Browsing by Author "Anderson, David"
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Item Open Access Daytime vertical E × B drift velocities inferred from ground‐based magnetometer observations at low latitudes(American Geophysical Union, 2004-11) Anderson, David; Anghel, Adela; Chau Chong Shing, Jorge Luis; Veliz, OscarThe daytime equatorial electrojet is a narrow band of enhanced eastward current flowing in the 100–120 km altitude region within ±2° latitude of the dip equator. A unique way of determining the daytime strength of the electrojet is to observe the difference in the magnitudes of the horizontal (H) component between a magnetometer placed directly on the magnetic equator and one displaced 6°–9° away. The difference between these measured H values provides a direct measure of the daytime electrojet current and, in turn, the magnitude of the vertical E × B drift velocity in the F region ionosphere. This paper discusses a recent study where 27 months of magnetometer H component observations and daytime, vertical E × B drift velocities were obtained in the Peruvian longitude sector between August 2001 and December 2003. In order to establish the relationships between ΔH and E × B drift velocities for the 270 days of observations, three approaches were chosen: (1) a linear regression analysis, (2) a multiple regression approach, and (3) a neural network approach. The neural network method gives slightly lower RMS error values compared with the other two methods. The relationships for all three techniques are validated using an independent set of E × B drift observations from the Jicamarca incoherent scatter radar (ISR) located at Jicamarca, Peru. The techniques presented here will be incorporated into a recently developed, real‐time Global Assimilation of Ionospheric Measurements (GAIM) model.Item Restricted Estimating daytime vertical ExB drift velocities in the equatorial F‐region using ground‐based magnetometer observations(American Geophysical Union, 2002-06-28) Anderson, David; Anghel, Adela; Yumoto, Kiyohumi; Ishitsuka, Mutsumi; Kudeki, ErhanThe daytime equatorial electrojet is a narrow band of enhanced eastward current flowing in the 100 to 120 km altitude region within ±2° latitude of the dip equator. A unique way of determining the daytime strength of the electrojet is to observe the difference in the magnitudes of the Horizontal (H) component between a magnetometer placed directly on the magnetic equator and one displaced 6 to 9 degrees away. The difference between these measured H values provides a direct measure of the daytime electrojet current, and in turn, the magnitude of the vertical ExB drift velocity in the F region ionosphere. This paper discusses a recent study that has established the quantitative relationship between the vertical ExB drift velocity in the ionospheric F region and the daytime strength of the equatorial electrojet in the South American (west coast) longitude sector.Item Open Access Global, low‐latitude, vertical E × B drift velocities inferred from daytime magnetometer observations(American Geophysical Union, 2006-08) Anderson, David; Anghel, Adela; Chau Chong Shing, Jorge Luis; Yumoto, KiyohumiNavigation and communication, Department of Defense and civilian, customers rely on accurate, low-latitude specification of ionospheric parameters, globally, that are not currently realistic on a day-to-day basis. This paper describes, demonstrates, and speculates about the data sets that are required inputs to the operational ionospheric models that will correct these deficiencies. In order to investigate quiet time, vertical E × B drift velocities at two different longitude sectors, magnetometer observations were obtained for the period between January 2001 and December 2004 from the magnetometers at Jicamarca (0.8N dip latitude) and Piura (6.8N dip latitude) in Peru and from Davao (1.4S dip latitude) and Muntinlupa (6.3N dip latitude) in the Philippine sector. We choose only geomagnetically “quiet” days, when the 3-hourly Kp value never exceeds a value of 3 over the entire day, and when the daily Ap value is less than 10. These are “binned” into three seasons, December solstice, equinox, and June solstice periods. A neural network trained for the Peruvian sector was applied to each of the days in both the Peruvian and Philippine sectors, providing ΔH-inferred vertical E × B drift velocities between 0700 and 1700 local time. For each season, the average E × B drift velocity curves are compared with the Fejer-Scherliess, climatological E × B drift velocity curves in both the Peruvian and Philippine sectors. In the Peruvian sector, the comparisons are excellent, and in the Philippine sector they are very good. We demonstrate that realistic magnetometer-inferred E × B drifts can be obtained in the Peruvian sector on a day-to-day basis and speculate that on the basis of the average, quiet day comparisons, realistic E × B drifts can be obtained on quiet days in the Philippine sector.Item Restricted Interplanetary electric fields and their relationship to low-latitude electric fields under quiet and disturbed conditions(Elsevier, 2007-07) Anghel, Adela; Anderson, David; Maruyama, Naomi; Chau Chong Shing, Jorge Luis; Yumoto, Kiyo; Bhattacharyya, Archana; Alex, S.Recent studies have demonstrated that ground-based magnetometer observations can be used to infer realistic, daytime vertical E×B drift velocities in the Peruvian and Philippine longitude sectors. It has also been demonstrated that under certain conditions the time variability in the interplanetary electric field (IEF)—minutes to hours—is reflected in the daytime, prompt penetration of high-latitude electric fields to low latitudes. In this paper, we incorporate magnetometer-inferred E×B drift techniques to extend this study to include the Indian sector E×B drift velocities and to investigate the relationships between IEF conditions and daytime, low-latitude electric field observations under both geomagnetically quiet and disturbed conditions. This paper addresses several basic questions related to the relationships between IEF conditions and low-latitude east west electric fields. (1) When low-latitude electric fields exhibit quiet-time, Sq-type behavior, what are the IEF conditions? (2) Under disturbed conditions, what are the relationships between the IEF parameters and the low-latitude electric fields in the Peruvian, Philippine, and Indian longitude sectors? (3) If the three longitude sector electric field