Solar Maximum Period Research Articles

Exposure to galactic cosmic radiation and solar energetic particles

Several investigations of the radiation field at aircraft altitudes have been undertaken during solar cycle 23 which occurred in the period 1993-2003. The radiation field is produced by the passage of galactic cosmic rays and their nuclear reaction products as well as solar energetic particles through the Earth's atmosphere. Galactic cosmic rays reach a maximum intensity when the sun is least active and are at minimum intensity during solar maximum period. During solar maximum an increased number of coronal mass ejections and solar flares produce high energy solar particles which can also penetrate down to aircraft altitudes. It is found that the very complicated field resulting from these processes varies with altitude, latitude and stage of solar cycle. By employing several active and passive detectors, the whole range of radiation types and energies were encompassed. In-flight data was obtained with the co-operation of many airlines and NASA. The EURADOS Aircraft Crew in-flight data base was used for comparison with the predictions of various computer codes. A brief outline of some recent studies of exposure to radiation in Earth orbit will conclude this contribution.

Mesoscale ionospheric anomalies not associated with space weather

Using the Austrian network of 10 two‐frequency GPS geodetic reference stations, whose location is known with high precision, a study of mesoscale anomalies in the ionosphere over central Europe has been made (and is still in progress for the following years). From the large number of individual observations during the period from 2000 to 2004, maps of small‐scale ionospheric behavior (with a pixel size of 0.5° in latitude and longitude) have been constructed from which local anomalies (at an equivalent height of 250 km) in terms of departure from the model total electron content (TEC) can be recognized. Not all magnetic storms which were most common during the 2000–2002 period of solar maximum showed local effects in the regional ionosphere, but mesoscale ionospheric anomalies which are not (necessarily) associated with space weather were also observed up to 20% of the days of observation, with their highest frequency of occurrence during the winter months (November–February). These anomalies have a typical duration of several hours and seem to be associated with traveling ionospheric disturbances (TIDs) whose origin could lie in gravity waves initiated in the lower atmosphere. Thus TIDs not associated with space weather may play a role in the regional ionosphere but can be detected only by virtue of the dense spatial coverage obtained by a network of GPS stations for precise positioning. From 2000 to 2004, the solar activity and the background ionization decreased considerably. Since TIDs depend on both the properties of atmospheric gravity waves and on the level of ionization, we observed smaller absolute TID amplitudes in 2003–2004 compared with 2000–2001. There is indication that the relative amplitudes have increased slightly, which is consistent with the influence of ion drag on thermosphere dynamics.

Solar cycle dependent characteristics of the equatorial blanketing E<sub><i>s</i></sub> layers and associated irregularities

Abstract. The occurrence of blanketing type Es (Esb) layers and associated E-region irregularities over the magnetic equatorial location of Trivandrum (8.5° N; 77° E; dip ~0.5°) during the summer solstitial months of May, June, July and August has been investigated in detail for the period 1986–2000 to bring out the variabilities in their characteristics with the solar cycle changes. The study has been made using the ionosonde and magnetometer data of Trivandrum from 1986–2000 along with the available data from the 54.95 MHz VHF backscatter radar at Trivandrum for the period 1995–2000. The appearance of blanketing Es layers during these months is observed to be mostly in association with the occurrence of afternoon Counter Electrojet (CEJ) events. The physical process leading to the occurrence of a CEJ event is mainly controlled by the nature of the prevailing electro dynamical/neutral dynamical conditions before the event. Hence it is natural that the Esb layer characteristics like the frequency of occurrence, onset time, intensity, nature of gradients in its top and bottom sides etc are also affected by the nature of the background electro dynamical /neutral dynamical processes which in turn are strongly controlled by the solar activity changes. The occurrence of Esb layers during the solstitial months is found to show very strong solar activity dependence with the occurrence frequency being very large during the solar minimum years and very low during solar maximum years. The intensity of the VHF radar backscattered signals from the Esb irregularities is observed to be controlled by the relative roles of the direction and magnitude of the prevailing vertical polarization electric field and the vertical electron density gradient of the prevailing Esb layer depending on the phase of the solar cycle. The gradient of the Esb layer shows a more dominant role in the generation of gradient instabilities during solar minimum periods while it is the electric field that has a more dominant role during solar maximum periods.

