G. D. Earle

Behavior of the O+/H+ transition height during the extreme solar minimum of 2008

[1]xa0Typically the solar radio emission at 10.7 cm is used to scale the critical euv radiation that is absorbed by the Earths neutral atmosphere. In the latter half of 2008 this radio emission from the Sun was at the lowest levels seen in the last 50 years and the persistence of these low levels has never been recorded before. Here we show that these uniquely low levels of solar radiation produce similarly unique behavior in the Earths ionosphere and the upper atmosphere. Most remarkably, the altitude extent of the ionosphere is significantly smaller than our present reference models would predict for these levels of solar activity. The transition height resides near 450 km at night and rises to only 850 km during the daytime. At night, this unusually contracted ionospheric shell around the equator has a temperature of only 600 K and prior to sunrise the ion number densities at the transition height fall below 104 cm−3.

Arecibo observations of ionospheric perturbations associated with the passage of Tropical Storm Odette

[1]xa0A suite of instruments including incoherent scatter radar, ionosonde, and a satellite-bourne GPS receiver observed the ionosphere immediately following the passage of a tropical storm. Tropical Storm Odette formed on 4 December 2003 and proceeded northeasterly over the next 4 days, passing within 600 km of the Arecibo Observatory (AO). On the night of 7–8 December AO measured F region plasma densities and velocities nearly coincident with the storm. Large velocity variations, 10–80 m/s, are evident in the plasma drift components. The variations appear wave-like with an average period of 90 min at 367 km. Zonal drifts were observed with magnitudes significantly greater than commonly observed for similar geomagnetic conditions. The Ramey ionosonde observed intense midlatitude spread F on the night following the closest passage of the storm. GPS occultations within the storm path showed an increase in gravity wave activity and F region scintillation. Combining the local increase in gravity wave activity with the large drift variations and dominant meridional electric field observed immediately following the storms traversal of the flux tube coincident with the AO observing volume provide insight into coupling between mesoscale weather systems and the ionosphere.

Medium‐scale equatorial plasma irregularities observed by Coupled Ion‐Neutral Dynamics Investigation sensors aboard the Communication Navigation Outage Forecast System in a prolonged solar minimum

[1]xa0The distribution of medium-scale irregularities in the total ion density at the equator is investigated. In the scale size range between 10 and 400 km, it is found that, as expected, these irregularities preferentially appear near 2100 local time (LT) in longitude regions that are selected by season according to an alignment between the magnetic meridian and the sunset terminator. However, these irregularities have a maximum occurrence frequency in the postmidnight sector and do not conform to the expected behavior seen for irregularities that appear after sunset. We suggest that the postmidnight peak in the occurrence frequency for these irregularities arose from the weak vertical drifts that prevail in the afternoon and evening during a prolonged solar minimum. It is also suggested that the observed longitude and seasonal dependence in the peak occurrence frequency is influenced by seeding from tropospheric sources, and therefore responds to the seasonal variations in the colocation of the magnetic equator and the Intertropical Convergence Zone. The irregularities appear throughout the nighttime period when the background density is declining rapidly. Thus, despite the postmidnight maximum in occurrence frequency, the maximum absolute perturbation density, most likely to be responsible for radio scintillation, occurs in the premidnight sector.

Ion temperature and density relationships measured by CINDI from the C/NOFS spacecraft during solar minimum

[1]xa0The Ion Velocity Meter (IVM), a part of the CINDI instrument package on board the C/NOFS spacecraft, makes in situ measurements of plasma temperature, composition, density, and velocity. The 16 April 2008 launch of C/NOFS coincided with the deepest solar minimum since the space age began with F10.7 cm radio fluxes in the 60–70 solar flux unit range. Because of the 13° inclination of the orbit the location of the perigee advances through all local times in about 66 days. This allows seasonal sampling of ionospheric temperature, density, and composition as a function of local time, magnetic latitude, and altitude. Measurements taken near the spacecrafts 402 km perigee altitude indicate an unusually cold low-density ionosphere with nighttime ion temperatures at the magnetic equator reaching as low as 600 K with an [O+]/[H+] ratio of 4 and maximum daytime temperatures of 1300 K. The O+ to H+ transition height is very low and at the highest altitudes measured H+ comprises over 75% of the ionospheric plasma at all local times. We compare average values of the measured parameters with those from the International Reference Ionosphere and with incoherent scatter radar measurements from Jicamarca.

Relative concentrations of molecular and metallic ions in midlatitude intermediate and sporadic‐E layers

[1]xa0A NASA sounding rocket launched from Wallops Island, VA (37.84 N, 75.48 W) on 1 July 2003 at 2:50 EST made the first in situ measurement of the relative concentrations of Fe+, Mg+, O2+, and NO+ within a nighttime intermediate layer below 140 km altitude. Ion composition measurements were made from 80–220 km altitude and included observations of three separate regions having high concentrations of metallic ions: the intermediate layer at 118 km, a sporadic-E layer at 105 km, and a third layer above 160 km altitude. These observations demonstrate that metallic ions may be a significant source of ionization in the nighttime E and F region ionosphere at midlatitudes.

