J.F. Valdés-Galicia

Solar Neutron Events of 2003 October-November

During the period when the Sun was intensely active in 2003 October-November, two remarkable solar neutron events were observed by the ground-based neutron monitors. On 2003 October 28, in association with an X17.2 large flare, solar neutrons were detected with high statistical significance (6.4 σ) by the neutron monitor at Tsumeb, Namibia. On 2003 November 4, in association with an X28-class flare, relativistic solar neutrons were observed by the neutron monitors at Haleakala in Hawaii and Mexico City and by the solar neutron telescope at Mauna Kea in Hawaii simultaneously. Clear excesses were observed at the same time by these detectors, with the significance calculated as 7.5 σ for Haleakala and 5.2 σ for Mexico City. The detector on board the INTEGRAL satellite observed a high flux of hard X-rays and γ-rays at the same time in these events. By using the time profiles of the γ-ray lines, we can explain the time profile of the neutron monitor. It appears that neutrons were produced at the same time as the γ-ray emission.

Long-lived solar neutron emission in comparison with electron-produced radiation in the 2005 September 7 solar flare

Strong signals of neutral emissions were detected in association with a solar flare that occurred on 2005 September 7. They were produced by both relativistic ions and electrons. In particular, relativistic neutrons were observed with the solar neutron telescopes (SNTs) located at Mount Chacaltaya in Bolivia and Mount Sierra Negra in Mexico and with neutron monitors (NMs) at Chacaltaya and Mexico City with high statistical significances. At the same time, hard X-rays and γ-rays, which were predominantly emitted by high-energy electrons, were detected by the Geotail and the INTEGRAL satellites. We found that a model of the impulsive neutron emission at the time of the X-ray/γ-ray peak can explain the main peaks of all the detected neutron signals, but failed to explain the long tailed decaying phase. An alternative model, in which the neutron emission follows the X-ray/γ-ray profile, also failed to explain the long tail. These results indicate that the acceleration of ions began at the same time as the electrons but that ions were continuously accelerated or trapped longer than the electrons in the emission site. We also demonstrate that the neutron data observed by multienergy channels of SNTs put constraints on the neutron spectrum.

FRACTALITY OF MONTHLY EXTREME MINIMUM TEMPERATURE

Interest in climate change has increased over the last 30 years due largely to global predictions associated with the greenhouse effect, which appear to lead to a substantial increase in planetary temperature. Implications of such results have led many scientists to examine climatic records from different regions of the world in order to understand temperature behavior. However, many researchers have noted that changes in temperature variability are also important in determining the future temperature distributions. In this context, we have analyzed a long-term record of monthly extreme minimum temperature registered at Guanajuato, Mexico. Data set was treated as a fractal profile to estimate the fractal dimension through variography (Dv) and power-spectral (Ds) approaches under two situations: (1) complete series, from January 1895 to December 1997 with 312 missing observations, and (2) partial series, from January, 1921 to April, 1963 with no missing values. In both cases, we obtained similar values for the two types of fractal dimensions meaning there is not a significant effect of missing values. The estimated fractal dimensions for the partial series (508 observations) are near 1.5 (Dv = 1.445 ± 0.06, Ds = 1.486 ± 0.155), which means monthly extreme minimum temperature is almost equally characterized by both short- and long-range variations. Evaluating through scaling arguments did not evidence multifractality in the scale range of two to 254 months. Then interpolation can make use of the fact that monthly extreme minimum temperature has a power-law spectrum. Interpolated data generated by this way may develop greater confidence in their capability to forecast near future climate.

