Board SOHO Research Articles

The Taylor‐Proudman Balance and the Solar Rotational Data

For the Sun (and because of the presence of buoyancy), the implications of the Taylor-Proudman balance (TPB, a balance between the pressure, Coriolis, and buoyancy forces in the radial and latitudinal momentum equations) differ fundamentally from those of an incompressible fluid. The TPB now only determines the latitudinal variations of the solar entropy in terms of the rotation law and known functions of r. As a consequence of the TPB, the energy equation is in fact an equation for the angular velocity, Ω(r, θ)=Ω0(ω0(r)+ω2(r)P2(cos θ)), where P2(cos θ) is the second-order Legendre polynomial. In agreement with data from the Solar Oscillations Investigations project (SOI) Michelson Doppler Imager (MDI) on board SOHO, we assume that ω0(r) is constant with r, and solve the equation for ω2(r) with a simple, heuristic expression for the convective flux [if ω0(r) is constant, then ∂Ω/∂r vanishes for θc=547, P2(cos θc)=0, in remarkable agreement with Kosovichev's results inferred from isocontours for Ω(r, θ)]. For values of the meridional motions that are not too large, solutions for ω2(r) exist that agree with these isocontours. These solutions are such that the latitudinal variation of the convective flux, arising from the latitudinal variations of the entropy, required by the TPB, are significantly reduced.

Multiresolution wavelet analysis of SUMER/SOHO observations in a solar prominence

We have studied through a multiresolution wavelet analysis the oscillations in a limb prominence. Intensity fluctuations in time and height corresponding to different lines of Si iv and O iv observed with SUMER on board SOHO have been analyzed in the wavelet bands of J3= 1 min 36 s to 3 min 12 s and J4=3 min 12 s to 6 min 24 s. For all species, oscillations in the J4 band were dominant. We found relevant differences between the behavior of line D1 (1393.76 A) corresponding to Si iv and the set D2 (1401.16 A), D3 (1404.81 A), D4 (1402.77 A) corresponding to O iv, O iv and Si iv respectively. We also report the identification of a pulse in the intensity of the line D1 that appears in the range of 15–20 min. This disturbance seems to travel with a speed of about 170 km s−1.

Effects on UV lines observations of stationary plasma flows confined in coronal loops

We describe a method which uses a rather detailed model of coronal loop hosting a siphon flow as a diagnostic tool to interpret solar UV and X-ray observations in selected bands and lines. We apply the method to investigate the deviations from ionization equilibrium induced by stationary plasma flows confined in coronal loops and their effects on the UV and EUV emission lines observed by the instruments on board SOHO. We present results on the detailed synthesis of loop emission in a set of selected emission lines observed by CDS and SUMER, taking into account the non-equilibrium of ionization effects.

Plasma Properties in Coronal Holes Derived from Measurements of Minor Ion Spectral Lines and Polarized White Light Intensity

Recent observations of the Lyα λ1216, Mg X λ625, and O VI λ1038 spectral lines carried out with the Ultraviolet Coronagraph Spectrometer (UVCS) on board SOHO at distances in the range 1.35-2.1 RS in the northern coronal hole are used to place limits on the turbulent wave motions of the background plasma and the thermal motions of the protons and Mg+9 and O+5 ions. Limits on the turbulent wave motion are estimated from the measured line widths and electron densities derived from white light coronagraph observations, assuming WKB approximation at radial distances covered by the observations. It is shown that the contribution of the turbulent wave motion to the widths of the measured spectral lines is small compared to thermal broadening. The observations show that the proton temperature slowly increases between 1.35 and 2.7 RS and does not exceed 3×10 K in that region. The temperature of the minor ions exceeds the proton temperature at all distances, but the temperatures are neither mass proportional nor mass-to -charge proportional. It is shown, for the first time, that collision times between protons and minor ions are small compared to the solar wind expansion times in the inner corona. At 1.35 RS the expansion time exceeds the proton Mg+9 collision time by more than an order of magnitude. Nevertheless, the temperature of the Mg ions is significantly larger than the proton temperature, which indicates that the heating mechanism has to act on timescales faster than minutes. When the expansion time starts to exceed the collision times a rapid increase of the O+5 ion spectral line width is seen. This indicates that the heavier and hotter ions lose energy to the protons as long as collision frequencies are high, and that the ion spectral line width increases rapidly as soon as this energy loss stops.

