Geomagnetic Flux Tubes Research Articles

Pitch-Angle Diffusion of Radiation Belt Electrons and Precipitating Particle Fluxes: Dependence on VLF Wavefield Parameters

The dependence of the pitch-angle diffusion efficiency of energetic electrons in the Earth’s magnetosphere on the distribution of the whistler wave field along the geomagnetic flux tube is quantitatively studied for parameters corresponding to the location of the Sura and HAARP HF heating facilities. The expansion of the precipitation energy range with the increase of the region of geomagnetic latitudes occupied by the waves is shown. Using the calculated pitch-angle diffusion coefficient for a given spectrum of waves and their distribution along the flux tube, the ratio of the fluxes of precipitating and trapped particles at low altitude is determined. It is shown that at typical wave intensities corresponding to chorus VLF waves and plasmaspheric hiss, the fluxes of precipitating and trapped electrons can be comparable to each other. At the same time, for the wave amplitudes observed as a result of the action of heating facilities, the flux of precipitating electrons is negligible.

Winter Nighttime Enhancement of the Midlatitude Ionosphere: Contribution From the Diffusive and Wind‐Driven Plasma Transport

AbstractThe characteristics of winter nighttime enhancement (WNE) of the midlatitude ionosphere at 300, 550, and 750 km altitude are presented, based on the radio occultation data from the Constellation Observing System for Meteorology, Ionosphere, and Climate. WNE at 750 km altitude has earlier start time and larger increasing rate than that at 550 km, possibly due to the downward plasma diffusion from above that dominates the most upper part of the ionosphere. Theoretical calculation of the plasma transport at 300 km altitude reveals the largest effect of downward diffusion at 45° geomagnetic latitude, consistent with the latitudinal distribution of WNE. Meanwhile, the wind‐driven plasma transport also contributes to WNE, at 30°W–180°W geographic longitude in the Southern Hemisphere. The strength and end time of WNE vary with longitude, and are correlated with the longitudinal variations of the enhancement in the conjugate (summer) hemisphere. This may be attributed to the transport processes in the interhemispheric geomagnetic flux tubes, which redistribute and balance the plasma between the conjugate hemispheres at night. The plasma flux from above is hence an important plasma source for further understanding of both the winter and summer nighttime enhancements.

Localization of the Source of Quasiperiodic VLF Emissions in the Magnetosphere by Using Simultaneous Ground and Space Observations: A Case Study

AbstractWe study quasiperiodic very low frequency (VLF) emissions observed simultaneously by Van Allen Probes spacecraft and Kannuslehto and Lovozero ground‐based stations on 25 December 2015. Both Van Allen Probes A and B detected quasiperiodic emissions, probably originated from a common source, and observed on the ground. In order to locate possible regions of wave generation, we analyze wave‐normal angles with respect to the geomagnetic field, Poynting flux direction, and cyclotron instability growth rate calculated by using the measured phase space density of energetic electrons. We demonstrate that even parallel wave propagation and proper (downward) Poynting flux direction are not sufficient for claiming observations to be in the source region. Agreement between the growth rate and emission bands was obtained for a restricted part of Van Allen Probe A trajectory corresponding to localized enhancement of plasma density with scale of 700 km. We employ spacecraft density data to build a model plasma profile and to calculate ray trajectories from the point of wave detection in space to the ionosphere and examine the possibility of their propagation toward the ground. For the considered event, the wave could propagate toward the ground in the geomagnetic flux tube with enhanced plasma density, which ensured ducted propagation. The region of wave exit was confirmed by the analysis of wave propagation direction at the ground detection point.

Quasiperiodic ELF/VLF Emissions Detected Onboard the DEMETER Spacecraft: Theoretical Analysis and Comparison With Observations

AbstractWe present results of numerical simulation of quasiperiodic (QP) extra low frequency/very low frequency emissions performed by using a theoretical model of flow cyclotron maser based on a self‐consistent set of equations of the quasi‐linear plasma theory averaged over oscillations of waves and particles in a geomagnetic flux tube. Calculations were made for a wide range of plasma parameters (i.e., cold plasma density, L‐shell, and energetic electron flux) in order to obtain a statistical relationship between various properties of QP emissions, such as the repetition period, the frequency bandwidth, the frequency drift rate, and the characteristic wave spectral energy density. The theoretical results are compared with the results of a statistical study of QP emissions measured by the DEMETER spacecraft (Hayosh et al., 2014, https://doi.org/10.1002/2013JA019731). The simulation results are in a good agreement with the observation data in the case of reasonable choice of cold plasma density value and its dependence on the QP‐source location (L‐shell). In particular, an increase in the frequency bandwidth of QP very low frequency waves with increasing central frequency of QP emissions, a decrease in the frequency drift rate of QP elements with increasing repetition period, and a decrease in the characteristic wave spectral energy density with increasing repetition period are confirmed.

