Solar Maximum Years Research Articles

Dependence of the equatorial anomaly and of equatorial spread F on the maximum prereversal E × B drift velocity measured at solar maximum

The relation of equatorial bubbles to the equatorial anomaly is important because scintillation that is most disruptive to transionospheric RF propagation occurs when it passes through the intersection of the two. However, measurement of the relation between the two and of the electric field from which both arise is difficult because of large separations in space and time. This first attempt to perform these measurements employs a latitudinal array of ionospheric sounders spanning 0° to 40° dip latitude (DLAT) in the Western American sector. Measured on each day of a solar maximum year are the following: (1) the maximum electron density of the postsunset equatorial anomaly, Ne, at 16° and at 20.3° DLAT at 2100 LT, the time when the anomaly crest is at its maximum latitude; (2) equatorial spread F (ESF), detected by the occurrence of macroscopic bubbles and of bottomside spread F (BSSF), the latter recorded at levels of none, weak and strong; (3) Kp averaged over the 6 hours before sunset. Ne and ESF are considered functions of the maximum prereversal F layer drift E × B drift velocity measured by the Jicamarca incoherent scatter radar also during solar maximum and at the same longitude. Parameters are averaged over two levels of Kp for the three seasons, the E months (March, April, September, and October), D months (November–February), and J months (May–August) to yield the following results: (1) Ne measured at 16°, at 20.3° DLAT or at the anomaly crest are linearly dependent on maximum E × B drift velocity. (2) Occurrence of each level of ESF increases with Ne approximately linearly during the E and J months but not during the D months. (3) ESF occurrence is dependent on and increases approximately linearly with maximum E × B drift velocity during the E and J months. During the D months this dependence is absent. Except for the D months, these results indicate that scintillation increases with maximum prereversal E × B drift velocity: at L‐band at the bubble‐anomaly intersection because bubble occurrence increases, Ne increases, and the latitudinal extent of the anomaly increases; and at VHF/UHF near the equator because the occurrence of strong BSSF increases.

Interplanetary shock parameters during solar activity maximum (2000) and minimum (1995-1996)

Interplanetary shock parameters are analyzed for solar maximum (year 2000) and solar minimum (years 1995-1996) activity. Fast forward shocks are the most usual type of shock observed in the interplanetary medium near Earth's orbit, and they are 88% of the identified shocks in 2000 and 60% in 1995-1996. Average plasma and magnetic field parameters for upstream and downstream sides of the shocks were calculated, and the parameter variations through the shock were determined. Applications of the Rankine-Hugoniot equations were made, obtaining shock speeds and Alfvenic Mach number. Static and dynamic pressures variations through the shocks were also calculated. Every parameter have larger variation through the shock in solar maximum than in solar minimum, with exception of the proton density. The intensity of shocks relative to the interplanetary medium, quantified by the Alfvenic Mach Number, is observed to be similar in solar maximum and minimum. It could be explained because, during solar maximum, in despite of the higher shock speeds, the Alfvenic speed of the interplanetary medium is higher than in solar minimum.

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

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.

Estimation of Minimum Separation of Geostationary Satellites for Satellite-Based Augmentation System (SBAS) from Equatorial Ionospheric Scintillation Observations

In Satellite-Based Augmentation Systems (SBAS), the correction messages are transmitted to the users' receivers via geostationary communication satellites (GEOS) at GPS L1 (1575·42 MHz) frequency. Severe scintillations in the equatorial zone disrupt geostationary satellite links even at L-band. Observations of scintillations at 1·5 GHz from Calcutta (22·58°N, 88·38°E geographic, 32°N magnetic dip), located near the crest of the equatorial anomaly in the Indian zone, show that scintillations occur in patches of duration varying from a few minutes to several hours. During the solar maximum years 1998–2000, severe scintillations (Scintillation Index [ges ]15 dB) were recorded for 48 hr 55 min (1·27%) out of the total observation time of 3868 hr 9 min in the local time interval 19 to 00 hrs. In order to have a fail-safe system, it is suggested that more than one geostationary satellite be used in SBAS so that, if one link is disrupted, the other can be used for transmission of correction messages to the GPS users. The minimum longitudinal separation between two GEOS required for reliable operation of SBAS has been estimated, from the cumulative distribution of scintillation patch duration, to be 57° in the Indian longitude zone.

