Large Solar Events Research Articles

Modeling the Transport of Relativistic Solar Protons along a Heliospheric Current Sheet during Historic GLE Events

There are many difficulties associated with forecasting high-energy solar particle events at Earth. One issue is understanding why some large solar eruptive events trigger ground-level enhancement (GLE) events and others do not. In this work we perform 3D test particle simulations of a set of historic GLEs to understand more about what causes these powerful events. Particular focus is given to studying how the heliospheric current sheet (HCS) affects high-energy proton transport through the heliosphere following an event. Analysis of ≥M7.0 flares between 1976 and 2020 shows that active regions located closer to the HCS (<10°) are more likely to be associated with a GLE event. We found that modeled GLE events where the source region was close to the HCS also led to increased heliospheric transport in longitude and higher count rates (when the Earth was located in the drift direction). In a model that does not include perpendicular diffusion associated with turbulence, the HCS is the dominant mechanism affecting heliospheric particle transport for GLE 42 and 69, and varying other parameters (e.g., a narrow, 10°, or wider, 60°, injection width) causes little change. Overall in our model, the HCS is relevant in 71% of our analyzed GLEs, and including it more accurately reproduces observed intensities near Earth. Our simulations enable us to produce model profiles at Earth that can be compared to existing observations by the GOES satellites and neutron monitors, as well as for use in developing future forecasting models.

Using Video Photometry to Measure the 3 MeV Proton Half Brightness Fluence for Tetrakis (Dibenzoylmethide) Europium (III) Triethylammonium

In recent years, luminescent materials have been proposed for use as smart sensors to detect structural damage using triboluminescence, measure incident radiation fluence using radioluminescence, and monitor surface temperature using phosphor thermometry. To sense structural damage using triboluminescence, appropriate materials are embedded in a composite host structure for various applications. When the damage occurs in the host structure, it leads to the fracture of the triboluminescent crystals resulting in light emission. This process provides a real-time warning that structural damage has occurred. The TL emission of the candidate material has to be sufficiently bright, so that the light signal reaching from the point of fracture to the detector, through a fiber optics cable, is strong enough to be detected. The majority of the known triboluminescent materials do not emit light with sufficient intensity to allow detection with compact and inexpensive detectors. Over the past forty years, some materials have been reported with TL of sufficient intensity for the light emission to be easily observed with the naked eye. Out of these triboluminescent materials, only few could satisfy the above criterion. Tetrakis (Dibenzoylmethide) Europium (III) Triethylammonium (EuD4TEA), one of the brightest known TL materials and is a potential candidate for application to this type of hybrid smart sensor in extreme environments. Recent measurements by the authors indicate that the decay time for EuD4TEA is bi-exponential and can be used to measure temperatures from -10 to 80 °C. Ionizing radiation in space poses a significant challenge to future human and robotic exploration missions. Any intelligent sensor material used in space must also be resistant to ionizing radiation. Sources and types of such radiation include galactic and solar electrons and protons, x-rays, ultraviolet light, and magnetically trapped charged particles in radiation belts. While much research has been devoted to characterizing material degradation due to electron exposure, surprisingly little work seems to have been done on proton damage. Research has shown that proton bombardment in the keV to MeV range, like that occurring in space, reduces the intensity of fluorescence. Thus, proton irradiation will likely reduce the ability of a candidate smart material to emit fluorescence.Practical space-based sensors based on luminescence will depend heavily upon research investigating the resistance of these materials to ionizing radiation and the ability to anneal or self-heal from damage caused by such radiation. In 1951, Birks and Black showed experimentally that the fluorescence efficiency of anthracene bombarded by alphas varies with total fluence (N) as (I/I0) = 1/(1 + AN), where I is the luminescence yield, I0 is the initial luminescence yield, and A is a constant. The half brightness fluence (N1/2) is a material figure of merit and is defined as the reciprocal of A. Broser and Kallmann developed a similar relationship to the Birks and Black equation for inorganic phosphors irradiated using alpha particles. From 1990 to the present, we have found that the Birks and Black relation describes the reduction in emission yield for every tested luminescent material except lead phosphate glass due to proton irradiation.These results indicate that radiation produced quenching centers compete with emission centers for absorbed energy. In 2016, the N1/2 for EuD4TEA irradiated with 3 MeV protons was measured to be about 3 x 1010 mm-2. Conversely, ZnS:Mn compounds tested were found to have N1/2 values for 3 MeV protons that are nearly one thousand times larger (~1013 mm-2). According to Tribble, a spacecraft at 1 AU from the Sun will receive a 1 MeV proton fluence of less than 1011 mm-2 from a large solar event. Likewise, 1 MeV proton fluences in the Earth’s radiation belts and the Earth-Moon-Sun Lagrange points will be even less than the 1011 mm-2 value from large solar events. For this reason, EuD4TEA might be a candidate sensor material to monitor large-scale solar events for astronauts in vehicles flying in near earth space. Since their N1/2 values are larger, ZnS:Mn compounds are more suited for use as sensors in higher radiation environments or for very long duration space missions. The purpose of this presentation is to present preliminary results from a 1U CubeSat payload using a EuD4TEA-based sensor to measure ionizing radiation in Earth orbit. This flight experiment is a natural outgrowth of earlier ground-based radiation exposure measurements that will be used to determine if EuD4TEA-based materials can be used as a real-time sensor for applications in space.

