* Astronomy

Blobrana · L

RE: Solar Activity Cycle

Blobrana · L

Title: The Solar Cycle
Author: David H. Hathaway

The Solar Cycle is reviewed. The 11-year cycle of solar activity is characterised by the rise and fall in the numbers and surface area of sunspots. A number of other solar activity indicators also vary in association with the sunspots including; the 10.7cm radio flux, the total solar irradiance, the magnetic field, flares and coronal mass ejections, geomagnetic activity, galactic cosmic ray fluxes, and radioisotopes in tree rings and ice cores. Individual solar cycles are characterised by their maxima and minima, cycle periods and amplitudes, cycle shape, the equatorward drift of the active latitudes, hemispheric asymmetries, and active longitudes. Cycle-to-cycle variability includes the Maunder Minimum, the Gleissberg Cycle, and the Gnevyshev-Ohl (even-odd) Rule. Short-term variability includes the 154-day periodicity, quasi-biennial variations, and double-peaked maxima. We conclude with an examination of prediction techniques for the solar cycle and a closer look at cycles 23 and 24.

Read more (1885kb, PDF)

Blobrana · L

Title: Asymmetric Solar Polar Field Reversals
Authors: Leif Svalgaard, Yohsuke Kamide

The solar polar fields reverse because magnetic flux from decaying sunspots moves towards the poles, with a preponderance of flux from the trailing spots. Let us assume that there is a strong asymmetry in the sense that all activity is in the Northern Hemisphere, then that excess flux will move to the North Pole and reverse that pole, while nothing happens in the South. If later on, there is a lot of activity in the South, then that flux will help reverse the South Pole. In this way, we get two humps in solar activity and a corresponding difference in time of reversals. Such difference was first noted by Babcóck (1959) from the very first observation of polar field reversal just after the maximum of the strongly asymmetric solar cycle 19. At that time, the Southern Hemisphere was most active before sunspot maximum and the South Pole duly reversed first, followed by the Northern Hemisphere more than a year later, when that hemisphere was most active. Solar cycles since then have had the opposite asymmetry, with the Northern Hemisphere being most active early in the cycle. Polar field reversals for these cycles have as expected happened first in the North. This is especially noteworthy for the present solar cycle 24. We suggest that the association of two peaks of solar activity when separated by hemispheres with correspondingly different times of polar field reversals is a general feature of the cycle.

Read more (10518kb, PDF)

Blobrana · L

Title: Hysteresis in a Solar Activity Cycle
Authors: Vinita Suyal, Awadhesh Prasad, Harinder P. Singh

We analyse in situ measurements of solar wind velocity obtained by the Advanced Composition Explorer (ACE) spacecraft during the solar activity cycle 23. We calculated a robust complexity measure, the permutation entropy (S) of solar wind time series at different phases of a solar activity cycle. The permutation entropy measure is first tested on the known dynamical data before its application to solar wind time series. It is observed that complexity of solar wind velocity fluctuations at 1 AU shows hysteresis phenomenon while following the ascending and descending phases of the activity cycle. This indicates the presence of multistability in the dynamics governing the solar wind velocity over a solar activity cycle.

Read more (495kb, PDF)