E. Kyrölä

Interplanetary Lyman α line profiles derived from SWAN/SOHO hydrogen cell measurements: Full‐sky Velocity Field

We present here an analysis of 1 year of data obtained by the solar wind anisotropies (SWAN) instrument on board the SOHO spacecraft orbiting around the Ll Lagrange point at 1.5 × 106 km sunward from Earth. This instrument is measuring the interplanetary Lyman α background due to solar photons backscattered by hydrogen atoms in the interplanetary medium. The interplanetary (IP) Lyman a line profile reflects the velocity distribution of H atoms projected onto the line of sight (LOS). Here we apply a new profile reconstruction technique using data from the two hydrogen absorption cells included in the SWAN instrument. For a LOS in a fixed celestial direction, the Doppler shift between the interplanetary emission profile and the H cell absorption profile varies by up to ±0.12 A during 1 year, owing to the Earths orbital velocity around the Sun, equal to 30 km s−1. Such a Doppler spectral scan across the emission line allows us to derive Lyman α line profiles, and hence the velocity distribution, in and out of the ecliptic independent of any modeling of the neutral hydrogen atom distribution in the heliosphere or of the multiple scattering of solar photons. The spatial distribution of the apparent velocity relative to the Sun as observed from the orbit of SOHO is derived for all directions, except within 5° of the ecliptic poles. This determination strongly constrains models of the interaction of the interstellar hydrogen with the solar wind. New estimates of the upwind direction (252.3° ± 0.73° and 8.7° ± 0.90° in J2000 ecliptic coordinates) show a small discrepancy by 3° – 4° with the direction of the helium flow, perhaps connected with an asymmetry of the heliosphere induced by the interstellar magnetic field. We find that the apparent velocity relative to the sun in the upwind direction is −25.4 ± 1 km/s, whereas it is equal to 21.6 ± 1.3 km s−1 in the downwind direction. A preliminary analysis shows that the Zero Doppler shift cone and the difference between the upwind and downwind velocities correspond to a ratio μ of Lyman α radiation pressure to solar gravity of 0.9–1.0. It follows that the observed upwind apparent velocity is compatible with a velocity at infinity of H atoms of the order of 21–22 km s−1. However, extensive modeling is required in order to get more definite conclusions. The velocity map presented here is the first ever obtained. For this reason, we discuss in detail the Doppler spectral scan method and the H cell data.

First simultaneous global measurements of nighttime stratospheric NO2 and NO3 observed by Global Ozone Monitoring by Occultation of Stars (GOMOS)/Envisat in 2003

The Global Ozone Monitoring by Occultation of Stars (GOMOS) stellar occultation instrument on board the Envisat European satellite provides global coverage of ozone and other stratospheric species with good vertical resolution and a self-calibrating method. In this paper we present the first simultaneous global distribution of stratospheric NO 2 and NO 3 from 1 year of nighttime GOMOS data in 2003. Most previous NO 2 satellite observations have been made using the solar occultation technique. They are difficult to interpret due to the fast photochemical evolution of NO 2 at sunrise and sunset. There are no published observations of NO 3 from space because this constituent is rapidly photodissociated during daytime and is not observable by solar occultation. It is shown that the NO 2 mixing ratio reaches a maximum around 40 km with values between 14 and 16 ppbv at low and middle latitudes. The global distribution of NO 2 observed by GOMOS is very similar to the NO + NO 2 Halogen Occultation Experiment climatology deduced from sunset measurements from 1999 to 2004. At high latitude a high mixing ratio is observed in the north vortex in November 2003 after a strong solar proton event and in the south vortex in July 2003. The NO 3 mixing ratio peaks at 40–45 km. NO 3 follows a semiannual variation at low latitudes with maxima at equinoxes and an annual variation at middle and high latitudes with a maximum in summer. In the upper stratosphere the mixing ratio of NO 3 is strongly correlated with temperature due to the thermal dependence of its formation rate. Citation: Hauchecorne, A., et al. (2005), First simultaneous global measurements of nighttime stratospheric NO 2 and NO 3 observed by Global Ozone Monitoring by Occultation of Stars (GOMOS)/Envisat in 2003

Comets in full sky

The SWAN instrument onboard the SOHO spacecraft is a Lyman α scanning photometer cabable of mapping the whole sky with

\mathsf{L_{\alpha}}

1\degr

maps of the SWAN instrument - I. Survey from 1996 to 1998

resolution. Since January 1996 the instrument has produced on average three full sky maps a week with the principal scientific objective of observing the distribution of heliospheric neutral hydrogen. In addition, these systematic observations are a valuable source for studying comets brighter than a visual magnitude of 7-11, the observing limit depending on the abundance ratios of produced radicals and the location of the comet relative to the galactic plane. When the data before the temporary loss of control of SOHO at the end of June 1998 were processed, altogether 18 comets were positively identified, of which one is a new discovery and another 5 can be detected on SWAN images before their actual discovery date. This demonstrates the feasibility of SWAN as an instrument for cometary surveys. The observations are used to estimate the water production rates of the detected comets near their perihelion passages.

