Relativistic proton production at the Sun in the 20 January 2005 solar event

Based on the concept of multiple acceleration of solar energetic particles (SEP) we analyzed the super-event of 20 January 2005 by the data of ground level, balloon and spacecraft observations. The main characteristics of relativistic solar protons (energy spectra, anisotropy directions and pitch-angle distributions) are derived and their dynamics during the event is studied. It is shown that the flux of relativistic solar protons may consist of two distinct components, the so-called prompt and delayed ones. Within a two-source model of particle generation, one of which is associated with an expanding magnetic loop, we solved the transport equation in energy phase space, including adiabatic losses simultaneously with the stochastic acceleration process, and calculate the expected spectra of the delayed component at the source. The confrontation of experimental spectra with theoretical ones shows that the delayed component may be correctly described by stochastic acceleration, but not the prompt component. The required acceleration efficiencies turned out to be rather high, so that, for this particular event, adiabatic cooling is practically negligible. Our results provide a new support to the existence of two populations of relativistic solar protons in some SEP events.     

A study of the ground level enhancement of 23 February 1956

One of the greatest and most famous increase of solar cosmic rays over the neutron monitor epoch is the ground level enhancement in 1956. All future proton events are inevitable when compared with this one and therefore it is necessary to provide the efficiency of such a comparison derived from the existing data. In this paper, we return to the analysis of ground level observations on 23 February 1956 in order to model more precisely the solar cosmic ray behaviour. The extremely high magnitude of this effect allowed various spectral characteristics of solar cosmic rays, their anisotropy, differential and integral proton fluxes, and angular distribution of the source of solar particle anisotropy to be obtained with sufficient accuracy on the basis of available data from 13 neutron monitors. The most outstanding feature of this event was a narrow and extremely intensive beam of ultra relativistic particles arriving at Earth at the beginning of the event. This unique beam was not long and its width did not exceed 30–40°, thus, its contribution to solar particle density was not significant. Many features of this GLE are apparently explained by the peculiarity of particle interplanetary propagation from a remote (limb or behind of limb) source.     

Re-analysis of the cosmic ray ground level enhancement of 4 May 1960.

The relativistic solar particle event of 4 May 1960, resulting in a cosmic ray ground level enhancement, occurred well before modern analysis techniques were available. We have located surviving data from 23 neutron monitors and have used these to estimate the spectrum, mean arrival direction and particle pitch angle distribution as the event progressed. We find that the apparent particle arrival direction was at equatorial latitudes, over northern South America, in contrast to contemporary analyses that proposed it to be over North America. Our modified power law spectra are broadly consistent with earlier results. Data from stations above sea level need to be corrected for altitude using a two-attenuation length technique. The standard method involves comparison of data from two relatively close stations at significantly different altitude. We have shown that this method may be unreliable in cases, such as this, of quite sharp anisotropy.     

Modeling the solar cosmic ray event of 13 December 2006 using ground level neutron monitor data

In order to understand the physics under extreme solar conditions such as those producing ground level enhancements of solar cosmic rays, it is important to use accurate and reliable models. The NM-BANGLE Model is a new cosmic ray model which couples primary solar cosmic rays at the top of the Earth’s atmosphere with the secondary ones detected at ground level by neutron monitors during GLEs. This model calculates the evolution of several GLE parameters such as the solar cosmic ray spectrum, anisotropy and particle flux distribution, revealing crucial information on the energetic particle propagation and distribution. The total output of the NM-BANGLE Model is a multi-dimensional GLE picture that gives an important contribution to revealing the characteristics of solar energetic particle events recorded at ground level. In this work, the results of the NM-BANGLE Model application to the recent GLE of 13 December 2006 are presented and discussed. Moreover, a comparison with the extreme event of 20 January 2005 (GLE69) has been realized.     