Geomagnetically Conjugate Observations of Equatorial Plasma Irregularities From Swarm Constellation and Ground‐Based GPS Stations

AbstractThe near‐polar orbit satellites of Swarm mission provide a good opportunity to investigate the conjugacy of equatorial plasma irregularities (EPIs) since their trajectories at low latitudes are basically aligned with fixed geographical longitude. However, the Swarm in situ electron density occasionally shows EPIs at only one hemisphere at this longitude. In this study, we provide detailed analysis of such EPI events from the in situ electron densities and onboard global positioning system (GPS) measurements of Swarm low pair satellites, and simultaneous GPS data from two geomagnetically conjugate ground stations at the Africa longitudes. The result indicates that when Swam in situ electron density sometime shows EPIs at only one hemisphere, the GPS scintillations are still observed from the Swarm onboard receiver and by the two conjugate ground stations. It implies that the EPIs should generally elongate along the geomagnetic flux tube. More than two‐year statistic results show that the onset time of scintillation in the northern station is on average 16 and 18 min earlier than that in the southern station for September equinox and December solstice in 2015, while for March equinox in 2016 the onset time of scintillation of northern station is about 11 min later than that of southern station, which indicates the asymmetry features of EPIs along the flux tube. Further analysis of nearly three‐year GPS data from two conjugate stations at the Asia longitudes, we find that during solar maximum years the local sunset time plays an important role for causing the difference of onset time of scintillation between two conjugate stations.

Rare Occurrence of Off‐Equatorial Edge Initiating and Equatorward Surging Plasma Depletions Observed in OI 630‐nm Imaging

AbstractUsual depletions in the Northern Hemisphere form a tree fork junction feature near the south (i.e., toward the equator) in all‐sky airglow images, and its branches surge toward the north (i.e., poleward). We report in this paper unusual airglow depletions in OI 630.0‐nm imaging over India that surged equatorward and formed an inverted tree fork junction feature on 3 January 2011. Airglow images showed faint signatures of medium‐scale traveling ionospheric disturbances in the beginning that were east‐west aligned and propagated toward equator. Within 06 to 18 min of its passing, turbulent structures were noted in the field of view, and two depleted patches appeared out of low airglow background over the off‐equatorial edge. The apex height of the associated geomagnetic flux tubes varied from ~ 1,400 to 1,600 km. Later, these dark regions intensified and surged equatorward while drifting slowly toward west. While remaining almost steady, one of them further intensified and continued to surge equatorward. The second dark patch got linked up with another isolated depletion to form an elongated depleted feature. Next, the southern end of this attached feature surged equatorward. When two depletions were well formed, an inverted tree fork junction was noticeable. During this time, the equatorward motion of the equatorial ionization anomaly structure is also seen. To the best of our knowledge, this is the first imaging observation of an inverted tree fork junction feature and brings out the unknown facets of ionospheric irregularities.

Morphological Investigation of Disturbed Ionosphere during Intense Geomagnetic Storms

Geomagnetic Storms are the disturbed magnetic conditions, influenced and induced by Interplanetary Magnetic Field and the Charged Particle's motion around the Earth, respectively, in Geospace. As the ionosphere is woven by the earth's magnetic field it responds to the change in that. During the geomagnetic storms the filled-in plasma between the magnetic field lines, geomagnetic flux tubes, redistributes itself in effect of the magnetic field forcing. In the present study we have done the investigation of the morphology of the ionosphere over the mid and high latitude regions during intense Geomagnetic Storms. We got fairly convincing results; in three cases decrease of the critical frequency of F2 layer (foF2) and in one case enhancement of the critical frequency of F2 layer (foF2) at mid and high latitudes.

