Moderate Solar Activity Years Research Articles

The variation of ionosonde-derived hmF2 and its comparisons with International Reference Ionosphere (IRI) and Empirical Orthogonal Function (EOF) over Pakistan longitude sector during solar cycle 22

Variations of maximum height of F2 ionospheric layer are analysed over ionospheric stations of Pakistan located at Islamabad (geog. coord. 33.75°N, 72.87°E), Multan (30.18°N, 71.48°E) and Karachi (24.95°N, 67.14°E) during the years, 1987 (LSA), 1990 (HSA) and 1993 (MSA) representing low, high and moderate solar activity years, respectively. Results showed that hmF2 in summer is higher than in winter during LSA and MSA at all stations. However, diurnal pattern is not consistently followed during winters of LSA and MSA. The lowest hmF2 values are observed around sunrise and sunset; whereas, higher values are observed around noon and midnight which may be due to upward and downward plasma drift velocity as diurnal behaviour of hmF2 is similar to diurnal behaviour of E×B drift. No specific latitudinal trend is observed in hmF2 during the period of this study over Pakistan which may be due to the combined effect of E×B drift and energy transferred to ionosphere by atmospheric planetary waves. Solar quiet current seems responsible for the variation of hmF2 over Multan and Islamabad. For Karachi equatorial electrojet seems controlling the daytime hmF2 during LSA. The hmF2 is found to have linear correlation with solar activity and this correlation is stronger with solar index F10.7 as compared to the sunspot numbers. The correlation coefficients for both solar indices are above 80%; whereas, geomagnetic indices Ap and Kp are weakly correlated having correlation coefficients less than 30%. Ionosonde-derived hmF2 are compared with regional model based on expansion of Empirical Orthogonal Function (EOF) with linear and spline interpolation of coefficients of time, seasons, magnetic dip & sunspot numbers and, global model, International Reference Ionosphere (IRI), with International Union of Radio Science (URSI) and Consultative Committee for International Radio (CCIR) options of IRI-2012 and AMTB-2013, SHU-2015 and BSE-1979 sub-models in combination with URSI and CCIR of IRI-2016 versions. The daytime hmF2 values over Pakistan modelled by AMTB of IRI-2016 (with URSI and CCIR) are found to be the most converging of all IRI options during all solar periods. AMTB-2013 also predicted nighttime values with least errors in most cases except during LSA. In addition, IRI showed better equinoctial and annual correlations with ionosonde-derived hmF2 as compared to EOF model. EOF based model showed higher nighttime Mean Absolute Errors (MAE) as compared to IRI. The lowest daytime and nighttime errors in hmF2 are observed over Multan during LSA shown by IRI-2016-AMTB-2013 (using URSI and CCIR) with MAE being 7.8 km and IRI-2016-SHU-2015 (using URSI and CCIR) with MAE being 4.7 km.

Nonlinear dependence study of ionospheric F2 layer critical frequency with respect to the solar activity indices using the mutual information method

This paper analyzes the dependence of the ionospheric F2 layer critical frequency, foF2, on two widely used solar activity indices (12-month running mean of the sunspot number (R12) and the solar radio flux of 10.7 cm wavelength (F10.712)) by using mutual information method at the ionospheric station of Darwin (geographic 12.4°S, 131.5°E) in Australia. The results show that the foF2 have different responses to R12 and F10.712 in different solar activity years. In the high and low solar activity years, the foF2 is more dependent on R12 rather than F10.712. In the moderate solar activity years, the foF2 is more dependent on F10.712 rather than R12. We further verify the effectiveness of the mutual information on the selection of the solar activity index by developing the prediction models of the foF2. Additionally, the results are compared against the International Reference Ionosphere (IRI) model. The data of training period used in the model is from 1998 to 2015, except 2000, 2009 and 2013. Data for 2000, 2009 and 2013 are used to validate the prediction accuracy, for high, low and moderate solar activity years, respectively. The results demonstrate that the favorable prediction results can be obtained when the solar activity index with large mutual information is used as the input variable of the foF2 prediction model. The proposed method not only can quantitatively analyze the nonlinear relationship between the foF2 and solar activity indices but also provides an effective way of selecting the solar activity indices for the foF2 prediction.

