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Search Results: 1 - 10 of 149796 matches for " H. Mavromichalaki "
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Study of the longitudinal expansion velocity of the substorm current wedge
A. Belehaki,I. Tsagouri,H. Mavromichalaki
Annales Geophysicae (ANGEO) , 2003,
Abstract: In this work we examine simultaneous observations from the two geosynchronous satellites GOES-5 and GOES-6 located at 282°E and 265°E respectively, and from middle and low latitude ground observatories located within 250°E and 294°E geographic longitude, during isolated substorms of moderate activity. The spatial distribution of our observation points allows us to make a detailed study of the azimuthal expansion of the substorm current wedge. The data analysis shows evidence that the substorm initiation and development mechanism include the cross-tail current diversion/ disruption, the substorm current wedge formation and the azimuthal expansion of the inner plasma sheet. The triggering mechanism is initially confined in a longitudinally narrow sector, estimated to be less than 15° and located very close to local midnight to the east or to the west. The current disruption region expands both eastward and westward in the magnetotail, so that the location of major field-aligned currents flowing into the ionosphere shifts successively eastward, and the location of the currents flowing out of the ionosphere shifts successively westward. Evidence was found that the perturbation travels toward the west with velocities greater than those expanding the wedge eastward. The drastic decrease of the velocity with the azimuthal distance from the location of the disturbance initiation, i.e., the onset sector, indicates that the energy release is a very localized phenomenon. Finally, the transient D perturbation observed by the geosynchronous satellites suggests that the field-aligned currents forming the wedge have a longitudunally limited extent. Key words. Magnetospheric physics (current systems; magnetosphere-ionosphere interactions)
Mapping of the cosmic ray events related to the solar activity for the period 2003-2005
A. Papaioannou,P. Makrantoni,H. Mavromichalaki
Physics , 2010,
Abstract: The relationship between cosmic ray intensity decreases and solar events is still an open field of space research. In this work a complete study of solar events occurred from January 2003 to December 2005, is considered. This three-years time period characterized by an unexpected activity of the Sun was divided into 27-day intervals starting from BR 2313 (06.01.2003) to 2353 (21.12.2005), generating diagrams of the cosmic ray intensity data recorded at the Athens Neutron Monitor Station. This station is working at an altitude of 260m and cut-off rigidity 8.53GV provided to the Internet high-resolution data in real-time. A mapping of all available solar and interplanetary events, such as solar flares with importance M and X, coronal mass ejections (Halo and Partial) was done. As we are going down from the solar maximum to the declining phase of the 23rd solar cycle, a statistical overview of the corresponding relationship among these phenomena, the significant percentage of the connection of Halo CMEs and solar flares and the respective connection to Forbush decreases on yearly and monthly basis are discussed. The close association, as well as a probable quantitative analysis, between solar events is being denoted. The role of extreme solar events occurred in October / November 2003 and January 2005 is also discussed. Obtained results may be useful for predictions of transient solar events and space weather forecasting.
Solar cosmic rays during the extremely high ground level enhancement on 23 February 1956
A. Belov, E. Eroshenko, H. Mavromichalaki, C. Plainaki,V. Yanke
Annales Geophysicae (ANGEO) , 2005,
Abstract: The 23 February 1956 ground level enhancement of the solar cosmic ray intensity (GLE05) is the most famous among the proton events observed since 1942. But we do not have a great deal of information on this event due to the absence of solar wind and interplanetary magnetic field measurements at that time. Furthermore, there were no X-Ray or gamma observations and the information on the associated flare is limited. Cosmic ray data was obtained exclusively by ground level detectors of small size and in some cases of a non-standard design. In the present work all available data from neutron monitors operating in 1956 were analyzed, in order to develop a model of the solar cosmic ray behavior during the event. The time-dependent characteristics of the cosmic ray energy spectrum, cosmic ray anisotropy, and differential and integral fluxes have been evaluated utilizing different isotropic and anisotropic models. It is shown that the most outstanding features of this proton enhancement were a narrow and extremely intense beam of ultra-relativistic particles arriving at Earth just after the onset and the unusually high maximum solar particle energy. However, the contribution of this beam to the overall solar particle density and fluency was not significant because of its very short duration and small width. Our estimate of the integral flux for particles with energies over 100 MeV places this event above all subsequent. Perhaps the number of accelerated low energy particles was closer to a record value, but these particles passed mainly to the west of Earth. Many features of this GLE are apparently explained by the peculiarity of the particle interplanetary propagation from a remote (near the limb) source. The quality of the available neutron monitor data does not allow us to be certain of some details; these may be cleared up by the incorporation into the analysis of data from muonic telescopes and ionization chambers operating at that time. Keywords. Interplanatary physics (Cosmic rays; Energetic particles) – Solar physics, astrophysics and astronomy (Flares and mass injections) Full Article (PDF, 1072 KB) Citation: Belov, A., Eroshenko, E., Mavromichalaki, H., Plainaki, C., and Yanke, V.: Solar cosmic rays during the extremely high ground level enhancement on 23 February 1956, Ann. Geophys., 23, 2281-2291, doi:10.5194/angeo-23-2281-2005, 2005. Bibtex EndNote Reference Manager XML
The burst of solar and geomagnetic activity in August–September 2005
A. Papaioannou, H. Mavromichalaki, E. Eroshenko, A. Belov,V. Oleneva
Annales Geophysicae (ANGEO) , 2009,
Abstract: 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.