Comparison between nighttime ionosonde, incoherent scatter radar, AE‐E satellite, and HF Doppler observations ofFregion vertical electrodynamic plasma drifts in the vicinity of the magnetic equator

NighttimeFregion vertical drifts were made using an ionosonde for the equatorial station Ibadan (7.4°N, 3.9°E, 6°S dip) from 1 year of data during 1957–1958 International Geophysical Year (IGY) that corresponds to a period of solar maximum for undisturbed condition. We compare the seasonal vertical drifts with measurements made by incoherent scatter radar, AE‐E satellite, and HF Doppler for equatorialFregion vertical drifts. We find a comparable variability pattern during periods of highFlayer heights during equinox and the December solstice, and the opposite behavior occurs during June solstice. The drifts are predominantly downward between 2000 and 0500 LT intervals. Ionosonde drifts are smaller in values by either a factor of two or three than other methods, except for consistent June solstice ionosonde and satellite magnitudes. The equinoctial average prereversal enhancements measured by the four techniques are roughly comparable (about 36 m/s) and occur at the same local time (1900 LT) for all the seasons. The evening reversal times are similar, apart from June solstice that exhibits large variations. The morning reversal times are also in accord except for the equinoctial Jicamarca drift. Our observations indicate that ionosonde drifts measurements are in better agreement with vertical drifts results at other equatorial stations.

Comparison of satellite measurements of the low-latitude nighttime upper ionosphere with IRI

In this paper, we report the results of our comparison study between satellite measurements and the International Reference Ionosphere (IRI) model on the seasonal and longitudinal changes of the low-latitude nighttime topside ionosphere during the period of solar maximum from June 2000 to July 2001. Satellite measurements were made by KOMPSAT-1 and DMSP F15 at 685 km altitude and 840 km altitude, respectively. The results show that the IRI2001 model gives reasonable density estimations for the summer hemisphere and the March equinox at both altitudes. However, the observed wintertime densities are smaller than the predictions of the IRI2001 model, especially at a higher (840 km) altitude, manifesting strong hemispheric asymmetries. The observed electron temperatures generally reside between the two estimations of IRI2001, one based on the Aeros–ISIS data and the other based on Intercosmos, and the latter estimation better represents the observations. With more or less monotonic increase with latitude, the temperature profiles of the IRI2001 model do not predict the enhancement seen around 15° magnetic latitude of the winter hemisphere. Longitudinal variation, probably caused by the zonal winds, is seen in all seasons at both altitudes, while the IRI2001 model does not show a large variation. The observed density and temperature show significant changes according to the F10.7 values in the whole low-latitude region from 40°S to 40°N geomagnetic latitude. The effect is manifested as increases in the density and temperature, but not in the hemispheric asymmetry or in the longitudinal variation.

Space weather drivers in the ACE era

We examine ACE observations of interplanetary disturbances and energetic particle activity for the period of solar maximum during solar cycle 23 and compare these measurements to the historical record as represented by the National Space Science Data Center Omnitape of near‐Earth observations. Magnetospheric response parameters, Dst and Kp, are similarly compared for the two epochs. We find that ACE observed an unexpectedly large number of the strongest transient events with the strength of magnetospheric drivers exceeding the historical record for the space flight era. This is not entirely true of the magnetospheric response, which demonstrates that the magnetospheric response is an integrated effect that is more subtle than examination of single hourly averages would indicate. For these and other reasons, we argue that satellites at L1 orbit, such as ACE, provide an excellent database for the study of magnetospheric storm drivers and their associated energetic particle populations from which space weather models may be refined. We also compare and contrast energetic particle measurements. While protons are a large space weather concern, heavy ions are also a radiation hazard for hardware in particular. We argue that heavy ion radiation predictions based on the measurements of protons are not reliable and that only by measuring the actual composition of the solar energetic particle population can we hope to understand and address the threat posed by these particles to machines and to obtain a more complete view of the radiation environment.

