Coronal Diagnostic Spectrometer Research Articles

Statistical study of extreme-ultraviolet nanoflares in the quiet-Sun transition region

Aims. We carried out a large statistical study of ubiquitous small-scale extreme-ultraviolet (EUV) brightenings in the nanoflare energy range in the quiet-Sun transition region to derive their properties, estimate their contribution to the heating of the solar atmosphere, and compare their numbers to the coronal events published in the literature. This is the first study of this magnitude at temperatures of about 2 × 105 K. Methods. We applied a numerical method for detecting small-scale transient events in long 1D image time series. We used data recorded by the SOHO Coronal Diagnostic Spectrometer (CDS) in the transition region line O V 62.97 nm (220 000 K) and analysed 702 h of sit-and-stare time series obtained with a cadence of 15.6 s and 50 h with a cadence of 20.5 s in different quiet-Sun areas at a fixed slit position. These data span from 1996 to 2011. This analysis used a different method and a vastly larger number of data than the previous high-cadence CDS study of small events. Results. We derive histograms of event durations, of the rise and decay time, of the peak intensity and thermal energy, and we obtain a continuous spectrum of their distributions for 117 000 events, spanning the nanoflare energy range with a linear spatial extent of 2−10 arcsec and with durations between 45 s and 40 min. The event peak intensity varied by a factor of 60. We demonstrated that all categories of small-scale events in the transition region are part of a continuum of activity. We obtain a total event rate of 460 s−1 on the entire surface of the Sun. This is more than four times greater than the coronal rate. The maximum value of the duration distribution occurs at 235 s, which is twice the duration of the coronal events. The decay time and rise time difference seen from the shortest to the longest events is symmetrical. We find two event populations: the power law of the smallest events that are confined to one pixel is far steeper for the peak count rates (index of −4.1) and thermal energy (index of −7) than the power law for combined larger events that extend over two or more pixels along the slit (thermal energy power-law index from −2.1 to −3.4). Conclusions. The power law of the thermal energy of the smallest events, extrapolated to lower energies (picoflares), may provide a huge amount of energy for heating the entire transition region plasma at temperatures of about 220 000 K. An extrapolation of only the flatter power law of the larger events can also account for the entire observed emission.

Temporal variations of the solar EUV network properties

The chromospheric network extended to the transition region as the solar EUV network disperses at the coronal level. The EUV emission lines from the transition region give information about different atmospheric heights. The network properties have been obtained from the daily spectroscopic data from the Coronal Diagnostic Spectrometer (CDS) on board the Solar and Heliospheric Observatory (SOHO) in two lines He i 584.5 A and O v 630 A. The synoptic changes in the line intensity, skewness of intensity distribution, mean contrast and network index with respect to the sunspot cycle were studied for a period of 11 years from 1996 to 2006. We have obtained the cross correlation of the monthly average of these quantities with the monthly sunspot number. The mean intensity and skewness of the two lines generally show a positive correlation with the sunspot cycle. The network contrast and network index of the He i shows a definite negative correlation with the sunspot cycle. The negative correlations shown by the He i line are unexpected which could be due to its anomalous behaviour. There are differences in the behaviour of the two lines regarding the time lag with the sunspot cycle and the magnitude of the correlation which is always higher for the He i line. The results have implications to the studies of solar irradiance, magnetic flux transfer and solar atmospheric dynamics.

