Pc3-4 geomagnetic pulsations at very low latitude in Brazil

In order to investigate Pc3-4 geomagnetic pulsations at very low and equatorial latitudes, L=1.0 to 1.2, we analyzed simultaneous geomagnetic data from Brazilian stations for 26 days during October-November 1994. The multitaper spectral method based on Fourier transform and singular value decomposition was used to obtain pulsation power spectra, polarization parameters and phase. Eighty-one (81) simultaneous highly polarized Pc3-4 events occurring mainly during daytime were selected for the study. The diurnal events showed enhancement in the polarized power density of about 3.2 times for pulsations observed at stations close to the magnetic equator in comparison to the more distant ones. The phase of pulsation observed at stations near the magnetic equator showed a delay of 48-62° in relation to the most distant one. The peculiarities shown by these Pc3-4 pulsations close to the dip equator are attributed to the increase of the ionospheric conductivity and the intensification of the equatorial electrojet during daytime that regulates the propagation of compressional waves generated in the foreshock region and transmitted to the magnetosphere and ionosphere at low latitudes. The source mechanism of these compressional Pc3-4 modes may be the compressional global mode or the trapped fast mode in the plasmasphere driving forced field line oscillations at very low and equatorial latitudes.     

Gravity waves in low latitude F-region

The paper presents a study of large scale travelling ionospheric disturbances detected by riometers operating at 30 MHz, over São José dos Campos (23°S, 45°W) and a nearby location in Sao Paulo, Brasil. The TID's are observed mainly at night and have wavelengths greater than 500 Km. In a few cases it is possible to determine the E-W component of the velocity of propagation, which is of the order of 450 Km per hour. Most of the events are characterized by disperions; the period is found to increase from half an hour to nearly two hours. These and other features are identified with the propagation of atmospheric gravity waves in the F-region, whose source might be located far away from the observing site. The results also suggest that a suitably designed riometer system could profitably be used for future investigation of gravity waves in the F-region in low latitude.     

Day time observations of precursors at low latitude

Whistler precursors observed during day time at low latitude ground station Gulmarg (Geomag. Lat. 24 10 N) and their morphological features are reported. Transverse resonance interaction between whistler mode wave and counter streaming energetic electrons as the probable generation mechanism has been worked out. Minimum anisotropy required for wave amplification, parallel energy of resonating electrons and wave growth rate relevant to generation mechanism is studied.     

An evaluation of duct-life times from low latitude ground observations of whistlers

On certain occasions, whistler rate occurrences at Gulmarg (24°N geomagnetic) and Naini Tal (19°N geomagnetic) are found to exhibit some periodicity. Power spectrum analyses of the occurrence rates yield a dominant period of about 1 hr. It is suggested that this period is an indication of the duct-life times at low L-values. Dispersion analyses of the whistlers have qualitatively confirmed the existence of separate ducts during the period of observation. It is pointed out that power spectrum analyses may not be applicable to whistler data corresponding to high L-values.     

Venus Express observations of ULF and ELF waves in the Venus ionosphere: Wave properties and sources

Electrical activity in a planetary atmosphere enables chemical reactions that are not possible under conditions of local thermodynamic equilibrium. In both the Venus and terrestrial atmospheres, lightning forms nitric oxide. Despite the existence of an inventory of NO at Venus like the Earth’s, and despite observations of the signals expected from lightning at optical, VLF, and ELF frequencies, the existence of Venus lightning still is met with some skepticism. The Venus Express mission was equipped with a fluxgate magnetometer gradiometer system sampling at rates as high as 128 Hz, and making measurements as low as 200 km altitude above the north polar regions of Venus. However, significant noise levels are present on the Venus Express spacecraft. Cleaning techniques have been developed to remove spacecraft interference at DC, ULF, and ELF frequencies, revealing two types of electromagnetic waves, a transverse right-handed guided mode, and a linearly polarized compressional mode. The propagation of both types of signals is sensitive to the magnetic field in ways consistent with propagation from a distant source to the spacecraft. The linearly polarized compressional waves generally are at lower frequencies than the right-handed transverse waves. They appear to be crossing the usually horizontal magnetic field. At higher frequencies above the lower hybrid frequency, waves cannot enter the ionosphere from below when the field is horizontal. The arrival of signals at the spacecraft is controlled by the orientation of the magnetic field. When the field dips into the atmosphere, the higher frequency guided mode above the lower hybrid frequency can enter the ionosphere by propagating along the magnetic field in the whistler mode. These properties are illustrated with examples from five orbits during Venus Express’ first year in orbit. These properties observed are consistent with the linearly polarized compressional waves being produced at the solar wind interface and the transverse guided waves being produced in the atmosphere.     