Neutron monitor asymptotic directions of viewing during the event of 13 December 2006

During the recent ground level enhancement of 13 December 2006, also known as GLE70, solar cosmic ray particles of energy bigger that ∼500 MeV/nucleon propagated inside the Earth’s magnetosphere and finally accessed low-altitude satellites and ground level neutron monitors. The magnitude and the characteristics of this event registered at different neutron monitor stations of the worldwide network can be interpreted adequately on the basis of an estimation of the solar particle trajectories in the near Earth interplanetary space. In this work, an extended representation of the Earth’s magnetic field was realized applying the Tsyganenko 1989 model. Using a numerical back-tracing technique the solar proton trajectories inside the magnetospheric field of the Earth were calculated for a variety of particles, initializing their travel at different locations, covering a wide range of energies. In this way, the asymptotic directions of viewing were calculated for a significant number of neutron monitor stations, providing crucial information on the Earth’s “magnetospheric optics” for primary solar cosmic rays, on the top of the atmosphere, during the big solar event of December 2006. The neutron monitor network has been treated, therefore, 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.     

Relationships between neutron fluxes and rain flows

Registration of secondary cosmic ray neutrons is a convenient tool for investigation of primary cosmic ray variations and meteorological effects as well. At present a large network of neutron monitors exists, providing the studies of cosmic ray variations related to the interplanetary conditions and geomagnetic activity. At the same time cosmic ray variations may be caused by some atmospheric processes. In this connection, using the data from standard and lead-free neutron monitors, and gamma and muon detectors, we studied relations between rain flows and neutron, gamma and ionization component behavior. To explain observable results the calculations of neutron and gamma absorption and albedo neutron spectra have been performed on the basis of universal software package FLUKA-2006. In this study we used hourly data on the neutron flux, corrected for barometric pressure and data from local meteorological stations. It was shown that secondary neutron radiation, recorded by lead-free NM, and gamma radiation as well are strongly effected by meteorological factors. The neutron component behavior depends on the moisture content in the soil, and above its surface.     

Electron acceleration and relativistic nucleon production in the 2003 October 28 solar event

A flare east of central meridian on 2003 October 28 produced a relativistic particle event at Earth, although it was located far from the footpoint of the nominal interplanetary Parker spiral. From a study of the onset times of the event at different neutron monitors we conclude that the earliest arriving solar particles may be neutrons. The first relativistic protons (prompt component) arrived a few minutes later. Metre wave imaging suggests that electrons are not only accelerated in the flaring active regions, but at several places far away, including the western hemisphere. Simultaneous Type III emission and associated Langmuir waves demonstrates that these regions are connected to the Earth. We suggest that, like in a few other nominally poorly connected particle events, promptly escaping relativistic protons were not accelerated in the flaring active region, but at remote places in relationship with the global magnetic restructuring in the course of a huge coronal mass ejection.     

Fluctuations of cosmic rays and IMF in the vicinity of interplanetary shocks

Fluctuations of cosmic rays and interplanetary magnetic field upstream of interplanetary shocks are studied using data of ground-based polar neutron monitors as well as measurements of energetic particles and solar wind plasma parameters aboard the ACE spacecraft. It is shown that coherent cosmic ray fluctuations in the energy range from 10 keV to 1 GeV are often observed at the Earth’s orbit before the arrival of interplanetary shocks. This corresponds to an increase of solar wind turbulence level by more than the order of magnitude upstream of the shock. We suggest a scenario where the cosmic ray fluctuation spectrum is modulated by fast magnetosonic waves generated by flux of low-energy cosmic rays which are reflected and/or accelerated by an interplanetary shock.     

The solar cosmic ray ground-level enhancements on 20 January 2005 and 13 December 2006

Close to the current solar activity minimum, two large solar cosmic ray ground-level enhancements (GLE) were recorded by the worldwide network of neutron monitors (NM). The enormous GLE on 20 January 2005 is the largest increase observed since the famous GLE in 1956, and the solar cosmic-ray event recorded on 13 December 2006 is among the largest in solar cycle 23. From the recordings of the NMs during the two GLEs, we determined the characteristics of the solar particle flux near Earth.     

Evaluation of solar proton spectra using balloon cosmic ray observations and Monte Carlo simulation results

We have developed a method to evaluate the spectrum of solar energetic protons at the top of the Earth’s atmosphere from the measurements of our balloon cosmic ray experiment. By using the Monte Carlo PLANETOCOSMICS code based on Geant4 we compute the interaction of solar protons [10 MeV–10 GeV] with the Earth’s atmosphere. We obtain the angular and energy distributions of secondary particles (p, e⁻, e⁺, photons, muons) at different atmospheric levels as a function of primary proton spectra. By comparing the calculated depth dependence of the particle flux with the data obtained by our balloon experiment we can deduce the parameters of the solar proton spectrum that best fit the observations. In this paper we discuss our solar proton spectrum estimation method, and present results of its application to selected solar proton events from 2001 to 2005.     