The EUV spectrum of the Sun: Quiet- and active-Sun irradiances and chemical composition

We benchmark new atomic data against a selection of irradiances obtained from medium-resolution quiet-Sun spectra in the extreme ultraviolet (EUV), from 60 to 1040 Å. We used as a baseline the irradiances measured during solar minimum on 2008 April 14 by the prototype (PEVE) of the Solar Dynamics Observatory Extreme ultraviolet Variability Experiment (EVE). We took into account some inconsistencies in the PEVE data, using flight EVE data and irradiances we obtained from Solar and Heliospheric Observatory (SoHO) Coronal Diagnostics Spectrometer (CDS) data. We performed a differential emission measure and find overall excellent agreement (to within the accuracy of the observations, about 20%) between predicted and measured irradiances in most cases, although we point out several problems with the currently available ion charge-state distributions. We used the photospheric chemical abundances of Asplund et al. (2009, ARA&A, 47, 481). The new atomic data are nearly complete in this spectral range for medium-resolution irradiance spectra. Finally, we used observations of the active Sun in 1969 to show that the composition of the solar corona up to 1 MK is nearly photospheric in this case as well. Variations of a factor of 2 are present for higher-temperature plasma, which is emitted within active regions. These results are in excellent agreement with our previous findings.

Spectroscopic EUV observations of impulsive solar energetic particle event sources

Context. Remote observations of solar flare ion acceleration are rather limited. Theoretical predictions for signatures of ion acceleration in extreme ultraviolet (EUV) line profiles have been made. Previous tests involve observations of flares with no evidence for energetic ions. Aims. We aim to examine a source flare of impulsive (or 3He-rich) solar energetic particle events with EUV line spectroscopy. Methods. We inspected all (more than 90) reported 3He-rich flares of the previous solar cycle 23 and found only 4 (recurrent) jets in the field of view of the Coronal Diagnostic Spectrometer (CDS) on board the Solar and Heliospheric Observatory (SOHO). The jet with the most suitable spatial and temporal coverage was analyzed in detail. Results. Two enhanced (nonthermal) line broadenings are observed in the cooler chromospheric and transition-region lines, and they are localized near the site where the closed magnetic loops reconnect with the open magnetic field lines. The enhanced broadenings are both found at the sites with redshifts in the lines, surrounded by the region with blueshifts. One enhanced line broadening is associated with a small flare without energetic particle signatures, while another occurs just after the particle acceleration signatures of the main flare terminated. Conclusions. The observed excess broadening does not appear to be directly related to the energetic ion production and motions. Further investigations are required that cover the critical impulsive phase of the flare, ideally with high-resolution spectrometers that are specifically pointed to the 3He-rich solar energetic particle source.

Element Abundance Ratios in the Quiet Sun Transition Region

Abstract Element abundance ratios of magnesium to neon (Mg/Ne) and neon to oxygen (Ne/O) in the transition region of the quiet Sun have been derived by re-assessing previously published data from the Coronal Diagnostic Spectrometer on board the Solar and Heliospheric Observatory in the light of new atomic data. The quiet Sun Mg/Ne ratio is important for assessing the effect of magnetic activity on the mechanism of the first ionization potential (FIP) effect, while the Ne/O ratio can be used to infer the solar photospheric abundance of neon, which cannot be measured directly. The average Mg/Ne ratio is found to be 0.52 ± 0.11, which applies over the temperature region 0.2–0.7 MK, and is consistent with the earlier study. The Ne/O ratio is, however, about 40% larger, taking the value 0.24 ± 0.05 that applies to the temperature range 0.08–0.40 MK. The increase is mostly due to changes in ionization and recombination rates that affect the equilibrium ionization balance. If the Ne/O ratio is interpreted as reflecting the photospheric ratio, then the photospheric neon abundance is 8.08 ± 0.09 or 8.15 ± 0.10 (on a logarithmic scale for which hydrogen is 12), according to whether the oxygen abundances of M. Asplund et al. or E. Caffau et al. are used. The updated photospheric neon abundance implies a Mg/Ne FIP bias for the quiet Sun of 1.6 ± 0.6.