Modeling and prediction of the magnetospheric dynamics during intense geospace storms

The nonlinear dynamical models of the coupled solar wind‐magnetosphere system derived from observational data have been used to yield efficient forecasts of the magnetospheric conditions. These data‐derived models have the advantage of capturing the essential features inherent in the data and thus can be improved as larger and better databases become available. Current models are based largely on the data of periods when the magnetosphere is weakly driven. For example, the widely used Bargatze et al. (1985) data set corresponds to a declining phase of the solar cycle and contains only a few weak storms. A correlated database of solar wind and magnetospheric time series data for the last solar cycle near its peak (year 2001) is compiled and used to model the magnetospheric dynamics under strong driving. The solar wind variables are obtained from ACE spacecraft, while the magnetospheric response consists of geomagnetic indices as well as magnetic perturbations measured by ground magnetometers. The dynamical models of the magnetosphere during superstorms developed with this database are used to forecast the geospace storms of October–November 2003 and April 2002 and yields improved forecasts of the intense storms. A new technique is introduced for data‐derived modeling based on the distances of the nearest neighbors from the current state. In this technique the contributions of the nearest neighbors are weighted by factors inversely proportional to the distances in the reconstructed phase space. This yields better predictions, especially during the strongly driven periods. The comparison of the models based on the year 2001 and Bargatze et al. (1985) databases show the similarities and differences in the magnetospheric states during solar maximum and minimum periods.

A mechanism for sun‐climate connection

Mechanisms by which small changes in the sun's energy output during the solar cycle can cause changes in weather and climate have been a puzzle and the subject of intense research in recent decades. Here we report that differences in surface circulation conditions during solar maximum and minimum periods are caused by differences in the frequencies with which circulation perturbations in the stratosphere reach the surface. A much greater fraction of stratospheric perturbations penetrate to the surface during solar maximum conditions than during minimum conditions. This difference is more striking when the zonal wind direction in the tropics is from the west: no stratospheric signals reach the surface when equatorial 50 hPa winds are from the west under solar minimum conditions, and over 50 percent reach the surface under solar maximum conditions. It has been previously shown that stratospheric circulation perturbations reaching the surface change weather patterns by imposing atmospheric pressure anomalies characteristic of the Arctic oscillation.

COSMIC RAY FLUXES IN THE MAXIMUM PHASE OF SOLAR ACTIVITY CYCLES

The homogeneous series of primary cosmic ray intensity with energy > 100 MeV is obtained in the long-term balloon measurements of charged particle fluxes performed by Lebedev Physical Institute. Supplementing these data with those of neutron monitors enables us to study the cosmic ray modulation on the wide energy base during more than four solar activity cycles. In the periods of solar activity maximum a transition from cosmic ray decrease to recovery occurs. Changes in fluxes of cosmic rays of lower energy lag behind the changes in fluxes of cosmic rays of higher energy producing an energy hysteresis. After a while cosmic ray fluxes of all energies start to recover. In the cycles 20 and 22 the periods of transition from decline to recovery were shorter than in the cycles 21 and 23. This may be indicative of more complicated passage from the cosmic ray drift in the A > 0 conditions to the drift in the A < 0 conditions than vice versa.

