Jingxiu Wang

Flux distribution of solar intranetwork magnetic fields

AbstractBig Bear deep magnetograms of June 4, 1992 provide unprecedented observations for direct measurements of solar intranetwork (IN) magnetic fields. More than 2500 individual IN elements and 500 network elements are identified and their magnetic flux measured in a quiet region of 300 × 235 arc sec. The analysis reveals the following results:(1)IN element flux ranges from 1016 Mx (detection limit) to 2 × 1018 Mx, with a peak flux distribution of 6 × 1016 Mx.(2)More than 20% of the total flux in this quiet region is in the form of IN elements at any given time.(3)Most IN elements appear as a cluster of mixed polarities from an emergence center (or centers) somewhere within the network interior.(4)The IN flux is smaller than the network flux by more than an order of magnitude. It has a uniform spatial distribution with equal amount of both polarities. It is speculated that IN fields are intrinsically different from network fields and may be generated from a different source as well.

Magnetic flux cancellation associated with the major solar event on 2000 July 14

The major solar event on 2000 July 14 is characterized by the simultaneous occurrence of a giant filament eruption, a great flare, and an extended Earth-directed coronal mass ejection. We examined in detail the magnetic evolution in its source active region, NOAA 9077, and found that the only obvious magnetic change in the course of the event is magnetic flux cancellation at many sites in the vicinity of the filament. Moreover, all the initial disturbance in the filament and the initial brightening around the filament took place at the cancellation sites. It is clearly indicated that the slow magnetic reconnection in the lower atmosphere, which is manifested as observed flux cancellation, is of overwhelming importance in leading to the global instability responsible for the major magnetic activity.

Correlation between halo coronal mass ejections and solar surface activity

We survey all the coronal mass ejections (CMEs) observed by the Large Angle and Spectrometric Coronagraph Experiment (LASCO) aboard the Solar & Heliospheric Observatory (SOHO) in the interval from 1997 to 2001, and select 197 frontside halo CMEs whose associated near-surface activity could be clearly identified from SOHO EUV Imaging Telescope (EIT) and other space-borne and ground-based observations. A statistical analysis has been made with the emphasis on the correlation between CMEs and solar surface activity.
We have found in our sample that all the CMEs were accompanied by local brightening in the CME source regions in EIT and/or H α images. However, if we only classify the events with a brightening increase more than 50% above the quiet background as flares, as measured from EIT data, approximately 88% of the earth-directed CMEs are associated with flares and more than 94% are associated with eruptive filaments. With regard to the locations of CME source regions, there are about 79% CMEs initiating from active regions, while only 21% originate outside active regions. We evaluate the symmetry between CMEs and associated solar surface activity in spatial distributions. We find that in about half the events the surface activity is asymmetric with respect to the CME. For the flares having GOES X-ray records in this study, allowing for the uncertainties specified by the data cadence, 59% of the CME initiations seem to precede the flare onset in the CME source regions, while 41% are preceded by flare onsets. The statistical correlation seems to suggest that CMEs are intrinsically related to surface activity.

Are homologous flare-coronal mass ejection events triggered by moving magnetic features?

Coronal mass ejections (CMEs) often present destabilization and eruption of a global (or large-scale) magnetic structure in the solar atmosphere. Furthermore, the Earth-directed CMEs are the primary driver of the disastrous space weather. Here we report on five homologous CMEs. They initiated in the early phase of five homologous X-class flares seen in NOAA Active Region 9236 on 2000 November 24-26. The flares appeared between the main sunspot with positive magnetic field and the moat of the active region. We examined the magnetic evolution in the source active region for the first three of the five flare-CME events and found that the main magnetic changes are magnetic flux emergence in the form of moving magnetic features (MMFs) in the vicinity of the main positive magnetic field. There are three peaks in the flux (number) distribution of the emerging MMFs. If each peak corresponds to an X-class flare and associated halo CME, there is a time lag of 10 hr between flux emergence and flaring. The flux appearing in the form of MMFs is 1.1 × 1022 Mx, with a net flux of -2.1 × 1021 Mx. Around the main spot region, about 9.1 × 1021 Mx of flux disappeared in the 2 day interval. This is indicative that the repeated flare-CME activities are triggered by the continuous emergence of MMFs.

