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Abstract:Magnetic field reconnection is believed to play a fundamental role in magnetized plasma systems throughout the universe 1 , including planetary magnetospheres, magnetars, and accretion discs around black holes. This letter presents extreme ultraviolet (EUV) and X-ray observations of a solar flare showing magnetic reconnection with a level of clarity not previously achieved. The multi-wavelength EUV observations from SDO/AIA show inflowing cool loops and newly formed, outflowing hot loops, as predicted. RHESSI X-ray spectra and images simultaneously show the appearance of plasma heated to >10 MK at the expected locations. These two data sets provide solid visual evidence of magnetic reconnection producing a solar flare, validating the basic physical mechanism of popular flare models. However, new features are also observed that need to be included in reconnection and flare studies, such as 3D non-uniform, non-steady, and asymmetric evolution. Main Text:The early concept of magnetic reconnection was proposed in the 1940s 1 to explain energy release in solar flares, the most powerful explosive phenomena in the solar system. The reconnection process reconfigures the field topology and converts magnetic energy to thermal energy, mass motions, and particle acceleration. The theories and related numerical simulations, especially 3D modelling, are still subjects of extensive research to obtain a full understanding of the process under different conditions. Meanwhile, observational studies have made progress in finding evidence of reconnection and deriving its physical properties to constrain and improve the theories.In-situ measurements of the magnetic field, plasma parameters, and particle distributions have shown the existence of magnetic reconnection in laboratory plasmas 2, 3 , fusion facilities, and magnetospheres of planets 4,5 . Such in-situ measurements are still not possible in the extremely hot solar atmosphere. Instead, observations are obtained through remote sensing of emissions across the entire electromagnetic spectrum from radio to X rays and gamma rays.However, in the corona 6 the magnetic field pressure dominates the plasma pressure (low plasma beta) and the magnetic flux is "frozen into" the highly conductive plasma. As a result, the emitting plasma trapped in coronal loops outlines the geometry of the magnetic field and their structural changes reflect the changes of the field connectivity (in general). Considerable pieces of evidence for features likely linked to reconnection in solar flares 7,8 and coronal mass ejections (CMEs 9 ) have been obtained so far. These include signatures of plasma inflow/outflow 10-14 , hot cusp structures 15 , current sheets [16][17][18] , fast-mode standing shocks 19 , and plasmoid ejection 20 .However, most evidence has been indirect and fragmented. Detailed observations of the complete picture are still missing due to the highly dynamic flare/CME process and limited observational capabilities.The launch of the Solar Dynamic Observatory (SDO 21 ) in 2010 sign...

Abstract. We report on a loop oscillation event observed by TRACE in the 195 Å bandpass at the solar limb. The difference images reveal the first evidence for vertical kink oscillations of the loop, i.e., alternately expanding and shrinking motions, in contrast to horizontal transverse loop oscillations reported before, which exhibit swaying motions. Based on the 3D geometry of the oscillating loop derived from the observation by fitting with a circular or elliptical loop model, we simulate these two kinds of global kink modes and find that only the vertical oscillations produce a signature in the difference images in agreement with the observations. We also find that the oscillating loop is associated with intensity variations. Based on the measured displacement amplitude, the simulation predicts an intensity variation of about 13% due to density changes produced by the change of the loop length. The observed intensity changes have the same sign but are considerably larger than the predictions although the error bars are also large. This suggests that these oscillations are compressible.

Abstract.We give an extensive overview of Doppler shift oscillations in hot active region loops obtained with SUMER. The oscillations have been detected in loops sampled 50−100 arcsec off the limb of the Sun in ultraviolet lines, mainly Fe  and Fe , with formation temperature greater than 6 MK. The spectra were recorded along a 300 arcsec slit placed at a fixed position in the corona above the active regions. Oscillations are usually seen along an extended section of the slit and often appear to be from several different portions of the loops (or from different loops). Different portions are sometimes in phase, sometimes out of phase and sometimes show phase shifts along the slit. We measure physical parameters of 54 Doppler shift oscillations in 27 flare-like events and give geometric parameters of the associated hot loops when soft X-ray (SXR) images are available. The oscillations have periods in the range 7−31 min, with decay times 5.7−36.8 min, and show an initial large Doppler shift pulse with peak velocities up to 200 km s −1 . The oscillation periods are on average a factor of three longer than the TRACE transverse loop oscillations. The damping times and velocity amplitude are roughly the same, but the derived displacement amplitude is four or five times larger than the transverse oscillation amplitude measured in TRACE images. Unlike TRACE oscillations, only a small fraction of them are triggered by large flares, and they often recur 2−3 times within a couple of hours. All recurring events show initial shifts of the same sign. These data provide the following evidence to support the conclusion that these oscillations are slow magnetoacoustic standing waves in hot loops: (1) the phase speeds derived from observed periods and loop lengths roughly agree with the sound speed; (2) the intensity fluctuation lags the Doppler shifts by 1/4 period; (3) The scaling of the dissipation time of slow waves with period agrees with the observed scaling for 49 cases. They seem to be triggered by micro-or subflares near a footpoint, as revealed in one example with SXR image observations. However other mechanisms cannot as yet be ruled out. Some oscillations showed phase propagation along the slit in one or both directions with apparent speeds in the range of 8−102 km s −1 , together with distinctly different intensity and line width distributions along the slit. These features can be explained by the excitation of the oscillation at a footpoint of an inhomogeneous coronal loop, e.g. a loop with fine structure.