Rocket and ground-based electron density soundings versus IRI representation

Monthly averages of F-region cut-off frequency foF2 and peak height hmF2 data obtained during the solar maximum years 1978 and 2000 through the use of ionosondes located at equatorial and low latitude regions in Brazil are compared with the IRI representations. The equatorial stations are Fortaleza (3°53'S, 38°25'W), and São Luís (2°20'S, 44°12'W), and the low latitude station was Cachoeira Paulista (22°41'S, 45°00'W). The ionosonde data analyzed here were taken at each 15 minutes, in the months of February, May, August and November, of 1978 and 2000. In a second part of this study, electron density profiles obtained by three rocket flights in the Brazilian equatorial region are also compared with the IRI representations. The rocket experiments were carried out on July 7, 1984, December 11, 1985 and October 14, 1994. The first and the second experiments were performed at the sub-equatorial station Natal (35°14'W, 5°55'S) and the third one at the equatorial station Alcântara (2°19'S, 44°22W). The results clearly show that IRI significantly underestimates the equatorial hmF2 during the pre-midnight period. On the other hand, the IRI representation for hmF2 over off-equatorial station Cachoeira Paulista is in rather better agreement with the experimental density profile. The IRI representation of the foF2 parameter is in reasonable agreement with the data for both the equatorial region and Cachoeira Paulista, although the model appears to systematically overestimate foF2 in August for both 1978 and 2000. The rocket electron density profiles show that the highly variable height distribution of the electron density during ionospheric plasma bubble events may always prevent the correct representation of electron density profiles in the equatorial region during the occurrence of those events.

Power Spectrum of Cosmic-ray Fluctuations During Consecutive Solar Minimum and Maximum Periods

Power spectral analysis of cosmic-ray intensity recorded by eight stations was carried out over a wide range of frequencies from 2.3 × 10−8 Hz to 5.8 × 10−6 Hz (2–500 days) during the period 1964–1995. Spectrum results of large-scale fluctuations have revealed the existence of a broad peak near 250–285 days and a narrower peak at 45–50 days during the studied epochs as a stable feature in all neutron monitors covering a wide rigidity range. The cosmic-ray power spectrum displayed significant peaks of varying amplitude with the solar rotation period (changed inversely with the particle rigidities) and its harmonics. The amplitudes of 27-day and 13.5-day fluctuations are greater during the positive-polarity epochs of the interplanetary magnetic field (qA>0) than during the qA 0 and qA<0 epochs are significantly large. Furthermore, the spectrum for even solar maximum years are higher and much harder than the odd years. There are significant differences in the individual spectra of solar maxima for different cycles.

The impact of magnetic storms on GPS receiver performance

During the past decade, geodetic measurements have been greatly improved through the use of space geodesy techniques based on the global positioning system (GPS). The GPS allows for precise surveys, where centimetre-level accuracies may be obtained through differential carrier phase-based positioning algorithms. GPS positioning techniques often rely on the availability of dual-frequency data for formation of ionosphere-free and widelane observables (in the case of ambiguity resolution). While GPS positioning results generally provide adequate accuracies for several precise positioning applications, users may experience degraded positioning accuracies over the next few years. Of particular concern are ionospheric phenomena called magnetic storms, which are characterized by an increased spatial decorrelation of ionosphere range delays and scintillation effects at low and high latitudes. During such events, degradations in differential positioning accuracies are observed, and availability of the L2 observations may be limited through loss of signal lock. The magnitude of such effects is estimated during a global magnetic storm event, and a performance comparison of two survey-grade receivers is conducted. This storm is representative of the activity at solar maximum and several years beyond.

Lunar tide in the equatorial F region vertical ion drift velocity

Vertical ion drift velocity data from Jicamarca have been analyzed for a lunar semidiurnal tide using a least squares fitting method. Amplitudes of up to 6 m s−1 are obtained with phases in agreement with lunar tidal determinations of other associated physical parameters. Variations between season, solar activity, and day to night are also examined. Generally, amplitudes are larger in the southern summer. Much of the phase variation with season is very similar for solar maximum and minimum years. There is a summer to winter phase change that is most distinct at solar maximum nighttime. A day‐to‐night phase reversal can also been seen in some seasons. Hints of this are also found in the lunar tide in the F region height and in the magnetic variations at Huancayo.

