James L. Scali

Investigations of thermospheric-ionospheric dynamics with 6300-Å images from the Arecibo Observatory

Pilot observations were conducted at Arecibo, Puerto Rico, using an all-sky, image-intensified CCD camera system in conjunction with radar, ionosonde, and Global Positioning System (GPS) diagnostic systems during the periods January 19–28, 1993, and February 21 to August 22, 1995. These represent the first use of campaign mode operations of an imager at Arecibo for extended periods of F region observations. The January 1993 period (the so-called “10-day run”) yielded a rich data set of gravity wave signatures, perhaps the first case of direct imaging of thermospheric wave train properties in the F region. The 6-month 1995 campaign revealed two additional optical signatures of F region dynamics. A brightness wave in 6300 A passing rapidly through the field of view (FOV) has been linked to meridional winds driven by the midnight temperature maximum (MTM) pressure bulge. On May 3, 1995, during a period of geomagnetic activity, a 6300-A airglow depletion pattern entered the Arecibo FOV. Such effects represent the optical signatures of equatorial spread F instabilities that rise above the equator to heights near 2500 km, thereby affecting Arecibos L = 1.4 flux tube.

Electrodynamics of midlatitude spread F: 1. Observations of unstable, gravity wave‐induced ionospheric electric fields at tropical latitudes

In part 1 of our series exploring the role of electrical forces in midlatitude spread F, we present observations of an elect.rolyiia.iuically driven traveling ionospheric disturbance which passed over Arecibo observatory between 22 and 24 AST on.January 26, 1993. The total electric potential differences driving the wave were of the order of 1 kV. Our analysis indicates that this disturbance is the result of a midlatitude F region plasma instability seeded by a therniospheric gravity wave. Two novel measurements, in addition to typical incoherent scatter observations, were crucial to this determinatiou: tie use of G300 A airglow images front the coupling, energetics, and dynamics of atmospheric regions (CEDAR) all-sky imager to track the two-dimensional, mesoscale dynamics of the disturbance and the rise of a portable ionosonde to simultaneonsly measure the field line integrated ionospheric conductivity in the conjugate hemisphere. we have also determined that this disturbance, like several previously observed midlatitude disturbances, is consistent with our theoretical knowledge of the basic instability of the midlatitude ionosphere described originally by Perkins [1973].

Ionospheric sounding in support of over‐the‐horizon radar

Precise coordinate registration for HF over-the-horizon (OTH) radar applications requires accurate knowledge of the ionospheric structure. In the mid-1980s Digisonde 256 systems were deployed in the American sector to provide this information from strategically located sites via telephone lines to the user. The mid-1990s saw the development of a new advanced system, the Digisonde portable sounder, or DPS, now being deployed in Australia in support of the Australian OTH radar system. A summary of the new features provided by the DPS is as follows: low radio frequency power (300 W); narrow transmission bandwidth; advanced automatic scaling; and control and data access via the Internet. The availability of real-time electron density profiles as function of time from a network of stations makes it possible to calculate the three-dimensional electron density distribution in the region of interest using Fourier transform techniques. The resulting density maps are the basis for the OTH radar coordinate registration. The DPS uses Doppler interferometry to determine the development of ionospheric irregularities.

Coordinated digisonde and incoherent scatter radar F region drift measurements at Sondre Stromfjord

Comparison of drift measurements made at Sondre Stromfjord show that the apparent velocities measured by the digisonde (DGS) are in good agreement with the drift velocities observed by the collocated incoherent scatter radar (ISR). Data from December 5 to 9, 1991, show the mean DGS velocities to be within 50 m s−1 of the ISR velocities, that is, within the uncertainty levels of each instrument. The analysis highlights the dominance of the electric field in controlling the plasma motion. The measured velocities are generally height independent, as would be expected for an E field mapped along the magnetic field lines from high altitudes to ionospheric heights. In addition, the comparative analysis is used to study an ionospheric event where a large section of ionization was removed from the daytime cusp region by a strong anti-Sunward drift when the interplanetary magnetic field (IMF) Bz component changed orientation.

Spread F and the structure of equatorial ionization depletions in the southern anomaly region

Combined optical and radio sensors provide a unique characterization of the structure of equatorial emission depletion regions connected to rising bubbles over the magnetic equator. In Chile, as part of the MISETA campaign in fall 1994, a CCD-enhanced all-sky imaging photometer provided optical images of the postsunset appearance and motions of the depletion bands at a magnetic dip latitude of 11°S. Concurrently, a Digisonde collocated with the photometer monitored the appearance of spread F. In between the ionograms, the sounder operated as a Doppler interferometer identifying the locations of F layer irregularities associated with the spread F. They were found to lie inside the emission depletion regions. The HF sounder, requiring orthogonality with the field-aligned F layer irregularities to generate the spread structure, tracked these irregularities inside the emission depletion bands as they drifted eastward. Ray tracing simulations show that the radio waves become trapped within the depletion regions when the depletions are within 300 km of the sounder site. Model calculations indicate that the sounder rays encounter orthogonality with the Earths magnetic field within the depletion bubble southward from the site, consistent with the local dip angle. The combination of optical images with HF radio sounding demonstrated that radio imaging in the equatorial ionosphere can be done with a digital ionosonde that operates as a Doppler interferometer. The Digisonde measurements and ray tracing show for the first time that the spread F signatures on ionograms are the result of coherent scatter from irregularities primarily within the walls of the depletion.

