Lower F-region Research Articles

Experimental Determination of Threshold Powers for the Onset of HF-Enhanced Plasma Lines and Artificial Ionization in the Lower F-Region Ionosphere

HF heating experiment, with linear power ramp from 50 to 92 dBW, was conducted to determine the HF threshold power levels for the onset of detectable plasma waves and the onset of artificial ionization inferred from HF-enhanced plasma lines (HFPLs). The threshold free-space electric field at 200-km altitude, where HFPLs were detected, is about 53 mV/m. However, the HF electric field near the reflection height is enhanced by a swelling factor ~ 3.76 and also conversion to a linear dipole pump mode ( $\surd 2$ ); the actual threshold field is about 281 mV/m that is consistent with theory. Artificial ionization in the lower region was detectable via UHF radar. Short wavelength upper-hybrid waves, which were excited parametrically by the HF heating wave at higher threshold, implement Doppler shifted harmonic-cyclotron resonance interaction, via finite Larmour radius effect, to effectively accelerate electrons. Monitoring the spectral power of the parametric decay instability (PDI) line in the HFPLs by radar was shown to determine the artificial ionization onset time more precisely than that by observing the start of a sharp downward trend in the altitude of the HFPLs; at this time, the HF free-space electric field slightly below 200-km altitude is about 550 mV/m. Langmuir cascade lines in the HFPLs are separated by intervals about double the ion-acoustic frequency (about 4–5 kHz) generated by the PDI. These lines, observed at lower power facilities, were not observed. The upper-hybrid OTSI and PDI excited at HighPower Active Auroral Research Program (HAARP) and the mode competition nonlinear-damping mechanism are suggested as the processes, suppressing cascade enhanced HFPLs.

Enhancement of negative capacitance effect in (CoFeZr)x(CaF2)(100−x) nanocomposite films deposited by ion beam sputtering in argon and oxygen atmosphere

The paper presents frequency f and temperature Tp dependences of phase shift angle Θ, admittance σ and capacitance Cp for the as-deposited and annealed (CoFeZr)x(CaF2)(100−x) nanocomposite films deposited by ion-beam sputtering of a compound target in a mixed argon–oxygen gas atmosphere in vacuum chamber. The studied films presented metallic FeCoZr “cores” covered with FeCo-based oxide “shells” embedded into oxygen-free dielectric matrix (fluorite). It was found for the metallic phase content within the range of 52.2at.%⩽x⩽84.3at.% in low-f region that Θ values were negative, while in the high-f region we observed the Θ<0o. It was obtained that the f-dependences of capacitance module displayed minimum at the corresponding frequency when the Θ(f) crossed its zero line Θ=0o. It was also observed that the σ(Tp) dependence displayed the occurrence of two minima that were related to the values of Θ1=90° (the first minimum) and of Θ2=−90° (the second one). Some possible reasons of such behavior of (CoFeZr)x(CaF2)(100−x) nanocomposite films are discussed.

Overview and summary of the Spread F Experiment (SpreadFEx)

Abstract. We provide here an overview of, and a summary of results arising from, an extensive experimental campaign (the Spread F Experiment, or SpreadFEx) performed from September to November 2005, with primary measurements in Brazil. The motivation was to define the potential role of neutral atmosphere dynamics, specifically gravity wave motions propagating upward from the lower atmosphere, in seeding Rayleigh-Taylor instability (RTI) and plasma bubbles extending to higher altitudes. Campaign measurements focused on the Brazilian sector and included ground-based optical, radar, digisonde, and GPS measurements at a number of fixed and temporary sites. Related data on convection and plasma bubble structures were also collected by GOES 12, and the GUVI instrument aboard the TIMED satellite. Initial results of our SpreadFEx analyses are described separately by Fritts et al. (2009). Further analyses of these data provide additional evidence of 1) gravity wave (GW) activity near the mesopause apparently linked to deep convection predominantly to the west of our measurement sites, 2) small-scale GWs largely confined to lower altitudes, 3) larger-scale GWs apparently penetrating to much higher altitudes, 4) substantial GW amplitudes implied by digisonde electron densities, and 5) apparent influences of these perturbations in the lower F-region on the formation of equatorial spread F, RTI, and plasma bubbles extending to much higher altitudes. Other efforts with SpreadFEx data have also yielded 6) the occurrence, locations, and scales of deep convection, 7) the spatial and temporal evolutions of plasma bubbles, 8) 2-D (height-resolved) structures in electron density fluctuations and equatorial spread F at lower altitudes and plasma bubbles above, and 9) the occurrence of substantial tidal perturbations to the large-scale wind and temperature fields extending to bottomside F-layer and higher altitudes. Collectively, our various SpreadFEx analyses suggest direct links between deep tropical convection and large GW perturbations at large spatial scales at the bottomside F-layer and their likely contributions to the excitation of RTI and plasma bubbles extending to much higher altitudes.