Variation in the Width of Transition Region Network Boundaries

The transition region network seen in solar extreme ultraviolet (EUV) lines is the extension of the chromospheric network. The network appears as an irregular web-like pattern over the solar surface outside active regions. The average width of transition region network boundaries is obtained from the two-dimensional autocorrelation function of SOlar and Heliospheric Observatory (SOHO)/Coronal Diagnostic Spectrometer (CDS) synoptic images of the Sun in two emission lines, He i 586 A and O v 630 A during 1996 – 2012. The width of the network boundaries is found to be roughly correlated with the solar cycle variation with a lag of about ten months. A comparison of the widths in the two emission lines shows that they are larger for the He i line. The SOHO/CDS data also show large asymmetry in boundary widths in the horizontal (x) and vertical (y) image directions, which is shown to be caused by image distortions that are due to instrumental effects. Since the network boundary widths are related to the magnetic flux concentration along the boundaries, the results are expected to have implications on the flux transport on the solar surface, solar cycle, and the mass and energy budget of network loops and jets.

The EUV spectrum of the Sun: SOHO, SEM, and CDS irradiances

We use calibrated extreme-UV (EUV) spectral irradiances obtained from observations with the Solar & Heliospheric Observatory (SOHO) Coronal Diagnostics Spectrometer Normal Incidence Spectrometer (NIS) to estimate the signal measured by the Solar EUV Monitor (SEM) first-order band, 260 to 340 A (SEM 1). The NIS observes the resonance lines He ii 304 A and Si xi 303 A directly in second order. The irradiances of the other lines in the band are estimated with a differential emission measure (DEM) modelling, using updated atomic data. The observations analysed here were obtained during 1998–2011, which means that they span the maximum and minimum of Cycle 23. The current knowledge of the SEM 1 degradation is used to find effective areas during the dates of the NIS observations and to predict the SEM 1 count rates across the band. The total count rates, estimated by folding the NIS-based spectra with the SEM 1 effective areas, agree very well (within 10–20%) with the observed ones during solar minimum conditions, when the He ii 304 A is the dominant contribution to the band. Excellent agreement with the Solar Dynamics Observatory (SDO) Extreme ultraviolet Variability Experiment (EVE) observations is also found. On the other hand, the predicted SEM 1 count rates during the Cycle-23 maximum are significantly (by about 30%) lower than the observed ones. The solar spectrum in the SEM 1 band changes significantly during maximum conditions, with the He ii 304 A only contributing about 40%. A significant fraction of the observed count rates comes from coronal emission in an off-band spectral region that has recently been discovered. An explanation for the discrepancy needs further investigation.

ON EXTREME-ULTRAVIOLET HELIUM LINE INTENSITY ENHANCEMENT FACTORS ON THE SUN

Helium lines in the solar EUV spectrum provide useful diagnostics of the solar atmosphere plasma. Helium is one of the few elements that exhibits strong emission lines formed in the lower transition region, and it is the second most abundant element in the Sun. However, the analysis of helium lines is complicated by their optical thickness and the unusual behavior of their intensities, with enhancements by a factor of up to 15 reported in the literature. Detailed study requires spatially and spectrally resolved observations in the EUV range, as well as sophisticated atomic modeling. The present work focuses on the application of the differential emission measure distribution to reproduce the observed fluxes of the He i and He ii lines observed by the Solar and Heliospheric Observatory (SOHO)/Coronal Diagnostic Spectrometer and Hinode/EIS spectrometers, using the latest atomic data. It is found that the comparison between observed and reconstructed intensities for He i resonance lines λλ537.03, 522.21, and 515.62 and the intercombination line λ591.41 does not show a real enhancement. By contrast, He i λ584.33, the first line of the resonance series, shows a depletion of a factor ~2, due to the opacity effect, as supported by non-LTE radiative transfer calculations. For single ionized helium lines 303.78 and 256.32 A, the enhancement factors obtained are higher and agree with those of previous work. The different behavior of He i and He ii lines suggests a mechanism that affects ionized helium only.

