Low Geomagnetic Latitudes Research Articles

Extended scaling factors for in situ cosmogenic nuclides: New measurements at low latitude

Production rates of cosmogenic nuclides at the earth's surface are controlled by the intensity of energetic cosmic-ray nucleons, which changes rapidly with elevation. An incomplete knowledge of how nucleon fluxes vary with elevation remains a major obstacle to utilizing cosmogenic nuclides as geochronometers in applications requiring highly accurate ages. One problem is that attenuation characteristics depend on nucleon energy. Measurements of high-energy (> 50 MeV) nucleon fluxes tend to give shorter attenuation lengths than low-energy (< 1 MeV) fluxes, but these differences are not well characterized due to a lack of data at lower energies. Another problem is that the atmospheric attenuation length for nucleon fluxes varies with the geomagnetic cutoff rigidity (a parameter related to geomagnetic latitude), RC, and that there has been an incomplete mapping of nucleon fluxes at high RC (low geomagnetic latitude). We report new measurements of nucleon fluxes from altitude transects in Hawaii (RC = 12.8 GV) and Bangalore, India (RC = 17.3 GV). Our measurements in Hawaii of low-energy neutrons (median energy 1 eV) and energetic nucleons (median energy 140 MeV) confirm that nucleon scaling functions are energy-dependent in the range of energies at which cosmogenic nuclides are produced. Our measurements in southern India extend our previously reported scaling model for spallation reactions [D. Desilets, M. Zreda, Spatial and temporal distribution of secondary cosmic-ray nucleon intensity and applications to in situ cosmogenic dating. Earth Planet. Sci. Lett. 206 (2003) 21–42] from RC = 13.3 GV to RC = 17.3 GV, nearly the highest cutoff rigidity on earth. The anomalously high cutoff rigidity over India provides a geomagnetic shielding condition that is effectively the same as would be observed at the geomagnetic equator in a dipole field with an intensity 1.2 times the modern value. This makes it possible to scale low-latitude production rates to paleomagnetic fields that are stronger than the present dipole field.

A modeling study of ionospheric F2-region storm effects at low geomagnetic latitudes during 17-22 March 1990

Abstract. We have presented a comparison between the modeled NmF2 and hmF2, and NmF2 and hmF2, which were observed in the low-latitude ionosphere simultaneously by the Kokubunji, Yamagawa, Okinawa, Manila, Vanimo, and Darwin ionospheric sounders, by the middle and upper atmosphere (MU) radar during 17-22 March 1990, and by the Arecibo radar for the time period of 20-22 March 1990. A comparison between the electron and ion temperatures measured by the MU and Arecibo radars and those produced by the model of the ionosphere and plasmasphere is presented. The empirical zonal electric field, the meridional neutral wind taken from the HWM90 wind model, and the NRLMSISE-00 neutral temperature and densities are corrected so that the model results agree reasonably with the ionospheric sounder observations, and the MU and Arecibo radar data. It is proved that the nighttime weakening of the equatorial zonal electric field (in comparison with that produced by the empirical model of Fejer and Scherliess (1997) or Scherliess and Fejer (1999)), in combination with the corrected wind-induced plasma drift along magnetic field lines, provides the development of the nighttime enhancements in NmF2 observed over Manila during 17-22 March 1990. As a result, the new physical mechanism of the nighttime NmF2 enhancement formation close to the geomagnetic equator includes the nighttime weakening of the equatorial zonal electric field and equatorward nighttime plasma drift along magnetic field lines caused by neutral wind in the both geomagnetic hemispheres. It is found that the latitudinal positions of the crests depend on the E×B drift velocity and on the neutral wind velocity. The relative role of the main mechanisms of the equatorial anomaly suppression observed during geomagnetic storms is studied for the first time in terms of storm-time variations of the model crest-to-trough ratios of the equatorial anomaly. During most of the studied time period, a total contribution from meridional neutral winds and variations in the zonal electric field to the equatorial anomaly changes is larger than that from geomagnetic storm disturbances in the neutral temperature and densities. Vibrationally excited N2 and O2 promote the equatorial anomaly enhancement during the predominant part of the studied time period, however, the role of vibrationally excited N2 and O2 in the development of the equatorial anomaly is not significant. The asymmetries in the neutral wind and densities relative to the geomagnetic equator are responsible for the north-south asymmetry in NmF2 and hmF2, and for the asymmetry between the values of the crest-to-trough ratios of the Northern and Southern Hemispheres. The model simulations provide evidence in favor of an asymmetry in longitude of the energy input into the auroral region of the Northern Hemisphere on 21 March 1990.

