Kwangsun Ryu

The “Spectroscopy of Plasma Evolution from Astrophysical Radiation” Mission

The Spectroscopy of Plasma Evolution from Astrophysical Radiation (SPEAR, also known as the Far-Ultraviolet Imaging Spectrograph) instruments, flown aboard the STSAT-1 satellite mission, have provided the first large-area spectral mapping of the cosmic far-ultraviolet (FUV; 900-1750 A) background. We describe the mission and its science motivation, the mission data and their processing, and the effects of mission performance on the science data. We present the first map of the cosmic FUV background (1360-1710 A) over most of the sky as an example of the mission results. These SPEAR data reveal diffuse radiation from warm and hot (104-106 K) plasma, molecular hydrogen fluorescence, and dust-scattered starlight. They allow for an unprecedented characterization of the spectral emission from a variety of environments, including the general interstellar medium (ISM), molecular clouds, supernova remnants, and superbubbles.

The SPEAR Instrument and On-Orbit Performance

The SPEAR (or FIMS) instrumentation has been used to conduct the first large-scale spectral mapping of diffuse cosmic far-ultraviolet (FUV; 900-1750 A) emission, including important diagnostics of interstellar hot (104-106 K) and photoionized plasmas, H2, and dust-scattered starlight. The instrumentations performance has allowed for the unprecedented detection of astrophysical diffuse FUV emission lines. A spectral resolution of λ/Δλ ~ 550 and an imaging resolution of 5 is achieved on-orbit in the Short (900-1150 A) and Long (1350-1750 A) bandpass channels within their respective 40 × 46 and 74 × 43 fields of view. We describe the SPEAR imaging spectrographs, their performance, and the nature and handling of their data.

FAR-ULTRAVIOLET OBSERVATIONS OF THE OPHIUCHUS REGION WITH SPEAR

We present the first far-ultraviolet (FUV; 1370-1670 A) image of the Ophiuchus molecular cloud region, observed with the SPEAR imaging spectrograph. The flux levels of the diffuse FUV continuum are in reasonable agreement with those of the Voyager observations in the shorter FUV wavelengths (912-1216 A), provided that the diffuse FUV emission is dominated by the spectra from late O- and early B-type stars. The observed region of the present study was divided into five subregions according to their FUV intensities, and the spectrum was obtained for each subregion with prominent H_2 fluorescent emission lines. A synthetic model of the H_2 fluorescent emission indicates that the molecular cloud has more or less uniform physical parameters over the Ophiuchus region, with a hydrogen density n_H of 500 cm^−3 and a H2 column density N(H_2) of 2 × 10^(20) cm^−2. It is notable that the observed diffuse FUV continuum is well reproduced by a single-scattering model with scattered starlight from the dust cloud located at ~120-130 pc, except at a couple of regions with high optical depth. The model also gives reasonable properties of the dust grains of the cloud with an albedo a of 0.36 ± 0.20 and a phase function asymmetry factor g of 0.52 ± 0.22.

Multisatellite observations of an intensified equatorial ionization anomaly in relation to the northern Sumatra earthquake of March 2005

Here we report multisatellite observations of ionospheric disturbances in relation to the noccurrence of the M8.7 northern Sumatra earthquake of 28 March 2005. The DEMETER (Detection of nElectro-Magnetic Emissions Transmitted from Earthquake Regions) and CHAMP (Challenging Minisatellite nPayload) satellite data were investigated to find possible precursory and postevent phenomena. It was nfound that EIA (equatorial ionization anomaly) strength expressed in the apex height, derived from the nCHAMP plasma density profile, was intensified along the orbits whose longitudes were close to the nepicenter within about a week before and after occurrence of the earthquake. Increases in electron and O+ ndensity along the orbits close to the epicenter were also observed in the DEMETER measurements. The nnormalized equatorial plasma density derived from the DEMETER measurements showed intensification nabout a week before and after the earthquake reaching maximum the day after the shock and afterward ndisappearing. In addition, similar behavior of the EIA enhancements related to the M8.0 Pisco earthquake of n15 August 2007 was observed. Surveys of space weather and geomagnetic activities excluded the npossibility that these fluctuations were caused by changes in space weather or by a geomagnetic storm. nStatistical analyses of the longitudinal variation revealed that the EIA was enhanced in the west of the nepicenter and reduced in the east of the epicenter, and this fits the “increased conductivity” model. Based non these observations, we proposed a revised view of seismo-ionospheric coupling in the region of the ngeomagnetic equator, to explain the EIA features observed in this study.

