Harmonic analysis and an empirical model for TEC over Palehua

Unnikrishnan, K. P.; Nair, R. Balachandran; Venugopal, Chandu

doi:10.1016/s1364-6826(02)00187-6

Cited by 33 publications

(32 citation statements)

References 13 publications

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“…This asymmetry in solar activity variations of the scale height could suppress the equinoctial difference in NmF2 increase rate that featured by higher increase rate at SE than at ME and result in larger increase of TEC with P at ME than at SE. It is mentionable that our result shows the increase rate of TEC with P is higher at ME than at SE, in contrast, the result of Unnikrishnan et al (2002) implies that the increase of TEC with solar activity is larger at SE than at ME, namely higher increase rate at SE than at ME. That maybe is due to that the influence of the inter-annual change of solar activity was not taken into account in Unnikrishnan et al (2002).…”

Section: Discussioncontrasting

confidence: 72%

“…It is mentionable that our result shows the increase rate of TEC with P is higher at ME than at SE, in contrast, the result of Unnikrishnan et al (2002) implies that the increase of TEC with solar activity is larger at SE than at ME, namely higher increase rate at SE than at ME. That maybe is due to that the influence of the inter-annual change of solar activity was not taken into account in Unnikrishnan et al (2002). The F 10.7 index obviously indicates the differences of solar activity between ME and SE in 1981 and 1984.…”

Section: Discussioncontrasting

confidence: 72%

“…They revealed that the equinoctial asymmetry is mainly a low-latitude phenomenon during solar minima. It is interesting that Unnikrishnan et al (2002) showed that TEC (total electron content) over Palehua (19 • N,206 • E) exhibits opposite equinoctial asymmetries between high and low solar activities. They showed that Palehua TEC is higher at SE than at ME during high solar activity year 1981, while it is higher at ME than at SE during low solar activity year 1984.…”

Section: Introductionmentioning

confidence: 99%

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Equinoctial asymmetry in solar activity variations of <I>Nm</I>F2 and TEC

et al. 2012

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Abstract. The ionosonde NmF2 data (covering several solar cycles) and the JPL TEC maps (from 1998 through 2009) were collected to investigate the equinoctial asymmetries in ionospheric electron density and its variation with solar activity. With solar activity increasing, the equinoctial asymmetry of noontime NmF2 increases at middle latitudes but decreases or changes little at low latitudes, while the equinoctial asymmetry of TEC increases at all latitudes. The latitudinal feature of the equinoctial asymmetry at high solar activity is different from that at low solar activity. The increases of NmF2 and TEC with the solar proxy P = (F 10.7 + F 10.7A )/2 also show equinoctial asymmetries that depend on latitudes. The increase rate of NmF2 with P at March equinox (ME) is higher than that at September equinox (SE) at middle latitudes, but the latter is higher than the former at the EIA crest latitudes, and the difference between them is small at the EIA trough latitudes. The phenomenon of higher increase rate at SE than at ME does not appear in TEC. The increase rate of noontime TEC with P at ME is higher than that at SE at all latitudes, and the difference between them peaks at both sides of dip equator. It is mentionable that the equinoctial asymmetries of NmF2 and TEC increase rates present some longitudinal dependence at low latitude. The influences of equinoctial differences in the thermosphere and ionospheric dynamics processes on the equinoctial asymmetry of the electron density were briefly discussed.

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Section: Discussioncontrasting

confidence: 72%

Section: Discussioncontrasting

confidence: 72%

Section: Introductionmentioning

confidence: 99%

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Equinoctial asymmetry in solar activity variations of <I>Nm</I>F2 and TEC

et al. 2012

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“…In recent years, a database of TEC in the ionosphere and plasmasphere, derived from a worldwide network of global positioning system (GPS) observations, has been used to investigate the local and regional characteristics of various anomalies (Huang and Cheng et al, 1996;Unnikrishnan et al, 2002;Wu et al, 2004). Since the early 1990s, a worldwide network of permanent GPS tracking stations has rapidly grown under the management of the International GNSS (Global Navigation Satellite Systems) Service, known as IGS.…”

