Maximum Occurrence Rate Research Articles

The occurrence of equatorial spread-F at conjugate stations in Southeast Asia

In this study, the probability of equatorial spread-F (ESF) occurrence at the conjugate stations in Southeast Asia: Chiangmai station (CMU), Thailand, and Kototabang station (KTB), Indonesia, and near the magnetic equator, Chumphon station (CPN), Thailand, is presented. We analyze the ionogram data recorded by the Frequency Modulated Continuous Wave (FM/CW) ionosondes for the periods of minimum solar activity from September 2008 to April 2009 and in the equinoctial months (March and April) from 2006 to 2013. The spread-F signatures are manually categorized into three types: the frequency spread-F (FSF), the range spread-F (RSF) and the mixed spread-F (MSF) and the monthly average percentage of the occurrence of each ESF type is presented. The results show that the percentage of RSF occurrence at CPN, which is located around the magnetic equator, is higher than at other stations and the RSF mostly occurs during the equinoctial months. On the other hand, the FSF occurrence at CMU and KTB, that are located in the northern and southern hemispheres, respectively, are higher than at CPN. The RSF occurrence typically has the peaks before midnight, while the maximum occurrence rate of FSF is after midnight. Furthermore, the RSF onsets normally precede the FSF onsets by about 1–2h. As the solar activity levels go up, the percentages of RSF occurrence increase, but the percentages of FSF tends to decrease. In addition, we compare the statistics of observed RSF occurrence with the prediction of the IRI-2012 model. The results show that the IRI model overestimates the observed RSF occurrence at all stations during most of the solar activity levels and seasons, except in June 2008 (21:00–03:00LT), March 2011 (23:30–01:00LT) and March 2013 (01:00–02:30LT) when the IRI model underestimates our observations. However, the IRI model gives closer probability of RSF occurrence to our observed values at CPN, especially in equinoctial months and during the periods of medium solar activity. This work is important for an improvement of the IRI model in the prediction of the spread F occurrence probability in the low-latitude region.

Climatology of nighttime medium-scale traveling ionospheric disturbances (MSTIDs) in the Central Pacific and South American sectors

Abstract. We present occurrence rate statistics for nighttime medium-scale traveling ionospheric disturbances (MSTIDs) in the Central Pacific and South American sectors using data collected by 630.0 nm filtered CCD imaging systems. The data were collected from September 2006 through December 2012. In general, the statistics are in good agreement with the basic linear theory of MSTIDs, with observations coinciding with low F10.7A values, representative of solar minimum. Overall, MSTIDs are observed in approximately 68% of the usable nights near the solstices at mid-latitudes and approximately 20% of the usable nights for equinox periods. Observations closer to the geomagnetic equator yielded a maximum occurrence rate of about 10–20% during the solstices and about 0–3% during the equinoxes. The lower number of MSTID observations near the low latitudes is attributed to limitations of MSTID growth rate, propagation, and/or geometrical observational effects. The relatively large number of MSTID occurrences during the solstices can be accounted for by the neutral wind contribution to the MSTID growth rate either at the local or magnetic conjugate point.

Gravity wave activity in the troposphere and lower stratosphere: An observational study of seasonal and interannual variations

AbstractAn 11 year (1998‐2008) temperature and wind data set obtained over Annette Island (55.03°N, 131.57°W) is used to examine gravity wave activity and their seasonal and interannual variations in gravity wave activity. Vertical wave number spectra of normalized temperature and wind fluctuations are calculated and are compared with the predictions of gravity wave saturation models. Results indicate that there are serious discrepancies between our measurements and earlier observational results at present stage of study of vertical wave number spectra. The correlation coefficients between tropospheric and stratospheric temperature spectral parameters are very small, suggesting that the result is in agreement with previous studies. Time series of total wave energy reveal clear seasonal and interannual variations. Maximum wave energy amplitudes occur near winter of each year in the troposphere and near summer of each year in the stratosphere. Specifically, the maximum wave energy amplitudes in the troposphere show a close correspondence with the maximum occurrence rate of dynamical instability. In addition, the maximum wave energy amplitudes in the stratosphere also show a close correspondence with the maximum occurrence rate of convective instability.

