F-region Parameters Research Articles

Asymmetric behaviour and geomagnetic dependencies of zonal winds in the middle latitude upper thermosphere

In this study we use the CHAMP wind observations from 2002 to 2004 to investigate features of mid-latitude thermospheric winds at low and high geomagnetic activity levels. The winds are observed between the geomagnetic latitudes 40–50° north or south within an altitude range of 370–450 Km and averaged longitudinally. The low geomagnetic activity level wind generally leads eastward in both hemispheres for all the seasons. Maximum westward speeds going above 100 m/s are observed at all levels of geomagnetic activity. Most investigations on F-region parameters assume the equinoxes are fundamentally the same. A first time separation of Equinox season using CHAMP winds reveals equinoctial asymmetric behavior of the wind speed at the two levels considered. Interhemispheric asymmetry in neutral wind behaviour is clearly evident in the solstice seasons with local winds considered in each of the hemispheres. This is consistent with asymmetry in the amount of solar illumination received in each of the hemispheres and differences in the magnetic field strength at magnetically conjugate regions.

Day-to-Day Variability of Low Latitude F-Region Ionosphere During Low Solar Activity

This paper presents an analysis of the F-region variability of the ionospheric parameters [foF2and M(3000)F2] at three low latitude stations during low solar activity periods from January 2006 to December 2010. The diurnal, seasonal and day-to-day characteristics of theseionospheric F-region parameters have been studied in detail. With the use of partial correlation analysis we have computed the correlation between the variation of these parameters and other factors such as the solar radio flux 10.7cm (F10.7)and the geomagnetic AP index. It has been observed that the magnitude of the variability of propagation parameter M(3000)F2aresmoother and smaller than in that case of foF2. It is also observed that for both parameters there are no remarkable changes in day-to-day variability with geomagnetic activity during low solar activity.

Studies of ionospheric F-region response in the Latin American sector during the geomagnetic storm of 21–22 January 2005

Abstract. In the present investigation, we have studied the response of the ionospheric F-region in the Latin American sector during the intense geomagnetic storm of 21–22 January 2005. This geomagnetic storm has been considered "anomalous" (minimum Dst reached −105 nT at 07:00 UT on 22 January) because the main storm phase occurred during the northward excursion of the Bz component of interplanetary magnetic fields (IMFs). The monthly mean F10.7 solar flux for the month of January 2005 was 99.0 sfu. The F-region parameters observed by ionosondes at Ramey (RAM; 18.5° N, 67.1° W), Puerto Rico, Jicamarca (JIC; 12.0° S, 76.8° W), Peru, Manaus (MAN; 2.9° S, 60.0° W), and São José dos Campos (SJC; 23.2° S, 45.9° W), Brazil, during 21–22 January (geomagnetically disturbed) and 25 January (geomagnetically quiet) have been analyzed. Both JIC and MAN, the equatorial stations, show unusually rapid uplifting of the F-region peak heights (hpF2/hmF2) and a decrease in the NmF2 coincident with the time of storm sudden commencement (SSC). The observed variations in the F-region ionospheric parameters are compared with the TIMEGCM model run for 21–22 January and the model results show both similarities and differences from the observed results. Average GPS-TEC (21, 22 and 25 January) and phase fluctuations (21, 22, 25, 26 January) observed at Belem (BELE; 1.5° S, 48.5° W), Brasilia (BRAZ; 15.9° S, 47.9° W), Presidente Prudente (UEPP; 22.3° S, 51.4° W), and Porto Alegre (POAL; 30.1° S, 51.1° W), Brazil, are also presented. These GPS stations belong to the RBMC/IBGE network of Brazil. A few hours after the onset of the storm, large enhancements in the VTEC and NmF2 between about 20:00 and 24:00 UT on 21 January were observed at all the stations. However, the increase in VTEC was greatest at the near equatorial station (BELE) and enhancements in VTEC decreased with latitude. It should be pointed out that no phase fluctuations or spread-F were observed in the Latin American sector during the post-sunset pre-reversal time in the geomagnetic disturbance (21 January). The disturbance dynamo electric field possibly resulted in downward drift of the F-region plasma and inhibited the formation of spread-F.

Dependence of the F-region peak electron density (foF2) on solar activity observed in the equatorial ionospheric anomaly region in the Brazilian sector

The long-term (solar cycle) changes in the Sun and how it affects the ionospheric F-region observed at São José dos Campos (23.2° S, 45.9° W), Brazil, a location under the southern crest of the equatorial ionospheric anomaly, have been investigated in this paper. The dependence of the F-region peak electron density (foF2) on solar activity during the descending phase of the 23rd solar cycle for the periods of high, medium, and low solar activity has been studied. The ionospheric F-region peak electron densities observed during high and medium solar activity show seasonal variations with maxima close to the equinox periods, whereas during the low solar activity the maxima during the equinox periods is absent. However, during the low solar activity only change observed is a large decrease from summer to winter months. We have further investigated changes in the different ionospheric F-region parameters (minimum virtual height of the F-region (h′F), virtual height at 0.834foF2 (hpF2), and foF2) during summer to winter months in low solar activity periods, 2006–2007 and 2007–2008. Large changes in the two ionospheric parameters (hpF2 and foF2) are observed during summer to winter months in the two low solar activity periods investigated.

