L. C. Gentile

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

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.

Electrodynamics of the inner magnetosphere observed in the dusk sector by CRRES and DMSP during the magnetic storm of June 4–6, 1991

We compare equatorward/earthward boundaries of convection electric fields and auroral/plasma sheet electrons detected by the DMSP F8 and CRRES satellites during the June 1991 magnetic storm. Measurements come from the dusk magnetic local time sector where the ring current penetrates closest to the Earth. The storm was triggered by a rapid increase in the solar wind dynamic pressure accompanied by a southward turning of the interplanetary magnetic field (IMF). Satellite data show the following: (1) all particle and field boundaries moved equatorward/earthward during the initial phase, probably in response to the strong southward IMF turning; (2) electric field boundaries were either at lower magnetic L shells or close to the inner edge of ring current ions throughout the main and early recovery phases. Penetration earthward of the ring current occurred twice as the polar cap potential increased rapidly; (3) electric potentials at subauroral latitudes were large fractions of the total potentials in the afternoon cell, twice exceeding 60 kV; and (4) the boundaries of auroral electron precipitation were more variable than those of electric fields and mapped to lower L shells than where CRRES encountered plasma sheet electrons. Observations qualitatively agree with predictions of empirical models for auroral electron and electric field boundaries.

C/NOFS observations of deep plasma depletions at dawn

[1]xa0The Communication/Navigation Outage Forecasting System (C/NOFS) satellite was launched in 2008, during solar minimum conditions. An unexpected feature in the C/NOFS plasma density data is the presence of deep plasma depletions observed at sunrise at all satellite altitudes. Ionospheric irregularities are often embedded within these dawn depletions. Their frequencies strongly depend on longitude and season. Dawn depletions are also observed in coincident satellite passes such as DMSP and CHAMP. In one example the depletion extended 50° × 14° in the N-S and E-W directions, respectively. These depletions are caused by upward plasma drifts observed in C/NOFS and ground-based measurements. The reason for these upward drifts is still unresolved. We discuss the roles of dynamo electric fields, over-shielding, and tidal effects as sources for the reported depletions.

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

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].

C/NOFS observations of plasma density and electric field irregularities at post-midnight local times

[1]xa0We report on plasma densities and electric fields measured by the C/NOFS satellite between 10 and 20 June 2008. Midway through the interval, geomagnetic conditions changed from quiescent to disturbed as a high speed stream (HSS) in the solar wind passed Earth. During the HSS passage C/NOFS encountered post-midnight irregularities that ranged from strong equatorial plasma bubbles to longitudinally broad depletions. At the leading edge of the HSS the interplanetary magnetic field rapidly intensified and rotated causing auroral electrojet currents to rise and fall within a few hours. As the electrojet relaxed, C/NOFS witnessed a rapid transition from a weakly to a strongly disturbed equatorial ionosphere that lasted ∼10 hours. Eastward polarization electric fields intensified within locally depleted flux tubes. We discuss relative contributions of gravity-driven currents, overshielding electric fields and disturbance dynamos as drivers of post-midnight depletions.

Climatology of plasma density depletions observed by DMSP in the dawn sector

[1]xa0Prior to the launch of the Communication/Navigation Outage Forecasting System (C/NOFS) satellite, equatorial plasma bubbles (EPBs) were regarded as postsunset phenomena. However, during this recent solar minimum the planar Langmuir probe (PLP) on the C/NOFS satellite has detected very few EPBs after sunset; most plasma density depletions have been observed between local midnight and dawn. We take advantage of the long history of plasma density measurements by a similar sensor on Defense Meteorological Satellite Program (DMSP) spacecraft to determine whether this change is typical of solar minima in general or unique to the present extended quiet time. In 2008 and 2009 the DMSP occurrence rates of topside plasma depletions in the dawn sector were unexpectedly high around the June and December solstices and extremely low near the March and September equinoxes. Dawn sector measurements from solar minimum years 1996–1997 exhibit similar seasonal and longitudinal distributions, but occurrence rates are significantly lower. While our analysis suggests that prevailing low levels of solar EUV flux and driving electric fields establish conditions favorable for the growth of postmidnight depletions, the primary causes of observed seasonal-longitudinal distributions remain unresolved.