An evaluation of the IRI-Plas-TEC for winter anomaly along the mid-latitude sector based on GIM-TEC and foF2 values

Total Electron Content (TEC) has become one of the most widely used observable parameters of the ionosphere. IRI-Plas is an empirical climatic model of the ionosphere and plasmasphere that extends up to GPS orbital height of 20,000 km. The purpose of the study is to reveal whether the winter anomaly (a phenomenon where the mid-latitude daytime NmF2 (or foF2) is greater in winter than in summer at approximately the same solar activity level) occurs in TEC over the Asian sector and how well it is “captured” by IRI-Plas. The study is based on GPS-TEC data derived from Global Ionospheric Maps (GIM-TEC) for January and July months of high (2000), low (2009) and moderate (2012) levels of solar activity. Ionosonde data recorded at a number of mid-latitude Russian and Kazakhstan ionospheric stations in winter and summer seasons for different levels of solar activity are used along with corresponding IRI-Plas calculations to illustrate some specific features of seasonal variations of the midlatitude ionosphere. A comparative study made between IRI-Plas-foF2 and ionosonde foF2 indicates that the IRI-Plas-foF2 variations are found to be in good agreement with the observations showing similar structures, similar magnitudes reflecting the winter anomaly in the diurnal variations. When the IRI-Plas-TEC values are examined for the difference between winter and summer seasons, it is found that their variations are mainly similar to those of IRI-Plas-foF2. Different from that of IRI-Plas-TEC, the winter anomaly in GIM-TEC is not so strong, appears for both high and moderate levels of solar activity, and it is more pronounced at higher latitudes. In low solar activity, the winter anomaly in GIM-TEC is almost absent at the region considered. Finally, the comparison of the IRI-Plas-TEC and GIM-TEC results for winter conditions show that the IRI-Plas-TEC values are significantly larger than those of daytime GIM-TEC at all locations of Russia and Kazakhstan and for all levels of solar activity that coincides with results of previous studies. However, unlike the previous studies, in summer solstice, IRI-Plas-TEC systematically underestimates the GIM-TEC values during high solar activity year and mostly overestimates them for low solar activity. In moderate solar activity year, IRI-Plas-TEC and GIM-TEC values are practically comparable. Thus, the IRI-Plas-TEC model represents the seasonal anomaly observed in foF2 in mid-latitude region for all levels of solar activity but there is a certain deviation from the behavior of GIM-TEC. One of the main reasons of this discrepancy may be the difference of the model Ne(h)-profile from a realistic profile in the upper ionosphere and plasmasphere, that demands additional updating of models for those upper atmosphere regions.

RESPONSE OF F2- REGION MAXIMUM ELECTRON DENSITY (NmF2/foF2) TO SOLAR ACTIVITY USING PEARSON PRODUCT MOMENT CORRELATION

Introduction: The ionosphere owes its origin primarily to ultraviolet radiation from the Sun. The ionosphere is an essential part of the Earth’s upper atmosphere. It is ionized by solar radiation and influences transionospheric radio wave propagation. Maximum electron density of the F2- layer (NmF2) is an important parameter for studying the ionosphere. The ionospheric F2-region maximum electron density (NmF2) depends strongly on solar activity, it also suffers temporal and spatial variations. Aim: The aim of this paper is to investigate the response of NmF2 to solar activity during high solar activity (HSA), moderate solar activity (MSA) and low solar activity (LSA) years using correlation analysis. Materials and Methods: The data used in this work are the hourly NmF2 values derived from foF2 data observed at Jicamarca (Lat.11.9 oS, Long.76.8 oW) and Puerto Rico (Lat.18.5 oN, Long.67.2 oW) during high solar activity HSA (2002), moderate solar activity MSA (2011) and low solar activity LSA (2006) years. The NmF2 data were evaluated using the relation in equation 1 NmF2 = 1.24 x 1010 (foF2)2 (1) Where NmF2 is in el/m3 and foF2 is in MHz. Pearson Product Moment Correlation (PPMC) was used to further analyse the NmF2 data. Results: Our results revealed two unequal NmF2 peaks. The NmF2 peaks values at Jicamarca (60 - 240; 63– 204) x 1010 el/m3 are observed to be higher in values than those at Puerto Rico (63 – 187; 57 – 164) x 1010 el/m3. The highest NmF2 peak values of 240 and 187x 1010 el/m3 occurred during March equinox at 09:00 and 14:00 hours at Jicamarca and Puerto Rico respectively during HSA year. Conclusion: Correlation analysis for the three epochs of solar activity revealed that NmF2 showed positive correlation with sunspot number with highest correlation values of 0.904 and 0.976 at Jicamarca and Puerto Rico stations respectively during MSA year.

