Sunspot Plumes Research Articles

Coronal condensation as the source of transition-region supersonic downflows above a sunspot

Context. Plasma loops or plumes rooted in sunspot umbrae often harbor downflows with speeds of 100 km s−1. These downflows are supersonic at transition region temperatures of ∼0.1 MK. The source of these flows is not well understood. Aims. We aim to investigate the source of sunspot supersonic downflows (SSDs) in active region 12740 using simultaneous spectroscopic and imaging observations. Methods. We identified SSD events from multiple raster scans of a sunspot by the Interface Region Imaging Spectrograph, and we calculated the electron densities, mass fluxes, and velocities of these SSDs. The extreme-ultraviolet (EUV) images provided by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory and the EUV Imager onboard the Solar Terrestrial Relations Observatory were employed to investigate the origin of these SSDs and their associated coronal rain. Results. Almost all the identified SSDs appear at the footpoints of sunspot plumes and are temporally associated with the appearance of chromospheric bright dots inside the sunspot umbra. Dual-perspective EUV imaging observations reveal a large-scale closed magnetic loop system spanning the sunspot region and a remote region. We observed that the SSDs are caused by repeated coronal rain that forms and flows along these closed magnetic loops toward the sunspot. One episode of coronal rain clearly indicates that reconnection near a coronal X-shaped structure first leads to the formation of a magnetic dip. Subsequently, hot coronal plasma catastrophically cools from ∼2 MK in the dip region via thermal instability. This results in the formation of a transient prominence in the dip, from which the cool gas mostly slides into the sunspot along inclined magnetic fields under the gravity. This drainage process manifests as a continuous rain flow, which lasts for ∼2 h and concurrently results in a nearly steady SSD event. The total mass of condensation (1.3 × 1014 g) and condensation rate (1.5 × 1010 g s−1) in the dip region were found to be sufficient to sustain this long-lived SSD event, which has a mass transport rate of 0.7 − 1.2 × 1010 g s−1. Conclusions. Our results demonstrate that coronal condensation in magnetic dips can result in the quasi-steady sunspot supersonic downflows.

A Hot Downflowing Model Atmosphere for Umbral Flashes and the Physical Properties of Their Dark Fibrils

Abstract We perform non-LTE inversions in a large set of umbral flashes, including the dark fibrils visible within them, and in the quiescent umbra by using the inversion code NICOLE on a set of full Stokes high-resolution Ca ii λ8542 observations of a sunspot at disk center. We find that the dark structures have Stokes profiles that are distinct from those of the quiescent and flashed regions. They are best reproduced by atmospheres that are more similar to the flashed atmosphere in terms of velocities, even if with reduced amplitudes. We also find two sets of solutions that finely fit the flashed profiles: a set that is upflowing, featuring a transition region that is deeper than in the quiescent case and preceded by a slight dip in temperature, and a second solution with a hotter atmosphere in the chromosphere but featuring downflows close to the speed of sound at such heights. Such downflows may be related, or even dependent, on the presence of coronal loops, rooted in the umbra of sunspots, as is the case in the region analyzed. Similar loops have been recently observed to have supersonic downflows in the transition region and are consistent with the earlier “sunspot plumes,” which were invariably found to display strong downflows in sunspots. Finally, we find, on average, a magnetic field reduction in the flashed areas, suggesting that the shock pressure is moving field lines in the upper layers.

OSCILLATORY RESPONSE OF THE SOLAR CHROMOSPHERE TO A STRONG DOWNFLOW EVENT ABOVE A SUNSPOT

ABSTRACT We report three-minute oscillations in the solar chromosphere driven by a strong downflow event in a sunspot. We used the Fast Imaging Solar Spectrograph of the 1.6 m New Solar Telescope and the Interface Region Imaging Spectrograph (IRIS). The strong downflow event is identified in the chromospheric and transition region lines above the sunspot umbra. After the event, oscillations occur at the same region. The amplitude of the Doppler velocity oscillations is 2 km s−1 and gradually decreases with time. In addition, the period of the oscillations gradually increases from 2.7 to 3.3 minutes. In the IRIS 1330 Å slit-jaw images, we identify a transient brightening near the footpoint of the downflow detected in the Hα+0.5 Å image. The characteristics of the downflowing material are consistent with those of sunspot plumes. Based on our findings, we suggest that the gravitationally stratified atmosphere came to oscillate with a three-minute period in response to the impulsive downflow event as was theoretically investigated by Chae & Goode.