Westward tilt of low‐latitude plasma blobs as observed by the Swarm constellation

AbstractIn this study we investigate the three‐dimensional structure of low‐latitude plasma blobs using multi‐instrument and multisatellite observations of the Swarm constellation. During the early commissioning phase the Swarm satellites were flying at the same altitude with zonal separation of about 0.5∘ in geographic longitude. Electron density data from the three satellites constrain the blob morphology projected onto the horizontal plane. Magnetic field deflections around blobs, which originate from field‐aligned currents near the irregularity boundaries, constrain the blob structure projected onto the plane perpendicular to the ambient magnetic field. As the two constraints are given for two noncoplanar surfaces, we can get information on the three‐dimensional structure of blobs. Combined observation results suggest that blobs are contained within tilted shells of geomagnetic flux tubes, which are similar to the shell structure of equatorial plasma bubbles suggested by previous studies.

Topside signature of medium-scale traveling ionospheric disturbances

Abstract. Plasma blobs, localized plasma density enhancements that occur singularly or in periodic groups, have been observed by in situ sensors in the lower- and middle-latitude nighttime ionosphere. Traditionally, creation of blobs has been thought to be connected to equatorial plasma bubbles, which are localized plasma depletions. Here, we report the association of blobs with medium-scale traveling ionospheric disturbances (MSTIDs). On 17 January 2010, an all-sky imager on the Caribbean island of Bonaire (geographic: 12.190° N, 68.244° W; geomagnetic 22.46° N, 7.099° E) observed a nighttime electrified MSTID propagating to the southwest. At the time of the MSTID's transit, the Coupled Ion-Neutral Dynamics Investigation instrument onboard the Communication/Navigation Outage Forecasting System satellite detected a group of blobs along the same geomagnetic flux tubes. The electron density variations measured at the satellite altitude, indicating the blobs, are anticorrelated with the intensity variations of the 630.0 nm dissociative recombination emission measured on the same magnetic field lines. This relationship is explained by a modulation of the O+ profile altitude due to electric fields generated within the MSTID. This idea is supported by in situ measurements of the vertical ion velocity. We argue that common climatology between blobs and MSTIDs reported in the literature, as well as this coincident observation, suggest that blobs may be the in situ signature of MSTIDs in the topside ionosphere.

Beta electron fluxes inside a magnetic plasma cavern: Calculation and comparison with experiment

We study the possibility of electrostatic blanking of beta electrons in the expanding spherical blob of a radioactive plasma in a rarefied ionosphere. From numerical studies on the dynamics of beta electrons departing a cavern, we obtain the form of a function that determines the portion of departing electrons and calculate the flux density of beta electrons inside the cavern in relation to the Starfish Prime nuclear blast. We show that the flux density of electrons in geomagnetic flux tubes and inside the cavern depend on a correct allowance for the quantity of beta electrons returning to the cavern. On the basis of a physical analysis, we determine the approximate criterion for the return of electrons from a geomagnetic flux tube to the cavern. We compare calculation results in terms of the flux density of beta electrons inside the cavern with the recently published experimental results from operation Starfish Prime.

Comparing plasma bubble occurrence rates at CHAMP and GRACE altitudes during high and low solar activity

Abstract. Based on the multi-year data base (2001–2009) of CHAMP Planar Langmuir Probe (PLP) data and GRACE K-Band Ranging (KBR1B) data, typical features of ionospheric plasma irregularities are studied at the altitudes of CHAMP (300–400 km) and GRACE (~500 km). The phenomena we are focusing on are the equatorial plasma bubbles (EPBs). Similar seasonal/longitudinal (S/L) distributions of EPB have been found at both CHAMP and GRACE altitudes during solar active and quiet years. Peak EPB occurrence rates, defined as number of events within an S/L bin divided by the number of passes over that bin, decrease from the high and moderate solar flux period (2001–2005) to the low solar flux period (2005–2009) from 80% to 60% and 60% to 40% at CHAMP and GRACE altitudes, respectively. On average the occurrence rate increases linearly with solar flux at about the same rate at CHAMP and GRACE. For high flux levels (P10.7>200) non-linear increases are observed at GRACE. The occurrence rate increases rapidly after 19:00 local time (LT) during high solar flux periods. Around solar minimum rates increase more gently and peak around 22:00 LT. The highest occurrence rates are encountered at latitudes around 10° north and south of the dip equator. Results from the two altitudes support the notion that EPBs form regions of depleted plasma along geomagnetic fluxtubes. It is shown for the first time that in regions of high occurrence rates EPBs are associated with fluxtubes reaching greater apex heights than those in regions of low rates.