MODULATION OF GALACTIC COSMIC RAYS OBSERVED AT L1 IN SOLAR CYCLE 23

We analyze a unique 15 yr record of galactic cosmic-ray (GCR) measurements made by the SOHO Coronal Diagnostic Spectrometer NIS detectors, recording integrated GCR numbers with energies above 1.0 GeV between 1996 July and 2011 June. We are able to closely reproduce the main features of the SOHO/CDS GCR record using the modulation potential calculated from neutron monitor data by Usoskin et al. The GCR numbers show a clear solar cycle modulation: they decrease by 50% from the 1997 minimum to the 2000 maximum of the solar cycle, then return to the 1997 level in 2007 and continue to rise, in 2009 December reaching a level 25% higher than in 1997. This 25% increase is in contrast with the behavior of Ulysses/KET GCR protons extrapolated to 1 AU in the ecliptic plane, showing the same level in 2008-2009 as in 1997. The GCR numbers are inversely correlated with the tilt angle of the heliospheric current sheet. In particular, the continued increase of SOHO/CDS GCRs from 2007 until 2009 is correlated with the decrease of the minimum tilt angle from 30° in mid-2008 to 5° in late 2009. The GCR level then drops sharply from 2010 January, again consistent with a rapid increase of the tilt angle to over 35°. This shows that the extended 2008 solar minimum was different from the 1997 minimum in terms of the structure of the heliospheric current sheet.

Velocities and Linewidths in the Network and Cell Interiors of a Polar Coronal Hole Compared with the Quiet Sun

The relative Doppler velocities and linewidths in a polar coronal hole and the nearby quiet-Sun region have been obtained from the {\it Solar and Heliospheric Observatory} (SOHO)/{\it Coronal Diagnostic Spectrometer} (CDS) observations using emission lines originating at different heights in the solar atmosphere from the lower transition region (TR) to the low solar corona. The observed region is separated into the network and the cell interior and the behavior of the above parameters were examined in the different regions. It has been found that the histograms of Doppler velocity and width are generally broader in the cell interior as compared to the network. The histograms of Doppler velocities of the network and cell interior do not show significant difference in most cases. However, in the case of the quiet Sun, the Doppler velocities of the cell interior are more blueshifted than those of the network for the lowermost line He {\sc ii} 304 {\AA}, and an opposite behavior is seen for the uppermost line Mg {\sc ix} 368 {\AA}. The histograms of line width show that the network--cell difference is more prominent in the coronal hole. The network has significantly larger linewidth than the cell interior for the lowermost TR line He {\sc ii} 304 {\AA} for the quiet Sun. For coronal hole, this is true for the three lower TR lines He {\sc ii} 304 {\AA}, O {\sc iii} 599 {\AA}, and O {\sc v} 630 {\AA}. Also obtained are the correlations between the relative Doppler velocity and the width. A mild positive correlation is found for the lowermost transition region line He {\sc ii} 304 {\AA} which further decreases or become insignificant for the intermediate lines. For the low coronal line, Mg {\sc ix} 368 {\AA}, the correlation becomes strongly negative.

Study of an Extended EUV Filament Using SoHO/SUMER Observations of the Hydrogen Lyman Lines

A filament and its channel close to the solar disk were observed in the complete hydrogen Lyman spectrum, and in several EUV lines by the SUMER (Solar Ultraviolet Measurement of Emitted Radiation) and CDS (Coronal Diagnostic Spectrometer) spectrographs on the SoHO satellite, and in Hα by ground-based telescopes during a multi-instrument campaign in May 2005. It was a good opportunity to get an overview of the volume and the density of the cold plasma in the filament channel; these are essential parameters for coronal mass ejections. We found that the width of the filament depends on the wavelength in which the filament is observed (around 15 arcsec in Hα, 30 arcsec in Lα, and 60 arcsec in EUV). In Lα the filament is wider than in Hα because cool plasma, not visible in Hα, is optically thick at the Lα line center, and its presence blocks the coronal emission. We have derived physical plasma properties of this filament fitting the Lyman spectra and Hα profiles by using a 1D isobaric NLTE model. The vertical temperature profile of the filament slab is flat (T≈7000 K) with an increase to ≈ 20 000 K at the top and the bottom of the slab. From an analysis of the Lα and Hα source functions we have concluded that these lines are formed over the whole filament slab. We have estimated the geometrical filling factor in the filament channel. Its low value indicates the presence of multi-threads.

