Engineering topics

Ai, Guoxiang

Publications and source records attributed to Ai, Guoxiang.

7 records

NASA NTRS1999

Phase Time and Envelope Time in Time-Distance Analysis and Acoustic Imaging

Time-distance analysis and acoustic imaging are two related techniques to probe the local properties of solar interior. In this study, we discuss the relation of phase time and envelope time between the two techniques. The location of the envelope peak of the cross correlation function in time-distance analysis is identified as the travel time of the wave packet formed by modes with the same w/l. The phase time of the cross correlation function provides information of the phase change accumulated along the wave path, including the phase change at the boundaries of the mode cavity. The acoustic signals constructed with the technique of acoustic imaging contain both phase and intensity information. The phase of constructed signals can be studied by computing the cross correlation function between time series constructed with ingoing and outgoing waves. In this study, we use the data taken with the Taiwan Oscillation Network (TON) instrument and the Michelson Doppler Imager (MDI) instrument. The analysis is carried out for the quiet Sun. We use the relation of envelope time versus distance measured in time-distance analyses to construct the acoustic signals in acoustic imaging analyses. The phase time of the cross correlation function of constructed ingoing and outgoing time series is twice the difference between the phase time and envelope time in time-distance analyses as predicted. The envelope peak of the cross correlation function between constructed ingoing and outgoing time series is located at zero time as predicted for results of one-bounce at 3 mHz for all four data sets and two-bounce at 3 mHz for two TON data sets. But it is different from zero for other cases. The cause of the deviation of the envelope peak from zero is not known.

Chou, Dean-Yi↗

NASA NTRS1994

Observations of vector magnetic fields in flaring active regions

We present vector magnetograph data of 6 active regions, all of which produced major flares. Of the 20 M-class (or above) flares, 7 satisfy the flare conditions prescribed by Hagyard (high shear and strong transverse fields). Strong photospheric shear, however, is not necessarily a condition for a flare. We find an increase in the shear for two flares, a 6-deg shear increase along the neutral line after a X-2 flare and a 13-deg increase after a M-1.9 flare. For other flares, we did not detect substantial shear changes.

Chen, Jimin↗

NASA NTRS1994

Vector magnetic field changes associated with X-class flares

We present high-resolution transverse and longitudinal magnetic field measurements bracketing five X-class solar flares. We show that the magnetic shear, defined as the angular difference between the measured field and calculated potential field, actually increases after all of these flares. In each case, the shear is shown to increase along a substantial portion of the magnetic neutral line. For two of the cases, we have excellent time resolution, on the order of several minutes, and we demonstrate that the shear increase is impulsive. We briefly discuss the theoretical implications of our results.

Wang, Haimin↗

NASA NTRS1991

Motions, fields, and flares in the 1989 March active region

The results of observations of NOAA AR 5395 are presented. The region was observed every day from limb to limb for significant periods, and nine of the ten class-X flares were recorded. The region was found to be a great Delta group, dominated by spots of following (f) polarity, which moved rapidly westward, producing large changes in magnetic structure which increased the shear and led to great flares. Aside from its great size, the region was unusual in that normally p spots dominate and move westward. In this case there was a 4:1 flux imbalance; 80 percent of the flux measured was of following polarity. The major following spot in the region was found to move with a near-constant acceleration, eventually reaching 0.25 km/s. Rapid spot motion was discovered in all other superactive regions. Small p and f spots move out from either side of the large f spot, and curl around it in curved trajectories. The moving penumbral material coalesces into new umbrae.

Wang, Haimin↗

NASA NTRS1991

Magnetic flux transport of decaying active regions and enhanced magnetic network

Several series of coordinated observations on decaying active regions and enhanced magnetic network regions on the sun were carried out jointly at Big Bear Solar Observatory and at the Huairou Solar Observing Station of the Bejing Astronomical Observatory in China. The magnetic field evolution in several regions was followed closely for three to seven days. The magnetic flux transport from the remnants of decayed active regions was studied, along with the evolution and lifetime of the magnetic network which defines the boundaries of supergranules. The magnetic flux transport in an enhanced network region was studied in detail and found to be negative. Also briefly described are some properties of moving magnetic features around a sunspot. Results of all of the above studies are presented.

Wang, Haimin↗

NASA NTRS1989

The association of flares to cancelling magnetic features on the sun

Previous work relating flares to evolutionary changes of photospheric solar magnetic fields are reviewed and reinterpreted in the light of recent observations of canceling magnetic fields. The results show that cancelation happens with fields spanning a wide range of magnetic field strengths. Flares of all magnitudes begin adjacent to cancelation sites, whether the associated active region as a whole is developing or decaying. Both small and big flares are initiated near canceling sites, from the microflares associated with ephemeral regions to the kernels of the great flares. Canceling magnetic flux is observed or deduced to be the common denominator among all observed associations of flares to changing magnetic fields. It is proposed that canceling magnetic fields are a necessary evolutionary condition for the initiation of solar flares.

Livi, Silvia H. B.↗

NASA NTRS1988

Coordinated videomagnetograph observations by the Big Bear and Huairou Observatories

A videomagnetograph patterned after the BBSO system was installed at Huairou in 1987, and five days of coordinated observing were carried out from September 24 to 29, 1987. The data were combined to make a continuous movie of the fields abound a stable spot. A 57 hour magnetograph run with two seven hour gaps were achieved from 2330 UT, September 24 to 0830 UT, September 27. The frames were reregistered and justified to eliminate the change of scale with meridian distance. The intensities were corrected for cosine effect. Preliminary examination of the data shows continuous decrease of the total magnetic field during this period by more than 50 percent. The principal loss of flux appears to be due to cancellation at the main neutral line. Some flux disappears due to fragmentation, which makes the elements fall below the threshold, while only a tiny loss due to diffusion can be detected. It is planned to continue this program during Max 1991, including transverse field measurements as well. Several long runs were already obtained in 1988.

Wang, Haimin↗