Atmosphere Explorer Research Articles

Preliminary Results From Probe of the Moon's Dust Environment

NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE), a robotic mission with the primary goal of gathering detailed information about the composition and structure of the thin lunar atmosphere, was launched on 6 September 2013. Since the spacecraft reached the Moon on 6 October, LADEE has returned streams of data important to understanding how dust is distributed above the lunar surface; where sources and sinks of lunar atmospheric dust can be found; and how dust distribution, sources, and sinks change through time.

Scattering properties of lunar dust analogs

The Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft is designed to characterize the exospheric dust environment using an on-board suite of specialized sensors. The objective of this paper is to present results from scattering experiments using an aqueous suspension of lunar simulants that contains a population of dust grains ranging in size from ~0.1μm to 10μm. The intensity of scattered light is measured with a commercial version of the ultraviolet–visible spectrometer (UVS) used in the LADEE mission. We show that our data is consistent with the fact that micron-sized particles tend to form agglomerates rather than remaining isolated entities and that certain characteristics of the target particles can be predicted from intensity measurements alone. These results can be used directly to assess general features of the lunar exosphere. Further analysis of particle properties from such remote sensing data will require more refined measurements such as polarization features or other components of the Stokes vector.

Production of neutral gas by micrometeoroid impacts

We present the first direct laboratory measurement of vapor produced by simulated micrometeoroid bombardment. New in situ observations from the Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft, and the anticipation of results from the Lunar Atmosphere and Dust Environment Explorer (LADEE), have highlighted the uncertainty surrounding the role of micrometeoroid impacts in sustaining planetary exospheres. In a recent series of experiments, the quantity of neutral molecules generated by impacts of simulated micrometeorids of 0.1–1μm radius was measured using a fast ion gauge, over a speed range of 1–10km/s. The quantity of neutrals released per unit projectile mass, N/m, is consistent with a power law N/m=vβ in the projectile speed v, with β∼2.4. At the highest speeds tested, the number of neutrals liberated is equivalent to 5% of the atoms in the projectile; complete vaporization is projected at speeds exceeding 20km/s.

ARTEMIS observations of lunar pickup ions: Mass constraints on ion species

Observations of heavy ions of lunar origin give important information regarding lunar exospheric processes, especially with respect to exospheric particle abundance and composition. Electrostatic analyzers without a time‐of‐flight section provide highly sensitive, absolute density detection but without mass discrimination. Here we place constraints on lunar ion species through inference of the average ion mass using such instruments. The technique is based on the plasma quasi‐neutrality requirement, an independent electron density measurement, and the fact that electrostatic analyzers underestimate the ion density by the square root of the ion mass. Applied to a case of such observations by ARTEMIS in the terrestrial lobe reported by Poppe et al. (2012), our technique suggests an average mass of 28 amu for lunar pickup ions. This result, consistent with the lower limit of 24 amu derived in the Poppe et al. model, suggests that the observed ions were most likely Al+ and Si+. The technique is also refined and applied to a more complicated event with a series of heavy ion surges in the plasma sheet, to show the spatial and/or temporal dependence of the observed lunar ion species. The technique is particularly timely given the planned conjunctions and coordinated lunar studies by NASA's Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) and Lunar Atmosphere and Dust Environment Explorer missions.

Metallic species, oxygen and silicon in the lunar exosphere: Upper limits and prospects for LADEE measurements

The only species that have been so far detected in the lunar exosphere are Na, K, Ar, and He. However, models for the production and loss of species derived from the lunar regolith through micrometeoroid impact vaporization, sputtering, and photon‐stimulated desorption, predict that a host of other species should exist in the lunar exosphere. Assuming that loss processes are limited to ballistic escape, photoionization, and recycling to the surface, we have computed column abundances and compared them to published upper limits for the Moon. Only for Ca do modeled abundances clearly exceed the available measurements. This result suggests the relevance of some loss processes that were not included in the model, such as the possibility of gas‐to‐solid phase condensation during micrometeoroid impacts or the formation of stable metallic oxides. Our simulations and the recalculation of efficiencies for resonant light scattering show that models for other species studied are not well constrained by existing measurements. This fact underlines the need for improved remote and in situ measurements of the lunar exosphere such as those planned by the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft. Our simulations of the LADEE neutral mass spectrometer and visible/ultraviolet spectrometer indicate that LADEE measurements promise to provide definitive observations or set stringent upper limits for all regolith‐driven exospheric species. We predict that observations by LADEE will constrain assumed model parameters for the exosphere of the Moon.