Whistler observations of the quiet time plasmasphere-ionosphere coupling fluxes at low latitude

Observations of whistlers during quiet times made at low-latitude ground station Nainital (geomag. lat. 19 1 N) are used to deduce plasmasphere-ionosphere coupling fluxes. The whistler data from 3 magnetically quiet days are presented that show a smooth decrease in dispersion with time. This decrease in dispersion is interpreted in terms of a corresponding decrease in electron content of tubes of ionization. The electron densities, electron tube contents (10¹⁶ el/m²-tube) and coupling fluxes (10 el m^–1 s^–2) are computed by means of an accurate curve fitting method developed by Tarcsai (1975) and are in good agreement with the results reported by other workers.     

Whistler observations of magnetospheric electric field in the night side plasma-sphere at low latitude

Whistlers recorded at low latitude ground stations of Gulmarg, Nainital and Varanasi were used to infer the east-west component of electric field on the nightside plasmasphere atL=1.2, 1.12, and 1.07 during magnetic storm periods. The method of measuring electric field from the observed cross-L motions of whistler ducts within the plasma-sphere, indicated by changes in nose frequency of whistlers has been outlined. The nose frequencies of the non-nose whistlers under consideration have been deduced from Dowden-Allocock linear Q-technique. The results show eastward electric fields of 0.7 mVm^–1 in the equatorial plane of Gulmarg and 0.3 mVm^–1 in the equatorial plane of Nainital in the premidnight local time sector. Near midnight, there is a sharp transition from eastward field to a westward electric field of 0.2–0.7 mV m^–1 for Gulmarg, 0.3–0.5 mV m^–1 for Nainital and 0.1–0.3 mVm^–1 for Varanasi.     

Excitation of ULF and VLF waves in the ionosphere

It is shown that a two-dimensional lower-hybrid wave structure can parametrically trigger the growth of VLF and ULF noises in a plasma. Analytical expressions for the increment and threshold of the instability are obtained. Application of our work to the auroral zones of the topside ionosphere is discussed.     

Generation mechanism of Pc3 magnetic pulsations at very low latitudes

Polarization properties of Pc3 magnetic pulsations at very low latitudes cannot be explained by existing theories which are based on the field line resonance model, because magnetic field lines at ¦Φ¦ < 22° are almost entirely in the ionosphere. In order to interpret Pc3 polarization characteristics observed at very low latitudes (¦Φ¦ < 20°), I would like to propose a possible, new qualitative model in which two superimposed ionospheric eddy currents, oscillating with slight differences in frequency in the Pc3 range and in azimuthal wave number, move azimuthally at very low latitudes. The equatorial ionospheric Pedersen eddy currents are believed to be predominantly caused by inductive electric fields of compressional Pc3 source waves which may possibly arrive in the equatorial ionosphere from the outer magnetosphere.     

An estimate of the duct life time from low latitude ground observations of whistlers at Varanasi

The observed periodicity in the whistler occurrence rate recorded at our low latitude ground station at Varanasi (geomagnetic latitude, 14°55'N) is interpreted in terms of duct life time at lowL values. Power spectrum analysis of the whistler data yields a period of about 50 min for the growth and decay of ducts. Further dispersion analysis of the whistlers has qualitatively confirmed the existence of separate ducts during the period of observations.     

Cluster observations of Pc 1–2 waves and associated ion distributions during the October and November 2003 magnetic storms