Interhemispheric asymmetries in the occurrence of magnetically conjugate sub-auroral polarisation streams

Abstract. Earthward injections of energetic ions and electrons mark the onset of magnetospheric substorms. In the inner magnetosphere (L4), the energetic ions drift westward and the electrons eastward, thereby enhancing the equatorial ring current. Wave-particle interactions can accelerate these particles to radiation belt energies. The ions are injected slightly closer to Earth in the pre-midnight sector, leading to the formation of a radial polarisation field in the inner magnetosphere. This maps to a poleward electric field just equatorward of the auroral oval in the ionosphere. The poleward electric field is subsequently amplified by ionospheric feedback, thereby producing auroral westward flow channels (AWFCs). In terms of electric field strength, AWFCs are the strongest manifestation of substorms in the ionosphere. Because geomagnetic flux tubes are essentially equi-potentials, similar AWFC signatures should be observed simultaneously in the Northern and Southern Hemispheres. Here we present magnetically conjugate SuperDARN radar observations of AWFC activity observed in the pre-midnight sector during two substorm intervals including multiple onsets during the evening of 30 November 2002. The Northern Hemisphere observations were made with the Japanese radar located at King Salmon, Alaska (57, and the Southern Hemisphere observations with the Tasman International Geospace Environment Radar (TIGER) located at Bruny Island, Tasmania (55. LANL geosynchronous satellite observations of energetic ion and electron fluxes monitored the effects of substorms in the inner magnetosphere (L6). The radar-observed AWFC activity was coincident with activity observed at geosynchronous orbit, as well as westward current surges in the ionosphere observed using ground-based magnetometers. The location of AWFCs with respect to the auroral oval was inferred from FUV auroral images recorded on board the IMAGE spacecraft. DMSP SSIES ion drift measurements confirmed the presence of AWFCs equatorward of the auroral oval. Systematic asymmetries in the interhemispheric signatures of the AWFCs probably arose because the magnetic flux tubes were distorted at L shells passing close to the substorm dipolarisation region. Transient asymmetries were attributed to the development of nearby field-aligned potential drops and currents.

Correction to “Flow angle dependence of 1‐m ionospheric plasma wave turbulence for near‐threshold radar echo electric fields” by E. E. Timofeev et al.

[1] In the paper ‘‘Flow angle dependence of 1-m ionospheric plasma wave turbulence for near-threshold radar echo electric fields’’ by E. E. Timofeev, M. K. Vallinkoski, P. Pollari, J. Kangas, T. Virdi, P. J. S. Williams, and E. Nielsen (Journal of Geophysical Research, 107(A10), 1286, doi:10.1029/2000JA005023, 2002), several errors appeared in the text. [2] The authors’ affiliations were not set correctly. The correct affiliations for each author are as follows: E. E. Timofeev V. A. Fock Institute of Physics, St. Petersburg State University, St. Petersburg, Russia M. K. Vallinkoski Sodankyla Geophysical Observatory, University of Oulu, Oulu, Finland P. Pollari Department of Physics, University of Oulu, Finland Now at Computer Services Center, University of Oulu, Finland J. Kangas Department of Physics, University of Oulu, Finland Now at Sodankyla Geophysical Observatory, Sodankyla, Finland T. Virdi Department of Physics, University of Aberystwyth, UK P. J. S. Williams Department of Physics, University of Aberystwyth, UK E. Nielsen Max-Planck-Institut fur Aeronomie, Katlenburg-Lindau, Germany [3] The correct complete address for P. Pollari is Computer Services Centre, University of Oulu, P.O. Box 7600, Oulu, FIN-90014, Finland (paivio.pollari@oulu.fi). [4] The captions for Figures 8 and 9 were incorrect. The correct captions for Figures 8 and 9 are as follows: Figure 8. The near-threshold STARE radar echo Doppler shifts versus the ionospheric electric field strength. Figure 9. The near-threshold STARE radar echo Doppler shifts versus the EISCAT plasma velocity component along the STARE radar line of sight. [5] The first sentence of paragraph 16 should read as follows: The special ERRRIS (E-region Rocket and Radar InstabilityStudy)campaign inthe twospringseasonsin1988– 1989 gives enough STARE andEISCATdata to study the 1-m ionospheric plasma wave turbulence for practically all possible flow angles within the eastward and westward electrojets. [6] The first sentence in paragraph 20 should read as follows: The experimental setup in the ERRRIS campaign intervals includes the STARE radars and the EISCAT facility, which allowed simultaneous coherent and incoherent scatter measurements along the geomagnetic flux tube intersected by the two STARE beams in the E-region over Tromso. [7] The last sentence in paragraph 62 should read as follows: According to Table 3, the ratio of jet averaged currents is between 1.6 and 1.8 for the D models, whereas for model C the ratio is about 1.5. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. A12, 1450, doi:10.1029/2002JA009771, 2002

