Guiping Zhou

Helicity Patterns of Coronal Mass Ejection-associated Active Regions

Coronal mass ejections (CMEs) are thought to originate from the overaccumulation of magnetic helicity. While recent studies have revealed the inability of active region (AR) fields to create enough helicity for CMEs, we have tried, on the other hand, to examine whether particular helicity patterns are retained by CME-associated ARs. Nine ARs, whose vector magnetic fields and CME-associated flares were observed at Huairou, are selected in this study. For the sampled ARs, contrary to the helicity-charging picture, we find evidence that the newly emerging flux often brings up helicity with a sign opposite to the dominant helicity of the AR. Moreover, the flare/CME initiation site is characterized by close contact with magnetic flux of the opposite helicity coinciding with observed flux cancellation. This revelation suggests that the interaction and reconnection of flux systems with opposite helicity are key elements in the magnetism of CME initiation.

A STUDY OF THE ORIENTATION OF INTERPLANETARY MAGNETIC CLOUDS AND SOLAR FILAMENTS

As a kind of eruptive phenomenon associated with coronal mass ejections (CMEs), solar eruptive filaments are thought to be parallel to the axis of surrounding arcade coronal magnetic fields that erupt and develop into interplanetary magnetic clouds (MCs). By investigating three events from 2000 August, 2003 October, and 2003 November, we estimate the axial orientations of the MCs and make a quantitative comparison with the filament orientations. By defining ‘‘tilt angle’’ as the angle between projected orientation on the plane of the sky and the ecliptic, wefind that the

MAGNETIC CHANGES IN THE COURSE OF THE X7.1 SOLAR FLARE ON 2005 JANUARY 20

Rapid magnetic changes in the course of the X7.1 solar flare on 2005 January 20 at the photosphere in the host active region (AR), NOAA AR 10720, are diagnosed. The database for this study consists of Huairou vector magnetograms, Transition Region and Coronal Explorer (TRACE) white light and UV/EUV images, RHESSI hard X-ray, and Solar and Heliospheric Observatory EUV observations. For such an event that is close to but not on the solar west limb (N12 W58), the projection effects in the observed vector magnetograms are untangled by combining an intuitive geometric analysis and a transformation of the magnetograms into the heliographic coordination system. The magnetic changes in the horizontal magnetic fields are emphasized. We find definitive evidence of weakening in the horizontal magnetic fields in a few isolated patches in the outskirts of this ?-sunspot group and strengthening in the horizontal fields (HFs) in an extended area centralized at the magnetic neutral line between major sunspots of opposite polarities. The rapid magnetic changes take place at the level of 100-300 G, several factors of ten of the noise level. The identified HF changes are consistent with the darkening of inner penumbrae and weakening of outer penumbrae in this ?-sunspot group. The enhanced HFs spatially coincide with the TRACE 1550 ? rope-like structures lying low above the magnetic neutral line. Unexpectedly, during the flare, the lower lying rope-like structures remain in place, though they exhibit episodic disturbance and brightening, while the outer EUV loops are impulsively expanding. The rapid magnetic changes manifest an impulsive input of free magnetic energy in the photosphere, resulting from an impulsive growth of a new emerging flux region (EFR) along the magnetic neutral line. The facts of the increasing core fields in magnetic nonpotentiality, the continued disturbance of the inner rope-like structures, and the breakout of the outer loops during the major flare cannot be interpreted by any single flare model. However, the nature of magnetohydrodynamical catastrophe is clearly implied for the flare triggering.

Transequatorial Filament Eruption and Its Link to a Coronal Mass Ejection

We revisit the Bastille Day flare/CME Event of 2000 July 14, and demonstrate that this flare/CME event is not related to only one single active region (AR). Activation and eruption of a huge transequatorial filament are seen to precede the simultaneous filament eruption and flare in the source active region, NOAA AR 9077, and the full halo-CME in the high corona. Evidence of reconfiguration of large-scale magnetic structures related to the event is illustrated by SOHO EIT and Yohkoh SXT observations, as well as, the reconstructed 3D magnetic lines of force based on the force-free assumption. We suggest that the AR filament in AR 9077 was connected to the transequatorial filament. The large-scale magnetic composition related to the transequatorial filament and its sheared magnetic arcade appears to be an essential part of the CME parent magnetic structure. Estimations show that the filament-arcade system has enough magnetic helicity to account for the helicity carried by the related CMEs. In addition, rather global magnetic connectivity, covering almost all the visible range in longitude and a huge span in latitude on the Sun, is implied by the Nancay Radioheliograph (NRH) observations. The analysis of the Bastille Day event suggests that although the triggering of a global CME might take place in an AR, a much larger scale magnetic composition seems to be the source of the ejected magnetic flux, helicity and plasma. The Bastille Day event is the first described example in the literature, in which a transequatorial filament activity appears to play a key role in a global CME. Many tens of halo-CME are found to be associated with transequatorial filaments and their magnetic environment.

ALTITUDE-DEPENDENT EMISSION OF TYPE III SOLAR RADIO BURSTS

The present discussion complements a preceding article in which a cyclotron-maser theory of type III solar radio bursts is proposed. One important issue, which has not been addressed in any of the existing theories, is that in the event of a F-H pair emission dynamic spectra usually show an initial time delay of the fundamental (F) component after the harmonic (H) component has commenced. Moreover, the ratio of the starting frequencies of the H waves to those of the F waves is generally higher than 2. A plausible interpretation is that the emission of H waves starts at an altitude lower than that for F waves. This notion leads to the present study. Although it is formulated within the context of the cyclotron-maser scenario, the model of the source electrons is different from that discussed previously.

