Jiansen He

Possible Evidence of Alfvén-cyclotron Waves in the Angle Distribution of Magnetic Helicity of Solar Wind Turbulence

The fluctuating magnetic helicity is considered an important parameter in diagnosing the characteristic modes of solar wind turbulence. Among them is the Alfv?n-cyclotron wave, which is probably responsible for the solar wind plasma heating, but has not yet been identified from the magnetic helicity of solar wind turbulence. Here, we present the possible signatures of Alfv?n-cyclotron waves in the distribution of magnetic helicity as a function of ?VB, which is the angle between the solar wind velocity and local mean magnetic field. We use magnetic field data from the STEREO spacecraft to calculate the ?VB distribution of the normalized reduced fluctuating magnetic helicity ?m. We find a dominant negative ?m for 1 s 150? in the solar wind inward magnetic sector. These features of ?m appearing around the Doppler-shifted ion-cyclotron frequencies may be consistent with the existence of Alfv?n-cyclotron waves among the outward propagating fluctuations. Moreover, right-handed polarized waves at larger propagation angles, which might be kinetic Alfv?n waves or whistler waves, have also been identified on the basis of the ?m features in the angular range 40? < ?VB < 140?. Our findings suggest that Alfv?n-cyclotron waves (together with other wave modes) play a prominent role in turbulence cascading and plasma heating of the solar wind.

DO OBLIQUE ALFVEN/ION-CYCLOTRON OR FAST-MODE/WHISTLER WAVES DOMINATE THE DISSIPATION OF SOLAR WIND TURBULENCE NEAR THE PROTON INERTIAL LENGTH?

To determine the wave modes prevailing in solar wind turbulence at kinetic scales, we study the magnetic polarization of small-scale fluctuations in the plane perpendicular to the data sampling direction (namely, the solar wind flow direction, ) and analyze its orientation with respect to the local background magnetic field . As an example, we take only measurements made in an outward magnetic sector. When is quasi-perpendicular to , we find that the small-scale magnetic-field fluctuations, which have periods from about 1 to 3 s and are extracted from a wavelet decomposition of the original time series, show a polarization ellipse with right-handed orientation. This is consistent with a positive reduced magnetic helicity, as previously reported. Moreover, for the first time we find that the major axis of the ellipse is perpendicular to , a property that is characteristic of an oblique Alfv?n wave rather than oblique whistler wave. For an oblique whistler wave, the major axis of the magnetic ellipse is expected to be aligned with , thus indicating significant magnetic compressibility, and the polarization turns from right to left handedness as the wave propagation angle (?kB) increases toward 90?. Therefore, we conclude that the observation of a right-handed polarization ellipse with orientation perpendicular to seems to indicate that oblique Alfv?n/ion-cyclotron waves rather than oblique fast-mode/whistler waves dominate in the dissipation range near the break of solar wind turbulence spectra occurring around the proton inertial length.

Electron acceleration in the reconnection diffusion region: Cluster observations

We present one case study of magnetic islands and energetic electrons in the reconnection diffusion region observed by the Cluster spacecraft. The cores of the islands are characterized by strong c ...

EXCITATION OF KINK WAVES DUE TO SMALL-SCALE MAGNETIC RECONNECTION IN THE CHROMOSPHERE?

The kink wave, which has often been observed in coronal loops, is considered as a possibly important energy source contributing to coronal heating. However, its generation has not yet been observed. Here, we report the first observation of kink-wave excitation caused by magnetic reconnection as inferred from Solar Optical Telescope measurements made in the Ca ii line. We observed transverse-displacement oscillations on a spicule which propagated upwardly along the spicule trace and originated from the cusp of an inverted Y-shaped structure, where apparently magnetic reconnection occurred. Such transverse oscillation of an individual spicule is interpreted by us to be the signature of a kink wave that was excited by magnetic reconnection. We present the height variations of the velocity amplitude, δv, and the phase speed, Ck, of the kink wave, starting from its source region. The kink wave is found to steepen with height and to evolve into a nonlinear state with a large relative disturbance, yielding a( δv/Ck) of 0.21 at 5.5 Mm. This nonlinear kink wave seems to be damped in velocity amplitude beyond 5.5 Mm, which may result from the conversion of transverse-fluctuation energy to longitudinal-motion energy required to sustain the spicule. We also estimate the energy flux density carried by the kink wave, and in spite of its attenuation in the transition region conclude it to be sufficient for heating the quiet corona. Our findings shed new light on future modeling of coronal heating and solar wind acceleration involving magnetic reconnection in the chromosphere.

What can We Learn about Solar Coronal Mass Ejections, Coronal Dimmings, and Extreme-ultraviolet Jets through Spectroscopic Observations?

