RESEARCH ACTIVITIES
From the first years of my undergraduate studies in Physics Department of Athens University I am a member of the Cosmic Ray Group of the Nuclear and Particle Physics Section of Athens University, under supervision of Prof. H. Mavromichalaki working on the collection and analysis of cosmic ray data from the Athens Neutron Monitor Station. The modern real-time Athens Neutron Monitor Station has been operating since 2000 providing real time measurements of the hadronic component of galactic cosmic rays ( http://cosray.phys.uoa.gr). Due to its cut-off rigidity (8.53 GV), it has a favourable position useful to separate galactic cosmic ray flux from solar proton enhancements. Athens Neutron Monitor already records data with 1sec resolution. Nowadays many stations of the Neutron Monitor Network are in a new level of collecting and processing continuously recorded information useful to space weather tasks. For this purpose in collaboration with the Cosmic ray group of the IZMIRAN, a special real time Network for collection and presentation of online data as well as an accurate and real time synchronization system of neutron monitors is established. This network will be useful for monitoring and forecasting intense solar events, dangerous to the Earth environment (Space Weather studies).
My field of scientific interests covers the areas of :
· Galactic and Solar Cosmic Ray Physics
· Magnetospheric Physics
· Solar Physics
· Astroparticle Physics
· Space Weather
Initially, I worked on the definition of an empirical model for predicting the value of the Coronal Index of solar activity. All possible changes of the solar activity can be expressed by the coronal index of solar activity that represents the averaged daily power of the green corona emitted from the Sun’s visible hemisphere. The representative character of this index allows us to study long-term, intermediate and short-term variations of the Sun as a star. This index can be expressed well as a function of other solar indices. As green line reflects the distribution of the photospheric magnetic fields in the solar corona, the dependence of this index on the solar magnetic field is confirmed by means of statistical analysis of these two parameters. Daily values of the coronal index, as well as of the magnetic field data obtained from the Wilcox Solar Observatory, have been analysed by Fast Fourier analysis and Wavelet Transform analysis for the time period 1966-1998 covering more than three solar cycles. Periodicities of 11.4, 3.2, 2.3, 1.7, 1, 0.29, 0.07 and 0.04 years have been found in both parameters that means once again that the coronal index is probably related to the underlying photospheric magnetic fields and it can be used as a global index of solar activity useful for Space Weather studies.
My research activity is mainly focused on the theoretical and experimental study of ground level enhancements (GLEs). GLEs are big solar proton events that can be detected at ground level by neutron monitors and muon detectors due to the fact that the primary relativistic solar cosmic ray particles at the top of the atmosphere are energetic enough to overcome the magnetic field obstacle and penetrate inside the atmosphere producing cascades. In collaboration with the members of Athens and IZMIRAN Cosmic Ray groups we have been analysing a variety of such events including :
· Three GLEs that took place in October-November 2003,
· The enhancement of solar cosmic rays in February 1956, which seems to be the
biggest GLE ever recorded by ground-based monitors,
· The recent GLE in January 2005.
Using data from the worldwide network of neutron monitors, we analyzed the three GLEs recorded at several locations around the globe almost at the end of solar cycle 23. During the extreme burst of solar activity in October–November 2003, a series of outstanding events distinguished by their magnitude and peculiarities were recorded by the ground based neutron monitor network. The biggest and most productive in 23rd solar cycle active region 486 generated the most significant series of solar flares among which the flare X28/3B on November 4, 2003 which was the most powerful over the history of X-ray solar observations. The fastest arrival of the interplanetary disturbance from the Sun after the flare event in August 1972 and the highest solar wind velocity and IMF intensity were observed during the exact period of these events. In one-week period three ground level enhancements (GLEs) of solar cosmic rays were recorded by the neutron monitor network on 28, 29 October and 2 November 2003. Maximum proton energy in these events seems to be ranged from 5 to 10 GeV. Joint analysis of data from ground level stations (neutron monitors) and satellite measurements allowed the estimation of the particle path length, the onset time of the injection on the Sun and some other proton flux characteristics.
The ground level enhancement of the cosmic ray intensity on February 23, 1956 (GLE05) is the most famous one among all proton events observed since 1942. The information relative to this event is of course very limited due to the fact that at that period there were neither solar wind data nor interplanetary magnetic field measurements. Furthermore, there were not any X-Ray or gamma observations and the information on flares was limited. Cosmic ray data was obtained exclusively by ground level detectors of small surface and in many cases of non-standard design. After having obtained the available data from neutron monitors operated in 1956 we analyzed them in order to create a model of the behavior of solar cosmic rays on February 23, 1956. The energy spectrum of cosmic rays as well as their anisotropy and their differential and integral fluxes, were calculated and presented. It was shown that the most outstanding feature of this proton enhancement was a narrow and extremely intensive beam of ultra relativistic particles arriving at Earth during the first minutes of the event. However, its contribution to the solar particle density and to their fluence was not significant because of its short duration and small width. The estimation of the integral flux for particles with energies bigger than 100MeV places this event over all others, but it does not outstand herewith from the common distribution. Perhaps, the number of accelerated low energy particles was close to record, but these particles passed mainly western of the Earth. Many features of this GLE are apparently explained by the peculiarity of the particle interplanetary propagation from remote limb or behind of limb source. The quality of the used neutron monitor data did not allow to us to be sure in some details which may be cleared up with incorporation to data analysis from muon telescopes and ionization chambers operated at that time.