Thermal electron measurement of the nighttime upper ionosphere by KOMPSAT-1 and comparison with the IRI

KOMPSAT-1 measured electron density and temperature of the pre-midnight upper ionosphere at 685 km altitude from June 28, 2000 to August 1, 2001. The data shows enhanced electron density compared to the IRI 2001 model during this solar maximum period along the magnetic equator, shifted toward the summer hemisphere. KOMPSAT-1 data also gives higher electron temperature in most of the low latitude regions than the IRI estimations. In general, KOMPSAT-1 measurements show more significant seasonal variations than the IRI model. It is seen that the longitudinal variation of the density and temperature distribution depends on the magnetic declination angle. Also, the KOMPSAT-1 data shows the solar activity dependence of the electron density and temperature with a time delay of two days.

Global distribution of equatorial plasma bubbles in the premidnight sector during solar maximum as observed by KOMPSAT‐1 and Defense Meteorological Satellite Program F15

We investigated the global distribution of equatorial plasma bubbles (EPBs) using in situ plasma density measurements from Korea Multipurpose Satellite‐1 (KOMPSAT‐1) and Defense Meteorological Satellite Program (DMSP) F15 during the solar maximum period from June 2000 to August 2001. The results were generally consistent with those of previous studies. EPBs were observed at all longitudes around the magnetic dip equator in the equinoctial seasons with the peak occurrence in the American‐Atlantic‐African regions. During the June solstice, EPBs occurred predominantly in the African sector, with enhancements in the magnetic north in the Indian and west Pacific regions, but were totally suppressed in the American‐Atlantic sector. During the December solstice, EPBs occurred frequently in the American‐Atlantic sector but were suppressed in the other longitude sectors, especially in the Pacific sector. The EPB occurrence probability was seen to be correlated with the observed topside plasma density and the model prereversal upward drift speed of ambient plasmas (Fejer et al., 1999), with their respective dominance dependent on the seasons. However, the peak EPB occurrence in the American‐Atlantic sector during the December solstice was displaced somewhat from the region of peak density and upward drift, probably due to a strong solar terminator influence on the flux tube–integrated E region Pedersen conductivity and due to anomaly morphology. The peak EPB occurrence in the African sector during the June solstice is consistent only with the high ambient density in that region, for there was no coincidence with the maximum vertical drift or the minimum E region Pedersen conductivity.

The quasi‐biennial oscillation: Analysis using ERA‐40 data

The ERA‐40 data set is used to examine the equatorial quasi‐biennial oscillation (QBO). The data set extends from the ground to 0.1 hPa (∼65 km) and covers a 44‐year period (January 1958 to December 2001), including 18.5 QBO cycles. Analysis of this data set of unprecedented spatial and temporal coverage has revealed a threefold structure in height in the QBO zonal wind anomalies at the equator. In addition to the well‐known twofold structure in the lower and middle stratosphere, that is, easterlies overlying westerlies or vice versa, there is a third anomaly in the upper stratosphere. The sign of this upper stratospheric anomaly is the same as the lower stratospheric anomaly, thus forming anomalies of alternating sign throughout the depth of the equatorial stratosphere. The amplitude of this upper stratospheric anomaly is ∼10 m s−1, approximately one third of the amplitude of the lower stratospheric signal. The frequency and descent rates of the east and west QBO phases are analyzed in detail, with particular attention to any 11‐year solar cycle influence. In addition to the observed solar modulation of the duration of the QBO west phase the analysis shows a solar modulation of the mean descent rate of the easterly shear zone. The mean time required for the easterly shear zone to descend from 20 to 44 hPa is 2 months less under solar maximum conditions than under solar minimum conditions (7.4 months versus 9.7 months). This rapid descent of the easterly shear zone cuts short the west phase of the QBO in the lower stratosphere during solar maximum periods.