Suspected seismo‐ionospheric coupling observed by satellite measurements and GPS TEC related to the M7.9 Wenchuan earthquake of 12 May 2008

Anomalous changes in the ionospheric conditions related to the Wenchuan earthquake of 12 May 2008 are investigated using electron density (Ne) from Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) and CHAMP satellites, electric field from DEMETER, and GPS-total electron content (TEC) maps. The normalized Ne from the DEMETER satellite reveal that the previously reported TEC increments before the earthquake can be considered as fragments of the gradual equatorial ionization anomaly (EIA) enhancements near the epicenter longitude that began approximately 1 month before the earthquake and reached its maximum with an exceptionally large strength index 8 days prior to the main shock. This feature is indirectly confirmed through the CHAMP Ne and GPS TEC data. Following the EIA intensity peak, disturbances in the Ne and O+ density were observed in the nightside. Based on the concurrent electric field and Ne changes, it is suggested that EIA intensification could be triggered by the E field disturbances over the epicenter.

Molecular Hydrogen Fluorescence in the Eridanus Superbubble

The first far-ultraviolet (1350-1750 A) spectral imaging observations of the Eridanus superbubble, obtained with the SPEAR/FIMS mission, have revealed distinct fluorescent emission from molecular hydrogen. Here we compare the observed emission features with those from a photodissociation region model with assumed illuminating stellar fields. The result shows rather high line ratios I1580/I1610, which may imply the existence of high-temperature molecular clouds in the region. The H2 fluorescence intensity shows a proportional correlation with Hα emission, indicating that the fluorescence and the recombination emission have similar physical origins.

ORFEUS Observations of the Foreground Gas toward HD 37903

We analyze H2 and CO absorption along the line of sight toward HD 37903 over the 1045-1060 A and 1086-1102 A wavelength regions, which were observed by the Berkeley Extreme and Far-Ultraviolet Spectrometer on the Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometer (ORFEUS) telescope. HD 37903 is a bright UV-emitting star embedded in the L1630 molecular cloud, creating the reflection nebula NGC 2023. Using the theory of formation and dissociation of molecular hydrogen in the far-UV spectral range, we derive the physical conditions of the foreground gas toward HD 37903, such as the density n, temperature T, and the UV intensity IUV, by analyzing the H2 lines from the J = 0-5 rotational levels. In addition, we identify the CO absorption band at 1088 A and calculate the abundance ratio [CO]/[H2] directly, which is found to be 1.3 × 10-7. The higher rotational levels of H2 are excited in the outer boundary of a molecular cloud where UV pumping dominates, while the lower J levels of H2 and other molecules, such as CO, prevail in the neutral core of the cloud. In this regard, our analysis indicates that the observed gas extends from the neutral region to the photodissociation region of NGC 2023 at a distance of 0.2 pc from HD 37903. These results are similar to those found in previous studies of the photodissociation region of NGC 2023 and chemical analysis of the foreground gas toward HD 37903. The total amount of molecular hydrogen, including the gas behind HD 37903, is estimated from the total visual extinction of the dark cloud and our H2 column density of the foreground gas. By measuring the CO and 13CO integrated intensities via radio observations, we find the conversion factor Ntot(H2)/W(CO) to be 1.5 × 1020 cm-2 (K km s-1)-1, which is in accord with canonical conversion factors.

