* Astronomy

Blobrana · L

RE: Solar cycle 24

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Title: The cause of the weak solar cycle 24
Author: Jie Jiang, Robert H. Cameron, Manfred Schuessler

The ongoing 11-year cycle of solar activity is considerably less vigorous than the three cycles before. It was preceded by a very deep activity minimum with a low polar magnetic flux, the source of the toroidal field responsible for solar magnetic activity in the subsequent cycle. Simulation of the evolution of the solar surface field shows that the weak polar fields and thus the weakness of the present cycle 24 are mainly caused by a number of bigger bipolar regions emerging at low latitudes with a `wrong' (i.e., opposite to the majority for this cycle) orientation of their magnetic polarities in the North-South direction, which impaired the growth of the polar field. These regions had a particularly strong effect since they emerged within ±10° latitude from the solar equator.

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Title: Solar Sources of ³He-rich Solar Energetic Particle Events in Solar Cycle 24
Author: Nariaki V. Nitta, Glenn M. Mason, Linghua Wang, Christina M. S. Cohen, Mark E. Wiedenbeck

Using high-cadence extreme-ultraviolet (EUV) images obtained by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory, we investigate the solar sources of 26 ³He-rich solar energetic particle (SEP) events at \lesssim1 MeV nucleon^-1 that were well-observed by the Advanced Composition Explorer during solar cycle 24. Identification of the solar sources is based on the association of ³He-rich events with type III radio bursts and electron events as observed by Wind. The source locations are further verified in EUV images from the Solar and Terrestrial Relations Observatory, which provides information on solar activities in the regions not visible from the Earth. Based on AIA observations, ³He-rich events are not only associated with coronal jets as emphasized in solar cycle 23 studies, but also with more spatially extended eruptions. The properties of the ³He-rich events do not appear to be strongly correlated with those of the source regions. As in the previous studies, the magnetic connection between the source region and the observer is not always reproduced adequately by the simple potential field source surface model combined with the Parker spiral. Instead, we find a broad longitudinal distribution of the source regions extending well beyond the west limb, with the longitude deviating significantly from that expected from the observed solar wind speed.

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Title: The Peculiar Behaviour of Halo Coronal Mass Ejections in Solar Cycle 24
Author: N. Gopalswamy, H. Xie, S. Akiyama, P. Mäkelä, S. Yashiro, G. Michalek

We report on a remarkable finding that the halo coronal mass ejections (CMEs) in cycle 24 are more abundant than in cycle 23, although the sunspot number in cycle 24 has dropped by about 40%. We also find that the distribution of halo-CME source locations is different in cycle 24: the longitude distribution of halos is much flatter with the number of halos originating at central meridian distance >/=60 degrees twice as large as that in cycle 23. On the other hand, the average speed and the associated soft X-ray flare size are the same in the two cycles, suggesting that the ambient medium into which the CMEs are ejected is significantly different. We suggest that both the higher abundance and larger central meridian longitudes of halo CMEs can be explained as a consequence of the diminished total pressure in the heliosphere in cycle 24 (Gopalswamy et al. 2014). The reduced total pressure allows CMEs expand more than usual making them appear as halos.

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Title: On the 27-day Variations of Cosmic Ray Intensity in Recent Solar Minimum 23/24
Author: R. Modzelewska, M.V. Alania

We have studied the 27-day variations and their harmonics of the galactic cosmic ray (GCR) intensity, solar wind velocity, and interplanetary magnetic field (IMF) components in the recent prolonged solar minimum 23 24. The time evolution of the quasi-periodicity in these parameters connected with the Suns rotation reveals that their synodic period is stable and is aprox 26-27 days. This means that the changes in the solar wind speed and IMF are related to the Suns near equatorial regions in considering the differential rotation of the Sun. However, the solar wind parameters observed near the Earths orbit provide only the conditions in the limited local vicinity of the equatorial region in the heliosphere (within in latitude). We also demonstrate that the observed period of the GCR intensity connected with the Suns rotation increased up to aprox 33-36 days in 2009. This means that the process driving the 27-day variations of the GCR intensity takes place not only in the limited local surroundings of the equatorial region but in the global 3-D space of the heliosphere, covering also higher latitude regions. A relatively long period ( aprox 34 days) found for 2009 in the GCR intensity gives possible evidence of the onset of cycle 24 due to active regions at higher latitudes and rotating slowly because of the Suns differential rotation. We also discuss the effect of differential rotation on the theoretical model of the 27-day variations of the GCR intensity.

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Title: 3He-rich SEP Events Observed by STEREO-A
Authors: R. Bucik, U. Mall, A. Korth, G. M. Mason, R. Gomez-Herrero

Using the SIT (Suprathermal Ion Telescope) instrument on STEREO-A we have examined the abundance of the rare isotope 3He during the rising activity phase of solar cycle 24 between January 2010 and December 2011. We have identified six solar energetic particle (SEP) events with enormous abundance enhancements of 3He (3He/4He >1). The events were short lasting, typically ~0.5-1 day and most of them occurred in association with high-speed solar wind streams and corotating interaction regions. With one exception the events were not associated with ~100 keV solar electron intensity increases. The events showed also enhanced NeS/O and Fe/O ratios. The solar images indicate that the events were generally associated with the active regions located near a coronal hole.

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