Engineering topics

Zhao, X. P.

Publications and source records attributed to Zhao, X. P..

4 records

NASA NTRS1997

Modeling boot-shaped coronal holes using SOHO-MDI magnetic measurements

The boot-shaped coronal hole observed between the 22 and 30 August 1996, reproduced by rigidly rotating the data computed on 27 August with the rotation rate near the solar equator, is discussed. The data were acquired by the Michelson Doppler imager (MDI) onboard the Solar and Heliospheric Observatory (SOHO). The results suggest that change in size and shape of the holes observed during the period is caused by the projection effect. It was found that the whole solar observational synoptic chart may be used to approximately reproduce boot-shaped holes in solar minimum.

Zhao, X. P.↗

NASA NTRS1996

Solar coronal structure: A comparison of NSO/SP ground-based coronal emission line intensities and temperatures with Yohkoh SXT and WSO magnetic data

The large-scale structure of the solar corona is investigated using synoptic maps produced from Fe XIV (530.3 nm), Fe X (637.4 nm) and Ca XV (569.4 nm) data obtained at National Solar Observatory (NSO/SP), Yohkoh/Soft X-ray Telescope (SXT) X-ray data and Wilcox Solar Observatory (WSO) 'source surface' maps. We find that the Fe XIV data are an excellent proxy for spatially-average Yohkoh/SXT data. Isolated emission features and large-scale structures are nearly identical in SXT and Fe XIV maps. In addition, coronal holes and other low-emission regions are very similar. Synoptic temperature maps, calculated from the Fe X/Fe XIV ratio, show a tendency for the highest temperatures to occur where the large-scale magnetic fields change polarity at high latitudes, while lower-latitude features, including active regions, have lower apparent tempertures. Regions of enhanced temperature generally follow the helisopheric current sheet (HCS) as defined by the WSO maps. Further, emission in Ca XV (formed at T is approximately equal to 3 MK), generally occurs only over low-latitude regions that are bright in both FE X (T approximately equal to 1 MK) and Fe XIV (T approximately equal to 2 MK). Thus, there is evidence for low (approximately equal to 1 MK), moderate (approximately equal to 2 MK) and high (approximately 3 MK) temperatures in close proximity in the low corona.

Altrock, R. C.↗

NASA NTRS1995

Prediction of coronal and heliospheric magnetic fields: The promise of SOI-MDI on SOHO

Models of the coronal magnetic field have been developed over the years that reproduce the static characteristics of coronal and heliospheric structures fairly well. Limitations of spatial and temporal resolution and nonuniform quality of the input data have made it particularly difficult to investigate the response of the corona to rapidly changing photospheric conditions. The Solar Oscillations Investigation (SOI) experiment on SOHO, scheduled for launch late in 1995, will produce a series of full-disk photospheric magnetic field observations with 4" resolution about every 2 hours for the next several years. Higher resolution observations of the center of the disk will be available several times per day. These data should provide a basis for predicting the coronal and heliospheric field and their changes with unprecedented accuracy during the rising phase of Solar Cycle 23.

Hoeksema, J. T.↗

NASA NTRS1995

Understanding the large-scale structure of magnetic fields in the corona and heliosphere

To understand the non-radial nature of the southwestern streamer in the 3 November 1994 eclipse and the near absence of a latitudinal gradient of the interplanetary magnetic field strength seen by Ulysses over the southern pole, we calculate the coronal and inter planetary magnetic field from WSO photospheric field data using both the horizontal current-current sheet model (Zhao and Hoeksema, 1994) and the current sheet-source surface model (Zhao and Hoeksema, 1995). These models account for the effects of large-currents and current sheets in the corona. Comparison of the calculated results with white light, X-ray, and IMF observations show that non-radial coronal streamers and a near-zero latitudinal gradient of the radial component of the IMF may be explained by a large-scale current sheet. To best match the observations, the height of the cusp point (above which current sheets form) differs from one streamer to another. While heliospheric volume currents flowing above the source surface certainly affect the interplanetary field, they have little or no influence on the structure of the corona.

Zhao, X. P.↗