Solar transition region above sunspots

We study the TR properties above sunspots and the surrounding plage regions, by analyzing several sunspot spectra obtained by SUMER in March 1999 and November 2006. We compare the SUMER spectra observed in the umbra, penumbra, plage, and sunspot plume regions. The Lyman line profiles averaged in each region are presented. For the sunspot observed in 2006, the electron densities, DEM, and filling factors of the TR plasma in the four regions are also investigated. The self-reversals of the Lyman line profiles are almost absent in umbral regions at different locations (heliocentric angle up to $49^\circ$) on the solar disk. In the sunspot plume, the Lyman lines are also not reversed, whilst the lower Lyman line profiles observed in the plage region are obviously reversed. The TR densities of the umbra and plume are similar and one order of magnitude lower than those of the plage and penumbra. The DEM curve of the sunspot plume exhibits a peak centered around $\log(T/\rm{K})\sim5.45$, which exceeds the DEM of other regions by one to two orders of magnitude at these temperatures. We also find that more than 100 lines, which are very weak or not observed anywhere else on the Sun, are well observed by SUMER in the sunspot, especially in the sunspot plume. We suggest that the TR above sunspots is higher and probably more extended, and that the opacity of the hydrogen lines is much smaller above sunspots, as compared to the TR above plage regions. Our result indicates that the enhanced TR emission of the sunspot plume is very likely to be the result of a large filling factor. The strongly enhanced emission at TR temperatures and the reduced continuum make many normally weak TR lines stick out clearly in the spectra of sunspot plumes.

Radio Measurements of the Height of Strong Coronal Magnetic Fields Above Sunspots at the Solar Limb

We measure coronal magnetic field strengths of 1750 G at a height of 8000 km above a large sunspot in NOAA AR 10652 at the west solar limb on 2004 July 29 using coordinated observations with the Very Large Array, the Transition Region and Coronal Explorer, and three instruments (CDS, EIT, MDI) aboard the Solar and Heliospheric Observatory. This observation is the first time that coronal radio brightness temperatures have been analyzed in a 15 GHz solar radio source projected above the limb. Observations at 8 GHz yield coronal magnetic field strengths of 960 G at a height of 12,000 km. The field strength measurements combine to yield a magnetic scale height LB = 6900 km. The radio brightness temperature maxima are located away from a sunspot plume that appears bright in EUV line emission formed at temperatures around several ×105 K. We use the density-sensitive emission-line intensity ratio of O IV 625.8 A/554.5 A to derive an electron density ne (in units of cm-3) of log ne = 10.1 ± 0.2 at the base of the plume.