Section: Introductionmentioning

confidence: 99%

Features of annual and semiannual variations derived from the global ionospheric maps of total electron content

et al. 2007

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Abstract.In the present work we use the NASA-JPL global ionospheric maps of total electron content (TEC), firstly to construct TEC maps (TEC vs. magnetic local time MLT, and magnetic latitude MLAT) in the interval from 1999 to 2005. These TEC maps were, in turn, used to estimate the annualto-mean amplitude ratio, A 1 , and the semiannual-to-mean amplitude ratio, A 2 , as well as the latitudinal symmetrical and asymmetrical parts, A and A of A 1 . Thus, we investigated in detail the TEC climatology from maps of these indices, with an emphasis on the quantitative presentation for local time and latitudinal changes in the seasonal, annual and semiannual anomalies of the ionospheric TEC. Then we took the TEC value at 14:00 LT to examine various anomalies at a global scale following the same procedure. Results reveal similar features appearing in NmF2, such as that the seasonal anomaly is more significant in the near-pole regions than in the far-pole regions and the reverse is true for the semiannual anomaly; the winter anomaly has least a chance to be observed at the South America and South Pacific areas. The most impressive feature is that the equinoctial asymmetry is most prominent at the East Asian and South Australian areas. Through the analysis of the TIMED GUVI columnar [O/N2] data, we have investigated to what extent the seasonal, annual and semiannual variations can be explained by their counterparts in [O/N2]. Results revealed that the [O/N2] variation is a major contributor to the daytime winter anomaly of TEC, and it also contributes to some of the semiannual and annual anomalies. The contribution to the anomalies unexplained by the [O/N2] data could possibly be due to the dynamics associated with thermospheric winds and electric fields.

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“…Compared with previous studies, (Zhao et al 2007;Unnikrishana et al 2002), we consider a more advanced model that includes not only the pure sinusoidal signals but also a few modulated ones. Then, the numerical results based on the application of the proposed method to the TEC time series are presented.…”

Section: Introductionmentioning

confidence: 99%

Harmonic analysis of total electron contents time series: methodology and results

Amiri-Simkooei

Asgari

2011

GPS Solut

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In an attempt to model regular variations of the ionosphere, the least-squares harmonic estimation is applied to the time series of the total electron contents (TEC) provided by the JPL analysis center. Multivariate and modulated harmonic estimation spectra are introduced and estimated for the series to detect the regular and modulated dominant frequencies of the periodic patterns. Two significant periodic patterns are the diurnal and annual signals with periods of 24/n hours and 365.25/n days (n = 1, 2, …), which are the Fourier series decomposition of the regular daily and yearly periodic variations of the ionosphere. The spectrum shows a cluster of periods near 27 days, thereby indicating irregularities at this solar cycle period. A series of peaks, with periods close to the diurnal signal and its harmonics, are evident in the spectrum. In fact, the daily signal harmonics of x i = 2pi are modulated with the annual signal harmonics of x j = 2pj/365.25 as x ijM = 2pi(1 ± j/365.25i). Among them, at low and midlatitudes, the largest variations belong to the diurnal signal modulated to the semiannual signal. Some preliminary results on the modulated part are presented. The maximum ranges of the modulated daily signal are ±15 TECU and ±6 TECU at high and low solar periods, respectively. A model consisting of purely harmonic functions plus modulated ones is capable of studying known regular anomalies of the ionosphere, which is currently in progress.

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Harmonic analysis and an empirical model for TEC over Palehua

Cited by 33 publications

References 13 publications

Equinoctial asymmetry in solar activity variations of <I>Nm</I>F2 and TEC

Equinoctial asymmetry in solar activity variations of <I>Nm</I>F2 and TEC

Features of annual and semiannual variations derived from the global ionospheric maps of total electron content

Harmonic analysis of total electron contents time series: methodology and results

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Harmonic analysis and an empirical model for TEC over Palehua

Cited by 33 publications

References 13 publications

Equinoctial asymmetry in solar activity variations of &lt;I&gt;Nm&lt;/I&gt;F2 and TEC

Equinoctial asymmetry in solar activity variations of &lt;I&gt;Nm&lt;/I&gt;F2 and TEC

Features of annual and semiannual variations derived from the global ionospheric maps of total electron content

Harmonic analysis of total electron contents time series: methodology and results

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Equinoctial asymmetry in solar activity variations of <I>Nm</I>F2 and TEC

Equinoctial asymmetry in solar activity variations of <I>Nm</I>F2 and TEC