A prototype system has been developed to monitor the instantaneous global distribution of ionospheric irregularities, using the worldwide network of Globa Positioning System (GPS) receivers. Case studies in this pape indicate that GPS receiver loss of lock of signal tracking may be associated with strong phase fluctuations. It is shown that a network-based GPS monitoring system will enable us to study the generation and evolution of ionospheric irregularities continuously around the globe under various solar and geophysical conditions, which is particularly suitable for studies of ionospheric storms, and for space weather research and applications.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97GL02273

Monitoring of global ionospheric irregularities using the Worldwide GPS Network

We use radar observations from the Jicamarca Observatory from 1968 to 1992 to study the effects of the F region vertical plasma drift velocity on the generation and evolution of equatorial spread F The dependence of these irregularities on season, solar cycle, and magnetic activity can be explained as resulting from the corresponding effects on the evening and nighttime vertical drifts In the early night sector, the bottomside of the F layer is almost always unstable The evolution of the unstable layer is controlled by the history of the vertical drift velocity When the drift velocities are large enough, the necessary seeding mechanisms for the generation of strong spread F always appear to be present The threshold drift velocity for the generation of strong early night irregularities increases linearly with solar flux The geomagnetic control on the generation of spread F is season, solar cycle, and longitude dependent These effects can be explained by the response of the equatorial vertical drift velocities to magnetospheric and ionospheric disturbance dynamo electric fields The occurrence of early night spread F decreases significantly during equinox solar maximum magnetically disturbed conditions due to disturbance dynamo electric fields which decrease the upward drift velocities near sunset The generation of late night spread F requires the reversal of the vertical velocity from downward to upward for periods longer than about half an hour These irregularities occur most often at ∼0400 local time when the prompt penetration and disturbance dynamo vertical drifts have largest amplitudes The occurrence of late night spread F is highest near solar minimum and decreases with increasing solar activity probably due to the large increase of the nighttime downward drifts with increasing solar flux

/pdf/effects-of-the-vertical-plasma-drift-velocity-on-the-2m25kbykcc.pdf

Effects of the vertical plasma drift velocity on the generation and evolution of equatorial spread F

Twenty one differences between CME-driven geomagnetic storms and CIR-driven geomagnetic storms are tabulated. (CME-driven includes driving by CME sheaths, by magnetic clouds, and by ejecta; CIR-driven includes driving by the associated recurring high-speed streams.) These differences involve the bow shock, the magnetosheath, the radiation belts, the ring current, the aurora, the Earth's plasma sheet, magnetospheric convection, ULF pulsations, spacecraft charging in the magnetosphere, and the saturation of the polar cap potential. CME-driven storms are brief, have denser plasma sheets, have strong ring currents and Dst, have solar energetic particle events, and can produce great auroras and dangerous geomagnetically induced currents; CIR-driven storms are of longer duration, have hotter plasmas and stronger spacecraft charging, and produce high fluxes of relativistic electrons. Further, the magnetosphere is more likely to be preconditioned with dense plasmas prior to CIR-driven storms than it is prior to CME-driven storms. CME-driven storms pose more of a problem for Earth-based electrical systems; CIR-driven storms pose more of a problem for space-based assets.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005JA011447

Differences between CME‐driven storms and CIR‐driven storms

In a test of the generally accepted Rayleigh-Taylor (R-T) instability mechanism for equatorial spread F (ESF) a linear instability growth rate γ RT 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 γ RT 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.