Pitch angle scattering of solar particles: Comparison of ‘particle’ and ‘field’ approach

In the quasi-linear theory of pitch angle scattering the power spectrum of magnetic field fluctuations is related to the shape of the pitch angle diffusion coefficient D(μ), the absolute value of the mean free path λ, and the rigidity dependence of the mean free path λ(R). We discuss these relations in detail during the solar particle event of 11 April, 1978 which was observed on HELIOS-2 at a distance of 0.49 AU from the Sun. Magnetic field measurements obtained during the time of the event are used as a basis for the ‘layer model’ in which the method of particle trajectories in an actually measured field is used to simulate pitch angle diffusion. The values of D(μ) and λ based on the trajectory simulation for 100 MeV protons (‘field’ approach) are compared with results obtained from solar proton data (‘particle’ approach) and with predictions from quasi-linear theory based on the additional assumption of the slab model for magnetic field fluctuations (‘QLT’ approach). The time of the event is characterized by a high level of field fluctuations, the observed mean free path of about 0.03 AU for 100 MeV protons is smaller than the average value near 1 AU. Results from the ‘field’ and ‘particle’ approaches agree surprisingly well. The remaining difference in the mean free path of about a factor of 2 could be due to tangential discontinuities which are measured by the magnetometer, but not seen by the real particles traveling along the average field. The results from the ‘field’ and ‘QLT’ approaches based on the same set of magnetic field measurements differ by about a factor of 4. One of the reasons for this discrepancy is that the conditions for resonance scattering are only marginally valid. In addition, the wave vectors representing Alfvén-type fluctuations may not be totally field aligned. This deviation from the slab model would cause an increase of the theoretically predicted mean free path and lead to better agreement with the other two approaches.

Galactic cosmic ray fluctuations during solar cycles 22 and 23 at high altitude neutron monitors

Abstract Cosmic ray detectors installed at high magnetic cutoff rigidity and mountain elevation are only a few. It is therefore important to have reliable data for active monitors with these characteristics. The Mexico City 6-NM64 has been in continuos operation since 1990. It is located at an altitude of 2274 m above sea level and has a vertical magnetic cutoff rigidity of 8.2 GV for the 1990 epoch (Shea and Smart, 1997). In this work we make a study of the cosmic ray intensity fluctuations observed in Mexico City and compare it with other mountain altitude neutron monitors of the american sector during the years 1990–1999. The period comprises the maximum and declining phase of solar cycle 22 and the beginning of cycle 23. Evolution of important variation periodicities compared with solar activity indicators are presented. We have also constructed a new solar activity index based on the measured hard x-rays flux.

Mean free paths of energetic particles at very large heliodistances (Pioneer 11 at 20 AU)

Pioneer 11 magnetic field data at 20 AU are analysed by the computational method of Moussas, Quenby, and Webb (1975), Moussas and Quenby (1978), and Moussas, Quenby, and Valdes-Galicia (1982a, b) to obtain the parallel mean free path λ∥, and the diffusion coefficient parallel to the magnetic field line K∥. This method is the most appropriate for the mean free path calculation at large heliodistances since the alternative method which is based on fitting of energetic particle intensities cannot be easily and accurately be used because the association of energetic particles with their parent flares is not precise. The results show that the mean free path has values between 0.85 and 0.98 AU, linearly increasing with energy according to λ∥(Tkinetic) = ∧ + MT, where Λ = 0.846 AU and M = 4.44 × 10 −5 AU MeV−1 for energies between 10 MeV and 3 GeV for protons. These values of the parallel mean free path are much larger than the values estimated by previous studies up to 6 AU. The diffusion coefficient dependence upon energy follows a relation which simply reflects an almost constant mean free path and a linear dependence on the velocity of the particle, so that at 20 AU heliodistance K∥(Tkin) = K∥, 1 MeV(Tkin)Tkineticα, with α = 1/2. The distance dependence of the parallel diffusion mean free path follows a power law, λ∥(R) = λ∥, 1 AURλ, where λ is 1 ± 0.1. While the parallel diffusion coefficient obeys a power-law relation with heliodistance R, K∥ (R, Tkin) = K∥, 1 AU(Tkin)Rβ, with β = 1 ± 0.1. The radial diffusion coefficient of cosmic rays is not expected to strongly depend upon the parallel diffusion coefficient because the nominal magnetic field at these large heliodistances (20 AU) is almost perpendicular to the radial direction and the contribution of the diffusion coefficient perpendicular to the magnetic field is expected to play a dominant role. However, the actual garden hose angle varies drastically and for long time periods and hence the contribution of the diffusion parallel to the field may continue to be important for the small scale structure of intensity gradients.