Diurnal and seasonal variations in h’F and foF2 over low (Indian) and mid (Japanese) latitude stations — a comparative study

Using the simultaneous data of ionosonde from a low latitude station, Waltair, India (17.7°N,83.3°E) in the Indian sector and a mid latitude station, Kokubunji, Japan (35.7°N,139.5°E) in the Japanese sector during the high sunspot year of 2001, a comparative study of the variations in the diurnal, seasonal and day to day characteristics of the ionospheric F-region parameters, such as the virtual height of the F-layer (h’F) and the critical frequency of the F2 layer (foF2), are studied. The effect of geomagnetic activity variations of the F-region parameters over the low and mid latitude stations is also presented.

Response of the equatorial and low-latitude ionosphere during the space weather events of April 2002

Abstract. A digital ionosonde, of the type known as Canadian Advanced Digital Ionosonde (CADI), has been in routine operation at São José dos Campos (23.2° S, 45.9° W; dip latitude 17.6° S), Brazil, since August 2000. A new CADI was installed at Palmas (10.2° S, 48.2° W; dip latitude 5.5° S), Brazil, in April 2002. The two CADIs are part of a new network being established in a collaborative program between UNIVAP and CEULP/ULBRA, to study the equatorial and low-latitude ionospheric regions in Brazil. In this paper we present and discuss the effects of the space weather events during 17-20 April 2002, as evidenced by the ionospheric parameter changes from simultaneous observations at São José dos Campos and Palmas. A comparison of the observed ionospheric parameters (h'F, hpF2 and foF2) at the two stations, separated only by about 1460km, shows both similarities and differences associated with the geomagnetic disturbances. Also, a comparison of the observed F-region parameters with the ASPEN-TIMEGCM model results is presented. The model results show reasonable agreement during the quiet period of observations, but some discrepancies have been observed during the geomagnetic disturbances.

The polar ionosphere

Photometers monitoring oxygen night airglow emissions and a Canadian advanced digital ionosonde (CADI) have given a complementary view of the polar ionosphere over Eureka, Canada (80°N, 86°W) through each winter since 1991. The 630 nm [O] emission intensity, in particular, is seen to be directly proportional to the ƒ oF2 critical frequency measured from the CADI. We report on the variations of these F-region parameters through the past solar cycle, drawing on observations from this site very near the north magnetic pole. [O] emission intensities even under quiet conditions are seen to vary over a factor of 10 or more through the solar cycle. During much of the time the airglow is enhanced by either polar auroras or F-layer patches (depending on the sign of the IMF B z ), each with their particular signatures. The extent and configuration of the polar cap, as a function of the B z component of the IMF and the characteristics of the F-laver hatches within will be illustrated.

Analysis of storm-time low-latitude simultaneous ionospheric and nightglow emission measurements

The first simultaneous ground-based observations of the permitted atomic oxygen OI 777.4 nm emission line and the forbidden OI 630.0 and 557.7 nm emission lines have been carried out from Cachoeira Paulista (22.7°S, 45.0°W) and Fortaleza (3.9°S, 38.4°W), Brazil, during the period November 1986–March 1988. In addition, N 2 + 391.4 nm emission observations have been carried out at Cachoeira Paulista during the same period. At both stations ionosonde measurements are carried out on a routine basis. These simultaneous multispectral optical and ionospheric measurements have made possible studies on the response of the equatorial and low-latitude ionospheric F-region to magnetic storms. A comparison of the observations obtained during magnetically quiet and disturbed conditions shows that the nocturnal intensity variations of the atomic oxygen emissions observed are well correlated with the variations of the F-region parameters ( ƒ0F2 and h'F), showing the dominant role played by the enhanced F-region dynamical processes during disturbed periods, possibly induced by substorm-associated electric fields and disturbed neutral winds in the low-latitude F-region. While the N 2 + emission is enhanced in disturbed periods, as expected from energetic neutral particle precipitation, any enhancements in the atomic oxygen emissions were not detectable against the airglow due to recombination processes.

Reliability of electron density profiles

Deviations from horizontal stratification in the F-region can cause significant errors in electron density profile calculations from ionograms. Such situations exist every day during sunrise and sunset. Angle of arrival measurements and studies of the variation of other F-region parameters indicate that gravity waves are frequently strong enough to produce effects of comparable magnitudes. Ray tracing model studies permit a first order estimate of the resulting errors which are largest for the peak parameters.