A statistical comparison of SuperDARN spectral width boundaries and DMSP particle precipitation boundaries in the afternoon sector ionosphere

Abstract. The open-closed magnetic field line boundary (OCB) is best measured at the foot points of the boundary in the Earth's ionosphere where continuous and extensive spatiotemporal measurements can be made. The ability to make routine observations of this type is crucial if accurate global measurements of energy transfer processes occurring at the boundary, such as magnetic reconnection, are to become a reality. The spectral width boundary (SWB) measured by the Super Dual Auroral Radar Network (SuperDARN) has been shown to be a reliable ionospheric proxy for the OCB at certain magnetic local times (MLTs). However, the reliability of the SWB proxy in the afternoon sector ionosphere (12:00-18:00 MLT) has been questionable. In this paper we undertake a statistical comparison of the latitudinal locations of SWBs measured by SuperDARN and particle precipitation boundaries (PPBs) measured by the Defense Meteorological Satellite Program (DMSP) spacecraft, concentrating on the PPB which best approximates the location of the OCB. The latitudes of SWBs and PPBs were identified using automated algorithms applied to 5 years (1997-2001) of data measured in the 12:00-18:00 MLT range. A latitudinal difference was measured between each PPB and the nearest SWB within a ±10 min universal time (UT) window and within a ±1 h MLT window. The results show that when the SWB is identified at higher geomagnetic latitudes (poleward of ~74), it is a good proxy for the OCB, with 76% of SWBs lying within 3 of the OCB. At lower geomagnetic latitudes (equatorward of ~74), the correlation is poor and the results suggest that most of the SWBs being identified represent ionospheric variations unassociated with the OCB, with only 32% of SWBs lying within 3 of the OCB. We propose that the low level of precipitating electron energy flux, typical of latitudes well equatorward of the OCB in the afternoon sector, may be a factor in enhancing spectral width values at these lower latitudes. A consequence of this would be low latitude SWBs unrelated to the OCB.

Duration and extent of the great auroral storm of 1859

The great geomagnetic storm of August 28 through September 3, 1859 is, arguably, the greatest and most famous space weather event in the last two hundred years. For the first time observations showed that the sun and aurora were connected and that auroras generated strong ionospheric currents. A significant portion of the world’s 200,000 km of telegraph lines were adversely affected, many of which were unusable for 8 h or more which had a real economic impact. In addition to published scientific measurements, newspapers, ship logs, and other records of that era provide an untapped wealth of first hand observations giving time and location along with reports of the auroral forms and colors. At its height, the aurora was described as a blood or deep crimson red that was so bright that one “could read a newspaper by.” At its peak, the Type A red aurora lasted for several hours and was observed to reach extremely low geomagnetic latitudes on August 28–29 (∼25°) and on September 2–3 (∼18°). Auroral forms of all types and colors were observed below 50° latitude for ∼24 h on August 28–29 and ∼42 h on September 2–3. From a large database of ground-based observations the extent of the aurora in corrected geomagnetic coordinates is presented over the duration of the storm event.

