Ground Level Event Research Articles

The atmospheric radiation response to solar-particle-events

High-energy solar particles, produced in association with solar flares and coronal mass ejections, occasionally bombard the earth's atmosphere,. resulting in radiation intensities additional to the background cosmic radiation. Access of these particles to the earth's vicinity during times of geomagnetic disturbances are not adequately described by using static geomagnetic field models. These solar fluxes are also often distributed non uniformly in space, so that fluxes measured by satellites obtained at great distances from the earth and which sample large volumes of space around the earth cannot be used to predict fluxes locally at the earth's surface. We present here a method which uses the ground-level neutron monitor counting rates as adjoint sources of the flux in the atmosphere immediately above them to obtain solar-particle effective dose rates as a function of position over the earth's surface. We have applied this approach to the large September 29–30, 1989 ground-level event (designated GLE 42) to obtain the magnitude and distribution of the solar-particle effective dose rate from an atypically large event. The results of these calculations clearly show the effect of the softer particle spectra associated with solar particle events, as compared with galactic cosmic rays, results in a greater sensitivity to the geomagnetic field, and, unlike cosmic rays, the near-absence of a “knee” near 60° geomagnetic latitude.

Flare- and Shock-accelerated Energetic Particles in the Solar Events of 2001 April 14 and 15

We report heavy-ion composition and spectra for the solar energetic particle (SEP) events of 2001 April 14 and 15, using the combined capabilities of the Advanced Composition Explorer (ACE), Wind, and the Interplanetary Monitoring Platform 8 (IMP-8) to cover the energy range from ∼30 keV nucleon^(-1) to ∼400 MeV nucleon^(-1). These two events are, respectively, the largest impulsive event and the largest ground-level event observed so far in solar cycle 23. These events arose from the same active region and launched into similar interplanetary conditions. Both were associated with large western flares and fast coronal mass ejections (CMEs). However, the two events are distinctly different, thereby providing useful reminders of the fundamental differences between flare- and shock-accelerated SEPs. The detailed observations present challenges for our theoretical understanding of SEP production. Of particular note is the fact that iron has a harder power-law energy spectrum than oxygen above ∼3 MeV nucleon^(-1) in the shock-dominated April 15 event. This spectral difference, which is seen in many other gradual events of various sizes and heliolongitudes, leads to enhanced Fe/O at high energies. Simple shock acceleration models predict the same power-law index for all species. Thus, understanding the origin of this spectral difference will significantly contribute to the resolution of the ongoing debate about the relative roles of CME-driven shocks and flares in producing high-energy solar heavy ions.

Ground level muons in coincidence with the solar flare of 15 April 2001

The counting rate of single muon tracks from the Project GRAND proportional wire chamber array is examined during the Ground Level Event (GLE) of 15 April 2001. The GLE was seen by neutron monitor stations shortly after the time of the solar X‐ray flare. GRAND's single muon data are presented and compared with neutron monitor data from Climax, Newark, and Oulu. The single muon data have mean primary hadron energies higher than those of these neutron monitor stations and so contain information about higher energy hadrons. For the single muon data for Project GRAND the GLE is detected at a statistical significance of 6.1σ.

On-board aircrew dosimetry using a semiconductor spectrometer.

Radiation fields on board aircraft contain particles with energies up to a few hundred MeV. Many instruments have been tested to characterise these fields. This paper presents the results of studies on the use of an Si diode spectrometer to characterise these fields. The spectrometer has been in use since spring 2000 on more than 130 return flights to monitor and characterise the on-board field. During a Czech Airlines flight from Prague to New York it was possible to register the effects of an intense solar flare, (ground level event, GLE 60), which occurred on 15 April 2001. It was found that the number of deposition events registered was increased by about 70% and the dose in Si by a factor of 2.0 when compared with the presence of galactic cosmic rays alone. Directly measured data are interpreted with respect to on-earth reference field calibration (photons, CERN high-energy particles): it was found that this approach leads to encouraging results and should be followed up.