On the effects of clouds and hazes in the atmospheres of hot Jupiters: semi-analytical temperature-pressure profiles

Motivated by the work of Guillot (2010), we present a semi-analytical formalism for calculating the temperature-pressure profiles in hot Jovian atmospheres which includes the effects of clouds/hazes and collision-induced absorption. Using the dual-band approximation, we assume that stellar irradiation and thermal emission from the hot Jupiter occur at distinct wavelengths ("shortwave" versus "longwave"). For a purely absorbing cloud/haze, we demonstrate its dual effect of cooling and warming the upper and lower atmosphere, respectively, which modifies, in a non-trivial manner, the condition for whether a temperature inversion is present in the upper atmosphere. The warming effect becomes more pronounced as the cloud/haze deck resides at greater depths. If it sits below the shortwave photosphere, the warming effect becomes either more subdued or ceases altogether. If shortwave scattering is present, its dual effect is to warm and cool the upper and lower atmosphere, respectively, thus counteracting the effects of enhanced longwave absorption by the cloud/haze. We make a tentative comparison of a 4-parameter model to the temperature-pressure data points inferred from the observations of HD 189733b and estimate that its Bond albedo is approximately 10%. Besides their utility in developing physical intuition, our semi-analytical models are a guide for the parameter space exploration of hot Jovian atmospheres via three-dimensional simulations of atmospheric circulation.

Reexamination of ionospheric photochemistry

[1] This paper reexamines the chemistry of N2+, NO+, O2+, and N+ by comparing densities from a photochemical model with data from the Atmosphere Explorer C satellite. These comparisons show that the measurements of N2+, NO+, O2+, and N+ density are well modeled with up-to-date reaction rates, solar EUV irradiances, and photoelectron fluxes. In particular, this study has resolved a long-standing problem wherein the previous investigations overestimated the measured N2+ density by a factor of 2. A new method of determining the important and controversial O+(2D) + N2 reaction rate from the Atmosphere Explorer C data is presented. This reaction rate determination agrees well with the latest laboratory measured reaction rate.

Comment on “Westward electric field penetration to the dayside equatorial ionosphere during the main phase of the geomagnetic storm on 22 July 2009” by V. Sreeja et al.