Storm time changes in total electron content in ionosphere measured by low orbiting topside sounder

Abstract. A new experimental technique is presented for the determination of the total electron content (TEC) below a low-orbiting satellite. According to this technique TEC can be obtained using the segment of a topside ionogram that only contains the traces of signals reflected from the Earth's surface. Possibilities of this technique were demonstrated using MIR station topside sounding data at the night time for both quiet and disturbed ionospheric conditions, and in particular, during the 14 November 1998 storm. An interesting fact was revealed with the help of this technique: after a series of relatively strong storms the main ionospheric trough on 14 November 1998 was detected at an abnormally low geomagnetic latitude (~43°). During this study some spatial variations of TEC were registered that can be interpreted as a TID-type wave structure.

Effects of solar activity on myocardial infarction deaths in low geomagnetic latitud regions

We study the effect of solar activity on the incidence of myocardial infarction deaths (MID) in Mexico. We work with 129,917 cases along 1996–1999, grouping the data by sex and age, and considering the solar cycle phases. At higher frequencies the circaseptan is the most persistent periodicity in MID occurrence. During solar minimum the circaseptan period is not detectable compared with solar maximum. During Forbush decreases and geomagnetic activity, most cases present a higher average MID occurrence. Furthermore the MID rate is higher as the level of the geomagnetic perturbation increases. Male MID rates are in general higher than female rates and the difference increases as the geomagnetic perturbation increases. The age group with the lowest MID incidence is 25 to 44 years, the age group of ≥65 years is the most vulnerable. We conclude that solar activity does affect MID at low geomagnetic latitudes and that the solar maximum is the most hazardous time for MID incidence.

Disturbances at F2-region heights of equatorial anomaly during geomagnetic storms

Neutral gas composition and ionospheric measurements taken by the Dynamic Explorer 2 satellite at F2-region heights during two geomagnetic storms are used to analyze the role of some possible physical mechanisms responsible for the changes of electron density at equatorial and low geomagnetic latitudes. The storms considered occurred on October 2, 1981 (storm 1) and July 13, 1982 (storm 2). During storm 1 (weak), vertical plasma drifts and equatorward storm-time winds operated increasing of the electron density at the trough of equatorial anomaly and the decreases at the crest region. During storm 2 (intense) changes of composition (increase of molecular nitrogen and atomic oxygen) played a fundamental role for the changes of electron density observed at low latitudes in summer hemisphere. It is concluded that different physical processes seem to have varying degrees of importance depending on the intensity of the storm.

Compressional wave events in the dawn plasma sheet observed by Interball-1

Abstract. Compressional waves with periods greater than 2 min (about 10-30 min) at low geomagnetic latitudes, namely compressional Pc5 waves, are studied. The data set obtained with magnetometer MIF-M and plasma analyzer instrument CORALL on board the Interball-1 are analyzed. Measurements performed in October 1995 and October 1996 in the dawn plasma sheet at -30 RE ≤ XGSM and |ZGSM| ≤ 10 RE are considered. Anti-phase variations of magnetic field and ion plasma pressures are analyzed by searching for morphological similarities in the two time series. It is found that longitudinal and transverse magnetic field variations with respect to the background magnetic field are of the same order of magnitude. Plasma velocities are processed for each time period of the local dissimilarity in the pressure time series. Velocity disturbances occur mainly transversely to the local field line. The data reveal the rotation of the velocity vector. Because of the field line curvature, there is no fixed position of the rotational plane in the space. These vortices are localized in the regions of anti-phase variations of the magnetic field and plasma pressures, and the vortical flows are associated with the compressional Pc5 wave process. A theoretical model is proposed to explain the main features of the nonlinear wave processes. Our main goal is to study coupling of drift Alfven wave and magnetosonic wave in a warm inhomogeneous plasma. A vortex is the partial solution of the set of the equations when the compression is neglected. A compression effect gives rise to a nonlinear soliton-like solution.Key words. Magnetosphere physics (magnetotail) · Space plasma physics (kinetic and MHD theory; non-linear phenomena)

