11 research outputs found
Modelling cosmic ray intensities along the Ulysses trajectory
Time dependent cosmic ray modulation in the inner heliosphere is studied by comparing results from a 2-D, time-dependent cosmic ray transport model with Ulysses observations. A compound approach, which combines the effects of the global changes in the heliospheric magnetic field magnitude with drifts to establish a realistic time-dependence, in the diffusion and drift coefficients, are used. We show that this model results in realistic cosmic ray modulation from the Ulysses launch (1990) until recently (2004) when compared to 2.5-GV electron and proton and 1.2-GV electron and Helium observations from this spacecraft. This approach is also applied to compute radial gradients present in 2.5-GV cosmic ray electron and protons in the inner heliosphere. The observed latitude dependence for both positive and negative charged particles during both the fast latitude scan periods, corresponding to different solar activity conditions, could also be realistically computed. For this an additional reduction in particle drifts (compared to diffusion) toward solar maximum is needed. This results in a realistic charge-sign dependent modulation at solar maximum and the model is also applied to predict charge-sign dependent modulation up to the next expected solar minimum
A study of L-dependent Pc3 pulsations observed by low Earth orbiting CHAMP satellite
Field line resonances (FLR) driven by compressional waves are an important
mechanism for the generation of ULF geomagnetic pulsations observed at all
latitudes during local daytime. References to observations of toroidal
standing Alfvén mode oscillations with clearly L-dependent frequencies
from spacecraft in the outer magnetosphere for L>3 are limited in the
literature. Such observations in the inner magnetosphere for L<3 have
not yet been reported in the literature. This study offers two interesting
case studies of observations of ULF waves by the low Earth orbiting CHAMP
satellite. The magnetic field measurements from CHAMP, which are of
unprecedented accuracy and resolution, are compared to Hermanus magnetometer
data for times when CHAMP crosses the ground station L-shell, namely for 13
February 2002 and 18 February 2003. The data were analysed for Pc3 pulsation
activity using the Maximum Entropy Spectral Analysis (MESA) method to
visualise FLRs in the vector magnetometer data. For the first time
observations of Pc3 toroidal oscillations with clearly L-dependent
frequencies for lower L-shell values (L<3) observed by an LEO satellite
are reported. These observations show FLR frequencies increasing as a
function of decreasing latitude down to L=1.6 and then decreasing as a
result of the larger plasma density of the upper ionosphere. The L-dependent
frequency oscillations were observed in the presence of a broadband
compressional wave spectrum. Our observations thus confirm the well-known
magnetohydrodynamic (MHD) wave theoretical prediction of a compressional
wave being the driver of the field line resonance
The structure of low-latitude Pc3 pulsations observed by CHAMP and on the ground
The structure of low-latitude continuous pulsations termed Pc3, which are
naturally occurring MHD waves in the Earth's magnetosphere, were studied by
comparing ground and satellite magnetic field measurements. Data from two
induction magnetometers, located at Hermanus and Sutherland in South Africa
were used in conjunction with Challenging Minisatellite Payload (CHAMP)
satellite observations to study a Pc3 event observed on 15 February 2003, at
a time when CHAMP was passing over the ground stations. We observed a number
of discrete frequency oscillations for the fast mode wave, one of which
drives a field line resonance (FLR) at characteristic latitude as detected
by both ground and satellite measurements. Consequently, our observations
confirmed the compressional wave as being the driver of the field line
resonance. The toroidal mode frequency observed on CHAMP experienced a
Doppler frequency shift due to the rapid motion across the resonance region.
Polarization hodograms in the resonance region clearly showed the expected
90° rotation of the field line resonant magnetic field components
Studies of Geomagnetic Pulsations Using Magnetometer Data from the CHAMP Low-Earth-Orbit Satellite and Ground-Based Stations: a Review
We review research on geomagnetic pulsations carried out using magnetic field measurements from the CHAMP low-Earth-orbit (LEO) satellite and ground-based stations in South Africa and Hungary. The high quality magnetic field measurements from CHAMP made it possible to extract and clearly resolve Pi2 and Pc3 pulsations in LEO satellite data. Our analyses for nighttime Pi2 pulsations are indicative of a cavity mode resonance. However, observations of daytime Pi2 pulsation events identified in ground station data show no convincing evidence of their occurrence in CHAMP data. We also studied low-latitude Pc3 pulsations and found that different types of field line resonant structure occur, namely discrete frequencies driven by a narrow band source and L-dependent frequencies driven by a broad band source