Unexpected burst of solar activity recorded by neutron monitors during October–November 2003

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 of which the flare X28/3B on November 4, 2003 was the mostly 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 these events. In one-week period three ground level enhancements (GLEs) of solar cosmic rays were recorded by neutron monitor network (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 allows the estimation of the particle path length, the onset time of the injection on the Sun and some other proton flux characteristics.     

Calculated cosmic rays dynamics before the geomagnetic storms onset caused by the interplanetary shocks

Dynamics of the galactic cosmic ray intensity caused by their interactions with a shock front in the September 9, 1992 event has been determined. Corresponding variations of the cosmic ray intensity have been calculated for different stations of the world network of neutron monitors and muon telescopes of stations Nagoya and Sakashita. Comparison calculated results with observational data shows, in general, satisfactory consensus both on amplitude and in time. The developed method can be used for investigation dynamics of the solar wind disturbances precursors in the cosmic rays.     

Real-time GLE alert in the ANMODAP Center for December 13, 2006

Within the last years, a real-time system to monitor high energy cosmic rays for space weather use has been operated at Athens cosmic ray station. Neutron monitors and satellite high resolution data in real time are used, making it possible to observe cosmic rays in dual energy range observations. In large solar energetic particle (SEP) events, ground level enhancement (GLE) can provide the earliest alert for the onset of the SEP event. This system watches for count rate increases recorded in real time by 23 neutron monitors, which triggers an alarm if a ground level enhancement (GLE) of cosmic ray intensity is detected.     

The onset of sunspot cycle 24 and galactic cosmic ray modulation

The annual mean sunspot number (SSN) has a minimum value in 2008, while the monthly mean SSN has a value of zero in August 2009. The galactic cosmic ray modulation for cycle 24 began at earth orbit in January 2010. We study the onset characteristics of the new modulation cycle using data from the global network of neutron monitors. They respond to time variations in different segments of the galactic cosmic ray rigidity spectrum. The corresponding temporal variations in the interplanetary magnetic field intensity (B) and solar wind velocity (V) as well as the tilt angle of the heliospheric current sheet are also studied. There is a lag of 3 months between a large, sharp increase of the tilt angle of the heliospheric current sheet and the onset of modulation. Some neutron monitors are undergoing long-term drifts of unknown origin.     

Solar proton events recorded in the stratosphere during cosmic ray balloon observations in 1957–2008

Long-term balloon observations have been performed by the Lebedev Physical Institute since 1957 up to the present time. The observations are taken several times a week at the polar and mid latitudes and allow us to study dynamics of galactic and solar cosmic ray as well as secondary particle fluxes in the atmosphere and in the near-Earth space. Solar energetic particles (120) – mostly protons – (SEP) events with >100 MeV proton intensity above 1 cm⁻² s⁻¹ s⁻¹ were recorded during 1958–2006. Before the advent of the SEP monitoring on spacecraft these results constituted the only homogeneous series of >100 MeV SEP events. The SEP intensities and energy spectra inferred from the Lebedev Physical Institute observations are consistent with the results taken in the adjacent energy intervals by the spacecraft and neutron monitors. Joint consideration of the SEP events series recorded by balloons and by neutron monitors during solar cycles 20–23 makes it possible to restore the probable number of events in solar cycle 19, which was not properly covered by observations. Some correlation was found between duration of SEP event production in a solar cycle and sunspot cycle characteristics.     