The extent of 3-min oscillations in regions other than sunspot plumes

The active region, AR0554, was observed with NIS/CDS on board SoHO to examine the extent and range of oscillations from a range of features. Among all the NIS spectral lines analysed, significant oscillations were found in Si xii 520 A, Mg x 625 A, O v 629 A, and He i 522 A. The periods of the strongest oscillations in these lines were ≈10−20 min. After the dominant 10−20 min oscillations were filtered out from these lines, only O v 629 A showed significant (i.e. above the 95% significance level) shorter-period oscillations. Specifically, we found that weak but significant 3-min oscillations are not confined to the umbra/plume but can be seen in many bright locations. The duration of these non-umbral 3-min oscillations is mostly ≈20 min. In contrast, the 3-min oscillations within the umbra are strong and stable for longer than 50 min. The duration could be related to the size of the oscillating source region, rather than the lifetime of the oscillation. To find the possible sources of these 3-min oscillations outside of the umbra, we compared the oscillations of single pixels in different regions. The results of our comparison indicate a possible connection between the magnetic fields and the oscillations. Therefore, we suggest that 3-min oscillations may exist in many magnetic structures, but are often too weak to be seen in an unfiltered signal. The detection of waves in the solar atmosphere has been of great interest to both solar and plasma physicists. Observations of oscillatory phenomena in the solar atmosphere have increased dramatically in the past few years. Detailed observational studies, especially with the Solar and Heliospheric Observatory (SoHO) and the Transition Region And Coronal Explorer (TRACE) have provided a strong stimulus to theoretical developments. A new field of study, namely, coronal seismology has emerged, which allows the determination of coronal parameters by using information about the properties of the waves that the medium supports. This field is rapidly developing and providing insights into the processes at work in the corona. Mode frequencies and damping rates are two main observable properties of the waves, and, hence, are important in linking theoretical models with observations. Most of the observations to date have suggested that 3-min oscillations are associated with sunspots. De Moortel et al. (2002) reported the detection of 3-min oscillations in the coronal loops above sunspots and 5-min oscillations in other coronal loops. The observational results in Banerjee et al. (2002) indicate that 3-min oscillations are localized within the umbra. Maltby et al. (1999) found 3-min oscillations in coronal loops that are rooted in the umbra. By examining TRACE and CDS data, Brynildsen et al. (2002) found that the 3-min oscillations of transition region lines are present in the whole umbral region but the 3-min oscillations of coronal lines tend to localize in small regions that coincide with the endpoints of coronal loops. In their study, the 3-min oscillations were not observed with CDS at coronal temperatures within the spots, but were seen in the TRACE 171 A band. In a different joint observing campaign with CDS and TRACE, O’Shea et al. (2002) reported the detection of 3-min oscillations over a larger sunspot as seen in Fe xvi 335 A, a high temperature coronal line in CDS. This could indicate that the detection of oscillations crucially depends on the extent of the oscillating region.

Properties of a Sunspot Plume Observed with the Coronal Diagnostic Spectrometer Aboard theSolar and Heliospheric Observatory

We used three instruments (CDS, EIT, MDI) aboard the Solar and Heliospheric Observatory spacecraft to observe the large sunspot in NOAA Active Region 8539 on 1999 May 9 and 13. The spot contained a bright plume, most easily seen in EUV emission lines formed at 5.2 log T 5.7 (where T is the temperature in K), in its umbra on both dates. The plume's differential emission measure (DEM) exhibited one and only one broad peak, centered around log T ~ 5.8 on May 9 and around log T ~ 5.6 on May 13, and exceeded the DEM of the quiet Sun by more than an order of magnitude at these temperatures. The high-temperature portion of the plume's DEM resembled that of nearby quiet-Sun areas. Intensity ratios of the O IV lines at 625.8 and 554.5 A yield log ne (where ne is the electron density in cm-3) of 9.6 in the plume on May 9 and 9.7 on May 13; values of 9.4 and 9.4 were obtained in the quiet-Sun areas on the same dates. Based on abundance enhancements derived from transition region emission lines of Ca, an element with low first ionization potential, elemental abundances in the plume appear to be coronal rather than photospheric. The plume plasma reveals a bipolar Doppler velocity flow pattern, in which maximum downflows in excess of 37 km s-1 are observed in the northeast portion of the plume, and maximum upflows that exceed 52 km s-1 are observed in the northwest.

Coronal oscillations in the vicinity of a sunspot as observed by GIS/CDS

In this paper, we present results from a time series analysis of data from the Grazing Incidence Spectrometer (GIS), onboard SoHO. Our observations were concentrated at the boundary between the quiet Sun and an active region, close to a sunspot plume. The dominant oscillations in all the lines studied are of periods longer than 5 min. Although our observations were not taken directly above the sunspot, the 3-min oscillations, which are normally associated with the umbra, are seen in several spectral lines, suggesting some leakage perhaps via plume structures. The oscillations of coronal lines form wavepackets, and are intermittent with no obvious decay. The average time scale of the intermittence is of the order of 20 min, which would correspond to a source size of 2 arcsec.