Space weather impact on the equatorial and low latitude F-region ionosphere over India

Abstract. For a detailed study of the space weather impact on the equatorial and low latitude F-region, the ionospheric response features are analysed during the periods of three recent and most severe magnetic storm events of the present solar cycle which occurred in October and November 2003, and November 2004. The F-layer base height (h'F), peak height (hmF2) and critical frequency (foF2) data, from Trivandrum, an equatorial station and Delhi, a low latitude location, are examined during the three magnetic storm periods. The results of the analysis clearly shows that the height of the F-region (both h'F and hmF2), at the equator and low latitude, simultaneously increases by 200 to 300 km, in association with maximum negative excursion of Dst values around the midnight hours with a large depletion of ionization over the equator, which is followed by an ionization enhancement at low latitude during the recovery phase of the storm. At Delhi, fast variations up to 200 m/s are also observed in the F-layer vertical upward/downward velocity, calculated using Doppler shifts, associated with the maximum negative excursion of Dst. This shows that during magnetic disturbances, the equatorial ionization anomaly (EIA) expands to a much wider latitude than the normal fountain driven by the E/F-layer dynamo electric fields. It is also observed that during the main phase of the storm, at low latitude there is generally an enhancement of F-region ionization with an increase in h'F/hmF2 but in the equatorial region, the ionization collapses with a decrease in h'F/hmF2, especially after sunset hours. In addition, at the equator the normal pre-sunset hours' enhancement in h'F is considerably suppressed during storm periods. This might be due to changes in magnitude and direction of the zonal electric field affecting the upward E×B drift and hence the plasma distribution in the form of a decrease in electron density in the equatorial region and an increase in the low latitude region. In association with disturbance electric fields, the enhanced storm-induced equatorward meridional winds in the thermosphere can also further amplify the F-layer height rise at low latitudes during the post-midnight hours, as observed in two of the storm periods.

Theoretical simulation of O&amp;lt;sup&amp;gt;+&amp;lt;/sup&amp;gt; and H&amp;lt;sup&amp;gt;+&amp;lt;/sup&amp;gt; densities in the Indian low latitude F-region and comparison with observations

Abstract. The O+ and H+ ion density distributions in the Indian low latitude F-region, within ± 15° magnetic latitudes, are simulated using a time dependent model developed on the basis of solution of the plasma continuity equation. The simulated ion densities for solar minimum June and December solstices are then compared with ionosonde observations from the period 1959–1979 and measurements made by the Indian SROSS C2 satellite during 1995–1996 at an altitude of ~ 500 km. The simulated O+ density has a minimum around pre-sunrise hours and a maximum during noontime. H+ density is higher at nighttime and lower during the day. The simulations reproduced the well-known equatorial ionization anomaly (EIA) observed in electron density at the peak of the F2-region in the Indian low latitude sector during solar minimum. In situ measurement of O+ density by the SROSS C2, however, showed a single peak of ionization around the equator.Key words. Ionosphere (equatorial ionosphere; modeling and forecasting)

Auroral field-aligned currents by incoherent scatter plasma line observations in the E region