Interplanetary shocks and sudden impulses during solar maximum (2000) and solar minimum (1995–1996)

In this work a study is performed on the correlation between fast forward interplanetary shock parameters at 1 Astronomical Unit and sudden impulse (SI) amplitudes in the H-component of the geomagnetic field, for periods of solar activity maximum (year 2000) and minimum (year 1995–1996). Solar wind temperature, density and speed, and total magnetic field, were taken to calculate the static pressures (thermal and magnetic) both in the upstream and downstream sides of the shocks. The variations of the solar wind parameters and pressures were then correlated with SI amplitudes. The solar wind speed variations presented good correlations with sudden impulses, with correlation coefficients larger than 0.70 both in solar maximum and solar minimum, whereas the solar wind density presented very low correlation. The parameter better correlated with SI was the square root dynamic pressure variation, showing a larger correlation during solar maximum ( r = 0.82) than during solar minimum ( r = 0.77). The correlations of SI with square root thermal and magnetic pressure were smaller than with the dynamic pressure, but they also present a good correlation, with r > 0.70 during both solar maximum and minimum. Multiple linear correlation analysis of SI in terms of the three pressure terms have shown that 78% and 85% of the variance in SI during solar maximum and minimum, respectively, are explained by the three pressure variations. Average sudden impulse amplitude was 25 nT during solar maximum and 21 nT during solar minimum, while average square root dynamic pressure variation is 1.20 and 0.86 nPa 1/2 during solar maximum and minimum, respectively. Thus on average, fast forward interplanetary shocks are 33% stronger during solar maximum than during solar minimum, and the magnetospheric SI response has amplitude 20% higher during solar maximum than during solar minimum. A comparison with theoretical predictions (Tsyganenko’s model corrected by Earth’s induced currents) of the coefficient of sudden impulse change with solar wind dynamic pressure variation showed excellent agreement, with values around 17 nT/nPa 1/2.

Statistical Characterization of the Ionosphere Using GPS Signals

In this paper, the variability of the ionosphere is examined usingVertical TEC (VTEC) estimates obtained from GPS sig nals. The statistical structure of the ionosphere is investigated by making use of a slidingwindow mean and a variance of the estimated VTEC data. The choice of the slidingwindow size is based on the estimated variance of the first and second order moments. The investigation technique is applied to a data set obtained from the solar maximum period duringApril 23–28, 2001, for typical hig h and mid latitude stations. Duringthe quiet day of this period, it is slowly time varying and locally stationary in a slidingwindow of 15 to 20 minutes. The wide sense stationarity period of the high latitude station, both for quiet and disturbed days, is shorter than those for the mid-latitude stations, which indicates the higher variability of ionosphere for high latitudes.

Understanding midlatitude space weather: Storm impacts observed at Bear Lake Observatory on 31 March 2001

On 30 March 2001 in the late evening an auroral display was observed over the United States of America. The Bear Lake Observatory (BLO) magnetometer in Utah measured changes of 550 nT in less than 30 min. During the same period, BLO ionosonde measurements showed deep high‐frequency radio wave absorption up to 7 MHz. BLO's GPS single‐frequency receiver experienced geolocation errors of 20 m for over 3 hours. These storm signatures were also accompanied by L‐band scintillation effects which approached an S4 value of 0.2, which is large for midlatitudes. Although such measurements have been have been made at midlatitude locations for many decades, our knowledge of the processes and couplings involved in such events remains incomplete and, at best, qualitative. The interpretation of key ionospheric parameters' storm response is discussed in the context of present‐day auroral and geospace electrodynamics understanding. We find that at BLO (L = 2.38) the available data raise more questions and can provide almost no answers without observational inputs from other locations. One solution to this impasse is to field a ground‐based sensor network to resolve the spatial scales of the geospace electrodynamics. On the basis of the instrument complement at BLO, we argue for a contiguous U.S. deployment of modest magnetic/optical/RF observatories to observe the next solar maximum period's geomagnetic storms and to use these data to explore the physical processes and couplings on space weather effective scales in assimilative models in conjunction with space‐based observations.

