K. J. Li

The Hα surges and EUV jets from magnetic flux emergences and cancellations

We analyzed multi-wavelength observations of three surges with a recurrent period of about 70 min in Ha, EUV, and soft X-ray, which occurred in the quiet-sun region on 2000 November 3. These homologous surges were associated with small flares at the same base, but their exact footpoints were spatially separated from the flare. Each surge consisted of a cool Ha component and a hot, EUV or soft X-ray component, which showed different evolutions not only in space but also in time. The EUV jets had slightly converging shapes, underwent more complicate development, showed clearly twisting structures, and appeared to open to space. The Ha surges, however, were smaller and only traced the edges of the jets. They always occurred later than the jets but had dark EUV counterparts appearing in the bright jets. These surge activities were closely associated with two emerging bipoles and their driven flux cancellations at the base region, and were consistent with the magnetic reconnection surge model. The possible cause of the delay between the surges and jets, of the dark structures in the jets are discussed, along with the possible role of flux cancellations in generation of these surges.

Regularity of the north-south asymmetry of solar activity

In the present work, the dominant hemisphere of solar activity in each of solar cycles 12 to 22 has been clarified by calculating the actual probability of the hemispheric distribution of several solar activity phenomena using long-term observational records. An attempt is made to demonstrate that a long characteristic time scale, about 12-cycle length, is inferred to occur in solar activity.

Detection of a planetary system orbiting the eclipsing polar HU Aqr

Using the precise times of mid-egress of the eclipsing polar HU Aqr, we discovered that this polar is orbited by two or more giant planets. The two planets detected so far have masses of at least 5.9 and 4.5M(Jup). Their respective distances from the polar are 3.6 and 5.4 au with periods of 6.54 and 11.96 yr, respectively. The observed rate of decrease of period derived from the downward parabolic change in the observed - calculated (O - C) curve is a factor of 15 larger than the value expected for gravitational radiation. This indicates that it may be only a part of a long-period cyclic variation, revealing the presence of one more planet. It is interesting to note that the two detected circumbinary planets follow the Titus-Bode law of solar planets with n = 5 and 6. We estimate that another 10 yr of observations will reveal the presence of the predicted third planet.

On long-term predictions of the maximum sunspot numbers of solar cycles 21 to 23

We have collected a set of predicted values of maximum sunspot numbers of solar cycles 22 and 23 published in the literature and examined the characteristics of predictions made by various methods. The precursor methods are found to be always superior to other prediction methods. The maximum sunspot number of solar cycle 23 is here inferred to be about 162.

Hemispheric variation in solar activity

Hemispheric solar activity is usually found to be asymmetric within each solar cycle, and this behavior is also found in its extended cycle. In addition to the familiar Il yr individual cyclic variation, a long-term hemispheric variation of at least 12-cycle length is inferred to exist.

On the Latitudinal Distribution of Sunspot Groups over a Solar Cycle

The latitudinal distribution of sunspot groups over a solar cycle is investigated. Although individual sunspot groups of a solar cycle emerge randomly at any middle and low latitude, the whole latitudinal distribution of sunspot groups of the cycle is not stochastic and, in fact, can be represented by a probability density function of the Γ distribution having maximum probability at about 15.5°. The maximum amplitude of a solar cycle is found to be positively correlated against the number of sunspot groups at high latitude (≥35°) over the cycle, as well as the mean latitude. Also, the relation between the asymmetry of sunspot groups and its latitude is investigated, and a pattern of the N-S asymmetry in solar activity is suggested.

Latitude Migration of Sunspot Groups

Characteristics of latitude migration of sunspot groups in the northern and southern hemispheres are investigated by using the Royal Greenwich Observatory data set from the years 1874 through 1999. Latitude migration of sunspot groups is found to be represented by a binomial formula, which quantitatively describes the law. Latitude migration velocity of sunspot groups is largest at the beginning of a Sporer solar cycle, and as the solar cycle progresses, it decreases with time, with an average of about 1-degrees6 yr(1) during a solar cycle. Two different methods have been employed to confirm that latitude migration in both hemispheres is statistically the same.

