Two Anomalies in the Ionosphere

Appleton, E.V.

doi:10.1038/157691a0

Cited by 664 publications

(378 citation statements)

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“…While the ionosphere as a whole can reasonably be defined as horizontally stratified, the electrodynamic coupling of the E and F regions produces complex structure and marked meridional gradients in the plasma distribution, thus affecting the distribution of electric currents. In our area of interest, the largest of these structures is the Appleton anomaly [Appleton, 1946], also known as the equatorial ionization anomaly (EIA), characterized by two "crests" of enhanced plasma density flanking the magnetic dip equator at tropical latitudes [Alken and Maus, 2010;Alken et al, 2011]. The EIA is the main ionospheric phenomenon resolved in this study-here we will briefly describe the electrodynamic environment affecting it.…”

Section: Low-latitude Ionospheric Electrodynamicsmentioning

confidence: 99%

Ionospheric midlatitude electric current density inferred from multiple magnetic satellites

et al. 2013

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[1] A method for inferring zonal electric current density in the mid-to-low latitude F region ionosphere is presented. We describe a method of using near-simultaneous overflights of the Ørsted and CHAMP satellites to define a closed circuit for an application of Ampère's integral law to magnetic data. Zonal current density from sources in only the region between the two satellites is estimated for the first time. Six years of mutually available vector magnetic data allows overlaps spanning the full 24 h range of local time twice. Solutions are computed on an event-by-event basis after correcting for estimates of main and crustal magnetic fields. Current density in the range˙0.1 A/m 2 is resolved, with the distribution of electric current largely matching known features such as the Appleton anomaly. The currents appear unmodulated at times of either high-negative Dst or high F 10.7 , which has implications for any future efforts to model their effects. We resolve persistent current intensifications between geomagnetic latitudes of 30 and 50 ı in the postmidnight, predawn sector, a region typically thought to be relatively free of electric currents. The cause of these unexpected intensifications remains an open issue. We compare our results with current density predictions made by the Coupled Thermosphere-Ionosphere-Plasmasphere model, a self-consistent, first-principles, three-dimensional numerical dynamic model of ionospheric composition and temperatures. This independent validation of our current density estimates highlights good agreement in the broad spatiotemporal trends we identify, which increases confidence in our results.

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Section: Low-latitude Ionospheric Electrodynamicsmentioning

confidence: 99%

Ionospheric midlatitude electric current density inferred from multiple magnetic satellites

et al. 2013

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“…Each of these effects has different weight depending on the geomagnetic/geographic location, solar and geomagnetic activity, season and local time. While production is mainly controlled by solar EUV radiation, transport at the low latitude region is dominated by the equatorial fountain effect (Appleton 1946;Martyn 1955;Duncan 1959;Ross 1966;Das Gupta & Basu 1973;Richmond et al 1976;Balan et al 1993;Doherty et al 2000). The electron production by photo-ionization of neutral atoms and molecules is proportional to the intensity of solar radiation, governed by the Beer Lambert law.…”

Section: Model Developmentmentioning

confidence: 99%

An empirical model of ionospheric total electron content (TEC) near the crest of the equatorial ionization anomaly (EIA)

Hajra

Chakraborty²,

Tsurutani

et al. 2016

J. Space Weather Space Clim.

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We present a geomagnetic quiet time (Dst > À50 nT) empirical model of ionospheric total electron content (TEC) for the northern equatorial ionization anomaly (EIA) crest over Calcutta, India. The model is based on the 1980-1990 TEC measurements from the geostationary Engineering Test Satellite-2 (ETS-2) at the Haringhata (University of Calcutta, India: 22.58°N, 88.38°E geographic; 12.09°N, 160.46°E geomagnetic) ionospheric field station using the technique of Faraday rotation of plane polarized VHF (136.11 MHz) signals. The ground station is situated virtually underneath the northern EIA crest. The monthly mean TEC increases linearly with F 10.7 solar ionizing flux, with a significantly high correlation coefficient (r = 0.89-0.99) between the two. For the same solar flux level, the TEC values are found to be significantly different between the descending and ascending phases of the solar cycle. This ionospheric hysteresis effect depends on the local time as well as on the solar flux level. On an annual scale, TEC exhibits semiannual variations with maximum TEC values occurring during the two equinoxes and minimum at summer solstice. The semiannual variation is strongest during local noon with a summer-to-equinox variability of 50-100 TEC units. The diurnal pattern of TEC is characterized by a pre-sunrise (0400-0500 LT) minimum and near-noon (1300-1400 LT) maximum. Equatorial electrodynamics is dominated by the equatorial electrojet which in turn controls the daytime TEC variation and its maximum. We combine these long-term analyses to develop an empirical model of monthly mean TEC. The model is validated using both ETS-2 measurements and recent GNSS measurements. It is found that the present model efficiently estimates the TEC values within a 1-r range from the observed mean values.

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“…2 that the ionosphere above the two sites chosen for the SKA are subject to the Equatorial Ionization Anomaly (EIA). These two regions of enhanced plasma density are located approximately 15 degrees north and south of the magnetic dip equator (McDonald et al 2011) and are the result of the equatorial fountain effect (Appleton 1946). We note that the SKA will suffer far higher levels of ionospheric Faraday rotation than at the LOFAR sites, as the southern component of the EIA can pass directly above both locations.…”

Section: Ionospheric Rm Variationmentioning

confidence: 99%

Calibrating high-precision Faraday rotation measurements for LOFAR and the next generation of low-frequency radio telescopes

Sotomayor-Beltran

Sobey

Hessels

et al. 2013

A&A

130

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Faraday rotation measurements using the current and next generation of low-frequency radio telescopes will provide a powerful probe of astronomical magnetic fields. However, achieving the full potential of these measurements requires accurate removal of the time-variable ionospheric Faraday rotation contribution. We present ionFR, a code that calculates the amount of ionospheric Faraday rotation for a specific epoch, geographic location, and line-of-sight. ionFR uses a number of publicly available, GPS-derived total electron content maps and the most recent release of the International Geomagnetic Reference Field. We describe applications of this code for the calibration of radio polarimetric observations, and demonstrate the high accuracy of its modeled ionospheric Faraday rotations using LOFAR pulsar observations. These show that we can accurately determine some of the highest-precision pulsar rotation measures ever achieved. Precision rotation measures can be used to monitor rotation measure variations -either intrinsic or due to the changing line-of-sight through the interstellar medium. This calibration is particularly important for nearby sources, where the ionosphere can contribute a significant fraction of the observed rotation measure. We also discuss planned improvements to ionFR, as well as the importance of ionospheric Faraday rotation calibration for the emerging generation of low-frequency radio telescopes, such as the SKA and its pathfinders.

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Two Anomalies in the Ionosphere

Cited by 664 publications

References 0 publications

Ionospheric midlatitude electric current density inferred from multiple magnetic satellites

Ionospheric midlatitude electric current density inferred from multiple magnetic satellites

An empirical model of ionospheric total electron content (TEC) near the crest of the equatorial ionization anomaly (EIA)

Calibrating high-precision Faraday rotation measurements for LOFAR and the next generation of low-frequency radio telescopes

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