Ground and Space-Based Measurement of Rocket Engine Burns in the Ionosphere

On-orbit firings of both liquid and solid rocket motors provide localized disturbances to the plasma in the upper atmosphere. Large amounts of energy are deposited to ionosphere in the form of expanding exhaust vapors which change the composition and flow velocity. Charge exchange between the neutral exhaust molecules and the background ions (mainly O+) yields energetic ion beams. The rapidly moving pickup ions excite plasma instabilities and yield optical emissions after dissociative recombination with ambient electrons. Line-of-sight techniques for remote measurements rocket burn effects include direct observation of plume optical emissions with ground and satellite cameras, and plume scatter with UHF and higher frequency radars. Long range detection with HF radars is possible if the burns occur in the dense part of the ionosphere. The exhaust vapors initiate plasma turbulence in the ionosphere that can scatter HF radar waves launched from ground transmitters. Solid rocket motors provide particulates that become charged in the ionosphere and may excite dusty plasma instabilities. Hypersonic exhaust flow impacting the ionospheric plasma launches a low-frequency, electromagnetic pulse that is detectable using satellites with electric field booms. If the exhaust cloud itself passes over a satellite, in situ detectors measure increased ion-acoustic wave turbulence, enhanced neutral and plasma densities, elevated ion temperatures, and magnetic field perturbations. All of these techniques can be used for long range observations of plumes in the ionosphere. To demonstrate such long range measurements, several experiments were conducted by the Naval Research Laboratory including the Charged Aerosol Release Experiment, the Shuttle Ionospheric Modification with Pulsed Localized Exhaust experiments, and the Shuttle Exhaust Ionospheric Turbulence Experiments.

Satellite‐based measurements of gravity wave‐induced midlatitude plasma density perturbations

[1] Large amplitude anticorrelated wave structures appear at midlatitudes in the nighttime ionospheric plasma and neutral density measurements made at altitudes between 250 and 300 km by the Dynamics Explorer-2 satellite. The wavelengths along the satellite orbit track are generally longer than a hundred kilometers, and the vertical perturbation velocities are about 20 m/s. These characteristics are consistent with plasma motions driven by gravity waves in the neutral atmosphere as they propagate upward from lower atmospheric source regions. The altitude and the horizontal wavelengths observed provide in situ empirical validation of a recently developed gravity wave propagation model that includes the effects of both kinematic viscosity and thermal diffusivity at high altitudes.

A comprehensive rocket and radar study of midlatitude spread F

[1]xa0An instrumented sounding rocket launched from Wallops Island Virginia has flown through a midlatitude spread F (MSF) event in conjunction with simultaneous ionosonde, HF radar, and 244 MHz scintillation observations from the ground. The in situ measurements include the electric field, horizontal neutral wind, and plasma density within the spread F region. The ground-based HF radar measurements of wave signatures in the bottomside F region ledge reveal the presence of waves propagating to the north and northwest prior to and during the spreading event. The periods of these bottomside waves range from 16 to 60 min, and they are shown to be associated with a strong tropical storm located ∼2000 km southeast of the launch site. Enhancements in the auroral current system occur about an hour before the MSF first appears, but none of the observed waves can be attributed to this source. The new phase-sensitive ionosonde system operated at Wallops Island during the experiment confirms the long-standing hypothesis that this particular spread F event arises from multipath echoes distributed over a wide field of view in the bottomside F region. Evidence of vertically displaced plasma that could produce such multipath echoes is observed in the rocket data at and above the F region peak over spatial scales smaller than the wavelengths observed on the bottomside ledge by the HF radar, but similar to the range separation given by the high resolution ionosonde echoes when the scale lengths of the structures are interpreted in magnetic coordinates. No significant plasma density structures smaller than a few kilometers are observed in the rocket data, and no unusual scintillation is observed along a path coincident with the rocket trajectory.

Metallic ion transport associated with midlatitude intermediate layer development

[1]xa0Although intermediate layers are frequently observed by the Arecibo Incoherent Radar Observatory and by ionosondes around the world, many questions still remain regarding their formation, structure, and composition. In this paper, we explore the effect of metallic ions, specifically Fe+, on intermediate layer development and evolution. Several studies have demonstrated that layers can form from either molecular or metallic ions. This paper extends these earlier studies by quantifying the effect of metallic ions on intermediate layer morphology. We show that the efficiency of metallic ion transport depends significantly on the amplitude and wavelength of the imposed horizontal wind field. Specifically, larger amplitudes and longer wavelengths result in increased ion transport in the direction of the propagating neutral wind field.

Ion Layer Separation and Equilibrium Zonal Winds in Midlatitude Sporadic E

In-situ observations of a moderately strong midlatitude sporadic-E layer show a separation in altitude between distinct sublayers composed of Fe+, Mg+, and NO+. From these observations it is possible to estimate the zonal wind field consistent with diffusive equilibrium near the altitude of the layer. The amplitude of the zonal wind necessary to sustain the layer against diffusive effects is less than 10 m/s, and the vertical wavelength is less than 10 km.