Solar eruptions, particularly coronal mass ejections (CMEs) and extreme-ultraviolet (EUV) jets, have rarely been investigated with spectroscopic observations. We analyze several data sets obtained by the EUV Imaging Spectrometer on board Hinode and find various types of flows during CMEs and jet eruptions. CME-induced dimming regions are found to be characterized by significant blueshift and enhanced line width by using a single Gaussian fit, while a red-blue (RB) asymmetry analysis and an RB-guided double Gaussian fit of the coronal line profiles indicate that these are likely caused by the superposition of a strong background emission component and a relatively weak (~10%), high-speed (~100 km s–1) upflow component. This finding suggests that the outflow velocity in the dimming region is probably of the order of 100 km s–1, not ~20 km s–1 as reported previously. These weak, high-speed outflows may provide a significant amount of mass to refill the corona after the eruption of CMEs, and part of them may experience further acceleration and eventually become solar wind streams that can serve as an additional momentum source of the associated CMEs. Density and temperature diagnostics of the dimming region suggest that dimming is primarily an effect of density decrease rather than temperature change. The mass losses in dimming regions as estimated from different methods are roughly consistent with each other, and they are 20%-60% of the masses of the associated CMEs. With the guide of RB asymmetry analysis, we also find several temperature-dependent outflows (speed increases with temperature) immediately outside the (deepest) dimming region. These outflows may be evaporation flows that are caused by the enhanced thermal conduction or nonthermal electron beams along reconnecting field lines, or induced by the interaction between the opened field lines in the dimming region and the closed loops in the surrounding plage region. In an erupted CME loop and an EUV jet, profiles of emission lines formed at coronal and transition region temperatures are found to exhibit two well-separated components, an almost stationary component accounting for the background emission and a highly blueshifted (~200 km s–1) component representing emission from the erupting material. The two components can easily be decomposed through a double Gaussian fit, and we can diagnose the electron density, temperature, and mass of the ejecta. Combining the speed of the blueshifted component and the projected speed of the erupting material derived from simultaneous imaging observations, we can calculate the real speed of the ejecta.

MULTI-SCALE ANTI-CORRELATION BETWEEN ELECTRON DENSITY AND MAGNETIC FIELD STRENGTH IN THE SOLAR WIND

This work focuses on the relation between the electron density and the magnetic field strength in the solar wind, and aims to reveal its compressive nature and to determine the level of compressibility. For this purpose, we choose a period of quiet solar wind data obtained at 1 AU by the Cluster C1 satellite. The electron density is derived with a sampling time as high as 0.2 s from the spacecraft-potential measurements made by the Electric Field and Waves instrument. We use the wavelet cross-coherence method to analyze the correlation between the electron density and the magnetic field strength on various scales. We find a dominant anti-correlation between them at different timescales ranging from 1000 s down to 10 s, a result which has never been reported before. This may indicate the existence of pressure-balanced structures (PBSs) with different sizes in the solar wind. The small (mini) PBSs appear to be embedded in the large PBSs, without affecting the pressure balance between the large structures. Thus, a nesting of these possible multi-scale PBSs is found. Moreover, we find for the first time that the relative fluctuation spectra of both the electron number density and the magnetic field strength look almost the same in the range from 0.01 Hz to 2.5 Hz, implying a similar cascading for these two types of fluctuations. Probable formation mechanisms for the multi-scale possible PBSs are discussed. The results of our work are believed to be helpful for understanding the compressive nature of solar wind turbulence as well as the connections between the solar wind streams and their coronal sources.

Numerical Simulations of Chromospheric Anemone Jets Associated with Moving Magnetic Features

Observations with the space-based solar observatory Hinode show that small-scale magnetic structures in the photosphere are found to be associated with a particular class of jets of plasma in the chromosphere called anemone jets. The goal of our study is to conduct a numerical experiment of such chromospheric anemone jets related to the moving magnetic features (MMFs). We construct a 2.5 dimensional numerical MHD model to describe the process of magnetic reconnection between the MMFs and the pre-existing ambient magnetic field, which is driven by the horizontal motion of the magnetic structure in the photosphere. We include thermal conduction parallel to the magnetic field and optically thin radiative losses in the corona to account for a self-consistent description of the evaporation process during the heating of the plasma due to the reconnection process. The motion of the MMFs leads to the expected jet and our numerical results can reproduce many observed characteristics of chromospheric anemone jets, topologically and quantitatively. As a result of the tearing instability, plasmoids are generated in the reconnection process that are consistent with the observed bright moving blobs in the anemone jets. An increase in the thermal pressure at the base of the jet is also driven by the reconnection, which induces a train of slow-mode shocks propagating upward. These shocks are a secondary effect, and only modulate the outflow of the anemone jet. The jet itself is driven by the energy input due to the reconnection of the MMFs and the ambient magnetic field.

Intermittent outflows at the edge of an active region - a possible source of the solar wind?

