Teemu Makinen

LYMAN-ALPHA OBSERVATIONS OF COMET HYAKUTAKE WITH SWAN ON SOHO

Abstract The SWAN instrument on board SOHO is a Lyman-α (Lα) photometer able to map the sky intensity with a resolution of 1°, and a capability of microstepping (0.1°). SWAN is primarily devoted to the study of the large scale distribution of solar wind from its imprints on the interplanetary sky background, but was in addition extensively used to map the Lα emission of several comets since its launch in December 1995. Here we report observations of comet C/1996 B2 (Hyakutake). Its Lα emission cloud extended over more than 60° while approaching the Earth at 0.102 AU. A comparison with a simple model allowed hydrogen and H2O production rates to be derived, while the comet approached closer to the Sun from 1.12 AU to 0.53 AU distance to the Sun, pre-perihelion. The derived H2O production rate was found in fair agreement with other derivations (IUE and ground-based in the IR and UV), validating the Lα method. The H2O production by SWAN was related to several other measurements of minor constituents in order to derive new values of abundance of CO, HCN, H2CO, CH3OH and CH3CN. Most important, the D H ratio in comet Hyakutake is now found at 3 × 10−4, as in comet Halley, while a previous estimate based on a wrong H2O number had indicated a value twice lower, with important cosmogonic consequences. The time evolution showed a fast surge on 21 March, coinciding with the time of fragmentation of the nucleus as detected 3 days later at Pic du Midi. This surge is also confirmed by the detailed comparison of H column densities (observed vs model) as a function of the distance to the nucleus, showing a larger ratio in the inner region (younger atoms) than in the outer region (older atoms) on 21 March, and then a progressive filling-in of the H envelope. After the surge, there was a plateau for 16 days around 1.8 × 1029 H2O mol s−1, and then an increase following approximately a R−2 law. This behavior is interpreted as the surge and plateau corresponding to the fragmentation and total disruption/evaporation of a fragment of the nucleus, of approximately 200 m. Finally, it is argued that the first detection of ethane C2H6 in this comet (IR observations) might have been the result of the special circumstances (a large fragment disrupted very near the Earth) rather than revealing a new special class of ethane-rich comets as argued by other authors.

Mars Science Laboratory relative humidity observations: Initial results

The Mars Science Laboratory (MSL) made a successful landing at Gale crater early August 2012. MSL has an environmental instrument package called the Rover Environmental Monitoring Station (REMS) as a part of its scientific payload. REMS comprises instrumentation for the observation of atmospheric pressure, temperature of the air, ground temperature, wind speed and direction, relative humidity (REMS-H), and UV measurements. We concentrate on describing the REMS-H measurement performance and initial observations during the first 100 MSL sols as well as constraining the REMS-H results by comparing them with earlier observations and modeling results. The REMS-H device is based on polymeric capacitive humidity sensors developed by Vaisala Inc., and it makes use of transducer electronics section placed in the vicinity of the three humidity sensor heads. The humidity device is mounted on the REMS boom providing ventilation with the ambient atmosphere through a filter protecting the device from airborne dust. The final relative humidity results appear to be convincing and are aligned with earlier indirect observations of the total atmospheric precipitable water content. The water mixing ratio in the atmospheric surface layer appears to vary between 30 and 75 ppm. When assuming uniform mixing, the precipitable water content of the atmosphere is ranging from a few to six precipitable micrometers. Key Points Atmospheric water mixing ratio at Gale crater varies from 30 to 140 ppm MSL relative humidity observation provides good data Highest detected relative humidity reading during first MSL 100 sols is RH75%

Swan Observations of the Solar Wind Latitude Distribution and Its Evolution Since Launch

SWAN is the first space instrument dedicated to the monitoring of the latitude distribution of the solar wind by the Lyman alpha method. The distribution of interstellar H atoms in the solar system is determined by their destruction during ionization charge-exchange with solar wind protons. Maps of sky Ly-α emission have been recorded regularly since launch. The upwind maximum emission region deviates strongly from the pattern that would be expected from a solar wind that is constant with latitude. It is divided in two lobes by a depression aligned with the solar equatorial plane, called the Lyman-alpha groove, due to enhanced ionization along the neutral sheet where the slow and dense solar wind is concentrated. The groove (or the anisotropy) is more pronounced in 1997 than in 1996, but it then decreases between 1997 and 1998.