Great solar bursts of October 19, 22 and 23, 1989

Itapetinga measurements at 48 GHz with the multibeam technique are used to determine the relative position of solar burst centroid of emission with high spatial accuracy and time resolution. For the Great Bursts of October 19,22, 1989, with a large production of relativistic particles, and October 23, it is suggested that, at 48 GHz, the bursts might have originated in more then one source in space and time. Additionally the October 19 and 22 Ground Level Events exhibited very unusual intensity-time profiles including double component structures for the onset phase. The Bern observatory spectral radio emission data show a strong spectral flattening typical for large source inhomogeneties. The interpretation for this is that large solar flares are a superposition of a few strong bursts (separated both in space and time) in the same flaring region.

Non-diffusive particle pulse transport – Application to an anisotropic solar GLE

A theory of the transport of an anisotropic pulse of charged particles injected into the interplanetary magnetic field is applied to an anisotropic ground level event on 24 May 1990. For this event the kinetic regime is considered when the mean free path is comparable with the distance from particle source. Both the source angular particle distribution and the angular dependence of a detector response are included. The theoretically predicted temporal profiles are compared with the particle intensity records measured by several neutron monitors with different asymptotic directions.

Milagro as a solar observatory

The Milagro Gamma Ray Observatory, a ground-based water Čherenkov detector designed primarily for observing very high energy gamma ray sources, can also be used to study the Sun. Measurements of high energy emission from solar flares can lead to an understanding of the solar energetic particle acceleration mechanism(s), and Milagro, with enhanced data acquisition electronics, will measure solar cosmic rays in the poorly studied energy range from 10 to 300 GeV. Monte Carlo simulations show that proton and neutron solar cosmic rays at these energies will be detectable at the ground level through the single muons and/or mini showers they produce. Milagro can detect these events in a scaler counting mode similar to that of a neutron monitor. Additionally, it can operate in a telescope mode that reduces background by pointing at the source region. We also report a preliminary detection, using the Milagro prototype, Milagrito, of a ground level event (GLE) on 6 November 1997 associated with an X-class solar flare.

Comment on the use of GOES solar proton data and spectra in solar proton dose calculations

There is a need to understand the calibration and response of the GOES solar particle detectors since the GOES data are being used to evaluate high energy solar particle events. We share some of our experience in utilizing these data in the analysis of solar particle ground-level events (GLEs). For the 29 September 1989 event, we have evaluated the solar proton and alpha particle spectral characteristics throughout the event. The results show that the solar cosmic ray spectrum is extremely hard at low energies with the magnitude of the slope increasing with increasing energy and with time.

Enhancement of stratospheric aerosols after solar proton event

Abstract. The lidar measurements at Verhnetulomski observatory (68.6°N, 31.8°E) at Kola peninsula detected a considerable increase of stratospheric aerosol concentration after the solar proton event of GLE (ground level event) type on the 16/02/84. This increase was located at precisely the same altitude range where the energetic solar protons lost their energy in the atmosphere. The aerosol layer formed precipitated quickly (1–2 km per day) during 18, 19, and 20 February 1984, and the increase of R(H) (backscattering ratio) at 17 km altitude reached 40% on 20/02/84. We present the model calculation of CN (condensation nuclei) altitude distribution on the basis of an ion-nucleation mechanism, taking into account the experimental energy distribution of incident solar protons. The meteorological situation during the event was also investigated.

Atmospheric cosmic rays and solar energetic particles at aircraft altitudes

Galactic cosmic rays, which are thought to be produced and accelerated by a variety of mechanisms in the Milky Way galaxy, interact with the solar wind, the earth's magnetic field, and its atmosphere to produce hadron, lepton, and photon fields at aircraft altitudes that are quite unlike anything produced in the laboratory. The energy spectra of these secondary particles extend from the lowest possible energy to energies over an EeV. In addition to cosmic rays, energetic particles, generated on the sun by solar flares or coronal mass ejections, bombard the earth from time to time. These particles, while less energetic than cosmic rays, also produce radiation fields at aircraft altitudes which have qualitatively the same properties as cosmic rays. The authors have calculated atmospheric cosmic-ray angular fluxes, spectra, scalar fluxes, and ionization, and compared them with experimental data. Agreement with these data is seen to be good. These data have been used to calculate equivalent doses in a simplified human phantom at aircraft altitudes and the estimated health risks to aircraft crews. The authors have also calculated the radiation doses from several large solar energetic particle events (known as GLEs, or Ground Level Events), which took place in 1989, including the very large event known as GLE 42, which took place on September 29th and 30th of that year. The spectra incident on the atmosphere were determined assuming diffusive shock theory. Unfortunately, there are essentially no experimental data with which to compare these calculations.