[1] Sreeja et al. [2011] have claimed that the “geomagnetic storm that occurred during the period from 21 to 25 July 2009 is anomalous because the storm main phase developed during northward interplanetary magnetic field (IMF)” and “westward electric field penetration to the dayside equatorial ionosphere during the main phase of the storm.” However, as discussed subsequently, the so‐called “anomalous” feature of geomagnetic storm is ill‐fully claimed due to their incorrect IMF Bz data. Since Sreeja et al. [2011] have made serious claims such as the geomagnetic storm of 21–25 July 2009 is anomalous which would have adverse impact on the future scientific investigations, it is highly necessary to clarify. [2] For example, Figures 1a–1d show the variation of interplanetary magnetic field (IMF Bz), interplanetary electric field (IEF) (IEFy = −Vsw × Bz), symmetric ring current (Sym‐H) index and the strength of equatorial electrojet (EEJ), respectively. The 1 min time resolution data of IMF Bz and IEFy are obtained from the Atmospheric Composition Explorer (ACE). The Sym‐H index (1 min values) data is obtained from the World data center for Geomagnetism, Kyoto University (http://wdc.kugi.kyoto‐u.ac.jp/). The strength of equatorial electrojet (1 min values) is obtained from the ground‐based magnetometers at Tirunelveli (8.7°N geog.latitude, 77.8°E geog.longitude and 0.6°N dip.latitude) and Alibagh (18.5°N geog.latitude, 72.9°N geog.longitude and 12.9°N dip.latitude) in India. The IMF Bz and IEFy parameters have been shifted to account for the delay from ACE location to the bow shock (http://omniweb.gsfc.nasa. gov/html/HROdocum.html). It could be clearly seen from Figure 1 that the IMF Bz is southward continuously from several hours before the onset of main phase at 0154 UT (0724 IST) of 22 July 2009. However, unlike Sreeja et al. [2011] claimed, the IMF Bz continues to be southward till 0544 UT (1114 IST). During the same period (0154 to 0544 UT) the ring current continues to be strengthened as evidenced by the variation of Sym‐H index which reached to a maximum negative excursion of −89 nT at 0544 UT. This clearly shows that the main phase of the storm is developed mainly under the southward orientation of the IMF Bz which is a well‐known phenomenon. Therefore, the principal finding of anomalous geomagnetic storm where the main phase is claimed to be developed during the northward IMF Bz is not true. While comparing the time variations of IMF Bz and IEFy with those from Sreeja et al. [2011, Figure 1], it can be clearly noticed that the IMF Bz and IEFy data shown in their figure is incorrect where the IMF Bz is turned northward around ∼0700 IST. Also in section 3.1 of Sreeja et al. [2011], they have claimed that “the IMF Bz is observed to be remain northward for a very long duration (∼8 h) from 0700 IST to 1600 IST.” This is incorrect as can be seen from Figure 1 that the IMF Bz is northward only for a period of 83 min from 0544 to 0707 UT (1114–1237 IST) and later turned southward from 0707 to 0849 UT (1237–1419 IST). Therefore, the anomalous main phase of the geomagnetic storm which developed under northward IMF Bz is not true and primarily arise due to incorrect data of IMF Bz used by Sreeja et al. [2011]. Though the authors have stated in sections 2 and 3.1 that the 1 min data of IMF Bz obtained from the Website of Atmospheric Composition Explorer spacecraft, it is observed that the data shown by Sreeja et al. [2011, Figure 1] do not match with the IMF data of ACE. Further, Sreeja et al. [2011] have stated that they have estimated a convection time delay of ∼40 min from an average solar wind velocity of 560 km/s. However, it is noticed that solar wind velocity never reached to a value of 560 km/s during the entire period of this storm event. Instead, the solar wind velocity varies between ∼300 and 370 km/s during the main phase and varies between ∼370 and 500 km/s during the recovery phase of the storm period. These publicly available data of ACE solar wind and magnetic parameters were also published in earlier studies [e.g., Fok et al. 2010]. In their reply to our comment, Sreeja et al. [2011] insisted that they have estimated the time delay based on the solar wind speed of ∼560 km/s on 24 July. However, their estimation of time delay is inappropriate because the geomagnetic storm is actually developed on 22 July. Research Institute for Sustainable Humanosphere, Kyoto University, Kyoto, Japan. Department of Earth and Planetary Sciences, Kyushu University, Fukuoka, Japan. Space Environment Research Center, Kyushu University, Fukuoka, Japan. Indian Institute of Geomagnetism, Navi Mumbai, India. Solar‐Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan.

A reanalysis of the Apollo light scattering observations, and implications for lunar exospheric dust

Conspicuous excess brightness, exceeding that expected from coronal and zodiacal light (CZL), was observed above the lunar horizon in the Apollo 15 coronal photographic sequence acquired immediately after orbital sunset (surface sunrise). This excess brightness systematically faded as the Command Module moved farther into shadow, eventually becoming indistinguishable from the CZL background. These observations have previously been attributed to scattering by ultrafine dust grains (radius ∼0.1 microns) in the lunar exosphere, and used to obtain coarse estimates of dust concentration at several altitudes and an order-of-magnitude estimate of ∼10 −9 g cm −2 for the column mass of dust near the terminator, collectively referred to as model “0”. We have reanalyzed the Apollo 15 orbital sunset sequence by incorporating the known sightline geometries in a Mie-scattering simulation code, and then inverting the measured intensities to retrieve exospheric dust concentration as a function of altitude and distance from the terminator. Results are presented in terms of monodisperse (single grain size) dust distributions. For a grain radius of 0.10 microns, our retrieved dust concentration near the terminator (∼0.010 cm −3) is in agreement with model “0” at z=10 km, as is the dust column mass (∼3–6×10 −10 g cm −2), but the present results indicate generally larger dust scale heights, and much lower concentrations near 1 km (<0.08 cm −3 vs. a few times 0.1 cm −3 for model “0"). The concentration of dust at high altitudes ( z>50 km) is virtually unconstrained by the measurements. The dust exosphere extends into shadow a distance somewhere between 100 and 200 km from the terminator, depending on the uncertain contribution of CZL to the total brightness. These refined estimates of the distribution and concentration of exospheric dust above the lunar sunrise terminator should place new and more rigorous constraints on exospheric dust transport models, as well as provide valuable support for upcoming missions such as the Lunar Atmosphere and Dust Environment Explorer (LADEE).

