Observational features of equatorial coronal hole jets

Collimated ejections of plasma called "coronal hole jets" are commonly observed in polar coronal holes. However, such coronal jets are not only a specific features of polar coronal holes but they can also be found in coronal holes appearing at lower heliographic latitudes. In this paper we present some observations of "equatorial coronal hole jets" made up with data provided by the STEREO/SECCHI instruments during a period comprising March 2007 and December 2007. The jet events are selected by requiring at least some visibility in both COR1 and EUVI instruments. We report 15 jet events, and we discuss their main features. For one event, the uplift velocity has been determined as about 200 km/s, while the deceleration rate appears to be about 0.11 km/s2, less than solar gravity. The average jet visibility time is about 30 minutes, consistent with jet observed in polar regions. On the basis of the present dataset, we provisionally conclude that there are not substantial physical differences between polar and equatorial coronal hole jets.Comment: 9 pages, 8 figures, 1 table, accepted for publication in Annales Geophysicae, Special Issue:'Three eyes on the Sun-multi-spacecraft studies of the corona and impacts on the heliosphere

Motion magnification in coronal seismology

We introduce a new method for the investigation of low-amplitude transverse oscillations of solar plasma non-uniformities, such as coronal loops, individual strands in coronal arcades, jets, prominence fibrils, polar plumes, and other contrast features, observed with imaging instruments. The method is based on the two-dimensional dual tree complex wavelet transform (DT$\mathbb{C}$WT). It allows us to magnify transverse, in the plane-of-the-sky, quasi-periodic motions of contrast features in image sequences. The tests performed on the artificial data cubes imitating exponentially decaying, multi-periodic and frequency-modulated kink oscillations of coronal loops showed the effectiveness, reliability and robustness of this technique. The algorithm was found to give linear scaling of the magnified amplitudes with the original amplitudes provided they are sufficiently small. Also, the magnification is independent of the oscillation period in a broad range of the periods. The application of this technique to SDO/AIA EUV data cubes of a non-flaring active region allowed for the improved detection of low-amplitude decay-less oscillations in the majority of loops.Comment: Accepted for publication in Solar Physic

Modelling observed decay-less oscillations as resonantly enhanced Kelvin-Helmholtz vortices from transverse MHD waves and their seismological application

In the highly structured solar corona, resonant absorption is an unavoidable mechanism of energy transfer from global transverse MHD waves to local azimuthal Alfv\'en waves. Due to its localised nature, a direct detection of this mechanism is extremely difficult. Yet, it is the leading theory explaining the observed fast damping of the global transverse waves. However, at odds with this theoretical prediction, recent observations indicate that in the low amplitude regime such transverse MHD waves can also appear decay-less, a yet unsolved phenomenon. Recent numerical work has shown that Kelvin-Helmholtz instabilities (KHI) often accompany transverse MHD waves. In this work, we combine 3D MHD simulations and forward modelling to show that for currently achieved spatial resolution and observed small amplitudes, an apparent decay-less oscillation is obtained. This effect results from the combination of periodic brightenings produced by the KHI and the coherent motion of the KHI vortices amplified by resonant absorption. Such effect is especially clear in emission lines forming at temperatures that capture the boundary dynamics rather than the core, and reflects the low damping character of the local azimuthal Alfv\'en waves resonantly coupled to the kink mode. Due to phase mixing, the detected period can vary depending on the emission line, with those sensitive to the boundary having shorter periods than those sensitive to the loop core. This allows to estimate the density contrast at the boundary.Comment: 8 pages, 4 figures; Accepted for publication in The Astrophysical Journal Letter

Non-damping oscillations at flaring loops

Context. QPPs are usually detected as spatial displacements of coronal loops in imaging observations or as periodic shifts of line properties in spectroscopic observations. They are often applied for remote diagnostics of magnetic fields and plasma properties on the Sun. Aims. We combine imaging and spectroscopic measurements of available space missions, and investigate the properties of non-damping oscillations at flaring loops. Methods. We used the IRIS to measure the spectrum over a narrow slit. The double-component Gaussian fitting method was used to extract the line profile of Fe XXI 1354.08 A at "O I" window. The quasi-periodicity of loop oscillations were identified in the Fourier and wavelet spectra. Results. A periodicity at about 40 s is detected in the line properties of Fe XXI, HXR emissions in GOES 1-8 A derivative, and Fermi 26-50 keV. The Doppler velocity and line width oscillate in phase, while a phase shift of about Pi/2 is detected between the Doppler velocity and peak intensity. The amplitudes of Doppler velocity and line width oscillation are about 2.2 km/s and 1.9 km/s, respectively, while peak intensity oscillate with amplitude at about 3.6% of the background emission. Meanwhile, a quasi-period of about 155 s is identified in the Doppler velocity and peak intensity of Fe XXI, and AIA 131 A intensity. Conclusions. The oscillations at about 40 s are not damped significantly during the observation, it might be linked to the global kink modes of flaring loops. The periodicity at about 155 s is most likely a signature of recurring downflows after chromospheric evaporation along flaring loops. The magnetic field strengths of the flaring loops are estimated to be about 120-170 G using the MHD seismology diagnostics, which are consistent with the magnetic field modeling results using the flux rope insertion method.Comment: 9 pages, 9 figures, 1 table, accepted by A&

Blobs in recurring EUV jets

In this paper, we report our discovery of blobs in the recurrent and homologous jets that occurred at the western edge of NOAA active region 11259 on 2011 July 22. The jets were observed in the seven extreme-ultraviolet (EUV) filters of the Atmospheric Imaging Assembly (AIA) instrument aboard the Solar Dynamics Observatory (SDO). Using the base-difference images of the six filters (94, 131, 171, 211, 193, and 335 {\AA}), we carried out the differential emission measure (DEM) analyses to explore the thermodynamic evolutions of the jets. The jets were accompanied by cool surges observed in the H$\alpha$ line center of the ground-based telescope in the Big Bear Solar Observatory. The jets that had lifetimes of 20$-$30 min recurred at the same place for three times with interval of 40$-$45 min. Interestingly, each of the jets intermittently experienced several upward eruptions at the speed of 120$-$450 km s$^{-1}$. After reaching the maximum heights, they returned back to the solar surface, showing near-parabolic trajectories. The falling phases were more evident in the low-$T$ filters than in the high-$T$ filters, indicating that the jets experienced cooling after the onset of eruptions. We identified bright and compact blobs in the jets during their rising phases. The simultaneous presences of blobs in all the EUV filters were consistent with the broad ranges of the DEM profiles of the blobs ($5.5\le \log T\le7.5$), indicating their multi-thermal nature. The median temperatures of the blobs were $\sim$2.3 MK. The blobs that were $\sim$3 Mm in diameter had lifetimes of 24$-$60 s. To our knowledge, this is the first report of blobs in coronal jets. We propose that these blobs are plasmoids created by the magnetic reconnection as a result of tearing-mode instability and ejected out along the jets.Comment: 22 pages, 10 figure