On the Role of Large-Scale Solar Photospheric Motions in the Cosmic-Ray 1.68-YR Intensity Variation

Analysis of cosmic-ray intensity time evolution has led to the identification of intensity variations with several periodicities, most of them correlated with one or another phenomenon of the Sun. Recently Valdés-Galicia, Pérez-Enriquez, and Otaola (1996) reported on a newly-found 1.68-yr variation, which seems to be correlated with periodicities in X-ray long-duration events and low-latitude coronal hole area variations. As those phenomena are related with magnetic flux emergence and transport, in this paper we investigate the possible relationship of the referred cosmic-ray variation with characteristic times of different tracers of meridional circulation. Our results indicate that several of the calculated times might be related to the 1.68-yr cosmic-ray variation. A physical mechanism through which this connection may operate is discussed.

Relatively stable, large-amplitude Alfvenic waves seen at 2.5 and 5.0 AU

Pioneer 11 and 10 observations of the wave structure seen in a corotating interaction region at 2.5 AU on day 284 of 1973 and 8 days later at 5 AU reveal large-amplitude Alfvénic structures with many detailed correlations seen between their features at the two radial distances. Hodogram analysis suggests the dominance of near plane polarized, transverse Alfvénic mode fluctuations with periods between 2 min and one hour or more. Some wave evolution close to the Corotating Interaction Region (CIR) shock is noticed, but waves towards the centre of the compression seem to propagate with little damping between the spacecraft observation positions.

Calibration and monitoring of water Cherenkov detectors with stopping and crossing muons

Abstract The Auger Observatory water Cherenkov detectors (WCD) will require that the initial calibration and subsequent monitoring of each of the WCDs be done in a remote way. We present a method to perform these tasks based on the detection of muons decaying inside the detectors and the application of adequate selection cuts. This technique may be complemented with another based on muons crossing the WCDs. Samples of decaying and crossing muon events were obtained with a WCD prototype to demonstrate the viability of the techniques. Three clear peaks of PMT charge distributions were identified. All of them are useful for calibration and monitoring of WCDs: one for stopping muons, one for decay electrons and one for crossing muons. The mean value of the peak found in the decay-electron charge distribution is 0.18 times the corresponding value for vertically crossing muons; likewise, the mean value of the peak in the charge distribution of crossing muons (excluding corner clipping muons) is 6.1 times the value for decay electrons in a tank of our dimensions; finally, the mean value of the peak in the charge distribution of stopping muons is 0.55 times the value for decay electrons. The techniques described can be applied equally well to unsegmented or segmented Auger tanks as each of the three PMTs of an Auger WCD can be self-triggered independently. The experimental data are well reproduced by numerical simulations.

A further search on waves generated by solar energetic protons

It has been suggested that in the interplanetary medium Alfvén waves may be significantly amplified or damped during large solar proton events. This implies the increase or decrease of the ambient magnetic fluctuations in concurrence with the presence of the streaming particles, that we have analysed in a first study at times of eight proton events observed by Helios spacecraft (Valdés-Galicia and Alexander, 1997). However, it is not possible with interplanetary magnetic field measurements only to distinguish between waves moving away or towards the Sun in the frame of reference of the spacecraft. Plasma data for these eight events have now been made available to us and hence the energetic content of inward and outward propagating waves may be found, which is an important aid in our search for signatures left by the energetic protons. In the present work we incorporate the new information into the analyses of those events that in our first study showed more favourable evidence and therefore try to give a more definite answer as to whether it might be observed. The new results do not reinforce the evidence of our previous work, as they seem to be mildly consistent with the presence of the proton self-generated waves in just one of the three cases studied.