E. O'Shea

GALEX high time-resolution ultraviolet observations of dMe flare events

Aims. We present near ultraviolet (NUV: 1750–2800 A) and far ultraviolet (FUV: 1350–1750 A) light-curves for flares on 4 nearby dMe-type stars (GJ 3685A, CR Dra, AF Psc and SDSS J084425.9+513830.5) observed with the GALEX satellite. Methods. Taking advantage of the time-tagged events recorded with the GALEX photon counting detectors, we present high temporal resolution (<0.01 s) analysis of these UV flare data. Results. A statistical analysis of 700 s of pre-flare quiescence data for both CR Dra and SDSS J084425.9+513830.5 failed to reveal the presence of significant micro-flare activity in time bins of 0.2, 1 and 10 s intervals. Using an appropriate differential emission measure for both the quiescent and flaring state, it is possible to reproduce the observed FUV:NUV flux ratios. A major determinant in reproducing this flux ratio is found to be the value of plasma electron density during the flare. We also searched the count rate data recorded during each of the four flare events for periodicity associated with magneto-hydrodynamic oscillations in the active region coronal loops. Significant oscillations were detected during the flare events observed on all 4 stars, with periodicities found in the 30 to 40 s range. Flare oscillations with this periodicity can be explained as acoustic waves in a coronal loop of length of ≈10 9 cm for an assumed plasma temperature of 5−20 × 10 6 K. This suggests a loop length for these M-dwarf flares of less than 1/10th of the stellar radii. We believe that this is the first detection of non-solar coronal loop flare oscillations observed at ultraviolet wavelengths.

Magnetoacoustic wave propagation in off-limb polar regions

Aims. To find evidence of magnetoacoustic waves in off-limb polar regions. Methods. Using temporal series data from the Coronal Diagnostic Spectrometer (CDS) on SOHO, we study oscillations found in radiant flux and velocity measurements from transition region and coronal spectral lines. We use Fourier techniques to measure phase delays between flux (“intensity”) oscillations and between velocity oscillations of different transition region-corona and corona-corona line pairs. We also measure the phase delays between flux and velocity oscillations (I − V). Results. The phase delays measured between different line pairs, when plotted over a –180 ◦ to +180 ◦ range, line up along diagonal lines corresponding to measurable and fixed time delays. The slopes of these diagonal lines suggest the outward propagation of waves. Using the measured time delays, we estimate propagation speeds for the different line pairs that indicate that the waves producing the observed phase delays are magnetoacoustic waves propagating at speeds close to the sound speed. In addition, we find that the phases occur at fixed integer frequencies of f / 4( 90 ◦ )a nd 3f /16 (67.5 ◦ ), instead of the expected interval of f (360 ◦ ), indicating that a “Doppler effect” is acting on the waves. Conclusions. From I − V measurements, we find evidence for fast magnetoacoustic waves to be predominantly present at coronal temperatures, while at transition region temperatures slow magnetoacoustic waves are more common. We find strong evidence for outwardly propagating slow magnetoacostic waves in off-limb polar regions and an indication that these propagating waves are influenced by some form of resonant cavity through which they pass.

On the widths and ratios of Mg x 609.79 and 624.94 Å lines in polar off-limb regions

Using measurements of Mg  609.78 and 624.94 A lines from the Coronal Diagnostic Spectrometer (CDS) on board SOHO, we seek to examine the variation of line width and line ratio in regions far off-limb at the Northern pole of the Sun. It is found that above ≈1150 �� the ratio of the two coronal Mg  resonance lines reduces to values that might be expected for a more radiatively dominant excitation mode. A comparison of the line widths with these ratios indicates that the line widths start to show a decrease in their values at the location where the dominant excitation changes from being collisionally to radiatively dominant, that is, at ≈1150 �� . We suggest that the decrease in the line widths above ≈1150 �� is likely to be due to a reduction in the non-thermal component of the line widths caused by a damping of upwardly propagating Alfven waves.