A Prediction Model of Ionospheric foF2 Based on Extreme Learning Machine

AbstractThe highly nonlinear variation of the ionospheric F2 layer critical frequency (foF2) greatly limits the efficiency of communications, radar, and navigation systems that employ high‐frequency radio waves. This paper proposes an effective method to predict the foF2 using the extreme learning machine (ELM). Compared with the previous neural network model based on feedforward algorithm, the ELM model offers the advantages of faster training speed and less manual intervention. The ELM model is trained with the daily hourly values of foF2 at Darwin (12.4°S, 131.5°E) in Australia. The training data are selected from 1995 to 2012, except 1997 and 2000, which includes all periods of quiet and disturbed geomagnetic conditions. The foF2 data to verify model performance are selected in 1997, 2000, and 2013, which are low, high, and moderate solar activity years, respectively. The prediction results have shown that the proposed ELM model can achieve faster training process while maintaining the similar accuracy compared with BPNN. In addition, the proposed ELM model is compared with the International Reference Ionosphere model prediction. The ELM model predicts the foF2 values more accurately than the International Reference Ionosphere model in low (1997), moderate (2013), and high (2000) solar activity years, as clearly seen on the yearly root‐mean‐square error. As far as the author's knowledge, this is the first time that the ELM model is applied to predict foF2.

Assessment of the NeQuick-2 and IRI-Plas 2017 models using global and long-term GNSS measurements

The global ionospheric models NeQuick and IRI-Plas have been widely used. However, their uncertainties are not clear at global scale and long term. In this paper, a climatologic assessment of the NeQuick and IRI-Plas models is investigated at a global scale from global navigation satellite system (GNSS) observations. GNSS observations from 36 globally distributed locations were used to evaluate performances of both NeQuick-2 and IRI-Plas 2017 models from January 2006 to July 2017, covering more than the 11-year period of a solar cycle. An hourly interval of diurnal profiles computed on monthly basis was used to measure deviations of the model estimations from corresponding GNSS VTEC observations. Results show that both models are fairly accurate in trends with the GNSS measurements. The NeQuick predictions were generally better than the IRI-Plas predictions in most of the stations and the times. The mean annual prediction errors for the IRI-Plas model typically varied from about 3 TECU at the high latitude stations to about 12 TECU at the low latitude stations, while for the NeQuick the values are respectively about 2–7 TECU. Out of a total 4497 months in which GNSS data were available for all the stations put together for the entire period covered in this work, the NeQuick model was observed to perform better in about 83% of the months while the IRI-Plas performed better in about 17% of the months. The IRI-Plas generally performed better than the NeQuick at certain locations (e.g. DAV1, KERG, and ADIS). For both models, the most of the deviations were witnessed during local daytimes and during seasons that receive maximum solar radiation for various locations. In particular, the IRI-Plas model predictions were improved during periods of increased solar activity at the low latitude stations. The IRI-Plas model overestimates the GNSS VTEC values, except during high solar activity years at some high latitude stations. The NeQuick underestimates the TEC values during the high solar activity years and overestimates it during local daytime for low and moderate solar activity years, but not as much as the IRI-Plas does.

