John D. Mathews

SPORADIC E: CURRENT VIEWS AND RECENT PROGRESS

Abstract Sporadic E (Es) and related processes are reviewed as functions of viewing system, latitude and altitude. We find that the “windshear theory”—perhaps with a small added external electric field—appears sufficient along with the tidal wind system to explain the mid-latitude layers which we refer to as tidal ion layers (TILs)—layers that have often been identified as sporadic E or as sequential sporadic E. Additionally, it has become clear that the high-latitude, altitude-narrow layers, also often identified as sporadic E, are consistently explained as being formed in appropriate large-scale convective electric field structures with the wind system playing a lesser role. Finally, we find that “true” sporadic E—an altitude-thin E region layer at an unpredictable altitude and/or an unexpected intensity—is found in a rich context of related phenomena that includes the tidal ion layers, electric-field-induced layers, HF/VHF radar quasi-periodic echoing (QPE) regions, and an apparently newly observed phenomenon descriptively termed “ion rain.” We conclude that the QPEs and “ion rain” indicate small horizontal scales and find considerable other evidence of order 10–100 km scale horizontal-structuring of layers which at least hints at an E/F region coupled-electrodynamic process, or processes. These processes apparently include instability-generated 10–100 km horizontal-scale E-fields that—we hypothesize—generate true sporadic E and the related complex layer structures (CLS) via horizontal redistribution of ions.

Electrical discharge from a thundercloud top to the lower ionosphere

For over a century, numerous undocumented reports have appeared about unusual large-scale luminous phenomena above thunderclouds and, more than 80 years ago, it was suggested that an electrical discharge could bridge the gap between a thundercloud and the upper atmosphere. Since then, two classes of vertically extensive optical flashes above thunderclouds have been identified—sprites and blue jets. Sprites initiate near the base of the ionosphere, develop very rapidly downwards at speeds which can exceed 107 m s-1 (ref. 15), and assume many different geometrical forms. In contrast, blue jets develop upwards from cloud tops at speeds of the order of 105 m s-1 and are characterized by a blue conical shape. But no experimental data related to sprites or blue jets have been reported which conclusively indicate that they establish a direct path of electrical contact between a thundercloud and the lower ionosphere. Here we report a video recording of a blue jet propagating upwards from a thundercloud to an altitude of about 70 km, taken at the Arecibo Observatory, Puerto Rico. Above an altitude of 42 km—normally the upper limit for blue jets and the lower terminal altitude for sprites—the flash exhibited some features normally observed in sprites. As we observed this phenomenon above a relatively small thunderstorm cell, we speculate that it may be common and therefore represent an unaccounted for component of the global electric circuit.

The micrometeoroid mass flux into the upper atmosphere : Arecibo results and a comparison with prior estimates

Radar micrometeor observations at Arecibo Observatory have enabled direct estimates of the meteoroid mass flux into the upper atmosphere. We report mass flux determinations from November 1997/1998 observations that are based on the observed number of meteor events per day in the 300-m diameter Arecibo beam and on particle mass determinations from that fraction of all particles for which deceleration is measured. The average mass of the Arecibo micrometeoroids that manifest observable deceleration is ∼0.32/0.76 µgm/particle with a resultant annual whole-Earth mass flux of 1.6 × 106/2.7 × 106 kg/yr over the ∼10−5−10² µgm mass range for 1997/1998, respectively. The annual whole-earth mass flux per decade of particle mass is calculated and compared with that of Ceplecha et al. [1998] (3.7 × 106 kg/yr) and with that derived by Love and Brownlee [1993] (LB) from small particle impact craters on the orbital Long Duration Exposure Facility (LDEF). We also give the LDEF results as significantly modified using the Arecibo-determined average particle velocity of ∼50 km/sec—much larger than the effective value of 12 km/sec used by LB. This modification results in a net LDEF mass flux of 1.8×106 kg/yr—7% of the value we determined from reanalysis of the LB data using their original 12 km/sec mean impact speed. These results may provoke some debate.