The aim of the Swedish-Japanese EISCAT campaign in February 1999 was to measure the ionospheric parameters inside and outside the auroral arcs. The ion line radar experiment was optimised to probe the E-region and lower F-region with as high a speed as possible. Two extra channels were used for the plasma line measurements covering the same altitudes, giving a total of 3 upshifted and 3 downshifted frequency bands of 25 kHz each. For most of the time the shifted channels were tuned to 3 (both), 4 (up), 5.5 (down) and 6.5 (both) MHz. Weak plasma line signals are seen whenever the radar is probing the diffuse aurora, corresponding to the relatively low plasma frequencies. At times when auroral arcs pass the radar beam, significant increases in return power are observed. Many cases with simultaneously up and down shifted plasma lines are recorded. In spite of the rather active environment, the highly optimised measurements enable investigation of the properties of the plasma lines. A modified theoretical incoherent scatter spectrum is used to explain the measurements. The general trend is an upgoing field-aligned suprathermal current in the diffuse aurora, There are also cases with strong suprathermal currents indicated by large differences in signal strength between up- and downshifted plasma lines. A full fit of the combined ion and plasma line spektra resulted in suprathermal electron distributions consistent with models.

High-resolution measurements of magnetospheric electric fields

Observations made at EISCAT suggest that the plasma velocity measured in the F- region above Tromsø can vary substantially on a timescale of a minute or so. The high-resolution measurements made using alternating codes during the ERRRIS experiment have confirmed this result by showing that the rapid variations of plasma velocity measured directly correspond exactly to the variations of ion temperature in the rmupper- E and lower-F region caused by frictional heating, and the variations of electron temperature in the E-region, caused by wave turbulence heating.

A new method of measuring the ion-neutral collision frequency using incoherent scatter radar

A new method of measuring the ion-neutral collision frequency using the Doppler shift instead of the shape of the incoherent scatter spectrum is introduced. The method is based on the fact that, in the absence of neutral wind, the relation between electric field and ion velocity is determined by collision frequency. If both the electric field and the ion velocity are measured, the collision frequency can be solved from the equation of ion motion. The method is tested using the EISCAT radar. The applied experiment gives profiles of vertical ion velocity in the E-region and electric fields in the lower F-region. Data from a period with a weak neutral wind and a sufficiently strong electric field are analysed and the results are compared with those obtained using the conventional method. It is found that, under favourable conditions, the new method allows the determination of the collision frequency up to 130 km altitude. This is a considerable improvement as compared to the conventional method, which usually gives the collision frequency profile no higher than to the 110 km level. At altitudes where both methods can be used, a good agreement is found. The main drawback in the new procedure is that it can be used only during periods of negligible neutral wind. It is suggested that this difficulty can be avoided by using an experiment with at least two transmitter beam directions.

EISCAT observations of plasma convection and the high-latitude, winter F-region during substorm activity

A 24 h period of observations by the EISCAT radar and other ground-based instrumentation is used to study the role of plasma convection in determining the morphology of the high-latitude F-region during winter. It is suggested that, in the afternoon sector of the polar convection pattern, rapid zonal (westward) flows caused low F-region electron densities due to an extension of the mid-latitude trough far into the sunlit hemisphere. Low densities on the dawn side prior to 0600 UT may also have been due to a trough-like feature. Although the generation mechanism is unclear, the trough may be the fossil remnant of a substorm. Around midnight, high F-region densities were seen, probably due to plasma flow emerging from the cap through soft particle precipitation in the auroral oval. Two substorms occurred at times when the radar was south of the auroral oval. Both caused enhanced convection speeds, a swing to equatorward flow, enhanced E-region densities and a depleted F-region. The first was seen as a Westward Travelling Surge, and the swing to purely southward flow which followed the surge front did not return to westward flows until 80–110 min later. The Harang discontinuity was observed co-rotating eastwards between the substorms, 65 ± 30 min before the separatrix between the dawn and dusk convection cells.

The equatorial ionospheric electric fields. A review

The equatorial electric fields have been extensively studied over the last decade using radio, radar and rocket observations. Radar measurements of vertical and east-west F-region drift velocities at Jicamarca have determined the seasonal and solar cycle variations of the F-region east-west and vertical electric fields, respectively. Rocket observations during twilight indicate the existence of a shear in the east-west drifts with eastward velocities near and above the F-region density peak and westward velocities in the lower F-region where incoherent scatter radar observations are not effective. The vertical drift (i.e. east-west electric field) changes considerably with magnetic activity, but the F-region east-west drift (vertical electric field) is not affected. During magnetically disturbed periods the zonal equatorial electric field shows frequent and large fluctuations indicating the penetration of magnetospheric electric fields into the low-latitude ionosphere. The coupling processes are not yet totally clear. Considerable progress has been made recently in the understanding of these quiet-time and disturbed equatorial electric fields.