Mass Flows in a Disappearing Sunspot Plume

We observed a large sunspot in NOAA Active Region 9535 on 2001 July 12, 15, 17, and 19 with three instruments (CDS, EIT, and MDI) on board SOHO. EUV emission lines of O IV (formed at logarithmic temperature of 5.2), O V (5.4), Ne IV (5.2), Ne V (5.5), Ne VI (5.6), Ne VII (5.7), and Ca X (5.9) revealed a large bright plume within the sunspot penumbra on July 12 and 15, a smaller, dimmer plume on July 17, and no plume on July 19. Downflows of 25 km s-1 or more were measured within the sunspot plume on July 12 and 15. By July 17 the downflow area had shrunk in size, the downflows had diminished in magnitude, and upflows were measured in the umbra and parts of the penumbra outside the plume. By July 19 downflows were no longer observed, but had been replaced entirely with upflows of 15-25 km s-1 in the umbra and portions of the penumbra. This is the first time that the disappearance of a sunspot plume has been observed to occur simultaneously with a dramatic change in flow velocity pattern in the sunspot plume and umbra. On July 12 upflows northeast of the sunspot were observed along with the downflows in the plume. Electron density measurements based on intensity ratios of the O IV lines at 625.8 and 554.5 A indicate a significantly greater value in the upflow zone than in the plume, consistent with siphon flow as the driver of the observed velocities; however, the line at 625.8 A is very weak and blended in the red wing of the Mg X line at 624.9 A, so derived densities are highly uncertain.

Oscillations in the Umbral Atmosphere

The results of simultaneous observations of oscillations in the chromosphere, transition region, and corona above nine sunspots are presented. The data are obtained through coordinated observing with the Solar and Heliospheric Observatory — SOHO and the Transition Region And Coronal Explorer — TRACE. Oscillations are detected above each umbra. The power spectra show one dominant frequency corresponding to a period close to 3 min. We show that the oscillations in the sunspot transition region can be modeled by upwardly propagating acoustic waves. In the corona the oscillations are limited to small regions that often coincide with the endpoints of sunspot coronal loops. Spectral observations show that oscillations in the corona contribute to the observed oscillations in the TRACE 171 A channel observations. We show that a recent suggestion regarding a connection between sunspot plumes and 3-min oscillations conflicts with the observations.

Close Association of an Extreme‐Ultraviolet Sunspot Plume with Depressions in the Sunspot Radio Emission

We obtained coordinated observations of the large sunspot in NOAA Region 8539 on 1999 May 9 and 13 with the Very Large Array and three instruments (CDS, EIT, MDI) aboard the Solar and Heliospheric Observatory satellite. The EUV observations reveal a plume in the sunspot umbra on both observing dates. The plume appears brightest in emission lines formed at temperatures between 1.6 × 105 and 5.0 × 105 K. Radio emission from the sunspot umbra is dominated by thermal gyroemission from the plume, which accounts for radio brightness temperatures <1 × 106 K in the umbra on both dates, as well as umbral brightness temperature depressions in the 4.535 and 8.065 GHz observations on May 13. A compact 14.665 GHz source persists near the umbra/penumbra boundary during our observing period, indicating a long-lived, compact flux tube with coronal magnetic field strength of at least 1748 G. It occurs in a portion of the sunspot that appears very dark in EUV emission.

Coronal Oscillations above Sunspots?

Observational data clearly indicate the presence of 3-min oscillations in sunspots in spectral lines covering a vast temperature range from the low chromosphere to those lines normally associated with coronal temperatures. We show that after folding in the sunspot plume emission measure distribution, the contribution functions for lines normally formed just below 1×106 K are shifted to lower temperatures. For example, the Fe ix 171 A line is shifted to 6×105 K for a Maxwellian distribution and to less than 5×105 K with a non-Maxwellian distribution. Other lines such as Mg ix 368 A will also be affected. This then questions some previous work regarding the suggested detection of 3-min oscillations in the corona above sunspots.