Influence of ionospheric conductivity on mid-latitude Pc 3–4 pulsations

Diurnal variations of the parameters of the magnetospheric Alfvén resonator at different latitudes have been calculated using a semi-empirical model of the ionosphere-magnetosphere plasma distribution. The ionospheric plasma density is taken from the IRI model, the electron density at the magnetospheric equator is based on the ISEE/whistler model, and the field-aligned magnetospheric plasma distribution is calculated under the assumption of diffusive equilibrium. It is shown that for the mid-latitude ionosphere the Hall conductivity has no effect on the parameters of the magnetospheric Alfvén resonator. The calculated values of damping rates of Alfvén oscillations at middle latitudes during the dark period are too high for the “free-end” and “quarter-wave” oscillation regimes to be realized. At low latitudes quality factors are relatively high both at daytime and nighttime conditions. An expected change of a field-aligned structure of Alfvén oscillations during the transition from dayside to nightside ionospheric conditions does not occur. The analysis of the experimental data recorded at middle and low latitude stations of the “210° Magnetic Meridian” magnetometer network and station l’Aquila gives the results, compatible with the predictions of the numerical model: (a) the pulsation amplitude in a frequency band near the fundamental harmonic of the Alfvén field line resonance has the strongest dependence on the ionospheric conductivity; (b) the influence of day/night ionospheric conditions on the Pc 3 amplitudes is less at low (L ≤ 2) geomagnetic latitudes than at middle latitudes; (c) the ionospheric conductivity control of the Pc 3 amplitude at middle latitudes weakens with increasing harmonic number.

Horizontal meridional thermospheric winds over King George Island, Antarctica, during the June 1991 Geomagnetic storm

Abstract Diurnal variations of the magnetic meridional component of the thermospheric neutral wind have been derived from both a servo model based algorithm with ionosonde data input, and the HWM-90 empirical wind model for the 4–6 and 11–13 June 1991 geomagnetic storm at King George Island (62.2°S, 58.8°W). While the HWM-90 winds are predominantly equatorward, the servo model winds are predominantly poleward during the storm, with especially strong poleward winds in the pre-noon hours. Such strong poleward winds are not expected to occur during a major storm at such a low geomagnetic latitude (51.4°S), which is equatorward of direct ion drag forcing by magnetospheric convection electric fields. We speculate that this atypical thermospheric wind response may be associated with particle heating in the South Atlantic Anomaly.

Space weather: Response of large‐scale geopotentials to an interplanetary magnetic cloud

The interaction of solar wind disturbances with the Earth's magnetosphere can produce disturbances, and at times complete disruptions, of technological systems on the Earth and in the space around the Earth. This brief report shows the changes induced in the large‐scale geopotentials of the Earth (as provided from measurements across transoceanic cables) produced by a well‐documented interplanetary magnetic cloud event. The study of such a well‐measured event can be used to begin to make empirical space weather phenomena more quantitative. We show that geopotentials at low geomagnetic latitudes can be used to infer the time derivative of the near‐equatorial magnetic disturbance index, Dst. At low geomagnetic latitudes, a peak geopotential of about 4 mV/km is found to correspond to a time rate of change of this index of about 50 nT/hr. Further, we show that in this event increases in the near‐equatorial geopotential are linearly related to the energy input to the magnetosphere from the solar wind as given by the ε parameter [e.g., Akasofu, 1979]. We find that an increase in the geopotential of about 4 mV/km corresponds to an energy input of about 2.8 × 1011 W for the event analyzed here.

An improved technique for reduction to the pole at low latitudes

Reduction to the pole at low latitudes based on a Wiener filtering approach has been improved by introducing a deterministic noise model. It is assumed that the noise power is a fixed fraction of the signal power. This allows the method to be fully automated. Further improvement is obtained by requiring the reduced‐to‐the‐pole field to map into the observed field when projected to the geomagnetic latitude of the observed field. This is done by iteratively minimizing the difference between the measured field and the reduced‐to‐the‐pole field projected to the geomagnetic latitude of the measured data. This results in a reduced‐to‐the‐pole magnetic map that, when projected to the geomagnetic latitude of the given data, closely matches the measured data. The final reduced‐to‐the‐pole field does not show any of the artifacts typical of reduction‐to‐the‐pole at low geomagnetic latitudes. The method is demonstrated on a data set from an aeromagnetic survey flown over north‐central Burkina Faso, West Africa.