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

[1] The Global Ultraviolet Imager (GUVI) instrument carried aboard the NASA TIMED satellite measures the spectral radiance of the Earth’s far ultraviolet airglow in the spectral region from 120 to 180 nm using a cross-track scanning spectrometer design. Continuous operation of the instrument provides images of the Earth’s disk and limb in five selectable spectral bands. Also, spectra at fixed scanning mirror position can be obtained. Initial results demonstrate the quantitative functionality of the instrument for studies of the Earth’s dayglow, aurora, and ionosphere. Moreover, through forward modeling, the abundance of the major constituents of the thermosphere, O, N2, and O2 and thermospheric temperatures can be retrieved from observations of the limb radiance. Variations of the column O/N2 ratio can be deduced from sunlit disk observations. In regions of auroral precipitation not only can the aurora regions be geographically located and the auroral boundaries identified, but also the energy flux Q, the characteristic energy Eo, and a parameter fo that scales the abundance of neutral atomic oxygen can be derived. Radiance due to radiative recombination in the ionospheric F region is evident from both dayside and nightside observations of the Earth’s limb and disk, respectively. Regions of depleted F-region electron density are evident in the tropical Appleton anomaly regions, associated with so-called ionospheric ‘‘bubbles.’’ Access to the GUVI data is provided through the GUVI website www.timed.jhuapl.edu\guvi. INDEX TERMS: 0310 Atmospheric Composition and Structure: Airglow and aurora; 0355 Atmospheric Composition and Structure: Thermosphere—composition and chemistry; 0358 Atmospheric Composition and Structure: Thermosphere—energy deposition; 2407 Ionosphere: Auroral ionosphere (2704); KEYWORDS: airglow, aurora, ultraviolet, imaging, satellite, atmosphere

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003JA009918

Initial observations with the Global Ultraviolet Imager (GUVI) in the NASA TIMED satellite mission

The Defense Meteorological Satellite Program (DMSP) flights F9 and F10 crossed postsunset local time sectors approximately 14 times per day in Sun-synchronous orbits at an altitude of ∼840 km. We have examined a large database of postsunset plasma density measurements acquired during ∼ 15,000 equatorial crossings made by DMSP F9 in 1989 and 1991 and DMSP F10 in 1991. On 2086 of these crossings equatorial plasma bubbles (EPBs) were observed as intervals of depleted and irregular plasma densities. We have analyzed these EPB events to determine their distributions with season, longitude (S/L), and levels of geomagnetic activity. The global S/L distributions of EPBs observed by the DMSP satellites are shown to be in general agreement with results from discrete ground-based measurements. That is, the seasonal variations detected at 840 km in longitude bins hosting radar/scintillation observatories appear similar to those reported from the ground. Over the Atlantic sector where EPBs occur frequently, we found good agreement with predictions of a simple model proposed by Tsunoda [1985]. In the Pacific sector the frequency of EPB occurrence is considerably lower, and poor counting statistics preclude confident predictions regarding the absolute value of seasonal variations. We suggest that relatively large equatorial magnetic fields at Flayer altitudes in the Pacific (∼0.34 G) sector more strongly inhibit the growth of the Rayleigh-Taylor instability than at Atlantic (∼0.25 G) longitudes. Contrary to general belief, we found that EPBs occurred regularly during geomagnetic storms, especially in the initial and main phases. EPB activity appears to have been suppressed from many hours to clays during storm recovery phases.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JA000319

DMSP observations of equatorial plasma bubbles in the topside ionosphere near solar maximum

The problem of day-to-day variability in the occurrence of equatorial spread F (ESF) is addressed using multidiagnostic observations and semiempirical modeling. The observational results are derived from a two-night case study of ESF onset conditions observed at Kwajalein Atoll (Marshall Islands) using the ALTAIR incoherent scatter radar and all-sky optical imaging techniques. The major difference between nights when ESF instabilities did not occur (August 14, 1988) and did occur (August 15, 1988) in the Kwajalein sector was that the northern meridional gradient of 6300-A airglow was reduced on the night of limited ESF activity. Modeling results suggest that this unusual airglow pattern is due to equatorward neutral winds. Previous researchers have shown that transequatorial thermospheric winds can exert a control over ESF seasonal and longitudinal occurrence patterns by inhibiting Rayleigh-Taylor instability growth rates. Evidence is presented to suggest that this picture can be extended to far shorter time scales, namely, that 'surges' in transequatorial winds acting over characteristic times of a few hours to a day can result in a stabilizing influence upon irregularity growth rates. The seemingly capricious nature of ESF onset may thus be controlled, in part, by the inherent variability of low-latitude thermospheric winds.