ANALYSES OF TOTAL ELECTRON CONTENT VARIATIONS OVER NORTHERN AND SOUTHERN NIGERIA

Total electron content (TEC) is a parameter of the ionosphere that produces great effect on radio signals. We present the diurnal and seasonal variations of vertical total electron content (vTEC) during the ascending phase of solar cycle 24. A moderate solar activity year (2011) with sunspot number, Rz = 55.7 is used in this study. Total electron content (TEC) deduced from the dual frequency GPS measurements obtained at two ground stations namely: ABUZ (Zaria) with longitude 7.39oE in the north and UNEC (Enugu) with longitude 7.30oE in the south are considered. Both stations are located within the same longitude and has a latitudinal difference of 4.74o in the Nigerian equatorial ionosphere (NEI). Comparison of diurnal and seasonal variations of TEC is carried out for both stations. The diurnal variation of TEC shows a steep increase starting from sunrise, reaching daytime maximum between 13 – 15 LT at UNEC and 14 – 16 LT at ABUZ, then falls to a minimum at sunset. Dawn depression occurred at the same local time of 04 LT at both stations. On a seasonal scale, Pre- and post-midnight values were highest during the Equinoxes, followed by December solstice and least in June Solstice season at ABUZ. Pre- and post-midnight values were also higher during the Equinoxes than the Solstice season at UNEC, although they are about the same range. Also, TEC values are observed to be slightly higher for all hours and seasons at Enugu in the south than Zaria in the north except during March equinox at Zaria where TEC values were higher during the daytime. This implies that there could be little variations in TEC even within the same latitudinal zone.

On the fresh development of equatorial plasma bubbles around the midnight hours of June solstice

AbstractUsing the 47 MHz Equatorial Atmosphere Radar (EAR) at Kototabang, Indonesia, the nocturnal evolution of equatorial plasma bubbles (EPBs) was examined during the moderate solar activity years 2011–2012. While the evolution of EPBs was mostly (86%) confined to post sunset hours (1900–2100 LT) during equinoxes, in contrast, the majority of EPBs (~71%) in June solstice found evolve around the midnight hours (2200–0300 LT). The mechanisms behind the fresh evolution of summer time midnight EPBs were investigated, for the first time, through SAMI2 model simulations with a realistic input of background ExB drift variation derived from CINDI IVM on board C/NOFS satellite. The term‐by‐term analysis of linear growth rate of RT instability indicates that the formation of high flux tube electron content height gradient (KF) (steep vertical gradient) region at higher altitudes is the key factor for the enhanced growth rate of RT instability. The responsible factors are discussed in light of relatively weak westward zonal electric field in the presence of equatorward neutral wind and bottomside recombination around the midnight hours of June solstice. The effects of neutral winds and weak westward electric fields on the uplift of equatorial F layer were examined separately using controlled SAMI2 simulations. The results indicate that relatively larger linear growth rate is more likely to occur around midnight during June solstice because of relatively weak westward electric field than other local times in the presence of equatorward meridional wind.

On new detailed ionogram signatures of Equatorial Plasma Bubbles from non-equatorial station during low solar activity

On new detailed ionogram signatures of Equatorial Plasma Bubbles from non-equatorial station during low solar activity

Explicit characteristics of evolutionary‐type plasma bubbles observed from Equatorial Atmosphere Radar during the low to moderate solar activity years 2010–2012

AbstractUsing the fan sector backscatter maps of 47 MHz Equatorial Atmosphere Radar (EAR) at Kototabang (0.2°S geographic latitude, 100.3°E geographic longitude, and 10.4°S geomagnetic latitude), Indonesia, the spatial and temporal evolution of equatorial plasma bubbles (EPBs) were examined to classify the evolutionary‐type EPBs from those which formed elsewhere and drifted into the field of view of radar. A total of 535 EPBs were observed during the low to moderate solar activity years 2010–2012, out of which about 210 (~39%) are of evolving type and the remaining 325 (~61%) are drifting‐in EPBs. In general, both the evolving‐type and drifting‐in EPBs exhibit predominance during the postsunset hours of equinoxes and December solstices. Interestingly, a large number of EPBs were found to develop even a few minutes prior to the apex sunset during equinoxes. Further, the occurrence of evolving‐type EPBs exhibits a clear secondary peak around midnight (2300–0100 LT), primarily, due to higher rate of occurrence during the postmidnight hours of June solstices. A significant number (~33%) of postmidnight EPBs generated during June solstices did not exhibited any clear zonal drift, while about 14% of EPBs drifted westward. Also, the westward drifting EPBs are confined only to June solstices. The responsible mechanisms for the genesis of fresh EPBs during postmidnight hours were discussed in light of equatorward meridional winds in the presence of weak westward electric fields.