Neutrinos from primordial black holes

The emission of particles from black holes created in the early Universe has detectable astrophysical consequences. The most stringent bound on their abundance has been obtained from the absence of a detectable diffuse flux of 100 MeV photons. Further scrutiny of these bounds is of interest as they, for instance, rule out primordial black holes as a dark matter candidate. We here point out that these bounds can, in principle, be improved by studying the diffuse cosmic neutrino flux. Measurements of near-vertical atmospheric neutrino fluxes in a region of low geomagnetic latitude can provide a competitive bound. The most favorable energy to detect a possible diffuse flux of primordial black hole origin is found to be a few MeV. We also show that measurements of the diffuse ${\ensuremath{\nu}}_{\mathrm{\ensuremath{\tau}}}$ flux is the most promising to improve the existing bounds deduced from \ensuremath{\gamma}-ray measurements. Neutrinos from individual black hole explosions can be detected in the GeV--TeV energy region. We find that the kilometer-scale detectors, recently proposed, are able to establish competitive bounds.

Diurnal trends in geomagnetic noise power in the Pc 2 through Pc 5 bands at low geomagnetic latitudes

Mean, integrated geomagnetic noise power spectra in six octaves between 0.002 and 0.128 Hz from the Air Force Geophysics Laboratory (AFGL) magnetometer network were grouped into seasonal ensembles. We found that the local time variation of the mean logarithm of this ensemble, with respect to both 3‐hr and 8.5‐min time blocks, consistently experienced a 5–10 dB maximum centered around local noon at higher frequencies and near dawn and dusk at lower frequencies, a factor of 3–10 in power. The shape of this trend differed with geomagnetic field component, geomagnetic latitude, and frequency octave. The high‐frequency Pc 3 and 4 waves have a broad maximum in power centered on local noon, but the longer‐period Pc 4 and 5 wave power maximizes away from noon toward dawn for the north‐south geomagnetic field component and toward dusk for the east‐west component. Two years of data did not provide a large enough sample to resolve a significant seasonal variation in the shape of the diurnal trend. These trends changed consistently during periods of enhanced global activity, as measured by the index Ap. These are among the first systematic observations of Pc 5 wave power at low geomagnetic latitudes (40°–55°), indicating that wave energy from the outer magnetosphere is coupling or propagating to low‐latitude locations. This statistical study extends the few previous event studies by providing an explicit parametrization of local time trends in wave power as a function of frequency subband, geographic location, and magnetic activity.

Geoelectric power spectra over oceanic distances

We present initial results of simultaneous measurements of the geoelectric potential as measured across two long cable routes in the Pacific (Guam to Philippines and California to Hawaii) separated by ∼6h in local time and ∼30° in latitude. Over the frequency range ∼10−5 to 1.67 × 10−3Hz the power levels tend to be enhanced near local noon along the two routes, especially during geomagnetic quiet intervals. During the seven days analyzed, geoelectric power levels up to 104 (mV/km)²/km were measured on a geomagnetically disturbed day. The power levels at the longest periods, ∼17m to ∼150m, are generally found to be largest along the route at lower geomagnetic latitudes (Guam to Philippines) when comparisons are made at equivalent local times. In contrast, the power levels for periods in the hydromagnetic wave region (periods ≲5m) tend to be larger at all local times for the higher geomagnetic latitude route. The nightside geoelectric power levels along the higher latitude route tend to increase with increasing geomagnetic activity.