Magnetic non-potentiality on the quiet Sun and the filigree

From the observed vector magnetic fields by the Solar Optical Telescope/Spectro–Polarimeter aboard the satellite Hinode, we have examined whether or not the quiet Sun magnetic fields are non-potential, and how the G-band filigrees and Ca II network bright points (NBPs) are associated with the magnetic non-potentiality. A sizable quiet region in the disk center is selected for this study. The new findings by the study are as follows. (1) The magnetic fields of the quiet region are obviously non-potential. The region-average shear angle is 40o, the average vertical current is 0.016A m−2, and the average free magnetic energy density, 2.7 × 102 erg cm−3. The magnitude of these non-potential quantities is comparable to that in solar active regions. (2) There are overall correlations among current helicity, free magnetic energy and longitudinal fields. The magnetic non-potentiality is mostly concentrated in the close vicinity of network elements which have stronger longitudinal fields. (3) The filigrees and NBPs are magnetically characterized by strong longitudinal fields, large electric helicity, and high free energy density. Because the selected region is away from any enhanced network, these new results can generally be applied to the quiet Sun. The findings imply that stronger network elements play a role in high magnetic non-potentiality in heating the solar atmosphere and in conducting the solar wind.

Magnetic Evolution and Temperature Variation in a Coronal Hole

We have explored the magnetic flux evolution and temperature variation in a coronal hole region, using Big Bear Solar Observatory deep magnetograms and SOHO EIT images observed from 2005 October 10 to 14. For comparison, we also investigated a neighboring quiet region of the Sun. The coronal hole evolved from its mature stage to its disappearance during the observing period. We have obtained the following results: (1) When the coronal hole was well developed on October 10, about 60% of the magnetic flux was positive. The EUV brightness was 420 counts pixel-1, and the coronal temperature, estimated from the line ratio of the EIT 195 and 171 A images, was 1.07 MK. (2) On October 14, when the coronal hole had almost disappeared, 51% of the magnetic flux was positive, the EUV radiance was 530 counts pixel-1, and the temperature was 1.10 MK. (3) In the neighboring quiet region, the fraction of positive flux varied between 0.49 and 0.47. The EUV brightness displayed an irregular variation, with a mean value of 870 counts pixel-1. The temperature was almost constant at 1.11 MK during the 5 day observation. Our results demonstrate that in a coronal hole less imbalance of the magnetic flux in opposite polarities leads to stronger EUV brightness and higher coronal temperatures.

Solar Intranetwork Magnetic Elements: Bipolar Flux Appearance

The Helioseismic and Magnetic Imager (HMI) and the Atmospheric Imaging Assembly (AIA) instruments onboard the Solar Dynamics Observatory satellite produce Doppler velocity and continuum intensity at 6173 Å as well as intensity maps at 1600 Å and 1700 Å, which can be used for helioseismic studies at different heights in the solar photosphere. We perform a Hankel-Fourier analysis in an annulus centered around sunspots or quiet-Sun regions, to estimate the change in power of waves crossing these regions of interest. We find that there is a dependence of power-reduction coefficients α on measurement height in the photosphere: Sunspots reduce the power of outgoing waves with frequencies ν lower than ν ≈ 4.5 mHz at all heights, but enhance the power of acoustic waves in the range ν ≈ 4.5−5.5 mHz toward chromospheric heights, which is likely the signature of acoustic glories (halos). Maximum power reduction seems to occur near the continuum level and to decrease with altitude. Sunspots also impact the frequencies of outgoing waves in an altitude-dependent fashion. The quiet Sun is shown to behave like a strong power reducer for outgoing f and p-modes at the continuum level, with a power reduction α ≈ 15 − 20%, and like a weak power enhancer for p-modes higher in the atmosphere. It is speculated that the surprising power reduction at the continuum level is related to granulation. In Doppler-velocity data, and unlike in intensity data, the quiet Sun behaves like a strong power reducer for granular flows.

Circular polarization in a solar filament

Integrating 26 624 pairs of video frames, the authors have mapped the circular polarization in an active-region filament against the solar disk by using a traditional magnetograph working at the Hβ line. This filament, offset the disk center, appeared at the boundary of three decayed active regions. It was quiet and away from any strong enhanced network. The mapped circular polarization in the filament has an average polarization degree of 1.1×10−3 with a measurement precision of 4×10−4. The mapping of circular polarization in a filament may provide a supplementary diagnosis of the filament magnetic field, in addition to the mapping of linear polarization via the Hanle effect. However, the interpretation of the circular polarization requires treatment of the full quantum problem of Zeeman and non-Zeeman effects of Stokes line profiles.

Low latitude magnetometer chain in China in the frame of the MERIDIAN project

MERIDIAN is a 120 degrees east ground based multi station and multi-instrument project proposed in China. TWF/OAW in Graz, Austria in cooperation with IGPP/UCLA in Los Angeles, USA and CSSAR/CAS in Beijing, China has developed variometers for magnetic field observations along this MERIDIAN chain. The variometer (CHIMAG) is a fluxgate magnetometer especially for Earth field variation and pulsation measurements. Three variometer stations have been installed in Beijing (40 degrees N), Hainan (19 degrees N) and Wuhan (30.5 degrees N), respectively. Two more will be installed in 1999. In this paper we will present the design and the scientific objectives of the CHIMAG as well as the results of the first observations