Unusual wave activity in the Pc 1–2 frequency band (0.1–5 Hz) was observed by the Cluster spacecraft in association with the two large geomagnetic storms of late 2003. During the onset of the Hallowe’en storm on October 29, 2003, intense broadband activity between ∼0.1 and 0.6 Hz appeared at all 4 spacecraft on both sides of the magnetic equator at perigee (near 1400 UT and 08:45 MLT). Power was especially strong and more structured in frequency in the compressional component: a minimum in wave power was observed at 0.38 Hz, corresponding to the oxygen ion cyclotron frequency. Poynting vector calculations indicated that wave power was primarily directed radially inward rather than along the magnetic field. Narrowband purely compressional waves near 0.15 Hz appeared at higher dayside latitudes in the southern hemisphere. CIS ion spectrometer data during this pass revealed that O⁺ was the dominant energetic ion. During the recovery phase of the November storm, on November 22, 2003, predominantly transverse 1.8 Hz waves with peak-to-peak amplitude of 10 nT were observed by all four spacecraft near perigee at L=4.4. During this more typical Pc 1 event, wave power was directed along B, toward the northern ionosphere. An unusually polarized 2.3 Hz emission (with power in the radial and compressional, but not azimuthal directions) was observed at L=5.4–5.9, 10–15° south of the magnetic equator. We infer that this wave event may have been generated on lower L shells and propagated obliquely to Cluster's location. Consistent with other recent observations, anisotropic plasma sheet/ring current proton distributions appeared to be a necessary condition for occurrence of waves during both passes, but was not always a sufficient condition. The transverse waves of November 22 occurred in regions which also contained greatly increased fluxes of cool ions (E<1 keV). On both days, Cluster observed features not previously reported, and we note that the purely compressional nature of the October 29 events was not anticipated in previous theoretical studies. The fact that these unusually polarized waves occurred in association with very intense geomagnetic storms suggests that they are likely to be extremely rare.     

Ground-based astrometric observations of latitude for improved reference frame

The Commission 19 (Earth Rotation) of the International Astronomical Union—IAU established the Working Group on Earth Rotation in the Hipparcos Reference Frame—WG ERHRF at 1995 to collect the optical observations of latitude and universal time variations, made during 1899.7-1992.0 in line with Earth orientation programmes (to derive Earth Orientation Parameters—EOP), with Dr. Jan Vondrák (Astronomical Institute of Academy of Sciences of the Czech Republic, Prague) as the head of WG ERHRF. Dr. Vondrák collected about 4.4 million optical observations of latitude/universal time variations made at 33 observatories. These data were used for EOP investigations, Hipparcos Catalogue—radio sources connection, etc. Nowadays, it is used to correct the positions and proper motions of stars of Hipparcos Catalogue (as an optical reference frame) using ground-based observations of some Hipparcos stars. After Hipparcos Catalogue, some new astrometric catalogues appeared (as ARIHIP, EOC-2, etc.) with better accuracy of proper motions. We use the latitude observations made by visual zenith-telescope (ZT), as classical astrometric instrument, at seven observatories (all over the world) of International Latitude Service—ILS. The observations were used in the programmes of monitoring the Earth orientation during the 20th century. We received the data from Dr. Vondrák via private communication. The observatories are Carloforte—CA, Cincinnati—CI, Gaithersburg—GT, Kitab—KZ, Mizusawa—MZZ, Tschardjui—TS and Ukiah—UK. The task is to improve proper motions in declination of the observed Hipparcos stars. The original method was developed. We removed from the instantaneous observed latitudes, all known effects (polar motion and some local instrumental errors), and the corrected latitudes were then the input data to calculate the corrections of Hipparcos proper motions in declination using the least squares method—LSM with the linear model. We did an improvement of Hipparcos proper motions in declination via mentioned latitude variations with time by using a long-term (a few decades) visual zenith-telescope data of ILS. The calculated results were compared with the ARIHIP and EOC-2 data, and the consistency were good. The main steps of the calculations and some of the results are presented here.     

Long-term predictions of Pc 1 geomagnetic pulsations: Comparison with observations

Comparison is made between the measured monthly occurrences of pc 1-active days in California during the interval 1970–1976 and predictions of these occurrences by Soviet and U.S. researchers. It is shown that successful long-term predictions of the monthly occurrences can be made for up to 3 yr ahead and probably also for longer intervals depending on the need. These predictions could have application in cooperative programs of observation, in magnetotelluric studies, and in communications.     

On the effects of lunar tides in the Tianjin latitude observations

In this paper we have analysed the effect of the lunar tide on the latitude observations of 25260 star-pairs with the zenith telescope, ZTL-180 of Tianjin Latitude Station during 1960–1966. For the M₂ wave, we found an amplitude of 0.0108 and hence a value of 1 + k − 1 = 1.34. When the effect of the ocean tide is subtracted, the value of 1 + k − 1 is reduced to 1.315. This is in very good agreement with the value 1.31, deduced by geophysicists for the Asia region.     