Comparison of GPS based TEC measurements with the IRI-2012 model for the period of low to moderate solar activity (2009–2012) at the crest of equatorial anomaly in Indian region

The measurements of total electron content (TEC) are conducted at Surat (21° 9′ N, 72° 47′ E) in India, which lies under the northern crest of the equatorial anomaly region, for a period of four years from low to moderate solar activity (2009–2012) using a Global Positioning System (GPS) receiver. These results are compared with the TEC derived from IRI-2012 using three different options of topside electron density: NeQuick, IRI01-corr and IRI-2001. As there is difference between the upper limit of integration in the GPS TEC (20,200km) and the IRI model (2000km), to have a fair comparison of measured TEC with that of modeled TEC, the plasmaspheric contribution to the GPS TEC is removed. The measured TEC are compared with the model derived TEC for different times of the day for all months of four years (2009–2012). The IRI-2012 estimates the TEC well for the dusk hours 1800LT and noon hour 1200LT from 2010 to 2012. However, the estimation shows discrepancies with the observed TEC in the year 2009. For 0600LT it is observed that, from 2009 to 2011 the predictions made by IRI-2012 (options NeQuick and IRI01-corr) shifts from over estimation (0–50%) to under estimation (50–75%) and estimate the TEC well in the year 2012. In general, from 2009 to 2012, it is observed that with ascending phase of solar cycle the discrepancies in IRI prediction decreases for 0600LT, 1200LT and 1800LT hours of the day. Further, the comparison is also done for TEC at peak hour 1430LT for four months i.e. April, June, October and December (representing four seasons). It is observed that the peak hour TEC obtained by model overestimates the TEC for a low solar activity year 2009 but estimates well in 2010 and 2012 (except June). Further, model underestimates the peak hour TEC for moderate solar activity year 2011.

GPS TEC near the crest of the EIA at 95°E during the ascending half of solar cycle 24 and comparison with IRI simulations

Total electron content (TEC) data obtained from GPS dual frequency measurements during the ascending half of the solar cycle 24 from 2009 to 2012 over Dibrugarh (27.5°N, 94.9°E; 17.6°N MLAT) have been used to study the diurnal, seasonal, annual and solar cycle variation of TEC. The measurements reported here are for the first time from the location situated at the poleward edge of the northern equatorial ionization anomaly (EIA) and within the peak region of the longitudinal wave number 4 (WN4) structure in EIA crest TEC. TEC exhibits a minimum around 0600LT and diurnal maximum around 1300–1600LT. In the low and moderate solar activity years 2009–2010 and 2010–2011, average daytime (1000–1600LT) TEC in summer was higher (25.4 and 36.6 TECU) compared to that in winter (21.5 and 26.1 TECU). However, at the peak of the solar cycle in 2011–2012, reversal in the level of ionization between winter and summer takes place and winter TEC becomes higher (50.6 TECU) than that in summer (45.0 TECU). Further, TEC in spring (34.1, 49.9 and 63.3 TECU respectively in 2009–10, 2010–11 and 2011–12) is higher than that in autumn (24.2, 32.3 and 51.9 TECU respectively) thus showing equinoctial asymmetry in all the years of observation. The winter anomaly in high solar activity years and equinoctial asymmetry all throughout may be largely attributed to changes in the thermospheric O/N2 density ratio. A winter to summer delay of ∼1h in the time of occurrence of the diurnal maximum has also been observed. Daytime maximum TEC bears a nonlinear relationship with F10.7 cm solar flux. TEC increases linearly with F10.7 cm solar flux initially up to about 140sfu (1sfu=10−22Wm−2Hz−1) after which it tends to saturate. On the contrary, TEC increases linearly with solar EUV flux (photons cm−2s−1, 0.5–50nm) during the same period. TEC predicted by the IRI 2012 are lower than the measured TEC for nearly 90% of the time.

