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The solar cycle 23 seems to be of great interest for the researchers due to many peculiarities. A study of the pa- rameters of the sixteen ground level enhancements recorded during the approximately 12-year period of it (1996-2008) together with the associated solar activity, including the main properties of the solar flares, the coronal mass ejections and the radio bursts has been realized, in an effort to understand the connection of these events. All studied cases seem to be connected with very intense flares of long duration, having a mean importance value of X5.9 and a mean duration of 164.5 min, with either halo or partial halo coronal mass ejec- tions with a mean linear velocity of 1876 km/sec, as well as with intense radio bursts. It is also noticed that the ground level enhancements of the 23rd solar cycle occurred after the onset time of the associated solar X ray flares with a mean time delay of about 38 min, very useful result for their mon- itoring and prediction.

The Standard Radiation Environment Monitor (SREM) is a particle detector developed by the European Space Agency for satellite applications with the main pur- pose to provide radiation hazard alarms to the host space- craft. SREM units have been constructed within a radiation hardening concept and therefore are able to register extreme solar particle events (SPEs). Large SPEs are registered at Earth, by ground based detectors as neutron monitors, in the form of Ground Level Enhancements of solar cosmic rays. In this work, a feasibility study of a possible radiation alert, deduced by SREM measurements was implemented for the event of 20 January 2005. Taking advantage of the neutron monitor's experience, the steps of the GLE alert algorithm were put into practice on SREM measurements. The out- come was that SREM units did register the outgoing SPE on-time and that these could serve as indicators of radiation hazards, leading to successful alerts.

During the August–September 2005 burst of solar activity, close to the current solar cycle minimum, a significant number of powerful X-ray flares were recorded, among which was the outstanding X17.0 flare of 7 September 2005. Within a relatively short period (from 22 August to 17 September) two severe magnetic storms were also recorded as well as several Forbush effects. These events are studied in this work, using hourly mean variations of cosmic ray density and anisotropy, derived from data of the neutron monitor network. During these Forbush effects the behavior of high energy cosmic ray characteristics (density and anisotropy) is analyzed together with interplanetary disturbances and their solar sources, and is compared to the variations observed in geomagnetic activity. A big and long lasting (~6 h) cosmic ray pre-decrease (~2%) is defined before the shock arrival on 15 September 2005. The calculated cosmic ray gradients for September 2005 are also discussed.

A new catalogue of 253 solar proton events (SPEs) with energy >10MeV and peak intensity >10 protons/cm2.s.sr (pfu) at the Earth's orbit for three complete 11-year solar cycles (1970-2002) is given. A statistical analysis of this data set of SPEs and their associated flares that occurred during this time period is presented. It is outlined that 231 of these proton events are flare related and only 22 of them are not associated with Ha flares. It is also noteworthy that 42 of these events are registered as Ground Level Enhancements (GLEs) in neutron monitors. The longitudinal distribution of the associated flares shows that a great number of these events are connected with west flares. This analysis enables one to understand the long-term dependence of the SPEs and the related flare characteristics on the solar cycle which are useful for space weather prediction.