https://iri.gsfc.nasa.gov/docs/iri_2007_asr.pdf

International Reference Ionosphere 2007: Improvements and new parameters

Electric-field-penetration events have been identified using F-region vertical-drift measurements obtained in the October 6-13, 1984 period by the Jicamarcan incoherent-backscatter radar and corresponding h-prime F measurements from ionosondes at Fortaleza, Cachoeira Paulista, and Dakar. Predictions made using the Rice Convection Model for the pattern, strength, and duration of the low-latitude electric field occurring in response to an increasing high-latitude convection agree with observations. The observed 1-2 h duration of the low-latitude response to decreased convection can be explained by the fossil-wind theory of Richmond (1983).

Penetration of high latitude electric fields effects tolow latitude during Sundial 1984

[1] We have used five years of measurements on board the ROCSAT-1 satellite to develop a detailed quiet time global empirical model for equatorial F region vertical plasma drifts. This model describes the local time, seasonal and longitudinal dependence of the vertical drifts for an altitude of 600 km under moderate and high solar flux conditions. The model results are in excellent agreement with measurements from the Jicamarca radar and also from other ground-based and in situ probes. We show that the longitudinal dependence of the daytime and nighttime vertical drifts is much stronger than reported earlier, especially during December and June solstice. The late night downward drift velocities are larger in the eastern than in the western hemisphere at all seasons, the morning and afternoon December solstice drifts have significantly different longitudinal dependence, and the daytime upward drifts have strong wave number-four signatures during equinox and June solstice. The largest evening upward drifts occur during equinox and December solstice near the American sector. The longitudinal variations of the evening prereversal velocity peaks during December and June solstice are anti-correlated, which further indicates the importance of conductivity effects on the electrodynamics of the equatorial ionosphere.

/pdf/quiet-time-equatorial-f-region-vertical-plasma-drift-model-262f231uag.pdf

Quiet time equatorial F region vertical plasma drift model derived from ROCSAT‐1 observations

[1] We use radar measurements from the Jicamarca Radio Observatory, magnetometer observations from the Pacific sector and ionosonde data from Brazil to study equatorial ionospheric electric fields during the November 2004 geomagnetic storm. Our data show very large eastward and westward daytime electrojet current perturbations with lifetimes of about an hour (indicative of undershielding and overshielding prompt penetration electric fields) in the Pacific equatorial region during the November 7 main phase of the storm, when the southward IMF, the solar wind and reconnection electric fields, and the polar cap potential drops had very large and nearly steady values. This result is inconsistent with the recent suggestion that solar wind electric fields penetrate without attenuation into the equatorial ionosphere for several hours during storm main phase. The largest daytime prompt penetration electric fields (about 3 mV/m) ever observed over Jicamarca occurred during the November 9 storm main phase, when large equatorial electrojet current and drift perturbations were also present in the Pacific and Brazilian equatorial regions. The rise and decay times of these equatorial electric fields were about 20 min longer than of the corresponding solar wind electric fields. The ratios of prompt penetration electric fields and corresponding solar wind electric field changes were highly variable even during the day, and had largest values near dawn. Also, the prompt penetration electric fields did not show polar cap potential drop saturation effects. Our results clearly highlight that the relationships of prompt penetration and solar wind electric fields, and polar cap potentials are far more complex than implied by simple proportionality factors.

https://works.bepress.com/bela_fejer/42/download/

Equatorial Ionospheric Electric Fields During the November 2004 Magnetic Storm

[1] We used equatorial measurements from the ROCSAT-1 satellite to determine the seasonal and longitudinal dependent equatorial F region disturbance vertical plasma drifts. Following sudden increases in geomagnetic activity, the prompt penetration vertical drifts are upward during the day and downward at night, and have strong local time dependence at all seasons. The largest prompt penetration drifts near dusk and dawn occur during June solstice. The daytime disturbance dynamo drifts are small at all seasons. They are downward near dusk with largest (smallest) values during equinox (June solstice); the nighttime drifts are upward with the largest magnitudes in the postmidnight sector during December solstice. During equinox, the downward disturbance dynamo drifts near sunset are largest in the eastern hemisphere, while the late night upward drifts are largest in the western hemisphere. The longitudinal dependence of the disturbance dynamo drifts is in good agreement with results from simulation studies.