Propagation characteristics of hydromagnetic waves in a cold plasma mixed with a hot plasma and right-hand polarized Pc1 and Pc5

Propagation characteristics of hydromagnetic waves in a cold plasma mixed with a hot plasma under a uniform static magnetic field are investigated. The existence of cold plasma seriously affects the polarization properties of the waves. The results are applied to the interpretation of Pcl and Pc5 with righthand polarizations guided along the geomagnetic field line.     

In search of a new ULF wave index: Comparison of Pc5 power with dynamics of geostationary relativistic electrons

A new ULF wave index, characterizing the turbulent level of the geomagnetic field, has been calculated and applied to the analysis of relativistic electron enhancements during space weather events in March–May 1994 and September 1999. This global wave index has been produced from the INTERMAGNET, MACCS, CPMN, and Greenland dense magnetometer arrays in the northern hemisphere. A similar ULF wave index has been calculated using magnetometer data from geostationary (GOES) and interplanetary (Wind, ACE) satellites. During the periods analyzed several magnetic storms occurred, and several significant increases of relativistic electron flux up to 2–3 orders of magnitude were detected by geostationary monitors. However, these electron enhancements were not directly related to the intensity of magnetic storms. Instead, they correlated well with intervals of elevated ULF wave index, caused by the occurrence of intense Pc5 pulsations in the magnetosphere. This comparison confirmed earlier results showing the importance of magnetospheric ULF turbulence in energizing relativistic electrons. In addition to relativistic electron energization, a wide range of space physics and geophysics studies will benefit from the introduction of the ULF wave index. The ULF index database is freely available via anonymous FTP for all interested researchers for further validation and statistical studies.     

Mode identification of terrestrial ULF waves observed by Cluster: A case study

Using multipoint measurements from the Cluster mission wave identification techniques are applied to observations of ULF waves made in the terrestrial foreshock with the aim of identifying the modes and properties of the waves taking into account the effects of a high beta plasma. The wave properties in the spacecraft and plasma rest frames are experimentally derived using minimum variance analysis. Two waves with periods of 30 and 3 s dominate the dynamic frequency spectrum. The results indicate that these waves propagate in the fast magnetosonic and Alfvén/Ion Cyclotron modes, respectively. Both waves propagate in the upstream direction in the plasma rest frame but are convected downstream in the spacecraft frame. The measured wave properties in the plasma rest frame are in good agreement with those obtained from the theoretical kinetic dispersion relation taking into account the effects of different plasma beta. The dispersion results show a rather significant deviation from fluid model, especially when high beta plasma conditions occur. These experimentally derived foreshock ULF wave properties are in good agreement with previous results but when the effects of a high beta plasma are considered it is not as straight forward to choose the correct wave mode branch.     

Multipoint observations of the dynamics of the sharp solar wind structure boundaries

In this paper we performed the selection of sharp and large solar wind ion flux changes and comparison of them by simultaneous observations onboard two or three spacecraft. It was shown that these sharp changes survive on the way up to million km or even sharpen on this way.     

The characteristics of Pc 3 and Pc 4 geomagnetic micropulsations recorded at Sanae,Antarctica

Digital dynamic spectra of micropulsations recorded at SANAE (L ～ 4) show that Pc 3 pulsations have frequencies which decrease throughout the day. Both the onset frequency and the rate of decrease of frequency depend on the level of magnetic activity during the previous night. The variation of Pc 3 amplitudes and frequencies is explained in terms of the position of the plasmapause and the associated Pc 3 resonance region in the plasmatrough.For Pc 4 pulsations a constant frequency is observed on most days and it is not possible to infer the presence of a Pc 4 resonance region.     

Multisatellite observations of resonant hydromagnetic waves

Most measurements of long period ULF pulsations have come from ground based and single satellite observations. The observations have given strong support to the idea that these waves are resonant standing hydromagnetic waves on geomagnetic field lines. Simultaneous ground-satellite observations provide further details of the pulsation structure and are useful for examining the effect of the ionosphere on the transmission of the waves to the ground. Recently, multisatellite observations have been used to provide further insight into the nature of pulsations and we review the results obtained using this technique. Among the results presented are those from the ISEE 1 and 2 spacecraft which are closely spaced in identical orbits, making it possible to distinguish temporal from spatial structure in waves. The ISEE spacecraft have made measurements of resonant region widths and resonance harmonics. In addition, examples are shown of recent multisatellite observations of the global nature of some pulsations and the localization of Pi2 pulsations in space.