H. Mavromichalaki

Modeling ground level enhancements: Event of 20 January 2005

[1] 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 this work a ground level enhancement model for getting the broadest possible picture, as well as for understanding the physics of solar cosmic ray particles under extreme solar conditions, is proposed. Neutron monitors responses from 41 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. The parameters of the primary solar particles outside the magnetosphere and their dynamics, as well as the characteristics of solar cosmic rays during this event are obtained and discussed.

A catalogue of high-speed solar-wind streams: Further evidence of their relationship to Ap-index

A reference catalogue of 430 well-defined high-speed plasma streams detected in solar-wind observations from 1972 to 1984 is presented. We have given the main characteristics of the streams as the beginning time and the duration of the stream, the interplanetary magnetic field polarity, etc. We have also separated them with respect to their origin into two categories: the corotating and the flare-generated streams. As a first application of this useful catalogue, a correlative study between the maximum speed of streams and the Ap-index of geomagnetic activity is carried out.

Positive and negative ionospheric disturbances at middle latitudes during geomagnetic storms

The morphology of middle latitude ionospheric disturbances in response to geomagnetic storms has been investigated to determine the phenomenological differences between positive and negative ionospheric storm effects, using foF2 observations from azimuthal chain of stations. To better organize the disturbance signatures, two ionospheric indices were introduced to describe the maximum positive (Dfu- index) and negative (Dfl-index) deviation observed during an ionospheric storm. A systematic appearance of nighttime positive effects was determined with a 24-hour recurrence. The thermospheric-ionospheric view associated with positive and negative storm effects proposed by Prolss (1993) was extended to encompass our observations. This test clearly demonstrates that such a model can capture most of the basic aspects of ionospheric storms, nevertheless the prominent feature of large nighttime enhancements in the ionization density have yet to be explained.

Time-lag of cosmic-ray intensity

The best correlation coefficient between the monthly cosmic-ray intensity of the Inuvik Station and various kinds of solar, interplanetary, and geophysical parameters has been found. It is calculated for different time-lags of cosmic-ray intensity with respect to these parameters. The maximum of these coefficients lead us to a useful empirical model for the 11-year cosmic-ray modulation.

Monitoring and forecasting of great solar proton events using the neutron monitor network in real time

Obtaining online information on the onset of great solar energetic particle (SEP) events from real-time data of the neutron monitor network (NMN) is considered and the corresponding algorithm and program are proposed. Determination of the particle energy spectrum outside the atmosphere at different moments of the flare is considered on the basis of coupling functions method. The spectra defined in diffusion and kinetic approaches are compared. Using this information, the time of the SEP ejection into solar wind, the energy spectrum of a SEP event in the source inside the solar corona, and the SEP diffusion coefficient in the interplanetary space during the flare can be estimated. In this work, the significant possibility of the expected SEP fluxes and the energy spectrum forecasting on the early part of the increasing SEP intensity (about 20-30 min after the onset) is considered. Available satellite data in real-time scale combined with real time-data from neutron monitors (NM) are used for extrapolation of this forecast to the region of very small energy particles. The method is checked on the SEP event of September 1989. It is important to note that the accuracy of the developed method sufficiently increases with the increasing dangerous level of the SEP event. The method is not CPU damaging and can run in real time, providing inexpensive means of SEP prediction.

Fast Plasma Streams Recorded Near the Earth During 1985–1996

A reference catalogue of 373 well-defined high-speed plasma streams identified in the solar wind measurements from 1985 to 1996 is reported. The data base for this study is the interplanetary plasma/magnetic field data compilation made available by the NSSDC/WDC-A for rockets and satellites (NASA/GSFC-Greenbelt). The main characteristics of those streams, such as the beginning time, the duration, the origin of them (corotating or flare-generated), the interplanetary magnetic field polarity (Stanford magnetic field), are given in the catalogue.The long-term variation in the occurrence rate of high-speed streams shows interesting differences between even and odd solar cycles when catalogues for solar cycles 20, 21, and 22 are considered together. Hence, this catalogue, extended now over three solar cycles, should be useful for studies connected with solar-interplanetary or solar-terrestrial phenomena, and to clarify solar activity in time.