Lidar, radar and airglow observations of a prominent sporadic Na/sporadic E layer event at Arecibo during AIDA-89

Abstract Sporadic Na (Na,) layer events were frequently identified during 160 h of lidar observations at Arecibo in January, March and April 1989. Most were accompanied by sporadic E ( E .) layers. The most spectacular Na s E s , event occurred on the night of 30–31 March when both the Na and electron abundances between 90 and 100 km increased by approximately 700% during a period of 2.25 h starting at 2100 LST. The maximum Na density was almost 42,000 cm 3 . The vertical and temporal structure of the Na and electron densities were remarkably similar during the event. The ratio of the average Na enhancement to the electron density varied from a maximum of 3.5 Na atoms/electron at 98 km to about 0.5 Na atoms/ electron below 94 km. Between 93 and 97 km the electron enhancement preceded the Na enhancement by 15–30 min. Above 97 km and below 93 km the Na and electron density variations were in phase. The data suggest that the E s , layer triggered the release of Na from a reservoir, but the E s layer was not the source of the major Na s layer. Two minor Na s layers were observed between 101 and 107 km after midnight LST which were also accompanied by intense s layers and enhancements of the O( 1 S) emission intensities. The abundances of these high altitude Na s layers were less than 1% of the electron abundances. These Na, layers appear to be caused by the conversion of Na in the E s layer to Na through a set of clustering reactions involving N 2 CO 2 and H 2 O.

Observations of ion layer motions during the AIDA campaign

Abstract The AIDA-89 campaign has yielded the most comprehensive set of low-latitude incoherent scatter radar power profiles and derived electron concentration results ever made. These results have been used to study the time-height trajectories of 80–150 km ion layers and serve to gauge both the periodicity and variability of ion layer structure throughout the campaign. Features of the AIDA ion layer trajectories point to a dynamics ‘zoo’ of processes ranging from multiday-period waves, tides and acoustic-gravity waves (AGWs) to geomagnetic storm effects and evidence of coupled neutral sodium and ion layer/plasma processes. The semidiurnal and diurnal tides are evidenced in the almost always present layers, the Tidal Ion Layers (TILs), which are identified by their regular and periodic trajectories that also display regions of variable mixing or confluence of the various tides. The TILs are contrasted with the truly sporadic layers that include sporadic E and sporadic intermediate layers. The sporadic layers may be formed due to interaction of the tidal wind system with AGWs. The formation process may involve horizontal as well as vertical ion convergence mechanisms and/or various non-linear effects. Limits to the study derive from volume undersampling due to use of the single radar beam.

Implications of meteor observations by the MU Radar

We report high resolution meteor echo observations using the Kyoto University Middle and Upper (MU) Atmosphere 46.5 MHz Radar. When the MU radar was pointed perpendicular to the geomagnetic field lines (B), numerous long-lived range spread trail echoes were observed which were largely absent when the beam was pointed in the vertical and parallel-to-B directions. This shows that this type of trail echo is largely due to scattering structures aligned along B. Additionally, nearly all the head echoes displaying an along-the-beam velocity component were followed by range spread echoes in the perpendicular-to-B pointing geometry. This demonstrates that meteoric field aligned irregularity is present in essentially all meteors up to the detection limit of the MU radar. Practically all the spectra are limited within a bandwidth corresponding to a Doppler shift of 320 m/s, suggesting that the two stream instability is absent most of the time. Meteoric field aligned structures can be a potential error source for aeronomical applications if they are not appropriately considered.

Incoherent Scatter Radar Probing of the 60-100-km Atmosphere and Ionosphere

All current incoherent backscatter radars can make mesospheric or D region measurements (60-90-km altitude) under at least some conditions. This paper, which is tutorial in nature, develops the basic concepts of incoherent scatter radar measurements in the D region and the conditions under which measurements are possible are derived and shown for each radar. Conditions examined include overall system sensitivity, electron and ion spectral line widths and power distributions, and time/height averaging effects. The appropriate form of the radar equation is derived and calibration of the radar system is discussed along with the various aspects of signal processing involved. Total power only and combined total power and ion line spectral measurements are described in terms of ease of use, applicability to various radars, and parameters of aeronomic interest derivable from the measurements.

