Liheng Yang

The Hα surges and EUV jets from magnetic flux emergences and cancellations

We analyzed multi-wavelength observations of three surges with a recurrent period of about 70 min in Ha, EUV, and soft X-ray, which occurred in the quiet-sun region on 2000 November 3. These homologous surges were associated with small flares at the same base, but their exact footpoints were spatially separated from the flare. Each surge consisted of a cool Ha component and a hot, EUV or soft X-ray component, which showed different evolutions not only in space but also in time. The EUV jets had slightly converging shapes, underwent more complicate development, showed clearly twisting structures, and appeared to open to space. The Ha surges, however, were smaller and only traced the edges of the jets. They always occurred later than the jets but had dark EUV counterparts appearing in the bright jets. These surge activities were closely associated with two emerging bipoles and their driven flux cancellations at the base region, and were consistent with the magnetic reconnection surge model. The possible cause of the delay between the surges and jets, of the dark structures in the jets are discussed, along with the possible role of flux cancellations in generation of these surges.

Nonlinear Interaction of Minor Heavy Ions with Kinetic Alfvén Waves and Their Anisotropic Energization in Coronal Holes

Some recent observations of the solar corona suggest that minor heavy ions undergo an anisotropic and mass-charge dependent energization in the extended corona. In this paper we investigate the nonlinear interaction of minor heavy ions with kinetic Alfven waves (KAWs) in the extended corona where plasma is low-β, in particular the ion energization by KAWs. We find that the cross-field ion energization depends on the ion mass and charge in the sense that the velocity is proportional to the ion mass-charge ratio and the field-aligned ion energy is directly proportional to the ion charge. Consequently, the effective ion temperature becomes strongly anisotropic, as well as much higher than that of protons. With an empirical model of plasma parameters in a coronal hole and an assumption of anisotropic spectrum for KAWs, we calculate the variations of the ion temperature anisotropy (δi ≡ T/T) and the ion-proton temperature ratio (γi ≡ T/T) with heliocentric distance. The results show that they both increase rapidly with heliocentric distances of 1.5-3.5 R☉. In the lower corona below 1.3 R☉, one has δi and γi 1, and in the higher corona above 4 R☉, they approach the saturation values δi 0.5A and γi 0.9AZi, where Ai is the ion mass-charge ratio in units of the proton mass-charge ratio mp/e, and Zi is the ion charge in units of the elementary charge e. We suggest that not only can the KAW-based model of the anisotropic and mass-charge dependent energization reasonably explain the recent observations of minor heavy ions in polar coronal holes, but KAWs may also be potentially important for understanding the microphysics of the wave-particle interaction in the extended solar corona, where the coronal plasma is heated and accelerated into the solar wind.

THE FORMATION AND MAGNETIC STRUCTURES OF ACTIVE-REGION FILAMENTS OBSERVED BY NVST, SDO, AND HINODE

To better understand the properties of solar active-region filaments, we present a detailed study on the formation and magnetic structures of two active-region filaments in active region NOAA 11884 during a period of four days. It is found that the shearing motion of the opposite magnetic polarities and the rotation of the small sunspots with negative polarity play an important role in the formation of two active-region filaments. During the formation of these two active-region filaments, one foot of the filaments was rooted in a small sunspot with negative polarity. The small sunspot rotated not only around another small sunspot with negative polarity, but also around the center of its umbra. By analyzing the nonlinear force-free field extrapolation using the vector magnetic fields in the photosphere, twisted structures were found in the two active-region filaments prior to their eruptions. These results imply that the magnetic fields were dragged by the shearing motion between opposite magnetic polarities and became more horizontal. The sunspot rotation twisted the horizontal magnetic fields and finally formed the twisted active-region filaments.