Morphology of the G condition occurrence over Irkutsk

Using Irkutsk digisonde data obtained in 2003–2011, a morphological analysis of the G condition occurrence has been made. The G condition was found to occur during daylight hours in summer; in winter, it is extremely rare, and its appearance is associated with intense magnetic storms. In the years of moderate solar activity, the G condition is most frequently registered at Kp⩾4, in the forenoon. During low solar activity, it can be observed under quiet geomagnetic conditions; in most cases, local time of its appearance shifts to afternoon hours. The highest percentage of the G condition occurrence (7.7–6.4%) was recorded in June and July 2008 when the levels of solar and geomagnetic activity were abnormally low.

Toward prediction of L band scintillations in the equatorial ionization anomaly region

[1] The first observations of the duration and spread of equatorial spread F (ESF) at the magnetic equator and their relationship with the L band scintillations in the equatorial ionization anomaly (EIA) region have been presented here. The analysis is done for the equinoctial months of low solar activity period 2005–2006 and the moderate solar activity year 2004. Ionosonde and CRABEX data from Trivandrum and GPS data from four stations in the EIA region centered around 77°E meridian have been used for the study. The results show that the maximum scintillation index (s4) in the EIA region is linearly dependent on the spread of ESF traces for both the equinoxes. The corresponding duration of L band scintillations is also found to be linearly dependent on the duration of ESF at the magnetic equator. Further, the study for the first time reveals the plausible use of the ESF prediction parameter during 1600–1845 IST period for predicting L band scintillations and its inverse relationship with F10.7 cm flux.

Electrodynamical control of the ambient ionization near the equatorial anomaly crest in the Indian zone during counter electrojet days

The variability of ambient ionization near the crest of the equatorial anomaly on the days of counter electrojet (CEJ) is studied using a long‐term (1980–1990) database of total electron content (TEC) obtained from Calcutta (geographic: longitude, 88.38°E; latitude, 22.58°N; dip: 32°N), situated virtually below the northern crest of the equatorial anomaly, in conjunction with the equatorial electrojet (EEJ) data of the Indian subcontinent and foF2 data from the equatorial stations. On the days of CEJ events the diurnal variations of TEC exhibit deviations (decreases) from monthly mean values, but the feature is not regular throughout the observing period. A larger percentage of CEJ days show decreases in the descending phase (1980–1985) of solar cycle compared to the ascending one (1986–1990). During low to moderate solar activity years the ionosphere near the anomaly crest is found to be more susceptible to CEJ related equatorial electrodynamics than in the high solar activity periods. On the seasonal basis, though the solstitial months of ascending phase exhibit higher percentage occurrence of decreases, the feature is not so prominent in the descending epoch. An attempt is made to relate the decreases in TEC with the prevailing electrodynamics at the magnetic equator for which EEJ, under quiet geomagnetic conditions, may be treated as a proxy index. A statistical analysis reveals positive correlations of TEC decreases with strength and duration of CEJ but anticorrelations with rising slope, strength, and time‐integrated value of EEJ as well as initiation time of CEJ. To represent better correspondence of the TEC variability with the equatorial electrodynamics a function combining various EEJ parameters has been generated, which seems to exhibit significant association with TEC decreases on the days of CEJ events.

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

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.

