Geophysical Research Letters

Kelley, Michael C.; Makela, Jonathan J.; Swartz, Wesley E.; Collins, Stephen C.; Thonnard, Stefan; Aponte, Nestor; Tepley, Craig A.

doi: 10.1029/2000GL000022pmid: N/A

A series of campaigns has been carried out in the Caribbean over a one‐year period to study intense mid‐latitude spread‐F events using a cluster of diversified instrumentation. These events are relatively rare but a number of them have now been captured and will be discussed in this and several companion papers. This paper focuses on 630 nm airglow images obtained by the Cornell All‐Sky Imager for two of the more spectacular cases that began on February 17, 1998 and February 17, 1999. In the latter case, and for the first time, a poleward surge of depletion/enhancement airglow zones was captured by radar as well as an airglow imager. In the former case structures grew in place overhead and produced strong VHF F‐region backscatter as observed by the CUPRI and University of Illinois radars; the other event, exactly one year later, did not result in detectable 3‐m backscatter. The two data sets show quantitatively that the low airglow region is elevated in height and depleted in plasma density and Pedersen conductivity. We suggest an enhanced eastward electric field inside the low conductivity zone may be responsible for the surge. The data also suggest small scale turbulence can only be observed in developing structures.

Swartz, Wesley E.; Kelley, Michael C.; Makela, Jonathan J.; Collins, Stephen C.; Kudeki, Erhan; Franke, Steve; Urbina, Julio; Aponte, Nestor; Sulzer, Michael P.; González, Sixto A.

doi: 10.1029/2000GL000021pmid: N/A

An intense mid‐latitude spread‐F event occurred over Puerto Rico during the night of February 17, 1998. Simultaneous observations were made with the Cornell University Portable Radar Interferometer (CUPRI) located near Isabela, PR, the University of Illinois VHF radar located at Salinas, PR, GPS receivers at Isabela and St. Croix, measuring total electron content, the Arecibo incoherent scatter radar, and the Cornell All‐Sky imager located at the Arecibo Observatory. This was the first time that such a broad range of complementary instrumentation captured a mid‐latitude spread‐F space weather event. It was the first (and still only) time that a spread‐F event over the Caribbean exhibited large Doppler shifts in the VHF spectra. This event was characterized with multiple filaments that initially produced receding Doppler velocities exceeding 300 m/s as seen by CUPRI and the Illinois radar. The Arecibo incoherent scatter radar recorded line‐of‐sight velocities exceeding 100 m/s that moved the F‐layer peak to over 400‐km altitude. Airglow images of 630.0 nm emissions from F‐region heights showed depleted structures oriented southeast to northwest. The large velocities observed with the radars suggest that we caught this event in a stage of explosive development. It is interesting that the first fully documented Caribbean event occurred during a magnetically active period.

Aponte, Néstor; González, Sixto A.; Kelley, Michael C.; Tepley, Craig A.; Pi, Xiaoqing; Iijima, Byron

doi: 10.1029/2000GL000025pmid: N/A

During highly disturbed geomagnetic periods, both the Arecibo radar and the Ramey ionosonde have recorded impressive nighttime ionospheric enhancements in which the peak electron density exceeded 1 × 1012 m−3 and the F2 peak height went above 400 km. In the past it has been suggested that these events could be caused by either a downward plasmaspheric flux that increases the density in the F region or by an eastward electric field that pushes the equatorial anomaly poleward to the Caribbean sector. On February 17–18, 1999 the Arecibo radar made observations during an event in which the electron density again rose to daytime values near midnight. For this event, the peaks in density were observed predominantly southeast of Arecibo while the ions sustained a northward‐eastward motion due to an eastward‐southward storm dynamo electric field. TEC maps from GPS for this night confirmed that the density enhancements were due to a poleward expansion of the equatorial anomaly.

Kelley, Michael C.; Makela, Jonathan J.; Saito, Akinori; Aponte, Nestor; Sulzer, Michael; González, Sixto A.

doi: 10.1029/2000GL000024pmid: N/A

Using a combination of airglow images and incoherent scatter radar data, we have explored the electrical structure of the airglow depleted, height layer bands over a mid‐latitude site. We find a reproducible electrical signature in both components of the electric field in all events studied. The most pronounced feature is a large northward/upward electric field in the heart of the structure. The latter is identical to the radially outward field reported for mid‐latitude conjugate electric fields (Saito et al., 1995), found to trace the poleward edge of the equatorial anomaly. We favorably compare the Arecibo drift to a typical satellite event. These electric fields may reflect a nonlinear evolved state of the Perkins instability or some, as yet, unexplained coupling between atmospheric motion and the plasma embedded in it. We show here that the F‐region Pedersen conductivity is much lower in these structures than outside and suggest that this is related to a polarization electric field inside the structure.

