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Register a new userHalo Coronal Mass Ejections during Solar Cycle 24: reconstruction of the global scenario and geoeffectiveness
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In this study we present a statistical analysis of 53 fast Earth-directed halo CMEs observed by the SOHO/LASCO instrument during the period Jan. 2009-Sep. 2015, and we use this CME sample to test the capabilities of a Sun-to-Earth prediction scheme for CME geoeffectiveness. First, we investigate the CME association with other solar activity features by means of multi-instrument observations of the solar magnetic and plasma properties. Second, using coronagraphic images to derive the CME kinematical properties at 0.1 AU, we propagate the events to 1 AU by means of the WSA-ENLIL+Cone model. Simulation results at Earth are compared with in-situ observations at L1. By applying the pressure balance condition at the magnetopause and a solar wind-Kp index coupling function, we estimate the expected magnetospheric compression and geomagnetic activity level, and compare them with global data records. The analysis indicates that 82% of the CMEs arrived at Earth in the next 4 days. Almost the totality of them compressed the magnetopause below geosynchronous orbits and triggered a geomagnetic storm. Complex sunspot-rich active regions associated with energetic flares result the most favourable configurations from which geoeffective CMEs originate. The analysis of related SEP events shows that 74% of the CMEs associated with major SEPs were geoeffective. Moreover, the SEP production is enhanced in the case of fast and interacting CMEs. In this work we present a first attempt at applying a Sun-to-Earth geoeffectiveness prediction scheme - based on 3D simulations and solar wind-geomagnetic activity coupling functions - to a statistical set of potentially geoeffective halo CMEs. The results of the prediction scheme are in good agreement with geomagnetic activity data records, although further studies performing a fine-tuning of such scheme are needed.
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Similar to the Sun, other stars shed mass and magnetic flux via ubiquitous quasi-steady wind and episodic stellar coronal mass ejections (CMEs). We investigate the mass loss rate via solar wind and CMEs as a function of solar magnetic variability represented in terms of sunspot number and solar X-ray background luminosity. We estimate the contribution of CMEs to the total solar wind mass flux in the ecliptic and beyond, and its variation over different phases of the solar activity cycles. The study exploits the number of sunspots observed, coronagraphic observations of CMEs near the Sun by SOHO/LASCO, in situ observations of the solar wind at 1 AU by WIND, and GOES X-ray flux during solar cycle 23 and 24. We note that the X-ray background luminosity, occurrence rate of CMEs and ICMEs, solar wind mass flux, and associated mass loss rates from the Sun do not decrease as strongly as the sunspot number from the maximum of solar cycle 23 to the next maximum. Our study confirms a true physical increase in CME activity relative to the sunspot number in cycle 24. We show that the CME occurrence rate and associated mass loss rate can be better predicted by X-ray background luminosity than the sunspot number. The solar wind mass loss rate which is an order of magnitude more than the CME mass loss rate shows no obvious dependency on cyclic variation in sunspot number and solar X-ray background luminosity. These results have implications to the study of solar-type stars.
We compare the properties of halo coronal mass ejections (CMEs) that originate close to the limb (within a central meridian distance range of 60 to 90 deg) during solar cycles 23 and 24 to quantify the effect of the heliospheric state on CME properties. There are 44 and 38 limb halos in the cycles 23 and 24, respectively. Normalized to the cycle-averaged total sunspot number, there are 42 percent more limb halos in cycle 24. Although the limb halos as a population is very fast (average speed 1464 km s-1), cycle-24 halos are slower by 26 percent than the cycle-23 halos. We introduce a new parameter, the heliocentric distance of the CME leading edge at the time a CME becomes a full halo; this height is significantly shorter in cycle 24 (by 20 percent) and has a lower cutoff at 6 Rs. These results show that cycle-24 CMEs become halos sooner and at a lower speed than the cycle-23 ones. On the other hand, the flare sizes are very similar in the two cycles, ruling out the possibility of eruption characteristics contributing to the differing CME properties. In summary, this study reveals the effect of the reduced total pressure in the heliosphere that allows cycle-24 CMEs expand more and become halos sooner than in cycle 23. Our findings have important implications for the space-weather consequences of CMEs in cycle 25 (predicted to be similar to cycle 24) and for understanding the disparity in halo counts reported by automatic and manual catalogs.
Solar activity, in particular coronal mass ejections (CMEs), are often accompanied by bursts of radiation at metre wavelengths. Some of these bursts have a long duration and extend over a wide frequency band, namely, type IV radio bursts. However, the association of type IV bursts with coronal mass ejections is still not well understood. In this article, we perform the first statistical study of type IV solar radio bursts in the solar cycle 24. Our study includes a total of 446 type IV radio bursts that occurred during this cycle. Our results show that a clear majority, $sim 81 %$ of type IV bursts, were accompanied by CMEs, based on a temporal association with white-light CME observations. However, we found that only $sim 2.2 %$ of the CMEs are accompanied by type IV radio bursts. We categorised the type IV bursts as moving or stationary based on their spectral characteristics and found that only $sim 18 %$ of the total type IV bursts in this study were moving type IV bursts. Our study suggests that type IV bursts can occur with both `Fast ($geq 500$ km/s) and `Slow ($< 500$ km/s), and also both `Wide ($geq 60^{circ}$) and `Narrow ($< 60^{circ}$) CMEs. However, the moving type IV bursts in our study were mostly associated with `Fast and `Wide CMEs ($sim 52 %$), similar to type II radio bursts. Contrary to type II bursts, stationary type IV bursts have a more uniform association with all CME types.
Coronal Mass Ejections (CMEs) contributes to the perturbation of solar wind in the heliosphere. Thus, depending on the different phases of the solar cycle and the rate of CME occurrence, contribution of CMEs to solar wind parameters near the Earth changes. In the present study, we examine the long term occurrence rate of CMEs, their speeds, angular widths and masses. We attempt to find correlation between near sun parameters, determined using white light images from coronagraphs, with solar wind measurements near the Earth from in-situ instruments. Importantly, we attempt to find what fraction of the averaged solar wind mass near the Earth is provided by the CMEs during different phases of the solar cycles.
The strength of the radial component of the interplanetary magnetic field (IMF), which is a measure of the Suns total open flux, is observed to vary by roughly a factor of two over the 11 yr solar cycle. Several recent studies have proposed that the Suns open flux consists of a constant or floor component that dominates at sunspot minimum, and a time-varying component due to coronal mass ejections (CMEs). Here, we point out that CMEs cannot account for the large peaks in the IMF strength which occurred in 2003 and late 2014, and which coincided with peaks in the Suns equatorial dipole moment. We also show that near-Earth interplanetary CMEs, as identified in the catalog of Richardson and Cane, contribute at most $sim$30% of the average radial IMF strength even during sunspot maximum. We conclude that the long-term variation of the radial IMF strength is determined mainly by the Suns total dipole moment, with the quadrupole moment and CMEs providing an additional boost near sunspot maximum. Most of the open flux is rooted in coronal holes, whose solar cycle evolution in turn reflects that of the Suns lowest-order multipoles.
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