The accuracy of ionogram-derived N(h) profiles

Abstract At the Millstone Hill station the Incoherent Scatter Radar (ISR) and a Digisonde 256 are simultaneously operating. Some characteristic true heights determined by both instruments are compared with each other, possible reasons for observed difference are indicated.

Equatorial F region irregularity morphology during an equinoctial month at solar minimum

A large number of instruments was used in October 1996 to record activities in the equatorial ionosphere above South America. In a month at solar minimum, data were obtained at various levels of magnetic activity and various levels of ionospheric irregularity development. With this multi-instrumented study, it was possible to utilize optical data, radar, GPS transmissions, and ionosondes at various sites in the equatorial region. The concept of this paper is to review the plethora of events which occurred during this month with a view to describing the interrelationship of the wide variety of irregularity developments. Data were obtained on nights when no irregularities were observed at any location in the equatorial region across South America. There were nights when only localized irregularity structures with relatively narrow latitudinal and longitudinal effects were noted close to the magnetic equator. We noted the occasional presence in the 02–06 local time period of plume structures with data available from optical observations as well as from phase and amplitude scintillation. During a major magnetic storm on one night, October 22–23, a long lasting high altitude plume was detected by the Jicamarca radar. On this night, irregularities were noted all across South America and even beyond the western and eastern coasts. This plume produced ionospheric effects which could be traced to turbulence at over 2000 km above the magnetic equator. With additional data from high latitude stations and from Guam and Kwajelein, it was possible to link and compare irregularity development in the same time period over a large portion of the globe. The aim of this paper is to give a day-to-day picture of the occurrence and intensity of equatorial irregularity development over a month-long period rather than a short case study or the converse, long term statistics over several seasons. Using this database and the modeling of total electron content as a function of solar flux, we outline the possibilities and limitations for forecasting irregularity activity in this region for a period of low solar flux. Forecasting is limited and calls for experimental data for necessary and sufficient gradients and wind conditions for plumes to fully develop.

Incoherent scatter radar and Digisonde observations at tropical latitudes, including conjugate point studies

In this paper we compare ionospheric measurements made during the January 1993 10-day World Day Campaign with three separate instruments: the Arecibo incoherent scatter radar, the Ramey Digisonde (located 50 km west of Arecibo), and the Puerto Madryn, Argentina, Digisonde (located 400 km from the magnetic conjugate point of Arecibo). F layer peak heights from Ramey agree well with those from Arecibo throughout the entire 10-day period, with a median difference of 4 km, and along with the F peak density show effects of a 4-hour periodic temporal variation. We relate this 4-hour variation to the familiar midday biteout in the electron density that is commonly observed at Arecibo in the winter months. However, this periodic temporal variation is not observed at Puerto Madryn, which is located at a higher geographic latitude. Only occasionally do the Arecibo incoherent scatter radar and the Ramey Digisonde velocity measurements agree. Of course, it must be remembered that while the Doppler shift in an incoherent scatter radar echo is a direct measure of the line-of-sight plasma velocity component, the Doppler shift of a Digisonde echo (or any ionosonde echo for that matter) is a measure of the time rate of change of the electrical distance to the reflection point. Differences between the velocity components obtained by the two techniques are indications of the importance of chemistry and divergent transport instead of simple motions. Over the ten-day campaign and in the average of the 10 days, the Arecibo and Ramey horizontal velocities show good agreement only for a short period at night from 0000 to 0700 LT (0400 to 1100 UT). During this time the HF radio waves were reflected from relatively large zenith angles and the ionosphere was fairly high. An increased westward component of the horizontal velocity measured at Arecibo and Ramey around 0330 LT (0730 UT), is correlated with sunrise in the summer conjugate hemisphere, but the drift velocity was smaller by about a factor of 2 at the southern station. Since we doubt that the field lines are not equipotentials, the difference observed could be due to Puerto Madryn not being located at the exact conjugate point for Arecibo and Ramey.

High latitude Digisonde measurements and their relevance to IRI

Abstract Digisondes operating at high latitudes provide new insights into the structure and dynamics of the polar cap and auroral ionosphere. These instruments derive the electron density profiles from vertical incidence ionograms, and the horizontal and vertical plasma velocities using the interferometric Doppler imaging drift technique. Multi-beam and polarization measurements in the ionogram unambiguously define the critical frequency of the E and F layers even in the presence of spread F and large scale patches. Occurrence of the patches is controlled by the configuration of the polar cap convection pattern which in turn is driven by the Bz and By components of the interplanetary magnetic field. To specify the UT and seasonal variations of the polar cap F region for IRI requires the knowledge of the prevailing convection pattern. This paper describes the applied techniques to measure profiles and drifts at high latitudes and the status of the analysis on polar cap patches and their transport across the polar cap.

Geomagnetic storm time studies using Digisonde data

Abstract The initial, main and recovery stages of three magnetic storms that occurred during March 1990, June 1991 and November 1993 are discussed in terms of the response of the ionosphere as measured by Digisondes in the polar, high latitude and equatorial regions. The results show that during increased geomagnetic activity, convection velocities at high latitudes and polar regions increase, while during the main phase of intense storms the E×B vertical drift in the equatorial region is reduced. Comparison of the three storm periods indicites seasonal differences in the response of the ionosphere to increased magnetic activity.