Makela, Jonathan J.; González, Sixto A.; MacPherson, Bryan; Pi, Xiaoqing; Kelley, Michael C.; Sultan, Peter J.

doi: 10.1029/2000GL000023pmid: N/A

We compare total electron content measurements made using the Arecibo Incoherent Scatter Radar (ISR) with those made using the Global Positioning System (GPS). The ISR measurements have a limited range for these observations, up to 1500 km. We extend these profiles to GPS heights of 20,200 km with the aid of a numerical model. We use a GPS receiver on St. Croix, which has been calibrated using JPL's Global Ionospheric Mapping (GIM) technique. In addition, we also use the ISR to calibrate GPS measurements made at Isabela, PR and see how the calibration holds up on the next day. The GIM technique gives very good results on both a quiet night and a night with a severe ionospheric depletion. Normalizing the Isabela receiver to the ISR also gives good results and shows promise as a way to independently calibrate nearby GPS receivers in the future. Finally, we give evidence that the severe depletion observed by the ISR on the night of June 25/26, 1998 was associated with an elongated TEC depletion. The structure may be related to a disturbance originating in the southern hemisphere.

MacPherson, B.; González, S. A.; Pi, X.; Kelley, M.; Bailey, G. J.; Sulzer, M. P.; Hajj, G.; Buonsanto, M.; Wang, C.

doi: 10.1029/2000GL000026pmid: N/A

A method of extrapolating electron densities (Ne) from Incoherent Scatter Radar (ISR) data to high altitudes is presented, and is used to estimate the vertical Total Electron Content (TEC) over Arecibo. Although primarily intended for use on the Ne profiles obtained using the new topside frequency hopping mode at Arecibo which provides measurements to over 2000 km altitude, we show that if the O+ ‐H+ transition altitude is low enough to be observed, then the method may also be applied to the data taken using the standard World Day multi‐radar mode, and thus to a great fraction of the World Day data from low solar flux conditions. This method is applied to Arecibo ISR data taken during the January 6–11 1997 CEDAR storm period, and compared with measured TEC from the Global Positioning System (GPS) satellite network. In addition, the TEC is compared to TEC calculated from the Sheffield University Plasmasphere Ionosphere Model (SUPIM) as part of an ongoing project of model validation.

Bust, Gary S.; Coco, David; Makela, Jonathan J.

doi: 10.1029/2000GL000053pmid: N/A

Results from the June 1998 combined ionospheric campaign (CIC) are presented. The CIC represents an attempt to focus a large number of different instruments on one interesting geophysical region. The Center for Ionospheric Research (CIR) at Applied Research Laboratories, the University of Texas at Austin (ARL:UT), has had several computerized ionospheric tomography (CIT) receivers deployed in the Caribbean region since July 1997. In this paper we compare CIT data, GPS TEC data and data from the incoherent scatter radar at Arecibo to try to obtain an understanding of the temporal and spatial distribution of ionospheric structure observed during the campaign. We use the three data sets as inputs to the 3DVAR tomography algorithm developed at CIR and present results of the 3DVAR “objectively analyzed” electron density field. An ionization wall was found near 40° latitude in agreement with previous Millstone Hill and DMSP observations in high Kp. Several elongated density depletions were also detected.

Urbina, Julio; Kudeki, Erhan; Franke, Steven J.; Gonzalez, Sixto; Zhou, Qihou; Collins, Stephen C.

doi: 10.1029/2000GL000028pmid: N/A

A 50 MHz radar interferometer was used near Salinas, Puerto Rico, to probe the meter‐scale E‐region plasma density irregularities during two campaigns conducted in 1998. During the February–April period E‐region echoing layers were primarily observed between 90 and 100 km heights. The layers were typically thin (∼1 km) and unstructured, although in several cases short period (∼90 s) layer oscillations were observed. During the June–July period E‐region echoes showed more varied characteristics. In addition to low altitude layers, quasi‐periodic structures with descending echoing layers were observed at altitudes above 100 km. Zonal motions detected during descending layer events were at times variable and oscillated between westward and eastward directions while the layer descent rates remained fixed.

Mishin, Evgeny; Carlson, Herbert C.; Hagfors, Tor

doi: 10.1029/2000GL000075pmid: N/A

The variation with height and plasma frequency of the distribution function of ambient electrons (EDF) in the F region ionosphere during high‐power, HF radio wave modification experiments is discussed. It is shown that the deviation of the EDF from a Maxwellian distribution (MD) in the high energy, >2 eV, range may be quite significant (decreasing by a factor of 2 to 5) depending on the background parameters. As a result, the cooling and excitation rates are reduced with respect to a MD calculation. This, for example, can improve agreement of the model based on excitation by heated thermal electrons with the observations of 630.0 nm airglow during HF modification experiments.

Tsunoda, Roland T.; Ecklund, Warner L.; Johnston, Paul E.

doi: 10.1029/2000GL003775pmid: N/A

We show that the vertical polarization electric field (Ep) in the topside of the equatorial electrojet can be determined from the Doppler spectra of type‐2 echoes obtained with a radar that is appropriately displaced in latitude from the magnetic dip equator. Using a 49.92 MHz radar on Christmas Island (2.9° magnetic dip latitude), we found unexpectedly large Ep at altitudes at least as high as 120 km over the dip equator. Another new and related finding is the transient appearance of type‐1 echoes at 99 km over Christmas Island, which likely was produced by an Ep of at least 11 mV/m that must have appeared at 114 km altitude over the dip equator. Elevated electron temperatures inferred from the type‐1 Doppler spectra are consistent with the presence of anomalous energetics.