Ultraviolet Coronagraph Spectrometer Observations Research Articles

Combined Analysis of Ultraviolet and Radio Observations of the 7 May 2004 CME/Shock Event

We report results from the combined analysis of UV and radio observations of a CME-driven shock observed on 7 May 2004 above the southeast limb of the Sun at 1.86 R ⊙ with the Ultraviolet Coronagraph Spectrometer (UVCS) on board the Solar and Heliospheric Observatory (SOHO). The coronal mass ejection (CME) was first detected in white-light by the SOHO’s Large Angle and Spectrometric Coronagraph (LASCO) C2 telescope and shock-associated type II metric emission was recorded simultaneously by ground-based radio spectrographs. The shock speed (∼ 690 km s−1), as deduced from the analysis of the type II emission drift in the radio spectra and the pre-shock local electron density estimated with the diagnostics provided by UVCS observations of the O vi λλ 1031.9, 1037.6 doublet line intensities, is just a factor ∼ 0.1 higher than the CME speed inferred by means of the white-light (and EUV) data in the middle corona. The local magnetosonic speed, computed from a standard magnetic field model, was estimated as high as ∼ 600 km s−1, implying that the CME speed was probably just sufficient to drive a weak fast-mode MHD shock ahead of the front. Simultaneously with the type II radio emission, significant changes in the O vi doublet line intensities and profiles were recorded in the UVCS spectra and found compatible with abrupt post-shock plasma acceleration and modest ion heating. This work provides further evidence for the CME-driven origin of the shocks observed in the middle corona.

UVCS Observation of Sungrazer C/2001 C2: Possible Comet Fragmentation and Plasma‐Dust Interactions

In this paper we analyze SOHO Ultraviolet Coronagraph Spectrometer (UVCS) observations of the sungrazing cometC/2001C2,amemberoftheKreutzfamily,observedon2001February7atheliocentricdistancesof4.98and 3.60 R� . This comet apparently went through sequential fragmentation events along its path: further indication of fragmentation processes is provided by UVCS observations, which show the presence of two separate tails in the 4.98 Rdata set, which we interpret as two fragments unresolved by LASCO images, one of which sublimates before reaching 3.60 R� . The cometary hydrogen Lysignal, decaying exponentially with time, has been inter- preted in terms of the H2O outgassing rate and the interactions of coronal protons with atoms created by the photodissociation of water. However, one of the fragments shows an additional Lycontribution, constant with time, which adds to the temporally decaying signal. This contribution has been ascribed to the sublimation of pyroxene dust grains, whose end products neutralize coronal protons via charge exchange processes. Hence, the twofragmentshavedifferentcomposition;differencesthroughoutthecometbodymayhavebeentheprimarycause for the comet fragmentation. Subject headingg comets: general — comets: individual (C/2001 C2) — ultraviolet: general

UVCS/SOHO observations of a CME-driven shock: Consequences on ion heating mechanisms behind a coronal shock

We report the observation of a 1100 km s - 1 CME-driven shock with the UltraViolet Coronagraph Spectrometer (UVCS) telescope operating on board SOHO on March 3, 2000. The shock speed was derived from the type II radio burst drift rate and from UVCS observations that can yield the density profile just before the passage of the shock. A CME projected speed of 920 km s - 1 was deduced from the Large Angle Spectrometric Coronagraph (LASCO) white light images, indicating that the CME leading edge was lagging behind at about 20% of the shock speed. The spectral profiles of both the O VI and Lya lines were Doppler dimmed and broadened at the passage of the shock by the emission from shocked material along the line of sight. The observed line broadening for both protons and oxygen ions was modeled by adopting a mechanism in which the heating is due to the nondeflection of the ions at the shock ramp in a quasi-perpendicular shock wave. This specific ion heating model was able to reproduce the observed spectroscopic properties of the shocked plasma.

Three‐Fluid 2.5‐dimensional Magnetohydrodynamic Model of the Effective Temperature in Coronal Holes

Recent SOHO Ultraviolet Coronagraph Spectrometer (UVCS) observations show that protons and minor ions are hot (Tp > 106 K, Ti > 107 K) and anisotropic in coronal holes. A possible cause of the large perpendicular motions is unresolved Alfvenic fluctuations in the solar wind. Using the three-fluid 2.5-dimensional MHD model, we have shown that the unresolved Alfvenic fluctuations lead to apparent proton temperature and anisotropy consistent with UVCS observations. However, Alfven waves with realistic amplitudes cannot reproduce the O5+ perpendicular temperature and anisotropy deduced from UVCS observations. This suggests that the minor ions are heated by a different mechanism than protons.

STUDY OF CORONAL TRANSIENTS IN UV LINE EMISSION

We investigate the expected emission from coronal transients in the following spectral lines observable with the Ultraviolet Coronagraph Spectrometer (UVCS) on board SOHO: Hi Lα 1216 A, Ovi 1032–1037 A, Nv 1239–1243 A, Mgx 610–625 A, Sixii 499–521 A, and Fexii 1242 A. We calculate line intensities and profiles for typical CME conditions, and we analyse their relation with the properties of the perturbed coronal region. We find that significant changes in UV line intensities are produced during a coronal transient. An overall decrease of the Hi Lα intensity is found, which is mainly due to the Doppler dimming produced by the increase in plasma outflow velocity. The emission from heavy ions is instead mainly affected by variations in plasma density and temperature. We expect to compare these results with the future UVCS observations of coronal transients.