Developing a Nowcasting Capability for X‐Class Solar Flares Using VLF Radiowave Propagation Changes.

AbstractA technique for analyzing very low frequency (VLF) radiowave signals is investigated in order to achieve rapid, real‐time detection of large solar flares, through the monitoring of changes in VLF radio signal propagation conditions. The reliability of the use of VLF phase and amplitude perturbations to determine the X‐ray fluxes involved during 10 large solar flare events (&gt;X1) is examined. Linear regression analysis of signals from the NPM transmitter in Hawaii, received at Arrival Heights, Scott Base, Antarctica, over the years 2011–2015 shows that VLF phase perturbations during large solar flares have a 1.5–3 times lower mean square error when modeling the long wavelength X‐ray fluxes than the equivalent short wavelength fluxes. The use of VLF amplitude observations to determine long or short wavelength X‐ray flux levels have a 4–10 times higher mean square error than when using VLF phase. Normalized linear regression analysis identifies VLF phase as the most important parameter in the regression, followed by solar zenith angle at the midpoint of the propagation path, then the initial solar X‐ray flux level (from 5 min before the impact of the solar flare), with F10.7 cm flux from the day beforehand providing the least important contribution. Transmitter phase measurements are more difficult to undertake than amplitude. However, networks of VLF receivers already exist which include the high quality phase capability required for such a nowcasting product. Such narrowband VLF data can be a redundant source of flare monitoring if satellite data is not available.

Particle Acceleration at the Pileup Collision of the Twin Shock

Abstract Ground-level enhancement (GLE) events are often associated with large gradual solar events such as fast coronal mass ejections (CMEs), but not all fast CMEs lead to GLE events. Is there a type of coordinated CME that could produce GLEs with larger intensity and higher energies than those in the normal fast isolated CMEs? Here we propose a twin-shock scenario driven by the twin CME coordinately, in which the posterior shock catches up with the preceding shock and has a pileup collision. In the present study, we chose the first GLE event of the solar cycle 24 occurring on 2012 May 17 as an example to investigate the probable association with the twin-shock scenario. We use a dynamic Monte Carlo method to examine the energy spectrum with relevance to the GLE event. In the twin-shock scenario, the seed energetic particles produced by the normal preceding shock can be injected into the posterior shock for reacceleration efficiently. As a result, we obtain the detailed energy spectrum of the solar energetic particles (SEPs) with different behaviors at the related episodes of the twin-shock evolution. Therefore, we predict that the pileup collision of the twin shock would dominate a concave energy spectral slope in the 2012 May 17 SEP event.