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Using data obtained by the EUV Imaging Spectrometer (EIS) onboard Hinode, we have performed a survey of obvious and persistent (without significant damping) Doppler shift oscillations in the corona. We have found mainly two types of oscillations from February to April in 2007. One type is found at loop footpoint regions, with a dominant period around 10 minutes. They are characterized by coherent behavior of all line parameters (line intensity, Doppler shift, line width and profile asymmetry), apparent blue shift and blueward asymmetry throughout almost the entire duration. Such oscillations are likely to be signatures of quasi-periodic upflows (small-scale jets, or coronal counterpart of type-II spicules), which may play an important role in the supply of mass and energy to the hot corona. The other type of oscillation is usually associated with the upper part of loops. They are most clearly seen in the Doppler shift of coronal lines with formation temperatures between one and two million degrees. The global wavelets of these oscillations usually peak sharply around a period in the range of 3-6 minutes. No obvious profile asymmetry is found and the variation of the line width is typically very small. The intensity variation is often less than 2%. These oscillations are more likely to be signatures of kink/Alfvén waves rather than flows. In a few cases there seems to be a π/2 phase shift between the intensity and Doppler shift oscillations, which may suggest the presence of slow mode standing waves according to wave theories. However, we demonstrate that such a phase shift could also be produced by loops moving into and out of a spatial pixel as a result of Alfvénic oscillations. In this scenario, the intensity oscillations associated with Alfvénic waves are caused by loop displacement rather than density change. These coronal waves may be used to investigate properties of the coronal plasma and magnetic field.

Abstract. An intensive field measurement study was conducted simultaneously at a site within the inland Pearl River Delta (PRD) region (WQS) and a site in Hong Kong (TC) between 22 October and 1 December 2007. Ambient air pollutants measured included O 3 , NO x , CO, SO 2 , NMHCs, and carbonyls. The purpose is to improve our understanding of the interplay among local and regional air pollutants in the Hong Kong area, and the influence of regional transport on local air pollutants. The results indicate that the mean levels of air pollutants at the WQS site were much higher than those at the TC site, except NO x . Thirteen O 3 episode days (daily O 3 peak in excess of 122 ppbv) were monitored at WQS during the study period, while only 2 days were recorded at TC. Diurnal variations of O 3 showed higher nighttime levels of O 3 at TC than at WQS as well as more photochemical activity at WQS than TC. Remarkable differences in diurnal variations were also found between high and low O 3 pollution days at each site, implying that Hong Kong is more acutely VOC-limited than the inland PRD region. Ratio analyses for trace gases and VOCs and back trajectory calculation revealed that the air masses arriving at WQS were more aged due to regional influence, whereas the air masses at TC were mainly affected by local emissions and/or regional transport. In addition, the influence of regional transport from Eastern China on the primary pollutants of Hong Kong was noticeable, whereas the air masses from the inland PRD region (e.g. Dongguan and Huizhou) had significant influence on the air pollutants at WQS, and the anthropogenic emissions Correspondence to: H. Guo (ceguohai@polyu.edu.hk) in Eastern PRD (e.g. Shenzhen) played an important role on the photochemical ozone pollution in Western Hong Kong. These results confirm that regional and sub-regional transport of air pollution has a complex and significant impact on local air pollutants in this region.

An observation from the Interface Region Imaging Spectrograph reveals coherent oscillations in the loops of an M1.6 flare on 2015 March 12. Both the intensity and Doppler shift of Fe xxi 1354.08Å show clear oscillations with a period of ∼25 seconds. Remarkably similar oscillations were also detected in the soft X-ray flux recorded by the Geostationary Operational Environmental Satellites (GOES). With an estimated phase speed of ∼2420 km s −1 and a derived electron density of at least 5.4×10 10 cm −3 , the observed short-period oscillation is most likely the global fast sausage mode of a hot flare loop. We find a phase shift of ∼π/2 (1/4 period) between the Doppler shift oscillation and the intensity/GOES oscillations, which is consistent with a recent forward modeling study of the sausage mode. The observed oscillation requires a density contrast between the flare loop and coronal background of a factor 42. The estimated phase speed of the global mode provides an lower limit of the Alfvén speed outside the flare loop. We also find an increase of the oscillation period, which might be caused by the separation of the loop footpoints with time.