Solar wind isotopic abundance ratios of ne, mg and si measured by SOHO/CELIAS/MTOF as diagnostic tool for the inner corona

Using the high-resolution mass spectrometer MTOF on board SOHO we have measured the solar wind isotopic abundance ratios of Ne, Mg, and Si in different solar wind regimes with bulk velocities ranging from 350 km/s to 650 km/s. Data indicate a systematic depletion of the heavier isotopes in the slow solar wind compared to their abundances in the fast solar wind from coronal holes. These variations in the solar wind isotopic composition represent a pure mass-dependent effect because the different isotopes of an element pass the inner corona with the same charge state distribution. The influence of particle mass on the acceleration of minor solar wind ions is discussed in the context of theoretical models and recent optical observations with other SOHO instruments.

Active region EUV transient brightenings – First Results by EIT of SOHO JOP 80

On 13 May 1998, the Extreme-Ultraviolet Imaging Telescope (EIT, on board SOHO) has produced a unique image sequence operating in 'shutterless mode' (SOHO JOP 80). In JOP 80, EIT is the lead instrument, followed by several space-born instruments (SXT, TRACE, MDI, CDS, SUMER), as well as two observatories on the ground (in La Palma and Sac Peak). The target of the campaign was a relatively small but rapidly evolving active region (AR 8218). For the EIT contribution, a 15 s cadence was achieved in the Fe xii bandpass at 195 A by leaving EIT's shutter open for 1 h and operating the CCD in frame-transfer mode. In this paper, we start the analysis of the huge data set, by making an inventory of the transients observed in the EIT image sequence. Besides scatter plots of duration, size and radiative output of the detected EUV brightenings, we discuss in full detail the morphology and evolution of several typical events. These transients range from a B3.5 flare producing a large plasma flow along pre-existing loops, to EUV versions of active region transient brightenings as previously observed by SXT on board Yohkoh. In addition, a new class of weaker footpoint brightenings is discussed that produce wave-like disturbances propagating along quasi-open field lines. This new class of propagating disturbances extends the wide variety of transient phenomena that we discovered in the EIT data, and makes the potential for inter-instrumental studies of the JOP 80 data all the more exciting. We stress the necessity of such forthcoming studies to reach an instrument-independent classification of small-scale solar transients.

Multiple-Thread Model of a Prominence Observed by SUMER and EIT on SOHO

A quiescent polar crown prominence was observed at Meudon in Hα and Ca ii lines, and by EIT and SUMER on board SOHO in UV lines from 9 to 10 March 1996. SUMER observed the prominence continuously in a scanning mode between 21:40 UT on 9 March, and 18:13 UT on 10 March, in the nitrogen line N v (λ1238) with a 1 arc sec2 resolution. Altogether 190 prominence images (121×108 pixels) were obtained. These are presented in a movie. The prominence is highly dynamic. Large-scale features, such as mixed loop systems and dark cavities are changing on time scales of a few hours. Filamentary structure is evident and is changing within a few frames of the movie. A lifetime of 20–25 min for the fine structure has been found by the autocorrelation method. We have statistically analysed the three moments of the N v line in the prominence: line intensity, Doppler shift and linewidth, in the context of a multiple-thread model. We find that the data are consistent with a model where the prominence is assumed to be an ensemble of small threads. In the brightest parts of the prominence it is possible that there are many unresolved threads (15–20) along the line of sight with diameters smaller than a few hundred kilometers. The filling factor is probably very small and in that case the structures occupy only a fraction of the volume.