Coronal and chromospheric dimmings during a halo-type CME event

We present complementary solar observations of a filament eruption occurring in association with a halo-type coronal mass ejection ( CME) on 2006 July 6 in AR 10898. This eruption was followed by an M2.5 flare and has led to the formation of a pair of dimmings, observed in H alpha, over network patterns in regions of opposite magnetic polarities located near the ends of the eruptive filament. These dimmings, also clearly visible in He I 10830 angstrom, EUV, and soft X-rays, were preceded by distinct H alpha and EUV brightenings that first mark the dimming sites during the rise phase of the flare and then form bright edges, observable in EUV, around the growing dim regions. These dimmings have been attributed to a magnetic flux rope that is considered the origin of the CME as proposed in recent theories. The brightenings reported here are a new aspect of the CME hydromagnetic process that has not been included in existing theories. The roles and implications of the network patterns, brightenings, and sharp edges in the dimming evolutions are discussed.

Observing the release of twist by magnetic reconnection in a solar filament eruption

Magnetic reconnection is a fundamental process of topology change and energy release, taking place in plasmas on the Sun, in space, in astrophysical objects and in the laboratory. However, observational evidence has been relatively rare and typically only partial. Here we present evidence of fast reconnection in a solar filament eruption using high-resolution H-alpha images from the New Vacuum Solar Telescope, supplemented by extreme ultraviolet observations. The reconnection is seen to occur between a set of ambient chromospheric fibrils and the filament itself. This allows for the relaxation of magnetic tension in the filament by an untwisting motion, demonstrating a flux rope structure. The topology change and untwisting are also found through nonlinear force-free field modelling of the active region in combination with magnetohydrodynamic simulation. These results demonstrate a new role for reconnection in solar eruptions: the release of magnetic twist.

Magnetic Interaction: An Erupting Filament and a Remote Coronal Hole

For the first time, we present a rare observation of direct magnetic interaction between an erupting filament and a coronal hole (CH). The small active region filament obliquely erupted toward the CH getting in the way, met and interacted with it, and then was deflected back. The erupting filament thus underwent a distinct to-and-fro motion in the visible disk, while the CH was clearly disturbed by the interaction. Brightenings in H alpha and EUV and darkenings in He I 10830 angstrom appeared at the boundaries and in the interior of the CH. This eruption was closely associated with the initiation of a halo-type coronal mass ejection (CME). The direction of the CME, despite being greatly different from that of the initial filament eruption, was consistent with that of the reflected filament. Moreover, when the CME was seen in the limb, the filament was still in the process of the return journey in the visible disk. Therefore, it appears that the large-scale structure of the CME was bounced against and then reflected away from the CH along with the filament, and the eruptive filament represented only a very small part in the CME.

Observations of EUV and soft X-ray recurring jets in an active region ∗

We present simultaneous observations of three recurring jets in EUV and soft X-ray (SXR), which occurred in an active region on 2007 June 5. By comparing their morphological and kinematic characteristics in these two different wavelengths, we found that EUV and SXR jets had similar locations, directions, sizes and velocities. We also analyzed their spectral properties by using six spectral lines from the EUV Imaging Spectrometer (EIS) onboard Hinode and found that these jets had temperatures from 0.05 to 2.0 MK and maximum electron densities from 6.6 x 10(9) to 3.4 x 10(10) cm(-3). For each jet, an elongated blue-shifted component and a red-shifted component at the jet base were simultaneously observed in the Fe XII lambda 195 and the He II lambda 256 lines. The three jets had maximum Doppler velocities from 25 to 121 km s(-1) in the Fe XII lambda 195 line and from 115 to 232 km s(-1) in the He II lambda 256 line. They had maximum non-thermal velocities from 98 to 181 km s(-1) in the Fe XII lambda 195 line and from 196 to 399 km s(-1) in the He II lambda 256 line. We also examined the relationship between averaged Doppler velocities and maximum ionization temperatures of these three jets and found that averaged Doppler velocities decreased with the increase of maximum ionization temperatures. In the photosphere, magnetic flux emergences and cancelations continuously took place at the jet base. These observational results were consistent with the magnetic reconnection jet model, implying that magnetic reconnection between emerging magnetic flux and ambient magnetic field occurred in the lower atmosphere.