High-resolution observations with ARTEMIS-JLS and the NRH

Aims. We study the characteristics of intermediate drift bursts (fibers) embedded in a large type-IV event. Methods. We used high-sensitivity, low-noise dynamic spectra obtained with the acousto-optic analyzer (SAO) of the ARTEMIS-JLS solar radiospectrograph, in conjunction with high time-resolution images from the Nançay radioheliograph (NRH) and extreme ultraviolet (EUV) images from the Transition Region and Coronal Explorer (TRACE) to study fiber bursts during the large solar event of July 14, 2000. We computed both 2D and 1D images and applied high pass time filtering to the images and the dynamic spectrum in order to enhance the fiber-associated emission. For the study of the background continuum emission we used images averaged over several seconds. Results. Practically all fibers visible in the SAO dynamic spectra are identifiable in the NRH images. Fibers were first detected after the primary energy release in a moving type-IV event, probably associated with the rapid eastward expansion of the flare and the post-flare loop arcade. We found that fibers appeared as a modulation of the continuum intensity with a root mean square value of the order of 10%. Both the fibers and the continuum were strongly circularly polarized in the ordinary mode sense, indicating plasma emission at the fundamental. We detected a number of discrete fiber emission sources along two parallel stripes of ∼300 Mm in length, apparently segments of large-scale loops encompassing both the EUV loops and the CME-associated flux rope. We found cases of multiple fiber emissions appearing at slightly different positions and times; their consecutive appearance can give the impression of apparent motion with supra-luminal velocities. Images of individual fibers were very similar at 432.0 and 327.0 MHz. From the position shift of the sources and the time delays at low and high frequencies, we estimated the exciter speed and the frequency scale length along the loops for a well-observed group of fibers; we obtained consistent values from imaging and spectral data, supporting the whistler origin of the fiber emission. Finally we found that fibers in emission and fibers in absorption are very similar, confirming that they are manifestations of the same wave train.

Multiradionuclide evidence for an extreme solar proton event around 2,610 B.P. (∼660 BC)

Recently, it has been confirmed that extreme solar proton events can lead to significantly increased atmospheric production rates of cosmogenic radionuclides. Evidence of such events is recorded in annually resolved natural archives, such as tree rings [carbon-14 (14C)] and ice cores [beryllium-10 (10Be), chlorine-36 (36Cl)]. Here, we show evidence for an extreme solar event around 2,610 years B.P. (∼660 BC) based on high-resolution 10Be data from two Greenland ice cores. Our conclusions are supported by modeled 14C production rates for the same period. Using existing 36Cl ice core data in conjunction with 10Be, we further show that this solar event was characterized by a very hard energy spectrum. These results indicate that the 2,610-years B.P. event was an order of magnitude stronger than any solar event recorded during the instrumental period and comparable with the solar proton event of AD 774/775, the largest solar event known to date. The results illustrate the importance of multiple ice core radionuclide measurements for the reliable identification of short-term production rate increases and the assessment of their origins.

Adaptability of GPS/BDS broadcast ionospheric models to solar activities

Solar activity induces ionospheric disturbances, which can reduce the performance of the Global Navigation Satellite System (GNSS). Broadcast ionospheric models (BIMs), used to correct ionospheric delay for single-frequency users, the features and performance of which under ionospheric disturbance conditions are still not understood clearly. In this study, we analyzed the adaptability of the Global Positioning System/BeiDou Navigation Satellite System (GPS/BDS) BIMs and ionospheric total electron content (TEC) anomalies over China during 12 X-class and M-class large solar flare events during the solar active period in terms of the TEC, rate of TEC (ROT), and relative accuracy. Our study showed that during the solar flare occurrence the GPS-derived TEC over China may increase up to 5.0 TECUs within minutes, whereas the ROT may exceed 0.5 TECU/min. Relative to the GPS-derived TEC and ROT, both BDS and GPS BIMs cannot response so actively to these solar flares. However, in terms of relative accuracy, the BDS BIM performed with weaker influence from solar flares due to its updating of coefficients with high frequency, though the absolute accuracy of the BDS BIM has to be improved further.

Using Exotic Materials like EuD4TEA to Monitor Damage and Radiation Exposure in Extreme Environments

