Republic Of China Satellite Research Articles

A statistical study on the occurrence characteristics of low-to-midlatitude ionospheric density enhancements (plasma blobs)

A statistical study on the occurrence characteristics of global low-to-midlatitude topside ionospheric density enhancements (plasma blobs) has been carried out using the ROCSAT-1 (Republic of China Satellite-1) data during high solar activity period of 1999 to 2004. The latitudinal distribution of plasma blobs indicates that the occurrence rates increase from the dip latitude ±15° to midlatitudes that appear to complement the latitudinal distribution of equatorial plasma bubbles (EPBs). For the seasonal distribution, it is found that the maximum occurrence rate appears during the June solstice in both northern and southern hemispheres. The local-time distribution indicates a maximum occurrence rate around midnight, and maintains a high occurrence rate until about 04 h in local time in contrast to a rapid decrease after midnight for the EPB occurrences. Negative correlation with the solar activity is found for the occurrence of plasma blobs that is opposite to the positive correlation with the solar activity for the EPB occurrences. All these different and contrasting occurrence characteristics between the low-to-midlatitude plasma blob occurrence and the EPBs imply that the occurrence mechanism for the low-to-midlatitude plasma blobs should have little relationship to that of EPBs. From the fact that an upward/outward plasma drift and the increase of O+ species are observed inside the plasma blob, an eastward polarization electric field should directly appear at topside ionosphere for the occurrence of plasma blobs. This polarization electric field should be mapped from the nighttime midlatitude Es layer instability process where the polarization electric field is generated.

Ion Velocity and Temperature Variation Around Topside Nighttime Irregularities: Contrast Between Low- and Mid-Latitude Regions.

Studies on low-/mid-latitude ionospheric irregularities have a long history, but ion temperature and drift velocity variations within the irregularities have been rarely investigated, especially at mid-latitudes. To fill this knowledge gap, we statistically investigate ion temperature and velocity of mid-latitude ionospheric irregularities and compare them to those of low-latitude ones. This study mainly relies on in situ plasma moment data from the newly launched Ionospheric Connection Explorer (ICON) satellite and is supplemented by Republic of China Satellite 1 (ROCSAT-1) data in 2000-2004. At low latitudes (|MLAT|<10°) plasma density irregularities encountered by ICON have the following properties: lower-density regions have cooler ions and more poleward/westward/outward velocity than (|MLAT| = 10°∼30°), the opposite trends are observed: lower-density regions have hotter ions and more the ambient plasma, which agrees with previous reports on equatorial plasma bubbles. At mid-latitudes equatorward/eastward/inward drift than the ambient, which is a new finding in this study. The mid-latitude observations can be put into the context of Medium-Scale Traveling Ionospheric Disturbance (MSTID) properties reported previously, which evidences that topside irregularities at nighttime mid-latitudes generally represent MSTIDs. ROCSAT-1 data support the contrast between low- and mid-latitude ionospheric irregularities.

An empirical model of the topside plasma density around 600 km based on ROCSAT‐1 and Hinotori observations

AbstractIt is an urgent task to improve the ability of ionospheric empirical models to more precisely reproduce the plasma density variations in the topside ionosphere. Based on the Republic of China Satellite 1 (ROCSAT‐1) observations, we developed a new empirical model of topside plasma density around 600 km under relatively quiet geomagnetic conditions. The model reproduces the ROCSAT‐1 plasma density observations with a root‐mean‐square‐error of 0.125 in units of lg(Ni(cm−3)) and reasonably describes the temporal and spatial variations of plasma density at altitudes in the range from 550 to 660 km. The model results are also in good agreement with observations from Hinotori, Coupled Ion‐Neutral Dynamics Investigations/Communications/Navigation Outage Forecasting System satellites and the incoherent scatter radar at Arecibo. Further, we combined ROCSAT‐1 and Hinotori data to improve the ROCSAT‐1 model and built a new model (R&amp;H model) after the consistency between the two data sets had been confirmed with the original ROCSAT‐1 model. In particular, we studied the solar activity dependence of topside plasma density at a fixed altitude by R&amp;H model and find that its feature slightly differs from the case when the orbit altitude evolution is ignored. In addition, the R&amp;H model shows the merging of the two crests of equatorial ionization anomaly above the F2 peak, while the IRI_Nq topside option always produces two separate crests in this range of altitudes.