ON THE DISTRIBUTION AND ASYMMETRY OF SOLAR ACTIVE PROMINENCES

The paper presents the results of a study of the distribution and asymmetry of solar active prominences (SAP) for the period 1957–1998 (solar cycles 19–23). The east-west (E-W) distribution study shows that the frequency of SAP events in the 81–90° slice (in longitude) near the east and west limbs is up to 10 times greater than in the 1–10° slice near the central meridian of the Sun. The north-south (N-S) latitudinal distribution shows that the SAP events are most prolific in the 11–20° slice in the northern and southern hemispheres. Further, the E-W asymmetry of SAP events is not significant. The N-S asymmetry of SAP events is significant and it has no relation with the solar maximum year or solar minimum year during solar cycles. Further, the present study also shows that the N-S asymmetry for cycles 19–23 follows and confirms the trend of N-S asymmetry cycles as reported by Verma (1992).

Equatorial spread F statistics in the American longitudes: Some problems relevant to ESF description in the IRI scheme

Ground based ionograms (followed by radio wave scintillation and satellite in situ measurements) provide the most abundant data base for generating equatorial spread F (ESF) irregularity occurrence statistics. For practical purposes it is necessary that the statistics of the spread F occurrence distribution (from ionograms) be based, separately, on the range spreading and frequency spreading components of the spread F. The well known features of spread F include its day-to-day, seasonal and solar cycle variations. There seems to exist general consensus on the geomagnetic declination-, longitude- and solar- control of seasonal occurrence of spread F, whereas solar flux seems to produce superimposed effects, including the solar activity cycle in its occurrence features. The day-to-day variability is certainly the least understood of all the known spread F variabilities. Empirical description of the spread F occurrence statistics in the IRI (International Reference Ionosphere) prediction scheme requires detailed examination of the available statistics not only over the magnetic equator but necessarily also over low latitudes extending to equatorial anomaly location. We have produced, and are continuing to improve upon, such statistics based on spread F data covering the solar activity minimum and maximum years for the two Brazilian stations, Fortaleza and Cachoeira Paulista. In this paper we present a comparative study of these results with the published statistics available by other technique and for other longitude sectors, and discuss the findings in terms of their implications for formulating an empirical description of the ESF for possible incorporation into the IRI model.

Long‐term observations of VHF scintillation and total electron content near the crest of the equatorial anomaly in the Indian longitude zone

Ionospheric VHF scintillation (SI ≥ 3 dB and saturated level) and total electron content (TEC) data obtained at Calcutta (subionospheric 21°N, 92.7°E geographic, 27°N dip) and ionosonde data at Kodaikanal ( 10.2°N, 77.5°E geographic, 3.5°N dip) for the period 1977‐1990 have been analysed to show the importance of electrodynamic drift near the magnetic equator in controlling nighttime ionospheric F region ionization and irregularities in the equatorial region. Such long‐term observations extending over a period of more than 13 years are possibly being reported for the first time from a location situated near the equatorial anomaly crest. Frequent and intense VHF scintillations near the equatorial anomaly crest during equinoctial and December solstice months around solar maximum years, have been identified with the equatorial F region irregularities. Simultaneous measurement of TEC at the same location shows that during solar maximum years the high F region ambient ionization is sustained for several hours in the postsunset period, often showing secondary enhancements during equinoctial months. Under solar minimum epoch, when scintillation is sparse, TEC in the above period shows a rapid decrease. At Kodaikanal, situated near the magnetic equator, during the equinoctial and December solstice months of solar maximum years, h/F values rise by more than 100 km in about an hour around sunset. These features are seldom observed during solar minimum epoch. A causative connection among h/F variation near the magnetic equator and the maintenance of high ambient ionization and occurrence of scintillation near the anomaly crest is established. Further, scintillation occurrence during the May‐July months shows a remarkable hysteresis effect with solar activity level.

Universal variation of the F2-layer critical frequency and solar activity

Regular variations of the F2-layer in the quiet ionosphere are studied using the universal variation of the F2-layer critical frequency, i.e. the dependence of the instantaneous mean-longitudinal value of foF2 at a fixed latitude on universal time (UT). Algorithms for the universal variation calculation have been developed and are used to process the ionospheric vertical sounding data for 1957–1990. It is shown that the foF2-universal variation forms are es-sentially different for solar minima and maxima years. In the solar minima years, the foF2-universal variation is similar to the known universal variation of the atmospheric electric field near the Earth’s surface which has a maximum at 19 UT. There is no foF2-universal variation seasonal change in solar minimum years. The foF2-universal variation form changes for solar maximum years have a minimum of foF2, at 16–17 UT, and there are small seasonal changes in the foF2-universal variation. Such behaviour of the foF2-universal variation suggests that under the quiet conditions in solar minimum years the electric field penetrates from the lower atmospheric layers up to the F2 layer of the ionosphere.