Abel Integral Inversion in Occultation Measurements

Occultation geometry under a spherical symmetry assumption leads to models described by the Abel integral equations. Analyzing general properties of the Abel transform, this work derives practical rules for discretization and for solution of the inverse problems, containing Abel-type integral equations. Two applications in remote sensing are considered: reconstruction of local densities from horizontal column densities (vertical inversion) in absorptive stellar occultation measurements and reconstruction of air density from refractive angle measurements. In the case of continuous functions, it is shown that the vertical inversion problem is ill-posed: small errors in measurements may cause errors of arbitrary size in retrieved quantities. The refractivity reconstruction problem is well posed: a noise in measurements is smoothed in inversion. In the reality of a finite number of measurements, the inverse problems can be made even-determined by discretization. The difficulties in discretization of the Abel-type integrals are the weak singularity at the lower limit and the upper limit initialization. Possible solutions to these problems are discussed together with different discretization schemes. The amplification of error coefficient is used as a criterion of ill- or well-posedness of the problems. Together with the averaging kernel, it also characterizes quality of the discretization schemes. For the vertical inversion problem, three matrix inversions: standard onion peeling, onion peeling with quadratic interpolation and discretization by trapezoidal rule in pole formulation are compared with discretization of inverse Abel transform. For refractivity inversion, the discretized inverse Abel transform is compared with two matrix inversions. Necessity of regularization for the considered inverse problems is also discussed.

The SPARC Data Initiative: A comparison of ozone climatologies from international satellite limb sounders

A comprehensive quality assessment of the ozone products from 18 limb-viewing satellite instruments is provided by means of a detailed intercomparison. The ozone climatologies in form of monthly zonal mean time series covering the upper troposphere to lower mesosphere are obtained from LIMS, SAGE I/II/III, UARS-MLS, HALOE, POAM II/III, SMR, OSIRIS, MIPAS, GOMOS, SCIAMACHY, ACE-FTS, ACE-MAESTRO, Aura-MLS, HIRDLS, and SMILES within 1978–2010. The intercomparisons focus on mean biases of annual zonal mean fields, interannual variability, and seasonal cycles. Additionally, the physical consistency of the data is tested through diagnostics of the quasi-biennial oscillation and Antarctic ozone hole. The comprehensive evaluations reveal that the uncertainty in our knowledge of the atmospheric ozone mean state is smallest in the tropical and midlatitude middle stratosphere with a 1σ multi-instrument spread of less than ±5%. While the overall agreement among the climatological data sets is very good for large parts of the stratosphere, individual discrepancies have been identified, including unrealistic month-to-month fluctuations, large biases in particular atmospheric regions, or inconsistencies in the seasonal cycle. Notable differences between the data sets exist in the tropical lower stratosphere (with a spread of ±30%) and at high latitudes (±15%). In particular, large relative differences are identified in the Antarctic during the time of the ozone hole, with a spread between the monthly zonal mean fields of ±50%. The evaluations provide guidance on what data sets are the most reliable for applications such as studies of ozone variability, model-measurement comparisons, detection of long-term trends, and data-merging activities.

SWAN/SOHO H cell measurements: The first year

Abstract We present here an analysis of one year of data obtained by the SWAN instrument on board the SOHO spacecraft orbiting around the L1 Lagrange point at 1.5 × 10 6 km sunward from Earth. This instrument is measuring the interplanetary Lyman α background due to solar photons backscattered by hydrogen atoms in the interplanetary medium. The interplanetary (IP) Lyman α line profile reflects the velocity distribution of H atoms, projected on the line of sight (LOS). Here, we apply a new profile reconstruction technique using data from the two hydrogen absorption cells included in the SWAN instrument. New estimates of the upwind direction (252.3° ± 0.73° and 8.7° ± 0.90° J2000 ecliptic) show a small discrepancy by 3° to 4° with the direction of the helium flow, perhaps connected with an asymmetry of the heliosphere induced by the interstellar magnetic field. We find that the apparent velocity relative to the sun in the upwind direction is −25.4 ± 1 km/s, whereas it is equal to 21.6 ± 1.3 km/s in the downwind direction.

Does a Priori Information Improve the Retrievals of Stellar Occultation Measurements

Stellar occultation technique provides global measurements of the atmospheric composition with a high vertical resolution. The main interest of the GOMOS (Global Ozone Monitoring by Occultation of Stars) instrument is to measure ozone density at 10–100 km. The correlations between the cross sections of ozone, neutral density and aerosols are rather strong in the UV/visible wavelength region, which is typically used in the stellar occultation measurements. Due to this correlation also the errors of the retrieved quantities are correlated. Therefore, improvements in, e.g., neutral density retrieval will also improve ozone retrievals. We will here discuss how the ozone and aerosol retrievals can be improved by introducing prior information for the neutral density. The inverse problem rising in stellar occultation measurements is ill-posed in the sense that there is a discrete set of measurements but continuous gas density profiles are wanted. Formally the problem can be transformed to a well-posed problem by simply discretizing the atmosphere into equally many layers as there are measurements and assuming, e.g., constant density inside each layer. We discuss here the advantages of requiring stronger smoothness for the gas profiles by using the Tikhonov regularization method.

Atmospheric Density, Pressure and Temperature Profile Reconstruction from Refractive Angle Measurements in Stellar Occultation

Stellar occultation instruments have high pointing accuracy and they follow point-like sources. These two features allow accurate measurements of the refractive angle in the limb viewing geometry. The determination of the stratospheric density and temperature profiles from the refractive-angle measurements by stellar occultation instruments is considered. The temperature reconstruction consists of the following steps. First, refractivity is reconstructed from the refractive angle measurements using inversion of the Abel-type integral. Refractivity is connected with air density via the Edlen formula. Then the pressure profile is calculated using the hydrostatic equation. Finally, the temperature profile is determined from the density and pressure data using the equation of state of a perfect gas. The error analysis was performed by the Monte-Carlo simulations with additive Gaussian noise. Main error sources are identified and sensitivity of the inverse procedure to them is studied. The accuracy attainable in the temperature profiling with the present design of the stellar occultation instruments is analyzed.