/pdf/seasonal-and-longitudinal-dependence-of-equatorial-290baw43w8.pdf

Seasonal and longitudinal dependence of equatorial disturbance vertical plasma drifts

[1] The global distribution of the occurrence rate for density irregularities at 600 km topside ionosphere between ±35° geographic latitudes has been studied with the ROCSAT data during moderate to high solar activity years of 1999 to 2004. The result indicates that the global occurrence distribution of the intermediate-scale (0.1 to 50 km) density irregularities can be grouped into two different populations, one in the equatorial region and the other in the middle-to-subauroral latitude region. The global seasonal/longitudinal (s/l) distribution of equatorial irregularities in the current report reproduces the result of McClure et al. (1998) obtained with the AE-E observations of the mesoscale (50 to 1000 km) plasma bubble structures during high solar activity years of 1978 to 1980, two solar cycles ago. This implies that the density irregularities of different scales from multistage cascading process of the large-scale (>1000 km) gravitational Rayleigh-Taylor instability have manifested in same global s/l distribution pattern. Furthermore, global variation in seeding mechanism and growth condition of the instability process that results in major features in global irregularity pattern seems to persist for past 25 years. In addition, the current result further indicates that an upper latitudinal limit of the equatorial irregularity distribution is located at about ±30°. A different kind of midlatitude irregularity distribution starts to fill in from this dip latitude. In other words, the equatorial density irregularity inside a depleted flux tube can only rise, on statistical average, to an apex height of ∼2000 km. Different magnetic and solar variability effects as well as the local time dependence are noted for the occurrences of density irregularities in the equatorial region versus that at midlatitudes. The occurrence frequency of equatorial density irregularities increases with solar flux intensity; whereas the midlatitude density irregularity is more likely to occur during low solar activity period. The equatorial density irregularities are more likely to occur during periods of low magnetic activity than during magnetic disturbed times. On the other hand, the occurrence of midlatitude density irregularities indicates little dependence on geomagnetic activity. The local time distribution of equatorial irregularity peaks before midnight while the midlatitude irregularity indicates a plateau of high occurrence rate after midnight. Such opposite characteristics in the occurrence pattern between these two spatially separated distributions suggest that different instability mechanisms are operated in two different latitude regions for the occurrence of intermediate-scale density irregularities.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005JA011330

Distribution characteristics of topside ionospheric density irregularities: Equatorial versus midlatitude regions

[1] Monthly variation of global equatorial density irregularity distribution has been obtained with the data taken by ROCSAT-1 at the 600 km topside ionosphere from March 1999 to June 2004 during high to moderate solar activity years. This global distribution of monthly irregularity occurrence rate not only provides the best spatial/temporal distribution in existence so far but also serves to fill the gap of irregularity distribution missing over some eastern Pacific region where no ground observation is available. The 5 1/2-year result of the monthly occurrence pattern indicates a smooth variation across the longitudes contrary to some beliefs that a drastic change in irregularity occurrence pattern has occurred across the eastern Pacific longitudes. Excellent agreement is noted for the current results with Aarons' (1993) conjectured sketch of the global scintillation distribution. Furthermore, the seasonal/longitudinal (s/l) variations of quiet time postsunset vertical drift velocities are found to track closely with the s/l variations of irregularity occurrences. Linear regression analysis between the vertical drift velocity and the irregularity occurrence rate indicates that the vertical drift velocities at three different longitude zones of magnetic declination have good correlations with the irregularity occurrences for all seasons. This implies that the averaged postsunset vertical drift velocity is a good indicator for the occurrences of equatorial density irregularities in a longitude region of similar magnetic declination. The smooth variation of monthly/seasonal/longitudinal distributions of the vertical drift velocities that results in the smooth variation of topside density irregularity occurrences implies that the global occurrence pattern of either distribution is controlled by the smooth global variation of the postsunset ionospheric condition, related to the magnetic declination effect and the seasonal variation of the ionospheric density level at the dip equator located with respect to the geographic equator. There seems little need to include the global distribution of instability perturbation seeds from atmospheric disturbances to complete the global distribution of topside irregularity occurrences.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2007JA012809