Long‐term analysis of ionospheric polar patches based on CHAMP TEC data

Total electron content (TEC) from LEO satellites offers great possibility to sound the upper ionosphere and plasmasphere. This paper describes a method to derive absolute TEC observations aboard CHAMP considering multipath effects and receiver differential code bias. The long‒term data set of 9 years GPS observations is used to investigate the climatological behavior of high‒latitude plasma patches in both hemispheres. The occurrence of polar patches has a clear correlation with the solar cycle, which is less pronounced in the Southern Hemisphere (SH). Summed over all years, we observed a higher number of patches in the SH. The maximum occurrence rate of patches has been found at the dayside polar cusp during 12:00–18:00 MLT (magnetic local time) supporting the mechanisms for patch creation by local particle precipitation and by intrusion of subauroral plasma into the polar cap through tongues of ionization (TOIs). The latter mechanism seems to be even more important in the SH. Investigating the patches in comparison with interplanetary magnetic field (IMF) conditions, we found that decreased IMF Bz and enhanced merging electric field preceded the patch observation; hence, patch creation follows a period of enhanced solar wind input into the magnetosphere/ionosphere. We further found an annual cycle in patch occurrence with maxima at equinox and December solstice and a June solstice minimum which reflects the global ionospheric seasonal asymmetry in electron density. We suggest that enhanced TEC at midlatitudes and low latitudes during December solstice provides a greater possibility to transport high‐density plasma to the polar region through the buildup of TOIs.

Geometry of duskside equatorial current during magnetic storm main phase as deduced from magnetospheric and low-altitude observations

Abstract. We present the results of a coordinated study of the moderate magnetic storm on 22 July 2009. The THEMIS and GOES observations of magnetic field in the inner magnetosphere were complemented by energetic particle observations at low altitude by the six NOAA POES satellites. Observations in the vicinity of geosynchronous orbit revealed a relatively thin (half-thickness of less than 1 RE) and intense current sheet in the dusk MLT sector during the main phase of the storm. The total westward current (integrated along the z-direction) on the duskside at r ~ 6.6 RE was comparable to that in the midnight sector. Such a configuration cannot be adequately described by existing magnetic field models with predefined current systems (error in B > 60 nT). At the same time, low-altitude isotropic boundaries (IB) of > 80 keV protons in the dusk sector were shifted ~ 4° equatorward relative to the IBs in the midnight sector. Both the equatorward IB shift and the current strength on the duskside correlate with the Sym-H* index. These findings imply a close relation between the current intensification and equatorward IB shift in the dusk sector. The analysis of IB dispersion revealed that high-energy IBs (E > 100 keV) always exhibit normal dispersion (i.e., that for pitch angle scattering on curved field lines). Anomalous dispersion is sometimes observed in the low-energy channels (~ 30–100 keV). The maximum occurrence rate of anomalous dispersion was observed during the main phase of the storm in the dusk sector.

GPS observations of medium-scale traveling ionospheric disturbances over Europe

Abstract. Two-dimensional structures of medium-scale traveling ionospheric disturbances (MSTIDs) over Europe have been revealed, for the first time, by using maps of the total electron content (TEC) obtained from more than 800 GPS receivers of the European GPS receiver networks. From statistical analysis of the TEC maps obtained 2008, we have found that the observed MSTIDs can be categorized into two groups: daytime MSTID and nighttime MSTID. The daytime MSTID frequently occurs in winter. Its maximum occurrence rate in monthly and hourly bin exceeds 70% at lower latitudes over Europe, whereas it is approximately 45% at higher latitudes. Since most of the daytime MSTIDs propagate southward, we speculate that they could be caused by atmospheric gravity waves in the thermosphere. The nighttime MSTIDs also frequently occur in winter but most of them propagate southwestward, in a direction consistent with the theory that polarization electric fields play an important role in generating the nighttime MSTIDs. The nighttime MSTID occurrence rate shows distinct latitudinal difference: The maximum of the occurrence rate in monthly and hourly bin is approximately 50% at lower latitudes in Europe, whereas the nighttime MSTID was rarely observed at higher latitudes. We have performed model calculations of the plasma density perturbations caused by a gravity wave and an oscillating electric field to reproduce the daytime and nighttime MSTIDs, respectively. We find that TEC perturbations caused by gravity waves do not show dip angle dependencies, while those caused by the oscillating electric field have a larger amplitude at lower latitudes. These dip angle dependencies of the TEC perturbation amplitude could contribute to the latitudinal variation of the MSTID occurrence rate. Comparing with previous studies, we discuss the longitudinal difference of the nighttime MSTID occurrence rate, along with the E- and F-region coupling processes. The seasonal variation, of the nighttime MSTID occurrence rate in Europe, is not consistent with the theory that the longitudinal and seasonal variations of the nighttime MSTID occurrence could be attributed to those of the Es layer occurrence.