Diffuse Far-Ultraviolet Observations of the Taurus Region

Diffuse far-ultraviolet (FUV; 1370-1670 A) flux from the Taurus molecular cloud region has been observed with the SPEAR/FIMS imaging spectrograph. An FUV continuum map of the Taurus region, similar to the visual extinction maps, shows a distinct cloud core and halo region. The dense cloud core, where the visual extinction Av > 1.5, obscures the background diffuse FUV radiation, while scattered FUV radiation is seen in and beyond the halo region, where Av < 1.5. The total intensity of H2 fluorescence in the cloud halo is I = 6.5 × 104 photons cm-2 s-1 sr-1 in the 1370-1670 A wavelength band. A synthetic model of the H2 fluorescent emission fits the present observation best with a hydrogen density nH = 50 cm-3, H2 column density N(H2) = 0.8 × 1020 cm-2, and incident FUV intensity IUV = 0.2. H2 fluorescence is not seen in the core, presumably because the required radiation flux to induce fluorescence is unable to penetrate the core region.

SPEAR far UV spectral imaging of highly ionized emission from the North Galactic Pole Region

Aims. We present far ultraviolet (FUV: 912–1750 A) spectral imaging observations recorded with the SPEAR satellite of the interstellar OVI (1032 A), CIV (1550 A), SiIV (1394 A), SiII* (1533 A) and AlII (1671 A) emission lines originating in a 60 ◦ × 30 ◦ rectangular region lying close to the North Galactic Pole. These data represent the first large area, moderate spatial resolution maps of the distribution of UV spectral-line emission originating the both the highly ionized medium (HIM) and the warm ionized medium (WIM) recorded at high galactic latitudes. Methods. By assessing and removing a local continuum level that underlies these emission line spectra, we have obtained interstellar emission intensity maps for the aforementioned lines constructed in 8 ◦ × 8 ◦ spatial bins on the sky. Results. Our maps of OVI, CIV, SiIV and SiII* line emission show the highest intensity levels being spatially coincident with similarly high levels of soft X-ray emission originating in the edge of the Northern Polar Spur feature. However, the distribution of the low ionization AlII emission does not show this spatial correlation, and suggests that warm-neutral and/or partially ionized gas with a temperature <20 000 K may be quite pervasive at high galactic latitudes. The variation of the emission line intensity ratios as a function of sky position is contrasted with theoretical predictions concerning the physical state of interstellar gas in the galactic halo. The observed line ratios alone unfortunately do not provide us with a clear diagnostic tool to distinguish between the various physical production mechanisms responsible for both high and low ion states. However, our results do favor the hybrid model of Shull & Slavin (1994, ApJ, 427, 784) which incorporates the combined effects of turbulent mixing layers and isobarically cooling supernova remnant gas. For this highly ionized gas, our results are best explained assuming that the observed OVI halo emission is somewhat clumpy in nature, consistent with its production at interfaces between warm (T = 10 3 −10 4 K) and hotter (T = 10 6 K) soft X-ray emitting gas. CIV emission at these interfaces occurs in the intermediate temperature (T = 10 5 K) gas, which seems always present whenever OVI is strongly detected. Alternately, the data are also consistent with CIV emission being ubiquitous throughout the halo with a fairly constant level of emission line intensity (of ∼4000 LU), and our observations mostly reflect the superposition of spatially separate OVI emission originating at the cloud interfaces of random clumps of high latitude gas.

Far-Ultraviolet Observations of a Thermal Interface in the Orion-Eridanus Superbubble

Diffuse far-UV emission arising from the edge of the Orion-Eridanus superbubble has been observed with the SPEAR imaging spectrometer, revealing numerous emission lines arising from both atomic species and H2. Spatial variations in line intensities of C IV, Si II, and O VI, in comparison with soft X-ray, Hα, and dust data, indicate that these ions are associated with processes at the interface between hot gas inside the bubble and the cooler ambient medium. Thus our observations probe the physical conditions of an evolved thermal interface in the interstellar medium.