Solar particle events with helium-over-hydrogen enhancement in the energy range up to 100 MeV nucl−1

Flux measurements of solar energetic particles (SEPs) in the ERNE instrument onboard SOHO indicate that the abundance of ⁴He-nuclei compared to protons in the energy range up to 100 MeV nucl^–1 was exceptionally high during the particle events on 27 May 1998 and 28 December 1999. The ⁴He/p ratio stayed between 0.15–0.50 for more than ten hours. There was also a prolonged enhancement in helium-3, ³He/⁴H 1%. Observations of EIT and LASCO on board SOHO confirm that the originators of both SEP events were western eruptions, flares and coronal mass ejections (CMEs). The onset of the SEP release took place close to the maximum of flares which were probably triggered by the rising CMEs. The observations suggest that the SEP events were started with the flare-(pre)accelerated particles, but impact of the CME-associated shocks might explain the continuation and modification of the helium and proton fluxes well after the flare production. These observations support the idea that the helium enhancements in the CME-associated events reflect the availability of seed particles that originate previously in flares.     

Statistical comparison of palaeomagnetic directional records from lake sediments

Summary. This paper presents three statistical methods for testing the reproducibility or compatibility of palaeomagnetic directional records in undated cores of sediment. These methods are based on: (a) the intrinsic equations (curvature and torsion as functions of arc-length) of the curve defining changes in direction; (b) an F -test derived from multiple linear regression theory and (c) the construction and comparison of confidence bands. All methods utilize a projection of palaeomagnetic unit vectors on to the tangent plane to their mean direction. Since preliminary alignment to a common depth scale is necessary both for method (b) and for subsequent pooling of data, some theoretical results are presented for this problem, allowing for the possibility of missing segments in either core. Methods (b) and (c) are applied to two cores from Loch Lomond, Scotland. Both methods show that, after suitable relative rotation and alignment, these palaeomagnetic directional records are indeed compatible, demonstrating that the Loch Lomond sediments can provide reliable palaeosecular variation data of igh quality.     

Active Regions Observed in Extreme Ultraviolet Light by the Coronal Diagnostic Spectrometer on Soho

We present observations of five active regions made by the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO). CDS observes the Sun in the extreme ultraviolet range 150–780 Å. Examples of active region loops seen in spectral lines emitted at various temperatures are shown. Several classes of loops are identified: those that are seen in all temperatures up to 2 x 10⁶ K; loops seen at 10⁶ K but not reaching 1.6 x 10⁶ K; those at temperatures 2– 4 x 10^-5 K and occasionally at 6 x 10^-5 K but not reaching 10⁶ K. An increasing loop size with temperature and the relationship between the cool and hot structures is discussed. CDS observations reveal the existence of loops and other unresolved structures in active regions, at temperatures between 1.5– 4 x 10^-5 K, which do not have counterparts in lines emitted above 8 x 10^-5 K. Bright compact sources only seen in the transition region lines are investigated. These sources can have lifetimes of up to several days and are located in the vicinity of sunspots. We study the variability of active region sources on time scales from 30 sec to several days. We find oscillatory behaviour of Hei and Ov line intensities in an active region on time scales of 5–10 min.     

Si-O stretch modes in amorphous silicates

The 9.7µm Trapesium silicate band is modelled as the sum of contributions from tetrahedral Si-O units with varying numbers of non-bridging oxygen atoms. In principle this will allow the silicate grain structure to be determined.     

SERTS-97 Measurements Of Relative Wavelength Shifts In Coronal Emission Lines Across A Solar Active Region

We used slit spectra from the 18 November 1997 flight of Goddard Space Flight Center's Solar EUV Rocket Telescope and Spectrograph (SERTS-97) to measure relative wavelength shifts of coronal emission lines as a function of position across NOAA active region 8108. The shifts are measured relative to reference wavelengths derived from spectra of the region's nearby quiet surroundings (not necessarily at rest) because laboratory rest wavelengths for the coronal EUV lines have not been measured to sufficient accuracy for this work. An additional benefit to this approach is that any systematic uncertainties in the wavelength measurements are eliminated from the relative shifts by subtraction. We find statistically significant wavelength shifts between the spatially resolved active region slit spectra and the reference spectrum. For He ii 303.78 Å the maximum measured relative red shift corresponds to a Doppler velocity +13 km s^–1, and the maximum relative blue shift corresponds to a Doppler velocity –3 km s^–1. For Si x 347.40 Å, Si xi 303.32 Å, Fe xiv 334.17 Å, and Fe xvi 335.40 Å the corresponding maximum relative Doppler velocities are +19 and –14, +23 and –7, +10 and –10, and +13 and –5 km s^–1, respectively. The active region appears to be divided into two different flow areas; hot coronal lines are predominantly red-shifted in the northern half and either blue-shifted or nearly un-shifted in the southern half. This may be evidence that material flows up from the southern part of the region, and down into the northern part. Qualitatively similar relative wavelength shifts and flow patterns are obtained with SOHO/CDS spectra.     