Very long period activity at the base of solar wind streams

Using time series data of spectral lines originating from a wide range of temperatures in the solar transition region, above a polar coronal hole, from SUMER (Solar Ultraviolet Measurements of Emitted Radiation) on SoHO (Solar and Heliospheric Observatory), we report on the detection of very long (≈170 min) periodic intensity fluctuations, above the limb. Our data also reveal long periodicities (10–90 min), previously observed with other SoHO instruments. With the acoustic cut-off frequency implying a maximum allowable period of ≈90 min, it is unclear whether these intensity fluctuations are due to waves or are the result of a recurrent magnetic reconnection process.

Plasma condensation in coronal loops

Aims. Using temporal series data from the Coronal Diagnostic Spectrometer (CDS) on SOHO, we seek to investigate plasma condensation through the use of spectroscopic analysis. Methods. Using a space-time (X-T) type plot we show, for a number of transition region (TR) and coronal lines, the variation of intensity (radiant flux) with time in an o -limb loop system. Noting a number of pixels where the intensity shows a sharp jump in value for the TR lines and a corresponding sharp decrease for the coronal lines, we investigate further and plot the intensity and velocity for these pixels as a function of time. Using standard Fourier techniques, together with a pre-whitening method, we measure all statistically significant frequencies present in the time series so created. Results. We find that, for the cooler lines of Hei 584 A and Ov 629 A, there are sharp increases in intensity, accompanied by large blueshifted velocities. In the case of Ov there is a substantial rebound to redshifted velocities when the strong brightening fades. The sharp increase in intensity and blueshifted velocity in the cooler lines is accompanied by a corresponding decrease in the intensity of the coronal lines (Mgx 624 A, Fexvi 360 A, Sixii 520 A), but with no corresponding change in their velocity values implying an evacuation of plasma. From the Fourier analysis, evidence is found for possible 1st and 2nd harmonics in the Hei and Ov lines, indicative of standing waves in loops. Conclusions. We conclude that what we are seeing is the first spectroscopic evidence of plasma condensation taking place in coronal loops.

On the statistical detection of propagating waves in polar coronal holes

Context. Waves are important to the study of dynamical processes in coronal holes and the acceleration of the fast solar wind. A spectral time series was taken with the SUMER spectrometer on-board SoHO on 20 October 1996. The observations were obtained in the N iv 765 A transition region line and the Ne viii 770 A line of the low corona. Aims. We detect the presence of waves and study their characteristic properties in terms of their propagation speeds and direction. Previous statistical studies, undertaken with data from the CDS spectrometer, report the presence of waves in these regions.We extend this analysis using SUMER observations. Methods. Using Fourier techniques, we measured the phase delays between intensity oscillations, as well as between velocity oscillations, in our two lines over the full range of available frequencies. From this, we were able to measure the travel time of the propagating oscillations, hence the propagation speeds of the waves that produce the oscillations. Results. We detect the long period oscillations in polar coronal holes on the disc. For network bright locations within coronal holes, our results indicate the presence of compressional waves with a dominant period of ≈25 min. However, we also find power at many other different frequencies, so we are able to study oscillations over a full range of frequencies. We find evidence of propagating waves with a fixed time delay in the coronal hole.We find, moreover, that there is a difference in the nature of the wave propagation in the bright (“network”), as opposed to the dark (“internetwork”) regions, with the latter sometimes showing evidence of downwardly propagating waves that are not seen in the former. From a measurement of propagation speeds, we find that all measured waves are subsonic in nature. Conclusions. Waves with different characteristics are found to be present at different locations in the observed coronal hole. The measured propagation speeds are subsonic, indicating that the majority of them are slow magneto-acoustic in nature. These waves, measured in the lower atmosphere, could accelerate farther at higher altitudes and may be important for the acceleration of the fast solar wind.

Blinker/macro-spicule activity in an off-limb polar region

Using measurements of Ov 629.73¯ from the Coronal Diagnostic Spectrometer (CDS) on board SOHO we report on blinker activity in o -limb regions above the Northern pole of the Sun. The blinkers are found to be occurring above a region showing strong dynamic activity, with evidence for evacuation of plasma. The presence of blinkers is discussed in terms of the heating of spicular material.