EXTREME-ULTRAVIOLET SPECTROSCOPIC OBSERVATION OF DIRECT CORONAL HEATING DURING A C-CLASS SOLAR FLARE

With the Coronal Diagnostic Spectrometer operating in rapid cadence (9.8 s) stare mode during a C6.6 flare on the solar disk, we observed a sudden brightening of Fe XIX line emission (formed at temperature T ≈ 8 MK) above the pre-flare noise without a corresponding brightening of emission from ions formed at lower temperatures, including He I (0.01 MK), O V (0.25 MK), and Si XII (2 MK). The sudden brightening persisted as a plateau of Fe XIX intensity that endured more than 11 minutes. The Fe XIX emission at the rise and during the life of the plateau showed no evidence of significant bulk velocity flows, and hence cannot be attributed to chromospheric evaporation. However, the line width showed a significant broadening at the rise of the plateau, corresponding to nonthermal velocities of at least 89 km s–1 due to reconnection outflows or turbulence. During the plateau He I, O V, and Si XII brightened at successively later times starting about 3.5 minutes after Fe XIX, which suggests that these brightenings were produced by thermal conduction from the plasma that produced the Fe XIX line emission; however, we cannot rule out the possibility that they were produced by a weak beam of nonthermal particles. We interpret an observed shortening of the O V wavelength for about 1.5 minutes toward the middle of the plateau to indicate new upward motions driven by the flare, as occurs during gentle chromospheric evaporation; relative to a quiescent interval shortly before the flare, the O V upward velocity was around –10 km s–1.

Comparison between observed and theoretical O IV line ratios in the UV/EUV solar spectrum as derived by SUMER, CDS and EIS

Aims. The joint use of SoHO Solar Ultraviolet Measurement of Emitted Radiation (SUMER), Coronal Diagnostic Spectrometer (CDS) and Hinode Extreme-ultraviolet Imaging Spectrometer (EIS) allow observation of several O iv line ratios, useful for temperature plasma diagnostics. Accurate atomic data are required to avoid interpretation errors in deriving the electron temperature from these ratios. Muglach et al. (2010) found that the measured ratio I(787.72 A)/I(279.93 A) is lower than the predicted value by a factor 2–5. Here the predicted value for this ratio is revised using updated atomic data. A comparison with other observed O iv line ratios is shown and the electron temperature is derived. Methods. The analysis is based on new observations made during the observational campaign of April 2009 and including three O iv multiplets. The theoretical ratios have been derived using the Atomic Data and Analysis Structure (ADAS) database and include comparison with the most recent calculations available in the literature. Results. The discrepancy for the O iv I(787.72 A)/I(279.93 A) ratio has been solved by adding transitions involving higher excited levels, which have been omitted in previous atomic models. This results in a decrease of the theoretical line ratio, providing electron temperatures in the range of logT = 5.17–5.39, close to the temperature expected from a plasma in ionisation equilibrium.