Investigating the possible association between thunderclouds and plasmaspheric ducts

It has been suggested that quasi‐electrostatic electric fields from thunderclouds might produce whistler ducts by driving plasma interchange of geomagnetic flux tubes. Using lightning activity as a proxy for the presence of electrified clouds, the relation between thunderclouds and duct number was investigated through the study of two periods of 10 days, that is from June 13–22, 1993, and August 6–15, 1993. Typical duct numbers observed using NAA transmissions (24.0 kHz, Cutler, Maine) received at Faraday during the nights of June and August 1993 were ∼ 7, while the maximum duct numbers were 9–12. Duct numbers dropped down to zero or near zero during the day. Times of increased duct number could not be clearly associated with increased thunderstorm activity in the duct footprint region. Duct numbers observed on the path from NLK (24.8 kHz, Seattle, Washington) to Dunedin, New Zealand, were also studied, where very low thunderstorm activity levels imply that thundercloud‐driven duct formation should not be significant. The typical nighttime peak was ∼ 5 at about 1000 UT (23 LT). The difference between duct numbers in areas with high thunderstorm activity and quiet areas might be thought to be due to thundercloud duct formation. However, the F region absorption on the NLK footprints at 1000UT (23 LT) in June was found to be equivalent to 0000 UT (20 LT) in June on the NAA duct footprints. The typical number of ducts observable on the NAA path at 0000 UT in June is ≈ 5, very similar to the number observed on NLK at 0930 UT. The results found in this study are consistent with the idea that ionospheric absorption is a significant influence on the number of ducts observed. It does not appear that quasi‐electronstatic fields from thunderclouds play a significant role in the formation of plasmaspheric whistler ducts. The similarity in quiet time midlatitude duct nuumbers at these two different locations suggest a globally consistent duct formation mechanism, such as the interchange of magnetospheric tubes of plasma induced by spatially varying electric fields in the E region.

Whistler–electron interactions in the magnetosphere: new results and novel approaches

Attention is focused here on the quasilinear and nonlinear physics of cyclotron interactions between magnetospheric whistler mode waves and energetic electrons on dipolar geomagnetic flux tubes. These interactions can lead to the generation of noise-like emissions or phase-coherent discrete signals in the frequency-time domain. In the magnetosphere noise-like emissions called hiss are accompanied by a smooth electron precipitation pattern. Examples of discrete emissions are ELF/VLF chorus or VLF emissions triggered by whistlers from lightning or by radio transmitters on the ground. The rapid temporal variations of these signals are associated with fine structure of the distribution function of the radiation belt electrons, such as a transient step-like deformation or a well-organized beam, which are prepared by initial noise-like emissions or by a quasimonochromatic whistler–wave packet, respectively. These cause the properties of the electrons, which may be observed on a satellite, to evolve rapidly in time and on relatively short spatial scales. Bursts of precipitating electrons occur, and can contribute significantly to depleting the radiation belts. Recent results on improvements in the theoretical understanding of such processes and on new observations of magnetospheric electrons and whistler-mode waves are presented.

POLAR spacecraft observations of helium ion angular anisotropy in the Earth's radiation belts

Abstract. New observations of energetic helium ion fluxes in the Earth's radiation belts have been obtained with the CAMMICE/HIT instrument on the ISTP/GGS POLAR spacecraft during the extended geomagnetically low activity period April through October 1996. POLAR executes a high inclination trajectory that crosses over both polar cap regions and passes over the geomagnetic equator in the heart of the radiation belts. The latter attribute makes possible direct observations of nearly the full equatorial helium ion pitch angle distributions in the heart of the Earth's radiation belt region. Additionally, the spacecraft often re-encounters the same geomagnetic flux tube at a substantially off-equatorial location within a few tens of minutes prior to or after the equatorial crossing. This makes both the equatorial pitch angle distribution and an expanded view of the local off-equatorial pitch angle distribution observable. The orbit of POLAR also permitted observations to be made in conjugate magnetic local time sectors over the course of the same day, and this afforded direct comparison of observations on diametrically opposite locations in the Earth's radiation belt region at closely spaced times. Results from four helium ion data channels covering ion kinetic energies from 520 to 8200 KeV show that the distributions display trapped particle characteristics with angular flux peaks for equatorially mirroring particles as one might reasonably expect. However, the helium ion pitch angle distributions generally flattened out for equatorial pitch angles below about 45°. Significant and systematic helium ion anisotropy difference at conjugate magnetic local time were also observed, and we report quiet time azimuthal variations of the anisotropy index.Key words. Magnetospheric physics (energetic particles · trapped; magnetospheric configuration and dynamics; plasmasphere)