Multiple CME Events Observed With LASCO

Observations are reported of two multiple CME events which were detected on 2–3 June 1997 and 9–10 June 1998, using the LASCO instrument on board SOHO. Each event consists of a group of four related CMEs which emerge from progressively higher latitudes over a time period of approximately 16 hours. In both cases there is on-disk activity visible in EIT EUV images which involves bright emission along the south polar crown filament and there is also ejection of mass from other regions of the corona during the time period of each event. We present a multi-wavelength view of these events (i.e. white-light, Hα, EUV and, in the case of the 2–3 June 1997 event, soft X-ray), which suggest that ejection of mass from one point in the corona can lead to a destabilization of a previously stable structure and the further ejection of mass from different regions of the corona, in a systematic way. The observations also show that the CME phenomenon is not always a localised event but can occur on a global level; and that complex CME activity can arise at relatively quiet-Sun periods as evidenced by the lack of significant X-ray flares or radio signatures.

Spatiotemporal Correlations and Turbulent Photospheric Flows fromSOHO/MDI Velocity Data

Time series of high-resolution and full-disk velocity images obtained with the Michelson Doppler Imager (MDI) instrument on board SOHO have been used to calculate the spacetime spectrum of photospheric velocity flow. The effects of different methods for filtering acoustic oscillations have been carefully studied. It is found that the spectra show contributions both from organized structures that have their origin in the convection zone and from the turbulent flow. By considering time series of different duration and cadence in solar regions with different line-of-sight projections, it is possible to distinguish the contributions of the spectra from the two different kinds of flows. The spectra associated with the turbulent velocity fields obey power laws characterized by two scaling parameters whose values can be used to describe the type of diffusion. The first parameter is the spectral exponent of the spatial correlation function and the second is a scaling parameter of the time correlation function. Inclusion of the time parameter is an essential difference between the present work and other solar studies. Within the confidence limits of the data, the values of the two parameters indicate that the turbulent part of the flow in the scale range 16-120 Mm produces superdiffusive transport.

Comparison of Hα and Heiiλ304 Macrospicules

Simultaneous observations of limb macrospicules were carried out in Hα by the Big Bear Solar Observatory (BBSO) and in He II λ304 by the Extreme-Ultraviolet Imaging Telescope (EIT) on board SOHO. For the first time, Hα and He II macrospicules are compared, with high spatial and temporal resolution and image enhancement. Data were obtained on 1996 October 6, 7, and 8. The target of the first and last day was the north pole; on October 7, it was the quiet west limb. BBSO uses a 12 bit digital camera to obtain high-resolution Hα filtergrams at -0.65 A, line center, and 0.65 A. The pixel resolution ranges between 017 and 033, and temporal resolution ranges between 30 and 90 s. EIT images have a fixed pixel resolution of 25 and temporal resolution between 1 and 7 minutes. We found the following: within the common field of view of BBSO and EIT, all 53 identified He II λ304 macrospicules have counterparts in Hα. However, morphologies of He II λ304 and Hα macrospicules are completely different. He II λ304 macrospicules are typically in the form of an elongated ejection, whereas Hα macrospicules are either looplike bright features or much shorter jets. In the polar region, 55 (over 50%) Hα macrospicules do not show any corresponding He II λ304 macrospicules. As expected, He II λ304 macrospicules occur much more frequently in the polar coronal hole area than in the equatorial region. However, Hα macrospicules occur at an equal rate in the pole and in the equatorial limb. Equatorial Hα macrospicules tend to be confined because of their looplike structure and the lack of jetlike structure. Based on common properties of different events, we propose that the disk counterparts of macrospicules might be Hα - 1.0 A jets or a subset of explosive events. If this assumption is true, macrospicules are the results of magnetic reconnections, likely due to the network-ephemeral region or network-intranetwork interactions. We propose that magnetic reconnection occurs at about the same rate in the polar coronal hole and in the quiet regions. Hα macrospicules are direct manifestation of magnetic reconnection. He II λ304 images detect substantially taller structures that are substantially hotter. Because of dominant vertical line configuration near the pole, reconnection tends to produce He II λ304 macrospicules; in the noncoronal hole regions, He II macrospicules become undetectable because of the inclined magnetic field configuration.