Observations of oscillations in the transition region above sunspots

Observations during two campaigns of the SUMER spectrograph and the EIT imager onboard SoHO were used to analyse oscillations in bright sunspot plumes. We report variations of both intensity and velocity seen in EUV emission lines originating in the sunspot upper chromosphere and the transition region. The wavelet analysis reveals rapidly changing conditions in the emitting volumes. Generally, oscillations in the 5 min range dominate in the chromosphere, while the transition region lines show oscillations at shorter periods (2 to 3 min). A drift of the oscillation period of the Doppler velocity from to within about 30 min can be explained by a strongly non-stationary behaviour of the upper chromosphere and transition region, probably related to downstream of material within the plume region. Synchronous EIT observations of the lower corona show no intensity oscillations. This hints either at a strong damping or a downward reflection in the upper transition region or lower corona.

Sunspot plume observations in the EUV

The electron density over a bright sunspot plume region was evaluated using lines within the O v 760 Å multiplet. The plume showed an intensity enhancement factor of ≈9 in the O v lines compared to regions outside the sunspot umbra. Internal agreement between the various ratios is excellent which would suggest that the O v lines do not suffer from blending problems. The derived mean electron densities for the sunspot plume is compared to in the surrounding area. The derived gas pressure in the plume compared to that outside leads weight to the suggestion that it is plasma flowing from outside the spot into the umbra at transition region temperatures that is the main cause of the down-flows. The plume non-thermal velocities are 5 to 10 smaller than those measured in regions external to the spot, suggesting significantly less turbulence within the umbra.

The O IV and S IV intercombination lines in the ultraviolet spectra of astrophysical sources

New electron density diagnostic line ratios are presented for the O IV 2s(2)2p P-2-2s2p(24)P and S IV 3s(2)3p P-2-3s3p(24)P intercombination lines around 1400 Angstrom comparison of these with observational data for the symbiotic star RR Telescopii (RR Tel), obtained with the Space Telescope Imaging Spectrograph (STIS), reveals generally very good agreement between theory and observation. However the S IV P-2(3/2)-P-4(1/2) transition at 1423.824 Angstrom is found to be blended with an unknown feature at 1423.774 Angstrom. The linewidth for the latter indicates that the feature arises from a species with a large ionization potential. In addition, the S IV P-2(1/2)-P-4(3/2) transition at 1398.044 Angstrom is identified for the first time (to our knowledge) in an astrophysical source other than the Sun, and an improved wavelength of 1397.166 Angstrom is measured for the O IV P-2(1/2)-P-4(3/2) line. The O IV and S IV line ratios in a sunspot plume spectrum, obtained with the Solar Ultraviolet Measurements of the Emitted Radiation (SUMER) instrument on the Solar and Heliospheric Observatory, are found to be consistent, and remove discrepancies noted in previous comparisons of these two ions. (Less)

Oscillations above sunspots: Evidence for propagating waves?

We present results of an analysis of time series data observed in sunspot umbral regions. The data were obtained in the context of the SOHO Joint Observing Program (JOP) 97 in September 2000. This JOP included the Coronal Diagnostic Spectrometer (CDS) and the Michelson Doppler Imaging (MDI) instrument, both part of SOHO, the TRACE satellite and various ground based observatories. The data was analysed by using both Fourier and wavelet time series analysis techniques. We find that oscillations are present in the umbra at all temperatures investigated, from the temperature minimum as measured by TRACE 1700 A up to the upper corona as measured by CDS $\ion{Fe}{xvi}$ 335 A ($\log T=6.4$ K). Oscillations are found to be present with frequencies in the range of 5.4 mHz (185 s) to 8.9 mHz (112 s). Using the techniques of cross-spectral analysis time delays were found between low and high temperature emission suggesting the possibility of both upward and downward wave propagation. It is found that there is typically a good correlation between the oscillations measured at the different emission temperatures, once the time delays are taken into account. We find umbral oscillations both inside and outside of sunspot plume locations which indicates that umbral oscillations can be present irrespective of the presence of these sunspot plumes. We find that a number of oscillation frequencies can exist co-spatially and simultaneously i.e. for one pixel location three different frequencies at 5.40, 7.65 and 8.85 mHz were measured. We investigate the variation of the relative amplitudes of oscillation with temperature and find that there is a tendency for the amplitudes to reach a maximum at the temperature of $\ion{O}{iii}$ (and less typically $\ion{O}{v}$ and $\ion{Mg}{ix}$) and then to decrease to reach a minimum at the temperature of $\ion{Mg}{x}$ ($\log T=6.0$ K), before increasing again at the temperature of $\ion{Fe}{xvi}$. We discuss a number of possible theoretical scenarios that might explain these results. From a measurement of propagation speeds we suggest that the oscillations we observe are due to slow magnetoacoustic waves propagating up along the magnetic field lines.