On monthly/seasonal/longitudinal variations of equatorial irregularity occurrences and their relationship with the postsunset vertical drift velocities

[1] The first Republic of China Satellite, ROCSAT-1 at 600 km altitude observed two groups of equatorial spread-F (ESF) plasma depletion structures located in either side of the midnight meridian during the 6–7 April 2000 magnetic storm recovery phase. In the group of postmidnight ESF irregularities, a large density dropout was observed in two consecutive ROCSAT orbits. This large density dropout structure is similar to the density depletion event observed by Defense Meteorological Satellite Program (DMSP) F9 spacecraft during the 13–14 March 1989 magnetic storm [Greenspan et al., 1991] except that the ROCSAT orbit traverses the magnetic field in the east-west direction. The postmidnight group of depletions together with the large density dropout was observed to drift westward with the background plasma at ∼170 m s−1. We examine the properties of the large density dropout through the plasma data taken from the drift meter (DM) and retarding potential analyzer (RPA) sensors in three consecutive orbits. The apparent widening of the density depletion is explained by the different locations of the satellite within the depletion region. The extreme low-density level inside the upwelling depletion indicates that of the nighttime F-peak passed over the ROCSAT orbit at 600 km. The existence of NO+ ions inside the dropout is consistent with such an elevation of the F-peak.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001JA007552

Observation of a large density dropout across the magnetic field at 600 km altitude during the 6–7 April 2000 magnetic storm

[1] The quiet-time low- and mid-latitude topside ionospheric ion temperature measured with ROCSAT-1/IPEI instrument is studied for local time, longitudinal, latitudinal, and seasonal variations for the solar maximum year of 2000. The statistical result shows two significant observations at the presunrise sector. Namely, the earliest presunrise ion temperature increase at 600 km low- and mid-latitude ionosphere always starts in the winter hemisphere for both summer and winter seasons; and the strongest presunrise ion-heating region is located in the longitudinal region between 165° and 195° during June summer and between 285° and 345° during December winter. Our simple calculation indicates that the temperature increase at the satellite altitude results from the heating process of photoelectrons that are produced at the magnetic conjugate-point where sunrise is at an earlier time. However, the mechanism to enhance the photoelectron heating at the strongest presunrise ion-heating region is still not clear, because the observed ion density and the field flow data fail to lend a clear support to the proposed heating mechanism for the current observations.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2002GL016268

Presunrise ion temperature enhancement observed at 600 km low‐ and mid‐latitude ionosphere

One of the Ionospheric Plasma and Electrodynamics Instrument (IPEI) payload onboard the Republic of China satellite 1 (ROCSAT-1) spacecraft is a Retarding Potential Analyzer (RPA) to measure the ionospheric plasma composition, temperature, and the ram velocity of the flow. A three-dimensional computer model has been developed to simulate the ion and electron motions inside the RPA of the IPEI payload. The dynamical changes in the charged particles’ motions corresponding to different electric potential profiles inside the RPA detector resulted in some distinctive measuring characteristics of the RPA detector, which show discrepancies in the measured of currents at the collector plate between the analytical model and the computer simulation. The important discoveries in the computer model are: (a) Ion transparency cannot be represented by the physical grid sizes in the detector. The alignments of the grid wires will also affect the ion transparency. The grid planes should be aligned for the maximum optical ...

Chi-Kuang Chao

Papers

Distribution characteristics of topside ionospheric density irregularities: Equatorial versus midlatitude regions

On monthly/seasonal/longitudinal variations of equatorial irregularity occurrences and their relationship with the postsunset vertical drift velocities

Observation of a large density dropout across the magnetic field at 600 km altitude during the 6–7 April 2000 magnetic storm

Presunrise ion temperature enhancement observed at 600 km low‐ and mid‐latitude ionosphere

Charged particle motion inside the retarding potential analyzer