In recent years, luminescent materials have been proposed for use as smart sensors to measure structural damage and to monitor radiation fluence. To sense structural damage using triboluminescence, appropriate materials are embedded in a composite host structure for various applications, such as crew launch or exploration vehicles. When the damage occurs in the host structure, it leads to the fracture of the triboluminescent crystals resulting in light emission. This process provides a real-time warning that structural damage has occurred. The TL emission of the candidate material has to be sufficiently bright, so that the light signal reaching from the point of fracture to the detector, through a fiber optics cable, is strong enough to be detected. The majority of the known triboluminescent materials do not emit light with sufficient intensity to allow detection with compact and inexpensive detectors. Over the past 40 years, some materials have been reported with TL of sufficient intensity for the light emission to be easily observed with the naked eye. Out of these triboluminescent materials, only few could satisfy the above criterion. Europium tetrakis dibenzoylmethide triethylammonium (EuD4TEA), one of the brightest known TL materials and is a potential candidate for application to this type of hybrid smart sensor in extreme environments. Ionizing radiation in the extreme environment of space poses a significant challenge to future human and robotic exploration missions. To be used in space, any intelligent sensor material must also be resistant to ionizing radiation. Sources and types of such radiation include galactic and solar electrons and protons, x-rays, ultraviolet light, and magnetically trapped charged particles in radiation belts. While much research has been devoted to characterizing material degradation due to electron exposure, surprisingly little work seems to have been done on proton damage. Research has shown that proton bombardment in the keV to MeV range, like that occurring in space, reduces the intensity of fluorescence. Thus, proton irradiation will likely reduce the ability of a candidate smart material to emit fluorescence. Practical space-based sensors based on luminescence will depend heavily upon research investigating the resistance of these materials to ionizing radiation and the ability to anneal or self-heal from damage caused by such radiation. In 1951, Birks and Black showed experimentally that the fluorescence efficiency of anthracene bombarded by alphas varies with total fluence (N) as (I/I0) = 1/(1 + AN), where I is the luminescence yield, I0 is the initial luminescence yield, and A is a constant. The half brightness fluence (N1/2) is a material figure of merit and is defined as the reciprocal of A. Broser and Kallmann developed a similar relationship to the Birks and Black equation for inorganic phosphors irradiated using alpha particles. From 1990 to the present, we have found that the Birks and Black relation describes the reduction in emission yield for every tested luminescent material except lead phosphate glass due to proton irradiation. These results indicate that radiation produced quenching centers compete with emission centers for absorbed energy. Recently, the N1/2 for EuD4TEA irradiated with 3 MeV protons was measured to be about 3 x 1010 mm-2. Conversely, ZnS:Mn compounds tested were found to have N1/2 values for 3 MeV protons that are nearly one thousand times larger (~1013 mm-2). According to Tribble, a spacecraft at 1 AU from the Sun will receive a 1 MeV proton fluence of less than 1011 mm-2 from a large solar event. Likewise, 1 MeV proton fluences in the Earth’s radiation belts and the Earth-Moon-Sun Lagrange points will be even less than the 1011 mm-2 value from large solar events. For this reason, EuD4TEA might be a candidate sensor material to monitor large-scale solar events for astronauts in vehicles flying in near earth space. Since their N1/2 values are larger, ZnS:Mn compounds are more suited for use as sensors in higher radiation environments or for very long duration space missions. The purpose of this presentation is to present results from all research completed in this area. Particular emphasis will be placed on synthesizing EuD4TEA material as well as the design and construction of a 1U CubeSat payload that will orbit the Earth carrying an innovative first generation sensor to monitor radiation fluence. This flight experiment is a natural outgrowth of earlier ground based radiation exposure measurements that will be used to determine if EuD4TEA-based materials can be used as a real-time structural damage and/or radiation sensor for applications in space or other extreme environments.

Multi-spacecraft observations and transport simulations of solar energetic particles for the May 17th 2012 event

Context. The injection, propagation and arrival of solar energetic particles (SEPs) during eruptive solar events is an important and current research topic of heliospheric physics. During the largest solar events, particles may have energies up to a few GeVs and sometimes even trigger ground-level enhancements (GLEs) at Earth. These large SEP events are best investigated through multi-spacecraft observations. Aims. We aim to study the first GLE-event of solar cycle 24, from 17th May 2012, using data from multiple spacecraft (SOHO, GOES, MSL, STEREO-A, STEREO-B and MESSENGER). These spacecraft are located throughout the inner heliosphere, at heliocentric distances between 0.34 and 1.5 astronomical units (au), covering nearly the whole range of heliospheric longitudes. Methods. We present and investigate sub-GeV proton time profiles for the event at several energy channels, obtained via different instruments aboard the above spacecraft. We investigated issues caused by magnetic connectivity, and present results of three-dimensional SEP propagation simulations. We gathered virtual time profiles and perform qualitative and quantitative comparisons with observations, assessed longitudinal injection and transport effects as well as peak intensities. Results. We distinguish different time profile shapes for well-connected and weakly connected observers, and find our onset time analysis to agree with this distinction. At select observers, we identify an additional low-energy component of Energetic Storm Particles (ESPs). Using well-connected observers for normalisation, our simulations are able to accurately recreate both time profile shapes and peak intensities at multiple observer locations. Conclusions. This synergetic approach combining numerical modelling with multi-spacecraft observations is crucial for understanding the propagation of SEPs within the interplanetary magnetic field. Our novel analysis provides valuable proof of the ability to simulate SEP propagation throughout the inner heliosphere, at a wide range of longitudes. Accurate simulations of SEP transport allow for better constraints of injection regions at the Sun, and thus, better understanding of acceleration processes.