Redshift in sunspot plumes

The non-uniform spatial distributions of emission and wavelength shift in ten EUV lines formed in the chromosphere, transition region and corona above 50 sunspots are investigated, based on observations with the Coronal Diagnostic Spectrometer on the Solar and Heliospheric Observatory. The sunspot plumes are the most prominent features in the transition region line emissions. We find that almost all sunspots show a plume when one magnetic polarity dominates the region out to a distance of 50″ from the sunspot. Both the enhanced emission and the redshift in sunspot plumes increase with the line formation temperature, T, reach a maximum close to log T = 5.5 and cease to exist close to log T = 6.0. The mass flow in the corona is too small to explain the observed redshift in the transition region. We present a working hypothesis where gas at transition region temperatures moves in flow channels from the surroundings into the sunspot.

Plumes and oscillations in the sunspot transition region

The suggestion that sunspot transition region oscillations are a typical feature of the sunspot plumes is examined. The present observations show 3 min oscillations in the umbra that end at the umbral rim. We nd that sunspot plumes located above the umbra show these oscillations, in contrast to plumes above the penumbra. These two ndings suggest that the oscillations may be a property of the umbral transition region.

Transition region oscillations above sunspots

We present observations of three sunspots made in the EUV wavelength range with the Coronal Diagnostic Spectrometer (CDS) on SOHO. We examine time series of line intensities obtained with a 15-s cadence and calculate their Fourier power spectra and the wavelet transforms. We find oscillations in the chromosphere and transition region above sunspots in the temperature range 2 10 4 -4 10 5 K. Most of the spectral power is contained in the 5-8 mHz frequency range, and the dominant frequency is 5.9 mHz (170 s period). We find a relationship between the oscillations and sunspot plumes which are compact features located above sunspots and bright in the transition region lines emitted between 1.7 10 5 and 4 10 5 K. When the CDS slit crosses a sunspot plume, the bright O IV 554.5 Aand O V 629.7 Alines show clear 3 min oscillations. An extreme alternative interpretation that excludes the presence of oscillations in sunspot plumes carries the requirement that the adjacent low-intensity plasma oscillate with high relative amplitudes greater than 50 percent. We also observe oscillations in a fainter area above a sunspot, outside the sunspot plume. The oscillations seen at different temperatures are compared. Using the wavelet transform, we find variability of the oscillation period in the range 120-200 s.

Sunspot Plumes and Flow Channels

It is well known that sunspots are dark. This statement is not correct in the sunspot atmosphere between the chromosphere and the corona, where sunspots often are brighter than their surroundings. The brightest feature in the sunspot transition region is called a sunspot plume. Not all sunspots contain a plume. We find that 20 out of 21 sunspots show a plume when one magnetic polarity dominates the sunspot region out to a distance of 50 ′′ from the sunspot. Most sunspots show downflows that exceed 25 km s−1 in the sunspot plumes at temperatures close to 250 000 K. This downflow is not maintained by inflow from the corona, but by gas at transition region temperatures, streaming in flow channels from locations well outside the sunspot. We suggest that this inflow is a necessary requirement for the sunspot plume to occur and present a working hypothesis for the origin of sunspot plumes. This paper is the first thorough spectral analysis of sunspot plumes. It is based on simultaneous observations of ten or six EUV emission lines in 42 sunspot regions with the Coronal Diagnostic Spectrometer – CDS on the Solar and Heliospheric Observatory – SOHO. The line profiles are studied in detail with another SOHO instrument, the Solar Ultraviolet Measurements of Emitted Radiation – SUMER.