Thermospheric heating by high‐speed streams in the solar wind

[1]xa0We compare thermospheric density (ρ) and exospheric temperature (T∞) responses measured by the Gravity Recovery and Climate Experiment (GRACE) satellites with systematic characteristics of recurring high-speed streams (HSSs) in the solar wind observed by the Advanced Composition Explorer (ACE) at L1 during the last four solar rotations of 2005. HSSs show remarkably similar features from one solar rotation to the next. Within corotating interaction regions (CIRs) at the leading edges of the HSSs plasma densities and magnetic fields steepen to excite low levels of geomagnetic and thermospheric activity. Consistent with origins in northern hemispheric coronal holes containing open, monopolar flux, interplanetary magnetic fields observed near L1 had average BX and BZ with the same polarities and opposite to that of BY. We show that a model used to estimate T∞ changes during large magnetic storms overpredicts CIR-driven thermospheric heating. However, large-amplitude Alfven waves in the interiors of HSSs generate regularly observed increases in T∞ as well as the auroral electrojet index. The regularity of interplanetary driving and thermospheric responses suggests the possibility of developing reliable 27-day alerts about impending increases in thermospheric drag exerted on objects in low Earth orbits during solar minimum.

Energy distributions of thruster pickup ions detected by the Shuttle Potential and Return Electron Experiment during TSS 1

Instrumentation to monitor the shuttle environment during the Tethered Satellite System (TSS 1) mission included the Shuttle Potential and Return Electron Experiment (SPREE) and the Quadrupole Ion-Neutral Mass Spectrometer (QINMS). SPREE measured fluxes of electrons and ions with energies between 10 eV and 10 keV; QINMS monitored neutral and ion species close to the shuttle. We report on energy distributions of pickup ions detected during and after thruster emissions while the shuttles velocity vector was nearly perpendicular to the Earths magnetic field. With SPREE looking in the ram direction and almost perpendicular to the magnetic field, fluxes >1011 ions cm−2 s−1 sr−1 were detected in the 10-to-60-eV energy range. Both prompt and prolonged ion flux enhancements were recorded. The prolonged fluxes lasted many seconds after the initiating thruster turned off. Only when thrusters fired close to the magnetic field direction were no pickup-ion flux enhancements detected. The SPREE measurements are compared with the predictions of a simple two-dimensional model of collisionless pickup-ion trajectories. Using the distribution of ion species recorded by QINMS, the model explains the main features of the ion energy spectra measured by SPREE. Comparison of the model with data also indicates that strong scattering of ejected materials must occur almost immediately after thruster emission and later between the time of pickup-ion creation and detection of the increased ion flux by SPREE.

Thruster effects on the shuttle potential during TSS 1

We have surveyed measurements taken by the Shuttle Potential and Return Electron Experiment (SPREE) in the shuttle payload bay during the satellite deployment phase of the Tethered Satellite System (TSS 1) mission. The shuttle was flying in an inverse airplane mode with its engine bells facing the ram direction to facilitate ion current collection. SPREE ion spectral measurements indicate that with no thrusters firing, the shuttle charged to, at most, a few tens of volts negative. We identified 13 intervals during which the tether was electrically connected to the shuttle by a low-impedance shunt and thruster firings occurred. In previously reported examples of neutral gas releases from charged spacecraft the magnitude of the vehicle potential decreased. Thruster firings during our events either left the TSS circuit unchanged or caused the current measured in the tether to diminish and the shuttle to become more negatively charged. In the latter case it appears that thruster gas impeded access of ambient ions to the current collecting surfaces of the shuttle. The changes in the distribution of the motionally induced potential between the tether and sheath calculated using Ohms law are in agreement with SPREE measurements.

Beam arc distributions of shuttle pickup ions and their instabilities

Data from the Shuttle Potential and Return Electron Experiment (SPREE) flown as part of the Tethered Satellite System (TSS 1) are used to determine the detailed characteristics of beam arc distributions of pickup ions due to molecules outgassed or ejected from the shuttle. These ion distributions are only detected near the plane perpendicular to the magnetic field direction in an angular range of ±45° about the minimum angle to ram. Their flux is largest when the angle between this plane and the shuttle ram direction is smallest. Generally, ion spectra peak in the range 19 to 25 eV at the minimum angle between the perpendicular plane and the ram direction. The peak energy decreases smoothly as this angle increases. Weak fluxes are measured above the peak, to energies as high as 150 eV. Within the SPREE energy range, two-dimensional distribution functions of beam arc ions in the perpendicular plane have teardrop shapes, symmetric about the minimum angle to shuttle ram with deep minima in the centers. Variations in the peak energies of differential number fluxes agree with collisionless trajectory analysis, assuming that the ions are H2O+ and allowing for different initial velocities before charge exchange. The lowest densities for beam arc ions occur during periods of purely residual outgassing from the shuttle. Ion densities increase by a factor of 5 during waste water dumps. The highest densities occur during operation of the flash evaporator system when the pickup ion densities in daylight can exceed 105 ions cm−3, about 30% of the estimated ambient plasma density. We also present a nonlinear numerical analysis to study the stability of beam arc generated plasma waves and explain electrostatic spectra measured during previous shuttle flights.