A Generalized Approach to Model the Spectra and Radiation Dose Rate of Solar Particle Events on the Surface of Mars

Abstract For future human missions to Mars, it is important to study the surface radiation environment during extreme and elevated conditions. In the long term, it is mainly galactic cosmic rays (GCRs) modulated by solar activity that contribute to the radiation on the surface of Mars, but intense solar energetic particle (SEP) events may induce acute health effects. Such events may enhance the radiation level significantly and should be detected as immediately as possible to prevent severe damage to humans and equipment. However, the energetic particle environment on the Martian surface is significantly different from that in deep space due to the influence of the Martian atmosphere. Depending on the intensity and shape of the original solar particle spectra, as well as particle types, the surface spectra may induce entirely different radiation effects. In order to give immediate and accurate alerts while avoiding unnecessary ones, it is important to model and well understand the atmospheric effect on the incoming SEPs, including both protons and helium ions. In this paper, we have developed a generalized approach to quickly model the surface response of any given incoming proton/helium ion spectra and have applied it to a set of historical large solar events, thus providing insights into the possible variety of surface radiation environments that may be induced during SEP events. Based on the statistical study of more than 30 significant solar events, we have obtained an empirical model for estimating the surface dose rate directly from the intensities of a power-law SEP spectra.

Potential of Using EuD4tea for Space-Based Damage and Radiation Sensors

In recent years, luminescent materials have been proposed as active elements for smart sensors to measure structural damage using triboluminescence (TL) and to monitor radiation fluence. To sense structural damage, these materials are embedded in a composite host structure for various applications, such as crew launch or exploration vehicles. When the damage/fracture occurs in the host composite structure, it will lead to the fracture of the TL crystals resulting in light emission. This will provide a warning, in real time, that structural damage has occurred. The TL emission of the candidate material has to be sufficiently bright, so that the light signal reaching from the point of fracture to the detector, through a fiber optics cable, is strong enough to be detected. The majority of the known TL materials do not emit light with sufficient intensity to allow detection with compact and inexpensive detectors. Over the past 40 years, some materials have been reported that TL with sufficient intensity for the light emission to be easily observed with the naked eye. Out of these TL materials, only few could satisfy the above criterion. Europium tetrakis dibenzoylmethide triethylammonium (EuD4TEA), one of the brightest known TL materials and is a potential candidate for application to this type of hybrid smart sensor. Space radiation poses a significant challenge to human and robotic exploration missions to deep space. To be used in space, any intelligent sensor material must also be resistant to ionizing radiation. Sources and types of such radiation include galactic and solar electrons and protons, x-rays, ultraviolet light, and magnetically trapped charged particles in radiation belts. While much research has been devoted to characterizing material degradation due to electron exposure, surprisingly little work seems to have been done on proton damage. Research has shown that proton bombardment in the keV to MeV range, like that occurring in space, reduces the intensity of fluorescence. Thus, proton irradiation will likely reduce the ability of a candidate smart material to emit fluorescence. Practical space-based sensors based on luminescence will depend heavily upon research investigating the resistance of these materials to ionizing radiation and the ability to anneal or self-heal from damage caused by such radiation. In 1951, Birks and Black showed experimentally that the fluorescence efficiency of anthracene bombarded by alphas varies with total fluence (N) as (I/I0) = 1/(1 + AN), where I is the luminescence yield, I0 is the initial luminescence yield, and A is a constant. The half brightness fluence (N1/2) is a material figure of merit and is defined as the reciprocal of A. Broser and Kallmann developed a similar relationship to the Birks and Black equation for inorganic phosphors irradiated using alpha particles. From 1990 to the present, we have found that the Birks and Black relation describes the reduction in emission yield for every tested luminescent material except lead phosphate glass due to proton irradiation. These results indicate that radiation produced quenching centers compete with emission centers for absorbed energy. Recently, we measured the N1/2 for EuD4TEA irradiated with 3 MeV protons to be about 3 x 1010 mm-2. Conversely, ZnS:Mn compounds tested were found to have N1/2 values for 3 MeV protons that are nearly one thousand times larger (~1013 mm-2). According to Tribble, a spacecraft at 1 AU from the Sun will receive a 1 MeV proton fluence of less than 1011 mm-2 from a large solar event. Likewise, 1 MeV proton fluences in the Earth’s radiation belts and the Earth-Moon-Sun Lagrange points will be even less than the 1011 mm-2 value from large solar events. For this reason, EuD4TEA might be a candidate sensor material to monitor large-scale solar events for astronauts in vehicles flying in near earth space. Since their N1/2 values are larger, ZnS:Mn compounds are more suited for use as sensors in higher radiation environments or for very long duration space missions. Late this year, a 1U CubeSat payload will be placed in Earth orbit carrying an innovative first generation EuD4TEA sensor that will be used to monitor radiation fluence. This flight experiment is a natural outgrowth of earlier ground based radiation exposure measurements. The purpose of this presentation is to present results from research completed in this area from 1990 to present. Particular emphasis will be placed on recent TL and radiation exposure measurements using EuD4TEA. This groundbreaking research can be used to help determine if EuD4TEA-based materials can be used as a real-time structural damage and/or radiation sensor for space applications.