Preliminary retrieval of solar wind latitude distribution from Solar Wind Anisotropies/SOHO observations

The Solar Wind Anisotropies (SWAN) instrument on board the SOHO spacecraft measures Lyman alpha radiation emanating mainly from neutral hydrogen gas in the solar neighborhood. This gas is part of the local interstellar cloud in which the Sun and the heliosphere are immersed. Measurements of Lyman alpha can be used to infer the local cloud characteristics like the velocity and the direction of the flow, gas temperature, and density. The strong interaction between the Sun and the neutral hydrogen gas also makes possible investigations of solar characteristics by Lyman alpha measurements. In this work we will concentrate on deriving the latitudinal distribution of solar-induced ionization from SWAN Lyman alpha maps measured in 1996 at a time of the solar minimum. From the ionization we derive the distribution of the solar wind mass flux. SWAN Lyman alpha data show that the ionization and the mass flux are nearly flat for all solar latitudes except the narrow belt from −20° to 20° around the solar equator. In this region, ionization and the solar wind mass flux show a definite increase, which can be seen as an intensity depression in the Lyman alpha data from directions near the ecliptic. These results confirm earlier in situ measurements by Ulysses during the present minimum and Lyman alpha measurements by Prognoz satellites 20 years ago.

LYMAN-ALPHA OBSERVATIONS OF COMET 46 P/WIRTANEN WITH SWAN ON SOHO: H2O PRODUCTION RATE NEAR 1997 PERIHELION

Abstract The SWAN instrument on board SOHO is a Lyman-α photometer able to map the sky intensity with a resolution of 1°, primarily devoted to the study of the large scale distribution of solar wind from its imprints on the interplanetary sky background. In addition SWAN was extensively used to map the Lyman α emission of several comets since launch in December 1995. Here we report observations of Comet 46 P/Wirtanen near perihelion. From the recorded Lyman α intensity the H 2 O production rate was derived for 45 observations from 21 December 1996–17 May 1997, with a peak of 1.6±0.4×10 28 mol/s just before perihelion. This should help to constrain the physical models of 46 P/Wirtanen for Rosetta mission planning purposes.

Scientific objectives and implementation of the Pressure Profile Instrument (PPI⧹HASI) for the Huygens spacecraft

Abstract The Huygens entry probe will be deployed into the Titan atmosphere by the Cassini spacecraft. During the 3 h descent the Huygens Atmospheric Structure Instrument (HASI) will observe a comprehensive set of variables and phenomena, encompassing pressure, temperature, density and atmospheric electricity. The Titan atmospheric vertical pressure profile will be recorded by the Pressure Profile Instrument (PPI:HASI) provided by the Finnish Meteorological Institute (FMI). The principal sections of the PPI are:• sensor boom extending out of the Huygens main body, • Kiel probe with pitot tube in the end of the sensor boom,• pressure sensors (Barocap ®) inside the Huygens body, and• pressure hose conveying the pressure signal from the Kiel probe to the pressure sensors.The decision to measure total pressure instead of static pressure and the design of the Kiel probe was based on aerodynamic simulations. Simulations were performed for the airflow around the Huygens probe and in the vicinity of the tip of the sensor boom. During the descent the Huygens probe is constantly changing its attitude. Hence a pitot tube alone would not give a reliable pressure reading. By using the Kiel probe the total pressure reading is insensitive to the angle between the streamlines and the Kiel probe up to 45°. The PPI uses pressure sensors with three different sensitivities to cover the pressure range of 0-180 kPa. The sensor technology is a heritage from a concept that has been applyed in earlier space and terrestrial applications. The PPI starts measurements at an altitude of 160 km, producing 28 bits of data per second. Measurements are designed to continue beyond the time of impact on the surface of Titan until Huygens stops operating. The flight unit has been integrated to the Huygens entry probe and tests have been successful. A special balloon test session of PPI and other HASI instruments simulating the actual mission of Huygens was carried out successfully in 1995.

Ppi results from the balloon drop experiment of the hasi pressure profile instrument

Abstract At December 1995 a balloon drop experiment was conducted at Leon, Spain, for the HASI (Huygens Atmospheric Structure Instrument) instrument of the Huygens probe. A part of HASI is the Pressure Profile Instrument (PPI) which will measure the atmospheric pressure profile of Titan during the descent at November 2004. The experiment platform was carried by a balloon to an altitude of 30 km and it made a subsequent parachute–assisted descent. The pressure instrument functioned basically as expected. The vertical flight trajectory and pressure profile was reconstructed by using the collected data of the pressure instrument and the simultaneous temperature measurements. The calculated flight trajectory was in agreement with independent measurements with Omega and GPS. Some turbulence was detected near the surface and other dynamic behaviour in the upper part of the trajectory. The experiment demonstrated the nominal performance of the PPI instrument and serves as a real-like test flight for the actual mission.