/pdf/onset-conditions-for-equatorial-spread-f-3rev4fmon3.pdf

Onset conditions for equatorial spread F

This paper extends a recent study of electric field penetration into the inner magnetosphere observed by the Combined Release and Radiation Effects (CRRES) satellite and the Defense Meteorological Satellite Program (DMSP) satellite F8 during the magnetic storm of June 4–6, 1991, to consider its ionospheric consequences. Effects include the development of > 1 km/s subauroral ion drift (SAID) structures, the formation of midlatitude density troughs, and the vertical transport of equatorial plasma, bubbles. Nearly simultaneous auroral electron and plasma drift measurements were acquired by three DMSP satellites with F8 and F9 in one hemisphere and FlO in the other. Moderate to strong SAID structures were consistently detected for ∼10 hours during the early main phase of the storm. Weak SAIDs were encountered during ∼8 hours of the early recovery phase. DMSP data show that SAIDs with similar characteristics developed at magnetically conjugate locations and extended for at least 3 hours in local time. Simultaneous measurements show that the SAIDs spanned temporally grooving but latitudinally narrow plasma density troughs. These observations suggest that the magnetospheric sources of SAIDs act more like voltage than current generators. Energetic electron fluxes, electric fields, and plasma waves measured by CRRES indicate that during this storm the ring current shielding charges and SAID sources were located in regions of high plasma density characteristic of the plasmasphere. The sequence in which DMSP detected equatorial plasma density irregularities is consistent with model predictions that stormtime electrodynamics at low latitudes operate on distinctive fast and slow timescales [Fejer and Schcrliess, 1997].

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999JA000188

Ionospheric disturbances observed by DMSP at middle to low latitudes during the magnetic storm of June 4–6, 1991

[1] Defense Meteorological Satellite Program (DMSP) spacecraft at 840 km observe a 27-day variation in plasma density and temperature at all subauroral latitudes. At the peak of the solar cycle, evening-sector variations are ∼40–50% in plasma density and ∼5–10% in electron temperature. The percent of variation decreases with decreasing solar activity to or below the threshold of detectability for the DMSP sensors. We compare in situ densities with simultaneous observations of total electron content but find that similar variations are not present in a consistent manner. Thus we conclude that the variations exist mostly as topside phenomena. However, comparisons with variations in the radio flux at 10.7 cm (F10.7), a standard proxy for solar EUV, indicate that the topside variations are driven by the solar EUV flux. When compared with the variations of several alternative proxies for the solar EUV flux, we find that only one of them correlates better than F10.7. Because the topside ionosphere couples with the plasmasphere, we suggest that similar 27-day variations should appear in plasmaspheric parameters.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2002JA009731

The 27-day variations of plasma densities and temperatures in the topside ionosphere

[1] The equatorial ionosphere is host to the most dramatic and enigmatic plasma instability mechanism in the geospace environment. Equatorial spread F (ESF) was discovered in early ionosonde measurements and interpreted theoretically using Rayleigh-Taylor theory. Subsequent diagnostic and modeling advances have improved substantially our understanding of ESF onset and evolution and its associated effects on the ionosphere throughout the low-latitude domain. The degree to which ESF mechanisms penetrate into the lower midlatitudes is a topic of current study, a reverse of the familiar concept of high-to-low latitude coupling for space weather phenomena. Optical diagnostic systems, first ground based and now space based, reveal the presence of ESF structures via images of airglow depletions that are aligned in the approximately north-south direction spanning the geomagnetic equator. Ground-based all-sky camera systems used to capture the two-dimensional horizontal patterns of airglow depletions are the main source of observations showing that ESF processes intrude to midlatitudes in the L ∼ 1.5 domain. In this paper we review the process of mapping airglow depletions along geomagnetic field lines to the equatorial plane, hence defining the maximum apex heights achieved. A case study comparison of simultaneous radar backscatter data from Kwajalein with optical data from Wake Island, sites that share common magnetic meridians in the Pacific section, confirms the utility of the approach and its applicability to sites at other longitudes. Modeling studies based on buoyancy arguments using flux tube–integrated mean density values versus L shell apex heights show that instability-induced plasma depletions starting at F layer bottomside heights easily reach altitudes above 2000 km in the equatorial plane, implying that ESF intrusions to lower midlatitudes should be a relatively frequent occurrence.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005JA011157

Peter J. Sultan

Papers

DMSP observations of equatorial plasma bubbles in the topside ionosphere near solar maximum

Onset conditions for equatorial spread F

Ionospheric disturbances observed by DMSP at middle to low latitudes during the magnetic storm of June 4–6, 1991

The 27-day variations of plasma densities and temperatures in the topside ionosphere

Observations and modeling of the coupled latitude-altitude patterns of equatorial plasma depletions