Typical comparisons between plasma bubble and blob distributions at different altitudes over the Indian sector using a unique combination of satellite-based observations – a case study

We report in this article, for the first time, on the altitudinal variations of ionospheric irregularities during post-sunset hours between 14 and 17 October 2001 at Indian longitudes by using a unique combination of Challenging Mini Satellite Payload (CHAMP), Republic of China Satellite (ROCSAT-1), and Defense Meteorological Satellite Programme (DMSP) satellites located at about 400, 650, and 850 km, respectively. It became possible for us to study the width of plasma bubbles and plasma blobs in the longitudinal (zonal) direction at different altitudes along with temporal variations of ion velocity components in horizontal (Vy) and vertical (Vz) directions as probed by ROCSAT-1 and DMSP satellites. The dominant features noticed in Vy and Vz components are their anti-correlation relationship at the ROCSAT-1 satellite’s altitude (mirroring effect). The most important observation from this study is that plasma bubbles are found to have occurred immediately after post-sunset hours, whereas plasma blobs were observed after three hours following the bubbles’ appearance time and continued until pre-sunrise hours at higher altitudes between around ±5° and ±20° from the magnetic equator, which indicate that the plasma blobs emanate from plasma bubbles that generated at the bottomside of the ionospheric F-layer shortly after sunset on similar lines to a few recent research studies. It is therefore believed that the polarized electric fields generated inside plasma bubbles might have played a role in the generation of higher-altitude plasma blobs. Importantly, the calculated power spectra of bubbles and blobs show a nearly equal trend, indicating that they are intricately connected with each other.

Features of highly structured equatorial plasma irregularities deduced from CHAMP observations

Abstract. In this study five years of CHAMP (Challenging Mini-satellite Payload) fluxgate magnetometer (FGM) data is used to investigate the characteristics of Equatorial Plasma Bubbles (EPBs). We filtered the FGM data by using band-passes with four different cut-off periods to get the EPBs with different maximum spatial scale sizes in the meridional plane ranging from 76–608 km. Associated with the EPB observations at about 400 km, the typical altitude of CHAMP during the year 2000–2005, we also investigate the post-sunset equatorial vertical plasma drift data from ROCSAT-1 (Republic of China Satellite 1). Since the height of the F-layer is highly correlated with the vertical plasma drift and solar flux, we sorted the ROCSAT-1 data into different groups by F10.7. From the integrated vertical drift we have estimated the post-sunset uplift of the ionosphere. By comparing the properties of EPB occurrence for different scale sizes with the global distribution of plasma vertical uplift, we have found that EPBs reaching higher altitudes are more structured than those which are sampled by CHAMP near the top side of the depleted fluxtube. Such a result is in accord with 3-D model simulations (Aveiro and Hysell, 2010). Small-scale EPB structures are observed by CHAMP when the irregularities reach apex heights of 800 km and more. Such events are encountered primarily in the Brazilian sector during the months around November, when the post-sunset vertical plasma drift is high.

Superrotation of the ionosphere and quiet time zonal ion drifts at low and middle latitudes observed by Republic of China Satellite-1 (ROCSAT-1)

[1] Local-time variations of zonal drifts at different latitudes, longitudes, and seasons are investigated using ion drift measurements from the Republic of China Satellite-1 (ROCSAT-1). We select a quiet time period defined when Dst > −100 nT and Kp ≤ 3. The ion drifts in the magnetic zonal direction are latitude (apex height) dependent and differ for different longitude sectors and seasons. Superrotation of the ionosphere is observed in the zonal drift measurements, particularly at the lower latitudes. The ionospheric superrotation generally decreases with increasing latitude and is attributed mainly to the F region dynamo where the features of the neutral wind and conductance affect the characteristics of the superrotation at different latitudes, longitudes, and seasons. We suggest that the latitudinal variation of the superrotation is mainly produced by the decrease in latitude of the neutral winds in the F region. A decrease of conductance with increasing latitude is also present, but evidence shows that neutral winds decrease faster with latitude than the conductance for high solar flux conditions. The longitudinal and seasonal variations of the superrotation are more influenced by changes in the F region conductance that is modified by vertical E × B ion drift motions and neutral wind-induced ion drift motions along magnetic field lines.