responses are similar under disturbed conditions, is the response consistent with the current ideas put forward at the Millstone Hill Workshop on promptly penetrating electric fields and over-shielding effects at low latitudes? We address the above questions by analyzing magnetometer-inferred E×B drift velocities between January 2001 and December 2004 when there exists more than 500 quiet days and more than 235 geomagnetically disturbed days, defined by daily Ap values greater than 20. It is suggested that the neural network approach that provides realistic E×B drift velocities based on magnetometer observations can be applied at any longitude where appropriately placed magnetometers exist. It is found that: (1) the average quiet, daytime upward E×B drift velocity vs. LT in the Indian sector is comparable to the average velocity vs. LT in the Peruvian sector and both are roughly 3 5 m/s less than the values in the Philippine sector; (2) under quiet conditions, the peak velocity occurs at 1100 LT in the Peruvian sector and at 1000 LT in both the Philippine and Indian sectors; and (3) during disturbed conditions, it is observed that daytime, promptly penetrating electric fields occur, simultaneously, in the Philippine, Indian and Peruvian sectors, in response to fluctuating IEF conditions.Item Restricted Relating the interplanetary-induced electric fields with the low-latitude zonal electric fields under geomagnetically disturbed conditions(American Geophysical Union, 2013-03-21) Anghel, Adela; Anderson, David; Chau Chong Shing, Jorge Luis; Yumoto, Kiyohumi; Bhattacharyya, ArchanaThe overall ionospheric variability with periods ranging from long-term, secular changes to days, hours, and even minutes and seconds, is influenced by the solar activity, geomagnetic activity, and processes originating in the lower atmospheric layers. Using a wavelet transform approach, in this paper, we study the short-term (minutes to hours) and day-to-day variability of the ionospheric low-latitude zonal electric fields (LLZEF) at three longitude sectors, Peruvian, Philippine, and Indian, during time intervals of increased geomagnetic activity and relate the LLZEF variability to changes in the dawn-to-dusk component of the interplanetary electric field (IEF). Continuous Morlet wavelet and cross-wavelet amplitude spectra with reduced and increased frequency resolutions were obtained to analyze and compare the oscillation activity in the LLZEF and IEF spectra, in the 10-min to 10-h and 1.25- to 12-d period ranges. For the 1.25- to 12-d period range, periodicities in the LLZEF spectrum were compared with similar periodicities in the IEF spectrum over 9 February to 9 June 2001, with our wavelet results indicating the geomagnetic activity as an important driver of LLZEF variability in this period range. For the 10-min to 10-h period range, four case studies were examined when concurrent observations of Jicamarca incoherent scatter radar zonal electric field and IEF, as calculated from the ACE satellite solar wind velocity and interplanetary magnetic field data, were available. We show that the wavelet transform represents a powerful tool to study the frequency dependence of the two specific mechanisms of ionospheric electric field variability, which are dominant during geomagnetic storms, namely penetration and disturbance dynamo.Item Restricted Sudden stratospheric warming event signatures in daytime ExB drift velocities in the Peruvian and Philippine longitude sectors for January 2003 and 2004(American Geophysical Union, 2010-08-28) Anderson, David; Araujo‐Pradere, Eduardo A.This paper first describes the technique that has been developed to obtain realistic, daytime, vertical ExB drift velocities in the equatorial ionosphere using two ground‐based magnetometers, one on the magnetic equator and the other located ±6°–9° away in latitude. This technique is then employed to study the unique ExB drift signatures associated with sudden stratospheric warming events (SSW) in both the Peruvian and Philippine longitude sectors, occurring in January 2003 and January 2004. It is found that the semidiurnal shaped signature first appears in the Peruvian sector and 3 days later appears in the Philippine sector. In both sectors, the ExB drift signature lasts for approximately 5 days.Item Restricted The spectral properties of low-latitude daytime electric fields inferred from magnetometer observations(Elsevier, 2007-03-24) Nicolls, Michael J.; Kelley, Michael C.; Chau Chong Shing, Jorge Luis; Veliz, Oscar; Anderson, David; Anghel, AdelaFour years of magnetometer data from two locations in Peru, one at the equator and one off the equator, have been converted to electric fields and their frequency characteristics (fluctuation spectra) examined. In the frequency range from 0.1 to 30 cycles per hour, the average spectrum monotonically decreases. However, it deviates from a power law in the range 0.3–3 cycles per hour especially for high levels of activity. The integrated power above 0.15 cycles per hour is a strong function of Kp indicating that much of the fluctuations in the ionospheric equatorial field are of solar wind or magnetospheric origin. This result is in agreement with a previous power spectral study of low, middle, and high latitude fields using radars. The observed field strengths are lower than the ones observed in a previous study using balloon data at middle and high latitudes when the fields are projected to the equatorial plane. Simultaneous interplanetary electric field (IEF) data are compared to the equatorial field to determine how strong a relationship exists and to determine the amplitude and phase of their ratio as a function of frequency—an estimate of the average transfer function of the system. This function displays a bandpass-like form with a peak near 0.5 cycles per hour. This peak and evidence for a ringing of the time domain response suggests a weakly resonant system indicating some capacitance in addition to the inductance of the ring current and the resistance of the ionosphere. Case studies show that application of this function to IEF data yields good results and supports the notion that the response of the equatorial field to long-duration IEF polarities can last for many hours. Application of the function to test inputs such as pulses and triangle waves support this result. At high frequencies, we suggest that mapping of small-scale MHD turbulence is less effective than high frequency related transitions in the IEF.