Ion heating, electron acceleration, and the self‐consistent parallel E‐field in downward auroral current regions

Downward auroral current regions are characterized by: upflowing field‐aligned electrons; transverse ion heating; depleted plasma densities; downward‐pointing parallel E‐fields and current densities; intense ELF and VLF plasma wave activity; and the absence of hot magnetospheric particles. In this paper, we present a kinetic model that explains all of these features. We imagine a geomagnetic flux tube of finite length imbedded in a downward current region and impose appropriate kinetic boundary conditions. We assume that broadband ELF waves exist on the flux tube and heat the ions by cyclotron resonance near the ion gyrofrequency. We carry out an iterative, steady‐state solution for the one‐particle distribution functions, fα(s, ν⟂, ν∥), and the self‐consistent parallel E‐field, E∥(s). When this theory is applied to the Gorney et al. [1985] data set, we find good agreement between theory and experiment for the ion conic, the electron beam, and the downward‐pointing E∥(s).

Are whistler ducts created by thunderstorm electrostatic fields?

Park and Helliwell [1971] suggested that whistler ducts might be caused by plasma interchange of geomagnetic flux tubes driven by thundercloud electrostatic fields. At present, this is the most favored whistler duct creation mechanism. Theoretical formulations have been developed to calculate high‐altitude electric fields due to electrified clouds, taking into account the presence of the ionosphere. Previous studies have calculated the values of these fields, making use of approximated conductivity profiles. Calculations are undertaken using more accurate representations of more modern conductivity profiles than those used previouly. Preference is given to experimental rather than theoretically derived conductivity profiles. These calculations indicate that under “typical” conductivity conditions the high‐altitude electric fields from even giant thunderclouds are too small to create a realistic whistler duct in a realistic period. This is the case for both day and night conditions.

Testing the formulation of Park and Dejnakarintra to calculate thunderstorm dc electric fields

Park and Dejnakarintra [1973] presented a formulation to calculate high‐altitude electric fields due to electrified clouds, taking into account the presence of the ionosphere. Calculation of these fields is necessary to test the hypothesis that whistler ducts might be caused by plasma interchange of geomagnetic flux tubes driven by thundercloud electrostatic fields. To test the accuracy of the formulation, the calculated dc electric fields are compared with those measured during a thunderstorm campaign reported by Holzworth et al. [1985]. Using a conductivity profile measured during their campaign, it is shown that the formulation of Park and Dejnakarintra produces electric field predictions which are consistent with the measurements of Holzworth et al. This indicates the formulation developed by Park and Dejnakarintra is valid. The comparisons agree with measurements which show that large negative screening charges commonly exist around the top of thunderclouds, reducing the high altitude fields. These calculations indicate that the effective thunderstorm charge used in such calculations needs to be significantly reduced to account for screening charges. Such reductions lead to serious increases (up to a factor of 10) in the time required for high‐altitude thunderstorm electric fields to produce whistler ducts.

Linear theory and modeling of the Rayleigh‐Taylor instability leading to the occurrence of equatorial spread F

In a test of the generally accepted Rayleigh‐Taylor (R‐T) instability mechanism for equatorial spread F (ESF), a linear instability growth rate γRT is derived following the formalism of Haerendel (preprint, 1973) which takes into account the variations of physical parameters along geomagnetic flux tubes. The resulting form of γRT extends the results of previous work by including direct dependencies on transequatorial neutral winds, zonal electric fields, vertical and horizontal ionospheric density gradients, the presence of an E region, and chemical recombination. Realistic atmospheric and ionospheric density model inputs are used for the first time to make quantitative calculations of R‐T growth rates for a range of geophysical conditions. The key result of this study is that time/altitude domains having positive calculated instability growth rates are found to coincide with observed time/altitude patterns of ESF occurrence over both a monthly and a yearly time frame. This success in being able to model the climatological occurrence of ESF lends support to the physical model adopted for the instability mechanism and opens up new avenues of research into ESF predictability on a night‐to‐night and even an hour‐to‐hour basis.