Properties of Ground Level Enhancement Events and the Associated Solar Eruptions During Solar Cycle 23

Abstract

Solar cycle 23 witnessed the most complete set of observations of coronal mass ejections (CMEs) associated with the Ground Level Enhancement (GLE) events. We present an overview of the observed properties of the GLEs and those of the two associated phenomena, viz., flares and CMEs, both being potential sources of particle acceleration. Although we do not find a striking correlation between the GLE intensity and the parameters of flares and CMEs, the solar eruptions are very intense involving X-class flares and extreme CME speeds (average ∼2000 km/s). An M7.1 flare and a 1200 km/s CME are the weakest events in the list of 16 GLE events. Most (80 %) of the CMEs are full halos with the three nonhalos having widths in the range 167 to 212 degrees. The active regions in which the GLE events originate are generally large: 1290 msh (median 1010 msh) compared to 934 msh (median: 790 msh) for SEP-producing active regions. For accurate estimation of the CME height at the time of metric type II onset and GLE particle release, we estimated the initial acceleration of the CMEs using flare and CME observations. The initial acceleration of GLE-associated CMEs is much larger (by a factor of 2) than that of ordinary CMEs (2.3 km/s 2 vs. 1 km/s 2 ). We confirmed the initial acceleration for two events for which CME measurements are available in the inner corona. The GLE particle release is delayed with respect to the onset of all electromagnetic signatures of the eruptions: type II bursts, low frequency type III bursts, soft X-ray flares and CMEs. The presence of metric type II radio bursts some 17 min (median: 16 min; range: 3 to 48 min) before the GLE onset indicates shock formation well before the particle release. The release of GLE particles occurs when the CMEs reach an average height of ∼3.09 R s (median: 3.18 R s ; range: 1.71 to 4.01 R s ) for well-connected events (source longitude in the range W20-W90). events, the average CME height at GLE particle release is ∼66 % larger (mean: 5.18 R s ; median: 4.61 R s ; range: 2.75-8.49 R s ). The longitudinal dependence is consistent with shock accelerations because the shocks from poorly connected events need to expand more to cross the field lines connecting to an Earth observer. On the other hand, the CME height at metric type II burst onset has no longitudinal dependence because electromagnetic signals do not require magnetic connectivity to the observer. For several events, the GLE particle release is very close to the time of first appearance of the CME in the coronagraphic field of view, so we independently confirmed the CME height at particle release. The CME height at metric type II burst onset is in the narrow range 1.29 to 1.8 R s , with mean and median values of 1.53 and 1.47 R s . The CME heights at metric type II burst onset and GLE particle release correspond to the minimum and maximum in the Alfvén speed profile. The increase in CME speed between these two heights suggests an increase in Alfvénic Mach number from 2 to 3. The CME heights at GLE particle r...

The Acceleration of High-energy Protons at Coronal Shocks: The Effect of Large-scale Streamer-like Magnetic Field Structures

“…There have been a number of factors regarding CME shock properties that have been found to be important for producing large SEP events. For example, large SEP events are often associated with fast and very energetic CMEs (e.g., Kahler 2001;Mewaldt et al 2008), especially the CMEs that produce GLE events, which are among the fastest ones, with speeds of ∼2000 km s −1 (e.g., Gopalswamy et al 2012Gopalswamy et al , 2013aThakur et al 2014). Those CMEs are usually in the western hemisphere and have a good magnetic connectivity to the Earth (e.g., Gopalswamy et al 2012Gopalswamy et al , 2014.…”

“…For example, large SEP events are often associated with fast and very energetic CMEs (e.g., Kahler 2001;Mewaldt et al 2008), especially the CMEs that produce GLE events, which are among the fastest ones, with speeds of ∼2000 km s −1 (e.g., Gopalswamy et al 2012Gopalswamy et al , 2013aThakur et al 2014). Those CMEs are usually in the western hemisphere and have a good magnetic connectivity to the Earth (e.g., Gopalswamy et al 2012Gopalswamy et al , 2014. However, not all CMEs that satisfy the two factors can lead to large SEP events or GLEs (e.g., Kahler & Vourlidas 2005;Ding et al 2013), indicating other processes are important for producing large SEP events as well.…”

“…Gopalswamy et al (2013b) found that CME shock formation can occur substantially below ∼1.5 R e from observations of type II radio bursts produced by electrons accelerated by shocks. Gopalswamy et al (2012) analyzed GLEs in solar cycle 23 and found that while the shock formation height can be ∼1.5 R e , the CME heights at GLE particle release times range from 1.7 to 4 R e , with an average of 3.1 R e for well-connected events, suggesting that particle acceleration occurs in this region. The results are in line with earlier studies for GLEs in both solar cycle 23 and previous cycles using the velocity dispersion analysis by Reames (2009aReames ( , 2009b.…”

Using global neutron monitor network data for GLE analysis: recent results

“…The second event presented in this study occurred during the declining phase of the solar cycle. On 13 December 2006, NOAA active region 10930, produced X3.4/4 B solar flare associated with full-halo CME accompanied by a SEP event [29,30]. The event was recorded by several NMs with maximum seen at Oulu NM ∼ 90 % (5 min data) and was classified as GLE 70.…”

Detection of Coronal Mass Ejections (CMEs) in the Period of March‑May 2012 at Moussala Peak

“…As flare is moving away from the Sun, the particles with the highest energy are moving in front of CME accelerated by shock mechanism. Thus the CME advance consists of solar energetic particles (SEP) from the ones with higher relativistic energy due to shock front [ 6 ]. When proton flux of particles with relativistic velocity enters the Earth atmosphere, they produce large showers.…”