A new inversion of solar rotational splitting data

The fine structure of the solar p-mode spectrum is used to obtain an estimate of the Sun's internal rotation rate, as a function of both latitude and depth, for fractional radii in the range 0.55 < r/R < 0.85. Because each piece of data is a weighted average of the rotation rate over an extended region of the solar interior and because the number of such measurements is finite, such an inversion unavoidably has limited resolution and suffers from other systematic errors. Accordingly, the results of the inversion presented here and other published inversions should be interpreted with this in mind. These problems are discussed in detail for the present inversion.Such systematic errors can be avoided by seeking not a functional form of the rotation rate but rather the values of suitably weighted averages of the rotation rate (or of functions derivable from it). Here it is shown that the value of a broad average over depth of the radial gradient of the solar rotation rate, concentrated in the convection zone, is consistent with the gradient being zero in this region but is apparently inconsistent with the picture of constant rotation on cylindrical surfaces aligned with the rotation axis. This result, which confirms the inference from the former type of inversion, is of importance for modelling the dynamics of the convection zone and also for dynamo models of the Sun's magnetic field.The National Center for Atmospheric Research is supported by the National Science Foundation.     

Eit Observations of the Extreme Ultraviolet Sun

The Extreme Ultraviolet Imaging Telescope (EIT) on board the SOHO spacecraft has been operational since 2 January 1996. EIT observes the Sun over a 45 x 45 arc min field of view in four emission line groups: Feix, x, Fexii, Fexv, and Heii. A post-launch determination of the instrument flatfield, the instrument scattering function, and the instrument aging were necessary for the reduction and analysis of the data. The observed structures and their evolution in each of the four EUV bandpasses are characteristic of the peak emission temperature of the line(s) chosen for that bandpass. Reports on the initial results of a variety of analysis projects demonstrate the range of investigations now underway: EIT provides new observations of the corona in the temperature range of 1 to 2 MK. Temperature studies of the large-scale coronal features extend previous coronagraph work with low-noise temperature maps. Temperatures of radial, extended, plume-like structures in both the polar coronal hole and in a low latitude decaying active region were found to be cooler than the surrounding material. Active region loops were investigated in detail and found to be isothermal for the low loops but hottest at the loop tops for the large loops. Variability of solar EUV structures, as observed in the EIT time sequences, is pervasive and leads to a re-evaluation of the meaning of the term ‘quiet Sun’. Intensity fluctuations in a high cadence sequence of coronal and chromospheric images correspond to a Kolmogorov turbulence spectrum. This can be interpreted in terms of a mixed stochastic or periodic driving of the transition region and the base of the corona. No signature of the photospheric and chromospheric waves is found in spatially averaged power spectra, indicating that these waves do not propagate to the upper atmosphere or are channeled through narrow local magnetic structures covering a small fraction of the solar surface. Polar coronal hole observing campaigns have identified an outflow process with the discovery of transient Fexii jets. Coronal mass ejection observing campaigns have identified the beginning of a CME in an Fexii sequence with a near simultaneous filament eruption (seen in absorption), formation of a coronal void and the initiation of a bright outward-moving shell as well as the coronal manifestation of a ‘Moreton wave’. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1004902913117     

Lunar and Venusian radar bright rings

Twenty-one lunar craters have radar bright ring appearances which are analogous to eleven complete ring features in the earth-based 12.5 cm observations of Venus. Radar ring diameters and widths for the lunar and Venusian features overlap for sizes from 45 to 100 km. Radar bright areas for the lunar craters are associated with the slopes of the inner and outer rim walls, while level crater floors and level ejecta fields beyond the raised portion of the rim have average radar backscatter. We propose that the radar bright areas of the Venusian rings are also associated with the slopes on the rims of craters.The lunar craters have evolved to radar bright rings via mass wasting of crater rim walls and via post impact flooding of crater floors. Aeolian deposits of fine-grained material on Venusian crater floors may produce radar scattering effects similar to lunar crater floor flooding. These Venusian aeolian deposits may preferentially cover blocky crater floors producing a radar bright ring appearance.We propose that the Venusian features with complete bright ring appearances and sizes less than 100 km are impact craters. They have the same sizes as lunar craters and could have evolved to radar bright rings via analogous surface processes.     

Intrinsic variations of interstellar extinction laws

The intrinsic ultraviolet colours of early type stars have been derived from the rocket data of J. W. Campbell and A. M. Smith. There exists a considerable spread in these colours. One possible source is the predicted rms variation due to the effect of rotation. It is necessary to recognise that rotation is an important source of uncertainty of the interstellar extinction curves derived from the comparison of pairs of stars.Predicted rms errors due to different sources (rotation, uncertainty of MK spectral type, photometric errors) have been compared with the dispersion of the observed extinction data and it is found that part of the large scatter could be due to intrinsic differences of the properties of interstellar grains.