M. L. Duldig

The October 22, 1989, solar cosmic ray enhancement: An analysis of the anisotropy and spectral characteristics

A solar cosmic ray ground-level enhancement was observed at Earth on October 22, 1989, with an extremely anisotropic onset. On the basis of neutron monitor data we have derived the arrival directions, spectra, and pitch angle distributions of the ≥1 GV solar protons. The results indicate an extended time period during which high-energy particles propagated past Earth with minimal scattering in the local interplanetary medium. The deduced pitch angle distributions of the particles propagating from the Sun show a relatively slow evolution from a focused particle flux, even though there are dramatic changes in the observed intensity-time profiles. Strong evidence is found that reverse scattering of particles beyond Earth resulted in bidirectional flow along the local interplanetary magnetic field during the second phase of the event.

An extended analysis of the September 1989 cosmic ray ground level enhancement

The September 29, 1989, cosmic ray ground level enhancement had a complex intensity-time profile with two distinct peaks observed at some neutron monitors. The event was detected by surface and some underground muon telescopes indicating the presence of particles up to ∼30 GV. We have modeled the response of neutron monitors and surface muon telescopes at the times of both intensity peaks and during the decay phase. We have determined particle rigidity spectra and pitch angle distributions centered on apparent arrival directions. Some evidence of bidirectional flow is present during the second peak. This may be due to backscattering of particles from beyond Earth. There was a high degree of scattering from all directions resulting in a significant isotropic component. Spectra derived from ground level responses are consistent with satellite observations. The spectrum between a few MeV and a few tens of GeV is consistent with shock acceleration.

Precursors of geomagnetic storms observed by the muon detector network

We report the first systematic survey of cosmic ray precursors of geomagnetic storms. Our data set comprises the 14 “major” geomagnetic storms (peak Kp ≥ 8−) identified by Gosling et al. [1990] together with 25 large storms (peak Kp ≥ 7−) observed from 1992 through 1998. After eliminating events for which the muon detector network had poor coverage of the sunward interplanetary magnetic field (IMF) direction, we determined that 15 of the remaining 22 events (68%) had identifiable cosmic ray precursors with typical lead times ranging from 6 to 9 hours prior to the storm sudden commencement (SSC). Of the 15 precursors, 10 were of the “loss cone” (LC) type which is characterized by an intensity deficit confined to a narrow pitch angle region around the sunward IMF direction. Cosmic rays in the loss cone presumably originate in the cosmic-ray-depleted region downstream of the approaching shock. The remaining five precursors were of the “enhanced variance” (EV) type which is characterized by intensity increases or decreases that do not systematically align with the IMF direction. The incidence of precursors increases with storm size; for instance, 89% of storms with peak Kp greater than or equal to 8.0 had precursors. Our results show that the muon detector network can be a useful tool in space weather forecasting. However, new detector(s) installed to fill major gaps in the present network are urgently required for better understanding the nature of precursors and for reliable space weather forecasting.

Contribution of obliquely incident particles to neutron monitor counting rate

We describe a new method for calculating geomagnetic cutoffs that incorporates obliquely incident primaries, and we use it to interpret a sea level neutron monitor latitude survey. Effects due to obliquely incident primaries are significant and may be responsible for anomalies observed in this and other latitude surveys. We define an “apparent” cutoff that takes these obliquely incident particles into account. Use of our apparent cutoff in a Dorman function fit to the 1994–1995 Bartol Research Institute-University of Tasmania latitude survey data results in a significant improvement over use of the more conventional effective vertical cutoff.

An Improved Model for Relativistic Solar Proton Acceleration Applied to the 2005 January 20 and Earlier Events

This paper presents results on modeling the ground-level response of the higher energy protons for the 2005 January 20 ground-level enhancement (GLE). This event, known as GLE 69, produced the highest intensity of relativistic solar particles since the famous event on 1956 February 23. The location of recent X-ray and γ-ray emission (N14° W61°) was near Sun-Earth connecting magnetic field lines, thus providing the opportunity to directly observe the acceleration source from Earth. We restrict our analysis to protons of energy ≥450 MeV to avoid complications arising from transport processes that can affect the propagation of low-energy protons. In light of this revised approach we have reinvestigated two previous GLEs: those of 2000 July 14 (GLE 59) and 2001 April 15 (GLE 60). Within the limitations of the spectral forms employed, we find that from the peak (06:55 UT) to the decline (07:30 UT) phases of GLE 69, neutron monitor observations from 450 MeV to 10 GeV are best fitted by the Gallegos-Cruz & Perez-Peraza stochastic acceleration model. In contrast, the Ellison & Ramaty spectra did not fit the neutron monitor observations as well. This result suggests that for GLE 69, a stochastic process cannot be discounted as a mechanism for relativistic particle acceleration, particularly during the initial stages of this solar event. For GLE 59 we find evidence that more than one acceleration mechanism was present, consistent with both shock and stochastic acceleration processes dominating at different times of the event. For GLE 60 we find that Ellison & Ramaty spectra better represent the neutron monitor observations compared to stochastic acceleration spectra. The results for GLEs 59 and 60 are in agreement with our previous work.