Seasonal modulation of GPS performance due to equatorial scintillation

Evening scintillation is an aspect of space weather occurring primarily in the equatorial region due to scattering of satellite signals by ionospheric F‐region irregularities. In order to quantify scintillation‐related performance degradation in GPS (Global Positioning System), we operated single frequency (L1) Ionospheric Scintillation Monitors (ISM), sampling at 50 Hz, at 6 sites in the South East Asia/Oceania region during the most recent solar maximum period 1998–2002. Scintillation indices, horizontal position, receiver lock time and dilution of precision were recorded every minute. Data reveal that individual one‐minute positional errors can increase from a few meters to 30 m during scintillation, whilst statistics at the hourly upper decile level reveal an underlying seasonal modulation from about 5 m to 15 m. Error maxima occur during the equinoxes and are more pronounced at the equatorial anomaly crests than at the magnetic equator, as expected. Such errors may concern users of systems that are dependent on GPS.

Wavelet analysis of long period oscillations in geomagnetic field over the magnetic equator

In this paper the complex Morlet Wavelet Transform is used to identify long period oscillations in the horizontal component ( H) of the geomagnetic field over the magnetic equatorial location of Trivandrum (8.5°N; 77°E; dip lat. 0.5°N) during the solar maximum period 1990/1991 and solar minimum period 1995/1996. The Morlet WT of the geomagnetic data set indicates the presence of multiple timescales, which are localized in both frequency and time. Extra long period oscillations are observed in the period range of 40–80, and 80–130 days during 1995/1996 (October–April), and 30–60, 130–180 days during 1990/1991 (October–April). This is one of the first such observations in the nature of long period oscillations in the earth's magnetic field. These observations on the existence of different long wave periods in the geomagnetic field are consistent with their findings in the lower and middle atmosphere (J. Atmos. Sol. Terr. Phys. 63 (2001) 835; J. Atmos. Sci. 29 (1972) 1109). This analysis also brings out clearly the merging of long period oscillations with short period oscillations during conditions of geomagnetic disturbance.

Observations of solar cyclical variations in geocoronal Hα column emission intensities

Observations of thermospheric + exospheric Hα column emissions by the Wisconsin Hα Mapper (WHAM) Fabry‐Perot (Kitt Peak, Arizona) over the 1997–2001 rise in solar cycle 23 show a statistically significant solar cyclical variation. The higher signal‐to‐noise WHAM observations corroborate suggestions of a solar cycle trend in the Hα emissions seen in Wisconsin observations over solar cycle 22. Here we compare WHAM 1997 and 2000–2001 winter solstice geocoronal Hα observations toward regions of the sky with low galactic emission. The observed variation in geocoronal hydrogen column emission intensities over the solar cycle is small compared with variations in hydrogen exobase densities. Higher Hα emissions are seen during solar maximum periods of the solar cycle. At a mid range shadow altitude (3000 km), WHAM geocoronal Hα intensities are about 45% higher during solar maximum than during solar minimum.

VLF wave activity in the solar wind and the photoelectron effect in electric field measurements: Ulysses observations

We present observations of very low frequency (VLF) waves (0.2 to 448 Hz) made by Ulysses during the solar maximum and compare the observations with those obtained when the solar activity was approaching a minimum. The occurrences and properties of the waves are found to be similar during the solar maximum and solar minimum periods for slow and intermediate speed solar wind. The maximum intensities of the electromagnetic waves for the two solar cycle periods are comparable. These similarities suggest that the plasma conditions for the waves' excitation are similar for the slow and intermediate solar wind in both solar maximum and minimum phases. It is also found that the electric field noise detected in the low band channels, which are measuring less than 10 Hz signals, is contaminated by the spin modulation of the electric field caused by the photoelectron cloud around the spacecraft, especially when the ambient plasma density is low. This effect enhances with increasing solar aspect angle.

03/01735 Analysis of generated power of ETS-VII during solar activity maximum period: Fujita, R. et al. Solar Energy Materials and Solar Cells, 2003, 75, (1–2), 319–325

03/01735 Analysis of generated power of ETS-VII during solar activity maximum period: Fujita, R. et al. Solar Energy Materials and Solar Cells, 2003, 75, (1–2), 319–325