Search for Brightness Variations in FexivCoronagraph Observations of the Quiescent Solar Corona

We use Fe XIV 5303 Å green line images obtained by the Large Angle Spectrometric Coronagraph (LASCO) on board SOHO to search for variability in the quiescent solar corona in the shortest observable timescales. The observing program obtained Fe XIV images of a small area of the inner corona every 2 minutes over a period of 1 hr. We present results from two executions of this program taken several months apart. The most obvious variability observed in the two sequences is in the form of quasi-steady brightening on timescales of at least an hour. Of particular interest are two compact loops that are observed to vary significantly during the course of the observations. Superposed on the long-term brightening in these loops are statistically significant variations on timescales of about 30 minutes. In both loops, the overall brightening is greatest at the apparent loop tops, where the intensity increases by at least 25%. In one loop there appears to be a flow up one of the legs of the loop. We place these observations in context with earlier observations of coronal variability, and we discuss the energy requirements for the observed brightening. Emission measures computed from Fe XIV intensities measured within the two brightening loops are over an order of magnitude lower than those typically found for active regions on the solar disk, which suggests significantly lower average densities. For one of the loops, we measure densities in the range ne = (5.0-7.4) × 108 cm-3. Lower than average densities are expected for the two loops, given that they reach more than 1 pressure scale height above the solar limb.

Assessment of degradation of VIRGO radiometers on board SOHO

The determination of the total solar irradiance (TSI) from the SOHO/VIRGO experiment is made by first correcting the time series for all a priori known influences, and then correcting the data for instrumental degradation, using the back-up instruments PMO6-VB and DIARAD-R. The long-term behaviour of PMO6-VA shows an exponential decrease in its sensitivity with a time constant of τ = 390 days and a 1/e amplitude of 528 × 10-6, whereas DIARAD-L exhibits a general increase in its sensitivity of about 0.25 × 10-6/day combined with an early exponential decrease of 405 days and an 1/e amplitude of 65 × 10-6. After correcting PMO6- VA and DIARAD-L for their degradation, the VIRGO TSI shows a minimum around July 1996 and has been increasing since then by about 0.4 W m-2.

Observations of galactic cosmic ray modulation during Earth‐directed coronal mass ejections

We describe three cosmic‐ray decreases observed by the energetic particle telescope ERNE on board SOHO spacecraft during the period January–May, 1997. The decreases in cosmic‐ray proton intensities in the energy range above 50 MeV were observed after the passage of interplanetary shocks on February 9, April 10, and May 15. All the shocks were produced by Earth‐directed coronal mass ejections. The amplitudes of the decreases compared to the counting rates just before the shock arrival were 8.5%, 14%, and 7%, respectively. The observed recovery times were 1, 4.2, and 5 days, respectively. After the February and May events the pre‐shock levels were reached, but in the April event the recovery was only to 3% below this level. In each case, there were also clear indications of the passage of the ejecta. The energy dependences of the cosmic‐ray helium flux decreases at 12–100 MeV/n were similar to each other in the April and May events, while in the February event the decrease was less pronounced.

The Adiabatic Exponent in the Solar Core

We present helioseismic inversions of solar p-mode frequencies obtained by the Michelson Doppler Imager instrument of the Solar Oscillation Investigation project on board SOHO, which indicate a lower central value for the adiabatic exponent Γ1 in the Sun than in current models. We show how the inclusion of relativistic effects in the equation of state removes the discrepancy and allows almost perfect agreement in Γ1 between the models and the Sun. This increases our confidence both in the reliability of the equation-of-state calculation and the determination of solar p-mode frequencies.

Lyman-alpha observations of comet Hyakutake with SWAN on SOHO

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 H 2O 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 H 2O production rate was found in fair agreement with other derivations (IUE and ground-based in the IR and UV), validating the Lα method. The H 2O production by SWAN was related to several other measurements of minor constituents in order to derive new values of abundance of CO, HCN, H 2CO, CH 3OH and CH 3CN. 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 H 2O 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 × 10 29 H 2O 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 C 2H 6 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.