Conjugate effects in the generation of travelling ionospheric disturbances (TIDs) in the F-region

A rapid onset of auroral absorptions was simultaneously recorded by a chain of standard riometers, situated in the northern and southern magneto-conjugate areas, during a period of pronounced substorm activity. The first absorption peak was followed by sequential disturbance patterns in the occurrence of the F-region parameters, virtual height ( h′ F) and spread - F, as deduced from the standard ionosonde data obtained over a wide range of latitudes in both hemispheres. The disturbances were consistent with the simultaneous occurrences of separate trains of large-scale ionospheric disturbances (TIDs), propagating equatorwards from the southern and northern auroral zones. It is suggested that TIDs were generated by an impulse-like increase in the conjugate particle precipitations, inferred from the riometer records. The precipitation pattern was limited to a high-latitude shell whose equatorwards edge was contained between L-values 5.0 and 5.3. The auroral sources of TIDs appeared to have large linear dimensions, extending at least 17 degrees in longitude.

Auroral riometer absorptions and the F-region disturbances observed over a wide range of latitudes

Standard riometer data from a southern auroral station were compared with ionograms obtained at five stations positioned from sub-auroral to equatorial latitudes. The rapid onset in riometer absorption, during intense substorm activities in an equinoctial period, was associated with a sequential propagation of ionospheric disturbances deduced from the F-region parameters h′F and range spread- F. The time shift between absorption maxima and extrapolated commencement times of the disturbances was consistent with the presence of large-scale travelling ionospheric disturbances (TIDs), propagating equatorwards with velocities lying typically in the range 600–900 m s −1, and with a median velocity of 720 m s −1. It is suggested that the onset of TIDs is associated with high-energy particle precipitation, manifested by the occurrence of auroral absorption events. Similarity of absorption increases at the southern and northern conjugate points, found from a previous riometer study, would indicate that large-scale TIDs are simultaneously generated in both hemispheres.

The effects of geomagnetic activity on the F-region of the ionosphere

A study is made of the effects of geomagnetic activity on F-region ionization by correlating the F-region parameters and the Kp and AE indices, using delays up to 33 h. The total electron content from a single station for the years 1971–74 as well as ƒ 0F2 from a longitudinal chain of stations around 150°E for the years 1968–74 were used. Initially the analyses were carried out using all the geomagnetic data. They were then divided according to high or low activity in order to separate magnetic storm from quiet magnetic effects. Data sets for each station consisted of 2-monthly segments for each hour of the day for each of the years. This enabled latitudinal, seasonal and sunspot cycle effects on the correlations to be detected. Significant positive correlations with delays greater than a few hours dominated the winter results at lower latitudes; the correlations being predominantly negative at higher latitudes. The summer results at all latitudes were dominated by negative correlations. Significant negative correlations with Kp at delays between 0–3 h were obtained for summer and autumn from higher latitude stations probably indicative of electric fields. The time lags greater than 3 h are consistent with changes in atmospheric neutral composition brought about by modification of the global thermospheric circulation following heating in the auroral zones. Analysis of hmF2 data for a mid latitude station showed a negative correlation of the peak height of the F-region with magnetic activity.

Geomagnetic effects on the F-region of the ionosphere

The effects of geomagnetic activity on F-region ionization are investigated by calculating correlations between F-region parameters and the Kp-index, including delays of up to 33 hours. At Kingston, Jamaica, total electron content values are used, while at Ottawa, Churchill and Resolute Bay tabulated values of foF2 are subjected to correlation. The correlations are evaluated for each hour of the day in groups of two months so that seasonal variations can also be detected. Results indicate that it may be possible to separate geomagnetic effects according to whether the lag between Kp and the ionospheric effect is short (0–6 h), medium (9–18 h), or long (21–30 h). The various processes which might correspond to these categories are discussed and applied to the results from the four stations mentioned above.

Effects of E-region electric fields on F-region parameters at the magnetic equator

The effects of E-region electric fields on F-region parameters like height of constant electron density contours ( h N ) and semi-thickness, at an equatorial station, Trivandrum, are investigated. The E-region east-west electric field ( E y ) has been deduced using the horizontal magnetic field values from the ground magnetograms. It is found that the semi-thickness parameter and h N follow closely the E-region electric field ( E y ) variations in the forenoon and noon periods. In the afternoon, it is shown that there is close association between h N and E y , when the electric field variations are large. On a day-to-day basis also, h N and the E-region electric field show very good correlation in the forenoon and noon hours. It is also shown that higher levels are affected more than the lower levels in the F-region by E y .