Simultaneous meteor echo observations by large‐aperture VHF and UHF radars

We report simultaneous meteor echo observations using the Arecibo 430-MHz and 46.8-MHz radars. Using identical data-taking and meteor selection criteria, 1868 and 367 meteors were found in the 430-MHz and 46.8-MHz beams, respectively, while 145 were found in both beams during the 7 hours of observation. Of the 367 VHF echoes, there were only 10 trail echoes, while the rest were head echoes, which was quite contrary to expectation. The smaller number of meteors detected by the VHF system and its wider beam width show that UHF meteors are far smaller than the VHF meteors. We estimate that VHF head echoes have a typical effective scattering cross section of the order of 10−3 m2, while the accompanying UHF echoes have an effective scattering cross section of the order of 10−6 m2. The paucity of VHF trail echoes observed leads us to suggest that the ratio of head echo power to the trail echo power increases with decreasing meteor size. When a meteor is too small, a radar can observe the head echo but not the trail echo. Of the 145 meteors observed by both radars, the powers received by the two systems were not correlated. Although antenna beam pattern contributes to the lack of correlation, it is also possible that UHF and VHF echoes may be enhanced by different scattering mechanisms.

A proposed temperature dependent mechanism for the formation of sporadic sodium layers

Abstract We examine the influence of temperature fluctuations on the formation of sporadic sodium layers (SSLs) with particular emphasis on AIDA (Arecibo Initiative in the Dynamics of the Atmosphere) results. We present evidence suggesting that sodium abundance is very sensitive to the temperature. A 10 K increase in mesopause temperature may double the sodium concentration. Thus the sodium profile may change significantly if appropriate thermal fluctuations due to tides and/or acoustic-gravity waves (AGWs) occur. Gravity wave theory predicts that the ion convergence node, without other influences, coincides with a temperature maximum for a westward propagating wave. In this case, the ion layer coincides with the temperature maximum which results in a higher sodium concentration at or near the ion layer height. This proposed temperature dependency can, for the tidal wind field, account for the observed correlation between sodium and ion column abundances and is supported by the average O 2 (0–1) rotational temperature determinations made at Arecibo. Specifically, we propose that the formation of SSLs is due to the temperature fluctuations induced by AGWs, or other wave processes, in conjunction with a background tidal wind system. Additionally, we argue that when an AGW propagates westward, the SSL coincides with an existing tidal ion layer or with a true sporadic- E layer which forms in the net wave field convergence zone. We also note that roughly the same processes may apply to the production of intense sporadic- E layers.

The incoherent scatter radar as a tool for studying the ionospheric D-region

The theory of the D-region incoherent scatter process and the techniques for incoherent scatter radar measurements in the D-region have both reached sufficient maturity that a synopsis of this area for the middle atmosphere community is appropriate. For example, all current UHF incoherent scatter radars can make useful D-region measurements, at least under some conditions, and the Arecibo 430 MHz radar is being used for a regular program of D-region measurements. Atmospheric and ionospheric parameters which are accessible, to varying degrees, as functions of time and height include electron concentration, ion-neutral collision frequency, neutral atmosphere temperature pressure and winds and mean negative and positive ion masses and concentrations. A qualitative view of the collision dominated incoherent scatter process is given as an aid in understanding the capabilities and limitations of these measurements. Included here is a discussion of some of the issues still surrounding the theory of collision dominated incoherent scattering. Also presented is an overview of data gathering, processing and interpretation techniques, with comments on how total power and power spectrum measurements should be combined to give optimum results. Finally, a comprehensive list of outstanding problems in D-region aeronomy is given, along with suggestions of how incoherent scatter radar along with other MAP measurements can address them. Included in this list are the winter absorption anomaly. D-region ‘ledge’ chemistry and high latitude D-region phenomena.