Influence of spacecraft outgassing on the exploration of tenuous atmospheres with in situ mass spectrometry

In situ mass spectrometry has been a powerful tool in many space missions to investigate atmospheres and exospheres of different bodies in the solar system. Applying new technologies, the mass spectrometers have become increasingly more sensitive. In this study, we show that spacecraft outgassing, which can never be completely prevented, will be the limiting factor in future missions that investigate very tenuous atmospheres and exospheres of moons, asteroids, or comets at large heliocentric distances. The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument on the European Space Agency Rosetta mission has monitored spacecraft outgassing for 6 years during the cruise phase with unprecedented instrument sensitivity. It is shown that diffusion of gas from materials and from the spacecraft interior plays an important role in maintaining a relatively permanent thin gas cloud around the spacecraft for many years. The density and composition of this gas cloud depends on location on the spacecraft, maneuvers, and payload activity. The main contaminants are water, which is adsorbed on cold surfaces, and organics from the spacecraft structure, electronics, and insulations. Decomposed lubricant material can give a significant contribution to the total background. Fortunately for Rosetta, outgassing of the spacecraft will play a minor role when the comet is close to perihelion; only in the early phase of the mission the outgassing may be larger than the cometary signature.

Water vapour profiles from SCIAMACHY solar occultation measurements derived with an onion peeling approach

Abstract. A new retrieval method has been developed to derive water vapour number density profiles from solar occultation measurements of the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY). This method is intentionally kept simple and based on a combination of an onion peeling approach with a modified DOAS (Differential Optical Absorption Spectroscopy) fit in the wavelength region around 940 nm. Reasonable resulting water vapour profiles are currently obtained in the altitude range 15–45 km. Comparisons of the SCIAMACHY profiles with water vapour data provided by the Atmospheric Chemistry Explorer Fourier Transform Spectrometer (ACE-FTS) show an average agreement within about 5% between 20 and 45 km. SCIAMACHY water vapour data tend to be systematically higher than ACE-FTS. These results are in principal confirmed by comparisons with water vapour profiles derived from model data of the European Centre for Medium Range Weather Forecasts (ECMWF), although ECMWF concentrations are systematicly lower than both corresponding SCIAMACHY and ACE-FTS data at all altitudes.

Optical scattering processes observed at the Moon: Predictions for the LADEE Ultraviolet Spectrometer

The Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft will orbit the Moon at an altitude of ≈50 km with a payload that includes the Ultraviolet Spectrometer (UVS) instrument, which will obtain high spectral resolution measurements at near-ultraviolet and visible wavelengths (≈231–826 nm). When LADEE/UVS observes the lunar limb from within the shadow of the Moon it is anticipated that it will detect a lunar horizon glow (LHG) due to sunlight scattered from submicron exospheric dust, as well as emission lines from exospheric gases (particularly sodium), in the presence of the bright coronal and zodiacal light (CZL) background. A modularized code has been developed at NMSU for simulations of scattered light sources as observed by orbiting instruments in lunar shadow. Predictions for the LADEE UVS and star tracker cameras indicate that LHG, sodium (Na) emission lines, and CZL can be distinguished based on spatial morphology and spectral characteristics, with LHG dominant at blue wavelengths (∼250–450 nm) and small tangent heights. If present, LHG should be readily detected by LADEE/UVS and distinguishable from other sources of optical scattering. Observations from UVS and the other instruments aboard LADEE will significantly advance our understanding of how the Moon interacts with the surrounding space environment; these new insights will be applicable to the many other airless bodies in the solar system.

Effects of the 16 February 1980 solar eclipse on the composition of the low-latitude ionosphere as seen by Atmosphere Explorer E

[1] Solar eclipses are known to locally disrupt the transport, production, and loss mechanisms in the ionosphere. Ion composition, ion temperature, and neutral temperature data from the Atmosphere Explorer E spacecraft are examined for the total solar eclipse of 16 February 1980. The spacecraft transited twice across the dayside face of the Earth during the course of the eclipse, allowing for examination of eclipse effects to be made over a wide longitude and local time range and for examination of posteclipse recovery of the ionosphere. One orbit from 14 February, occurring over a longitude and local time range similar to that of the first eclipse orbit, is used as control data. The eclipse had a significant effect on the concentrations of both O+ and N+, which both dropped. The concentration of H+ seems to show an eclipse effect, but the concentrations are too low to draw definite conclusions. Signatures of charge exchange between H+ and neutral oxygen are seen in the data from the second eclipse orbit. The ion temperature drops by as much as 60 K. The neutral atmosphere shows no change in temperature during the course of the eclipse. The second eclipse orbit occurred closer to the path of the eclipse than did the first orbit, and the perturbations caused by the eclipse are greater in the second orbit. The control and second eclipse orbit data are compared to results from the International Reference Ionosphere 2000 model. The model results show good qualitative agreement with the ion concentration data.