Highly relativistic radiation belt electron acceleration, transport, and loss: Large solar storm events of March and June 2015.

Two of the largest geomagnetic storms of the last decade were witnessed in 2015. On 17 March 2015, a coronal mass ejection‐driven event occurred with a Dst (storm time ring current index) value reaching −223 nT. On 22 June 2015 another strong storm (Dst reaching −204 nT) was recorded. These two storms each produced almost total loss of radiation belt high‐energy (E ≳ 1 MeV) electron fluxes. Following the dropouts of radiation belt fluxes there were complex and rather remarkable recoveries of the electrons extending up to nearly 10 MeV in kinetic energy. The energized outer zone electrons showed a rich variety of pitch angle features including strong “butterfly” distributions with deep minima in flux at α = 90°. However, despite strong driving of outer zone earthward radial diffusion in these storms, the previously reported “impenetrable barrier” at L ≈ 2.8 was pushed inward, but not significantly breached, and no E ≳ 2.0 MeV electrons were seen to pass through the radiation belt slot region to reach the inner Van Allen zone. Overall, these intense storms show a wealth of novel features of acceleration, transport, and loss that are demonstrated in the present detailed analysis.

Impact of inverter loading ratio on solar photovoltaic system performance

Due to decreasing solar module prices, some solar developers are increasing their projects’ inverter loading ratio (ILR), defined as the ratio of DC module capacity to AC inverter capacity. In this study, we examine the operational impacts of this trend. Using minute-level solar data, we examine the relationship between inverter induced clipping losses and AC generation. We find minimal clipping losses at an ILR of 1.25; at an ILR of 2.0, we observe that 16% of potential annual generation is lost. Minute-level data prove to be essential in determining the generation lost to clipping, as hourly data mask key clipping and ramping events. Higher ILRs lead to a greater frequency of time spent at maximum generation, but also a greater frequency and magnitude of large solar ramping events. Module degradation can attenuate the impacts of inverter clipping over time. We observe that the effective degradation rate (net of any changes to inverter clipping losses) can be as little as half the actual degradation rate for projects with high ILRs. The diurnal and seasonal trends in clipping correspond with solar insolation patterns, with the highest clipping occurring around noon. For fixed tilt installations with tilt angles at latitude, we observe the highest clipping near the autumnal and vernal equinoxes. Increasing the tilt angle leads to more winter clipping, while lower tilt angles shifts the clipping to summer months. Shifting from fixed tilt to north–south single axis tracking increases the generation lost to clipping significantly. At an ILR of 1.25, annual clipping approximately doubles to 1% compared to fixed tilt at latitude, while clipping under an ILR of 2.0 increases to 22%, compared to 16% for the fixed configuration. As expected, more clipping occurs during the hours preceding and following noon when using single axis tracking.