Anomalous multiple traces below foE in high gain ionograms recorded at Sodankylä

Anomalous multiple trace patterns below f oE have been observed in the high gain ionograms recorded at Sodankylä (67°22' N, 26°38' E). The patterns normally contain 3–6 traces with a virtual height separation of 50–80 km. A relationship between the daily variations of the occurrence of the multiples and the high type sporadic E-layers has been found. Model calculations are presented and it is concluded that the phenomenon is caused by multiple transits of z-mode radio pulses between a semitransparent sporadic E-layer and the lower F-region.

Night-time radial plasma drifts and coupling fluxes at L = 2.3 from whistler mode measurements

A method recently reported for measuring radial drifts in the equatorial plane and ionosphere-magnetosphere coupling fluxes from the Doppler shifts and group delays on whistler mode signals is applied to VLF transmissions from station NLK on 18.6 kHz. Data from 22 nights, primarily during the months November to February, are analysed. When averaged over a time of about 90 min, drifts found are accurate to ±20 ms −1, corresponding to an equatorial electric field accuracy of ±0.05 mVm −1 and fluxes, to ±1.5 × 10 12 el m −2 s −1 (two hemisphere total). Given currently accepted values of coupling fluxes, the flux accuracy is of marginal value on individual nights, but useful information on average behaviour may be obtained. We find fluxes generally contribute less than 20% to the measured Doppler shift, most of which is therefore produced by cross- L drifts. To an accuracy of about 20% then, Doppler data alone may give information on these drifts. Doppler shift data previously accumulated over a number of years and relating to signals in ducts near L = 2.3 are re-examined. Dominating the nightly behaviour is an inward drift which reaches a maximum of ∼ 100 ms −1 as the duct ends cross the dusk terminator and an outward drift at dawn of the same magnitude which is initiated when the duct end crosses the terminator in the E or lower F-region. In some months, separate effects can be seen corresponding to sunrise at each end of the duct. During the night, there are clear differences between December and June solstice drift behaviour which are as yet unexplained. Two-hemisphere-total coupling fluxes found from the 22 nights of data are ∼ 1−2 × 10 12 elm −2s −1. At the time of the evening maximum of inward drift the flow is to the ionosphere, but later in the night upfluxes were measured.

A simple ion source for ionospheric experiments

It is suggested that the presence of ‘parallel’ electric fields in the auroral ionosphere could be detected by measuring the acceleration of He 3+ ions produced from the β-decay of tritium gas released at low F-region altitudes in the form of a neutral cloud. A numerical example is given, which shows that detectable fluxes of He 3+ ions (with energies between 10 eV and 1 keV, depending on the strength of the assumed parallel electric field) could be obtained at 1200 km altitude from the release of ~5 g of Tritium at 200 km altitude.

Fresnel-type fading on satellite records at low latitudes

Fresnel-type fading patterns have been observed on recordings of geostationary and travelling satellites made at three low-latitude stations in East Africa. The occurrence is greatest in the time periods 05–08 and 18–22 local time, at the June solstice. The patterns seem to be produced by patches of increased ionization some 1.5–25 km in extent at heights corresponding to sporadic- E levels and the lower F-region. The sizes show an increase with height. On occasions the patterns exhibit repetition with intervals of 4–30 min. The equations used in determining the height of the patches are reviewed. The results are compared with those from middle latitudes. A possible connection with sporadic- E is discussed.

Field-aligned current observations in the polar cusp ionosphere

Vector magnetic field measurements made during a sounding rocket flight in the polar cusp ionosphere show field fluctuations in the lower F-region which are interpreted as being caused by the payload's passage through a structured field aligned current system. The field aligned currents have a characteristic horizontal scale size of one kilometer. Analysis of one large field fluctuation gives a current density of 0.0001 amp/m sq.

