H. S. Ogawa

First Solar EUV Irradiances Obtained from SOHO by the CELIAS/SEM

The first results obtained with the Solar EUV Monitor (SEM), part of the Charge, Element, and Isotope Analysis System (CELIAS) instrument, aboard the SOlar and Heliospheric Observatory (SOHO) satellite are presented. The instrument monitors the full-disk absolute value of the solar He II irradiance at 30.4 nm, and the full-disk absolute solar irradiance integrated between 0.1 nm and 77 nm. The SEM was first turned on December 15, 1995 and obtained ‘first light’ on December 16, 1995. At this time the SOHO spacecraft was close to the L-l Lagrange point, 1.5 x 106 km from the Earth towards the Sun. The data obtained by the SEM during the first four and a half months of operation will be presented. Although the period of observation is near solar minimum, the SEM data reveal strong short-term solar irradiance variations in the broad-band, central image channel, which includes solar X-ray emissions.

First‐year continuous solar EUV irradiance from SOHO by the CELIAS/SEM during 1996 solar minimum

The CELIAS/SEM photodiode spectrometer aboard SOHO continuously monitors the full-disk EUV solar irradiance in an 8-nm wavelength band centered at 30.4 nm (first order), and in a broad wavelength band between 0.1 and 77 nm (central order). We present the absolute solar EUV irradiances for the 1996 solar EUV minimum year at 1 AU. The uncertainty in absolute flux for each channel is approximately ± 14%. The accuracy and stability of the instrument make the data extremely useful in modeling the upper terrestrial atmosphere during this period of “low” solar activity. The data show evidence of persistent solar EUV/soft X ray active regions throughout this solar minimum period which give rise to both 27-day and short-term (minutes to hours) solar EUV irradiance variations. The lowest value of solar flux in the first order 30.4-nm band occurred on November 6, 1996, with a photon flux of 9.8 × 109 cm−2 s−1. Using previously obtained solar spectra, we infer a photon flux of 4.7 × 109 cm−2 s−1 within a 1-nm bandpass centered on the solar He II 30.4-nm emission line at this time. The irradiance variation of the first order channel was between +15% and −10% as measured from a smoothed quadratic least squares fit to the entire first-order channel database for 1996. The lowest central-order EUV photon flux occurred on the same day (November 6, 1996) with an absolute flux of 2.2 × 1010 cm−2 s−1. When sharp increases of short-term flux variability are ignored, a variation between +45% and −30% from the smoothed least squares fit to the central-order database is obtained. The long-term solar cycle valuation during the 12-month smoothed data in both channels indicates that the solar EUV minimum was reached during mid 1996. Large short-term sudden increases monitored by both channels correspond to solar flares observed from the ground and from the GOES satellites. New data for two isolated flares obtained from both CELIAS/SEM channels are also presented and compared with GOES 0.1 to 0.8-nm soft X ray data.

The SOHO CELIAS/SEM EUV database from SC23 minimum to the present

Abstract The SOHO Solar EUV Monitor has been in operation since December 1995 onboard the SOHO spacecraft. This instrument is a highly stable transmission grating solar extreme ultraviolet spectrometer. It has made nearly continuous full disk solar irradiance measurements both within an 8 nm bandpass centered at 30.4 nm and throughout the 0.1 to 50 nm solar flux region since launch. The 30.4 nm flux, the 0.1 to 50 nm flux and the extracted soft X-ray (0.1 to 5 nm) flux are presented and compared with the behavior of solar proxies.

Evidence of a nearby solar wind shock as obtained from distant Pioneer 10 ultraviolet glow data

Pioneer 10 UV photometric data are used to study the nature of the heliospheric boundary interactions between solar wind and inflowing partially ionized interstellar wind and the interstellar magnetic field. The radial intensity dependence of solar Ly-alpha photons backscattered by inflowing interstellar hydrogen atoms is observed. The data in the downstream direction at distances greater than about 37 AU show a distortion of the incoming hydrogen atom density profile. It is suggested that this is characteristic of a nearby solar wind shock probably located about 50 AU from the sun in January 1987. 58 references.

Absolute solar 30.4 nm flux from sounding rocket observations during the solar cycle 23 minimum

A transmission grating extreme ultraviolet (EUV) spectrometer, nominally identical to the Charge, Element, and Isotope Analysis System/Solar EUV Monitor (CELIAS/SEM) instrument on the Solar and Heliospheric Observatory (SOHO), has obtained accurate measurements of the integrated absolute solar extreme ultraviolet irradiance in an 8 nm band pass centered at 30.4 nm. The spectrometer also measured the EUV/soft X-ray flux, but those data will be reported in a later paper. The instrument was launched on two sounding rocket flights from White Sands Missile Range, New Mexico, on June 26, 1996, and again on August 11, 1997, to provide a SOHO underflight calibration database in the EUV. The full disk solar 30.4 ± 40 nm fluxes measured by it on the above 2 days were 1.21 × 1010 and 1.42 × 1010 photons cm−2 s−1 at 1 AU, respectively. These measurements have an absolute 1σ uncertainty of 8.1%.

