Solar and Galactic Cosmic Rays observed by SOHO

Both the Cosmic Ray Flux (CRF) and Solar Energetic Particles (SEPs) have left an imprint on SOHO technical systems. While the solar array efficiency degraded irreversibly down to ~77% of its original level over roughly 1 1/2 solar cycles, Single Event Upsets (SEUs) in the solid state recorder (SSR) have been reversed by the memory protection mechanism. We compare the daily CRF observed by the Oulu station with the daily SOHO SEU rate and with the Degradation curve of the solar arrays. The Oulu CRF and the SOHO SSR SEU rate are both modulated by the solar cycle and are highly correlated, except for sharp spikes in the SEU rate, caused by isolated SEP events, which also show up as discontinuities in the otherwise slowly decreasing solar ray efficiency. This allows to discriminate between effects with solar and non-solar origin and to compare the relative strength of both. We find that during solar cycle 23 (1996 Apr 1 -- 2008 Aug 31) only 6% of the total number of SSR SEUs were caused by SEPs; the remaining 94% were due to galactic cosmic rays. During the maximum period of cycle 23 (2000 Jan 1 -- 2003 Dec 31), the SEP contribution increased to 22%, and during 2001, the year with the highest SEP rate, to 30%. About 40% of the total solar array degradation during the 17 years from Jan 1996 through Feb 2013 can be attributed to proton events, i.e. the effect of a series of short-lived, violent SEP events is comparable to the cycle-integrated damage by cosmic rays.Comment: 10 pages, 5 figures accepted for publication in Cent. Eur. Astrophys. Bul

The coronal convection

We study the hydrogen Lyman emission in various solar features - now including Lyman-alpha observations free from geocoronal absorption - and investigate statistically the imprint of flows and of the magnetic field on the line profile and radiance distribution. As a new result, we found that in Lyman-alpha rasters locations with higher opacity cluster in the cell interior, while the network has a trend to flatter profiles. Even deeper self reversals and larger peak distances were found in coronal hole spectra. We also compare simultaneous Lyman-alpha and Lyman-beta profiles. There is an obvious correspondence between asymmetry and redshift for both lines, but, most surprisingly, the asymmetries of Lyman-alpha and Lyman-beta are opposite. We conclude that in both cases downflows determine the line profile, in case of Lyman-alpha by absorption and in the case of Ly-beta by emission. Our results show that the magnetically structured atmosphere plays a dominating role in the line formation and indicate the presence of a persisting downflow at both footpoints of closed loops. We claim that this is the manifestation of a fundamental mass transportation process, which Foukal back in 1978 introduced as the 'coronal convection'.Comment: 8 pages, 5 figures, accepted for publication in Cent. Eur. Astrophys. Bul

Cool and hot components of a coronal bright point

We performed a systematic study of the Doppler shifts and electron densities measured in an EUV bright point (hereafter BP) observed in more than 10 EUV lines with formation temperatures from log (T/K) p 4.5 to 6.3. Those parts of a BP seen in transition region and coronal lines are defined as its cool and hot components, respectively. We find that the transition from cool to hot occurs at a temperature around log (T/K) p 5.7. The two components of the BP reveal a totally different orientation and Doppler-shift pattern, which might result from a twist of the associated magnetic loop system. The analysis of magnetic field evolution and topology seems to favor a two-stage heating process, in which magnetic cancellation and separator reconnection are powering, respectively, the cool and hot components of the BP. We also found that the electron densities of both components of the BP are higher than those of the surrounding quiet Sun, and comparable to or smaller than active region densities.Comment: 4 pages, 4 figure