On the nature of moving magnetic feature pairs around sunspots

Employing data recorded by the Michelson Doppler Imager (MDI) instrument on the Solar and Heliospheric Observatory (SOHO), we have identified 144 pairs of opposite magnetic polarity moving magnetic features (MMFs) in two active regions (NOAA ARs 8375 and 9236). The following results are obtained: (1) The majority of MMF pairs first appears at a distance of 1000 to 5000 km from the outer boundary of the sunspot, although MMF pairs appearing closer to the sunspot may be missed. (2) MMF bipoles are not randomly oriented. The member of an MMF pair further from the sunspot has the polarity of the parent sunspot in 85% of the cases. Furthermore, the orientations of MMF pairs are associated with the twist of the sunspot superpenumbra deduced from HIT images. (3) The mean lifetime of the studied MMFs is around 4 hours. (4) The separation between the two polarities of the MMFs falls in the range of 1100-1700 km. This separation remains almost unchanged, even decreases slightly as the MMF pairs move outwards. (5) MMFs are observed to cluster at particular azimuths around the parent sunspot, in particular in AR 8375. (6) MMF pairs move approximately radially outward from sunspots at an average speed of around 0.5 km s - 1 . Their motion is deflected towards large concentrations of magnetic flux of opposite polarity to that of the parent sunspot. A qualitative model based on these and other observations is presented. MMF pairs are proposed to be part of a U-loop emanating from the sunspots magnetic canopy. Possible mechanisms leading to the formation of such a loop are discussed.

The velocities of intranetwork and network magnetic fields

We analyzed two sequences of quiet-Sun magnetograms obtained on June 4, 1992 and July 28, 1994. Both were observed during excellent seeing conditions such that the weak intranetwork (IN) fields are observed clearly during the entire periods. Using the local correlation tracking technique, we derived the horizontal velocity fields of IN and network magnetic fields. They consist of two components: (1) radial divergence flows which move IN fields from the network interior to the boundaries, and (2) lateral flows which move along the network boundaries and converge toward stronger magnetic elements. Furthermore, we constructed divergence maps based on horizonal velocities, which are a good representation of the vertical velocities of supergranules. For the June 4, 1992 data, the enhanced network area in the field of view has twice the flux density, 10% higher supergranular velocity and 20% larger cell sizes than the quiet, unenhanced network area. Based on the number densities and flow velocities of IN fields derived in this paper and a previous paper (Wang et al., 1995), we estimate that the lower limit of total energy released from the recycling of IN fields is 1.2 × 1028 erg s−1, which is comparable to the energy required for coronal heating.

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.

Magnetic Interaction: A Transequatorial Jet and Interconnecting Loops

We present, to our knowledge for the first time, a rare observation of direct magnetic interaction between a transequatorial jet and interconnecting loops (IL) in the southern hemisphere. The jet originated from a flare and appeared to move outward along open field lines, but it passed so close to the IL that its edge met with one of the IL ends. As a result, the IL began to erupt, weak brightenings appeared at the meeting site, and a nearby dark feature was disturbed. After the eruption, in addition to a looplike dimming due to the disappearance of the IL, a dimming region was formed around its another end, which was very probably caused by the expansion or opening of its field lines and represented its evacuated feet. Two coronal mass ejections (CMEs) were observed within 2 hr in association with the event. One was related to the flare and the jet, while the other was due to the IL eruption. These observations suggest that a sole flare can not only trigger a CME but also simultaneously trigger an IL eruption by means of its interaction with a jet, so can lead to two interdependent CMEs, i. e., a sympathetic CME pair physically connected by the jet/IL interaction.

JOINT VECTOR MAGNETOGRAPH OBSERVATIONS AT BBSO, HUAIROU STATION AND MEES-SOLAR-OBSERVATORY

Joint vector magnetograph observations were carried out at Big Bear Solar Observatory (BBSO), Huairou Solar Observing Station (Huairou), and Mees Solar Observatory (MSO) in late September 1989. Comparisons of vector magnetograms obtained at the three stations show a high degree of consistency in the morphology of both longitudinal and transverse fields. Quantitative comparisons show the presence of noise, cross-talk between longitudinal field and transverse field, Faraday rotation and signal saturation effects in the magnetograms. We have tried to establish how the scatter in measurements from different instruments is apportioned between these sources of error.

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