Normal incidence spectrophotometer with high-density transmission grating technology and high-efficiency silicon photodiodes for absolute solar extreme-ultraviolet irradiance measuremen

New developments in transmission grating and photodiode technology now make it possible to realize spectrometers in the extreme ultraviolet (EUV) spectral region (λ<1000A), which are expected to be virtually constant in their diffraction and detector properties. Time-dependent effects associated with reflection gratings are eliminated through the use of free-standing transmission gratings. These gratings together with recently developed and highly stable EUV photodiodes have been utilized to construct a highly stable normal incidence spectrophotometer to monitor the variability and absolute intensity of the solar 304-A line. Owing to its low weight and compactness, such a spectrometer will be a valuable tool for providing absolute solar irradiance throughout the EUV. This novel instrument will also be useful for cross-calibrating other EUV flight instruments and will be flown on a series of Hitchhiker shuttle flights and on SOHO. A preliminary version of this instrument has been fabricated and characterized, and the results are described.

Direct evidence of the interstellar gas flow velocity in the pickup ion cut‐off as observed with SOHO CELIAS CTOF

He+ pickup ions as observed with SOHO CELIAS CTOF have been analyzed for the time period DOY 160–190, 1996. During this time of the year the Earth is on the upwind side of the interstellar gas flow with respect to the sun. The high-speed cut-off in the frame of the sun is significantly higher v/Vsw = 2, predicted for pickup ions. The difference increases with lower solar wind speeds. This behavior is interpreted as an effect of the local interstellar gas flow velocity (inflow at large distances including gravitational acceleration by the sun) on the pickup ion distribution. The neutral velocity is added to the solar wind velocity in the determination of the pickup ion cut-off on the upwind side and subtracted on the downwind side of the gas flow. This new observation will provide a valuable tool to determine the interstellar gas flow and will thus complement direct neutral gas measurements.

The hydrogen density of the local interstellar medium and an upper limit to the Galactic glow determined from Pioneer 10 ultraviolet photometer observations

Pioneer 10 Lyman-alpha data obtained beyond the heliocentric distance of 30 AU are used here to determine the local ISM hydrogen density and downwind Galactic glow. The resulting asymptotic interstellar hydrogen density obtained using a conventional interplanetary model is 0.06/cu cm, and the Galactic Lyman-alpha glow is found to be negligible in the downwind direction. Discrepancies are found between observations and the conventional theoretical predictions of the glow dependence on radial distance. Based solely on data obtained between 30 and 39 AU, a better fit is obtained with a constant density distribution beyond 30 AU. In this case, a density of 0.05/cu cm and Galactic glow of 1.5 R is obtained. These results suggest that a complex density distribution may be more appropriate than either the conventional exponential distribution or the constant distribution. 43 references.

Magnesium isotopic composition as observed with the CELIAS/MTOF experiment on the SOHO spacecraft

Solar wind abundance ratios of magnesium isotopes measured with the high resolution Mass Time-of-Flight spectrometer (MTOF) of the Charge, Element, and Isotope Analysis System (CELIAS) experiment on board the Solar and Heliospheric Observatory (SOHO) are presented. MTOF, as part of CELIAS, is, because of its high time and mass resolution, an excellent tool for isotope abundance measurements in the solar wind. From the data analysis we have found that the isotopic composition of magnesium in the solar wind agrees with the terrestrial composition within the experimental uncertainty. We have obtained isotopic ratios of 24Mg/25Mg = 7.7 ± 0.4 and 24Mg/26Mg = 7.0 ± 0.5. These values are consistent with the terrestrial values of 24Mg/25Mg = 7.90 ± 0.01 and 24Mg/26Mg = 7.17 ± 0.03. Furthermore, these investigations also show that with the given uncertainties the abundance ratios do not vary significantly within a solar wind velocity range from 375 km/s to 530 km/s.

Solar photoionization as a loss mechanism of neutral interstellar hydrogen in interplanetary space

Two primary loss mechanisms of interstellar neutral hydrogen in interplanetary space are resonance charge exchange ionization with solar wind protons and photoionization by solar EUV radiation. The later process has often been neglected since the average photoionization rate has been estimated to be as much as 5 to 10 times smaller than the charge exchange rate. These factors are based on ionization rates from early measurements of solar EUV and solar wind fluxes. Using revised solar EUV and solar wind fluxes measured near the ecliptic plane we have reinvestigated the ionization rates of interplanetary hydrogen. The result of our analysis indicates that indeed the photoionization rate during solar minimum can be smaller than charge exchange by a factor of 5; however, during solar maximum conditions when solar EUV fluxes are high, and solar wind fluxes are low, photoionization can be over 60% of the charge exchange rate at Earth orbit. To obtain an accurate estimate of the importance of photoionization relative to charge exchange, we have included photoionization from both the ground and metastable states of hydrogen. We find, however, that the photoionization from the metastable state does not contribute significantly to the overall photoionization rate.