Observing the reconnection region in a transequatorial loop system

A vertical current sheet is a crucial element in many flare/coronal mass ejection (CME) models. For the first time, Liu et al. reported a vertical current sheet directly imaged during the flare rising phase with the EUV Imaging Telescope (EIT) onboard the Solar and Heliospheric Observatory (SOHO). As a follow-up study, here we present the comprehensive analysis and detailed physical interpretation of the observation. The current sheet formed due to the gradual rise of a transequatorial loop system. As the loop legs approached each other, plasma flew at ∼ 6 km s−1 into a local area where a cusp-shaped flare loop subsequently formed and the current sheet was seen as a bright, collimated structure of global length (≥ 0.25 R⊙) and macroscopic width ((5–10)×103km), extending from 50 Mm above the flaring loop to the border of the EIT field of view (FOV). The reconnection rate in terms of the Alfvén Mach number is estimated to be only 0.005–0.009, albeit a halo CME was accelerated from ∼ 400 km s−1 to ∼ 1300 km s−1 within the coronagraph FOV. Drifting pulsating structures at metric frequencies were recorded during the impulsive phase, implying tearing of the current sheet in the high corona. A radio Type III burst occurred when the current sheet was clearly seen in EUV, indicative of accelerated electrons beaming upward from the upper tip of the current sheet. A cusp-shaped dimming region was observed to be located above the post-flare arcade during the decay phase in EIT; both the arcade and the dimming expanded with time. With the Coronal Diagnostic Spectrometer (CDS) aboard SOHO, a clear signature of chromospheric evaporation was seen during the decay phase, i.e., the cusp-shaped dimming region was associated with plasma upflows detected with EUV hot emission lines, while the post-flare loop was associated with downflows detected with cold lines. This event provides a comprehensive view of the reconnection geometry and dynamics in the solar corona.

The EUV spectrum of the Sun: SOHO CDS NIS irradiances from 1998 until 2010

We present extreme-ultraviolet (EUV) irradiances of the Sun taken during the 1998‐2010 period from the Solar & Heliospheric Observatory (SOHO) Coronal Diagnostic Spectrometer (CDS) Normal Incidence Spectrograph (NIS). They were obtained from NIS full-Sun radiance observations, and represent the first set of EUV spectral observations spanning a solar cycle. We compare the CDS line irradiances with those obtained from rocket measurements, one that flew in May 1997 and one in April 2008, together wi th the Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) Solar EUV Experiment (SEE) EUV Grating Spectrograph (EGS) and various historical records. Excellent agreement (to within a relative 20%) is found in most cases, with a few notable exceptions. Lines formed in the transition region show very small changes with the solar cycle, with the exception of the helium lines. The irradiances of lines formed around 1 MK already change during the cycle by a factor∼ 5; for hotter lines (2.5 MK) the variability reaches factors of the order of 40. For lines formed around 1‐3 MK, and to a less extent, the helium lines, we find a good linea r correlation between CDS irradiances and the 10.7 cm radio flu x, although each line has a different coeffi cient. No correlation is found for the transition-region lines. Significant discrepancie s between the observed irradiances and those modelled is found. This confirms the importance in obtaining EUV spectral measurements of the solar irradiance.

Radiative and magnetic properties of solar active regions

Context. Global relationships between the photospheric magnetic flux and the extreme ultraviolet emission integrated over active region area have been studied in a previous paper by Fludra & Ireland (2008, A&A, 483, 609). Spatially integrated EUVline intensities are tightly correlated with the total unsigned magnetic flux, and yet these global power laws have been shown to be insufficient for accurately determining the coronal heating mechanism owing to the mathematical ill-conditioning of the inverse problem. Aims. Our aim is to establish a relationship between the EUV line intensities and the photospheric magnetic flux density on small spatial scales in active regions and investigate whether it provides a way of identifying the process that heats the coronal loops. Methods. We compare spatially resolved EUV transition region emission and the photospheric magnetic flux density. This analysis is based on the O V 62.97 nm line recorded by the SOHO Coronal Diagnostic Spectrometer (CDS) and SOHO MDI magnetograms for six solar active regions. The magnetic flux density φ is converted to a simulated O V intensity using a model relationship I(φ,L) = Cφ δ L λ , where the loop length L is obtained from potential magnetic field extrapolations. This simulated spatial distribution of O V intensities is convolved with the CDS instrument’s point spread function and compared pixel by pixel with the observed O V line intensity. Parameters δ and λ are derived to give the best fit for the observed and simulated intensities. Results. Spatially-resolved analysis of the transition region emission reveals the complex nature of the heating processes in active regions. In some active regions, particularly large, local intensity enhancements up to a factor of five are present. When areas with O V intensities above 3000 erg cm −2 s −1 sr −1 are ignored, a power law has been fitted to the relationship between the local O V line intensity and the photospheric magnetic flux density in each active region. The average power index δ from all regions is 0.4 ± 0.1 and λ = −0.15 ± 0.07. However, the scatter of intensities in all regions is significantly greater than ±3σ from the fitted model. We therefore determine for the first time an empirical lower boundary for the IOV − φ relationship that is the same for five active regions. We postulate that it represents a basal heating. Because this boundary is present in the spatially-resolved data, this is compelling proof that the magnetic field is one of the major factors contributing to the basal component of the heating of the coronal plasma. We discuss the implications for the diagnostics of the coronal heating mechanism.