Geometry of an interplanetary CME on October 29, 2003 deduced from cosmic rays

A coronal mass ejection (CME) associated with an X17 solar flare reached Earth on October 29, 2003, causing an ∼11% decrease in the intensity of high-energy Galactic cosmic rays recorded by muon detectors. The CME also produced a strong enhancement of the cosmic ray directional anisotropy. Based upon a simple inclined cylinder model, we use the anisotropy data to derive for the first rime the three-dimensional geometry of the cosmic ray depleted region formed behind the shock in this event. We also compare the geometry derived from cosmic rays with that derived from in situ interplanetary magnetic field (IMF) observations using a Magnetic Flux Rope model. Copyright 2004 by the American Geophysical Union.

Detection of 0.5–15 GEV solar protons on 29 September 1989 at Australian stations

A major cosmic ray ground-level enhancement, the largest in 33 years, occurred on 29 September 1989 during which intensity enhancements at Australian observatories ranged up to a maximum of 344% at Mt. Wellington. The Darwin neutron monitor (cutoff rigidity 14.1 GV) recorded an ∼13% increase in the five-minute counting rate indicating that solar particles up to at least 15 GeV must have been present. Surface muon detectors at Hobart and Mawson recorded increased fluxes, but the event was not recorded by underground muon detectors at either station. Preliminary analysis of the solar particle flux during the initial phase of the event shows a hard spectrum approximated equally well by an exponential spectra with a Po of 2.0 GV or by a modified power law spectra of exponent ∼−2.9. Particles arriving at the detectors from non-vertical directions make a significant contribution to the total increase recorded at mid and low latitude stations.

Drift Effects and the Cosmic Ray Density Gradient in a Solar Rotation Period: First Observation with the Global Muon Detector Network (GMDN)

We present for the first time hourly variations of the spatial density gradient of 50 GeV cosmic rays within a sample solar rotation period in 2006. By inversely solving the diffusive flux equation, including the drift, we deduce the gradient from the anisotropy that is derived from the observation made by the Global Muon Detector Network (GMDN). The anisotropy obtained by applying a new analysis method to the GMDN data is precise and free from atmospheric temperature effects on the muon count rate recorded by ground-based detectors. We find the derived north-south gradient perpendicular to the ecliptic plane is oriented toward the heliospheric current sheet (HCS; i.e., southward in the toward sector of the interplanetary magnetic field [IMF] and northward in the away sector). The orientation of the gradient component parallel to the ecliptic plane remains similar in both sectors, with an enhancement of its magnitude seen after the Earth crosses the HCS. These temporal features are interpreted in terms of a local maximum of the cosmic ray density at the HCS. This is consistent with the prediction of the drift model for the A<0 epoch. By comparing the observed gradient with the numerical prediction of a simple drift model, we conclude that particle drifts in the large-scale magnetic field play an important role in organizing the density gradient, at least in the present A<0 epoch. We also found that corotating interaction regions did not have such a notable effect. Observations with the GMDN provide us with a new tool for investigating cosmic-ray transport in the IMF.

The influence of polar-cap convection on the geoelectric field at Vostok, Antarctica

Abstract Vertical geoelectric field measurements at Vostok, Antarctica ( 78.5° S , 107° E ; corrected geomagnetic latitude, 83.4°S) made during 1998 are compared with both Weimer (1996) and IZMEM (1994) model calculations of the solar-wind-induced, polar-cap potential differences with respect to the station. By investigating the correlations between these parameters for individual UT hours, we confirm and extend the diurnal range over which significant correlations have been obtained. Nineteen individual UT hours are significantly correlated with the Weimer model predictions and nine with the IZMEM model predictions. Diurnal variation in the slopes of the linear regressions allows us to comment on each model, demonstrating that Antarctic polar plateau geoelectric field measurements can be used to investigate polar convection. Seasonal variations in the diurnal electric field variations at Vostok are compared with the Carnegie global electric circuit diurnal curves, after allowance is made for the solar-wind-induced, polar-cap potential difference patterns.

Relativistic Proton Production during the 2000 July 14 Solar Event: The Case for Multiple Source Mechanisms

Protons accelerated to relativistic energies by transient solar and interplanetary phenomena caused a ground-level cosmic-ray enhancement on 2000 July 14, Bastille Day. Near-Earth spacecraft measured the proton flux directly, and ground-based observatories measured the secondary responses to higher energy protons. We have modeled the arrival of these relativistic protons at Earth using a technique that deduces the spectrum, arrival direction, and anisotropy of the high-energy protons that produce increased responses in neutron monitors. To investigate the acceleration processes involved we have employed theoretical shock and stochastic acceleration spectral forms in our fits to spacecraft and neutron monitor data. During the rising phase of the event (10:45 and 10:50 UT) we find that the spectrum between 140 MeV and 4 GeV is best fitted by a shock acceleration spectrum. In contrast, the spectrum at the peak (10:55 and 11:00 UT) and in the declining phase (11:40 UT) is best fitted with a stochastic acceleration spectrum. We propose that at least two acceleration processes were responsible for the production of relativistic protons during the Bastille Day solar event: (1) protons were accelerated to relativistic energies by a shock, presumably a coronal mass ejection (CME); and (2) protons were also accelerated to relativistic energies by stochastic processes initiated by magnetohydrodynamic (MHD) turbulence.