Tuula Summanen

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.

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.

Latitudinal distribution of solar wind as deduced from Lyman alpha measurements: An improved method

In this work we examine the possibility of deducing the latitudinal distribution of the solar ionization rate using Prognoz 6 Lyman alpha data in a more general and flexible way than previously examined. Using so-called hot model for the hydrogen distribution and the optically thin model for the resonance scattering, theoretical Lyman alpha intensity for the interstellar hydrogen is calculated and compared with the intensity data measured by Prognoz 6. Varying the latitudinal dependence of the ionization rate, the distributions, which produce the best fit with the data, are analyzed for four different measuring sessions. As a result, we get four ionization rate distributions that have two common features. The ionization rate is enhanced near the solar equator, and large broad plateaus exist around heliographic latitudes ±30° to ±70°. The latitudinal distribution of the average ionization rate about the solar minimum deviates clearly from the spherically symmetric and sinusoidally (harmonically) with the latitude-varying models used so far. The growth of the solar wind mass flux from the solar polar areas toward the equator corresponds to the earlier results concluded from Lyman alpha measurements. The method used in this work allows a higher latitudinal resolution of the ionization rates. However, there are several uncertainties both in the simulations and in the measurements. The exclusion of time-dependent effects as well as multiple scattering requires that the results be considered only suggestive.

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.

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.

Solar wind proton flux latitudinal variations: Comparison between Ulysses in situ data and indirect measurements from interstellar Lyman α mapping

We compare the solar wind proton flux latitude dependence derived in the past from the interstellar neutral H distribution in the inner heliosphere from Lyman α observations with recent in situ solar wind observations by Ulysses [Goldstein et al., 1995, Phillips et al., 1995, 1996, J.L. Phillips, private communication, 1996]. We find common features, such as a significant proton flux decrease with increasing heliographic latitude (about 30%) in the low-latitude regions and broad plateaus of low particle fluxes ( 2.0 - 2.5 x 10 8 protons cm -2 s -1 ) around the poles. We use our model of interstellar H distribution under the influence of a multiparameter, latitude dependent solar wind to investigate the effects of a solar wind distribution matching as closely as possible the south heliographic Ulysses observations. For the first time, multiple scattering is included in such an anisotropic model. The Ulysses-type wind is found to produce a secondary minimum of Lyman α intensity in the upwind direction, something already observed near solar minimum of activity. However, the modeled feature has a larger amplitude than the observed one, probably an indication of smoothing due to the combination of solar rotation and waviness of the neutral sheet. The total solar wind particle flux and the full Sum-averaged ionization rate of the interstellar neutral H are estimated in various cases. For identical equatorial and polar fluxes, the existence of broad plateaus results in a significant reduction of the average neutral H ionization (and then of the ionization cavity) when comparing with models using a classical harmonic dependence with latitude. As a result, the downwind cavity is less depleted. This may partially explain some discrepancies between the expected and observed Lyman α emissions from the interplanetary hydrogen cavity, in particular, the excess of emission from the downwind cavity compared with the classical model.

Discovery of a comet by its Lyman-alpha emission

Several searches for near-Earth objects have recently been initiated, as a result of increased awareness of the hazard of impacts on the Earth. These programs mainly search for asteroids, so amateur astronomers can still contribute to the discovery of comets, especially out of the orbital plane of the Solar System. An ideal way to search for comets would be to use a spaceborne instrument capable of imaging the whole sky on a daily basis in a systematic and repeatable way. Such an instrument already exists on the solar observatory SOHO; it operates at the Lyman-α wavelength of neutral hydrogen, which is the main component of the emission cloud of a comet. Here we report the discovery, using archival data from this satellite, of a hitherto unnoticed comet which reached a perihelion of 1.546 a.u. on 26 June 1997. We derive the water production rate of the comet as a function of time and find that it increases after perihelion, like that of comet Halley.

The Solar Ionization Rate of the Interplanetary Hydrogen as a Function of a Heliomagnetic Latitude: A New Model for the Interplanetary Lyman Alpha Studies

In this work we have modelled the solar wind proton flux which varies as a function of distance to the heliomagnetic equator and its effects on the interplanetary Lyman α radiation. The results imply that a groove observed in Lyman α intensity patterns toward the upwind direction Bertaux et al. disappears when the tilt angle of the heliomagnetic equator is larger than 20°.The observations by Bertaux et al. were measured during the solar wind minimum when the tilt angle of the streamer belt is low. During the solar wind maximum when the tilt angle of the streamer belt is large the Lyman α groove should disappear according to our results.