EARLY CHROMOSPHERIC RESPONSE DURING A SOLAR MICROFLARE OBSERVED WITHSOHO's CDS ANDRHESSI

We observed a solar microflare with RHESSI and SOHO's Coronal Diagnostic Spectrometer (CDS) on 2009 July 5. With CDS we obtained rapid cadence (7 s) stare spectra within a narrow field of view toward the center of AR 11024. The spectra contain emission lines from ions that cover a wide range of temperature, including He I (< 0.025 MK), O V (0.25 MK), Si XII (2 MK), and Fe XIX (8 MK). The start of a precursor burst of He I and O V line emission preceded the steady increase of Fe XIX line emission by about 1 minute, and the emergence of 3-12 keV X-ray emission by about 4 minutes. Thus the onset of the microflare was observed in upper chromospheric (He I) and transition region (O V) line emission before it was detected in high temperature flare plasma emission. Redshifted O V emission during the precursor suggests explosive chromospheric evaporation, but no corresponding blueshifts were found with either Fe XIX (which was very weak) or Si XII. Similarly, in subsequent microflare brightenings the O V and He I intensities increased (between 49 s and almost 2 minutes) before emissions from the hot flare plasma. Although these time differences likely indicate heating by a nonthermal particle beam, the RHESSI spectra provide no additional evidence for such a beam. In intervals lasting up to about 3 minutes during several bursts, the He I and O V emission line profiles showed secondary, highly blueshifted ( approximately 200 km/s) components; during intervals lasting nearly 1 minute the velocities of the primary and secondary components were oppositely directed. Combined with no corresponding blueshifts in either Fe XIX or Si XII, this indicates that explosive chromospheric evaporation occurred predominantly at either comparatively cool temperatures (< 2 MK) or within a hot temperature range to which our observations were not sensitive (e.g., between 2 and 8 MK).

The EUV spectrum of the Sun: long-term variations in the SOHO CDS NIS spectral responsivities

We present SOHO Coronal Diagnostic Spectrometer (CDS) normal incidence, extreme-ultraviolet spectra of the Sun taken from the beginning of the mission in 1996 until now. We use various methods to study the performance of the instrument during such a long time span. Assuming that the basal chromospheric-transition region emission in the quiet parts of the Sun does not vary over the cycle, we find a slow decrease in the instrument sensitivity over time. We applied a correction to the NIS (Normal Incidence Spectrograph) data, using as a starting reference the NIS absolute calibration obtained from a comparison with a rocket flight in May 1997. We then obtained NIS full-Sun spectral irradiances from observations in 2008 and compared them with the EUV irradiances obtained from the rocket that flew on April 14, 2008 a prototype of the Solar Dynamics Observatory EVE instrument. Excellent agreement is found between the EUV irradiances from NIS and from the EVE-prototype, confirming the NIS radiometric calibration. The NIS instrument over 13 years has performed exceptionally well, with only a factor of about 2 decrease in responsivity for most wavelengths.