Occurrence rate of earthward‐propagating dipolarization fronts

The occurrence rate of earthward‐propagating dipolarization fronts (DFs) is investigated in this paper based on the 9 years (2001–2009) of Cluster 1 data. For the first time, we select the DF events by fitting the characteristic increase inBzusing a hyperbolic tangent function. 303 earthward‐propagating DFs are found; they have on average a duration of 4 s and aBz increase of 8 nT. DFs have the maximum occurrence at ZGSM ≈ 0 and r ≈ 15 RE with one event occurring every 3.9 hours, where r is the distance to the center of the Earth in the XYGSM plane. The maximum occurrence rate at ZGSM ≈ 0 can be explained by the steep and large increase of Bz near the central current sheet, which is consistent with previous simulations. Along the r direction, the occurrence rate increases gradually from r ≈ 20 to r ≈ 15 RE but decreases rapidly from r ≈ 15 to r ≈ 10 RE. This may be due to the increasing pileup of the magnetic flux from r ≈ 20 to r ≈ 15 RE and the strong background magnetic field at r <∼13 RE, where the magnetic field changes from the tail‐like to dipolar shape. The maximum occurrence rate of DFs (one event per 3.9 hours) is comparable to that of substorms, indicating a relation between the two.

EMIC wave activity during geomagnetic storm and nonstorm periods: CRRES results

Electromagnetic ion cyclotron (EMIC) waves have been observed during geomagnetic storms and are thought to contribute to ring current and radiation belt particle loss during the main phase. Ground‐based storm time studies alternatively observe the majority of storm time Pc1‐2 pulsations during the recovery phase. In this study we look at the occurrences of EMIC waves during 119 storms occurring throughout the CRRES mission. The storms were defined using the Sym‐H index. The storm was divided into three phases: pre‐onset, main, and recovery (80% of minimum Sym‐H value). The majority, 56.25%, of storm time EMIC waves were found to occur during the main phase, while 35.57% were observed in the recovery phase. The recovery phase definition was then extended to 6 days after the minimum in order to compare with past ground studies. Although more EMIC waves were observed during this extended recovery phase, the maximum occurrence rate of EMIC waves remains in the main phase. A slight increase does appear in the 4–6 days after the minimum Sym‐H value. The mean occurrence location of EMIC waves was at L = 6.07, MLT = 15.12. This suggests that EMIC waves may have been generated when the ring current and plasmasphere or plasma plume particle populations overlap, often observed during the main phase due to storm dynamics. The rise in the occurrence rate in the extended recovery phase potentially is dominated by the process of plasmaspheric expansion after the end of the storm.

Neutral density depletions associated with equatorial plasma bubbles as observed by the CHAMP satellite

Using CHAMP observations from 2002 to 2005 we investigate neutral density depletions (NDDs) associated with equatorial plasma bubbles (EPBs). The seasonal–longitudinal distribution of NDDs generally follows that of EPBs. However, there are several important differences between them. The maximum NDD occurrence rate is much smaller than the maximum EPB occurrence. NDDs occur at latitudes north and south of the dip equator with an offset of about 15 ∘ , which is collocated with the Appleton anomaly peaks and slightly poleward of EPB occurrence maxima. The NDD occurrence maximizes around 21 LT, and has nearly died out after 23 LT. Meanwhile, the EPB occurrence shows a broad maximum between 20 and 24 LT. NDD distribution deviates slightly from that of EPBs shifted toward the region of high ion–neutral interaction. Based on our statistical results, as well as on some physics-based calculations, we suggest that an enhanced friction between ions and neutrals is needed for the NDD generation.

Morphology and seasonal characteristics of low latitude E‐region quasiperiodic echoes studied using large database of Gadanki radar observations

In this paper we study the morphology, seasonal features, and statistics of low latitude quasiperiodic (QP) echoes using an unprecedented data set of 147 nights of observations spanning over all seasons made using the Gadanki radar. The QP echoes have periods 2–20 min and occur on more than 50% nights. They are often found embedded in descending echoing layers whose characteristics resemble to those of tidal winds. Their occurrence shows strong seasonal dependence with 48% of the time in summer, 26–32% in equinox, and 14% in winter. The QP echoes have a tendency to appear first during 21–00 LT, which could last for 2–8 h, leading to their maximum occurrence rate during postmidnight hours. Height‐time occurrences of radar echoes and QP structures both show common descending behavior, which agree extremely well with the observed descending behavior of the Es layers. We also find that the seasonal variations of QP echo occurrence are in very close agreement with that of the Es activity, tidal activity in the same height region, and also gravity wave activity in the mesosphere. Interestingly, the occurrence statistics are much more than their midlatitude counterpart; and time of first appearance, time of maximum occurrence, and duration of QP echoes all strongly contrast the midlatitude features. These observations have been discussed in detail in the light of current understanding of QP echoes.