Day-to-day variability of the quiet-day daily range of equatorial geomagnetic field and its relationship with ionospheric dynamics

To study the day-to-day variability of the daily range ΔH = H max − H min of the H component of geomagnetic field at the Equator, quiet days ( A p = 0–8) in the quiet sun year 1964 were divided into four ranges viz. 0–69 gamma, 70–89 gamma, 90–109 gamma and 110 or more gamma of ascending ΔH values at Trivandrum for the three seasons J, D, E separately. Average daily variation patterns of several parameters for these 12 groups were studied. It was found that enhanced ΔH at Trivandrum was well correlated with the northern S q current strength only in winter when the latter was weak. ΔH at Trivandrum was poorly correlated with ΔH at Alibag as also with solar radiation parameters (10.7 cm and X-ray fluxes) in all seasons, and moderately correlated with the southern component of interplanetary magnetic field B z t. In contrast, ΔH at Alibag was moderately correlated with the B x and B v components. Sevral other peculiarities of corresponding ΔY and ΔZ components and associated upheavals in ionospheric F-region parameters at equatorial and middle latitudes are presented and discussed. It is concluded that the day-to-day variability of ΔH at equatorial and middle latitudes is primarily connected with ionospheric upheavals which seem to be connected with different interplanetary parameters for different latitudes.

The lunar variations in the F2-region of the ionosphere at low latitudes

The suggestion by Martyn that the lunar tidal variations of the electron concentration in the F-region near the magnetic equator are due to the superposition of a small (lunar) vertical velocity on a large (solar) velocity is exterrded to examine the lunar variations in the F2-region over the range of latitudes within 25° of the magnetic equator. Because the form of the equatorial anomaly is modified by the lunar component, the lunar variations in electron density show a large latitudinal dependence. The variations of various F-region parameters have been calculated assuming equilibrium conditions and a comparison with experimental data shows that most of the observed features can be explained satisfactorily.

Electron concentrations and electron and ion temperatures in theFregion for magnetically quiet and disturbed conditions

Incoherent backscatter measurements of the F-region electron concentrations and electron and ion temperatures were made at Arecibo during the sudden commencement magnetic storm of June 15–16, 1965, and a few magnetically quiet days in the same season. The results obtained are used to describe the quiet and disturbed day diurnal behavior of the shape of the electron concentration profile, the contours of constant plasma frequency, the plasma density scale heights in the topside ionosphere, and the electron and ion temperatures. Storm-day deviations of some of the important F-region parameters were also calculated, and their temporal variations are presented. In response to the strong magnetic storm, the electron concentrations in the topside ionosphere are enhanced by a factor of 1.5–2.0 and the nighttime plasma density scale heights by a factor of 2.0–3.0. The electron temperatures under disturbed conditions are 200–300°K higher than the quiet day values during nighttime, whereas the daytime changes are within the limits of day to day variations. The changes in the ion temperatures, which are found to be appreciable during the daytime, suggest that under disturbed conditions the ion temperature is coupled to the neutral temperature to a greater height level than during quiet conditions. A significant amount of storm-associated heating of the nighttime F region is evident from the electron-to-ion temperature ratios that are found to be above 1.5 throughout the night in the height range 225–450 km.

Photoelectron heating efficiency in the ionosphere

Thomson scatter data taken at the Jicamarca Radar Observatory1 (11.95°S., 76.87°W.; surface dip ∼2°) during January 1967 are used to derive the heating efficiency of photoelectrons. It is found that a limiting value of approximately 15 ev per ion-pair formed is deposited in the electron gas at altitudes above 300 km. This value decreases to less than 1 ev per photoelectron near 200 km. The rapid change of heating efficiency with altitude causes the peak in the electron temperature to be at a rather higher altitude than previously estimated. Thermal coupling beween ions and electrons is the dominant electron heat loss mechanism both above and below the altitude where the peak value of Te occurs. To obtain reasonable values for the various F-region parameters it is necessary to assume that the electron loss coefficient decreases with temperature if laboratory rate coefficients measured at room temperature are used.

Seasonal variation of some F-region parameters at sun-spot minimum

Eleven months' data, consisting of midday values of the electron production, q45, and the ‘linear’ loss coefficient, β45, at χ = 45° for Godley Head and Campbell Island are presented. The latitudinal and seasonal variations in both q45 and β45 are discussed; the latter in terms of the effects of travelling ionospheric disturbances. Unlike sun-spot maximum data which show a broad winter minimum in β45, the sun-spot minimum data exhibit considerable fine-scale structure; and a late summer sudden decrease in β45 is included.

Variations of some ionospheric parameters over a solar cycle

Abstract The basic F-region parameters, rates of electron production and loss and the scale height of atomic oxygen are followed through a solar cycle. The flux of ionizing radiation shows a lag on the flux of solar radio noise around the maximum of the cycle.