Seasonal and latitudinal distributions of the dominant light ions at 600 km topside ionosphere from 1999 to 2002

Data taken by the Republic of China satellite (ROCSAT‐1) during moderate to high solar activity years from 1999 to 2002 have been studied for the statistical distribution of the dominant light ion species, either hydrogen or helium ions, at 600 km topside ionosphere. The results indicate some interesting seasonal and longitudinal/latitudinal distributions of the dominant light ions in the topside ionosphere during the magnetic quiet periods. Each light ion species can become the dominant ion species at 600 km topside ionosphere but only at night when the ion temperature is cooler than during the day. More cases of H+ dominance have been observed than those of He+ dominance. Except for the March equinox the distribution of dominant H+ shows a strong hemispheric asymmetry for the other three seasons. When H+ dominance is observed in one hemisphere during the solstice season, the low latitude limit of this transition region is a constant dip latitude in the winter hemisphere. This statistical minimum of the transition latitude shows little dependence on the seasonal averaged solar flux intensity. Similar hemispherically asymmetric distribution for dominant He+ in the winter hemisphere during the solstice season has also been noted except that the asymmetrical pattern is not as prominent as in the dominant H+ case because much fewer cases have been observed for dominant He+. The asymmetrical distribution of the dominant light ions seems to be related to the observed hemispheric field‐aligned ion flow pattern. Thus it is concluded that the downward field‐aligned ion flow together with the nighttime lower ion temperature in the winter hemisphere compose a possible cause for the occurrence distribution of the hemispheric asymmetry in the dominant light ion species. This can be understood from the fact that the field‐aligned flow is related to the hemispheric asymmetry of the ionospheric F peaks and serves to enhance or retard the nocturnal redistribution of the light ions along the field line.

Ionospheric effects of major magnetic storms during the International Space Weather Period of September and October 1999: GPS observations, VHF/UHF scintillations, and in situ density structures at middle and equatorial latitudes

In this paper we present a study of the ionospheric effects of a halo coronal mass ejection (CME) initiated on the Sun on September 20, 1999, and causing the largest magnetic storm during this month on September 22–23, 1999, with the hourly Dst index being −167 nT at ∼2400 UT on September 22. The recurrent CME on October 18 caused an even larger magnetic storm on October 22, 1999, with Dst of −231 nT at ∼0700 UT. The ionospheric effects of these two major magnetic storms are studied through their effects on a prototype of a Global Positioning System (GPS)‐based navigation system called Wide Area Augmentation System (WAAS) being developed by the Federal Aviation Administration for use in the continental United States and their impact on global VHF/UHF communication systems. It is shown that the penetration of transient magnetospheric electric fields equatorward of the shielding region at midlatitudes, which have been well‐correlated in the past with rapid changes in the well‐known Dst index (or through its recently available high resolution 1‐min counterpart the SYM‐H index), can cause large increases of total electron content (TEC), TEC fluctuations, and saturated 250‐MHz scintillation, and these, in turn, may have significant impacts on WAAS. The local time of Dst changes (and not just Dst magnitude) was found to be very important for WAAS, since the largest effects on TEC are seen near dusk. The prompt penetration of these magnetospheric electric fields all the way to the magnetic equator causes augmentation or inhibition of equatorial spread F. The global ionospheric response to these storms has been obtained from ground‐based TEC observations with a GPS network and space‐based in situ density and electric field measurements using the Republic of China Satellite‐1 (ROCSAT‐I) and several Defense Meteorological Satellite Program satellites. These prompt penetration electric fields cause VHF/UHF scintillations and GPS TEC variations at low latitudes in the specific longitude sector for which the early evening period corresponds to the time of rapid Dst variations and maximum Dst phase. The effects of the delayed ionospheric disturbance dynamo and those of decreased magnetospheric convection on postmidnight irregularity generation are shown to be confined to a part of the same longitude range that actively responded to the prompt penetration of electric fields in the early evening sector.

Charged particle motion inside the retarding potential analyzer

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 transparency to detect light ion fluxes. (b) Cut-off energy for protons is not as sharp as indicated in the analytical model. The contamination in the collector current from lighter ion species is possible. (c) Electron contamination at the collector plate is noticed if the mesh sizes are larger than the electron Debye length.