Context. It has already been established that the solar wind may originate at the edges of active regions (ARs), but the key questions of how frequently these outflows occur, and at which height the nascent solar wind originates have not yet been addressed. Aims. We study the occurrence rate of these intermittent outflows, the related plasma activities beneath in the low solar atmosphere, and the interplanetary counterparts of the nascent solar wind outflow. Methods. We use the observations from XRT/Hinode and TRACE to study the outflow patterns. The occurrence frequency of the intermittent outflow is estimated by counting the occurrences of propagating intensity enhancements in height-time diagrams. We adopt observations of SOT/Hinode and EIS/Hinode to investigate the phenomena in the chromosphere associated with the coronal outflows. The ACE plasma and field in-situ measurements near Earth are used to study the interplanetary manifestations. Results. We find that in one elongated coronal emission structure, referred to as strand, the plasma flows outward intermittently, about every 20 min. The flow speed sometimes exceeds 200 km s −1 , which is indicative of rapid acceleration, and thus exceeds the coronal sound speed at low altitudes. The inferred flow speed of the soft-X-ray-emitting plasma component seems a little higher than that of the Fe ix/x-emitting plasma component. Chromospheric jets are found to occur at the root of the strand. Upflows in the chromosphere are also confirmed by blue-shifts of the He ii line. The heliospheric plasma counterpart close to the Earth is found to be an intermediate-speed solar wind stream. The AR edge may also deliver some plasmas to a fraction of the fast solar wind stream, most of which emanate from the neighboring CH. Conclusions. The possible origin of the nascent solar wind in the chromosphere, the observed excessive outflow speed of over 200 km s −1 in the lower corona, and the corresponding intermediate-speed solar wind stream in interplanetary space are all linked in our case study. These phenomena from the low solar atmosphere to the heliosphere near Earth in combination shed new light on the solar wind formation process. These observational results will constrain future modeling of the solar winds originating close to an AR.

Signatures of magnetic reconnection at boundaries of interplanetary small-scale magnetic flux ropes

The interaction between interplanetary small-scale magnetic flux ropes and the magnetic field in the ambient solar wind is an important topic in the understanding of the evolution of magnetic structures in the heliosphere. Through a survey of 125 previously reported small flux ropes from 1995 to 2005, we find that 44 of them reveal clear signatures of Alfvenic fluctuations and thus classify them as Alfven wave trains rather than flux ropes. Signatures of magnetic reconnection, generally including a plasma jet of ~30 km s–1 within a magnetic field rotational region, are clearly present at boundaries of about 42% of the flux ropes and 14% of the wave trains. The reconnection exhausts are often observed to show a local increase in the proton temperature, density, and plasma beta. About 66% of the reconnection events at flux rope boundaries are associated with a magnetic field shear angle larger than 90° and 73% of them reveal a decrease of 20% or more in the magnetic field magnitude, suggesting a dominance of anti-parallel reconnection at flux rope boundaries. The occurrence rate of magnetic reconnection at flux rope boundaries through the years 1995-2005 is also investigated and we find that it is relatively low around the solar maximum and much higher when approaching solar minima. The average magnetic field depression and shear angle for reconnection events at flux rope boundaries also reveal a similar trend from 1995 to 2005. Our results demonstrate for the first time that boundaries of a substantial fraction of small-scale flux ropes have properties similar to those of magnetic clouds, in the sense that both of them exhibit signatures of magnetic reconnection. The observed reconnection signatures could be related either to the formation of small flux ropes or to the interaction between flux ropes and the interplanetary magnetic fields.

Reproduction of the Observed Two-component Magnetic Helicity in Solar Wind Turbulence by a Superposition of Parallel and Oblique Alfvén Waves

The angular distribution of the normalized reduced magnetic helicity density (sigma(r)(m)) in solar wind turbulence reveals two components of distinct polarity in different angle ranges. This kind of two-component sigma(r)(m) m may indicate the possible wave modes and power spectral densities (PSDs) of the turbulent fluctuations. Here we model the measured angular distribution of sigma(r)(m) m by assuming a PSD distribution for Alfven fluctuations in wavevector space, and then fit the model results to the observations by adjusting the pattern of the PSD distribution. It is found that the two-component form of the PSD, which has a major and minor component close to k(perpendicular to) and k(parallel to), respectively, seems to be responsible for the observed two-component sigma(r)(m). On the other hand, both an isotropic PSD and a PSD with only a single component bending toward k(perpendicular to) fail to reproduce the observations. Moreover, it is shown that the effect of gradual balance between outward and inward wave-energy fluxes with decreasing spatial scale needs to be considered in order to reproduce the observed diminishing of vertical bar sigma(r)(m)vertical bar at shorter scales. Therefore, we suggest that the observed two-component sigma(r)(m) in the solar wind turbulence may be due to a superposition of Alfven waves with quasi-perpendicular (major part) and quasi-parallel (minor part) propagation. The waves seem to become gradually balanced toward shorter scales.