Ozone “miniholes” initiated by energetic solar protons

In this study, it is shown that during four Solar Proton Events (SPE), mostly of the Ground Level Event (GLE) type (May 1990, September and October 1989, and March 1989), inside the polar cap in the Arctic (or the Antarctic) short-term depletions were observed (up to 20%) in the ozone total content. These depletions or so-called ozone “miniholes” seem to be caused by energetic solar protons with energies of 150–300 MeV. For May 1990, the gas phase photochemical model includes only 1% ozone depletion compared with 18% observed at Barentsburg (Svalbard), and for none of the other events can homogeneous processes explain the observed depletion. The problem seems to be solved considering heterogeneous reactions in the presence of increased amounts of aerosol particles in the stratosphere which may be triggered by penetrating solar protons, or through an additional decrease of temperature, or through an increase of volume electric charge in the stratosphere (or even troposphere).

Ozone “miniholes” initiated by energetic solar protons

In this study, it is shown that during four Solar Proton Events (SPE), mostly of the Ground Level Event (GLE) type (May 1990, September and October 1989, and March 1989), inside the polar cap in the Arctic (or the Antarctic) short-term depletions were observed (up to 20%) in the ozone total content. These depletions or so-called ozone “miniholes” seem to be caused by energetic solar protons with energies of 150–300 MeV. For May 1990, the gas phase photochemical model includes only 1% ozone depletion compared with 18% observed at Barentsburg (Svalbard), and for none of the other events can homogeneous processes explain the observed depletion. The problem seems to be solved considering heterogeneous reactions in the presence of increased amounts of aerosol particles in the stratosphere which may be triggered by penetrating solar protons, or through an additional decrease of temperature, or through an increase of volume electric charge in the stratosphere (or even troposphere).

Cosmic ray decreases and solar wind disturbances during late October 1989

We describe the interplanetary phenomena (energetic particles, solar wind plasma, and magnetic field) seen at IMP 8 and at ICE, located 65° west of IMP 8, during the period October 19‐31, 1989, when neutron monitors observed three ground level events originating in one active region when it was in the longitude range E09° to W57°. At least four shocks, associated with energetic particle enhancements, which can be attributed to a sequence of coronal mass ejections from the same active region, were seen at both spacecraft. An additional shock was observed only at ICE late in this period when the active region was behind the west limb. Considering all the data (which unfortunately suffer from large gaps), it appears that the ejecta associated with the shocks were detected only when the spacecraft and solar source longitude were separated by less than 50°. The shocks extended over a greater range of longitudes. The cosmic ray record at Earth is consistent with this picture such that only the first two shock‐associated cosmic ray decreases had the signature expected for intercepting ejecta material. This same time period was also examined by Bavassano et al. (1994). However, we do not agree with their conclusion that “magnetic clouds” extending at least 75° from the source longitude were present.

Large ground level events in solar cycle 22 and some peculiarities of relativistic proton acceleration

Some of the Ground Level Enhancements of Solar Cosmic Rays (SCR) recorded by neutron monitors during solar cycle 22 are analyzed. The events appeared in series, and their occurence rate in 1989–1991 was a factor of 4 larger than the average value (≅1/yr) for the total observation interval (since 1942). The events of Sept. 29 and Oct. 24, 1989 proved to be the more intensive. The analyzed events show no peculiarities as to the distribution of the T 1 2 parameter, whereas the shape of the profile of some events is notable for a two peak structure. The later implies the possibility of a two component SCR ejection from two different sources in the solar atmosphere. For the event of Sep. 29 we have estimated the ejection rigidity spectrum of protons to be D ⊙ (R) = (1–2) 10 32R −2.9 Gv −1 at R⩾ 1 Gv. As to its proton flux, this event proved to be by 1–2 orders less intensive than the well known event of Feb. 23 1956.

