Typically the solar radio emission at 10.7 cm is used to scale the critical euv radiation that is absorbed by the Earth's 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 Earth's 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.

Spectral analyses have been performed upon a number of incoherent scatter radar data sets obtained at Jicamarca, Peru; Chatanika, Alaska; and Arecibo, Puerto Rico, with the goal of understanding the sources of electric fields with periods in the range of 1–10 hours. Two distinct sources are identified and studied in some detail. In quiet times, atmospheric gravity waves seem the most likely source of the ionospheric electric field. In fact, both in an average sense and in the single case study available the mesospheric winds measured at Poker Flat, Alaska, in this frequency range are remarkably similar in magnitude to the quiet time thermospheric plasma drifts measured overhead by the nearby Chatanika radar. Such drifts are driven by electric fields which, we argue, could easily be generated by the observed wind fields. Comparison with the spectra of electric field measurements at other latitudes suggests that such a source is worldwide and determines the geophysical noise level of low‐ and mid‐latitude electric field measurements. Turning to active times, we present a measure of the transfer function for electric field penetration between high‐ and low‐altitude L shells. At the very lowest frequencies (periods of ≥10 hours) the low‐altitude sites are well shielded, presumably by an Alfvén layer at the inner edge of the ring current. Higher frequency fluctuations penetrate very easily to low latitudes. A response peak seems to occur in the 3‐ to 5‐hour range of periods, with a lower response occurring at 1 cycle/hour, although this result must be viewed as preliminary for now. Between L = 5.5 and L = 1.4 the zonal electric field component as projected to the equatorial plane of the magnetosphere penetrates with little or no attenuation. At Jicamarca the field is reduced to about 75% of the high‐latitude source. Even so, the very high sensitivity of the Jicamarca measurement of zonal electric fields makes this component very easy to detect. We find that for fluctuation periods less than 5 hours the magnetospheric electric field source dominates atmospheric sources for Kp ≥ 3 at Jicamarca. In one remarkable event, six cycles of a 1‐ cycle/hour signal were recorded with nearly identical spectral features in the interplanetary medium, at auroral zone latitudes, and on both sides of the earth at equatorial ionospheric heights. The ratio of the relative magnitude of this oscillation at L = 5.5 to its value at L = 1.04 is in good agreement with the value of the average transfer function at 1 cycle/hour found in the 24‐hour runs.

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 Manuscript

We present and analyze sounding rocket and HILAT satellite measurements of the low frequency (< 1 Hz) electric and magnetic fields δE and δB perpendicular to the Earth's magnetic field B0 in the auroral oval. By examining the time‐domain field data it is often difficult to distinguish temporal fluctuations from static structures which are Doppler shifted to a non‐zero frequency in the spacecraft frame. However, we show that such a distinction can be made by constructing the impedance function Z(f) = μ0|δE(f)/δB(f)|. Using Z(f) we find agreement with the static field interpretation below about 0.1 Hz in the spacecraft frame, i.e. Z(f) = Σp−1 where Σp is the height‐integrated Pedersen conductivity of the ionosphere. Above 0.1 Hz we find Z(f) > Σp−1, which we argue to be due to the presence of Alfvén waves incident from the magnetosphere and reflecting from the lower ionosphere, forming a standing wave pattern. These waves may represent an electromagnetic coupling mechanism between the auroral acceleration region and the ionosphere.

[1] The 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.