Characteristics of thunderstorm systems producing winter sprites in Japan

In this work, we analyze 67 sprite events observed during the winter sprite campaign carried out in Japan from 1998 to 2003. Our aim is to clarify the relationship between winter sprites in Japan and the characteristics of their parent thunderstorm systems. Combining optical data of sprites, various meteorological data such as cloud maps obtained from the Geostationary Meteorological Satellite‐5, and precipitation maps obtained from Doppler radars as well as lightning detection network data, we classify sprite‐producing thunderstorm systems into four categories: (1) those associated with the cold front over the Sea of Japan, (2) those developing in the Japan Sea Polar Air Mass Convergence Zone, (3) those developing in local low‐pressure areas over the Pacific Ocean, and (4) those developing ∼200–900 km behind the cold front over the Pacific Ocean. These thunderstorm systems have average cloud top temperatures ranging from −25 to −40°C and average horizontal areas of 8500–40,500 km2 at the −20°C isotherm. They are much smaller than thunderstorm systems over the U.S. high plains in the summertime. The average maximum occurrence rate of sprites is 7.7 per hour in category 1, while the occurrence rates in categories 2–4 are only 2–3 per hour. In each of the four categories of thunderstorm systems, winter sprites in Japan tend to be induced by positive cloud‐to‐ground lightning discharges occurring in the convective core region during the developing stage where and when convection becomes most active spatially and temporally.

Lidar observations of sporadic Na layers over Wuhan (30.5°N, 114.4°E)

Based on lidar measurements between March and September 2001, the characteristics of sporadic Na layers (Nas layers) over Wuhan (30.5°N, 114.4°E) are presented. Nas layers were observed on 29 occasions from 275 h observational data. They tended to show a seasonal variation with a maximum occurrence rate in July. The maximum Na density was 24080 cm−3 which was comparable with those observed at low and high latitudes. Different from the observations at all other sites over the world, most Nas layers over Wuhan tended to occur around the peak altitudes of the normal Na layers (∼92 km). However, a Nas layer was observed at 112.9 km altitude. The formation of the Nas layers over Wuhan generally needed a longer time than those at low and high latitudes. The formation processes usually consisted of consecutive density enhancement bursts. The Nas layers often exhibited broader layer widths than those at low and high latitudes.

Statistical characteristics of bursty bulk flow events

Using a common methodology to analyze data from the AMPTE/IRM and ISEE 2 satellites we report on the statistical properties of bursty bulk flow events (BBFs) in the inner plasma sheet (IPS). A positive correlation between BBFs and the AE index suggests that BBFs are predominantly geomagnetically active time phenomena. Earthward BBFs are more frequent close to midnight and away from Earth, up to a distance of ∼19 RE. Tailward BBFs are very infrequent in the IRM data set and somewhat less infrequent in the ISEE 2 data set in the region of the satellites' spatial overlap, possibly due to the more active conditions prevailing during the ISEE 2 mission in that region. However, in both data sets the ratio of tailward to earthward BBFs increases with distance from Earth; more than 20% of all BBFs are anti‐sunward tailward of X = −19 RE in the ISEE 2 data set. BBFs are responsible for 60‐100% of the measured earthward transport of mass, energy and magnetic flux past the satellite in the regions of maximum occurrence rate, even though they last approximately 10‐15% of the IPS observation time there. Thus BBFs represent the primary transport mechanism at those regions. The one‐to‐one correspondence between BBFs and substorm phase, as well as the relative contribution of BBFs to the total transport observed during substorms are questions that await further investigation based on multi instrument studies of individual events.

Role of the Mode Theory on Lightning Generated Waves Propagating in Venus-Ionosphere Waveguide.

The wave mode either electromagnetic or electrostatic of the OEFD measured signals is unknown and needs to be ascertained by indirect means. Considering these waves to be generated by global Venus lightning, the wave attenuation along the Venus-ionosphere wave-guide has been computed. The effective wave attenuation versus frequency shows a “knee” characterizing minimum attenuation 6 kHz. This “knee” is consistent with the amplitude spectral peak of OEFD signals around this frequency. This feature of the OEFD signals implies that the Venus lightning may have global occurrence. The lightning generated signals may travel considerable distance within the Venus-ionosphere waveguide. The encounter of these signals with plasma “holes” or ion “troughs” in the nightside of Venus wake region results in the leakage of the signals out of the shielding ionosphere. These signals are occasionally recorded by the OEFD. The occurrence rate statistics shows that the leakage of 5.4 kHz signals through the plasma “holes” in the vicinity of PVO periapsis are most effective. The maximum occurrence rate of higher frequency signals change in altitude slightly from one PVO season to another.

Lightning-generated waves escaping out through plasma holes in the nightside Venus ionosphere

The plasma waves in the Venus ionosphere measured by OEFD aboard PVO are analysed. It is shown that these waves are generated by lightning like cloud-to-cloud discharges anywhere in the Venus ionosphere-surface waveguide. The theoretical minimum attenuation for waveguide mode propagation at 5.4 kHz is consistent with the maximum occurrence rate at this frequency. The lightning-generated and globally-propagating signals when encountered with plasma holes or ion-trough structures escape out partially and are detected by the OEFD aboard PVO. The 100 Hz signals can propagate upwards in whistler mode. Even the localized electrostatic mode waves would be converted into electromagnetic waves in the plasma holes and ion-trough regions.