Injection profiles of solar energetic particles as functions of coronal mass ejection heights

Previous studies with Skylab and Solwind coronal mass ejections (CMEs) have shown that nearly all large E greater than 10 MeV solar energetic particle (SEP) events are associated with fast (v greater than 400 km/sec) CMEs. We compare heights of CMEs observed on the SMM spacecraft with the flux-time profiles of five associated SEP events observed by the GOES spacecraft, including three events observed as ground-level events (GLEs) by neutron monitors. The SEP injection profiles as functions of the CME heights are estimated from the 'solar release times' and the effects of interplanetary scattering. We find that the peaks of the 470 MeV to 4 GeV injection profiles of the GLEs occur when CME heights reach 5 to 15 R(sub 0) or greater and that the onsets occur no earlier than the maxima of the flare impulsive phases. In those events SEP injection appears to result only from a single CME-driven shock and not from the flare impulsive phase or from separate coronal and interplanetary shocks. In one small SEP event an impulsive flux-time profile is consistent with injection during the flare impulsive phase but could also be due to injection from a coronal shock over a limited time.

Ground-level energetic solar particle event on 24 May 1990: Possibility of direct solar neutron detection

The first increase in neutron monitor count rate during the ground-level event on 24 May 1990 was interpreted by Shea et al. (1991) as a consequence of an arrival of flare neutrons. Debrunner et al. (1991) rejected the neutron hypothesis and proposed that the first neutron monitor increase was due to the arrival of primary protons. We have show that neutron monitor data do not contradict the hypothesis of a neutron origin of the first increase of ground-level event on 24 May 1990.

Arctic ozone abundance and solar proton events

In this study it is documented for the first time that the atmospheric ozone column density is decreased during solar proton events, GLEs (Ground Level Events), events with protons having energy exceeding 50 MeV and penetrating down to the ground. The ozone decrease was significant at the polar cap stations, Longyearbyen and Barentsburg at 78° N, whereas it was not detectable at the auroral zone stations Murmansk (69° N) and Tromsø (70° N). The duration of the depletion was limited to the days of the proton precipitation, and the depletion may be explained by the ability of energetic protons to produce nitric and hydrogen oxides in the stratosphere and their catalytic reactions with ozone. Similar events are identified in the Antarctic.

Changes in the cosmic-ray power spectrum during solar flares

The power spectrum analyses of cosmic-ray neutron-monitor data at two different cutoff rigidity stations, Calgary (1.09 GV) and Lomnicky Stit (4.0 GV), exhibit similar changes in power spectral form in the frequency interval 10−5–10−3 Hz before and after the six solar flare events that occurred from January 1989 to June 1990. The power spectrum index n at both stations, which showed significantly lower values one day before the flare onset, indicated a sharp steepening of the spectral shape following the flare event. The change in the power spectrum is attributed to the redistribution of inhomogeneities near the sun, produced before the solar flares in which cosmic-ray particles are accelerated to high energies, to produce a ground level event.

The ground-level cosmic-ray event of September 29, 1989 as recorded by the rome detectors

An extraordinary increase in cosmic-ray intensity has been recorded on September 29, 1989 by the Rome detectors (rigidity threshold ∼6.2 GV). We show the time evolution of the event as recorded at Rome in the nucleonic component and in the total ionizing and muon components; as well as at Campo Imperatore (2300 m a.s.l. near Rome) in the nucleonic component by a special monitor designed for the detection of solar-neutron events. The one-minute time resolution of the Campo Imperatore data offers the possibility to follow in detail the development of the event for most of the time. The possible contribution of solar neutrons to the early phase of the Campo Imperatore event has been not clarified so far, further results may be obtained by the analysis of the multiplicity data registered in the event.

Unique series of increases in cosmic-ray intensity due to solar flares

THE second half of 1989 was the most prolific period of relativistic particle production by the Sun since continuous monitoring of cosmic-ray activity by neutron detectors was begun in 1957. Some of last year's events were among the largest bursts of particles observed throughout the last three solar cycles. Only when the particles released by the Sun during flares have energies greater than 0.5 gigaelectronvolt (GeV) are the secondary particles generated through interactions in the upper atmosphere able to reach ground level. Six different ground-level events were detected by the super neutron monitor in Calgary (geomagnetic latitude and longitude 51.08° N 245.91°, altitude 1,128 m, cut-off 1.07 GV) in the period July to October 1989. The intensity of energetic particle fluxes during these ground-level events is high enough to be a potential hazard, especially for passengers in aircraft at altitudes of 10 to 15 km.