Atmospheric expansion from Joule heating

An expression for the vertical velocity of the neutral atmosphere in the F-region is derived for Joule heating by the electric field that drives the auroral electrojet. When only vertical expansion is allowed, it is found that the vertical wind must always increase monotonically with altitude. The heating rate is proportional to the F-region ion density, so that appreciable heating, even during high electric fields, requires some production mechanism of ionization such as auroral secondary ionization or solar photoionization, in the lower F-region. Once started at night, when an ionizing source is present in the lower F-region, the expansion of the atmosphere transports ionization upward, thereby increasing the heating rate, and hence the expansion rate, i.e. positive feedback. Electric field strengths and F-region ion densities of 50 mV/m and 2 × 10 11 e/ m 3, respectively, will produce vertal neutral wind speeds of several tens of m/sec in the 300–500 km altitude range. During periods of high magnetic activity, i.e. high electric field, Joule heating can produce large increases in the relative N 2 concentration in the upper F-region; computations made with a simple model suggest that tenfold increases can occur at 400 km altitude 1 2−1 hr after the onset of magnetic activity, a result in agreement with satellite observations. When the Joule heating theory is applied to incoherent scatter data taken during one period of high heating, the horizontal electric field in the F-region is found to decrease markedly, possibly approaching zero as the field penetrates a weak, discrete auroral arc; the decrease began 10–20 km from the arc.

The electron heating rate and ion chemistry in the thermosphere above Wallops Island during the solar eclipse of 7 March 1970

An identical pair of thermosphere probes measured the N 2 concentration and temperature, the ion composition and concentrations and the electron temperature up to 290 km about 30 min and 5 min before totality during the 7 March 1970 eclipse at Wallops Island. The rockets travelled similar trajectories thus permitting the purely temporal changes between nights to be resolved. The neutral temperature and N 2 concentration changed little but the electron temperature decreased by as much as 20 per cent in the lower F-region. The ion concentration decreased by about 30 per cent in the F-region and about 50 per cent in the E-region, with little change in relative ion composition. The electron cooling rates decreased by a factor of 6 in the lower F-region, approximately in proportion to the change in visible solar disc. A smaller than expected decrease in the cooling rate below 150 km between the two flights indicates a hardening of the solar spectrum and suggests a significant heat contribution from the solar corona near totality. The ion composition measurements were consistent with solutions of the ion continuity equations. A proper fit required a factor of three enhancement of the flux below 200 Å, an amount also consistent with the electron heat balance analysis. Reactions involving the minor ions N + and N 2 + were found to be important for the ion chemistry of the major ions O 2 + and NO +, especially at the time of eclipse. The negligible response of the neutral atmosphere to the eclipse is reasonable considering the long time constants for the conductive and convective transport processes and the local nature of the disturbance.

Effect of the solar eclipse of may 20, 1966, on the ionospheric absorption of cosmic noise on 25 Mc/s

Measurements of cosmic noise absorption (A2 method) on 25 Mc/s in south-east Bulgaria during the solar eclipse of May 20, 1966, show a significant decrease of absorption by about 0.5 dB at the time of maximum eclipse. The contribution of the D- and E-regions of the ionosphere to this eclipse effect is only 0.22 decibels, determined by a comparison with simultaneous Al absorption measurements (pulse reflection) on 3.86 Mc/s. There remains a significant eclipse effect which must be ascribed to a decrease of electron density in the lower F-region. The eclipse variation of the maximum F2-layer electron density appears not to be representative for this F-region effect.

A limited survey of the F1-region in summer at solar minimum

Ionograms obtained at Campbell Island during December 1963 and January 1964 have been analysed by the ‘method of overlays’ (King and Lawden, 1964) to provide measures of the rates of electron production and loss, and of tho scalo height, in the lower F-region. The results indicate that the ratio between the concentrations of atomic oxygen and of molecules, which determines the rate of loss, changed suddenly during the period investigated; it also changed with (even weak) storm activity in such a way as to increase the rate of loss.

Viscosity in the high atmosphere

The kinematic viscosity η/ρ approaches infinity as the density ρ approaches zero, since the results of kinetic theory show that the molecular viscosity η is independent of density. The elementary viscosity derivations use a linear velocity profile which implies near infinite streaming velocities as the density becomes small. The streaming velocities must be finite, and the velocity profile must have an upper and a lower limit in the atmosphere. It is shown that the viscous stress must approach zero with decreasing density when the velocity profile is bounded. Either the velocity gradients or the corrected viscosity coefficients become small in the higher atmosphere. The viscous effect depends on a length scale representative of the wind shear and is a maximum in the lower F-region for scales comparable to those representative of the air currents studied in synoptic meteorology.