Upflows in funnel-like legs of coronal magnetic loops

The prominent blue shifts of Ne viii associated with the junctions of the magnetic network in the quiet Sun are still not well understood. By comparing the coronal magnetic-field structures as obtained by a potential-field reconstruction with the conspicuous blue-shift patches on the dopplergram of Ne viii as observed in an equatorial quiet-Sun region, we find that most of the regions with significant upflow are associated with the funnel-like legs of magnetic loops and co-spatial with increments of the line width. These quasi-steady upflows can be regarded as the signatures of mass supply to coronal loops. By using the square-root of the line intensity as a proxy for the plasma density, the mass flux of the upflow in each funnel can be estimated. We find that the mass flux is anti-correlated with the funnel's expansion factor as determined from the extrapolated magnetic field. One of the loop systems is associated with a coronal bright point, which was observed by several instruments and exhibited various morphologies in different wavelengths and viewing directions. A remarkable agreement between its magnetic structure and the associated EUV emission pattern was found, suggesting an almost potential-field nature of the coronal magnetic field. We also report the direct detection of a small-scale siphon flow by both STEREO satellites. However, this transient siphon flow occurred in a weak mixed-polarity-field region, which was outside the adjacent magnetic funnel, and thus it is perhaps not related to plasma upflow in the funnel. Based on these observations, we suggest that at upper-TR temperatures the dominant flows in quiet-Sun coronal loops are long-lasting upflows rather than siphon flows. We also discuss the implications for coronal heating and unresolved magnetic structures.Comment: 20 pages, 5 figures, accepted by Ap

Molecular absorption in transition region spectral lines

Aims: We present observations from the Interface Region Imaging Spectrograph (IRIS) of absorption features from a multitude of cool atomic and molecular lines within the profiles of Si IV transition region lines. Many of these spectral lines have not previously been detected in solar spectra. Methods: We examined spectra taken from deep exposures of plage on 12 October 2013. We observed unique absorption spectra over a magnetic element which is bright in transition region line emission and the ultraviolet continuum. We compared the absorption spectra with emission spectra that is likely related to fluorescence. Results: The absorption features require a population of sub-5000 K plasma to exist above the transition region. This peculiar stratification is an extreme deviation from the canonical structure of the chromosphere-corona boundary . The cool material is not associated with a filament or discernible coronal rain. This suggests that molecules may form in the upper solar atmosphere on small spatial scales and introduces a new complexity into our understanding of solar thermal structure. It lends credence to previous numerical studies that found evidence for elevated pockets of cool gas in the chromosphere.Comment: accepted by A&A Letter

The SUMER Data in the SOHO Archive

We have released an archive of all observational data of the VUV spectrometer Solar Ultraviolet Measurements of Emitted Radiation (SUMER) on SOHO that has been acquired until now. The operational phase started with 'first light' observations on 27 January 1996 and will end in 2014. Future data will be added to the archive when they become available. The archive consists of a set of raw data (Level 0) and a set of data that are processed and calibrated to the best knowledge we have today (Level 1). This communication describes step by step the data acquisition and processing that has been applied in an automated manner to build the archive. It summarizes the expertise and insights into the scientific use of SUMER spectra that has accumulated over the years. It also indicates possibilities for further enhancement of the data quality. With this article we intend to convey our own understanding of the instrument performance to the scientific community and to introduce the new, standard-FITS-format database.Comment: 38 pages, 9 figures, accepted for publication by Solar Physic

Spectroscopic evidence for helicity in explosive events

We report spectroscopic observations in support of a novel view of transition region explosive events, observations that lend empirical evidence that at least in some cases explosive events may be nothing else than spinning narrow spicule-like structures. Our spectra of textbook explosive events with simultaneous Doppler flow of a red and of a blue component are extreme cases of high spectro-scopic velocities that lack apparent motion, to be expected if interpreted as a pair of collimated, linearly moving jets. The awareness of this conflict led us to the alternate interpretation of redshift and blueshift as spinning motion of a small plasma volume. In contrast to the bidirectional jet scenario, a small volume of spinning plasma would be fully compatible with the observation of flows without detectable apparent motion. We suspect that these small volumes could be spicule-like structures and try to find evidence. We show observations of helical motion in macrospicules and argue that these features - if scaled down to a radius comparable to the slit size of a spectrometer - should have a spectroscopic signature similar to that observed in explosive events, while not easily detectable by imagers. Despite of this difficulty, evidence of helicity in spicules has been reported in the literature. This inspired us to the new insight that the same narrow spinning structures may be the drivers in both cases, structures that imagers observe as spicules and that in spectrometers cross the slit and are seen as explosive events. We arrive at a concept that supports the idea that explosive events and spicules are different manifestations of the same helicity driven scenario. Consequently, in such a case, a photospheric or subphotosperic trigger has to be assumed.Comment: 4 pages, 4 figures, accepted for publication as A&A lette