Meridional wind effect on anti‐solar activity correlation of equatorial density irregularity distribution

Contrasting longitudinal variation in equatorial spread‐F (ESF) density irregularity distribution has been observed by ROCSAT‐1 during high solar activity year of 2000 against moderate solar activity year of 2003. The monthly averaged solar flux intensity F10.7 was higher every month in 2000 than in 2003, but the irregularity occurrence rate did not always correlate with solar activity at every longitude in a season. For the equinox seasons, the occurrence rate was higher at every longitude in 2000 than in 2003. Higher occurrence rate was also observed in 2000 than in 2003 during a solstice season at longitudes of the so‐called ESF longitudes. On the other hand, higher occurrence rates were observed in 2003 at the so‐called low‐ESF longitudes from 230° to 10° during the June solstice season and from 90° to 260° during the December solstice season, where the magnetic flux tube makes a large‐angle alignment with the sunset terminator during the respective solstice season. Such anti‐solar activity correlation of ESF irregularity occurrences at these low‐ESF longitudes could be due to transequatorial meridional wind‐induced instability suppression agent which becomes more effective to offset the Rayleigh‐Taylor instability growth rate in a solstice season during a high solar activity period than during a low solar activity period.

An empirical model for IEC over Lunping

The diurnal, seasonal and solar cycle variations of ionospheric electron content (IEC) are studied. An attempt has been made to develop an empirical model for the IEC at Lunping (25°N,121.2°E) using harmonic analysis. The proposed model predicts the diurnal variation of the monthly mean values of IEC for any month for sun spot numbers up to 150. The amplitudes and phases of all harmonic components show seasonal and solar cycle dependences. The daily mean IEC and the amplitude of the first order harmonic depend seriously on the solar activity. A set of 81 coefficients of the zero and first four orders is determined and found to be sufficient for modelling the IEC. The agreement between observed and modelled IEC values is rather good; maximum deviation is limited to ±15% in winter and 10% in summer and equinox. The agreement between observed and modelled IEC values is remarkably good in May and is generally better in years of moderate solar activity.

Variation of cosmic rays and solar wind properties with respect to the heliospheric current sheet: 1, Five‐GeV protons and solar wind speed

We investigate the two‐dimensional, steady state distributions of solar wind speed and 5‐GeV cosmic ray flux with respect to the heliospheric current sheet and their variation with the solar cycle from late 1964 through mid‐1982. Synoptic K coronameter data are used to locate the current sheet, taken as the center of the “band of coronal streamers,” during years of low and intermediate solar activity. We confirm the conclusion that the profile of the solar wind at 1 AU during these years consists of (1) a minimum in mean speed of ∼400 km s−1 at the heliospheric current sheet, (2) a rise to a mean speed of ∼600 km s−1 at an angular distance from the current sheet of 20° to 40°, and (3) a plateau of high mean speed of ∼600 km s−1 over the polar coronal holes. During those years of moderate solar activity when the current sheet provides a simple organization to the heliosphere, the mean velocities at the current sheet and in the plateau are approximately independent of phase in the solar cycle. The principal change in the profile is that the breadth of the minimum decreases with subsiding solar activity and reaches its narrowest at sunspot minimum. The profile of the flux of 5‐GeV cosmic rays shows a maximum at the current sheet, with a decrease of 2 to 3% out to a displacement in latitude from the current sheet of ∼45°. There is no evidence for change in this gradient with either the level of solar activity or the overall modulation of cosmic ray flux within a given solar cycle. The data are unable to demonstrate the existence of any dependence of either the sign or the magnitude of the gradient with the polarity of the large‐scale heliospheric magnetic field. The observed distribution in cosmic ray flux is compared to the predictions of several theoretical models in an effort to resolve the question of the role played by particle drifts in cosmic ray propagation. Although drift‐dominated models with inclined current sheets naturally produce negative gradients, which could be made to coincide with the observations by a suitable choice of parameters, such models also predict a dramatic dependence of the magnitude of the gradient upon the polarity of the large‐scale heliospheric magnetic field. This dependence is not apparent in the currently available data. Diffusion‐convection models adjusted to yield a constant radial gradient of cosmic ray flux in the ecliptic also produce a negative gradient with displacement in latitude from the current sheet. However, these latter models require considerable arbitrary “tuning” of the latitudinal dependence of the propagation parameters to reproduce the observations. The models which explain recurrent fluctuations in cosmic ray flux as the in‐ecliptic consequences of corotating interaction regions (rather than those of a gradient in displacement in latitude from the current sheet) are shown to produce effects quite contrary to the observations. We conclude that a satisfactory, quantitative explanation of the gradient of cosmic rays with respect to the current sheet is not yet available.