G. R. Gupta

Propagating waves in polar coronal holes as seen by SUMER & EIS

Context. To study the dynamics of coronal holes and the role of waves in the acceleration of the solar wind, spectral observations were performed over polar coronal hole regions with the SUMER spectrometer on SoHO and the EIS spectrometer on Hinode. Aims. Using these observations, we aim to detect the presence of propagating waves in the corona and to study their properties. Methods. The observations analysed here consist of SUMER spectra of the Ne VIII 770 angstrom line (T = 0.6 MK) and EIS slot images in the Fe XII 195 angstrom line (T = 1.3 MK). Using the wavelet technique, we study line radiance oscillations at different heights from the limb in the polar coronal hole regions. Results. We detect the presence of long period oscillations with periods of 10 to 30 min in polar coronal holes. The oscillations have an amplitude of a few percent in radiance and are not detectable in line-of-sight velocity. From the time distance maps we find evidence for propagating velocities from 75 km s(-1) (Ne VIII) to 125 km s(-1)(Fe XII). These velocities are subsonic and roughly in the same ratio as the respective sound speeds. Conclusions. We interpret the observed propagating oscillations in terms of slow magneto-acoustic waves. These waves can be important for the acceleration of the fast solar wind.

Propagating MHD Waves in Coronal Holes

Coronal holes are the coolest and darkest regions of the upper solar atmosphere, as observed both on the solar disk and above the solar limb. Coronal holes are associated with rapidly expanding open magnetic fields and the acceleration of the high-speed solar wind. During the years of the solar minima, coronal holes are generally confined to the Sun’s polar regions, while at solar maxima they can also be found at lower latitudes. Waves, observed via remote sensing and detected in-situ in the wind streams, are most likely responsible for the wind and several theoretical models describe the role of MHD waves in the acceleration of the fast solar wind. This paper reviews the observational evidences of detection of propagating waves in these regions. The characteristics of the waves, like periodicities, amplitude, speed provide input parameters and also act as constraints on theoretical models of coronal heating and solar wind acceleration.

Sunspot waves and triggering of homologous active region jets

We present and discuss multiwavelength observations of five homologous recurrent solar jets that occurred in active region NOAA 11133 on 2010 December 11. These jets were well observed by the Solar Dynamic observatory (SDO) with high spatial and temporal resolution. The speed of the jets ranged between 86 and 267 km s−1. A type III radio burst was observed in association with all the five jets. The investigation of the overall evolution of magnetic field in the source regions suggested that the flux was continuously emerging on longer term. However, all the jets but J5 were triggered during a local dip in the magnetic flux, suggesting the launch of the jets during localized submergence of magnetic flux. Additionally, using the PFSS modelling of the photospheric magnetic field, we found that all the jets were ejected in the direction of open field lines. We also traced sunspot oscillations from the sunspot interior to foot-point of jets and found presence of ∼3 min oscillations in all the SDO/AIA (Atmospheric Imaging Assembly) passbands. The wavelet analysis revealed an increase in amplitude of the oscillations just before the trigger of the jets, that decreased after the jets were triggered. The observations of increased amplitude of the oscillation and its subsequent decrease provides evidence of wave-induced reconnection triggering the jets.

The dynamical behaviour of a jet in an on-disk coronal hole observed with AIA/SDO

Aims. Extreme ultraviolet (EUV) jets situated in coronal holes are thought to play an important role in supplying heated material to the corona and solar wind. The multi-wavelength capabilities and high signal-to-noise ratio of detectors on board the Solar Dynamic Observatory (SDO) allow for detailed study of these jets’ evolution. We aim to exploit SDO’s capabilities to reveal information on the jet dynamics and to obtain estimates for plasma properties associated with the jets. Methods. We studied the dynamics of an EUV jet with SDO at a coronal hole boundary. The details of the jet evolution are discussed and measurements of the jet’s parameters, e.g. length, width, life time, and outward speed, are obtained. Furthermore, automated emission measure analysis is exploited to determine estimates for the temperature and density of the jet. A propagating transverse wave supported by the jet spire is also observed. Measurements of the wave properties are exploited for magneto-seismology and are used in conjunction with the emission measure results to estimate the magnetic field strength of the jet. Results. We present a detailed description of the jet’s evolution, with new evidence of plasma flows, prior to the jet’s initiation, along the loops at the base of the jet and also find further evidence that flows along the jet spire consist of multiple, quasi-periodic smallscale plasma ejection events. In addition, spectroscopic analysis reveal that the jet has temperatures of log 5.89 ± 0.08 K and electron densities of log 8.75 ± 0.05 cm −3 . Measured properties of the transverse wave provide evidence that a strong damping of the wave occurs as it propagates along the jet spire with speeds of ∼110 km s −1 . The magneto-seismological inversion of the wave parameters provides values of B = 1.21 ± 0.2 G along the jet spire, which is in line with previous estimates for open fields in coronal holes.