Comparison of Prominences in Hα and He II 304 Å

In this letter, we bring attention to prominences which show different morphology in Hα and He ii 304 A, as observed simultaneously by BBSO and EIT on board SOHO. Those two lines have been thought to represent similar chromospheric structures although they are formed at significantly different temperatures. We give two examples representing two kinds of anomaly: (1) prominences showing strong Hα emissions in the lower part and strong He ii emissions in the upper part, and (2) erupting prominences showing extensive He ii emission, but nothing in Hα. Our results indicate that a part or the whole of a prominence may be too hot to emit Hα radiation, possibly due to heating or thermal instability. Please note that these are not just two isolated cases, many other prominences show the similar differences in Hα and He ii 304 A.

Spherical and Aspherical Structure of the Sun: First Year of Soho/Mdi Observations

We report the initial results of one year of continuous observations of the Sun‘s internal structure from the Michelson Doppler Imager (MDI) on board SOHO. The results have been obtained by inverting frequencies of p and f modes determined with two different methods of averaging over split multiplets. Small systematic differences between the two frequency sets depend primarily on mode frequencies, and, thus, did not significantly affect the inversions. A preliminary study of the systematic effects resulting from asymmetry of oscillation power peaks has also shown no significant influence on the inversion results. The inferred sound-speed profile is in general agreement with the previous data from MDI and ground-based networks. In the energy-generating core, the resolution is substantially improved, and the inversion results indicate a sharp negative perturbation of the sound speed in the core, tending to a positive value near the center. High-precision measurements of the f-mode frequencies have been used to determine the seismic radius of the Sun. The global asphericity estimated from frequency variation across the split mode multiplets has been found to be small, and is consistent with the asphericity during the previous activity minimum. Variations of the solar frequencies during the first year of MDI observations have also been detected.

Loi and Gong Low-Degree Rotational Splittings

Two different data sets have been used to derive low-degree rotational splittings. One data set comes from the Luminosity Oscillations Imager of VIRGO on board SOHO; the observation starts on 27 March 96 and ends on 26 March 97, and are made of intensity time series of 12 pixels (Appourchaux et al, 1997, Sol. Phys., 170, 27). The other data set was kindly made available by the GONG project; the observation starts on 26 August 1995 and ends on 21 August 1996, and are made of complex Fourier spectra of velocity time series for l = 0 − 9. For the GONG data, the contamination of l = 1 from the spatial aliases of l = 6 and l = 9 required some cleaning. To achieve this, we applied the inverse of the leakage matrix of l = 1, 6 and 9 to the original Fourier spectra of the same degrees; cleaning of all 3 degrees was achieved simultaneously (Appourchaux and Gizon, 1997, these proceedings).

High Frequency Signal in Golf Data

The analysis of ≈ 460 days of high quality data provided by the GOLF experiment on board SOHO, has unambiguously revealed the presence of signal in the power spectrum in the region 5.8 to 7.5 mHz, well above the p-modes cut-off frequency. In this contribution, the observed structure of these full-disk observations is presented. High frequency peaks (hereafter HFPs) well above the acoustic cutoff frequency of the solar p modes (≈ 5.5 mHz) have been already observed in intermediate and high spatial resolution oscillation data (Duvall et al. 1991). The different interpretations of the observed pattern (Kumar 1994 and references therein; Rast and Gough 1995) agree that it provides direct information on the location and some characteristics of the source of the acoustic oscillations (see also Vorontsov et al. 1997).

Atomic Physics for Atmospheric Composition Measurements

The atomic physics relevant to the interpretation of solar spectra produced by plasmas at temperatures ≳ 105 K are discussed. Methods for determining relative abundance ratios are presented and examples provided from the Coronal Diagnostic Spectrometer on board SOHO. In particular, the Fe/Si ratio in the corona is found to be close to photospheric; the Mg/Ne ratio in the transition region is found to vary by an order of magnitude in different solar features. The Mg/Ne ratios in supergranule cell centres and the network are separated for the first time, although no significant differences are found.