The solar cosmic ray event associated with an X7.1 class solar flare on 20 January, 2005 was one of the greatest enhancements ever recorded by the ground level worldwide network of neutron monitors. The event occurred during a Forbush decrease, almost at the end of the 23rd cycle of solar activity. In order to study this specific event, as well as other past or future GLEs, we proposed a new ground level enhancement model for getting the broadest possible picture of the respective events and for understanding the physics of solar cosmic ray particles under extreme solar conditions. Neutron monitors responses from forty-one stations widely distributed around the Earth have been modeled to an anisotropic solar proton flux, using an optimization method based on the Levenberg-Marquardt algorithm. This new GLE-model, the so called NM-BANGLE model (Plainaki et al., 2006) proposes a generalized technique for revealing several characteristics of solar cosmic rays during solar extreme events using exclusively ground based data. The parameters of the primary solar particles outside the magnetosphere, their dynamics, as well as the characteristics of solar cosmic rays can be obtained and discussed.
In order to make attainable the application of the NM-BANGLE model to a GLE event detected by the ground based neutron monitors of the worldwide network we obtained at first some information on the viewing directions of each station. Cosmic ray particles arriving at the vicinity of the Earth propagate inside the magnetosphere and finally access low-altitude satellites or ground level neutron monitors, if their energy is sufficiently high. In order to define the exact relation between primary solar cosmic rays at the top of the atmosphere with the secondary ones detected at ground level, we applied a numerical back-tracing technique for revealing the proton trajectory inside the magnetospheric field of the Earth. In this way we defined the details of the cosmic-ray particle transport defining the asymptotic windows of allowed trajectories for each neutron monitor of the worldwide network during a particular time-period. Moreover using the Tsyganenko-1989 magnetospheric field model we obtained crucial information on the Earth’s “magnetospheric optics” for primary cosmic rays. Especially for the big solar cosmic ray event of January the asymptotic directions of viewing for forty-one neutron monitors stations widely distributed around the globe covering a wide range of longitudes and rigidities have been calculated. In this case the neutron monitor network has been treated as a multidimensional tool that gives insights into the arrival directions of solar cosmic ray particles as well as their spatial and energy distributions during extreme solar events.
For the event of 20 January 2005 it was found that there was a complex structure with two maxima. The first maximum appeared due to the extremely anisotropic beam of solar particles arriving during the initial phases of the event, whereas the second one is probably related to SCR density maximum. The time evolution of the rigidity spectrum has a rather complicated behaviour. In the beginning of the event it appears hard. In the second time-interval it softens abruptly and then it hardens again. During the later phases the spectral index varied between -6.6 and -7. The extremely intensive narrow beams of solar relativistic particles arriving at the Earth during the time interval 6:50 UT – 6:55 UT had a width that did not exceed 10-40 degrees. The neutron monitors whose asymptotic directions at that time viewed the anisotropy source recorded enhancements of thousand of percents (e.g. McMurdo, South Pole). These initially narrow particle beams widened with time resulting in big enhancements recorded by all other high latitude NMs. Anisotropy remained in relatively high levels during the first hour of the event. The source of anisotropic flux was located in southern hemisphere. The position of the anisotropy source changed with time, moving to more northern locations. The estimation of the integral flux for particles with energy >100 MeV on the basis of our model is in good agreement with the satellite observations. Moreover, it ranks this event among the largest proton enhancements ever recorded. Moreover, peculiarities and differences between the intensities of secondary solar particles occurring between different neutron monitor stations were found to be related to their different asymptotic directions of viewing.
The results of modeling applied to the GLE of 20 January 2005 were satisfactory enough. Calculated values for the integral proton flux of particles with energy >100 MeV were found to be in very good agreement with the satellite observations. This implies that the proton fluxes obtained from our model using ground level NM data are very much consistent with the real fluxes recorded by space-instruments. This result can be utilized in means of the space weather monitoring and/or prognosis. Application of this model to as many GLEs as possible is of great importance in order to reveal the characteristics of the solar proton flux distribution and magnitude in the vicinity of the Earth in as many cases as possible. For this reason historical GLE datasets, collected from the worldwide network of neutron monitors, should be modeled using upgraded techniques based on effective algorithms. Incorporation of data from satellite particle detectors can also provide important information on particles of the lower rigidity range as well as on the X-Ray flux. Therefore, an extended integrated analysis combining all the above mentioned potentials may drive our current knowledge to a point that the forecasting of solar extreme phenomena will be attainable.