“风云一号（B）”卫星探测到的太阳质子事件

我国“风云一号(B)”气象卫星于1990年9月3日发射入轨,该星载有粒子成分监测器,用来探测空间粒子辐射环境,其中包括测量太阳耀斑时产生的太阳质子事件及其重粒子丰度;银河宇宙线异常成分与强度;内辐射带磁异常区的粒子通量及重粒子成分,“风云一号(B)”卫星运行半年来,我们已获取了上述有关的粒子辐射资料,在卫星上获得这些资料在我国尚属首次,本文主要分析观测到的太阳质子事件。     

FEM code simulation of the magnetospheric proton fluxes

We report a study of the numeric solution to the diffusive transport equation for energetic protons magnetically trapped in the Earth's equatorial magnetosphere. The analysis takes into account the pertinent physical processes in this region, including deceleration of protons by Coulomb collisional interactions with free and bound electrons, the charge exchange process, cosmic ray albedo neutron decay source, and electric and magnetic radial diffusion. These results were obtained using the Finite Element Method with magnetic moment and geomagnetic L-shell as free variables. Steady state boundary conditions were imposed at L=1 as zero distribution function and at L=7 with proton distribution function extracted from ATS 6 satellite observations. The FEM-code yields unidirectional proton flux in the energy range of 0.1–1000 MeV at the equatorial top of the geomagnetic lines, and the results are found to be in satisfactorily agreement with the empirical NASA AP-8 model proton flux within the energy range of 0.5–100 MeV. Below 500 keV, the empirical AP-8 model proton fluxes are several orders of magnitude greater than those computed with the FEM-code at L<3. This discrepancy is difficult to explain by uncertainties of boundary spectrum parameters or transport coefficients.     

The scaler mode in the Pierre Auger Observatory to study heliospheric modulation of cosmic rays

The impact of the solar activity on the heliosphere has a strong influence on the modulation of the flux of low energy galactic cosmic rays arriving at Earth. Different instruments, such as neutron monitors or muon detectors, have been recording the variability of the cosmic ray flux at ground level for several decades. Although the Pierre Auger Observatory was designed to observe cosmic rays at the highest energies, it also records the count rates of low energy secondary particles (the scaler mode) for the self-calibration of its surface detector array. From observations using the scaler mode at the Pierre Auger Observatory, modulation of galactic cosmic rays due to solar transient activity has been observed (e.g., Forbush decreases). Due to the high total count rate coming from the combined area of its detectors, the Pierre Auger Observatory (its detectors have a total area greater than 16,000 m²) detects a flux of secondary particles of the order of ∼10⁸ counts per minute. Time variations of the cosmic ray flux related to the activity of the heliosphere can be determined with high accuracy. In this paper we briefly describe the scaler mode and analyze a Forbush decrease together with the interplanetary coronal mass ejection that originated it. The Auger scaler data are now publicly available.     

The local time dependence of the anisotropic solar cosmic ray flux.

The distribution of the solar cosmic radiation flux over the earth is not uniform, but the result of complex phenomena involving the interplanetary magnetic field, the geomagnetic field and latitude and longitude of locations on the earth. The latitude effect relates to the geomagnetic shield; the longitude effect relates to local time. For anisotropic solar cosmic ray events the maximum particle flux is always along the interplanetary magnetic field direction, sometimes called the Archimedean spiral path from the sun to the earth. During anisotropic solar cosmic ray event, the locations on the earth viewing "sunward" into the interplanetary magnetic field direction will observe the largest flux (when adjustments are made for the magnetic latitude effect). To relate this phenomena to aircraft routes, for anisotropic solar cosmic ray events that occur during "normal quiescent" conditions, the maximum solar cosmic ray flux (and corresponding solar particle radiation dose) will be observed in the dawn quadrant, ideally at about 06 hours local time.     

太阳宇宙线在电离层D层中的电离

本文根据带电粒子对D层大气电离的理论,导出了太阳宇宙线在D层的电子产生率Q(h)的表达式,并计算了不同级别的太阳宇宙线事件、不同能谱参数下,Q(h)在极区随高度的分布。结果表明,不同级别、不同能谱的太阳宇宙线事件在极区产生的电离有显著的差别。同一级别,能谱指数γ越大,在较高的高度上电子产生率越大;能谱指数越小,在较低的高度上电子产生率越大。电子产生率的分布曲线出现明显的双峰,一个峰位于60公里左右,另一个峰位于85公里左右。前一个峰主要由太阳宇宙线中质子产生的,后一个峰主要是z≥2的重粒子成分产生的。本文所得结果明显好于Velinov等人的结果。