Short Term Ionospheric Variability in the Northern Hemisphere for Two Consecutive Solar Cycles

Hourly values of the total electron content (TEC-Faraday content) collected at five stations in the northern hemisphere extending from the low to high latitudes lying in the narrow longitude zone of (270 ± 20) °E during the solar maximum years 1981 and 1989 respectively of the last 21 and 22nd solar cycles and also during the common solar minimum year 1985 are used to study the latitudinal variations of the day to day TEC variability. The effect of solar phase change in the variability is found to be the least for the midlatitude station of Boulder and most during the nighttime for the high latitude station of Goose bay. The magnetic disturbance generally enhances the TEC variability for the five stations during the three seasons of both solar phases and this effect seems to be somewhat higher for the mid and high latitudes during the equinox season of the solar maximum years. A spectral investigation of the TEC variability reveals narrowing of the spectrum from low to high latitudes during the solar minimum phase while the reverse seems to be the case during the solar maximum. While the low latitude TEC variability spectrum is not correlated to either the variability spectrum of S10.7 solar flux or to the mean equatorial Dst variations, the mid latitude TEC variations show a good positive correlation with the variability of Ap and the higher latitude TEC variations are negatively correlated with the variability spectrum of the daily mean Auroral electrojet (AE) indices.

Effects of intense storms and substorms on the equatorial ionosphere/thermosphere system in the American sector from ground‐based and satellite data

Equatorial ionospheric responses to magnetospheric storm/substorm‐associated electric fields are investigated for a few intense events of the equinoctial months of solar maximum years 1978–1979. All the magnetic storms considered here are the result of the transit at Earth of interplanetary magnetic clouds. The interplanetary magnetic field data Bz from the ISEE 3 satellite, the auroral electrojet activity index AE, and the ring current index Dst are used as indicators of the disturbed magnetospheric conditions, and the ionospheric response features are analyzed using the F layer critical parameters h′F, h′F3, hpF2, and ƒ0F2. Focus is given to identify, when a large number of sequential substorms occurs, (1) the responses to prompt penetration electric field (from individual substorm events) as different from the delayed effect from the disturbance dynamo electric field and (2) the verification of local time dependences of the disturbance electric field polarity as predicted from the existing theoretical models. We have found evidence of near‐midnight polarity reversal of prompt penetration disturbance electric field during the course of a developing substorm. Evidence is provided also on the near‐midnight polarity reversal for the disturbance dynamo electric field. The prereversal enhancement electric field at sunset, produced by the F layer dynamo, is found to undergo drastic day‐to‐day variations in the course of a disturbed interval. However, the competing influences of the prompt versus delayed electric fields after a series of substorms could result at times in partial, or even complete, cancellation of the effects, so that the prereversal enhancement in the vertical drift could appear unaffected by the disturbances. There are indications that the disturbance dynamo electric field effects on the equatorial ionosphere last for one more day past the end of the substorm recovery.

Evidence for orographic wave heating in the equatorial thermosphere at solar maximum

Fabry Perot interferometer observations of equatorial 630 nm airglow thermospheric winds and temperatures were made at Arequipa, Peru (16.5 S, 71.5 W) during the solar maximum years of 1989 and 1990. Examination of the thermospheric winds measured from Doppler shifts of the 630 nm airglow emission in the two directions east and west of Arequipa shows that the zonal wind is often significantly weaker over the Andes (east) than over the Pacific (west) in winter. Between 19 LT and 24 LT, the typical reductions in speed found were ∼40 to 70 m/s. In the same time period the temperature results show elevated values of ∼200‐500 K for the three directions (north, east, and zenith) over mountainous terrain compared with those of the two directions (west and south) over Water. This effect fades near equinoctial periods. Examination of the standard deviation of the temperature observations for the five directions showed an increase in this value above the measurement error by a factor of three for the direction toward the Andes (East) but not for the direction over water (South); this elevation is consistent with wave activity as a source of the heating. Viscous dissipation of waves propagating into the thermosphere region from below is suggested as an explanation for the localized regions of heating.