On oscillations found in an active region with EIS on Hinode

Aims. Using temporal series (40 �� slot) data in the Fe xii 195 A line from the extreme-ultraviolet imaging spectrometer (EIS) on board the Hinode satellite, we seek to carry out a statistical study of oscillations in an active region. Methods. Using wavelet techniques, we measure the frequency and duration of oscillations found in the time series data. Stokes I and V data from Hinode’s SOT are used to measure photospheric magnetic fields, while an EIS raster is used to provide estimates of LOS velocity and electron density. The measured frequencies are displayed as maps in different bands to aid their analysis. Results. Oscillations over a broad range of frequencies (2–154 mHz) are found throughout the active region at the temperature of Fe xii (log T = 6.1 K). Oscillations with frequencies between 2–8 mHz are concentrated in bright plage areas, while oscillations of higher frequency preferentially group at the edges of these areas. Evidence is found for harmonics on the boundary of the active region loop, indicating the presence of standing waves. From a measurement of the lengths and electron density of loops appearing in our active region field-of-view (FOV), together with a knowledge of the periods (frequencies) of the oscillations within the area of these loops, we find a magnetic field value of between 12.2 ± 2.08−18.9 ± 3.40 G, assuming standing fast kink waves. Conclusions. We conclude that waves, which the observed oscillations are signatures of, are common in the active region studied, over a broad range of frequencies. The change in the distribution of the oscillations present at frequencies less than 8 mHz and those at all frequencies above this indicates a variation of the wave mode with frequency, e.g., possibly from slow magnetoacoustic waves in the bright plage regions to fast magnetoacoustic waves at the boundaries of these regions. The preference for higher frequency oscillations to cluster on the edges of regions of high magnetic field is considered indicative of resonant absorption. Based on our measurements of magnetic field, we conclude that the oscillations found by us in the active region loop are fast kink waves. Previous studies such as O’Shea et al. (2001 )a ndIreland et al. (1999) carried out statistical studies of the oscillatory characteristics of active regions using transition region and coronal lines observed with CDS on board the SOHO spacecraft. Here, we seek to carry out a similar statistical study but with the superior temporal resolution and imaging capabilities of the EUV imaging spectrometer (EIS) on board the Hinode spacecraft (Culhane et al. 2007). In this work we will present a global statistical analysis of all intensity oscillations, measured in a Fe xii 195 A line, present in an active region on the disk. From this statistical study we will seek to identify different wave modes present in the active region and its structures. We note that O’Shea et al. (2001) found evidence for non-compressive (transverse) waves to be more prevalent in coronal lines (at temperatures between Log T = 6.0−6.3 K) than in lower temperatures lines. We also, therefore, seek to find further evidence for these non-compressive waves. From the measured oscillation frequencies (periods), together with the observed physical quantities of loop length, radius and electron density, we will seek to provide estimates of the magnetic field in the corona. We note that Nakariakov & Ofman (2001), using a similar technique to that which will be used by us, obtained values of 13 ± 9 G for global standing kink waves in an active region.

Transition region counterpart of a moving magnetic feature

Context. While moving magnetic features have been studied extensively at the photospheric level, the effect they have on the upper atmosphere remains largely unknown, and it is this which we seek to address in this work. Aims. In this work we aim to investigate the chromospheric and transition-region dynamics associated with a moving magnetic monopole by using spectral time-series and images. Methods. Cross correlation was applied to images taken by different instruments and at different times in order to spatially correlate brightenings seen at transition region temperatures with moving magnetic features seen in magnetograms. We used wavelet analysis to examine and compare the periodicities of time-series signals in different regions. Results. Oscillations with a multitude of frequencies are found in the chromospheric and transition-region brightenings associated with a moving magnetic monopole. The region of the brightenings shows a tendency to be blue-shifted when compared to the average motion of the entire field of view. The results indicate the presence of waves and/or flows carrying energy from the monopole to the higher atmosphere. Conclusions. We studied the influence of a moving magnetic monopole, as recorded by magnetograms, up to transition region temperatures. This suggests that the magnetic monopole, despite being small, can influence dynamics in the upper atmospheric layers.

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.