VARIATIONS OF SOLAR ROTATION AND SUNSPOT ACTIVITY

The continuous wavelet transformation is used to study the temporal variations of the rotational cycle length of daily sunspot numbers from 1849 January 1 to 2010 February 28, from a global point of view. The rotational cycle length of the Sun is found to have a secular trend, which statistically shows a linear decrease by about 0.47 days during the time interval considered. The empirical mode decomposition analysis of the temporal variations of the rotational cycle length shows an acceleration trend for the surface rotation rate from cycles 11 to 19, but a deceleration trend from the beginning of cycle 20 onward. We cannot determine whether the rotation rate around the maximum times of the Schwable cycles should be faster or slower than that around the minimum times, implying no Schwable cycle in the long-term variations of rotation. The results obtained are compared to those from the literature. It is inferred that the variation of the rotational cycle length may be related to the variation of sunspot activity in the long run.

Latitude migration of solar filaments

The Carte Synoptique catalogue of solar filaments from 1919 March-1989 December, corresponding to complete cycles 16-21, is utilized to show the latitudinal migration of filaments at low latitudes (less than 50 degrees), and the latitudinal drift of solar filaments in each hemisphere in each cycle of the time interval is compared with the corresponding drift of sunspot groups. The latitudinal drift of filaments obviously differs from that of sunspot groups. At the beginning of a cycle, filaments (sunspot groups) migrate from latitudes of about 40 degrees (28 degrees) with a drift velocity of about 2.4 m s-1 (1.2 m s-1) toward the solar equator, reach latitudes of about 25 degrees (20 degrees) 4 yr later at the cycle maximum with a drift velocity of about 1.0 m s-1 (1.0 m s-1) and halt at about 8 degrees (8 degrees) at the end of the cycle. When solar activity is programmed into a solar cycle, the difference between the latitudes of appearance of filaments and sunspot groups becomes smaller and smaller. The difference rapidly decreases in the first and last similar to 4 yr of a cycle, but barely decreases at all in the similar to 4 yr after the cycle maximum. For filaments, the latitudinal drift velocity decreases in the first similar to 7.5 yr of a cycle, but increases in the last similar to 3.5 yr of the cycle. However, for sunspot groups the drift velocity always decreases over the whole cycle. The difference between the latitude drift of filaments in the northern and southern hemispheres is found not to be obvious. The latitudinal drift velocity of filaments differs slightly in the northern and southern hemispheres within a cycle. The physical implication behind the latitudinal drift of filaments is explored.

SYNCHRONIZATION OF HEMISPHERIC SUNSPOT ACTIVITY REVISITED : WAVELET TRANSFORM ANALYSES

Three kinds of wavelet transform methods-continuous wavelet transform, cross-wavelet transform, and wavelet coherence-have been proposed to investigate the phase synchrony of the smoothed monthly mean sunspot areas in the time interval of 1874 May to 2008 March in the solar northern and southern hemispheres. For both time series, the Schwabe cycle is the only period of statistical significance, whose mean value is 10.61 yr. The length of the Schwabe cycle for the smoothed monthly mean sunspot areas in the northern hemisphere actually differs from that in the southern hemisphere, which should lead to phase asynchrony between the two series. Both the cross-wavelet transform and wavelet coherence analyses show an asynchronous behavior with phase mixing in the high-frequency components of hemispheric sunspot activity and a strong synchronous behavior with coherent phase angles in the low-frequency components corresponding to period scales around the Schwabe cycle. Although a phase coherence is found at timescales of about 8.5-13.5 yr (which is similar to those of Donner & Thiel, but within a shorter period), phases are not always coherent at the timescales in the considered time interval. The availability of a physical, meaningful phase definition depends crucially on the appropriate choice of reference frequencies. At the coherent period scales, the leading role is found from those conditions where processes of sunspot formation in the northern hemisphere occur earlier than in the southern one (except some exceptions in several years around the year 1900) during the years of about 1874-1926 to those where the opposite is true during the years of about 1926-1966, and returning back again during the years of about 1966-2008. The mean phase synchronization values at the coherent timescales given by wavelet coherence represent the running trend of the line of synchronization given by a cross-recurrence plot.