Keywords

We applied principal component analysis (PCA) to the study of five ground level enhancements (GLEs) of cosmic ray (CR) events. The nature of the multivariate data involved makes PCA a useful tool for this study. A subroutine program written and implemented in the R software environment generated interesting principal components. Analysis of the results shows that the method can distinguish between neutron monitors (NMs) that observed Forbush decreases from those that observed GLEs at the same time. The PCA equally assigned NMs with identical signal counts with the same correlation factor (r) and those with close r values equally have a close resemblance in their CR counts. The results further indicate that while NMs that have the same time of peak may not have the same r, most NMs that had the same r also had the same time of peak. Analyzing the second principal components yielded information on the differences between NMs having opposite but the same or close values of r. NMs that had the same r equally had the tendency of being close in latitude.

We present the results of analyses of the ground level enhancements (GLEs) of cosmic ray (CR) events on 1989 September 29; 2001 April 15 and 2005 January 20. This involves examination of hourly raw CR counts of an array of neutron monitors (NMs) spread across different geographical latitudes and longitudes. Using awk script and computer codes implemented in R software, the pressure corrected raw data plots of the NMs were grouped into low-, mid- and high-latitudes. The results show both similarities and differences in the structural patterns of the GLE signals. In an attempt to explain why the CR count during the decay phase of GLEs is always higher than the count before peak, we interpreted all counts prior to the peak as coming from direct solar neutrons and those in the decay phase including the peak as coming from secondary CR neutrons generated by the interactions of primary CRs with the atoms and molecules in the atmosphere. We identified NMs that detected these primary neutrons and found that they are close in longitude. Previous authors seemingly identified these two species as impulsive and gradual events. Although there are a number of unexplained manifestations of GLE signals, some of the results suggest that geomagnetic rigidity effectively determines the intensity of CRs at low- and mid-latitudes. Its impact is apparently insignificant in high-latitude regions. Nevertheless, the results presented should be validated before making any firm statements. Principally, the contributions of the ever-present and intractable CR diurnal anisotropies to GLE signals should be accounted for in future work.

We present numerical modeling of particle acceleration at coronal shocks propagating through a streamer-like magnetic field by solving the Parker transport equation with spatial diffusion both along and across the magnetic field. We show that the location on the shock where the high-energy particle intensity is the largest, depends on the energy of the particles and on time. The acceleration of particles to more than 100 MeV mainly occurs in the shock-streamer interaction region, due to perpendicular shock geometry and the trapping effect of closed magnetic fields. A comparison of the particle spectra to that in a radial magnetic field shows that the intensity at 100 MeV (200 MeV) is enhanced by more than one order (two orders) of magnitude. This indicates that the streamer-like magnetic field can be an important factor in producing large solar energetic particle events. We also show that the energy spectrum integrated over the simulation domain consists of two different power laws. Further analysis suggests that it may be a mixture of two distinct populations accelerated in the streamer and open field regions, where the acceleration rate differs substantially. Our calculations also show that the particle spectra are affected considerably by a number of parameters, such as the streamer tilt angle, particle spatial diffusion coefficient, and shock compression ratio. While the low-energy spectra agree well with standard diffusive shock acceleration theory, the break energy ranges from ∼1 MeV to ∼90 MeV and the high-energy spectra can extend to ∼1 GeV with a slope of ∼2–3.

Observations have shown that type III radio bursts (RBs) are generated by 1–10 keV flare electrons ejected from the exhaust of a magnetic reconnection site in a coronal (loop-top) source region. Surprisingly, it is generally accepted without question that the injection of low-energy electrons occurs significantly earlier than the onset of the type III RBs. Therefore, it is necessary to re-examine the timing of flare electrons. For this, we observed a "normal" event in which the injection of low-energy electrons coincided with the injection of high-energy electrons, and "abnormal" events in which the low-energy electrons seemed to arrive earlier. A high background of low-energy particles lacking any evidence of velocity dispersion characterizes an abnormal event. Due to the existence of a reconnection acceleration that results in similar enhancements at magnetic islands confined by the heliospheric current sheet (HCS), HCS observations are used to establish the empirical criteria for the reconnection acceleration in impulsive electron events. Observations show that 2–8 keV electrons accelerated by magnetic reconnection can change the pitch-angle distribution of background electrons for a time interval of approximately 0.5 hr before or after the time of current-sheet crossing. Therefore, this reconnection acceleration in the solar wind can influence the onset time analysis of electrons by emulating the effect of the earlier arrival of flare electrons. In addition, a technique is developed for estimating the phase velocity of whistler waves in the ion dissipation range, which may significantly affect the pitch-angle scattering analysis of low-energy electrons.

The high-temperature (T > 4 MK) emissions of nonflaring active regions are investigated in the context of the coronal heating problem. We study the role of emerging flux, nonpotential magnetic fields, and sunspots in the heating of active-region loops. Using extreme ultraviolet images from the Atmospheric Imaging Assembly on the Solar Dynamic Observatory (SDO), we construct intensity maps in Fe xviii 94 Å for 48 active regions. We also use the corresponding magnetograms from the Helioseismic and Magnetic Imager on SDO to measure the total magnetic flux. The Fe xviii 94 Å emission intensity of the brightest loops is found to be correlated with the presence of sunspots and emerging or canceling magnetic flux in the photosphere below. We conclude that sunspots and emerging flux play an important role in the process of coronal heating and the production of high-temperature plasmas. We suggest that energy may be injected into the corona as a result of the dynamics of magnetic fields associated with sunspots and/or emerging flux. These processes may cause the large magnetic disturbances (δB_⊥ ∼ 10 G) needed to produce strong nanoflare-heating events.

