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4. fs04.pdf.
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5. fs05.pdf.
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