Engineering topics
Publications and source records attributed to Tang, Frances.
We examine 3 years of interplanetary data and geomagnetic activity indices (1973-1975) to determine the causes of geomagnetic storms and substorms during the descending phase of the solar cycle. In this paper, we specifically studied the year 1974 where two long lasting coronating streams existed.
Vector magnetograms, H-alpha, D3, and white-light filtergrams were obtained at the Big Bear Solar Observatory immediately after the X-9 flare on March 22, 1991. These observations show that the umbral area increased by 2 x 10 exp 7 sq km, together with a magnetic flux increase of 2 x 10 exp 20 Mx. The magnetic shear increased by 40 deg along the neutral line. It is indicated, by the study of the evolution of spot and magnetic structure of the March 22, 1991 region, that a pair of new umbrae emerged suddenly on either side of the neutral line coinciding with the shear increase immediately after the flare.
The five largest magnetic storms that occurred between 1971 to 1986 are studied to determine their solar and interplanetary causes. All of the events are found to be associated with high speed solar wind streams led by collisionless shocks. The high speed streams are clearly related to identifiable solar flares. It is found that: (1) it is the extreme values of the southward interplanetary magnetic fields rather than solar wind speeds that are the primary causes of great magnetic storms, (2) shocked and draped sheath fields preceding the driver gas (magnetic cloud) are at least as effective in causing the onset of great magnetic storms (3 of 5 events) as the strong fields within the driver gas itself, and (3) precursor southward fields ahead of the high speed streams allow the shock compression mechanism (item 2) to be particularly geoeffective.
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.
The optical and morphological properties of impulsive flares are examined, showing that all impulsive flares occur close to sunspots and on magnetic inversion lines with steep gradients. Impulsive flares are defined and flare brightness variations and the magnetic structure of flares are discussed. The basic characteristics of impulsive flares are described and the evolution of a typical impulsive flare is outlined.
Recent data on the interplanetary and solar causes of geomagnetic activity are reviewed. Emphasis is given to the interplanetary source of southward Bz for magnetic storms, the solar sources of magnetic storms, and the roles of 'great' solar flares, substorms, the magnetopause boundary layer, and the dayside aurora in causing geomagnetic activity. A background information for interplanetary phenomena is also provided.
The solar wind-magnetosphere coupling problem during intense magnetic storms was investigated for ten intense magnetic storm events occurring between August 16, 1978 to December 28, 1979. Particular attention was given to the dependence of the ring current energization on the ISEE-measured solar-wind parameters and the evolution of the ring current during the main phase of the intense storms. Several coupling functions were tested as energy input, and several sets of the ring current decay time-constant were searched for the best correlation with the Dst response. Results indicate that a large-scale magnetopause reconnection operates during an intense storm event and that the solar wind ram pressure plays an important role in the energization of the ring current.
The solar sources of interplanetary southward Bz events responsible for major magnetic storms observed in the August 1978-December 1979 period were studied using a full complement of solar wind plasma and field data from ISEE 3. It was found that, of the ten major storms observed, seven were initiated by active region flares, and three were associated with prominence eruptions in solar quiet regions. Nine of the storms were associated with interplanetary shocks. However, a comparison of the solar events' characteristics and those of the resulting interplanetary shocks indicated that standard solar parameters did not correlate with the strengths of the resulting shocks at 1 AU.
Simultaneous ISEE-3 field and plasma data were used to examine interplanetary phenomena associated with 10 major magnetic storms detected from August 16, 1978, to December 28, 1979, in a study of Gonzalez and Tsurutani (1987), and, in particular to determine the origins of the southward magnetic fields which caused the storms. In nine of the 10 cases, the responsible interplanetary events were found, as expected, to be associated with the high magnetic fields in the stream-stream interaction regions (sheaths) or driver gases, with the events following the interplanetary shocks. The tenth event was found to be associated not with a high-speed stream, but with a noncompressional density-enhancement event. The results of this study indicate the equal importance of both the sheath fields or draped fields and the driver gas fields for the generation of major geomagnetic storms.
An investigation of two years of quiescent prominences shows that substantially more (20 percent in 1973 and 96 percent in 1979) quiescent prominences were formed on neutral lines between bipolar regions than on neutral lines inside bipolar regions. An examination of the associated solar activity of the seven white light coronal transients observed by Skyklab indicates that regardless of where the prominences were formed, their eruptions can cause observable coronal mass ejections. The present results suggest that a prominence model is needed in which the evolution begins at the boundary of two adjacent bipolar regions.
Using daily full-disk magnetograms and He I 10830 spectroheliograms to study the count and surface distribution of ephemeral regions over the solar cycle, Harvey (1985) concluded that the small dark structures seen in 10830, thought to correspond to X-ray bright points, were more often associated with magnetic bipoles that appeared to result from an encounter of already existing opposite polarity magentic flux than with emerging small magnetic bipoles (ephemeral regions). Such encounters would be more likely to occur in areas of mixed polarity. The fractional area of the sun covered by mixed polarity fields varies anti-correlated with the solar cycle leading to a possible explanation for the 180 degrees out of phase solar cycle variation of X-ray bright points. To establish the validity of this suggestion, a detailed study of time-sequence magnetic field, He I wavelength 10830, Ha, C IV, and Si II observations of selected areas of the quiet sun was initiated about 2 years ago. The preliminary results of this study are reported.