The dearth of observations between 1 and 3 au limits our understanding of energetic particle acceleration processes in interplanetary space. We present first-of-their-kind observations of the energetic particle acceleration in a corotating interaction region (CIR) using data from two vantage points, 1 au (near Earth) and 1.5 au (near Mars). The CIR event of 2015 June was observed by the particle detectors aboard the Advanced Composition Explorer satellite as well as the Solar Energetic Particle (SEP) instrument aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft situated near 1.5 au. We find that a CIR shock can accelerate a significant number of particles even at 1.5 au. During this event the acceleration by the shocks associated with the CIR could cause an enhancement of around two orders of magnitude in the SEP energetic ion fluxes in the ∼500 keV to 2 MeV range when the observations near 1 and 1.5 au are compared. To demonstrate the differences between SEP acceleration in CIR and other impulsive events, we show the energetic ion flux observations during an intense coronal mass ejection period in March 2015, in which case the enhanced SEP fluxes are seen even at 1 au. These observations provide evidence that CIR shock can accelerate particles in the region between Earth and Mars—that is, only within the short heliocentric distance of  0.5 au—in interplanetary space.

The study of the acceleration of particles is an essential element of research in heliospheric science. Here, we discuss the predisposition to the particle acceleration around shocks driven by coronal mass ejections (CMEs) with corrugated wave-like features. We adopt these attributes on shocks formed from disturbances due to the bimodal solar wind, CME deflection, irregular CME expansion, and the ubiquitous fluctuations in the solar corona. In order to understand the role of a wavy shock in particle acceleration, we define three initial smooth shock morphologies each associated with a fast CME. Using polar Gaussian profiles we model these shocks in the low corona. We establish the corrugated appearance on smooth shock by using combinations of wave-like functions that represent the disturbances from the medium and CME piston. For both shock types, smooth and corrugated, we calculate the shock normal angles between the shock normal and the radial upstream coronal magnetic field in order to classify the quasi-parallel and quasi-perpendicular regions. We consider that corrugated shocks are predisposed to different processes of particle acceleration due to irregular distributions of shock normal angles around the shock. We suggest that disturbances due to CME irregular expansion may be a decisive factor in origin of particle acceleration. Finally, we regard that accepting these features on shocks may be the starting point for investigating some questions regarding the sheath and shock, like downstream jets, instabilities, shock thermalization, shock stability, and injection particle processes.

Little is known about the origin of the high-energy and sustained emission from solar long-duration gamma-ray flares (LDGRFs) identified with the Compton Gamma Ray Observatory, the Solar Maximum Mission, and now Fermi. Though the Fermi Large Area Telescope (LAT) has identified dozens of flares with LDGRF signatures, the nature of this phenomenon has been a challenge to explain due to both extreme energies and long durations. The highest-energy emission has generally been attributed to pion production from the interaction of ≳300 MeV protons with the ambient matter. The extended duration suggests that particle acceleration occurs over large volumes extending high in the corona, either from stochastic acceleration within large coronal loops or from back precipitation from coronal mass ejection–driven shocks. It is possible to test these models by making a direct comparison between the properties of the accelerated ion population producing the γ-ray emission derived from the Fermi/LAT observations and the characteristics of solar energetic particles (SEPs) measured by the Payload for Matter-Antimatter Exploration and Light Nuclei Astrophysics spacecraft in the energy range corresponding to the pion-related emission detected with Fermi. For 14 of these events, we compare the two populations—SEPs in space and the interacting particles at the Sun—and discuss the implications in terms of potential sources. Our analysis shows that the two proton numbers are poorly correlated, with their ratio spanning more than 5 orders of magnitude, suggesting that the back precipitation of shock-acceleration particles is unlikely to be the source of the LDGRF emission.

Type III solar radio bursts are generated by streams of energetic electrons accelerated at the Sun during periods of solar activity. The generation occurs in two steps. Initially, electron beams generate electrostatic Langmuir waves and then these waves are transformed into electromagnetic emissions. Recent studies showed that the level of density fluctuations in the solar wind and in the solar corona is so high that it may significantly affect beam–plasma interaction. Here, we show that the presence of intense density fluctuations not only crucially influences the process of beam–plasma interaction, but also changes the mechanism of energy transfer from electrostatic waves into electromagnetic. Reflection of the Langmuir waves from the density inhomogeneities may result in partial transformation of the energy of electrostatic waves into electromagnetic around plasma frequency. We show that the linear wave energy transformation for the level of fluctuations of the order of 1% or higher is efficient enough to produce radio bursts with a brightness temperature of 10¹⁴–10¹⁵ K.

Using three nonstationary solar series, the solar flare index (FS), the sunspots index (SS), and the solar flux (F10.7) index, we apply the Morlet wavelet analysis to determine the most dominant harmonics of solar activity, 1.73, 3.27, 4.9, 10.4, and 11 yr. The periodicities obtained are processed by the fuzzy logic method, which allows us to reproduce the occurrence dates of ground level enhancements (GLE), since 1942–2006, which we use as a training baseline of these spectral techniques to determine the occurrence of solar particle enhancements in solar cycles. Then, the result of fuzzy logic is extended to periods later than the training period so as to cover the end of cycle 24 and the beginning of cycle 25. In addition to the forecastable aspect of this work, the obtained results are of high interest in view of the recent controversy that has arisen in relation to the occurrence of small GLE (namely sub-GLE), during cycle 24.