A comparison of ground and satellite observations of F region vertical velocity near the dip equator

Nighttime F region vertical electrodynamic drifts were made at the magnetic equatorial stations in Africa, Ibadan (7.4°N, 3.9°E; 6°S dip), and Ouagadougou (12°N, 1.5°W; 5.9°N dip) using ionosondes hmF2 data during 1957/1958 International Geophysical Year (IGY) and 1990 periods, respectively; for high solar flux and geomagnetic quiet time conditions. We compare the seasonal averages of vertical drifts with observations made by Incoherent Scatter Radar (ISR) at Jicamarca (11.95°S, 76.87°W; 2°N dip) and Atmospheric Explorer E (AE‐E) satellite for equatorial F layer vertical drifts reported by Fejer et al. (1995). The results indicate good accord between the three techniques at periods when convection dominates other factors (e.g., around prereversal enhancement) except for June solstice drifts. However, when the drifts are completely downward (negative) between 2000 and 0500 LT sector, the mean discrepancies between ionosondes and ISR drifts are of 8–11 m/s (December solstice), 14–17 m/s (equinoxes) but comparable during June solstice with a value of about 14 m/s. Conversely, the typical values of the differences among ionosondes and AE‐E downward velocities are 7–10 m/s (December solstice), 9–12 m/s (equinoxes), yet again similar in winter with a significantly smaller value of about 3 m/s. The evening reversal times are in excellent agreement, apart from June solstice drifts, which exhibits large fluctuations. The morning reversal times show small variations. Equinoctial prereversal enhancement velocities have amplitudes of approximately 17–35 m/s between the methods. Our data are useful for global ionospheric modeling and for the predictions of development of nighttime equatorial F region irregularities at the African region where there is paucity of data.

An Atmospheric Visual Analysis and Exploration System

Meteorological research involves the analysis of multi-field, multi-scale, and multi-source data sets. In order to better understand these data sets, models and measurements at different resolutions must be analyzed. Unfortunately, traditional atmospheric visualization systems only provide tools to view a limited number of variables and small segments of the data. These tools are often restricted to two-dimensional contour or vector plots or three-dimensional isosurfaces. The meteorologist must mentally synthesize the data from multiple plots to glean the information needed to produce a coherent picture of the weather phenomenon of interest. In order to provide better tools to meteorologists and reduce system limitations, we have designed an integrated atmospheric visual analysis and exploration system for interactive analysis of weather data sets. Our system allows for the integrated visualization of 1D, 2D, and 3D atmospheric data sets in common meteorological grid structures and utilizes a variety of rendering techniques. These tools provide meteorologists with new abilities to analyze their data and answer questions on regions of interest, ranging from physics-based atmospheric rendering to illustrative rendering containing particles and glyphs. In this paper, we will discuss the use and performance of our visual analysis for two important meteorological applications. The first application is warm rain formation in small cumulus clouds. Here, our three-dimensional, interactive visualization of modeled drop trajectories within spatially correlated fields from a cloud simulation has provided researchers with new insight. Our second application is improving and validating severe storm models, specifically the Weather Research and Forecasting (WRF) model. This is done through correlative visualization of WRF model and experimental Doppler storm data.