A Statistical Study of Distant Consequences of Large Solar Energetic Events

Large solar flares and eruptions may influence remote regions through perturbations in the outer-atmospheric magnetic field, leading to causally related events outside of the primary or triggering eruptions that are referred to as "sympathetic events." We quantify the occurrence of sympathetic events using the full-disk observations by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory associated with all flares of GOES class M5 or larger from 01 May 2010 through 31 December 2014. Using a superposed-epoch analysis, we find an increase in the rate of flares, filament eruptions, and substantial sprays and surges more than 20 degrees away from the primary flares within the first four hours at a significance of 1.8 standard deviations. We also find that the rate of distant events drops by two standard deviations, or a factor of 1.2, when comparing intervals between 4 hours and 24 hours before and after the start times of the primary large flares. We discuss the evidence for the concluding hypothesis that the gradual evolution leading to the large flare and the impulsive release of the energy in that flare both contribute to the destabilization of magnetic configurations in distant active regions and quiet-Sun areas. These effects appear to leave distant regions, in an ensemble sense, in a more stable state, so that fewer energetic events happen for at least a day following large energetic events.

The Annual Cosmic-Radiation Intensities 1391 – 2014; The Annual Heliospheric Magnetic Field Strengths 1391 – 1983, and Identification of Solar Cosmic-Ray Events in the Cosmogenic Record 1800 – 1983

The annual cosmogenic $^{10}\mathrm{Be}$ ice-core data from Dye 3 and the North Greenland Ice-core Project (NGRIP), and neutron-monitor data, 1951 – 2014, are combined to yield a record of the annual cosmic-ray intensity, 1391 – 2014. These data were then used to estimate the intensity of the heliospheric magnetic field (HMF), 1391 – 1983. All of these annual data are provided in the Electronic Supplementary Material. Analysis of these annual data shows that there were significant impulsive increases in $^{10}\mathrm{Be}$ production in the year following the very large solar cosmic-ray events of 1942, 1949, and 1956. There was an additional enhancement that we attribute to six high-altitude nuclear explosions in 1962. All of these enhancements result in underestimates of the strength of the HMF. An identification process is defined, resulting in a total of seven impulsive $^{10}\mathrm{Be}$ events in the interval 1800 – 1942 prior to the first detection of a solar cosmic-ray event using ionization chambers. Excision of the $^{10}\mathrm{Be}$ impulsive enhancements yields a new estimate of the HMF, designated B(PCR-2). Five of the seven $^{10}\mathrm{Be}$ enhancements prior to 1941 are well correlated with the occurrence of very great geomagnetic storms. It is shown that a solar cosmic-ray event similar to that of 25 July 1946, and occurring in the middle of the second or third year of the solar cycle, may merge with the initial decreasing phase of the 11-year cycle in cosmic-ray intensity and be unlikely to be detected in the $^{10}\mathrm{Be}$ data. It is concluded that the occurrence rate for solar energetic-particle (SEP) events such as that on 23 February 1956 is about seven per century, and that there is an upper limit to the size of solar cosmic-ray events.

Plasmaspheric hiss properties: Observations from Polar

AbstractIn the region between L = 2 to 7 at all Magnetic Local Time (MLTs) plasmaspheric hiss was detected 32% of the time. In the limited region of L = 3 to 6 and 15 to 21 MLT (dusk sector), the wave percentage detection was the highest (51%). The latter plasmaspheric hiss is most likely due to energetic ~10–100 keV electrons drifting into the dusk plasmaspheric bulge region. On average, plasmaspheric hiss intensities are an order of magnitude larger on the dayside than on the nightside. Plasmaspheric hiss intensities are considerably more intense and coherent during high‐solar wind ram pressure intervals. A hypothesis for this is generation of dayside chorus by adiabatic compression of preexisting 10–100 keV outer magnetospheric electrons in minimum B pockets plus chorus propagation into the plasmasphere. In large solar wind pressure events, it is hypothesized that plasmaspheric hiss can also be generated inside the plasmasphere. These new generation mechanism possibilities are in addition to the well‐established mechanism of plasmaspheric hiss generation during substorms and storms. Plasmaspheric hiss under ordinary conditions is of low coherency, with small pockets of several cycles of coherent waves. During high‐solar wind ram pressure intervals (positive SYM‐H intervals), plasmaspheric hiss and large L hiss can have higher intensities and be coherent. Plasmaspheric hiss in these cases is typically found to be propagating obliquely to the ambient magnetic field with θkB0 ~30° to 40°. Hiss detected at large L has large amplitudes (~0.2 nT) and propagates obliquely to the ambient magnetic field (θkB0 ~70°) with 2:1 ellipticity ratios. A series of schematics for plasmaspheric hiss generation is presented.