Short term variabilities of ionospheric electron content (IEC) and peak electron density (NP) during solar cycles 20 and 21 for a low latitude station

Hourly values of IEC and of f 0F 2 (critical frequency) for a low latitude station, Hawaii (21.2°N, 157.7°W), during the solar maxima (1969 and 1981) and minima (1965 and 1985) years of two consecutive solar cycles, 20 and 21, are used to study the day to day variabilities of the ionospheric parameters IEC and NP. It is found that there is good correspondence in the day to day variations of IEC and NP from one solar cycle to the other for both solar maximum and minimum years in the two solar cycles. Depending on solar phase and season, while the mean daytime IEC and NP variations range from about 20% to 35%, the mean night time values vary from about 25% to 60%. The mean daytime variations in NP for the solar minimum phase are remarkably higher in all the three seasons compared to the solar maximum phase. However, no such increase is observed in the mean daytime IEC variations, indicating the highly variable nature of the daytime ionospheric F region compared to the topside during solar minimum for this low latitude station. The winter night time IEC also seems to be a relatively stable parameter during the solar minimum. The short term day to day variabilities of the day time peak values of IEC and NP (ie IEC max and NP max) are not closely associated with the variations in F 10.7 solar flux. Contrary to the common expectation, the variabilities in both the parameters, particularly in NP max, are somewhat reduced during the solar maximum (when the variability in F 10.7 solar flux is much higher compared to the solar minimum) which is more evident in the stronger 21 solar cycle. A larger number of significant components are seen in the spectra of the percentage variation of both IEC max and NP max during both solar phases of the two solar cycles compared to the corresponding F 10.7 solar flux spectra. The number of additional components for both the parameters with periods less than 15 days are more for the low solar activity years than for the solar maximum years.

Relations of field line resonances and upstream waves and the winter attenuation of pulsations

Abstract. Using data on the occurrence frequency of geomagnetic pulsations of different periods from three observatories in Central Europe, conclusions are drawn about the occurrence of field line resonances and pulsations directly driven by upstream waves at L-values below 3. It was found that both types occur during the interval studied (first 6 months of the year 1991), but both the occurrence frequency of the two types and the characteristic period of the field line resonance change significantly as compared to other intervals. During Northern winter, pulsation activity is severely damped in solar maximum years, including the year 1991. The decrease in the activity of the pulsations is more significant at shorter periods.

A new aspect of magnetic declination control of equatorial spread F and F region dynamo

Range spread F (RSF) data for a 12‐year period (1978–1990) over the Brazilian low‐latitude station, Cachoeira Paulista, are analyzed to determine the fine structure of the seasonal pattern of the irregularity occurrence, which appears consistent throughout an entire solar cycle. The RSF occurrence for one of these years is compared also with that over the equatorial station, Fortaleza, to show that the seasonal pattern discussed here corresponds to that of plasma bubble irregularity developments. A striking result that has come out of the present analysis concerns two secondary minima, during the epoch of broad annual maximum, in the RSF that occur in association with the two nodal points at the magnetically conjugate E layer sunset local times, that is, at the perfect alignment of the sunset terminator with the magnetic meridian of the station. The occurrence of these minima, seen in individual solar minimum and solar maximum years (as also in the overall mean behavior), is interpreted on the basis of a simplified F region dynamo development model that considers also asymmetric conjugate E layer decay conditions. Competing roles of a prereversal zonal electric field enhancement and of its height gradient seem to determine the occurrences of these secondary minima in RSF.

Sociology of the SMY

Since Professor de Jager described the history and organization of the Solar Maximum Year (SMY) at the Kansas City Workshop on Max’91 Science, and since the SMY scientific highlights were summarized in Solar Physics, Vol. 114, and in the published proceedings of the SMY and SMA (Solar Maximum Analysis) workshops, I hope you will allow me to reflect on the sociology of the SMY. I served as the SMY science coordinator and as the Solar Maximum Mission (SMM) observatory coordinator in 1979-81.

Observations of beam propagation

We discuss the observational evidence for propagating electron distributions in the solar corona as obtained from type III observations. The location of the beam acceleration region together with the effects of the ambient medium on beam propagation are discussed in the framework of recent observational and theoretical developments. The results of combined X-ray and radio observations obtained during and after the Solar Maximum Year are summarized. Special attention is paid to the effect of the small-scale structure of the corona on the observed radio emission. We outline various desirable developments which are necessary to improve the understanding of the behaviour of charged particle beams in magnetized plasmas.