THE FORMATION OF AN INVERSE S-SHAPED ACTIVE-REGION FILAMENT DRIVEN BY SUNSPOT MOTION AND MAGNETIC RECONNECTION

We present a detailed study of the formation of an inverse S-shaped filament prior to its eruption in active region NOAA 11884 from October 31 to November 2, 2013. In the initial stage, clockwise rotation of a small positive sunspot around the main negative trailing sunspot formed a curved filament. Then the small sunspot cancelled with negative magnetic flux to create a longer active-region filament with an inverse S-shape. At the cancellation site a brightening was observed in UV and EUV images and bright material was transferred to the filament. Later the filament erupted after cancellation of two opposite polarities under the upper part of the filament. Nonlinear force-free field (NLFFF) extrapolation of vector photospheric fields suggests that the filament may have a twisted structure, but this cannot be confirmed from the current observations.

WALEN TEST AND DE HOFFMANN-TELLER FRAME OF INTERPLANETARY LARGE-AMPLITUDE ALFVEN WAVES

In this study, three methods of analysis are compared to test the Walen relation. Method 1 requires a good de Hoffmann-Teller ( HT) frame. Method 2 uses three components separately to find the frame that is slightly modified from Method 1. This method is intended to improve the accuracy of the HT frame and able to demonstrate the anisotropic property of the fluctuations. The better the relation is, the closer the slope of a regression fitting the data of plasma versus Alfven velocities is to 1. However, this criterion is based on an average HT frame, and the fitted slope does not always work for the Walen test because the HT frame can change so fast in the high-speed streams. We propose Method 3 to check the Walen relation using a sequence of data generated by taking the difference of two consecutive values of plasma and Alfven velocities, respectively. The difference data are independent of the HT frame. We suggest that the ratio of the variances between plasma and Alfven velocities is a better parameter to qualify the Walen relation. Four cases in two solar wind streams are studied using these three methods. Our results show that when the solar wind HT frame remains stable, all three methods can predict Alfvenic fluctuations well, but Method 3 can better predict the Walen relation when solar wind contains structures with several small streams. A simulated case also demonstrates that Method 3 is better and more robust than Methods 1 and 2. These results are important for a better understanding of Alfvenic fluctuations and turbulence in the solar wind.

MEASUREMENT OF INTERMITTENCY OF ANISOTROPIC MAGNETOHYDRODYNAMIC TURBULENCE IN HIGH-SPEED SOLAR WIND

We investigate the intermittency of anisotropic magnetohydrodynamic (MHD) turbulence in high-speed solar wind. Using the data recorded by the Ulysses spacecraft, we apply the Castaing function to model the probability density functions of the fluctuating magnetic field and calculate the magnetic structure functions (SFs) S-p of the order p in the coordinates (r, Theta), with r being the length scale and Theta the direction of the local mean field. The scaling exponent zeta, from S-p(r, Theta) alpha r(zeta(p, Theta)), has an anomalous nonlinear dependence on p, implying the intermittent scaling of solar wind turbulence, which has been observed for decades. Furthermore, we study the anisotropy of solar wind turbulence introduced by the strong mean magnetic field. From S-p(Theta = 0) alpha S-p(Theta = pi/2), we obtain r perpendicular to alpha r(parallel to)(alpha p) with alpha(p) = zeta(parallel to)/zeta(perpendicular to) denoting the perpendicular-parallel spatial correlation of the moment of the pth order. For the magnetic field difference dB, we find alpha(2) = 1.78 +/- 0.26, consistent with recent theories and observations. However, when the contribution from the intermittent fluctuations begins to dominate the scaling, alpha is not a constant but increases with p, e.g., alpha(5) = 1.97 +/- 0.41 and alpha(8) approximate to 2.42 +/- 0.64. This complication of the perpendicular-parallel spatial correlation due to the intermittency raises new questions for MHD turbulence theory.