Space radiation environment in low earth orbit during influences from solar and geomagnetic events in December 2006

In low earth orbit, the SAA region is the dominant contributor to both proton environment and electron environment from the standpoint of radiation dose for spacecraft lifetime. However, the polar region and the horn region are sometimes strongly disturbed due to large solar and geomagnetic events. During large disturbances, enhancements in proton flux are measured in the polar region, which gives temporary more severe space radiation environment than that given in the SAA region. On the other hand, enhancements in electron flux are measured mainly in the horn region corresponding to the outer radiation belt, which are likely sources of high-energy electrons in the inner radiation belt. These short-term disturbances have another radiation hazard to spacecraft such as single event and electrostatic discharge.

PARTICLE ENERGY SPECTRA AT TRAVELING INTERPLANETARY SHOCK WAVES

We have searched for evidence of significant shock acceleration of He ions of ~1-10 MeV amu–1 in situ at 258 interplanetary traveling shock waves observed by the Wind spacecraft. We find that the probability of observing significant acceleration, and the particle intensity observed, depends strongly upon the shock speed and less strongly upon the shock compression ratio. For most of the 39 fast shocks with significant acceleration, the observed spectral index agrees with either that calculated from the shock compression ratio or with the spectral index of the upstream background, when the latter spectrum is harder, as expected from diffusive shock theory. In many events the spectra are observed to roll downward at higher energies, as expected from Ellison-Ramaty and from Lee shock-acceleration theories. The dearth of acceleration at ~85% of the shocks is explained by (1) a low shock speed, (2) a low shock compression ratio, and (3) a low value of the shock-normal angle with the magnetic field, which may cause the energy spectra that roll downward at energies below our observational threshold. Quasi-parallel shock waves are rarely able to produce measurable acceleration at 1 AU. The dependence of intensity on shock speed, seen here at local shocks, mirrors the dependence found previously for the peak intensities in large solar energetic-particle events upon speeds of the associated coronal mass ejections which drive the shocks.

Multi-spacecraft Study of the 8 November 2000 SEP Event: Electron Injection Histories 100° Apart

We present the analysis of a large solar near-relativistic electron event observed by the Ulysses and the ACE spacecraft on 8 November 2000, when Ulysses was located at a heliocentric distance of 2.4 AU and at a heliographic latitude of ∼80° S. We use a particle propagation model to infer the local interplanetary transport conditions and the injection histories of the near-relativistic electrons observed by both spacecraft. We find different local transport conditions for each set of observations. The inferred injection profiles for both spacecraft extend for several hours; but the injection at Ulysses was smaller and started later. The association with type II radio emission suggests that the heliospheric electrons were provided by coronal shock acceleration. An analysis of the in situ magnetic field and plasma measurements indicates that the global configuration of the heliosphere (disturbed by transient structures) could play a role in shaping the characteristics of solar energetic particle events observed from different locations.

The impact of large solar events on the total electron content of the ionosphere at mid latitudes

Ionospheric disturbances associated with solar activity may occur via two basic mechanisms. The first is related to the direct impact on the ionosphere of EUV photons from a flare, and the second by prompt electric field penetration into the magnetosphere during geomagnetic storms. In this paper we examine the possibility that these two mechanisms may have an impact at mid latitudes by calculating the total electron content (TEC) from GPS stations in Mexico during several large X-ray flares. We have found that indeed large, complex flares, which are well located, may affect the mid latitude ionosphere. In fact, in the solar events of July 14, 2000 and April 2001 storms, ionospheric disturbances were observed to increase up to 138 and 150 TECu, respectively, due to the influence of EUV photons. Also, during the solar events of July 2000, April 2001, Halloween 2003, January 2005 and December 2006, there are large ionospheric disturbances (up to 393 TECu in the Halloween Storms), due to prompt penetration electric field, associated with CME producing geomagnetic storm.