Observations Supporting the Role of Magnetoconvection in Energy Supply to the Quiescent Solar Atmosphere

Identifying the two physical mechanisms behind the production and sustenance of the quiescent solar corona and solar wind poses two of the outstanding problems in solar physics today. We present analysis of spectroscopic observations from the Solar and Heliospheric Observatory that are consistent with a single physical mechanism being responsible for a significant portion of the heat supplied to the lower solar corona and the initial acceleration of the solar wind; the ubiquitous action of magnetoconvection-driven reprocessing and exchange reconnection of the Sun's magnetic field on the supergranular scale. We deduce that while the net magnetic flux on the scale of a supergranule controls the injection rate of mass and energy into the transition region plasma it is the global magnetic topology of the plasma that dictates whether the released ejecta provides thermal input to the quiet solar corona or becomes a tributary that feeds the solar wind.Comment: 34 pages, 13 figures - In press Astrophysical Journal (Jan 1 2007

Hydrogen Lyman-alpha and Lyman-beta radiances and profiles in polar coronal holes

The hydrogen Lyman-alpha plays a dominant role in the radiative energy transport in the lower transition region, and is important for the stud- ies of transition-region structure as well as solar wind origin. We investigate the Ly-alpha profiles obtained by SUMER in coronal holes and quiet Sun. In a subset of these observations, also the Hi Lyman-beta, Si iii, and O vi lines were (quasi-) simultaneously recorded. We find that the distances between the two peaks of Ly-alpha profiles are larger in coronal holes than in the quiet Sun, indicating a larger opacity in coronal holes. This difference might result from the different magnetic structures or the different radiation fields in the two regions. Most of the Ly-beta profiles in the coronal hole have a stronger blue peak, in contrast to those in quiet-Sun regions. Whilst in both regions the Ly-alpha profiles are stronger in the blue peak. Although the asymmetries are likely to be produced by differential flows in the solar atmosphere, their detailed formation processes are still unclear. The radiance ratio between Ly-alpha and Ly-beta decreases towards the limb in the coronal hole, which might be due to the different opacity of the two lines. We also find that the radiance distributions of the four lines are set by a combined effect of limb brightening and the different emission level between coronal holes and quiet Sun.Comment: 13 pages,4 figures, 1 talbe, accepted by Ap

New views on the emission and structure of the solar transition region

The Sun is the only star that we can spatially resolve and it can be regarded as a fundamental plasma laboratory of astrophysics. The solar transition region (TR), the layer between the solar chromosphere and corona, plays an important role in solar wind origin and coronal heating. Recent high-resolution observations made by SOHO, TRACE, and Hinode indicate that the TR is highly nonuniform and magnetically structured. Through a combination of spectroscopic observations and magnetic field extrapolations, the TR magnetic structures and plasma properties have been found to be different in coronal holes and in the quiet Sun. In active regions, the TR density and temperature structures also differ in sunspots and the surrounding plage regions. Although the TR is believed to be a dynamic layer, quasi-steady flows lasting from several hours to several days are often present in the quiet Sun, coronal holes, and active regions, indicating some kind of plasma circulation/convection in the TR and corona. The emission of hydrogen Lyman lines, which originates from the lower TR, has also been intensively investigated in the recent past. Observations show clearly that the flows and dynamics in the middle and upper TR can greatly modify the Lyman line profiles.Comment: This paper has been withdrawn by the authors. This is a repetition of another record in ADS: New Astronomy Reviews, Volume 54, Issue 1-2, p. 13-3