Convective Ionospheric Storms: A Major Space Weather Problem

Convective Ionospheric Storms: A Major Space Weather Problem

Numerical models of the E-region ionosphere

Although the ambient E-region of the ionosphere has been fairly well characterized in the non-auroral regions with respect to density, ion composition, and variability, it is not as well understood as might be expected from a theoretical point of view. This is due primarily to uncertainties in the solar irradiance that produces E-region ionization (mainly the H Lyman-β line and the soft X-ray region), and to the variability of odd-nitrogen species that control ion composition. Recent measurements from the SNOE, TIMED, and SORCE satellites have greatly improved our quantitative understanding of both solar spectral irradiance and lower thermosphere nitric oxide, which in turn improves our ability to accurately model the E-region. The new measurements indicate higher solar soft X-ray irradiance but lower flux in the H Lyman-β line than earlier estimates by Hinteregger et al. [Hinteregger, H.E., Fukui, K., Gilson, B.R., Observational, reference, and model data on soalr EUV, from measurements on AE–E. Geophys. Res. Lett. 8 (1981) 1147] based on measurements from Atmosphere Explorer and from rocket flights. However, preliminary results from the Solar EUV Experiment (SEE) on the TIMED satellite are in better agreement with the EUVAC model [Richards, P.G., Fenenelly, J.A., Torr, D.G., EUVAC: a solar EUV flux model for aeronomic calculations. J. Geophys. Res. 99 (1994) 8981–8992]. This raises the question of whether either of these representations of the solar spectrum can adequately represent E-region electron density and variation. Model simulations of the E-region using these two solar model inputs are compared to the International Reference Ionosphere (IRI) empirical model, which has been validated using measurements from the Boulder ionosonde. Calculations using the Hinteregger et al. solar inputs significantly underestimate the peak electron densities at all levels of activity, while using EUVAC solar inputs produces better agreement but slightly more variability than the empirical representations. As improved estimates of variability from TIMED/SEE over a complete solar cycle become available, these remaining discrepancies may be resolved.

An empirical topside electron density model for calculation of absolute ion densities in IRI

An empirical model of the electron density in the topside ionosphere for solar maximum and minimum is introduced. This model is intended for use in combination with the relative ion composition (TTS) model that will be included in the forthcoming version of the IRI code to calculate the absolute ion composition in the topside ionosphere. The validation of the model using satellite data from five different satellites for different altitudes and high and medium solar activity is presented. The proposed model is based on the Langmuir Probe measurements onboard the Atmosphere Explorer satellites for low solar activity and Retarding Potential Analyzer measurements onboard the Intercosmos satellites for high solar activity. Examples of vertical profiles of the absolute density of the most important ions following from the mentioned models are shown.

Manifestation of solar activity in the global topside ion composition − a study based on satellite data

Abstract. The solar cycle variation of the most important parameters characterizing the ion composition in the topside ionosphere is studied. For this purpose data from the ACTIVE mission (the IK-24 satellite) for the maximum of solar cycle 22 (aver F10.7~200), complemented by data available from the Atmosphere Explorer (AE) satellites, for the minimum of solar cycle 21 (average F10.7~85), were processed. OGO-6 data from the low maximum of solar cycle 20 (average F10.7~150) were used for medium solar activity conditions. The results for the equinox from the recently developed empirical model of ion composition are analyzed and presented, and typical vertical profiles from solar maxima and minima are shown. It was found that the logarithm of the O+, H+, He+, and N+ densities in the topside ionosphere at a fixed altitude, latitude, and local time is, in the first approximation, a linear function of solar activity characterized by the daily F10.7. On the other hand, the upper transition height is generally a non linear function of the daily F10.7, the deviation from linear dependence increases with latitude. Keywords. Ionosphere (Plasma temperature and density; Ion chemistry and composition; Modeling and forecasting)

HEUVAC: A new high resolution solar EUV proxy model

This paper presents a new high-resolution version of the solar EUV irradiance model for aeronomic calculations (HEUVAC) that is designed to facilitate comparisons with measured spectra and enable more accurate calculations of ionization rates, airglow emission rates, and photoelectron calculations. The HEUVAC model bins can range from 0.1 to 100 nm and extends the EUV model below 5 nm. The new solar EUV irradiance calculations with the high resolution irradiance model show good agreement with the most recent solar EUV irradiance measurements from the solar EUV experiment (SEE) instrument on the thermosphere, ionosphere, mesosphere, energetics, and dynamics satellite. Also, photoelectron fluxes calculated from both the SEE measured and EUVAC modeled solar EUV irradiances agree well with photoelectron flux measurements by the FAST satellite. The good agreement of the EUVAC and SEE derived photoelectron fluxes with the FAST measured fluxes at solar maximum lends support to an earlier finding that the previous reference solar EUV irradiances from the Atmosphere Explorer measurements need to be adjusted upward by a factor of 2–3 below 25 nm wavelength. This result is important for remote sensing of the ionosphere and thermosphere because, as this paper shows, the airglow emission rates calculated using the SEE and HEUVAC models are 50% higher than those based on earlier solar EUV irradiance models. The calculations also show that for solar maximum conditions on 21 April 2002, most of the degradation of the escaping photoelectron flux takes place below 1000 km altitude.