Intensity Distribution and Contrast of the Solar EUV Network

Intensity distributions of the EUV network and the cell interior in the solar atmosphere have been obtained in fourteen emission lines from Solar and Heliospheric Observatory (SOHO)/Coronal Diagnostic Spectrometer (CDS) observations. The formation temperature of the observed lines is in the range log T=4.90 – 6.06 (T in Kelvin), and hence they represent increasing heights in the solar atmosphere from the upper chromosphere and the transition region to the low corona. Intensity distributions of the cell interior have been found to be different in the quiet Sun and the coronal hole even at the lower transition region, which is at variance with some earlier results. The intensity contrast of the network with respect to the cell interior has been obtained for each line, and differences in the quiet Sun and the coronal hole have been examined. The network contrast, in general, is lower for the coronal hole as compared to the quiet Sun, but becomes equal to it in the upper transition region. The maximum contrast for both the regions is at about log T=5.3. Also obtained are the relative contributions of the network and the cell interior to the total intensity. The implications of the results for models of the transition region are briefly mentioned.

Multi-wavelength fine structure and mass flows in solar microflares

Aims. We study the multi-wavelength characteristics at high spatial resolution, as well as chromospheric evaporation signatures of solar microflares. To this end, we analyze the fine structure and mass flow dynamics in the chromosphere, transition region and corona of three homologous microflares (GOES class Methods. A multi-wavelength analysis using temporally and spatially highly resolved imaging data from the Dutch open telescope (H α , Ca ii H), the transition region and coronal explorer (17.1 nm), the extreme-ultraviolet imaging telescope (19.5 nm), and the Reuven Ramaty high energy solar spectroscopic imager ( ≳ 3 keV) was carried out. EUV line spectra provided by the coronal diagnostic spectrometer are searched for Doppler shifts in order to study associated plasma flows at chromospheric (He i, $T\sim3.9\times 10^4$ K), transition region (e.g. O v, $T\sim 2.6\times 10^5$ K), and coronal temperatures (Si xii, $T\sim 2\times 10^6$ K). RHESSI X-ray spectra provide information about non-thermal electrons. Results. The multi-wavelength appearance of the microflares is in basic agreement with the characteristics of large flares. For the first event, a complex flare sequence is observed in TRACE 17.1 nm images ($T\approx 1$ MK), which show several brightenings, narrow loops of enhanced emission, and an EUV jet. EIT 19.5 nm data ($T\approx 1.5$ MK) exhibit similar features for the third event. DOT measurements show finely structured chromospheric flare brightenings for all three events, loop-shaped fibrils of increased emission between H α brightenings, as well as a similar feature in Ca ii. For all three events, a RHESSI X-ray source (3–8 keV, $T \gtrsim 10$ MK) is located in between two chromospheric brightenings situated in magnetic flux of opposite polarity. We find the flow dynamics associated with the events to be very complex. In the chromosphere and transition region, CDS observed downflows for the first ($v \lesssim 40$ km s -1 ), and upflows for the second event ($v \lesssim 40$ km s -1 ). During the third microflare, we find upflows of $\lesssim $20 km s -1 and also weak downflows of ≲ 20 km s -1 in two separate brightenings. For all three microflares, multi-component fitting is needed for several profiles of He i, O v, and Ne vi lines observed at the flare peaks, which indicate spatially unresolved, oppositely directed flows of ≲ 180 km s -1 . We interpret these flows as twisting motions of the flare loops. Loop-shaped fibrils in between H α brightenings showing opposite flow directions ($v\approx5$ km s -1 ) are also observed in DOT H α Dopplergrams. RHESSI X-ray spectra show evidence of non-thermal bremsstrahlung for two of the three microflares. The electron beam flux density deposited in the chromosphere for these events is estimated to straddle the threshold heating flux between gentle and explosive evaporation.