Observations of dissipation of slow magneto-acoustic waves in a polar coronal hole

We focus on a polar coronal hole region to find any evidence of dissipation of propagating slow magneto-acoustic waves. We obtained time-distance and frequency-distance maps along the plume structure in a polar coronal hole. We also obtained Fourier power maps of the polar coronal hole in different frequency ranges in 171~\AA\ and 193~\AA\ passbands. We performed intensity distribution statistics in time domain at several locations in the polar coronal hole. We find the presence of propagating slow magneto-acoustic waves having temperature dependent propagation speeds. The wavelet analysis and Fourier power maps of the polar coronal hole show that low-frequency waves are travelling longer distances (longer detection length) as compared to high-frequency waves. We found two distinct dissipation length scales of wave amplitude decay at two different height ranges (between 0--10 Mm and 10--70 Mm) along the observed plume structure. The dissipation lengths obtained at higher height range show some frequency dependence. Individual Fourier power spectrum at several locations show a power-law distribution with frequency whereas probability density function (PDF) of intensity fluctuations in time show nearly Gaussian distributions. Propagating slow magneto-acoustic waves are getting heavily damped (small dissipation lengths) within the first 10~Mm distance. Beyond that waves are getting damped slowly with height. Frequency dependent dissipation lengths of wave propagation at higher heights may indicate the possibility of wave dissipation due to thermal conduction, however, the contribution from other dissipative parameters cannot be ruled out. Power-law distributed power spectra were also found at lower heights in the solar corona, which may provide viable information on the generation of longer period waves in the solar atmosphere.

Spectroscopic Observations of Propagating Disturbances in a Polar Coronal Hole: Evidence of Slow Magneto-acoustic Waves

Aims. We focus on detecting and studying quasi-periodic propagating features that have been interpreted both in terms of slow magneto-acoustic waves and of high speed upflows. Methods. We analyze long duration spectroscopic observations of the on-disk part of the south polar coronal hole taken on 1997 February 25 by the SUMER spectrometer aboard SOHO. We calibrated the velocity with respect to the off-limb region and obtain time‐distance maps in intensity, Doppler velocity and line width. We also perform a cross correlation analysis on different time series curves at different latitudes. We study average spectral line profiles at t he roots of propagating disturbances and along the propagating ridges, and perform a red-blue asymmetry analysis. Results. We find the clear presence of propagating disturbances in int ensity and Doppler velocity with a projected propagation speed of about 60± 4.8 km s −1 and a periodicity of≈14.5 min. To our knowledge, this is the first simultaneous det ection of propagating disturbances in intensity as well as in Doppler velocity in a cor onal hole. During the propagation, an intensity enhancement is associated with a blue-shifted Doppler velocity. These disturbances are clearly seen in intensity also at higher latitudes (i.e. c loser to the limb), while disturbances in Doppler velocity becomes faint there. The spectral line profiles averaged along the propagating r idges are found to be symmetric, to be well fitted by a single Gaussian, and hav e no noticeable red-blue asymmetry. Conclusions. Based on our analysis, we interpret these disturbances in terms of propagating slow magneto-acoustic waves.

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.

IRIS AND SDO OBSERVATIONS OF RECURRENT EXPLOSIVE EVENTS

Observations of recurrent explosive events (EEs) with timescales of 3?5 minutes are reported. These EEs have been observed with the Interface Region Imaging Spectrograph (IRIS) and have a spatial dimension of along the slit. The spectral line profiles of C ii ? and Si iv ? become highly broadened both in red as well as blue wings. Several absorption lines on top of the broadened profiles were identified. In addition, emission lines corresponding to neutral lines such as Cl i 1351.66 ?, C i 1354.29 ?, and C i 1355.84 ? were identified. The C i 1354.29 ? and C i 1355.84 ? lines were found only during the EEs, whereas Cl i 1351.66 ? broadens during the EEs. The estimated lower limit on electron number density obtained using the line ratios of Si iv and O iv is about cm?3, suggesting that the observed events are most likely occurring at heights corresponding to a lower chromosphere. To the best of our knowledge, for the first time we have detected short-period variability (30 s and 60?90 s) within the EE bursts. Observations of the photospheric magnetic fields underneath EEs indicate that a negative polarity field emerges in the neighborhood of oppositely directed positive fields that undergo repetitive reconnection (magnetic flux cancellation) events. The dynamic changes observed in AIA 1700 ?, 1600 ?, C ii 1330 ?, and Si iv 1400 ? intensity images corresponded very well with the emergence and cancellation of photospheric magnetic field (negative polarity) on a timescale of 3?5 minutes. The observations reported here suggest that these EEs are formed due to magnetic reconnection and are occurring in the lower chromosphere.

Spectroscopic Observations of a Coronal Loop: Basic Physical Plasma Parameters Along the Full Loop Length

Coronal loops are the basic structures of the solar transition region and corona. Understanding of the physical mechanisms behind the loop heating, plasma flows, and filling are still considered a major challenge in solar physics. The mechanism(s) should be able to supply mass to the corona from the chromosphere and to heat the plasma over 1 MK within a small distance of a few hundred kilometers from the chromosphere to the corona. This problem makes coronal loops an interesting target for detailed study. In this study, we focus on spectroscopic observations of a coronal loop observed in its full length in various spectral lines as recorded by the Extreme-ultraviolet Imaging Spectrometer on board Hinode. We derive physical plasma parameters such as electron density, temperature, pressure, column depth, and filling factors along the loop length from one footpoint to the another. The obtained parameters are used to infer whether the observed coronal loop is overdense or underdense with respect to gravitational stratification of the solar atmosphere. These new measurements of physical plasma parameters, from one footpoint to another, provide important constraints on the modeling of the mass and energy balance in coronal loops.

Characteristics of polar coronal hole jets

High spatial- and temporal-resolution images of coronal hole regions show a dynamical environment where mass flows and jets are frequently observed. These jets are believed to be important for the coronal heating and the acceleration of the fast solar wind. We studied the dynamics of two jets seen in a polar coronal hole with a combination of imaging from EIS and XRT onboard Hinode. We observed drift motions related to the evolution and formation of these small-scale jets, which we tried to model as well. We found observational evidence that supports the idea that polar jets are very likely produced by multiple small-scale reconnections occurring at different times in different locations. These eject plasma blobs that flow up and down with a motion very similar to a simple ballistic motion. The associated drift speed of the first jet is estimated to be