Phase relationships between total electron content variations, Doppler velocity oscillations and geomagnetic pulsations

The phase relationship between variations of ionospheric total electron content (TEC) and ground‐level ULF geomagnetic pulsations has been examined for the advection and compression mechanisms. To determine the causal mechanism, several earlier studies have examined the phase difference between oscillations of Doppler velocity in ionospherically reflected radio waves and simultaneous ULF geomagnetic pulsations. In most instances it was found that the phase relation varied from event to event. With the application of Euler’s formula this study shows that in low geomagnetic and midgeomagnetic latitudes the phase differences between variations of TEC and ULF pulsations in the northward component of the geomagnetic field due to the advection and compression mechanisms are 0o and 180o, respectively. We also found that TEC variations tend to lead ionospheric Doppler velocity oscillations by 90o. Furthermore, it is shown that the phase relationship between ionospheric Doppler velocity oscillations and ULF pulsations of the northward component of the geomagnetic field, caused by the advection and compression mechanisms, are functions of the scale length, frequency of ULF waves, and geomagnetic dip.

An overview of the Solar Anomalous, and Magnetospheric Particle Explorer (SAMPEX) mission

The scientific objectives of the NASA small-class explorer mission SAMPEX are summarized. A brief history of the Small Explorer Program is provided along with a description of the SAMPEX project development and structure. The spacecraft and scientific instrument configuration is presented. The orbit of SAMPEX has an altitude of 520 by 670 km and an 82 degrees inclination. Maximum possible power is provided by articulated solar arrays that point continuously toward the sun. Highly sensitive scientific instruments point generally toward the local zenith, especially over the terrestrial poles, in order to measure optimally the galactic and solar cosmic ray flux. Energetic magnetospheric particle precipitation is monitored at lower geomagnetic latitudes. The spacecraft uses several innovative approaches including an optical fiber bus, powerful onboard computers, and large solid state memories (instead of tape recorders). Spacecraft communication and data acquisition are discussed and the space- and ground-segment data flows are summarized. >

External Sq currents in the India‐Siberia region

The Sq current systems for quiet days of 1976 and 1977 were studied for observatories established near 76° E. longitude in the northern hemisphere. We used a special spherical harmonic analysis technique to separate the internal and external contributions of the quiet field variations and to determine the equivalent external current contours responsible for the Sq field variations on the sixth and twenty‐first of each month. The average ratio of the external to internal spectral power was 6.7 in 1976 and 7.4 in 1977. Focus positions for the India‐Siberia region external Sq current vortex, near 22° to 29°, were found to be at lower geomagnetic latitudes than for Europe and North America. The usual mid‐latitude vortex current, reaching 11.0 to 13.2 × 104 A in summertime, disappeared completely during winter months in both the years. The Sq current position was affected clearly by the off‐spin axis position of the north geomagnetic pole. Throughout the 2 years, we also found small current vorticies near 70° to 80° geomagnetic latitude, which we ascribed to auroral region activity on quiet days.

Pc3 activity at low geomagnetic latitudes: A comparison with solar wind observations

On an hourly time-scale the different roles of the solar wind and interplanetary magnetic field (IMF) parameters on ground micropulsation activity can be better investigated than at longer timescales. A long-term comparison between ground measurements made at L'Aquila ( L ≌ 1.6) and IMP 8 observations confirms the solar wind speed as the key parameter for the onset of pulsations even at low latitudes, although additional control of the energy transfer from the interplanetary medium to the Earth's magnetosphere is clearly exerted by the cone angle. Above ≌-20 mHz the frequency of pulsations is confirmed to be closely related to the IMF magnitude while, in agreement with model predictions, the IMF magnitude is related to the amplitude of the local fundamental resonant mode. We provide an interesting example in which high resolution measurements simultaneously obtained in the foreshock region and on the ground show that external transversal fluctuations do not penetrate deep into the low latitude magnetosphere.

Relationship between field line resonance at low geomagnetic latitudes and solar wind structures

The link between the pulsation activity in the local resonant band at a low latitude station and the macroscale structure of the interplanetary medium has been investigated. The experimental results show a clear dependence of the amplitude of the local resonant events on the interplanetary magnetic field magnitude and a preferable occurrence in correspondence of the compression regions preceding the high velocity solar wind streams. These features appear consistent with an upstream wave source for these events.