Statistical analysis of solar proton events
V. Kurt, A. Belov, H. Mavromichalaki,M. Gerontidou
Annales Geophysicae (ANGEO) , 2004,
Abstract: 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.
Energy dissipation during a small substorm
A. Belehaki,H. Mavromichalaki,D. V. Sarafopoulos,E. T. Sarris
Annales Geophysicae (ANGEO) , 2003,
Abstract: The relative importance of the two most likely modes of input energy dissipation during the substorm of 8 May 1986, with an onset at 12:15 UT (CDAW 9E event), is examined here. The combination of data from the interplanetary medium, the magnetotail and the ground allowed us, first of all, to establish the sequence of phenomena which compose this substorm. In order to calculate the magnetospheric energetics we have improved the Akasofu model, by adding two more terms for the total magnetospheric output energy. The first one represents the energy consumed for the substorm current wedge transformation, supplied by the asymmetric ring current. This was found to be 39% of the solar wind energy entering the magnetosphere from the start of the growth phase up to the end of the expansion phase. The second term represents the energy stored in the tail or returned to the solar wind. Our results suggest that the substorm leaves the magnetosphere in a lower energy state, since, according to our calculations, 23% of the energy that entered the magnetosphere during the whole disturbance was returned back to the solar wind. Finally, it is interesting to note that during the growth phase the driven system grow considerably, consuming 36% of the solar wind energy which entered the magnetosphere during this early phase of the substorm.
Low- and high-frequency spectral behavior of cosmic-ray intensity for the period 1953–1996
H. Mavromichalaki,P. Preka-Papadema,B. Petropoulos,I. Tsagouri
Annales Geophysicae (ANGEO) , 2003,
Abstract: A study of the cosmic-ray intensity power spectrum using the Climax Neutron Monitor data in the frequency range from 10-9 Hz to 10-7 Hz (which corresponds to periodicities from 11 years to a few months) during the period 1953–1996, was carried out by means of the successive approximations method of analysis and was compared against the power spectrum and the maximum entropy methods. The contributions of the time evolution of several peaks to the global one were obtained. Except for the well-known 11-year and the 1-year variations, peaks at 7.7, 5.5, 2 and 1.7 years are found. Several peaks with periods less than 10 months have appeared in our analysis, while the occurrence of 5.1 months is obtained in all the examined solar cycles with a strong signature in cycle 21. Transitions of these quasi-periodicities are seen in power spectra plots. Some of them can be attributed to the modulation of the cosmic ray intensity by solar activity. Others are sporadic and have been previously attributed to the interplanetary magnetic field. The results obtained support once again the argument regarding the difference in the solar activity between odd and even solar cycles. Key words. Interplanetary physics (Cosmic rays, Interplanetary magnetic fields)
Solar activity and the associated ground level enhancements of solar cosmic rays during solar cycle 23
M. Andriopoulou, H. Mavromichalaki, P. Preka-Papadema, C. Plainaki, A. Belov,E. Eroshenko
Astrophysics and Space Sciences Transactions (ASTRA) , 2011, DOI: 10.5194/astra-7-439-2011
Abstract: The solar cycle 23 seems to be of great interest for the researchers due to many peculiarities. A study of the parameters 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 ejections 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 monitoring and prediction.
Solar particle event analysis using the standard radiation environment monitors: applying the neutron monitor's experience
A. Papaioannou, H. Mavromichalaki, M. Gerontidou, G. Souvatzoglou, P. Nieminen,A. Glover
Astrophysics and Space Sciences Transactions (ASTRA) , 2011, DOI: 10.5194/astra-7-1-2011
Abstract: The Standard Radiation Environment Monitor (SREM) is a particle detector developed by the European Space Agency for satellite applications with the main purpose to provide radiation hazard alarms to the host spacecraft. 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 outcome 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.
The effect of cosmic ray intensity variations and geomagnetic disturbances on the physiological state of aviators
M. Papailiou, H. Mavromichalaki, K. Kudela, J. Stetiarova, S. Dimitrova,E. Giannaropoulou
Astrophysics and Space Sciences Transactions (ASTRA) , 2011, DOI: 10.5194/astra-7-373-2011
Abstract: Over the last few years various researches have reached the conclusion that cosmic ray variations and geomagnetic disturbances are related to the condition of the human physiological state. In this study medical data regarding 4018 Slovak aviators were analyzed in relation to daily variations of cosmic ray and geomagnetic activity. Specifically daily data concerning mean values of heart rate which were registered during the medical examinations of the Slovak aviators, were related to daily variations of cosmic ray intensity, as measured by the Neutron Monitor Station on Lomnicky Stit (http://neutronmonitor.ta3.sk/realtime.php3) and the high resolution neutron monitor database (http://www.nmdb.eu) and daily variations of Dst and Ap geomagnetic indices. All subjects were men in good health of age 18–60 yrs. This particular study refers to the time period from 1 January 1994 till 31 December 2002. Statistical methods were applied to establish a statistical significance of the effect of geomagnetic activity levels and cosmic ray intensity variations on the aforementioned physiological parameters for the whole group. The Pearson r-coefficients were calculated and the Analysis of Variance (ANOVA) method was applied to establish the statistical significance levels (p-values) of the effect of geomagnetic activity and cosmic ray intensity variations on heart rate up to three days before and three days after the respective events. Results show that there is an underlying effect of geomagnetic activity and cosmic ray intensity variations on the cardiovascular functionality.
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