Asymmetric variations of the coronal green line intensity

The analysis of the daily measurements of the coronal green line intensity, which have been extensively tested for homogeneity and freedom of trends observed at the Pic-du-Midi observatory during the period 1944–1974, has revealed some characteristic asymmetric variations. A north-south asymmetry of the green line intensity is the main feature of the period 1949–1971 while a south-north one is obvious within 1972–1974 and the minor statistical significance span 1944–1948. On the other hand a significant W-E asymmetry has been confirmed in the whole period 1944–1974. It is noteworthy that the period 1949–1971, where the N-S asymmetry takes place consists a 22-yr solar cycle which starts from the epoch of the solar magnetic field inversion of the solar cycle No. 18 and terminates in the relevant epoch of the cycle No. 20.The combination of N-S and S-N asymmetry with a W-E one makes the NW solar-quarter to appear as the most active of all in the 22-yr cycle 1949–1971, while in the periods 1944–1948 and 1972–1974 the SW quarter is the most active. Finally, from the polar distribution of the green line intensity has been derived that the maximum values of the asymmetries occur in heliocentric sectors ± 10°–20° far from the solar equator on both sides of the central meridian.Physical mechanisms which could contribute to the creation of both N-S and E-W asymmetries of the solar activity and the green line intensity as an accompanied event, like different starting time of an 11-yr solar cycle in the two solar hemispheres, the motion of the Sun towards the Apex, and short-lived ‘active’ solar longitudes formed by temporal clustering of solar active centers, have been discussed.

Short-term variations of cosmic-ray intensity and flare related data in 1981–1983

Abstract A statistical analysis of the cosmic-ray intensity (CR) daily means, registered at three Neutron Monitor stations with different cut-off rigidities (Deep River, Climax and Alma-Ata), as well as, of the solar hard X-ray flares fluence recorded by Venera-13, -14 space-probes, has been performed for the time interval 1981–1983. Various methods of time series spectrum analysis, such as Fast Fourier Analysis (FFT) and Maximum Entropy (MESA), accompanied by appropriate statistical tests, have been employed to detect periodicities, while the method of Successive Approximations (SA) is used independently in order to define the amplitude and the phase of each fluctuation. New short-term periodicities of 100, 70, 50 and 32 days, in addition to the known ones of 152, 27 and 14 days, appeared in cosmic ray data. During this particular time interval, similar spectral behaviour has been reported in the solar hard X-ray flares data. The influence of the solar hard X-ray flares variability in the energy range 50–500 keV, expressed by their fluence values, upon the cosmic-ray modulation, is discussed.

Frequency distributions of solar proton events

Abstract A study has been coordinated of 147 solar proton events (SPEs) with proton energies >10 MeV and peak intensities >10 protons cm −2 s −1 sr −1 “particle flux units (pfu)” at the Earths orbit that were measured between 1976 and 1999. This study has been done in association with other related activities such as sunspot numbers, solar flare index, H-alpha solar flares, magnetic field of the Sun, high-speed solar wind streams, and galactic cosmic-rays. The time frequency and size distributions of the peak intensities of the SPEs have been obtained over the entire period and over the ascending–descending phases of each solar activity cycle. For the threshold intensity of >10 pfu the differential size distributions have a power-law form with a slope of −1.3±0.2 with no evidence of any change with time, which is in agreement with the results of other authors in different time intervals. An updated catalogue of the solar proton events with energy >10 MeV and peak flux >10 pfu is presented. It is based on the Moscow University catalogue (Catalogue of Solar Proton Events 1987–1996, Moscow University, Moscow) for the period 1987–1996. The events are separated into two categories (ordinary and anomalous) with respect to their sources at the Sun, their peak intensity, and their ground level enhancements. The relation between high-energy H-alpha flares and proton events associated with neutron monitor enhancements is discussed in order to be useful for the determination of acceleration processes at the flare site and in very rare interplanetary magnetic conditions.

APPLICATION OF DIFFUSION - CONVECTION MODEL TO DIURNAL ANISOTROPY DATA

The diurnal anisotropy of cosmic-ray intensity observed over the period 1970–1977 has been analysed using neutron-monitor data of the Athens and Deep River stations. Our results indicate that the time of the maximum of diurnal variation shows a remarkable systematic shift towards earlier hours than normally beginning in 1971. This phase shift continued until 1976, the solar activity minimum, except for a sudden shift to a later hour for one year, in 1974, the secondary maximum of solar activity.This behavior of the diurnal time of maximum has been shown to be consistent with the convective- diffusive mechanism which relates the solar diurnal anisotropy of cosmic-rays to the dynamics of the solar wind and of the interplanetary magnetic field. Once again we have confirmed the field-aligned direction of the diffusive vector independently of the interplanetary magnetic field polarity. It is also noteworthy that the diurnal phase may follow in time the variations of the size of the polar coronal holes. All these are in agreement with the drift motions of cosmic-ray particles in the interplanetarty magnetic field during this time period.